Patellar shape can be a predisposing factor in patellar instability.

PURPOSE: Predisposing factors to objective patellar instability include trochlear dysplasia, patella alta, patellar tilt and elevated tibial tuberosity-femoral groove distance. The shape of the patella is classically not considered a predisposing factor. Anomalies of dynamic and static factors, including excessive patellar height, tibial tubercle lateralisation or trochlear dysplasia, may influence the development of the patella. METHODS: One hundred and five patients (140 knees) with objective patellar instability were retrospectively reviewed to identify a possible association between the above-mentioned predisposing factors and patellar shape. All patients were evaluated with static and dynamic CT scans, and plain lateral and antero-posterior radiographs, and skyline patellar views. RESULTS: Evidence of a significant association emerged between patellar shape and patellar tilt in static (r(s) = 0.20, P = 0.019) or dynamic conditions (r(s) = 0.18, P = 0.031) and a significant association between Wiberg patellar shape type C and trochlear dysplasia grade 3 (χ(2) = 4.5, P = 0.035). Also, we found a significant association between trochlear dysplasia stage 3 and tibial tuberosity-trochlear groove (TT-TG) and patellar tilt relaxed (P < 0.01 and P < 0.05, respectively). There is an association between patellar shape and patellar tilt. CONCLUSION: Increased lateral stresses may produce a Wiberg type C patella, with a hypoplastic medial facet and a more developed lateral facet. Unbalance between dynamic medial and lateral stabilisers may act as an additional factor. A rehabilitation program aiming to reduce this unbalance may decrease the incidence of type C patella in young patients