Chronic instability of the hip

The hip joint is a particularly stable (constrained type) joint with a strong negative-pressure seal, which is of great importance to its function. The joint capsule of the hip is one of the most important static stabilisers of the joint. As in the other joints of the human body, instability of the hip is generally defined as movement of the hip beyond its physiological limits that causes pain, with or without symptoms of instability. However, this condition is not well defined, as no objective criteria have been proposed for characterising a condition as microinstability of the hip.
The cause of the pain and of the symptoms may be minor anatomical abnormalities, and in most cases it is accompanied by repetitive movements involving rotation and axial loading. In addition, generalised hyperlaxity of the ligaments or muscle weakness can mimic microinstability of the hip. As a result of the change in the biomechanics between the femoral head and the acetabulum, the neighbouring structures such as the labrum of the acetabulum, the joint capsule and the articular cartilage may be placed at risk. The symptoms associated with this pathology are persistent pain in the groin, tendinitis due to overloading of the active stabilisers, and a piston-like movement of the hip joint.
Source: Bolia I, Chahla J, Locks R, Briggs K, Philippon MJ. Microinstability of the hip: a previously unrecognized pathology. Muscles Ligaments Tendons J., 21 December 2016• 6 (3):354-360