Accurate femoral tunnel placement is of great importance during medial patellofemoral ligament (MPFL) reconstruction. Purpose of the present study was to investigate the influence of trochlear dysplasia on the accuracy of fluoroscopic guided femoral tunnel placement.
CT-Scans of 30 knees (five with regular shaped trochlea, 10 with a Type A and five each with a Type B, C, or D trochlear dysplasia) were imported into the image analysis platform MeVisLab. A 3D Bone Volume Rendering (VR) and a virtual lateral radiograph was created. The anatomic femoral MPFL insertion was identified on the 3D VR. On virtual lateral radiographs, the MPFL insertion was identified based on landmarks described by Schöttle et al. using three different perspectives: Best possible overlap of the femoral condyles (BC) and a tangent along posterior border of the posterior femoral cortex (pBC); a tangent along the anterior border of the posterior cortex (aBC); and best possible overlap of the distal part of the posterior femoral cortex (BF). Distances between the anatomic attachment and radiographically obtained insertions were measured on the 3D VR and compared according to the type of trochlear dysplasia.
Significantly lower accuracy of fluoroscopy guided tunnel placement in MPFL reconstruction was found in knees with Type C and D dysplasia. This effect was observed irrespectively from the radiologic perspective (pBC, aBC, and FC). In the pBC view (highest accuracy), the mean distance from the centre of the anatomic MPFL attachment to the radiographically defined location was 4.3 mm in knees without trochlear dysplasia and increased to 4.8 mm in knees with Type A dysplasia, 3.8 mm in knees with Type B dysplasia, 6.7 mm (p < 0.001) in knees with Type C dysplasia, and 7.3 mm (p < 0.001) in knees with Type D dysplasia.
Radiographic landmark-based femoral tunnel placement in the pBC view provides highest accuracy in knees with a normal shaped trochlea or low grade trochlear dysplasia. In patients with severe dysplasia, fluoroscopy guided tunnel placement has a low accuracy, exceeding a critical threshold of 5 mm distance to the anatomic MPFL insertion irrespective of the radiographic perspective. In these patients, utilization of anatomic landmarks may be beneficial.