The inferomedial patellar protuberance and medial patellar ossicle in patellar instability

The purpose of the study was to define the frequency of an inferomedial patellar protuberance in patients presenting to a specialist Patella Clinic and to characterise the clinical and radiological features as well the association between the inferomedial patellar protuberance and the medial patellar ossicle.

A cohort of 163 patients (166 knees) was reviewed from a prospectively collected clinical database and radiological imaging. This included a record of patellar tracking.

An inferomedial patellar protuberance was found in 62 (37 %) knees. A medial patellar ossicle was noted in 56 (34 %) knees. In all, an inferomedial patellar protuberance or medial patellar ossicle or both was found in 90 (54 %) knees. The association between inferomedial patellar protuberance and significant trochlear dysplasia was highly significant (p = 0.01), but not for the medial patellar ossicle (n.s.). The presence of an inferomedial patellar protuberance was significantly less likely in patients with hypermobility syndrome (p = 0.001); however, there was no significant association between hypermobility syndrome and medial patellar ossicle (n.s.), or the presence of either or both an inferomedial patellar protuberance and medial patellar ossicle (n.s.). All patients with a clunk at 20°-30° flexion had significant trochlear dysplasia and an inferomedial patellar protuberance.

Radiological changes consistent with an inferomedial patellar protuberance were found in about one-third of patients presenting to a specialist Patella Clinic. Patellar maltracking and a clunk at 20°-30° flexion are associated with significant trochlear dysplasia plus an inferomedial patellar protuberance. If undertaking an operative correction, both deformities should be considered in order to avoid joint incongruity.

III.