Mechanics of hip joint movement
The hip joint is the largest and most stable joint in the body. It is a ball and socket joint which is held tightly between the head of the femur and the cuplike acetabulum on the side of the pelvis. A number of powerful ligaments surround the outside of the joint capsule. Inside of the joint is a smooth layer of cartilage, that cushions the joint and allows the head of the femur to move smoothly around the circumference of the joint socket.
The hip joint is a major weight bearing structure that transmits forces through the pelvis from the lower limb to the upper body and vice versa. The hip is moved by a number of large, powerful muscles including the gluteus maximus in the buttock and also the quadriceps and hamstring muscles in the thigh.
Some people are born with a shallow hip joint socket. This is known as hip displacia which results in a misalignment of the joint, with congenital deformation and recurrent dislocation of the head of the femur.
Congenital abnormalities of the hip can compromise the supporting ligamentous structures and can lead to developmental issues in young children. Early arthritic degenerative changes of the hip joint can develop due to faulty joint mechanics and compensatory gait patterns. Leg length differentials, scoliosis (curvature of the spine) and weakness in the gluteal muscles, are common occurrences in people with a history of hip displacia. Weakening of the gluteal muscles on the side of the hip, can cause the pelvis to tilt on one side, resulting in what is known as trendelenburg gait. This causes the effected individual to lurch towards the weakened side when walking, in an attempt to try and level the pelvis during the gait cycle.
Hip fractures are common in people aged 65 and over, especially in woman with osteoporosis and those who have balance problems and are prone to falls. A fracture of the femur or hip can be potentially life threatening if it compromises the femoral artery. Road traffic accidents can also be associated with injuries to the hip; a direct impact to the front of the knee can force the femur back into the socket causing serious trauma to the soft tissues and the artery.
Other causes of hip pain
The most common reasons people develop hip pain includes the following:
- Osteoarthritis- wear and tear in the hip joint effecting the joint space and causing a thinning of the cartilage with osteophytic bone spurs on the joint margins. Typical symptoms of O/A in the hip include stiffness, crepitus, pain and inflammation. Pain can radiate around the buttock, over the hip and into the groin. Sometimes pain is referred to the inside of the knee. Another very common sign of an arthritic hip is to see the effected person walking with a limp and with painful hip held in extension and rotated out to the side, so as to try and keep more weight off of the painful joint.
- Trochanteric bursitis- inflammation of the fluid sacs that cushion and lubricate the structures around the joint. This causes tenderness directly over the outside of the hip joint and is painful to lie on.
- Labral tear- which is damage to the cartilage inside of the joint and can cause a piece of the cartilage to get trapped between the joint surfaces. Impingement of the joint capsule and synovitis can also cause hip pain.
- Snapping hip syndrome, which can cause an audible noise as the connective tissues pass over the hip joint whilst walking, is painful and causes inflammation and a repetitive strain injury to the underlying bursae and ligaments of the hip joint.