Transient lateral patellar dislocation resulting in osteochondral lesion of the lateral femoral condyle: a case series of findings on cone beam CT.

Transient lateral patellar dislocation (TLPD) occurs most often in young active individuals. Although relatively common, they tend to spontaneously reduce and can be difficult to accurately diagnose. The typical pattern of magnetic resonance imaging (MRI) findings following TLPD has been well documented in the literature and are known to frequently result in osteochondral lesions (OCL) in up to 70% of cases. However, the emergence of cone beam computed tomography (CBCT) as an imaging technique is in its relative infancy and the associated corresponding findings are less well established. We describe a series of three cases of TLPD, two first-time post-traumatic dislocations and one recurrent dislocation, and the similar pattern of findings on CBCT, which were used as an adjunct to imaging with MRI. The series of images demonstrate that OCL from the lateral femoral condyle following TLPD are visible on CBCTas a distinctive curvilinear fragment of bone within the joint space and, in some cases, are at risk of being misdiagnosed as an avulsion fracture fragment from the medial patellar facet. Although MRI offers better assessment of soft tissues, it has been found to over-estimate the size of OCL due to the surrounding oedema and fluid. CBCT can be an excellent adjunct to MRI and given the ease of access, could be performed prior to MRI rather than afterwards for surgical planning as has largely been the case to date.