Interesting study out this week that tries to tie lack of blood flow in the knee cap to why some patients have chronic knee cap pain after a total knee replacement.The study authors found that in about 1/4 of the knee replacement patients with chronic knee cap pain, blood flow in the knee cap area markedly decreased with flexing the knee. To be honest, I was surprised to learn that knee cap or patellar pain is a common complication of knee replacement. A 1995 study identified that chronic pain in the knee cap after total knee replacement occurred in about 13% of knees. The doctors who first reported the side effect believed that the type of metal or plastic knee used by the surgeon may make a difference. Since then there have been many studies trying to figure out why these patients still hurt when they should all be be pain free after having their knee replaced. So why do some patients develop pain in the patella after knee replacement? One study looked at wear on the patella after knee replacement and found much more strain on the inside of the patella in a total knee replacement knee when compared to a knee without a prothesis. Another study looked at many different causes of this knee replacement side effect including shortening of the patellar tendon and that the knee replacement prosthesis may be misaligned in some patients. One surgeon postulated that knee cap pain after knee replacement may be due to instability in the patella due to the metal prosthesis not being properly installed. Whatever the cause of this knee replacement complication, the fact that more than 1 in 10 knee replacement patients is walking around with chronic knee cap pain is concerning. It’s maybe a little more concerning that there isn’t consensus about what causes the problem.