Biomechanical analysis of a centralization procedure for extruded lateral meniscus after meniscectomy in porcine knee joints

Abstract The recently developed arthroscopic centralization for lateral meniscal extrusion has obtained satisfactory short‐term clinical and radiological results and improves the meniscus biomechanical properties. However, the effectiveness of treatment for meniscus extrusion after partial meniscectomy still requires elucidation. This study investigated the effect of centralization with modifications from a mechanical viewpoint. Porcine knee joints (N = 6) were set in a universal tester under the following conditions: (1) Intact; (2) Meniscectomy: Inner half of the posterior half meniscus was removed; (3) Extrusion: Posterior meniscus was dislocated laterally by transecting the posterior root and the meniscotibial ligament; (4) Centralization‐1: Centralization procedure using one anchor; (5) Centralization‐2: Centralization procedure using two anchors; and (6) Centralization‐ad: Centralization with capsular advancement using two anchors. Load distributions and contact pressure in the meniscus and tibial cartilage were evaluated with an axial compressive force of 200 N. After meniscectomy, the tibial cartilage load increased and that of the medial margin of the posterior part of the meniscus decreased. When the meniscus was extruded, the load was concentrated only on the tibial cartilage. Centralization‐1 increased the load on the meniscus, while Centralization‐2 further increased the meniscus load but decreased the tibial cartilage load. Centralization‐ad further decreased the load on the tibial plateau. The average contact pressure of the tibial cartilage was significantly higher in the Extrusion group than in the Intact group or the Centralization‐ad group. From a biomechanical viewpoint, centralization with capsular advancement was the most effective of the tested procedures for treatment for an extruded meniscus after partial meniscectomy.


| INTRODUCTION
Meniscal extrusion induces dysfunction of load distribution [1][2][3] and is often observed after meniscectomy 4,5 and meniscus root tears, 6 as well as with aging. [7][8][9] Even if the meniscectomy is not extended to the popliteal hiatus, the fragility of the attachment site of the meniscotibial capsular ligament just anterior to the popliteal hiatus results in loosening of the capsular ligament and leads to extrusion. 10 This extrusion correlates with symptoms of the knee joints, especially knee pain, and initiates knee osteoarthritis (OA) as the extrusion progresses. [11][12][13] An association has been reported between a poorer clinical outcome after partial meniscectomy and a greater severity of meniscal extrusion. 5 Therefore, restoring the loss of function of the meniscus can improve the symptoms and delay knee OA progression. 14 An arthroscopic centralization technique has been developed to centralize the mid-body of the lateral meniscus (LM) to reduce/prevent extrusion. 15 The advantage of this technique is that the function of the LM in distributing loads can be restored if the mid-body of the meniscus is retained. Therefore, this technique is applicable to cases with osteochondral injury or OA progression caused by extrusion of the LM after partial meniscectomy, as well as cases with symptomatic lateral discoid meniscus where meniscoplasty is planned. This procedure has shown satisfactory short-term clinical and radiological results 16,17 and improves the biomechanical properties of the meniscus for load distribution, [18][19][20][21] making it one of the potential surgical options that can prevent the progression of OA due to meniscal extrusion. However, treatment of meniscus extrusion remains difficult after partial meniscectomy, the major surgical treatment for the meniscus, 22 despite recent improvements in the techniques for meniscus repair. 23 Meniscoplasty of the LM by capsular advancement has recently been reported as a treatment for meniscus extrusion with meniscus defects. With this technique, the meniscotibial capsule is released from the tibia and advanced with the remaining meniscus onto the rim of the tibial plateau to reform a meniscus-like configuration.
This surgical intervention was shown to improve clinical and radiographic outcomes at a 2-year follow-up in patients with lateral compartment OA attributed to LM defects. 24 The purpose of the present study was to investigate the effect of centralization with modifications from the mechanical viewpoint using an existing porcine knee joint model. 18,21 2 | METHODS

| Porcine knee joints
We used the knee joints of approximately 100 kg of 6-month-old commercial pork pigs (strain and gender unknown; Tokyo Shibaura Zouki). The porcine knee joints were fresh-frozen and only right-side joints. Any knees with a damaged meniscus or cartilage were excluded. The lateral compartments were analyzed in six right knees.

