Background: Several studies have reported varus-valgus stability in the extension position after total knee arthroplasty (TKA). However, few studies have evaluated joint laxity in the flexion position postoperatively. The purpose of the study was to evaluate joint laxity against distal traction force on flexion after cruciate-retaining and posterior-stabilized total knee arthroplasties.
Methods: A total of 44 knees (22 knees cruciate-retaining, 22 knees posterior-stabilized) in 40 patients with osteoarthritis were tested in this study. The subjects were seated at a table and their knee joints were fixed at 80 degrees of flexion to avoid overlapping images of condyles and the femoral shaft. Tibial shafts were adjusted to be parallel to the radiographic films, and posteroanterior radiographs were obtained. Flexion stress tests were performed with a distal traction of 100 N at a neutral foot position. Radiographs were obtained at neutral and traction positions. The distance from the perpendicular line of the top of the polyethylene insert to the midpoint on the tangential line of the femoral condyle was measured (joint space distance) at each side.
Results: In the flexion-neutral position, average joint space distances were 0.1 +/- 0.2 mm in cruciate-retaining (CR) TKA knees and 0.2 +/- 0.3 mm in posterior-stabilized (PS) TKA knees. With flexion-traction stress tests, the average joint space distances were 0.5 +/- 0. 5 mm in CR TKA knees 2.4 +/- 1.2 mm in PS TKA knees. Average changes of joint space distances between the two positions were 0.3 +/- 0.4 mm (CR TKA) and 2.2 +/- 1.5 mm (PS TKA). The changes in joint space distances between neutral and traction positions of PS TKA knees were significantly larger than those of CR TKA knees in flexion stress tests (P < 0.01).
Conclusion: The posterior cruciate ligament acted as a stabilizer against distal traction force in the CR-TKA knees. However, the laxity of PS-TKA knees against distal force differed among individual cases.