The Open Dentistry Journal


ISSN: 1874-2106 ― Volume 12, 2018

The Aponeurotic Tension Model of Craniofacial Growth in Man



Richard G Standerwick1, *, W. Eugene Roberts2
1 20159 - 88th Ave, Suite E207, Langley BC V1M 0A4, Canada
2 5955 S. Emerson Avenue, Ste. 200, Indianapolis, IN 46237, USA

Abstract

Craniofacial growth is a scientific crossroad for the fundamental mechanisms of musculoskeletal physiology. Better understanding of growth and development will provide new insights into repair, regeneration and adaptation to applied loads. Traditional craniofacial growth concepts are insufficient to explain the dynamics of airway/vocal tract development, cranial rotation, basicranial flexion and the role of the cranial base in expression of facial proportions. A testable hypothesis is needed to explore the physiological pressure propelling midface growth and the role of neural factors in expression of musculoskeletal adaptation after the cessation of anterior cranial base growth.

A novel model for craniofacial growth is proposed for: 1. brain growth and craniofacial adaptation up to the age of 20; 2. explaining growth force vectors; 3. defining the role of muscle plasticity as a conduit for craniofacial growth forces; and 4. describing the effect of cranial rotation in the expression of facial form.

Growth of the viscerocranium is believed to be influenced by the superficial musculoaponeurotic systems (SMAS) of the head through residual tension in the occipitofrontalis muscle as a result of cephalad brain growth and cranial rotation. The coordinated effects of the regional SMAS develop a craniofacial musculoaponeurotic system (CFMAS), which is believed to affect maxillary and mandibular development.

Key Words: Brain, Aponeurotic, Airway, Mandible, Rotation, Muscle, SMAS.


Article Information


Identifiers and Pagination:

Year: 2009
Volume: 3
First Page: 100
Last Page: 113
Publisher Id: TODENTJ-3-100
DOI: 10.2174/1874210600903010100

Article History:

Received Date: 5/1/2009
Revision Received Date: 26/2/2009
Acceptance Date: 26/3/2009
Electronic publication date: 22/5/2009
Collection year: 2009

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© Standerwick and Roberts; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http: //creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.


* Address correspondence to this author at the 20159 - 88th Ave, Suite E207, Langley BC V1M 0A4, Canada; Tel: (604) 888-3450; E-mail: rjstanderwick@gmail.com




INTRODUCTION

The soft tissue matrix, in which skeletal elements are embedded, is the primary determinant of growth, while both the bone and cartilage are secondary growth sites. Growth centers display inherent growth versus growth sites which are reactive [1Proffit WR, Fields HW. Contemporary orthodontics. 3rd ed. St. Louis: Mosby 2000.,2Roberts WE, Hartsfield JK Jr. Bone development and function: Genetic and environmental mechanisms Semin Orthod 2004; 10(2): 100-22.].

This is the fundamental premise of the Functional Matrix Theory of Melvin Moss [3Moss ML. A theoretical analysis of the functional matrix Acta Biotheor 1968; 18(1): 195-202.]. The soft tissue matrix (muscles, connective tissue, neural tissue) models the bone, rather than bone morphology being genetically determined.

Proposed is a descriptive model of craniofacial growth based on the principle that late brain growth and cranial rotation create a residual tension in the occipitofrontalis muscles which in turn loads the facial superficial musculoaponeurotic systems (SMAS) via connected fascia, muscle and ligaments and reflect a craniofacial musculoaponeurotic system (CFMAS).

PURPOSE

The purpose of this review is to provide a comparative, biologically accurate and clinically effective framework for understanding [4Carlson DS. Theories of craniofacial growth in the postgenomic era Semin Orthod 2005; 11(4): 172-83.] the coordination of brain and craniofacial growth (CFG), and the relationship of brain growth to cranial rotation, airway and vocal development. There is a strong belief that the musculoaponeurotic system of the skull has a direct effect on maxillary and mandibular development, and forms the underlying theme of the proposed aponeurotic tension model of craniofacial growth.

TRADITIONAL CONCEPTS

Moss introduced the functional matrix theory describing skeletal growth as a secondary, compensatory, and mechanically obligatory response to temporally and morphogenetically prior growth changes in specially related tissues and organs [5Moss ML, Young RW. A functional approach to craniology Am J Phys Anthropol 1960; 18: 281-92.-7Moss ML, Salentijn L. The primary role of functional matrices in facial growth Am J Orthod 1969; 55(6): 566-77.]. A solitary growth matrix for the entire head is difficult to explain, therefore Moss divided the head into areas (capsular matrices) such as the neurocranial capsular matrix and the orofacial capsular matrix [8Moss ML. Ontogenetic aspects of cranio-facial growth. 1st ed. Oxford New York: Pergamon Press 1971.], the latter being comprised of the teeth, sinus spaces, muscles and connective tissue (blood vessels, etc...). Neurocranial capsular matrix enlargement resulting from neural growth seems self-evident. However, the nature of the orofacial capsular matrix growth is more elusive. It is believed that the orofacial capsular matrix enlargement is driven by airway enlargement and that the direction of this facial growth is caudad and ventral (Fig. 1).

Fig. (1)

(A) Displayed is the ventral and caudad growth direction of the ethmomaxillary complex Illustration from [1Proffit WR, Fields HW. Contemporary orthodontics. 3rd ed. St. Louis: Mosby 2000.]. (B) Houston [16Houston WJ. Mandibular growth rotations, their mechanisms and importance Eur J Orthod 1988; 10(4): 369-73.] combined his CFG model based on cervical vertebrae growth, with (C) the soft-tissue stretching of Solow [17Solow B, Kreiborg S. Soft-tissue stretching: a possible control factor in craniofacial morphogenesis Scand J Dent Res 1977; 85(6): 505-7.] and cranial posture changes [114Solow B, Tallgren A. Dentoalveolar morphology in relation to craniocervical posture Angle Orthod 1977; 47(3): 157-64.]. Illustrations from [16Houston WJ. Mandibular growth rotations, their mechanisms and importance Eur J Orthod 1988; 10(4): 369-73., 17Solow B, Kreiborg S. Soft-tissue stretching: a possible control factor in craniofacial morphogenesis Scand J Dent Res 1977; 85(6): 505-7.]. Houston’s model cannot explain forward/counter-clockwise mandibular rotation. (D) An anatomical drawing of the aponeurotic tension model of craniofacial growth. Shown are: the force of gravity (black arrows) and CFMAS tension (white arrows); the frontalis muscle (frontalis), the occipitalis muscle (occipitalis) and the area between is the location of the galea aponeurotica (aponeurosis). The modiolus (muscular confluence joining the upper portion of the muscle mask with the lower portion) is found vertically between the black arrows overlying the cheek and chin. Illustration adapted from [115Liebgott B. The anatomical basis of dentistry Rev Rep ed. St. Louis: C.V. Mosby 1986.].



The nasal septum model of Scott [9Scott JH. The cartilage of the nasal septum: a contribution to the study of facial growth Br Dent J 1953; 95: 37-43.] describes the nasal septum as a growth center forcing the viscerocranium caudad and ventral relative to the cranial base until the facial sutures have become stabilized by dense connective tissue [9Scott JH. The cartilage of the nasal septum: a contribution to the study of facial growth Br Dent J 1953; 95: 37-43.,10Dixon A, Hoyte D, Rönning O. Fundamentals of craniofacial growth. Boca Raton: CRC Press 1997.]. The nasal septum directs prenatal and some postnatal growth to the approximate age of 4 years, [4Carlson DS. Theories of craniofacial growth in the postgenomic era Semin Orthod 2005; 11(4): 172-83.] but the brain is believed to be the primary growth center for CFG until approximately the age of 8 [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.]. After neural growth is complete, the more inferior portions of the anterior cranial base (ACB) are considered to continue “growing” caudad and ventral, causing drift and displacement of the ethmomaxillary (midface) complex [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.-13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.] in conjunction with growth at the sphenooccipital synchondrosis (SOS: a hyaline cartilage growth center between the clivus of the occipital bone and sphenoid bone) [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.]. The anteroposterior force of the SOS displacement is believed by some to be transmitted through the nasal septum acting as a strut connecting to the midface [4Carlson DS. Theories of craniofacial growth in the postgenomic era Semin Orthod 2005; 11(4): 172-83.,13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.,14Ranly DM. Craniofacial growth Dent Clin North Am 2000; 44(3): 457-70.]. Alternatively, the advancement of the midface could be a result of physical growth forces of enclosing soft tissues. For example, displacement of facial sutures could result from the enlargement of muscles, [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.] or from growth of the brain temporal lobe, infraorbital and retromaxillary fat pads, and infratemporal fossa contents (pterygoid muscles, fat pads) creating a laterally transmitted force to the midface [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.,15Latham RA, Scott JH. A newly postulated factor in the early growth of the human middle face and the theory of multiple assurance Arch Oral Biol 1970; 15(11): 1097-0.].

