Chondromalacia is due to an irritation of the undersurface of the kneecap. The undersurface of the kneecap, or patella, is covered with a layer of smooth cartilage. This cartilage normally glides effortlessly across the knee during bending of the joint. However, in some individuals, the kneecap tends to rub against one side of the knee joint, and the cartilage surface becomes irritated, and knee pain is the result.

The term chondromalacia sometimes is used to describe abnormal-appearing cartilage anywhere in the body. For example, a radiologist might note chondromalacia on an MRI of an ankle.

Pain at the front/inner side of the knee is common in young adults, especially soccer players, gymnasts, cyclists, rowers, tennis players, ballet dancers, basketball players, horseback riders, volleyball players, and runners. The pain of chondromalacia patellae is typically felt after prolonged sitting, like for a movie. Snowboarders and skateboarders are prone to this injury, particularly those specializing in jumps where the knees are under great stress. Skateboarders most commonly receive this injury in their non-dominant foot due to the constant kicking and twisting that is required of it during skateboarding.

The condition may result from acute injury to the patella or from chronic friction between the patella and the groove in the femur through which it passes during motion of the knee. Possible causes include a tight iliotibial band, neuromas, bursitis, overuse, malalignment, core instability, and patellar maltracking.