How exercise affects knees: bone spurs vs. cartilage

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Conventional wisdom tells us that running and other forms of vigorous exercise will ruin your knees. A number of studies, on the other hand, have found that long-term runners are actually less likely to develop osteoarthritis in their knees than non-runners, as I’ve written about before. So how we reconcile these conflicting beliefs?

Australian researchers have just performed a large-scale analysis of existing research in an attempt to answer this question, in a paper that was posted online by Medicine & Science in Sports & Exercise earlier this month. They started by scanning 1,362 relevant studies, and eventually winnowed that number down to 28 high-quality studies (which gives you some idea of the number of lower-quality studies out there!). Six of them were MRI studies, with the remaining using X-rays to assess cross-sectional and longitudinal links between exercise and knee joint status.

Here’s what they found:

i) strong evidence (from multiple high quality cohort studies) that there is a positive relationship between osteophytes and physical activity;

ii) strong evidence (from multiple high quality cohort studies) that there is no relationship between joint space narrowing, as a surrogate for cartilage thickness, and physical activity;

iii) limited evidence (from a cohort study and two cross-sectional studies) that there is a positive relationship between cartilage volume and physical activity;

iv) strong evidence (from multiple high quality cohort studies) that there is an inverse relationship between cartilage defects and physical activity.

In other words, exercise results in fewer cartilage defects, and may actually result in increased cartilage volume, but also results in more osteophytes (also known as bone spurs).

So what does this mean? Bone spurs aren’t generally a good thing, but decreased cartilage (as measured by joint space narrowing) is the most common way of monitoring the progression of degenerative conditions like arthritis. The researchers suggest that, in this case, the bone spurs may simply be a reaction to the forces of exercise, and don’t indicate that the joint is degenerating:

Thus, in response to mechanical stimuli, such as physical activity, osteophytes may enhance the functional properties of the joint by increasing joint surface area for the greater distribution of load or reducing motion at a joint and improve joint stability… While it is possible the higher prevalence of osteophytes identified in people exercising may be detrimental to the knee joint, it could also be argued, in the absence of cartilage destruction, that physical activity is beneficial and osteophytes are simply a response to mechanical stimuli.

To be perfectly honest, it sounds like they’re stretching a bit to make things sound good. “What, exercise causes bone spurs? Well, maybe bone spurs are actually good for us!” But for most people, loss of cartilage is a far more serious consideration, so overall these results are (once again) good news for people who exercise regularly and don’t want to have to replace their knees when they turn 60.