Giới hạn của chuyển động có thể là một hậu quả của hạn chế quá mức. Đối với chuyển động sinh lý trong một đầu gối bình thường, cơ chế roll-back là cần thiết. Cơ chế này di chuyển phía trước xương chày trong chổ cong tối đa, thay thế xương chày từ condyles đùi và do đó ngăn ngừa sự rung động giữa hai phía sau. | 71 11 Chapter 11 Failure in Constraint Too Much - N. Wulker M. Ludemann Limitation of motion may be another consequence of excessive constraint. For physiological motion in a normal knee the roll-back mechanism is necessary. This mechanism moves the tibia anteriorly in maximum flexion displacing the tibia away from the femoral condyles and thereby preventing impingement between the two posteriorly. If roll-back is prevented by constraint this may limit motion. Patellar tracking on the femoral shield depends on physiological motion. Femoral roll-back reduces patellofemoral forces by improving the efficiency of the extensor mechanism. Lack of femoral roll-back may increase pressures at the patellofemoral joint during flexion with or without surface replacement of the patella. Lack of rotation around a vertical axis may result in patellar displacement which occurs particularly in a lateral direction. Knee function during gait significantly differs between hinged prostheses and a standard surface replacement. These differences were clearly demonstrated during gait and stair climbing 4 . Finally constrained knee designs generally require more bone resection than surface replacements. In hinged or rotation designs resection must be sufficient to accommodate the hinges or rotation pegs of the implant. With spines at the tibial insert room for a cam must be created by femoral bone resection. Also because these implants are generally larger they may be more prone to infection. Too Much Constraint In a knee prosthesis with excessive constraint forces at the implant and at the interface between implant and bone will be greater than necessary. Without the energy absorption described above excessive forces may accelerate wear and loosening of the prosthesis. These considerations have led to a general agreement to implement as little constraint as necessary in knee joint replacements. However in spite of the logic behind this approach the available data are still somewhat .