The facial tissues have previously been described by Houston [16Houston WJ. Mandibular growth rotations, their mechanisms and importance Eur J Orthod 1988; 10(4): 369-73.] and Solow [17Solow B, Kreiborg S. Soft-tissue stretching: a possible control factor in craniofacial morphogenesis Scand J Dent Res 1977; 85(6): 505-7.] (Fig. 1), as influencing facial growth but from the aspect of growth restriction and postural change through soft tissue forces by facial application of a dorsally directed force to the underlying skeletal structures. The Servosystem model of Petrovic [18Petrovic A. Control of postnatal growth of secondary cartilages of the mandible by mechanisms regulating occlusion Cybernetic model Trans Eur Orthod Soc 1974; 69-75.] assumes the displacement of the midface through nasal septum growth in conjunction with direct thrust of labionarinary muscles, and through the superior labial frenum and septopremaxillary ligament [19Petrovic A, Stutzmann J, Lavergne J. Mechanisms of craniofacial growth and modus operandi of functional appliances: a cell-level and cybernetic approach to orthodontic decision making In: Carlson DS, Ed. Craniofacial growth theory and orthodontic treatment: Craniofacial growth series 23. Ann Arbor: Center for Human Growth & Development, The University of Michigan 1990.]. As the maxilla is moved ventrally there is believed to be compensation maintaining the mandibular relationship to the maxilla.

Although each of the models has added to our attempt at understanding CFG, none of the CFG models seem to directly neither address nor provide a model for the differing patterns of maxillomandibular rotation in hypo- and hyperdivergent individuals as described by Bjork [20Bjork A. Facial growth in man, studied with the aid of metallic implants Acta Odontol Scand 1955; 13(1): 9-34.,21Bjork A, Skieller V. Normal and abnormal growth of the mandible: a synthesis of longitudinal cephalometric implant studies over a period of 25 years Eur J Orthod 1983; 5(1): 1-46.]. There are inconsistencies found in earlier theories that are believed more effectively addressed with the proposed CFG model. The proposed model explains why the airway enlarges sagittally, despite a backward slide of the vomer between infancy and adulthood [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.]. If it were the only issue, the relative descent of the larynx would be plausible with traditional caudad and ventral CFG model as there is significant growth of the cervical vertebrae, for which the heights of the vertebrae roughly double by growth at their respective epiphyseal plates [22Wang JC, Nuccion SL, Feighan JE, Cohen B, Dorey FJ, Scoles PV. Growth and development of the pediatric cervical spine documented radiographically J Bone Joint Surg Am 2001; 83-A(8): 1212-8.]. However, Houston’s cervical growth model [16Houston WJ. Mandibular growth rotations, their mechanisms and importance Eur J Orthod 1988; 10(4): 369-73.] cannot explain forward mandibular rotation as the tissue are oriented to provide a dorso-caudad force.

Also, the maxilla is believed to display “growth” at the maxillary tuberosity causing pressure against the pterygoid plate of the sphenoid bone. This pressure is believed to displace the maxilla forward. However, between the pterygoid plate and maxilla there is a fibrous suture rather than synchondrosis. Compression in sutures has been shown to display resorption [23Cohen MM Jr. Sutural biology and the correlates of craniosynostosis Am J Med Genet 1993; 47(5): 581-616.-25Roberts W. Bone Physiology, metabolism, and biomechanics in orthodontic practice Orthodontics Current principles & techniques In: Graber TM, Vanarsdall RL, Vig KWL, Eds. Orthodontics : Current principles & techniques. 4 ed. St. Louis: Mo.: Elsevier Mosby 2005; pp. 221-92.] and the bone (pterygoid plates) in the area is much too pliable in that environment to withstand the pressures [26Ranly DM. A synopsis of craniofacial growth. New York: Appleton-Century-Crofts 1980.]. The lack of inherent growth potential of sutures negates any ventral thrust of midface growth by the circumaxillary suture system [2Roberts WE, Hartsfield JK Jr. Bone development and function: Genetic and environmental mechanisms Semin Orthod 2004; 10(2): 100-22.,4Carlson DS. Theories of craniofacial growth in the postgenomic era Semin Orthod 2005; 11(4): 172-83.,14Ranly DM. Craniofacial growth Dent Clin North Am 2000; 44(3): 457-70.]. The viscerocranial units (maxilla, mandible) seem suspended in CFMAS relative to cranial rotation concomitant with allometric brain growth. Further investigation of the degree of viscerocranial suspension by the CFMAS relative to elastic fiber and collagen fiber resistance within the suture itself seems warranted.

The relative fixed position of the zygomatic processes as the remainder of the maxilla is thrust forward is assumed to be a result of maxillary resorption anteriorly and deposition posteriorly [26Ranly DM. A synopsis of craniofacial growth. New York: Appleton-Century-Crofts 1980.]. Problematic is that the anterior resorption is superficial as the anterior surface displays stability relative to endosseous implants. Essentially, all of the increase in maxillary length occurs posteriorly [27Bjork A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method Br J Orthod 1977; 4(2): 53-64.-29Bjork A. Sutural growth of the upper face studied by the implant method Acta Odontol Scand 2007; 29: i82-.]. Considering the limitations of microscopic histology, this process requires further study with intravital markers [30Roberts WE, Roberts JA, Epker BN, Burr DB, Hartsfield Jr JK. Remodeling of mineralized tissues, Part I: the frost legacy Semin Orthod 2006; 12(4): 216-37.]. Cranial rotation better explains the observed pattern of surface resorption anteriorly due to pressure applied by CFMAS weight to the anterior region during the rotation. Surface resorption is created as bone advances into the drape of the CFMAS pressure is created on the anterior bone leading to collapse of the vasculature, stimulating compensatory modeling. Deposition posteriorly is stimulated by tension created within the suture by cranial rotation and the associated force of the CFMAS. Cranial rotation and the facial block concept [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 31McCarthy RC, Lieberman DE. Posterior maxillary (PM) plane and anterior cranial architecture in primates Anat Rec 2001; 264(3): 247-60.] would rectify the conflict of Bjork’s observations displaying a stable zygomatic surface relative to implants [27Bjork A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method Br J Orthod 1977; 4(2): 53-64.], while Enlow believed the zygoma relocated dorsally [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.]. Both were correct. The rotation of the maxilla relative to the zygoma would display a stable zygoma surface relative to an implant, while slight surface resorption of the zygoma would be observed due to pressure of the facial muscle mask. The rotation of the maxilla would be displaced forward relative to the zygoma leading to the conclusion that the zygoma must be posteriorly displaced with growth. This also explains the observation that the mandible growth rotation is greater that maxillary growth rotation; at the same time, the mandible is also rotating around the maxilla [32Bjork A, Skieller V. Postnatal growth and development of the maxillary complex In: Moyers RE, McNamara JA, Eds. Factors affecting the growth of the midface, proceedings of a sponsored symposium honoring Professor Robert E Moyers, held February 6 and 7, 1976, in Ann Arbor, Michigan. Ann Arbor: Craniofacial Publications, CHGD 1976; pp. 61-99.].

ALTERNATIVE MODEL INTRODUCED TO SUPPLEMENT THE CURRENT UNDERSTANDING OF CRANIOFACIAL GROWTH IN MAN

Proposed is an alternative model of CFG based on musculoaponeurotic tension enveloping the head. This CFG model describes the affect of late cephalad growth of the brain pushing on the occipitofrontalis muscle (Figs. 1 and 2), which places the muscle in tension, the peak in temporal and occipital lobe gray matter being at 16-20 of years age [33Giedd JN, Blumenthal J, Jeffries NO, et al. Brain development during childhood and adolescence: a longitudinal MRI study Nat Neurosci 1999; 2(10): 861-3.]. The tension force is transmitted from the occipitofrontalis down through the mask of muscles overlying the face due to individual muscle fiber blending with adjacent muscles and the associated superficial musculoaponeurotic systems (SMAS; investing connective tissues) [34Kushima H, Matsuo K, Yuzuriha S, Kitazawa T, Moriizumi T. The occipitofrontalis muscle is composed of two physiologically and anatomically different muscles separately affecting the positions of the eyebrow and hairline Br J Plastic Surg 2005; 58: 681-7.-40Spiegel JH, DeRosa J. The anatomical relationship between the orbicularis oculi muscle and the levator labii superioris and zygomaticus muscle complexes Plast Reconstr Surg 2005; 116(7): 1937-42. discussion 43-4]. This facial muscle mask and associated regional SMAS are believed to be part of a CFMAS important in directing craniofacial development and jaw rotation by acting as a conduit for the brain derived force. Cranial rotation [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 41Bjork A. Cranial base development: : a follow-up x-ray study of the individual variation in growth occuring between the ages of 12 and 20 years and its relation to brain case and face development Am J Orthod 1955; 41: 198-225.] is also believed to occur sagittally around the atlantooccipital joint as a result of allometric brain growth and progressive facial bone pneumatization with sinus development.

