Lesions that are covered by cartilage or other formations, and thus, cannot be visualised on traditional radiographs or via direct examination methods, such as arthroscopy, are referred to as occult bone injuries, occult interosseous fractures, or bone bruises.

Since the late 1980s, occult bone injuries have been diagnosed in association with soft tissue injuries of the knee, particularly with the rupture of the anterior cruciate ligament, and less commonly, with injuries of the hip joint and foot. Such occult bone injuries, also called bone bruise, present with bleeding, oedema, and infarct in the subchondral bone marrow due to micro-trabecular fractures [1]. Diagnosis of such lesions has become more common, particularly with the use of magnetic resonance (MR) imaging methods [2-5]. A typical occult bone injury is observed as a signal loss on T1 images and increased signal content on T2 images, due to the increased water content in the damaged bone marrow. Bone bruise and bone marrow oedema could be distinguished primarily based on trauma history, as their radiological features are almost indistinguishable. The sensitivity and specificity of MR imaging methods in detecting bone bruises of the knee have been reported to be 83% - 96% and 86% - 96%, respectively [6].

Bone bruises of the knee may cause substantial clinical entities, as they cause acute pain and functional loss in the joint [7, 8]. In terms of the natural history of bone bruises, it has been reported that such lesions resolve within a period ranging from 6–12 weeks [9] to 12–14 weeks [10], and even to 24 months [11]. Despite the lack of a definite consensus on the follow-up periods of such lesions via MR imaging methods, the clinical reflections and importance of long-term, non-healing lesions are not known. The present study aimed to assess occult bone injuries using MR imaging in patients presenting with minor knee trauma.

In the present study, the data of nine male and three female patients with a mean age of 32 years (range, 24–51 years) were reviewed. All of the patients were admitted with minor traumatic knee injury. The etiological factors of the injuries before the patients complained of knee pain were walking a long distance in six patients, falling from a 1-meter height in three patients, and minor knee trauma in three patients. Moreover, one of the patients had a history of surgery for rupture of the anterior cruciate ligament eight years prior; the other patients had no history of surgery on the knee or other extremities.

The mean restriction of ROM in patients was 15° (range, 10–40°). Minimal joint effusion was observed in three patients. Joint puncture performed on these patients revealed no microbiological or histological pathology. No pathological signs were detected on the clinical and radiological evaluations of the patients (Fig. 1). The mechanical axis was within the normal range in all patients.

Fig. (1). Post-traumatic direct radiographs: a) lateral view; b) anteroposterior view.

Bone marrow changes detected on the MR images were classified as Type I lesions in nine patients and Type II lesions in three patients (Figs. 2, 3).

Fig. (2). Magnetic resonance images: a) post-traumatic coronal MR image of the knee (bone bruise in the lateral condyle); b) post-traumatic sagittal MR image of the knee (bone bruise in the lateral condyle); c) coronal MR image of the knee obtained at the last control visit (after nine months); d) sagittal MR image of the knee obtained at the last control visit (after nine months).

Fig. (3). Magnetic resonance images: a) post-traumatic coronal MR image of the knee (bone bruise in the medial condyle); b) post-traumatic sagittal MR image of the knee (bone bruise in the medial condyle); c) coronal MR image of the knee obtained at the last control visit (after 12 months); d) sagittal MR image of the knee obtained at the last control visit (after 12 months).

Diagnostic arthroscopy was performed in five patients in whom improvement was not achieved with conservative therapy. In these patients, the lesions were classified as Type I according to the International Cartilage Repair Society (ICRS) classification [13]. All patients treated conservatively. The lesions improved in all patients within a mean period of six months (range, 4–12 months) after the trauma.

Currently, there is no definite consensus on the natural history of bone injuries secondary to minor knee traumas. Injuries that occur after a low-energy trauma generally resolve in a short time [9], and in such patients, precautions that prevent weight load on the knee joint are sufficient for the treatment. Improvements observed in our patients within a mean period of six months after the trauma were consistent with previous reports.

Radiographs, which are beneficial in detecting bone fractures following traumatic knee injuries, are inadequate, particularly in detecting soft tissue pathologies. On the other hand, MR imaging, a non-invasive method that shows knee joint injuries quite accurately, has begun to be used widely in the last few decades.

No remarkable pathology was noted on the radiographs obtained after minor knee trauma in our patients. MR images revealed bone contusions characterised by low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, although a remarkable fracture line was not detected on the bony parts exposed to minor trauma. Contusions that could not be detected on the direct radiographs were the most common lesions detected on the MR images.

Based on the available data on biomechanics of the knee joint, it is known that intensive mechanical stress could cause an overload on the subchondral region and lead to contusion-type injuries in the event of weakness of the muscle mass protecting the joint structure. This might explain the injuries that occurred due to walking for a longer time than normal in six patients in our series. Bruise injuries due to minor traumas in the other patients were confirmed by MR imaging findings.

In their study, Maurer et al., [14] found that MR imaging in acute knee trauma improved the diagnostic certainty of clinicians and reduced the number of diagnostic arthroscopies. This finding was also confirmed by Bui-Mansfield et al., [15], and it was reported that MR imaging performed before arthroscopy of the knee reduced costs. In the present study, positive results were obtained via MR images of the patients in whom radiographs showed no pathological findings, and these patients were classified as Type I, based on the arthroscopic findings [16]. Therefore, it can be assumed that using MR imaging in acute minor knee traumas might prevent unnecessary diagnostic arthroscopy.