In this paper, the effect of the patellar tendon release of the upper end, the lower end, and both end release of 30% on the movement characteristics of the tibiofemoral joint squat was investigated. Furthermore, the data of the no-released model was compared with the data of the released model and other research results of knee joint.
Medial-lateral translation of the femur relative to the tibia
In the knee joint movement after TKA of Koji [19], with the knee joint flexion, the femur first translated medially and then translated laterally relative to the tibia, and there is a trend of lateral translation for the femur. In the natural knee joint study of Michal Kozanek [20] and Chi-hui Chen [21], the femur relative to the tibia also showed a trend of lateral translation. Their results of the general movement trend of femur relative to the tibia were basically consistent with the results of the released model in this paper. The TKA study results of Koji[19] showed that the lateral translation was the largest (18 mm) at 120 degrees of flexion; at 135 degrees of flexion in this paper, the maximal lateral translation of the femur relative to the tibia was about 11.71 mm before the release of the patellar tendon, the maximal lateral translation was about 10.57 mm after the release of the patellar tendon; the maximal lateral translation results of Koji[19] was different from this paper, the reason maybe related to the using different knee prosthesis types.(the patients in Koji’s paper had undergone bi-cruciate stabilized fixed-bearing TKA). In the natural knee joint study of Chi-hui Chen[21], the maximal lateral translation was about 3 mm, which was also basically consistent with the maximal lateral translation (2.39 mm) after the release of the patellar tendon in this paper. Meantime, the knee flexion’s improvement level was directly decided by the limitation flexion capacity after 100 degrees flexion. In order to study the relationship between medial-lateral translation and the improvement of high flexion, we focus on this relationship when the flexion is greater than 100 degrees for released and no-released models. So we extract the relationship from Fig. 4, as shown in Figure.7.
As shown in Figure.7, for all above four models, the medial-lateral translation of the femur relative to the tibia exhibited an increasing trend. It is obvious that the lateral translation of no-released model was larger than that of released model at the same flexion angle. This means that only the small amount of lateral translation was required to achieve the same flexion angle than the no-release one. The average value of lateral translation of the upper-end-release, the lower-end-release, and both-ends-release was taken as the lateral translation of the released model, and compared with the lateral translation of the no-released model, as shown in Figure.8 (a). When the degree of flexion is greater than 100°, the lateral translation of the released and no-released model increases approximately linearly and its variation trend is stable, in which N-trendline means no-released trendline and R-trendline means released trendline. According to the trend line in Figure.8(a), it can be concluded that: at the same translation(T), the corresponding flexion degree of released model (β) is larger than that of no-released model (α), therefore, there are larger flexion capability for released TKA knees.
Superior-inferior translation and internal-external rotation
For the superior-inferior translation of the femur relative to the tibia, the nature knee studies of Michal Kozanek [20] and Chi-hui Chen [21] showed that the femur translated upward in the early flexion. Their results of the femoral superior translation were basically consistent with that of the released model in this paper. Related studies [22] also showed that the femur translated upward with the knee flexion, and the maximum flexion angle of the knee was 90 degree, at the same time, the maximum superior translation was about 6.17 mm in their researches, which was basically consistent with this paper (the maximum knee flexion was 135 degrees, and the maximum superior translation was 4.92 mm). Similarly, we focus on this relationship when the flexion is greater than 100 degrees for released and no-released models. It can also be predicted that the released flexion degrees was larger 5°than the no-released one after 100 degrees of flexion at table 2.
For the internal-external rotation of the femur relative to the tibia, the post-TKA knee studies of Koji[19] showed, with the flexion of the knee joint, the femur is always rotated externally relative to the tibia. The femur of the released model showed rotated externally except for 0–10 degrees in this paper, which consistent with the general trend of Koji [19]. In this paper, the external rotation of the femur of upper-released and lower-released reached the maximum at 80 degrees of flexion, the both-ends release reached the maximum at 90 degrees of flexion, and the lateral rotation trend of the four models were first increased and then decreased. It can also be predicted that the released flexion degrees was larger 4.8° than the no-released one after 100 degrees of flexion at table 2.
Anterior-posterior translation and the adduction- abduction
For the anterior-posterior translation of the femur relative to the tibia, Silvia Pianigiani [23] studied knee squatting after four TKA respectively, the results showed that the femur translated anteriorly relative to tibia. In this paper, within 0-110 degrees of flexion, the femur of released models also translated anteriorly, which was consisted with the no-released model; however, within 110–135 degrees of flexion, the femur translated posteriorly. In the natural knee joint study of Kesys [24], with the flexion of the knee joint, the femur continued to translated posteriorly relative to the tibia; while the femur of released and no-released models translated posteriorly until 110 degrees; different reason may be the femoral anterior-posterior translation has changed after TKA. In this study when the knee flexion is greater than 110°, the posterior translation of the released and no-released models increased linearly and their variation trend were stable. It can also be predicted by that the released flexion degrees was larger 1°than the no-released one after 100 degrees of flexion, as shown in table 2.
For the adduction-abduction of the femur relative to the tibia, the results of Silvia Pianigiani [23] showed that the use of different knee prostheses resulted in a great difference in adduction- abduction of femur relative to tibia, which indicated that the type of prosthesis had a great influence on the movement of knee joint. The results of K.E. Moglo [25]showed that the femur adducted and then abducted, In this paper, within 30–80 degrees, the femur adducted relative to the tibia; within 90-135degrees,the femur abducted, which was basically constited with the results of K.E. Moglo. In this study, similarly, we focus on this relationship when the flexion is greater than 100 degrees for released and no-released models. It can also be predicted by that the released flexion degrees was larger 4° than the no-released one after 100 degrees of flexion at table 2.
By comparative analysis, it can be concluded that the release of patellar tendon was favorable for high flexion(flexion angle greater than 120°). And the femur of released models relative to tibia for the medial-lateral translation, superior-inferior translation, internal-external rotation, anterior-posterior translation, and adduction-abduction were favorable for high flexion. As shown in table 3.
By comparative analysis, the change of flexion angle of the released and no-released models was obtained. Among them, when the flexion is greater than 100 degrees,at the same lateral translation, the flexion degree of released model increased by 5.4° on average compared with no-released model; At the same superior translation, the flexion degree of released model increased by 5° on average; At the same external rotation, the flexion degree of released model increased by 4.8° on average; At the same posterior translation, the flexion degree of released model increased by 1° on average; At the same adduction, the flexion degree of released model increased by 4° on average as shown in table 4 below. In this paper the total average ROM of the released models increased by 4°, indicating that the release of the patellar tendon is favorable for the high flexion.
The limitations of this study should be considered. Only 30% of the model was released, and other degrees of releasing were not considered. In this paper, the FEM of the knee after TKA was established on the basis of considering the main bone tissue and soft tissue. Although the medial and lateral transverse patellofemoral ligament was added, the joint capsule, hamstring or gastrocnemius muscle did not take into account for the knee joint activity. Subsequent research should take these shortcomings into account to make the model more physiologically compatible.