Fig. (2)

Algorithmic adaptation of the proposed CFG model. Displayed is brain growth modulation of cranial rotation and CFMAS tension. Brain growth can display temporal regional growth and mylenization with normal development and as a result of trauma (concussion, drug use). Cranial rotation modulates CFMAS tension and itself is influenced by brain development and postural control. CFMAS tension manifests as as a strong or weak phenotype. A strong CFMAS phenotype will be expected to develop a counter-clockwise/forward maxillomandibular rotation, while a weak CFMAS phenotype will be expected to develop a clockwise/backward maxillomandibular rotation pattern.



Brain extension consists of uprighting of the cerebral portion of the brain, and therefore also the head, relative to the body axis during growth and development. This pattern is mimicked by cranial base (basicranial) flexure [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 41Bjork A. Cranial base development: : a follow-up x-ray study of the individual variation in growth occuring between the ages of 12 and 20 years and its relation to brain case and face development Am J Orthod 1955; 41: 198-225.] and airorhynchy (posterior and upper portions of the face rotate dorsally relative to the posterior cranial base by extension of the ACB relative to the posterior cranial base (PCB), analogous with the “facial block”) [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.] (Fig. 3). Cranial rotation being intimately integrated with the latter, the term cranial rotation will be used collectively for them from this point. It is possible that the proposed model applies to only humans as it seems coordinated with cranial base flexion during growth which is unique to humans [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.].

Fig. (3)

(A) The “facial block” showing the effects of angular invariance between the back of the face (summarized by the PM plane) and the top of the face, which is also the bottom of the anterior cranial base (S-FC). Changes in cranial base angle (NHA-BaS or alternatively S-FC-BaS) cause the top and back of the face to rotate together around an imaginary axis through the PM point [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.]. Illustration from [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.]. (B) Osteocutaneous ligaments that originate from periosteum and insert directly into dermis (zygomatic and mandibular ligaments are shown; zygomaticus minor and major muscles, masseteric cutaneous ligaments and risorius are also shown). Note the modiolus as a confluence of muscles located at the angle of the mouth. Illustration from [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1]. (C) Skull in norma lateralis shows the asymmetric growth of the spheno-occipital synchondrosis. Tension on the aponeurosis (arrows at glabella and occipital nuchal line) are expressed as an inverted V with the net force at the center of the skull, superior to the occipital condyle (large arrow). This is expected to create a lever system (dark lines) pivoting at the large arrow and extending to the smaller arrows. These are the effective forces of the muscles on the dorsal and ventral skull surface. (D) The divergence of the facial planes (sella-nasion at the top and then descending; orbitale-porion or the Frankfort horizontal; palatal plane; occlusal plane; mandibular plane).



Brain Growth and Craniofacial Adaptation up to the Age of 20

Lateral cranial expansion is limited after approximately 1 year of age [10Dixon A, Hoyte D, Rönning O. Fundamentals of craniofacial growth. Boca Raton: CRC Press 1997.,14Ranly DM. Craniofacial growth Dent Clin North Am 2000; 44(3): 457-70.] and ACB anteroposterior growth is limited by 8 years of age with the fusion of sphenoethmoid, frontoethmoid and intersphenoid synchondroses [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.]. The brain temporal lobe continues growing for several more years after brain anterior lobe growth has ceased [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.]. However, elongation of the middle cranial fossa (MCB) nearly ceases by 10 years of age [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.]. Therefore, it is proposed that most brain growth must be in a cephalad direction [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.].

The concept of a cephalad growth component is derived from the works of Bergersen [42Bergersen EO. The directions of facial growth from infancy to adulthood Angle Orthod 1966; 36(1): 18-43.], Bjork [21Bjork A, Skieller V. Normal and abnormal growth of the mandible: a synthesis of longitudinal cephalometric implant studies over a period of 25 years Eur J Orthod 1983; 5(1): 1-46., 32Bjork A, Skieller V. Postnatal growth and development of the maxillary complex In: Moyers RE, McNamara JA, Eds. Factors affecting the growth of the midface, proceedings of a sponsored symposium honoring Professor Robert E Moyers, held February 6 and 7, 1976, in Ann Arbor, Michigan. Ann Arbor: Craniofacial Publications, CHGD 1976; pp. 61-99., 41Bjork A. Cranial base development: : a follow-up x-ray study of the individual variation in growth occuring between the ages of 12 and 20 years and its relation to brain case and face development Am J Orthod 1955; 41: 198-225.], Broadbent [43Broadbent BH. The face of the normal child Angle Orthod 1937; 7(4): 183-208.], Coben [44Coben SE. Growth concepts Angle Orthod 1961; 31(3): 194-201.], Kanomi (K point), Melsen [45Melsen B. The cranial base : the postnatal development of the cranial base studied histologically on human autopsy material Thesis Acta Odontol Scand 1974; 32(Suppl 62).] and current observations superimposing overlay tracings of sequential cephalometric radiographs referenced on the occipital condyles [46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0., 47Standerwick RG, Roberts WE, Hartsfield JK Jr, Babler WJ, Katona TR. Comparison of the bolton standards to longitudinal cephalograms superimposed on the occipital condyle (I-point) J Orthod 2009; 36: 23-35.]. All of these data demonstrate cephalad movement of sella turcica relative to the PCB of the skull [46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0., 47Standerwick RG, Roberts WE, Hartsfield JK Jr, Babler WJ, Katona TR. Comparison of the bolton standards to longitudinal cephalograms superimposed on the occipital condyle (I-point) J Orthod 2009; 36: 23-35.]. Microscopic evidence of cephalad brain movement is evident as bone apposition along the surfaces of sella turcica, including the anterior curvature of sella turcica, which is traditionally considered stable yet displays pubertal apposition [10Dixon A, Hoyte D, Rönning O. Fundamentals of craniofacial growth. Boca Raton: CRC Press 1997., 13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996., 45Melsen B. The cranial base : the postnatal development of the cranial base studied histologically on human autopsy material Thesis Acta Odontol Scand 1974; 32(Suppl 62)., 48Enlow DH, Kuroda T, Lewis AB. The morphological and morphogenetic basis for craniofacial form and pattern Angle Orthod 1971; 41(3): 161-88., 49Ghafari J, Engel FE, Laster LL. Cephalometric superimposition on the cranial base: a review and a comparison of four methods Am J Orthod Dentofacial Orthop 1987; 91(5): 403-13.]. Late brain growth beyond what is traditionally accepted is consistent with more recent observations. For example, brain allometry has been observed with peak development of gray matter at approximately the age of 12 for the frontal and parietal lobes, age 16 for the temporal lobes, and through 20 for the occipital lobe [33Giedd JN, Blumenthal J, Jeffries NO, et al. Brain development during childhood and adolescence: a longitudinal MRI study Nat Neurosci 1999; 2(10): 861-3.]. Moreover, gender related pubertal spurts in brain growth suggest gonadal hormone influence, [33Giedd JN, Blumenthal J, Jeffries NO, et al. Brain development during childhood and adolescence: a longitudinal MRI study Nat Neurosci 1999; 2(10): 861-3.] and coincide with the somatic growth spurt.

This late brain growth may not have been previously apparent as traditional methods of intercranial volume measurement could be misleading. Radiographic midline structures may not be an adequate reference, [50Bastir M, Rosas A. Correlated variation between the lateral basicranium and the face: a geometric morphometric study in different human groups Arch Oral Biol 2006; 51(9): 814-24.] anthropometric measurements may not be reliable, [51Sahin B, Acer N, Sonmez OF, et al. Comparison of four methods for the estimation of intracranial volume: a gold standard study Clin Anat 2007; 20(7): 766-3.] and the stability of the ACB structural midline at age 10 is only valid for the ethmoid region [41Bjork A. Cranial base development: : a follow-up x-ray study of the individual variation in growth occuring between the ages of 12 and 20 years and its relation to brain case and face development Am J Orthod 1955; 41: 198-225.].

