More arthritis in amputation-side knees
If you’ve lost a leg below the knee to trauma and amputation, which knee is at greater risk for osteoarthritis?
Many people, including a lot of experts, would guess the knee of the intact leg, “particularly affecting the medial compartment of the intact-side knee due to increased joint contact forces possibly resulting from altered gait biomechanics.” That is the basis for the conventional wisdom.
But a new study by Watson et al shows the opposite: “significantly worse [radiographic] osteoarthritis on the amputation-side knee 11-years post-amputation.”
A couple caveats before we interpret that: the sample is modest (36) and drawn from a modern military population with good rehabilitation and prosthetic access, and it just might not go like this for civilian amputees. And the study reports a difference seen on x-ray only. Actual symptoms were the same despite the fact that amputation-side knee cartilage was objectively more degraded. (But… care to bet on how they’ll all feel in another decade?)
So, why would the amputation side degenerate faster?
Many people are now aware that osteoarthritis has never been a simple biomechanical wear-and-tear disease, but a complex physiological/inflammatory one. Some other posts and articles on this theme:
- Bone on Bone
- Preventing arthritis and tendinitis is easier than you think (with patience)
- Ozempic works for arthritis, and probably other pain (Member Post)
- Running does not wreck knees (probably)
Load is involved, but it’s not the essence of the thing. This modern view of arthritis has taken root enough now that a lot of people will claim that they are not surprised by this result. Instead, their assumption is that it is explained by the lack of loading on the amputation side.
Not so fast! As the authors explain, the intensity of loading of the amputation side knee is known to be either similar or just a little bit less. It’s probably variable, and in some cases it may even be more, as reported by one PainSci reader, a below-knee amputee: “I load bear more on my amputated side.”
Awkward loading of the amputation side knee, possibly worsened by janky prosthetics in some cases, is another possible explanation.
But, again, loading is just generally not the major driver of arthritis. It might combine poorly with the physiology; it isn't irrelevant. But it’s probably not what really matters.
What (probably) really explains the result
All load-based explanations — under, over, and/or awkward loading — are probably overshadowed by the most plausible explanation: long-term inflammation and physiological dysfunction associated with major trauma, both the original injury and the amputation. Specifically and fascinatingly, it could also reflect previously under-appreciated consequences of high-energy injuries, lots of microtrauma throughout the leg:
“We hypothesise that blast injury could instigate degenerative processes in the cartilage (42) within the amputation-side knee leading to early increased radiographic knee OA. These high-energy injuries cause all the reported predictors of poor outcome: primary high-energy insult to the articular cartilage, comminuted joint surface, articular bone loss, soft-tissue damage, and open and contaminated wounds susceptible to infectious complications.”
But it’s complicated and there are caveats every which way. This paper is well-written, and there's a good and thorough “Discussion” section with much more nuance than I’ve provided here, and the full text is available.
I was particularly interested in this paper because my father, although not an amputee, is a veteran with a major gunshot wound, a shattered femur. He is quite keen on evidence about the effects of high-energy trauma! You can read his story here.