Trochlear dysplasia influences patellotrochlear index when measuring patella alta

OBJECTIVES: Patella alta is a risk factor for patellar instability and its recurrence after treatment. The patellotrochlear index (PTI) is a measurement used to quantify patella alta, describing patellar engagement in the trochlea through an index of trochlear overlap with patellar articular surface based on sagittal imaging. Values less than 20% have been reported to indicate patella alta, yet this measurement has primarily been described in normal knees. Trochlear dysplasia is also a common risk factor in patellar instability and can affect the length of the trochlea. Therefore the purpose of this study was to compare PTI measurements in knees with and without trochlear dysplasia, and to describe its relationship with trochlear length. METHODS: Knees with trochlear dysplasia (defined by trochlear depth<4mm) based on CT imaging were identified from a series of knees with symptomatic patellar instability and compared to age- and gender matched asymptomatic controls. Measurements of patellar height including PTI and Caton Deschamps Index (CDI) were performed, as well as additional measurements of trochlear dysplasia including lateral trochlear inclination [LTI] and sulcus angle [SA]. Trochlear length was quantified using a previously described method of trochlear center-edge angle (TCEA) measurement, an angle between the posterior femoral cortical line with its intersection with Blumensaat’s line to the most proximal margin of the trochlea, with a smaller angle indicating more proximal extension of the trochlea. TCEA was measured at the central, medial, and lateral trochlea. Measurements were compared between dysplastic and control groups using t-test. Linear and multivariate regression analyses were performed to assess the relationship between PTI and measurements of trochlear dysplasia. RESULTS: 21 knees with trochlear dysplasia (13 female and 8 male) were included in this study, and were compared with 21 age- and gender-matched controls. In the dysplastic group, trochlear depth (TD) was 0.7+/-2.4mm vs 6.8+/-2.5mm (p<0.001) in the control group, SA was 154.2+/-11.5 degrees vs 134.0+/-8.3 degrees (p<0.001), and LTI was 12.1+/-6.2 vs 23.7+/-4.4 degrees (p<0.001) . Despite CDI being greater in the symptomatic group (1.19+/-0.23 vs 1.06+/-0.19, p=0.053), PTI was greater (with more overlap) in the symptomatic group (34.92+/-15.23% vs 25.43+/-21.55%, p=0.076). In the symptomatic group, the trochlear groove extended more proximally at the central (TCEA 58.4 degrees vs 68.6 degrees, p<0.001) and medial aspects (68.9 degrees vs 77.9 degrees, p<0.001). In dysplastic knees, PTI demonstrated a weak negative correlation with TCEA (R=-0.40) and with LTI (R=-0.45). CONCLUSION: In this study, knees with trochlear dysplasia had a PTI of 35% vs 25% in the control group, indicating greater patellar engagement despite having greater measurements of patella alta based on CDI (1.19 vs 1.06). PTI correlated with increased proximal extension of the trochlea which may account for the greater patellofemoral overlap. As abnormal trochlear length can associated with supratrochlear spurs, greater values on PTI may not necessarily indicate functional patellofemoral engagement. Further studies are needed to identify the functional relationship between trochlear length and patella alta.