Traditionally, weight of the growing brain is expected to cause surface resorption on the endocranial aspect of the human cranial base, [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996., 48Enlow DH, Kuroda T, Lewis AB. The morphological and morphogenetic basis for craniofacial form and pattern Angle Orthod 1971; 41(3): 161-88.] but this has not been experimentally tested [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.]. Intravital staining and an accurate specimen age are still needed to establish the quantity and temporal nature of deposition or resorption for the endocranial compartments with growth and development [30Roberts WE, Roberts JA, Epker BN, Burr DB, Hartsfield Jr JK. Remodeling of mineralized tissues, Part I: the frost legacy Semin Orthod 2006; 12(4): 216-37.] as assessment of these is impossible with microscopic histology.

Cranial base flexion during growth is unique to humans and complements inferior drift in the PCB by moving the floor of the PCB caudad relative to the middle cranial fossa [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.]. The surface resorption of the cranial fossa seems negligible when comparing the growth of the PCB relative to the position of the occipital condyles, MCB and ACB [46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0., 47Standerwick RG, Roberts WE, Hartsfield JK Jr, Babler WJ, Katona TR. Comparison of the bolton standards to longitudinal cephalograms superimposed on the occipital condyle (I-point) J Orthod 2009; 36: 23-35., 52Broadbent BH, Broadbent BH, Golden WH. Bolton standards of dentofacial developmental growth. Saint Louis: Mosby 1975.]. The occipital condyles are a stable structure to reference due to the lack of an epiphyseal growth plate in the presence of a large pressure gradient [2Roberts WE, Hartsfield JK Jr. Bone development and function: Genetic and environmental mechanisms Semin Orthod 2004; 10(2): 100-22., 46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0., 47Standerwick RG, Roberts WE, Hartsfield JK Jr, Babler WJ, Katona TR. Comparison of the bolton standards to longitudinal cephalograms superimposed on the occipital condyle (I-point) J Orthod 2009; 36: 23-35.]. The observed movement with the occipital condyle reference seems to correlate with the observation that PCB displays relative inferior drift which positions the PCB below the middle cranial fossa [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.]. Growth of the basilar region of the skull must be limited to avoid impinging on critical neural structures [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.] and therefore dural “slings” cradle the brain and restrict growth that might compromise neurologic function (Fig. 4). The desmocranial capsule (dura) is anisotropic, being thicker at its base which grows slowly and resists the enlargement of the brain in the developing cranial base. Over the calvarial region it is thinner and less resistant, allowing the cerebral hemispheres, and to a lesser extent, the cerebellar hemispheres to expand more rapidly [10Dixon A, Hoyte D, Rönning O. Fundamentals of craniofacial growth. Boca Raton: CRC Press 1997., 53Friede H. Normal development and growth of the human neurocranium and cranial base Scand J Plast Reconstr Surg 1981; 15(3): 163-9.]. Also, cephalad brain growth better explains why the entire cranial base surface is not resorptive, but displays areas of bone deposition: the petrous portion of the temporal bone, crista galli/foramen cecum, between the occipital lobes, and sella turcica [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996., 48Enlow DH, Kuroda T, Lewis AB. The morphological and morphogenetic basis for craniofacial form and pattern Angle Orthod 1971; 41(3): 161-88.] (Fig. 4). Continued relative growth of the PCB into the adolescent years is an important factor that has not been adequately appreciated. The relative effects of cephalad brain growth warrant further study.

Fig. (4)

Cranial resorption (dark stipple) and deposition patterns (light stipple) are illustrated on sectioned skulls. Areas of bone deposition are along the petrous portion of the temporal bone, crista galli/foramen cecum, between the occipital lobes, and sella turcica. The anterior (ACB), middle (MCB) and posterior (PCB) cranial fossa are shown (bottom left). The proposed normal distribution of facial bone resorption and deposition is shown (top). Bottom right displays the desmocranial lining of the cranial base; the lining is continuous with the falx cerebri but shown are the tentorium cerebelli (TC) and its attachment to the anterior clinoid process (ACP). This connection spans the SOS creating tension on the endocranial aspect of the SOS and contributing to differential growth of the SOS. Illustration from [48Enlow DH, Kuroda T, Lewis AB. The morphological and morphogenetic basis for craniofacial form and pattern Angle Orthod 1971; 41(3): 161-88.].



Explaining Growth Force Vectors

Mitz and Peyronie (Mitz and Peyronie, 1976) originally coined the term SMAS, yet despite numerous publications on this subject, there remain significant variations in the anatomic descriptions of facial fascial anatomy, and descriptions of the relationship between the superficial and deep facial fascia remain imprecise [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1]. The SMAS is a composite fibro-fatty layer comprising collagen and elastic fibers interspersed with fat cells. It microscopically displays a considerable amount of elastic fibers in close relationship to the collagen fibers, and the collagen fibers display a convoluted appearance similar to that found in the dermis [55Har-Shai Y, Bodner SR, Egozy-Golan D, et al. Mechanical properties and microstructure of the superficial musculoaponeurotic system Plast Reconstr Surg 1996; 98(1): 59-70. discussion 1-3]. The SMAS invests the superficially lying mimetic muscles (muscles of facial expression; e.g. platysma, orbicularis oculi, zygomaticus major, and risorius) and forms a continuous sheath throughout the head and neck, extending into the temporal region, forehead, scalp, malar areas, nose and upper lip. Thus the superficial facial fascia is intimately associated with the mimetic muscles [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1, 56Mitz V, Peyronie M. The superficial musculo-aponeurotic system (SMAS) in the parotid and cheek area Plast Reconstr Surg 1976; 58(1): 80-.]. The mimetic muscles and SMAS function as a single anatomic unit in producing movement of facial skin [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1] and the low viscoelastic properties of the SMAS are the reason for incorporation of the SMAS as a standard part of the rhytidectomy (facial lift) procedure [55Har-Shai Y, Bodner SR, Egozy-Golan D, et al. Mechanical properties and microstructure of the superficial musculoaponeurotic system Plast Reconstr Surg 1996; 98(1): 59-70. discussion 1-3, 57Saulis AS, Lautenschlager EP, Mustoe TA. Biomechanical and viscoelastic properties of skin, SMAS, and composite flaps as they pertain to rhytidectomy Plast Reconstr Surg 2002; 110(2): 590-8. discussion 9-600]. Also deserving further investigation and consideration are the relatively thick osteocutaneous retaining ligaments that anchor periosteum to dermis, notable being the zygomatic and mandibular ligaments [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1] (Fig. 3).

The coordinated effect of facial muscle and regional SMAS blending develops a CFMAS. The proposed model provides a more biologically correct explanation for observations that earlier models have been unable to provide satisfactorily. The tension conducted through the CFMAS explains the enlargement of airway and maxillomandibular rotation patterns consistent with the observations of implant studies [28Bjork A, Skieller V. Facial development and tooth eruption. An implant study at the age of puberty Am J Orthod 1972; 62(4): 339-83., 58Gans BJ, Sarnat BG. Sutural facial growth of the Macaca rhesus monkey: a gross and serial roentgenographic study by means of metallic implants Am J Orthod 1951; 37(11): 827-41.] and cephalometric superimposition referenced at I-point on the occipital condyles in norma lateralis [46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0., 47Standerwick RG, Roberts WE, Hartsfield JK Jr, Babler WJ, Katona TR. Comparison of the bolton standards to longitudinal cephalograms superimposed on the occipital condyle (I-point) J Orthod 2009; 36: 23-35.].

Growth associated force transmittance through mimetic muscles has previously been described by Delaire [59Delaire J, Ed. The potential role of facial muscles in monitoring maxillary growth and morphogenesis. Ann Arbor: Craniofacial Publications, CHGD 1978., 60Delaire J. Maxillary development revisited: relevance to the orthopaedic treatment of Class III malocclusions Eur J Orthod 1997; 19(3): 289-311.]. CFMAS tension may also be related to resting muscle tonus, [61Palla S. The vertical dimension: a prosthodontist's perspective In: McNamara J Jr, Ed. The enigma of the vertical dimension: craniofacial growth series 36. Ann Arbor: Craniofacial Publications, CHGD 2000; pp. 75-101.] which opposes gravity effects resulting from increase in tissue mass (e.g. muscle, fascia, skin, bone, connective tissue, associated hydration of these structures and any cantilever developed). During normal growth it is postulated that the tension through the CFMAS resists the effects of gravity until CFMAS attenuation with late aging [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1]. Hemifacial paralysis, (e.g. Bell’s Palsy) conveniently displays effect of muscular atonicity resulting in uncompensated gravitational forces upon tissue mass, which is tissue sag (Fig. 5). CFMAS tension transmittance is amplified with muscular growth and development with puberty, along with densification and increased crosslinking of connective tissue component [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36.] (Fig. 5). An increase in muscle size and fat deposit deep to the SMAS may also have a tendency to place the overlying associated SMAS in tension which exhibits some similarity to the work of Solow [17Solow B, Kreiborg S. Soft-tissue stretching: a possible control factor in craniofacial morphogenesis Scand J Dent Res 1977; 85(6): 505-7., 54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1, 56Mitz V, Peyronie M. The superficial musculo-aponeurotic system (SMAS) in the parotid and cheek area Plast Reconstr Surg 1976; 58(1): 80-., 63Ferrario VF, Dellavia C, Tartaglia GM, Turci M, Sforza C. Soft tissue facial morphology in obese adolescents: a three-dimensional noninvasive assessment Angle Orthod 2004; 74(1): 37-42.].

