The patella (knee cap) is sesamoid bone which is attached the the extensor mechanism of the knee joint (Quadriceps muscles), its function is to increase efficiency of the muscles by upto 40%. In doing so the articulation (joint) formed by the patella and the bottom end of the thigh bone (Trochlea) endures some of the highest contact stresses (pressures) in any of the joints in the body. These can be as much as 4 to 6 times your body weight in stair climbing and higher in some sporting activities.
The articular cartilage can be damaged by activities which put extensive and repetitive pressures on the patellofemoral joint or by a simple injury where the impact force causes damage to the articular cartilage. This then gives rise to anterior knee pain, swelling, a feeling of grating (crepitus) and episodes of catching and giving way.
Most of population over the age of 40 years will notice a degree of crepitus on stair climbing, squatting and getting out of a chair and have no associated symptoms.
The majority of cases have simple softening of the articular cartilage a condition called chondromalacia patellae (CMP) and are treated symptomatically, with analgesia and anti inflamatory medication, physiotherapy and avoiding activities which exacerbate the symptoms.
However it is important that other structural causes such as malalignment and maltracking problems are assessed, investigated and treated on an individual basis. The hip, knee, ankle and foot need to be assessed in detail and managed as needed.
Patellar articular damage
Crab meat appearance of CMP
More information: http://orthoinfo.aaos.org/topic.cfm?topic=A00680