THE SULCUS ANGLE AND EXTENSOR MECHANISM MALALIGNMENT OF THE KNEE

Abstract

Anterior knee pain attributable to the patellofemoral joint and extensor mechanism dysplasia is a common presentation to Orthopaedic surgeons. Plain radiology is likely to remain the primary investigation of the knee in most centres, but most of the radiological features of extensor mechanism dysplasia are time consuming and difficult to measure reproducibly. 137 consecutive symptomatic knees aged under 30, referred to an Orthopaedic surgeon were studied in order to identify a rapid and reproducible marker for those knees worthy of further in-depth analysis.

Overall, 67 knees (49%) had at least one radiological abnormality and 70 (51%) were considered ‘normal’. There were 5 Dejour Type3 dysplasias of the femoral trochlea, 9 Type2 and 12 Type1. There were 49 cases of patella alta and 5 of patella infera. Four knees had an abnormal lateral patellofemoral (patellar tilt) angle. 15 knees had more than one abnormality. The classification of trochlear dysplasia was difficult and showed poor reproducibility. This was also true for the measurement of lateral patellofemoral angles. Patellar height was more easily measured but took time. The sulcus angle emerged as an easily and rapidly measurable feature that was reproducible and was closely related to the other features of extensor mechanism dysplasia.

The sulcus angle offers a rapid and reliable ‘screening’ measurement on knee radiographs. A normal sulcus angle suggests that seeking the other radiological markers of extensor mechanism malalignment is unlikely to reveal additional useful information. Other diagnoses can then be sought. The more abnormal the sulcus angle, the more severe the other features of extensor mechanism dysplasia are likely to be. Further detailed measurements can then help to define the most appropriate surgical correction.