Fig. (5)

(Top) Hemifacial paralysis is shown. Notice the tissue sag (left), and the resulting alar contour and relative vertical position between the paralyzed and normal sides (middle). There is arguably a more inferior position of the patient’s left orbit and eye. Photograph from [116Rubin LR. Reanimation of the paralyzed face: new approaches. St. Louis: C.V. Mosby 1977.] (Bottom left) There is a progressive age-related fiber thickening, densification and cross-linking of the connective tissue component of muscles (endomysium, perimysium, and epimysium) (bottom right) [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36.]. Age dependant directional restriction caused by the process requires further study. Photograph from [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36.]). (Bottom right) The original pterygomasseteric attachment may be preserved as the ramus grows posteriorly and new muscle attachment heads are developed; numbered from [1Proffit WR, Fields HW. Contemporary orthodontics. 3rd ed. St. Louis: Mosby 2000.] initial/oldest attachment to [7Moss ML, Salentijn L. The primary role of functional matrices in facial growth Am J Orthod 1969; 55(6): 566-77.] more recent attachment. Photograph from [103El Haddioui A, Bravetti P, Gaudy JF. Anatomical study of the arrangement and attachments of the human medial pterygoid muscle Surg Radiol Anat 2007; 29(2): 115-24.].



Defining the Role of Muscle Plasticity as a Conduit for Craniofacial Growth Forces

The CMFAS is an unstable conduit for force transmittance and its development is age dependant, reactive and inherent (genetic).

The traditional view is that bone reacts to muscle forces but that muscle does not react to bone modeling. However, there is increasing evidence that the muscle response is also adaptive to underlying skeletal development [64Hunt N, Shah R, Sinanan A, Lewis M. Muscling in on malocclusions: current concepts on the role of muscles in the aetiology and treatment of malocclusion. Northcroft memorial lecture 2005 J Orthod 2006; 33(3): 187-97.]. The effect of gravity with increase in bone and muscle mass may stimulate muscle lengthening. In addition to functional muscle development, genetic properties may determine the number of muscle myofibrils, myofibers and myotubes [65Goldspink G. The proliferation of myofibrils during muscle fibre growth J Cell Sci 1970; 6(2): 593-603., 66Pierson CR, Agrawal PB, Blasko J, Beggs AH. Myofiber size correlates with MTM1 mutation type and outcome in X-linked myotubular myopathy Neuromuscul Disord 2007; 17(7): 562-8.] and quality of the supporting connective tissue, thereby displaying individual variation.

During muscle growth, there is an increase in length by addition of sarcomeres at the muscle tendon junction for which the rate of sarcomeres addition may vary temporally depending on the individual muscle [67Williams PE, Goldspink G. Longitudinal growth of striated muscle fibres J Cell Sci 1971; 9(3): 751-67.]. At a certain point, the addition of sarcomeres and associated increase in muscle fiber length with growth ceases. Any further increase in muscle belly length is presumed to be a reorganization of muscle fibers as insertion of myofibers into the tendon are not uniform but instead stagger [67Williams PE, Goldspink G. Longitudinal growth of striated muscle fibres J Cell Sci 1971; 9(3): 751-67.]. However, the muscle continues to increase in girth due to myofibril splitting as a result of oblique forces within the sarcomeres when a critical diameter is reached [68Goldspink G. Changes in striated muscle fibres during contraction and growth with particular reference to myofibril splitting J Cell Sci 1971; 9(1): 123-37.]. Immobilization of limbs in both extended or contracted muscular positions displays a decrease in the number of sarcomeres relative to controls, presumably due to the restriction of function [67Williams PE, Goldspink G. Longitudinal growth of striated muscle fibres J Cell Sci 1971; 9(3): 751-67., 69Goldspink G. Sarcomere length during post-natal growth of mammalian muscle fibres J Cell Sci 1968; 3(4): 539-48.] because the bone length in immobilized specimens is not significantly different than controls. This is assumed to be a result of increase in tendon length [67Williams PE, Goldspink G. Longitudinal growth of striated muscle fibres J Cell Sci 1971; 9(3): 751-67.]. Immobilization is obviously not physiologic as it overpowers the normal function of the Golgi tendon apparatus and occult muscle tonus, thereby stressing ligaments and tendons beyond the normal viscoelastic limits [70Woo SL, Debski RE, Zeminski J, Abramowitch SD, Saw SS, Fenwick JA. Injury and repair of ligaments and tendons Annu Rev Biomed Eng 2000; 2: 83-118.]. Therefore, certain observations from immobilization experiments may not be representative of the normal growth process.

As muscle ages, there is a rapid increase in quantity and quality of muscle associated connective tissue and therefore CFMAS; [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36., 71Gao Y, Kostrominova TY, Faulkner JA, Wineman AS. Age-related changes in the mechanical properties of the epimysium in skeletal muscles of rats J Biomech 2008; 41(2): 465-9.-73Alnaqeeb MA, Al Zaid NS, Goldspink G. Connective tissue changes and physical properties of developing and ageing skeletal muscle J Anat 1984; 139 (Pt 4): 677-89.] this increase may be a phenomenon throughout the connective tissue in the body. Skeletal muscle growth is believed to be rate limited by connective tissue growth which controls myofiber diameter and length [73Alnaqeeb MA, Al Zaid NS, Goldspink G. Connective tissue changes and physical properties of developing and ageing skeletal muscle J Anat 1984; 139 (Pt 4): 677-89.-75Lewis MP, Machell JR, Hunt NP, Sinanan AC, Tippett HL. The extracellular matrix of muscle-implications for manipulation of the craniofacial musculature Eur J Oral Sci 2001; 109(4): 209-1.] increasingly as the intramuscular connective tissue arrangement becomes thicker and increasingly cross-linked with age [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36.] (Fig. 5). The latter is resistant to lengthening compared to the rather compliant muscle fibers [71Gao Y, Kostrominova TY, Faulkner JA, Wineman AS. Age-related changes in the mechanical properties of the epimysium in skeletal muscles of rats J Biomech 2008; 41(2): 465-9., 73Alnaqeeb MA, Al Zaid NS, Goldspink G. Connective tissue changes and physical properties of developing and ageing skeletal muscle J Anat 1984; 139 (Pt 4): 677-89.] and relatively small increases in the muscle collagen content increases muscle rigidity due to the extremely low compliance of collagen [62Nishimura T, Ojima K, Liu A, Hattori A, Takahashi K. Structural changes in the intramuscular connective tissue during development of bovine semitendinosus muscle Tissue Cell 1996; 28(5): 527-36., 73Alnaqeeb MA, Al Zaid NS, Goldspink G. Connective tissue changes and physical properties of developing and ageing skeletal muscle J Anat 1984; 139 (Pt 4): 677-89., 76Martin RB, Burr DB, Sharkey NA. Skeletal tissue mechanics. New York: Springer 1998., 77Fang SH, Nishimura T, Takahashi K. Relationship between development of intramuscular connective tissue and toughness of pork during growth of pigs J Anim Sci 1999; 77(1): 120-30.]. A decreased range of motion during distraction osteogenesis of bone (sectioning of a bone, allowing callus formation/primary healing and then applying a force across the wound to stretch the tissues thereby stimulating bone and tissue formation to elongate bones) seems a function of the perimysium adaptation rather than of the muscle fibers [75Lewis MP, Machell JR, Hunt NP, Sinanan AC, Tippett HL. The extracellular matrix of muscle-implications for manipulation of the craniofacial musculature Eur J Oral Sci 2001; 109(4): 209-1., 78De Deyne PG, Meyer R, Paley D, Herzenberg JE. The adaptation of perimuscular connective tissue during distraction osteogenesis Clin Orthop Relat Res 2000; 379: 259-69.].

