Meniscus--friend or foe: epidemiologic observations and surgical implications.

The meniscus has long been appreciated as a source of knee symptoms and disability. In fact, through much of the 20th century total meniscectomy was performed relatively frequently for patients with persistent, disabling mechanical knee symptoms. The procedure relieved mechanical symptoms but at a cost: Total meniscectomy was associated with accelerated degenerative radiographic changes, establishing the importance of the meniscus in the pathogenesis of osteoarthritis.1-3 Biomechanical studies clarified these clinical observations. The meniscus distributes substantial mechanical load throughout the tibiofemoral joint surfaces that would otherwise be transmitted directly and focally to the weight-bearing portions of the femur and tibia. Damage to or resection of the meniscus disrupts this load sharing function, leading to excess loading of cartilage and subchondral bone and attendant changes of osteoarthritis.1, 4-6 These concerns assumed public health importance with the advent of two key technologies, arthroscopy and MRI. The optical work of Watanabe and others in the 1960s led to the introduction of arthroscopy in the 1970s and its widespread dissemination by the 1980s.7 The arthroscope made meniscal surgery much simpler and less costly. Widespread application of arthroscopic meniscectomy required noninvasive diagnostic techniques to facilitate recognition of meniscal tears. MRI filled this need, with sensitivities and specificities for meniscal tear on the order of 90% as compared with a surgical gold standard.8 9, 10 The combination of MRI and arthroscopy permitted noninvasive diagnosis and minimally invasive treatment of symptomatic meniscal tears.11 By the end of the 20th century, arthroscopic knee surgery had become the most common procedure performed by orthopedic surgeons, with up to one million knee arthroscopies performed annually in the US at present.12, 13 The majority of these procedures appear to be done for meniscal tear. The lack of a comprehensive ambulatory surgery database in the US precludes more precise estimates of arthroscopy utilization. Technology development and dissemination often subtly change the spectrum of diagnosed and treated disease. The typical recipient of meniscal surgery prior to the age of arthroscopy and MRI was a younger person with relatively normal knee joint and an injury-related meniscal tear. Over time, the spectrum shifted to include older patients with underlying osteoarthritis and superimposed degenerative meniscal tears. This population is difficult to evaluate since the vast majority of patients with osteoarthritis also have meniscal tears, as do a third of asymptomatic adults.14-16 The capacity to identify a torn meniscus noninvasively, and to address it minimally invasively, has emboldened surgeons and patients to opt for arthroscopic partial meniscectomy. The efficacy of arthroscopic partial meniscectomy in the setting of osteoarthritis has received surprisingly little study. Herrin and colleagues performed the only RCT addressing this question and found no difference in function of pain between patients treated with APM or with a nonoperative regimen for symptomatic meniscal tear and concomitant osteoarthritis.17. Thus, we know relatively little about the efficacy of arthroscopic partial meniscectomy in patients with concomitant osteoarthritis and even less about the effects of meniscal damage and meniscal resection on the underlying degenerative process. As noted above, total meniscectomy is clearly associated with degenerative changes and accelerated osteoarthritis.3 Observations over the last two decades have shown that partial meniscectomy is also associated with accelerated osteoarthritis.18 These findings have lead to the development of surgical approaches that preserve or replace meniscal tissue, such as meniscal repair and meniscal allograft implantation.5, 19, 20 Partial meniscectomy has evolved in the same manner, with increasing emphasis on trimming only the unstable portion of the tear and leaving the stable portion intact.4 It is unclear whether the osteoarthritic changes that follow total or partial meniscectomy arise from the surgery itself or the underlying condition. Specifically, does the resection of meniscal tissue lead to joint instability and attendant joint damage (as is presumed to occur following total meniscectomy)? Or is the problem not surgery but the underlying meniscal lesion itself and its associated abnormalities in cartilage, bone, and synovium? This question cannot be addressed simply by evaluating the outcome of patients who have had meniscal surgery since the role of the underlying meniscal lesion and the surgical procedure cannot be disentangled in such studies. Englund et al address this question with an elegant nested case control study in the present issue of Arthritis and Rheumatism. The parent cohort included participants aged 50-79 in the Multicenter Osteoarthritis (MOST) Study, which was designed to identify predictors of osteoarthritis incidence and progression in a large community based sample. The nested case control study was restricted to 1863 MOST subjects who were at risk but initially free of radiographic osteoarthritis at the baseline evaluation. After 30 months of follow up, 121 of these subjects developed definite osteoarthritis, defined as Kellgren-Lawrence Grade 2, and had readable MRI examinations at baseline. These patients served as cases and were compared with 294 controls who were also free of OA at the outset but did not go on to develop OA. The authors examined the baseline MRIs for meniscal changes and found that three times more cases than controls had meniscal damage at baseline (54% vs 18%). Thus, among older patients who did not have knee surgery of any sort, the presence of meniscal damage is associated with subsequent development of osteoarthritis. It could be argued that much of the population under study already had osteoarthritis at study entry that had not yet become evident on radiographs. The authors addressed this issue by examining a subset of patients with no cartilage findings at baseline and they confirmed that in this group as well that the presence of baseline meniscal damage predicted subsequent development of cartilage lesions. Thus, this study establishes that meniscal damage is a risk factor for the development of radiographic osteoarthritis. Returning to the conundrum of whether the osteoarthritis that follows APM is due to partial meniscectomy or to the underlying meniscal lesion, the work of Englund and colleagues indicates that the meniscal lesion itself likely plays a part. How large a part? The relative contributions from the meniscal lesion, from associated joint abnormalities and from APM surgery can only be resolved with a randomized controlled trial of APM versus nonoperative therapy among patients with symptomatic meniscal tear and concomitant OA. The randomized controlled trial would permit us to estimate the counterfactual: What would have happened if the same patient who we observe several years after arthroscopic partial meniscectomy had not undergone surgery? The work of Englund and colleagues prompts several suggestions for research priorities and clinical management. First, these data support continued work on strategies to preserve meniscal tissue in the management of meniscal tear. Rigorous studies are needed to address both symptomatic response to meniscal sparing surgery and also the effects of these surgeries on development and progression of OA. The work of Englund and colleagues warns that control observations will be critical to testing the hypothesis that meniscal preserving surgery slows development of OA. A second implication is that ongoing research is needed on the determinants of meniscal degeneration, as the damaged meniscus appears to be a potent risk factor for osteoarthritis. In particular, knee injury prevention may have an even more important role in reducing the population burden of osteoarthritis than previously envisioned. Injury prevention is typically oriented toward reducing the growing number of anterior cruciate ligament tears, as these lesions clearly lead to accelerated osteoarthritis if not managed surgically. If meniscal tears are also potent risk factors for accelerated osteoarthritis, knee injury prevention assumes an even greater public health priority. Patients will continue to present with symptoms suggestive of meniscal injury. Those with compelling mechanical symptoms and functional disability will continue to undergo arthroscopic partial meniscectomy. The work of Englund and colleagues adds to the call for critical evaluation of the indications and rates of surgery for meniscal lesions in middle aged subjects to ensure we are doing more good than harm.