Arthritis of the joint behind the kneecap typically causes pain at the front of the knee that is worse when walking up, but especially down, stairs. This is because during this action the kneecap takes a force across it equivalent to 5x your body weight. Pain from the patello-femoral joint may also be felt behind the knee. Often the joint can be felt to grind (crepitus) and crack. It commonly affects people who kneel a lot in the course of their work. It can also follow patellar fractures.
If the pain is severe enough, an arthroscopic debridement ("clean-up") may improve the symptoms and injections may be appropriate in the early stages. If the arthritis is advanced, a patello-femoral joint replacement may be required.