The lateral condyle index: a new index for assessing the length of the lateral articular trochlea as predisposing factor for patellar instability.

The central trochlea has been considered as the major location of dysplasia. The purpose of this study was to investigate the influence of the lateral trochlea on patellar stability and to establish a new method for measuring the lateral trochlea on sagittal magnetic resonance (MR) images. Twenty-eight knees of 23 patients suffering from lateral patellar subluxation (12 knees with radiological signs of central trochlear dysplasia) and of 46 patients without patellofemoral complaints (without central trochlear dysplasia) were analysed. The lateral condyle index was designed to measure the lateral trochlea by comparing the anterior cartilaginous trochlea (a) and the posterior aspect (p) [(a:p)× 100]. The lateral condyle index showed high interrater reliability (r  = .94) and was significantly (p < 0.001) lower in symptomatic patients (86%) than in the control group (93%). These results show high clinical relevance of the lateral trochlea as another factor for patellar instability.