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

Abstract

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.

Fig. 1

Magnetic resonance measurements of the lateral condyle index [5]. a Measurement of the central axis (Ca): C1 proximal circle in the femoral shaft, C2 distal circle in the femoral shaft. b Measurements of the anterior and posterior cartilage length: d baseline distal condyle (perpendicular to Ca), A superior most aspect of anterior cartilage of the lateral condyle, P superior most aspect of the posterior cartilage of the lateral condyle, a length of the anterior articular cartilage of the lateral condyle (red line), p length of the posteror articular cartilage of the lateral condyle (blue line)

Fig. 2

Magnetic resonance measurements with normal length of the anterior articular cartilage of the trochlea lateral facet. The lateral condyle index is 95%. Ca central axis, C1 proximal circle in the femoral shaft, C2 distal circle in the femoral shaft

Fig. 3

Magnetic resonance (MR) measurement showing an anterior articular cartilage of the trochlea lateral facet that is too short. The lateral condyle index on sagittal MR is 63%. Ca central axis, A superior most aspect of anterior cartilage of the lateral condyle, P superior most aspect of the posterior cartilage of the lateral condyle, a length of the anterior articular cartilage of the lateral condyle (red line), p length of the posteror articular cartilage of the lateral condyle (blue line), d baseline distal condyle (perpendicular to Ca)

Fig. 4

Intra-articular inspection shows an articular cartilage of the trochlear lateral facet that is too short (arrow), with signs of patellar subluxation (cartilage destruction)