With increasing understanding of the biological and biomechanical basis of bone fusion, the quest for better and more efficient bone fusion technologies in spinal surgery continues. While autologous iliac crest bone is recognized as the most successful grafting material due to its osteoconduc-tive, osteogenic and osteoinductive properties [1, 2], compli-cation rates of this invasive procedure remain high [3-5], with protracted postoperative pain at the donor site being the most commonly reported complication [6]. In an attempt to limit such complications by either reducing or eliminating iliac crest autograft harvest, demineralized bone matrix (DBM) and synthetic osteoconductive bone graft extenders (BGEs) are often used in spinal fusion procedures as an adjunct to patient-derived autograft [7-11]. Similarly, local bone may be used in lieu of iliac crest grafts, although current evidence suggests that while local bone is equally effective as iliac crest bone in single-level fusion [6, 12], it has been associated with significantly lower fusion rates when utilized in multi-level fusion due to volume limitations [1].

A number of bone graft substitutes are available, the most common of which are ceramic- or calcium-phosphate-based synthetic materials. While these products provide a suitable scaffolding upon which to build new bone, they are inherently unable to stimulate bone growth without the patient’s own bone or marrow to augment the process. Alternate options such as tricalcium phosphate, calcium sulfate and hydroxyapatite, while possessing osteoconduc-tive properties, are associated with equivocal results regarding fusion rates [13]. To address the shortcomings of the currently available technology, a fully-resorbable hybrid biosynthetic autograft extender for use in spinal fusion was developed (InQu^®, ISTO Technologies, Inc., St. Louis, MO). This extender, which has been commercially available since 2008, contains no ceramic component and no animal-derived protein. Instead, it consists of poly(D,L-lactide-co-glycolide) (PLGA, 75:25) combined by lyophilization with unmodified high molecular weight hyaluronic acid (10-12% wt:wt) to create a hybrid interwoven scaffold. The PLGA component provides compressive resistance to maximize the product's bulking properties. The rate of polymer resorption (3-6 months) is consistent with the rate of bone remodeling at the site of implantation, and this scaffolding support has been shown to direct new bone growth via endochondral ossification [14, 15], in contrast to ceramic-based materials, which function through creeping substitution [16].

In order to evaluate the safety and clinical effectiveness of the new hybrid interwoven biosynthetic bone extender, we retrospectively analyzed a consecutive series of posterior lumbar interbody fusion (PLIF) procedures with supplemental instrumentation.

All patients who underwent elective PLIF procedures at a single academic medical center between March 2008 and March 2011 utilizing the new hybrid interwoven biosynthetic bone extender were included in this analysis. All patients signed an institutional consent form that explained all treatment modalities offered to these patients. No Institutional Review Board (IRB) review was sought as all procedures were performed under our center’s standard of care and did not involve any additional research-oriented procedure or products.

One author (MKC) performed a retrospective chart review of eligible patients. The goal of the review and data collection was to document the rate of fusion progression at approximately 18 months of follow-up together with the rate of complications and changes in pain scores and other patient-reported outcomes.

One hundred nine (109) procedures were performed on 108 patients involving 245 individual vertebral levels. Patient background characteristics and pre-surgical diagnoses are summarized in Table 1. The mean age was 59.4 (±12.4 yrs, range 28-85 yrs); 56% of patients were female. Only 39 patients (36%) reported working prior to the surgery. Mean body mass index (BMI) was 30.2 (±5.7). Mean clinical follow-up was 19.2 (±11.7) months, whilemean radiological follow-up was 17.7 (±10.4) months. Three patients (2.8%), accounting for a total of 9 fusion levels, did not return for full follow-up. Of the initial 245 levels fused, complete data are available for 236 levels.

PLIF procedures at all 245 levels were instrumented. A detailed summary of the distribution of index levels is provided in Table 2; procedural details are summarized in Table 3. Spinal fusion was performed at a single level in 32 patients (29.4%), at two levels in 37 patients (33.9%) and at three or more levels in 40 patients (36.7%).

Clinical Outcomes, Time to Fusion and Fusion Rate

Data at final follow-up were available for 236 of 245 individual levels in 105 patients. A total of 221 of 236 vertebral levels (93.6%) fulfilled the success criteria for fusion at final follow-up by demonstrating partial or comp-lete fusion (see Table 4). Partial fusion was demonstrated at 68 of 236 fusion levels (28.8%) at final follow-up while 153 of 236 levels (64.8%) demonstrated complete fusion. Only 15 of 236 levels (6.4%) showed no evidence of fusion at final follow-up. The mean time to fusion for the entire group was 10.2 months (±4.1). An example of solid fusion is demonstrated in Fig. (1).