As brain growth and cranial rotation decrease with early aging (≤ 25-30 years of age), the influence of CFMAS tension is expected to increase through decreased connective tissue compliance relative to gravity affects on tissue mass [71Gao Y, Kostrominova TY, Faulkner JA, Wineman AS. Age-related changes in the mechanical properties of the epimysium in skeletal muscles of rats J Biomech 2008; 41(2): 465-9., 73Alnaqeeb MA, Al Zaid NS, Goldspink G. Connective tissue changes and physical properties of developing and ageing skeletal muscle J Anat 1984; 139 (Pt 4): 677-89.] until the age at which brain growth and cranial rotation cease and CFMAS attenuation [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1] becomes an issue.

Tension is Transmitted from the Galea Aponeurotica/ Occipitofrontalis to the Cranial Base

It is postulated that the anteroposterior tension within the occipitofrontalis is the cause of the asymmetric separation of the SOS. The weight of the brain on the desmocranial capsule (dural slings traverse the SOS; Fig. 3) and cranial base, and anteroposterior tension muscle tension within the CFMAS, cause a pivot point at the superior aspect of the SOS creating a greater relative separation of the pharyngeal side of the SOS relative to the endocranial aspect [45Melsen B. The cranial base : the postnatal development of the cranial base studied histologically on human autopsy material Thesis Acta Odontol Scand 1974; 32(Suppl 62)., 79Melsen B. The postnatal growth of the cranial base in Macaca rhesus analyzed by the implant method Tandlaegebladet 1971; 75(12): 1320-9.]. A biomechanical lever system would allow smaller increments of brain growth a greater significance in directing facial tension, (Fig. 3) however, a finite element analysis is needed to adequately describe the stresses and distinguish effects.

Asymmetric growth at the SOS has been demonstrated with and without implants as radiologic markers [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 41Bjork A. Cranial base development: : a follow-up x-ray study of the individual variation in growth occuring between the ages of 12 and 20 years and its relation to brain case and face development Am J Orthod 1955; 41: 198-225., 45Melsen B. The cranial base : the postnatal development of the cranial base studied histologically on human autopsy material Thesis Acta Odontol Scand 1974; 32(Suppl 62)., 79Melsen B. The postnatal growth of the cranial base in Macaca rhesus analyzed by the implant method Tandlaegebladet 1971; 75(12): 1320-9.-81Vilmann H, Kirkeby S, Moss ML. Studies on orthocephalization. IV. differential growth of the sphenooccipital synchondosis in the rat Anat Anz 1980; 148(2): 97-104.]. Hyaline cartilage becomes anabolic when induced by tensile stress, which also accelerates endochondral ossification at the pharyngeal surface of the SOS [80Baume LJ. Principles of cephalofacial development revealed by experimental biology Am J Orthodont 1961; 47(12): 881-901., 82Lei WY, Wong RWK, Rabie ABM. Factors regulating endochondral ossification in the spheno-occipital synchondrosis Angle Orthod 2008; 78(2): 215-0.-86Wang X, Mao JJ. Chondrocyte proliferation of the cranial base cartilage upon in vivo mechanical stresses J Dent Res 2002; 10: 701-5.]. Therefore, it is difficult to demonstrate the asymmetric growth of the SOS radiographically because of concomitant bone modeling. Consideration must be given to the SOS as a reactive site of growth rather than a primary growth center during the adolescent and early adult years [87Thilander B, Ingervall B. The human spheno-occipital synchondrosis. II. a histological and microradiographic study of its growth Acta Odontol Scand 1973; 31(5): 323-4.]. Premature fusion of a cranial suture, craniosynostosis, causes flattening of the basicranium, [88Babler WJ, Persing JA. Experimental alteration of cranial suture growth: effects on the neurocranium, basicranium, and midface Prog Clin Biol Res 1982; 101: 333-45.] while inhibition of SOS growth creates a more flexed cranial base [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.].

Implants demonstrate that growth of the face does not follow straight lines, but rather curves in association with sutural plane rotation [58Gans BJ, Sarnat BG. Sutural facial growth of the Macaca rhesus monkey: a gross and serial roentgenographic study by means of metallic implants Am J Orthod 1951; 37(11): 827-41.]. Rotation of the cranium [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.] seems related to allometric brain development [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 33Giedd JN, Blumenthal J, Jeffries NO, et al. Brain development during childhood and adolescence: a longitudinal MRI study Nat Neurosci 1999; 2(10): 861-3., 89Gogtay N, Giedd JN, Lusk L, et al. Dynamic mapping of human cortical development during childhood through early adulthood Proc Natl Acad Sci USA 2004; 101(21): 8174-9.] and possibly the progression of brain myelination patterns (the water content of fat being less than grey matter), brain capillary blood volume, [90Courchesne E, Chisum HJ, Townsend J, et al. Normal brain development and aging: quantitative imaging at In vivo MR imaging in health volunteers Radiology 2000; 216(3): 672-82.] and progressive pneumatization of the facial bones [46Standerwick R, Roberts E, Hartsfield J Jr, Babler W, Kanomi R. Cephalometric superimposition on the occipital condyles as a longitudinal growth assessment reference: I-point and I-curve Anat Rec (Hoboken) 2008; 291(12): 1603-0.] which collectively allow the head to remain balanced as facial tissues enlarge with growth. The physiologic result is a circular growth pattern around the basioccipital portion of the occipital condyles. The occipital condyles in norma lateralis have been observed slightly dorsal to the calculated center of mass for preserved head specimens, [91Vital JM, Senegas J. Anatomical bases of the study of the constraints to which the cervical spine is subject in the sagittal plane: a study of the center of gravity of the head Surg Radiol Anat 1986; 8(3): 169-73.] however this is due to artifacts [90Courchesne E, Chisum HJ, Townsend J, et al. Normal brain development and aging: quantitative imaging at In vivo MR imaging in health volunteers Radiology 2000; 216(3): 672-82., 92Quester R, Schroder R. The shrinkage of the human brain stem during formalin fixation and embedding in paraffin J Neurosci Methods 1997; 75(1): 81-9.]. MRI displays a 4% greater brain volume and weight resulting from the volume of blood in gray matter capillaries in the living brain, [90Courchesne E, Chisum HJ, Townsend J, et al. Normal brain development and aging: quantitative imaging at In vivo MR imaging in health volunteers Radiology 2000; 216(3): 672-82.] which may cause the center of mass to be located directly over the occipital condyles. Maintenance of head balance is important as it seems that CFG emanates from the occipital condyles [93Frankel R. The applicability of the occipital reference base in cephalometrics Am J Orthod 1980; 77(4): 379-95.] which are along the central growth axis of the body and proximate the brainstem, around which the brain grows centripetally. Balance of the head would reduce any unnecessary metabolic demand required of the musculature for an upright posture; conservation of energy from an evolutionary standpoint.

The proposed CFG model is able to explain the developmental and functional observations of airorhynchy and the facial block hypothesis [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69., 31McCarthy RC, Lieberman DE. Posterior maxillary (PM) plane and anterior cranial architecture in primates Anat Rec 2001; 264(3): 247-60.]. The concept of brain temporal lobe growth displacing the ACB forward must deal with the asymmetric growth of the SOS, lack of brain temporal lobe/middle cranial fossa elongation, [11Hoyte DA. The cranial base in normal and abnormal skull growth Neurosurg Clin North Am 1991; 2(3): 515-37.] and lack of a direct articulation between the MCB and maxilla due to separation by the infraorbital fissure. Additionally, current consensus seems to be that the nasal septum functions to support the roof of the nasal chamber rather than actively participate in the displacement of the palate itself by approximately 4 years of age [4Carlson DS. Theories of craniofacial growth in the postgenomic era Semin Orthod 2005; 11(4): 172-83., 13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996., 14Ranly DM. Craniofacial growth Dent Clin North Am 2000; 44(3): 457-70., 94Thilander B. Basic mechanisms in craniofacial growth Acta Odontol Scand 1995; 53(3): 144-51.]. On the whole, the tissue(s) that displaces the maxilla downward with craniofacial growth postnatally have not yet been satisfactorily defined relative to the competing nasal septum theory and functional matrix theory [26Ranly DM. A synopsis of craniofacial growth. New York: Appleton-Century-Crofts 1980.].

Airway and Speech Development Through CFMAS Tension

Sagittal rotation of the cranium at the occipital condyles and asymmetric growth at the SOS are proposed to cause the face to rotate cephalad and ventral, opening the airway with normal jaw rotation and extension of tissues (lingual tonsil, velum) [95Moss ML, Rankow RM. The role of the functional matrix in mandibular growth Angle Orthod 1968; 38(2): 95-103., 96Tourne LP. Growth of the pharynx and its physiologic implications Am J Orthod Dentofacial Orthop 1991; 99(2): 129-39.] (Fig. 6). The resultant relative descent of the larynx develops the hyolaryngeal complex, [12Lieberman DE, Ross CF, Ravosa MJ. The primate cranial base: ontogeny, function, and integration Am J Phys Anthropol 2000; (suppl 31)117-69.] creating a resonance chamber to allow voice production for speech.