| Experimental setup
The experimental setup was described previously. 18,21 The muscles around the knee joint were removed before cutting the bone. In brief, the femur bone was cut obliquely at 45°at 7 cm proximal from the joint and the tibia bone was cut horizontally at 3 cm distal to the joint. The resulting joint was then fixed to a tester using poly-

| Contact area and force measurements
The load-distributing force on the lateral compartment was quantified using a pressure mapping sensor system that enabled the measurement of the real-time force and contact area (Tekscan, Inc.). [26][27][28] The sensor was placed on the femoral side of the LM and recorded the load distribution, as well as average contact pressure, maximum load, and contact area. The data were analyzed with MATLAB® (MathWorks).

| Statistics
The Friedman one-way non-parametric test and Dunn's multiple comparisons test were used as post hoc tests and calculated using Prism 8 software (GraphPad Inc.). P values less than 0.05 were considered statistically significant. All data were shown as means with 95% confidence intervals (CIs).  Figure 3A and Table S1).
The maximum load decreased after meniscectomy and further decreased significantly after extrusion, whereas it increased after centralization step by step in every part of the LM. However, the maximum load was not fully restored even after centralization with advancement in the posterior LM ( Figure 3B and Table S2).
Similar results to those for maximum load were obtained for the average contact pressure in every part of LM. This pressure was fully restored after centralization with advancement, even in the posterior LM ( Figure 3C and Table S3). In the tibial cartilage, the average contact pressure significantly increased after extrusion, whereas it significantly decreased after centralization with advancement ( Figure 4A and Table S4). The distribution of the average contact pressure showed a similar pattern between the intact group and the group after centralization with advancement ( Figure 4B).

| DISCUSSION
This biomechanical study showed that the centralization procedure for meniscus extrusion after partial meniscectomy can reduce extrusion of the LM; however, this procedure was not sufficient for restoration of the load distribution function of the LM.
Conversely, the centralization with advancement significantly decreased the average contact pressure on the tibial cartilage and restored a distribution pattern similar to that of the intact knee joint.
Meniscal defects after meniscectomy of the LM are one of the main causes of secondary knee OA of the lateral compartment. Koga et al. 24 reported that meniscoplasty of the LM by capsular advancement improved the clinical and radiographic outcomes at the 2-year follow-up in patients with lateral compartment OA attributed to lateral meniscal defects. However, the biomechanics of this procedure remains unclear.
Therefore, we created our extruded LM model after partial meniscectomy and analyzed the centralization procedure using one or two anchors, or two anchors with capsular advancement in porcine knee joints.
Centralization with one or two anchors did not fully recover the contact area in the LM; however, centralization with advancement restored both the maximum load and the contact pressure. In the middle LM, the contact area, maximum load, and contact pressure increased after centralization with one anchor and these were further increased with two anchors, whereas they were fully restored after centralization with advancement. In the posterior LM, none of the centralization groups showed full recovery of the contact area and maximum load, as only the contact pressure was recovered after centralization with advancement.
These findings indicated that centralization with one or two anchors was useful for restoration of the meniscus function to some extent; however, adding the procedure of capsular advancement was more effective for the recovery of most of the meniscus functions in the knee with meniscus extrusion after partial meniscectomy.
In regard to the tibial cartilage, meniscus extrusion significantly in- The Tekscan device is one of the most popular biomechanical testing devices and is able to measure the real-time contact forces of the knee joint. [26][27][28] We used the Tekscan Sensor Model 5027, and the pressure sensor film was calibrated and equilibrated using the Tekscan pressure calibration unit and correction software for each specimen. The maximum load (N) was the value measured in one cell (0.3 × 0.3 mm 2 ) of the sensor.
The curved interface might possibly impact the measurements because the sensor was placed between the femoral cartilage and the LM.
However, the sensor did not move throughout the testing because it was stabilized by the pressure within the joint space. We also repeatedly added saline mist throughout the experiment to avoid wrinkling of the film with time due to the dry environment. 18 Nevertheless, the cells in the sensor film did not react to the loading force. Therefore, the data set was adjusted with respect to the neighboring cells.
This study had some limitations. One was that the LCL was cut to insert a sensor from the lateral side, thereby raising concerns that in-

| CONCLUSIONS
The centralization procedure could reduce extrusion of the LM and restore the load distribution function of the LM in a porcine model. From a biomechanical viewpoint, centralization with capsular advancement was the most effective among the tested centralization procedures as a treatment for an extruded meniscus after partial meniscectomy.

ACKNOWLEDGMENTS
The authors thank all the members of the Center for Stem Cells and