Fig. (1). Sagittal (A, C) and coronal (B, D) CT images at 9 months (A, B) and 16 months (C, D) post-op in a 54-year-old male who underwent 2-level posterior lumbar interbody fusion at L4-S1 (yellow arrows). The patient complained of low back pain and left buttock pain after failed previous fusion at L5-S1. Nine-month imaging demonstrates progression to fusion (A, B), with complete fusion realized by 16-month imaging at the time fusion was extended to include L2-L4 (C, D). Low back pain and radiculopathy resolved following re-operation.

The objective of this retrospective review was to investigate the safety and clinical effectiveness of InQu, a novel biosynthetic osteoconductive scaffold used as a local autograft extender in PLIF with supplemental instrumentation. This retrospective case series with radiologic assessment of fusion comprised 109 cases in 108 subjects at 245 individual lumbar spine levels. Local bone harvested during the decompression procedure was combined with the hybrid interwoven biosynthetic bone extender and this mixture was hydrated with concentrated bone marrow harvested from the posterior iliac crest. Posterolateral fusion was then achieved utilizing the same grafting materials placed within the interbody (IB) device and into the disc space.

The new hybrid interwoven biosynthetic bone extender appears to safely support bone formation when applied to the axial skeleton. A total of 64.8% of the 245 individual vertebral levels demonstrated complete and stable fusion at final follow-up, with an additional 28.8% of vertebral levels demonstrating progression toward fusion. Thus, a total of 221 of 236 vertebral levels (93.6%) fulfilled the success criteria for fusion, while only 15 of 236 levels (6.4%) showed lack of fusion at final follow-up. The mean time to fusion was an acceptable 10.2 months. No safety concerns were raised with use of the product, including the lack of heterotopic ossification. There were no reported cases of implant failure, deep wound infection or seroma formation post-surgery. No loosening or failure of the hardware was noted.

While technically more difficult and typically associated with greater complication rates than anterior lumbar interbody fusion (ALIF) and transforaminal lumbar interbody fusion (TLIF) procedures [24-26], the PLIF procedure – first pioneered in the 1950s [27-29] – is associated with high fusion rates, which has resulted in its remaining a staple of lumbar fusion. The current case series demonstrated satisfactory fusion rates in patients necessitating mechanical stabilization of single, double and multiple levels without neurological complication. Previous reports have suggested that the use of local bone in cases of multiple level fusion is associated with poor fusion outcomes, necessitating the use of iliac bone grafts [1, 30]. The current study, however, demonstrates successful fusion in 37 of 38 patients undergoing fusion at 3 or more levels (representing 129 of 132 levels). This indicates that the use of local bone in multi-level fusion is feasible when combined with an extender such as InQu. These data complement previous studies that have shown similar levels of fusion success when utilizing local bone. Coutour et al. [31] utilized the HYDROSORB bio-absorbable implant packed with locally harvested autograft and segmental internal fixation in a group of 27 patients undergoing PLIF and showed comparable efficacy, with 95.5% (42 of 44) of vertebral levels demonstrating successful fusion. They noted, however, that the rate of fusion was inversely proportional to the number of levels being fused, whereas we observed a nearly equivalent rate of fusion in single-, double- and multi-level fusion cases.

The fusion rates demonstrated in this report are comparable to those observed with the use of other bone graft extenders and fusion performed without BGE. Fusion rates above 85% at 2 to 3 years post-procedure have been demonstrated for both local and iliac crest bone grafts [12, 32]. Synthetic BGEs have been associated with similar rates of fusion success, with one recent systematic review [7] reporting an 86.4% fusion rate when ceramic materials were utilized as the BGE, while another systematic review [13] found pooled fusion rates ranged from 75% with calcium sulphate to 98.2% with a combination of hydroxyapatite and tricalcium phosphate. The current study included supplemental posterolateral fusion for which greater quantities of bone graft are required. Despite the finite volume of autograft that could be harvested locally, concentrated bone marrow aspirate combined with cancellous allograft and biosynthetic extender appeared to provided adequate osteogenic and osteoconductive support of new bone growth.

In conclusion, the clinical experience of a single clinical site in a 109 case series of subjects requiring PLIF procedures using a fully-resorbable osteoconductive scaffold made of PLGA with integrated hyaluronic acid demonstrates that this biosynthetic bone graft extender provides good to excellent radiographic outcomes in approximately 94% of patients. Most importantly, this study demonstrates that the use of the InQu extender allows for the use of local bone and resulted in a 97.7% fusion rate in 38 patients undergoing multiple level (>3) fusion. Additional prospective studies evaluating its clinical success for one- and two-level transforaminal lumbar interbody fusion procedures will add to these clinical findings.