Fig. (6)

Illustrated are the components of Waldeyer’s ring of lymphoid tissue: the palatine tonsils, adenoid tissue mass and lingual tonsil. Enlargement of the nasal lining with rhinitis can alter laminar airflow. Normal mandibular rotation is expected to direct the tongue forward through muscle attachment at the genial tubercles (internal surface of the mandibular symphysis). Cranial rotation serves to direct the velum forward in coordination with differential SOS growth.



Transverse development of the maxilla and nasal airway is believed to be a result of muscle mass increases resulting in greater CFMAS tension which causes the teeth to be compressed (keeping in mind the relative separation of the jaws with growth). Developed is a greater posterior relative to anterior transverse increase of the maxilla and nasal airway with growth (molar force is greater due to the Class 3 lever system of the jaws) [27Bjork A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method Br J Orthod 1977; 4(2): 53-64.]. The functional occlusion of the teeth displaces the maxillary halves, thereby increasing the transverse airway and providing room for the tongue. The tongue has been believed to play a major role in transverse airway enlargement, however the apposition observed on the bony palate during growth [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996., 97Enlow DH, Bang S. Growth and remodeling of the human maxilla Am J Orthod 1965; 51: 446-64.] seems dismissive of a pressure large enough to displace the maxillary shelves. Also, the comparable posterior face height of individuals with hyperdivergent/leptoprosopic (simply, long faced) profiles to those with the hypodivergent/europrosopic (simply, short faced) profiles also raises questions of relative posterior tongue posture [98Nanda S. Patterns of vertical growth in the face Am J Orthod Dentofacial Orthop 1988; 93(2): 103-6.].

Lateral midfacial muscle attachments may also play a minor role in developing transverse nasal dimension. Craniofacial muscles are bilateral and, through lateral attachment relative to the midfacial bone centroids (center rotation), may cause lateral rotation below the centroid and medial rotation above the centroids. This may contribute to the inverted “V” shape [13Enlow DH, Hans MG. Essentials of facial growth. Philadelphia: Saunders 1996.] of the nasal aperture, in conjunction with occlusal forces transmitted across the palate and mid-palatal suture. This must be weighed against the reciprocal forces from the muscles of mastication.

SUPPORT FOR THE PROPOSED MODEL

The Effect of Cranial Rotation in the Expression of Facial form

Maxillomandibular rotation with craniofacial growth (CFG) had not been apparent until the Bjork implant studies of the mid-fifties [20Bjork A. Facial growth in man, studied with the aid of metallic implants Acta Odontol Scand 1955; 13(1): 9-34.]. Yet, as a whole, the interrelationship between the growth of the maxilla and mandible is still not fully understood, and remains one of the great challenges of craniofacial biologists [14Ranly DM. Craniofacial growth Dent Clin North Am 2000; 44(3): 457-70.].

The CFMAS tension is not only a dorsal restrictive pressure as with the soft tissue stretching CFG model proposed by Solow, [17Solow B, Kreiborg S. Soft-tissue stretching: a possible control factor in craniofacial morphogenesis Scand J Dent Res 1977; 85(6): 505-7.] but displays a relative cephalad tension capable of directing anabolic growth. The CFMAS tension is also opposite to the dorsal/caudad force expected from suprahyoid muscle attachment relative to cervical vertebrae growth of the Houston CFG model [16Houston WJ. Mandibular growth rotations, their mechanisms and importance Eur J Orthod 1988; 10(4): 369-73.] (Fig. 1).

Maxillomandibular Rotation and TMJ Modeling Resulting from Differences in CFMAS Somatotype

The inherent (genetic) contribution of the CFMAS in the brachycephalic and dolichocephalic individual is believed to influence maxillomandibular rotation [75Lewis MP, Machell JR, Hunt NP, Sinanan AC, Tippett HL. The extracellular matrix of muscle-implications for manipulation of the craniofacial musculature Eur J Oral Sci 2001; 109(4): 209-1., 99Tippett HL, Dodgson LK, Hunt NP, Lewis MP. Indices of extracellular matrix turnover in human masseter muscles as markers of craniofacial form: a preliminary study Eur J Orthod 2008; 30(2): 217-5.]. Brachycephalic individuals tend to be europrosopic, strong muscled [100Pepicelli A, Woods M, Briggs C. The mandibular muscles and their importance in orthodontics: a contemporary review Am J Orthod Dentofacial Orthop 2005; 128(6): 774-80.] and display a hypodivergent radiographic profile, while dolichocephalic individuals tend to be leptoprosopic, weak muscled [100Pepicelli A, Woods M, Briggs C. The mandibular muscles and their importance in orthodontics: a contemporary review Am J Orthod Dentofacial Orthop 2005; 128(6): 774-80.] and display a hyperdivergent radiographic profile (see Dale [101Dale JG, Dale HC Eds, Eds. Interceptive guidance of occlusion with emphasis on diagnosis. 4th ed. St. Louis: Mo.: Elsevier Mosby 2005.] for a detailed comparison).

In general, CFMAS tension is believed to be related to resting muscle tonus [61Palla S. The vertical dimension: a prosthodontist's perspective In: McNamara J Jr, Ed. The enigma of the vertical dimension: craniofacial growth series 36. Ann Arbor: Craniofacial Publications, CHGD 2000; pp. 75-101.] and the connective tissue compliance, and is greater in the europrosopic versus leptoprosopic individual resulting from muscle morphology [100Pepicelli A, Woods M, Briggs C. The mandibular muscles and their importance in orthodontics: a contemporary review Am J Orthod Dentofacial Orthop 2005; 128(6): 774-80.].

During normal CFG rotation, the tension through the CFMAS is expected to modify the effects of gravity on the viscerocranium as muscle growth increases with puberty, along with progressive densification and crosslinking of the associated connective tissue component (Fig. 5) until age related CFMAS tension attenuation occurs [54Stuzin JM, Baker TJ, Gordon HL. The relationship of the superficial and deep facial fascias: relevance to rhytidectomy and aging Plast Reconstr Surg 1992; 89(3): 441-9. discussion 50-1]. The effect of the CFMAS resisting gravity may result in the anterior displacement of the midface by maxillary contact with the mandible through a functional occlusion of the teeth. The divergence of the facial planes in combination with the opposing gravity and CFMAS forces seem to create a wedge effect (Fig. 3). Hence, leptoprosopic facial types which display a hyperdivergent mandibular plane angle usually display a sagittally retruded maxilla, as will be further explored.

The Anatomic Perspective of the Proposed Mandibular Rotation Around the Pterygomasseteric Sling

The rotation axis of the mandible has been postulated but not definitively ascertained. Therefore, based on current anatomical understanding, the pivot point for mandibular rotation is proposed at the pterygomasseteric sling (PtmS) which is formed by the blended aponeuroses of the masseter and medial pterygoid muscles at the angle of the mandible [102Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection: experimental study in rats Am J Orthod Dentofacial Orthop 1995; 108(4): 367-79., 103El Haddioui A, Bravetti P, Gaudy JF. Anatomical study of the arrangement and attachments of the human medial pterygoid muscle Surg Radiol Anat 2007; 29(2): 115-24.]. Horizontal motion of the mandible is allowed as the sling cradles the mandible, in addition to mandibular rotation around the “true” pterygomasseteric sling (PtmS). The PtmS is proposed as original neonatal muscle attachment site that approximates the neutral zone of the mandibular periosteal sleeve where little periosteal migration occurs [104Frankenhuis-Van Den Heuvel TH, Maltha JC, Kuijpers-Jagtman AM, Van 't Hof MA. A longitudinal radiographic study of the periosteal migration along the growing rabbit mandible J Dent Res 1992; 71(2): 398-402.] (Fig. 7). As children grow, there is a migration of the periosteum and associated muscle attachments; [105Dorfl J. Migration of tendinous insertions. I: cause and mechanism J Anat 1980; 131(Pt 1): 179-95.-107Covell DA Jr, Herring SW. Periosteal migration in the growing mandible: an animal model Am J Orthod Dentofacial Orthop 1995; 108(1): 22-9.] muscle portions nearer this neutral zone would be expected to remain positionally stable and/or new muscle attachment/heads developed [103El Haddioui A, Bravetti P, Gaudy JF. Anatomical study of the arrangement and attachments of the human medial pterygoid muscle Surg Radiol Anat 2007; 29(2): 115-24., 108Grant PG. The effect of position on the migration of muscle J Anat 1978; 127(Pt 1): 157-62.] (Fig. 5).

