Knees don’t wear out evenly. In fact, they don’t wear out at all unless there is an underlying injury. This could be a meniscus or ligament tear, or a disease process such as osteoarthritis. The cartilage in the knee is extremely durable and, if not damaged, lasts a lifetime.
Once injury sets in, however, the wear patterns depend on the type and location of the injury. Many patients with arthritic knees are told they need a full knee replacement, when in fact resurfacing the worn out part alone would do just fine. Partial knee replacement—also called unicondylar replacement—wasn’t commonly performed in the past because of the difficulty in accurately matching the resurfaced parts to the uninjured knee. Computerized models and robotic arms have changed all that. Today, with robotic assistance, we can resurface each part of the knee independently, on an outpatient basis.
The results of partial knee replacement have become so good that my patients often ask me, “Can I run, can I play sports, can I return to my full range of activities?” My answer is often “Yes, but…”. Here is why.
If you walk a mile or run a mile, which has more force on the knee? It turns out that the total force is the same, since you take fewer steps when running. Peak forces are higher with running, though. So if you are going to run on an injured, arthritic or repaired knee, it makes sense to refine your running style. Lowering the peak forces can be done by short strides, mid-foot landing, running on soft surfaces such as grass or sand, and smooth styles such as Chi running.
By the time many patients come in for resurfacing of the knee, however, they have limped for years. Their muscles are weak, their hip and back mechanics have compensated for the arthritic knee, and their range of motion is limited. In order to return to sports without damaging their knee joint, each of these limitations must be addressed with extensive physical therapy and fitness training.
The most common next question I’m asked is, “Will I wear out or loosen my implant by sports?”
The answer is, “Not likely.” While loosening has a number of causes, we believe that one of the most frequent is weakening of the bone with aging and disuse. People have traditionally been told to limit their activities as they get older. In fact, by increasing weight training and loading of the joint, the bone mass increases, thus the risk of loosening decreases. But even if things go south, the plastic-based inserts between the metal resurfacings are now replaceable if they wear out—though they rarely do.
So with modern outpatient techniques, partial knee replacements and sports can be complimentary. But as with other activities, the quality of the motion affects the outcome.