In this study, the biomechanics of the extruded and centralized LM were analyzed in porcine knee joints at different flexion angles. In the anterior and middle LM, both the contact pressure and area decreased in extrusion, increasing close to the intact status after the centralization procedure. In this model, the effectiveness of centralization to restore the lost function of the meniscus has been demonstrated in the anterior and middle LM.
We set knee flexion angles at 30°, 45°, 60°, and 90°. Since the most extended position is approximately at 30° and the most flexed position is approximately at 90° in a pig knee joint, we first set the flexion angles every 30° between 30° and 90°. We also set 45° because we set that angle in our previous report . Biomechanical analysis was therefore performed at 30°, 45°, 60°, and 90°.
We applied 200 N as an axial compressive force at each setting. The body weight of the pigs which knees we used was approximately 80-100 kg. The pig's center of gravity is near the forelegs and the load on the hind legs is lower than on the forelegs. When standing on a quadruped, the load on single hind leg of an 80-100 kg pig is approximately 160-200 N . 200 N might be too small as an axial compressive force for biomechanical studies. However, in this study, 200 N was sufficient to examine the effects of meniscal extrusion and the effects of centralization. In our previous report, 200 N was the force applied in a similar setting . Furthermore, reported that, in their model, which used similar porcine knees, the in situ force of the LM with a complete radial tear significantly decreased even under an axial load of 150 N . Therefore, the axial compressive force applied in this study would be large enough to yield clinically significant findings.
Although the deviation distance of the LM, which increased in extrusion, was restored to the intact status in centralization at all angles, the contact pressure and area, decreased in extrusion, were not fully restored in the posterior LM, even after centralization. This was possibly because a 1 cm width of the posterior root deficiency was left untreated. These results suggest that hoop function should also be reconstructed, if possible, in order to fully restore the load distribution function of the posterior LM. Even so, centralization decreased the contact pressure in the tibial cartilage, and this effect became more obvious as the flexion angle became larger.
The distances between the two markers increased with knee flexion angle in each setting, although no significances were found. This can be explained from the results of the current study; the load distribution moved posteriorly as the flexion angle increased. A previous magnetic resonance imaging (MRI) study also supports our results, showing that the lateral femoral condyle and LM consistently displayed a marked posterior translation .
To our knowledge, previous reports of biomechanical analysis for the centralization of the extruded meniscus are limited. Nakamura et al. used the centralization procedure in an ACL-reconstructed porcine knee with an irreparable lateral meniscus defect to evaluate the effects of knee biomechanics; they reported that using arthroscopic centralization for the capsular support of the middle segment of the lateral meniscus improved the residual rotational laxity of the ACL-reconstructed knee, which had lateral meniscus dysfunction due to massive meniscal defect . Daney et al., in the only report other than ours , measured meniscal extrusion and tibiofemoral contact mechanics at the medial compartment in human cadaveric knees . The anatomic transtibial pull-out root repair and the anatomic transtibial pull-out root repair with centralization suture techniques best restored the contact mechanics of the knee and meniscal extrusion when compared with root tear and nonanatomic repair states. However, the degree of extrusion increased as the knee was flexed to 90°. Their study differs from ours in terms of using human knees, examining the inner compartment, and performing the centralization with pullout techniques; both studies, however, showed the effectiveness of centralization.
We previously reported the effect of centralization in a porcine model . The methods used in the two studies were similar in that the experimental settings were the same. The difference between the two studies was that, in the current study, biomechanical analysis was performed at 30°, 60°, and 90° as well as at 45°. Similar results were obtained at 45° and the new findings were revealed at 30°, 60°, and 90° in knee flexion. Although significant differences of contact area and contact pressure at different angles were not detected, the following trends were observed: Contact area and contact pressure in the anterior and middle LM reached their maxima at 30°, 45°, and 60°, while those at the posterior LM reached theirs at 90°.
For limitations, we cut the lateral collateral ligament to insert a sensor from the lateral side; this raised a concern that instability caused by LCL deficiency could have affected the results. We also inserted a pressure mapping sensor between the femoral cartilage and the LM, rather than between the LM and the tibial cartilage, which would have impaired the load-distribution measurements for the entire tibial cartilage. However, the knee joint was stabilized and the loading force applied in the vertical direction; the comparison of evaluated values under intact, extrusion, and centralization settings at each flexion angle will therefore provide important information.