Fig. (7)

(A) The site of the PtmS is shown with arrows, which corresponds to the original/neonate location of the pterygomasseteric sling. Illustration from [117Enlow DH. Wolff's law and the factor of architectonic circumstance Am J Orthod 1968; 54(11): 803-22.] (B) Shown is the neutral zone observed in the periosteum of the rabbit mandible, which is an area of little expected muscle attachment migration. Illustration from [104Frankenhuis-Van Den Heuvel TH, Maltha JC, Kuijpers-Jagtman AM, Van 't Hof MA. A longitudinal radiographic study of the periosteal migration along the growing rabbit mandible J Dent Res 1992; 71(2): 398-402.] (C) CW and CCW rotation is illustrated; with backward (CW) rotation there is inhibition/resorption at the condyle and with forward (CCW) rotation there is distraction of the condyle articular surface and associated anabolic modeling; compensatory modeling at menton and gonion, and tooth eruption are not shown.



The Normal to Brachycephalic Individual (Forward/Counter-Clockwise Rotation)

In the mesocephalic to brachycephalic individual, tension in the CFMAS passes through the anterior of the mandible via the modiolus and osteocutaneous ligaments, (Figs. 1 and 3) and is believed to cause a forward/counter-clockwise (CCW) maxillomandibular rotation described by Bjork [20Bjork A. Facial growth in man, studied with the aid of metallic implants Acta Odontol Scand 1955; 13(1): 9-34., 109Bjork A. Prediction of mandibular growth rotation Am J Orthod 1969; 55(6): 585-99.].

The effect of gravity on the CFMAS suspended viscerocranial elements includes other elements resting on the skeletal units; for example, the eyeballs. The reason that the viscerocranial elements can be suspended relative to the cephalad force of the functional occlusion and not provide compression to the circumaxillary sutures may be in part that, unlike the rather direct distraction of the sutures resulting from the force of gravity, the force of the functional occlusion is redistributed throughout the skull due to buttressing [110Atkinson S. Balance- the magic word Am J Orthod 1964; 50(3): 189-202.] (Fig. 8) and compression of the bone. Due to proximity, orientation and the initial buttressing, the direct affect of functional occlusion on the palatal suture is not common to all the circumaxillary sutures.

Fig. (8)

The force vector resulting from the functional occlusion of teeth is not equal and opposite to the force vector of gravity; therefore, functional occlusion would not be expected to place circumaxillary sutures in compression. Figure from [110Atkinson S. Balance- the magic word Am J Orthod 1964; 50(3): 189-202.].



CCW mandibular rotation causes distraction (separation) of the articular surfaces of the temporomandibular joint, resulting in bone apposition on the anterior surface of the condylar head (Fig. 7) and the articular eminence of the temporal bone [102Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection: experimental study in rats Am J Orthod Dentofacial Orthop 1995; 108(4): 367-79.]. This results in caudad movement of the gonial angle leading to increased ramal height relative to anterior face height, [102Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection: experimental study in rats Am J Orthod Dentofacial Orthop 1995; 108(4): 367-79.] a more caudally positioned glenoid fossa and steeper articular eminence [111Baccetti T, Antonini A, Franchi L, Tonti M, Tollaro I. Glenoid fossa position in different facial types: a cephalometric study Br J Orthod 1997; 24(1): 55-9.]. CCW rotation of the mandible also results in increased overbite and progressive restriction of the mandibular dentoalveolar complex on the basal bone36 and causes overjet to open and the incisors impinge on the tongue. The wedge cross-section shape of the teeth results in an increased overjet as the overbite increases, and the mandibular incisors tend to drift labially relative to implants due to the tongue impingement, as demonstrated by Bjork [21Bjork A, Skieller V. Normal and abnormal growth of the mandible: a synthesis of longitudinal cephalometric implant studies over a period of 25 years Eur J Orthod 1983; 5(1): 1-46.]. CCW rotation increases the space anterior to the ramus for tooth eruption relative to the position of the PtmS; therefore tooth crowding is less probable in these individuals.

Dolichocephalic Individuals (Backward/Clockwise Mandibular Rotation)

Dolichocephalic individuals often display a decreased biting strength and backward/clockwise (CW) maxillomandibular rotation compared to mesocephalic individuals, which is believed to be a result of decreased CFMAS tension. Airway obstruction is often a concern with these individuals resulting in a neuromuscular response that positions the tongue relatively forward [101Dale JG, Dale HC Eds, Eds. Interceptive guidance of occlusion with emphasis on diagnosis. 4th ed. St. Louis: Mo.: Elsevier Mosby 2005.] of the PtmS causing the gravitational effects to overly dominate CFMAS effects. This results in a more pronounced CW rotation of the mandible, a narrow and longer face, and posterior rotation in nose shape (dorsal hump). Condylar growth is restricted by compression at the articular surfaces of the mandibular condyle and articular eminence resulting in a glenoid fossa that is relatively more cephalad and shallow [111Baccetti T, Antonini A, Franchi L, Tonti M, Tollaro I. Glenoid fossa position in different facial types: a cephalometric study Br J Orthod 1997; 24(1): 55-9., 112Schellhas KP, Wilkes CH, Fritts HM, Omlie MR, Lagrotteria LB. MR of osteochondritis dissecans and avascular necrosis of the mandibular condyle AJR Am J Roentgenol 1989; 152(3): 551-60.]. The CFMAS causes superficial pressure induced resorption of the anterior maxilla, [27Bjork A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method Br J Orthod 1977; 4(2): 53-64.] maxillary retrusion, resorption at menton and the mandibular teeth are posteriorly inclined, as gravity’s effect on the mandible is resisted by the CFMAS. This reduces the relative depth of B-point and anterior orientation of the symphysis. The mandible tends to grow ventral less in the europrosopic individual and the caudad force on the anterior mandible predominates. The anterior mandible bends over the PtmS as a result of the forward tongue posture and restriction of the PtmS which accentuates the antegonial notch in combination with matrix apposition at gonion as a result of tissue stretch to the periosteum. The issue of Wolff’s law must be analyzed in light of non-traditional microscopy patterns on the bony surfaces of necropsy specimens [113Hans MG, Enlow DH, Noachtar R. Age-related differences in mandibular ramus growth: a histologic study Angle Orthod 1995; 65(5): 335-40.]. Since there is a CW rotation around the PtmS, (Fig. 7) the ramus does not move dorsal relative to the PtmS, resulting in an increase probability of tooth crowding.

Experimental Observations in the Rat

In rats, this muscle dependant mandibular rotation and associated articular cartilage change overlying the head of the mandibular condyle has been demonstrated by Navarro [102Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection: experimental study in rats Am J Orthod Dentofacial Orthop 1995; 108(4): 367-79.]. Increased thickness of the condylar cartilage, length of the ramus after temporalis muscle resection (TR) and opening of the Stutzmann’s angle (estimates the direction of condylar growth) were described as unexpected; however, the observations fit well with the proposed model of rotation around the PtmS. With masseter muscle resection (MR), there was a backward rotation of the mandible and with TR, there was a forward rotation. Considering rotation of the mandible around the PtmS, MR releases the restrictive element of the PtmS, while TR releases tension posterior to the PtmS allowing forward rotation of the mandible. The authors [102Navarro M, Delgado E, Monje F. Changes in mandibular rotation after muscular resection: experimental study in rats Am J Orthod Dentofacial Orthop 1995; 108(4): 367-79.] concluded that alterations in masticatory musculature can modify “articular growth”. Observation of the mandibular condyle articular cartilage displayed an increase in thickness with TR and a decrease in thickness with MR. A thinner ramus was observed with MR and an increased ramus was noted with TR. These are expected with the proposed mechanism of craniofacial growth.

CONCLUSIONS

The ability to model growth patterns will lead to further understanding and insights. Cephalometric study based on the occipital condyles as the craniofacial growth axis leads to a more biologically correct craniofacial growth model. This aponeurotic tension model of craniofacial growth, in which brain growth and cranial rotation create cephalad tension within the inter-related superficial musculoaponeurotic systems of the head and face, adds clarity to the mechanism of maxillomandibular rotation and TMJ development. The modeling of craniofacial growth is important when considering surgical intervention during growth, prediction of condylar resorption, post-surgical relapse, temporomandibular joint dynamics/growth, understanding of airway, future beneficial surgical procedures and age specific plasticity of tissues.

ACKNOWLEDGEMENT

We would like to thank Professor Manuel Chanavaz, (Oral and Maxillofacial Implantology, Faculty of Medicine, University of Lille, France) for his guidance with muscle growth concepts.

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