As more and more aging Baby Boomers start to develop knee osteoarthritis, researchers are focusing on the whys and wherefores of this condition. As you have found out, studies have already shown that removing a torn meniscus (knee cartilage) puts people at a significantly greater risk of developing knee osteoarthritis later on.
But what happens to those patients who have a torn meniscus that doesn’t get treated? Can the untreated injury also contribute to the development of knee osteoarthritis? That is what remains unclear at this time. It makes sense that a torn meniscus that goes clear down to the bone can disrupt the normal knee joint biomechanics, alignment, and movement. And without a completely normal joint, the risk goes up of joint degeneration and eventual osteoarthritis.
Specific characteristics of meniscal tears also appear to be very important in whether or not the person develops knee osteoarthritis later. For example, the location, severity, and complexity of meniscal damage (as shown on MRIs) are predictive factors.
It has been shown that the larger and more complex the meniscal tear, the greater the likelihood of osteoarthritic changes in that knee. Tears across the meniscus (rather than horizontal tears along the length of the cartilage) were a greater risk factor when the tear went through at least one-third of the meniscus. It is also the case that if more than half of the torn meniscus is observed outside the boundaries of the joint, it is considered “severe” and a risk for the development of OA.
Of course, the next question was: what makes an untreated meniscal tear a risk factor for damage to the joint surface and resultant osteoarthritic changes of the knee joint? It turns out that people who have a meniscal tear of the posterior horn of the meniscus are at increased risk of osteoarthritis developing in that knee. As the name suggests, the posterior horn is the curved section of the meniscus along the very back of the knee.
Having both a posterior horn tear and severe extrusion does not appear to increase the risk of developing OA. However, extrusion seemed to be a risk factor for longer meniscal tears. These two combined together are seen more often in the patients who did develop osteoarthritis.
Once risk factors (such as meniscal tears) for osteoarthritis (OA) are identified, treatment that will yield the best results can be determined. Not all meniscal injuries result in OA, so efforts to find the most significant characteristics of meniscal injuries are important.
For now, it looks like complex tears, meniscus that extrude out of the joint, and injuries that are wider and/or longer than one-third of the meniscus are present in patients who develop X-ray evidence of osteoarthritis later.