The main finding of this study is that the meniscus with slight and severe posterior root tears can still produce circular tension to maintain the consistency of the tibial-femoral joint, that is, there is no significant difference in contact mechanics, kinematics and contact area among the meniscus and the healthy meniscus. There are similarities in biomechanical behavior between the entire tear model and partial meniscectomy model, and meniscus repair can restore the mechanical properties of medial meniscus to some extent.
The relationship between medial meniscus root tear and the progression of knee osteoarthritis has been fully proved in clinical studies [28]. Injuries that lead to tears at the root of the meniscus often occur during squats or activities involving flexion, usually at the same time as some type of rotation [29]. The circumferential fibers of the meniscus disperse the compression force in the vertical direction uniformly and effectively improve the axial stress overload [30]. This dissipation of axial load is essential for the viability and function of articular cartilage [31]. Root tearing will lead to the destruction of the circular structure of meniscus, the increase of contact pressure of tibial and femoral articular cartilage, and the acceleration of joint degeneration [32]. In order to observe this phenomenon, our team vividly simulated the changes of the biomechanical properties of the knee joint before and after the medial meniscus posterior root tear by constructing a complete gait analysis simulation model of the knee joint. it more visually shows the effect of meniscus circumferential fiber damage on the biomechanics of knee joint.
The classical teaching is that the extension of meniscus tear injury to the periphery will reduce the structural strength of meniscus hoop [33]. However, it is not completely clear whether there is a "critical size" in the radial tear of the posterior root of the medial meniscus. Furthermore, it has a negative effect on the contact mechanics of the medial compartment of the knee joint. In our study, two pathological models were established according to the severity of radial tear of the posterior root of the medial meniscus. The injured areas of slight and severe tear models extend 30% and 60% of the contour width from the inner edge of the meniscus, respectively, compared with the intact meniscus during the gait cycle. the radial tear of the medial meniscus extends to 60% of the width of the contour does not significantly increase the load amplitude of the average or peak contact pressure in the medial and medial compartments of the knee joint. Tearing of 30% and 60% of the radial width of the meniscus does not result in significant changes in meniscus displacement and contact area as well, reflecting that the torn meniscus still retains some load transfer and distribution functions, which is similar to the previous research results [34]. However, the change of stress transfer in the medial compartment is not reflected by the difference of global peak stress, and the local poor performance may be masked by the global overall slight illusion, and the effect of this pressure position change is not clear, but the work of Andriacchi et al. [35] and Li et al. [36] suggests that joint injury may be related to the mild change of articular cartilage load transfer from weight-bearing area to non-weight-bearing area. It is not related to the absolute size of the transmission load. in order to test this hypothesis, we measured the local pressure of the posterior root of the meniscus of each model. The results show that the change of local stress of meniscus in the case of slight tear is not obvious, but when the injury is further deteriorated to the condition of severe tear, the average stress of posterior root under gait load increases sharply by 267.1%. In the course of the development of meniscus posterior root tear, the aggravation rate of tear injury increases exponentially, and the development of the disease will quickly enter the symptomatic stage.
Historically, partial meniscectomy for root tears has usually provided short-term relief. Krych et al. [4] followed up 52 patients with MMPRTs who underwent partial meniscectomy for 2.3–9.3 years, with an average IKDC score of 67.8. Lee et al. [37] treated 288 patients, the overall improved Lysholm score increased from 64.4 to 81.3. However, arthroscopic partial meniscectomy for irreparable meniscal tears is only a pain relief operation, not to prevent the progression of osteoarthritis [38, 39], and because this procedure does not fully restore meniscus tension, in most cases it will eventually develop into degenerative osteoarthritis [40]. In order to further understand the role of partial meniscectomy in the evolution of osteoarthritis, the intra-articular stress changes and tibial plateau overload caused by partial meniscectomy were demonstrated in our study. Compared with the entire tear model, the stress in the medial compartment of the partial meniscectomy model decreased as a whole, which was related to the fact that part of the root tissue was retained after the operation was removed. The residual meniscus ring still plays a braking role in the process of gait, which is of positive significance for the prevention of meniscal extrusion. However, it is believed that the resection area of meniscectomy is an important factor, and the posterior 1/3 meniscectomy and meniscectomy may lead to poor imaging results [41]. A similar phenomenon was found in this biomechanical study. Both the partial meniscectomy model and the entire tear model showed stress concentration in the anterior horn of the medial meniscus in the footprint area of the tibial plateau in the middle of the swing phase. this reveals that the posterior root of the medial meniscus lost most of its ability to share the extrusion stress after partial meniscectomy, which seriously affected the physiological function of the medial meniscus. This was also verified in the test of the contact area of cartilage of femur and tibia. Compared with the model of entire tear, the model of partial meniscectomy did not show the effect of improving cartilage contact. Considering that partial meniscectomy is not designed to restore biomechanics, and there is a high conversion rate of joint replacement [4], it is very significant to choose meniscal repair strategy in the treatment of non-degenerative MMPRTs patients to restore the integrity of the posterior root of meniscus.
