Patellar shape can be a predisposing factor in patellar instability

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. 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. 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 (chi(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. 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.