This was the first study to investigate the impact of posterior slope on flexion angle in the same knee using experimental study protocol. The results of the current study showed that flexion against gravity did not increase when the posterior slope was increased by an additional 10°. Thus, an increase in the posterior slope did not affect the intra-operative flexion against gravity in the model used in this study.
As to an appropriate posterior slope angle, a negative or anterior slope reportedly led to the subsidence of the anterior tibia and dislocation of the insert [18, 19]. In PS TKA, an excessive posterior slope led to anterior post-cam impingement [20]. A recent paper recommended, based on a computer simulation, that the posterior tibial slope should be less than 5° [21]. Furthermore, the study reported that abnormal kinematics, such as anterior sliding of the tibial component and the anterior impingement of the tibial post, were observed when the posterior slope was greater than 5°, arguing that an excessive posterior slope of the tibia in a PS knee should be avoided to prevent damage to the post-cam mechanism.
For PS TKA, some studies investigated the relationship between the posterior tibial slope and the post-operative flexion angle. A previous study reported that the flexion angle improved by 1.8° per degree increase in the posterior slope [15]. Hence, we had expected that the flexion angle would increase by 18° due to the slope being increased by 10°. However, another study reported that there were no differences in the post-operative range of motions between the two groups after using a cutting block with a tilt of 0° and 5°, respectively [12]. Yet another study found that there was no significant difference in the post-operative range of motion between the group with a posterior slope < 10° and the group with a slope ≥ 10° [14]. These studies were of observational nature, and it was not possible to exclude factors affecting the post-operative flexion angle other than the posterior tibial slope.
In this study, the specially-designed 10° posterior slope insert moved the femur more posteriorly than the normal insert. Theoretically, this shift occurred when the bone cut was sloped, and the positive effect increased in the flexion angle because the distance between the posterior surface of the femoral bone and posterior edge of the tibial component increased [4]. The effect of the shift might be obscured by the high conformity of the insert to the femoral component in this study. The prosthesis used in this study had a mobile mechanism and a very conforming upper articulating surface. The rotational freedom probably affected the flexion angle, as high flexion was associated with significant internal rotation of the tibia [22]. Thus, the results obtained in this study may not be directly extrapolated to other PS designs. Further study comparing mobile insert and fixed insert in the same knee will clarify the effect of insert mobility on flexion angle.
In CR TKA, tight flexion gap knee requires PCL release to increase or maintain the flexion angle [7]. The tight PCL causes excessive roll-back of the femur and decreases flexion angle [23]. The most popular method to ease the tightness of the PCL is to increase the posterior slope of tibia. However, the posterior slope itself was reported not to reflect anteroposterior kinematics during deep flexion (90°–120°) [24]. Therefore, the key to deal with tight CR TKA is the adequate tensioning of the PCL and not the increase of the posterior slope.
The merit of this study lies in that the direct effect of the posterior slope of the tibia on the flexion angle was evaluated intra-operatively in the same knee. Two measurements were performed continuously, as the soft tissue balance was corrected before the measurements and remained unchanged between the two measurements. This way, factors other than the insert shape could be completely eliminated, so that the influence on the flexion angle of the tibial slope could be appropriately evaluated.
This study had some limitations. First, the intra-operative flexion angle was examined only against gravity. In agreement with our findings, a previous study reported that there existed a correlation between the flexion against gravity and the post-operative flexion angle when the same implant was used [25]. Therefore, the results of this study also apply to the post-operative flexion angle. Second, the flexion angles were measured manually using a goniometer. Although this is an easy and straightforward method, it often fails to provide accurate and reproducible results. Recently, the standard error of measurement using this device has been reported to be 1.56° (range, 0.52–2.66) [26]. In this study, three markers (the lateral condyle of femur, the head of the fibula, and the lateral malleolus of the foot) were determined and measured. It is unknown whether these markers can be used to reproduce the femoral/tibial axis. However, this measurement evaluates the difference in the flexion angles based on the two types of inserts, and not the flexion angle itself. We believe that the three markers will not detach and will not significantly influence the results of this study. Third, thigh-calf contact, which might limit higher flexion, was not investigated in this study [27]. However, the main conclusion would not be affected by the existence of posterior flesh, which should be identical between the repeated measurements. Finally, this study did not include a large number of samples. However, the sample size was calculated as described in statistical analysis section and 21 knees would allow for the detection of a 5° difference. According to a previous report [15], the flexion angle was expected to increase by 18° due to a 10° slope increase. For these reasons, the number of samples in this study was adequate to serve the purpose of this study.