Comparison of native axial radiographs with axial MR imaging for determination of the trochlear morphology in patients with trochlear dysplasia

A dysplastic trochlea. may be missed on conventional roentgenography due to superimposition effects of the distal femoral groove. Measurements of (1) the sulcus angle, (2) the lateral trochlear slope angle and (3) the lateral to medial trochlear ratio were compared between plain axial radiographs and axial magnetic resonance images (MRI) in 24 patients suffering from clinical patellofemoral instability. Trochlear dysplasia was classified into type A through D on MR images. Measurements for (1) were significantly different among all dysplasia types. It was 135.6A degrees A A +/- A 19.8A degrees on X-ray while it was above 180A degrees on MRI within dysplasia type D patients. Measurements for (2) were different among all dysplasia types, reaching statistical significance in type B dysplasia patients. Differences between radiographs and MRI for (3) were not significant, while only on MRI the ratio increased with increasing degree of dysplasia. The investigation illustrates that plain axial radiographs do not represent the natural bony trochlear morphology and may mislead further clinical management.