Chung et al. [9] followed up MMPRTs patients who underwent partial meniscectomy and meniscus repair for 10 years. The Lysholm and IKDC scores at the last follow-up in the meniscus repair group were significantly higher than those in the partial meniscectomy group, and 56% of the patients in the latter group received total knee arthroplasty, compared with 22% in the former group. The results show that root repair is better than partial meniscectomy. From a long-term point of view, it is more valuable to repair the annular structure of meniscus. Previous biomechanical studies have shown that repairing the posterior root of the medial meniscus can restore the ability of the posterior root of the meniscus, absorb circumferential stress and reduce the contact pressure of the tibiofemoral joint, which is equivalent to that of the natural knee joint [11]. Unfortunately, meniscus repair of knee osteoarthritis has a higher progressive rate than the natural incidence of healthy knee [42]. An earlier cadaver study compared the contact mechanics of the tibiofemoral joint between anatomical and non-anatomical repair of the root tear. The results showed that the contact pressure of the non-anatomical meniscus was 33% higher than that of the anatomically repaired meniscus [43]. The results are similar to our experimental results. Therefore, we can make a hypothesis that the changes in mechanical properties caused by meniscus repair compared with healthy meniscus may result from the decrease of meniscus fibrous tissue and the shift of fixed position. In order to test this hypothesis, our study mainly focused on the changes of contact mechanics of meniscus repair model, and found that the knee flexion angle increased gradually at the end of the swing phase. The extrusion of the articular surface causes overload in the articular cavity. the analysis of this result is related to the shape of the model tested in this experiment. when we made the meniscus repair model, we removed the stump of the injured posterior root tissue and fixed the posterior end of the medial meniscus on the tibial plateau with sutures, which inevitably reduced the perimeter of the meniscus ring. It affects the ability of the meniscus to convert the tibial-thigh load into circumferential tension [44], which stiffens the tissue and reflects the increased stress in the articular cavity. The corresponding result is the increase of local overall stress in the posterior root, and the stress concentration around the suture hole can be observed obviously, which is attributed to the use of rivet suture instead of the normal posterior root of meniscus. The hardness of suture materials and the concentration of fixed points are mainly responsible for the increase of local stress in the posterior root. In addition, there are distinct changes in stress conduction in the meniscus repair model, which is reflected in the increase of stress in the tibial plateau. It is gratifying to note that the overall displacement and contact area of the model almost returned to normal after meniscus repair, consistent with previous reports [11].
Considering the above factors, whether we can restore rationality from the classical teaching of meniscus repair for non-degenerative MMPRTs, compared with partial meniscectomy, meniscus repair may delay the medium and long-term progression of radiological osteoarthritis. However, non-anatomical root repair can not fully restore the joint contact force [43]. Our study reinforces the fact that the tendency to develop osteoarthritis is caused by undesirable surgical intervention. In view of the fact that surgical decisions are affected by many factors, including symptoms, age, activity level, treatment expectations and cost-effectiveness, individualized and voluntary principles should be followed scientifically and cautiously when drawing up treatment plans for MMPRTs patients who meet surgical guidelines.
This study has some limitations: first of all, the subjects of the study are only based on the CT data provided by one volunteer to establish a three-dimensional knee joint model. As we all know, the evolution of the disease is affected by many factors, such as the baseline data of patients, the proficiency of surgeons, postoperative rehabilitation management and so on. Whether this conclusion is applicable to explain the efficacy of surgical intervention in patients with different MMPRTs needs to be further observed and confirmed. Secondly, at present, there are abundant technical means to restore the integrity of the posterior root of meniscus, including rivet suture fixation technology, pull-out suture fixation technology, meniscus transplantation technology and so on. In this study, the rivet suture fixation technology is selected for the model of meniscus repair, so the conclusion can not completely cover the scope of meniscus repair. Third, it is also ideal in the aspect of dynamic simulation. In order to simplify the experimental design, the dynamic tests of different models only load the ISO standard gait load. However, the knee joint plays a very active role in people's daily activities, such as up and down stairs, squatting, jumping and other activities on the knee joint under different conditions need to be further studied and explored. Fourth, the articular cartilage in this study is considered to be a linear elastic material, without considering the effects of viscoelasticity and anisotropy. In view of the limitations of the research design, the author's research team plans to coordinate computer simulation and biomechanical experiments of cadaveric specimens in future experiments to test several groups of knee joint samples, to observe the knee biomechanical changes of each group in different activity scenes, so as to facilitate the scientific evaluation of different surgical methods. And carry out prospective and retrospective studies on different surgical methods to delay the progression of osteoarthritis, which can further verify the conclusions of this study.