This study suggests that major MME is associated with significantly worse clinical and radiologic outcomes and survival for at least 4-year follow-up after arthroscopic surgery. Since we demonstrated that major MME is largely related to the arthroscopic surgery outcome of KOA, it is important to know what causes MME. Several studies have reported a significant correlation between MME and severity of chondropathy.[18,23] Costa et al also found that major MME was associated with meniscus degeneration, radial, oblique, complex, and root tears, accordingly, longitudinal and horizontal tears were not associated with major MME which is similar to what we observed in our study. However, Lee et al reviewed 102 knees with medial meniscus posterior horn degenerative tears that underwent a partial meniscectomy and showed that the incidence and degree of major extrusion were similar in knees with root tears and non-root tears, but a radial component and KOA severity were similarly predictive of absolute and relative extrusion. They suggested that arthroscopic meniscal procedures should be cautiously considered in patients with meniscal extrusion. Intra-articular injuries are associated with pain. Average medial meniscal extrusions of the knees with and without pain were 7.58 mm and 5.88 mm, respectively. Pain was associated with greater medial meniscal extrusions in KOA, which was similar to what we observed.
Although more and more randomized trials including patients with symptomatic KOA and meniscal tears found no greater benefit with arthroscopic surgery followed by physical therapy,[30,33] compared with physical therapy alone, our observational study showed high rates of functional improvement, despite of serious radiologic KOA progression within patients after arthroscopic partial meniscectomy and debridement. We believe that meniscus surgery can promote radiologic KOA progression; however, patient's conscious symptoms and radiological presentations are not completely consistent and even contradictor each other, and some patients were well satisfied with surgical effect while imaging revealed a severe degree of KOA progression. The degree of joint space narrowing does not fully represent the degree of degeneration of the joint. This could also be possibly attributed to the difference in the extent of meniscus resection.
Furthermore, the most important finding of our study was that the presence of major MME could be predictive of clinical and radiologic progression of KOA, even for a long-term increase in progression of KR after arthroscopic surgery. This result was further confirmed after adjustment for age, sex, and BMI, because of the significant baseline BMI differences between the study groups, and a higher age and BMI is related to meniscal extrusion,[35,36] even increased age, female gender, and greater BMI were modestly associated with poorer self-reported outcomes after undergoing arthroscopic meniscal surgery. We did not adjust the results of our analysis for knee structural pathology parameters such as tear meniscus type, cartilage lesions. The above may be part of the causes of the major MME, given that the statistical results showed that those variables were significantly related to meniscal extrusion in our study, combining all those variables prevented us from analyzing the effect of major MME on outcomes, which would have required much larger sample size. Interestingly, there was an unexpected result that patients with major MME treated with surgery had even more benefit in terms of pain relief, although not reaching statistical significance (adjusted MD: −0.200; 95% CI: −3.300, 3.000, P = 0.831). The cause of this finding is unknown. One explanation might be that patients with more pre-operative pain are likely to realize more significant pain relief after surgery.
There is more and more investigation into the longitudinal relevance of MME in OA progression, but a lack of data in arthroscopic KOA patients. Choi et al measured MME in 56 cases that were associated with cartilage degeneration of ipsilateral medial femoral condyle after 2-year follow-up. Van der Voet et al showed that meniscal extrusion was associated with a significantly higher incidence of radiographic KOA (K-L Grade 2 or higher) and medial joint space narrowing of >1.0 mm after 30 months in a high-risk population of overweight and obese women free of clinical and radiological KOA at baseline. Teichtahl et al extracted data from osteoarthritis initiative cohort (either with presence or absence of radiographic KOA) during 72 months follow-up, the presence of a baseline meniscal extrusion (independent of bone marrow lesions) was associated with accelerated cartilage volume loss, progressive radiological KOA, and total KR. According to arthroscopic patients, Krych et al had concerned meniscus extrusion on the effect of arthroscopic surgery, they recently found that meniscus extrusion was associated with worse outcome, but they only focused on partial meniscectomy patients with symptomatic degenerative medial meniscus posterior root tears and did not adjust for baseline covariates. Kim et al recently retrospectively reviewed 208 medial meniscus tear patients who were treated with arthroscopic partial meniscectomy and had a minimum 7-year follow-up. Consistent with our findings, their result showed that the pre-operative extrusion of the medial meniscus was negatively correlated with outcomes of partial meniscectomy, but they just focused on non-osteoarthritic knee.
Our study does suggest that major MME is the important risk factor for radiological OA progression and poor post-operative outcomes after arthroscopic surgery. However, it was impossible to determine whether MME was a cause or consequence of KOA in our present study. Our retrospective information does not prove cause and effect, because the MME may be a marker of other risk factors, such as more severe internal meniscus and cartilage degeneration. The best evidence of a causal relationship between MME and KOA is improved radiological and clinical outcomes after centralization of the extruded medial meniscus. A rat model data support a link between pre-existing MME and OA development and centralization of the extruded medial meniscus by the pull-out suture technique delayed cartilage degeneration. In practice, if we consider that MME is a potential predictive factor of structural progression in OA, pre-operative detection of MME suggests active cartilage breakdown requiring an appropriate treatment such as the medial meniscus centralization procedure and a thorough follow-up. Our study suggests that arthroscopic surgery may have better post-operative outcomes for KOA with symptomatic meniscal tears among those with pre-operative non-major MME; however, in terms of pain relief, arthroscopic surgery in patients with major MME is also beneficial as well as in patients with non-major MME. Thus MME provides an interesting target for patient selection and counseling for arthroscopic surgery in degenerative KOA with a medial meniscus tear.
The present study had some limitations. First, we presented only one outcome score (WOMAC) and did not include knee scores such as the Knee Injury and Osteoarthritis Outcome Score or Lysholm. Also, we had no comparative data between pre- and post-operative MME. Second, the definition for meniscal extrusion used for the present study corresponds to extrusion of 3 mm or more, and we arbitrarily divided the meniscus into two groups which not allow us to account for within-grade progressions of knee OA. Moreover, this semi-quantitative scoring method for the meniscus does not take into account the proportion of the tibial cartilage surface covered by the meniscus. Third, the study involved a retrospective analysis with a relatively small population based on self-reported outcomes and this might have resulted in recall bias which makes comparison less reliable. The missing and unmeasured covariates such as bone marrow lesions,[39,42] and synovitis could also drive the association. Furthermore, pre-operative major MME may involve more arthroscopic meniscus resection, all of which could have affected outcomes, rather than the MME itself, even if our analyses had been adjusted for age sex and BMI to control for the potential confounding. Lastly, we were unable to calculate minimal clinically important differences for our outcome measures; therefore, we cannot evaluate whether our statistically significant findings are clinically meaningful.
This study was supported by a grant from the Beijing Natural Science Foundation (No. 7191010).
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