The postoperative, comparative analysis of the TKAs in the two groups showed that the mean medial femorotibial articular contact point in group 1 was significantly more anterior than in group 2. The medial femoral condyle of the knees in group 2 developed paradoxical anterior motion during the 1-year recovery interval, whereas group 1 knees did not. There were also significant differences between the groups regarding pain and the functional Knee Society Score and the Western Ontario and McMaster universities osteoarthritis index scores (P ≤ 0.05). Thus, group 1 had better postoperative results than group 2. A proper understanding of the reduced PTS in patients undergoing PCR-TKA, with the accompanying improved femorotibial articular contact kinematics, could contribute to better outcomes after PCR-TKA.
Previous studies focused on the influence of the tibial slope on extension and flexion gaps associated with TKA [9–11]. In recent years, some authors studied the influence of the tibial slope on knee kinematic patterns [12, 13]. Fujimoto stated that the mean medial femorotibial articular contact point in a large-PTS group was significantly more posterior than that in a small-PTS group, although there was no significant difference at the lateral femorotibial articular contact point during both weight bearing and non-weight bearing . Previous studies have reported that changes in the knee kinematic patterns after TKA could affect outcomes, such as polyethylene wear and prosthetic loosening. Medial femoral condyle paradoxical anterior movement after PCR-TKA was also reported [14, 15]. Another study noted a greater reduction in PTS after TKA compared with the preoperative PTS, thereby reducing the paradoxical mediofemoral condylar movement. The authors proposed a possible explanation for this mechanism .
The paradoxical movement after TKA has potential negative consequences, such as pathomechanics and decreased quadriceps efficiency, which could affect the outcome [16, 17].The biomechanics of a normal knee are changed by the paradoxical motion, which is non-physiological . The dynamic balance of soft tissue—including the joint capsule, the lateral collateral ligament, and the patellofemoral joint—was disrupted. It causes discomfort during knee flexion, which in turn influences the postoperative recovery of knee function. Some believe that the paradoxical movement reduces the distance between the femorotibial articular contact point and the patellar tendon ligament tibial insertion during knee flexion, reduces the lever arm extensor mechanism, and leads to the requirement of greater quadriceps muscle force to extend the knee at higher flexion angles . A large PTS could lead to the femorotibial articular contact point being in a posterior location, which would increase the efforts of the quadriceps of the lever arm and might cause movement inclination of the weight borne by the tibia [20, 21]. This could lead to higher quadriceps efficiency, possibly resulting in discomfort during knee extension [22–24]. Furthermore, a more posterior femorotibial articular contact point than normal could cause the joint contact force to arrive at eccentric positions, resulting in abnormal compressive or tensile strain at the tibial bearing point [18, 25, 26]. It could therefore result in discomfort during weight bearing at mid-flexion.
There were both weaknesses and strengths in this study. One weakness was that the number of subjects was small. All patients underwent bilateral knee replacement, however, which reduced the influence of the small number of subjects. One strength was that all operations were completed by one senior surgeon using the same size PCR-TKA, applying a consistent, controllable technique. This reduced the random influences of the surgeon, prosthesis, and surgical technique.
In conclusion, a proper understanding of the reduced PTS in subjects undergoing PCR-TKA could improve the outcomes by affecting femorotibial articular contact kinematics. The reduced posterior tibial slope, however, could also lead to decreased knee flexion range of motion and increase the probability of direct impingement of the prosthesis on the posterior femur postoperatively [27–29]. Hence, the degree of reduction of the posterior tibial slope is a subject worthy of further study.