Background  

The joint gap is set rectangular at 90° flexion during total knee arthroplasty (TKA). However, the condition of the joint gap in deep knee flexion is obscure.

Methods  

The method for obtaining a posteroanterior view radiograph of the knee at 90° flexion (the epicondylar view) was modified, and a method to obtain an anteroposterior view radiograph at 120° flexion (deep flexion view) was established. With this method, subjects lie on the radiography table with their thighs placed on a device so their lower legs hang down in neutral rotation with a 1.5-kg weight attached to the ankle. The joint gap angle and medial and lateral joint space widths were measured on epicondylar view and deep flexion view radiographs in 20 normal male subjects, 20 normal female subjects, and 20 subjects after TKA.

Results  

The joint gap was almost rectangular at two flexion angles in normal subjects. In the implanted knees, the gap angle was 1.4° varus ± 3.3° (mean ± standard deviation), and no significant difference was found between medial and lateral joint space widths at 90° flexion. In contrast, the gap angle was 2.5° varus ± 2.5° and the lateral joint space width was significantly wider than the medial joint space width at 120° flexion (P < 0.001). The gap angle was more varus with a significant difference in the implanted knees than that in the normal subjects at 120° flexion (P < 0.001).

Conclusions  

The joint gap was trapezoidal with a wider lateral side at 120° flexion even though it was almost rectangular at 90° flexion after TKA.