Trochlear dysplasia and patellar instability in patients with Down syndrome☆

OBJECTIVE To analyze occurrences of trochlear dysplasia in patients with Down syndrome in the presence and absence of femoropatellar instability. METHODS Eleven knees with stable patellae and thirteen with unstable patellae in patients with Down syndrome were compared. Radiographs were produced to evaluate patellar height, trochlear angle and femoropatellar congruence angle. RESULTS The prevalence ratio for a high patella between the unstable and the stable patients was 1.01 using the Insall-Salvati index and 0.68 using the Caton-Deschamps index. For an abnormal congruence angle, the prevalence ratio was 2.04. An increased congruence angle was only found in four cases, all presenting instability. CONCLUSIONS Trochlear dysplasia was only found in cases of instability. The trochlear groove angle and the femoropatellar congruence angle correlated with the presence of patellar instability.