The thigh bone, femur, and the pelvis, acetabulum, join to form the hip joint. The hip joint is a ball and socket joint. The ball is the head of the femur, or thigh bone, and the socket is the cup shaped acetabulum. The joint surface is covered by a smooth articular surface that allows pain free movement in the joint.

The cartilage cushions the joint and allows the bones to move on each other with smooth movements. This cartilage does not show up on X-ray, therefore you can see a joint space between the femoral head and acetabular socket.

The acetabulum faces an angle of 30° outwards and anteriorly. The normal neck-shaft angle of the femur is 125° in adults, with 15° of anteversion. The neck is made up of spongy bone with aggregation of bony trabeculae along the lines of stress. The most important of these is the medial longitudinal trabecular stream. These run from the lesser trochanter, along the medial cortex of the neck to the postero-medial quadrant of the head. A thin vertical plate of bone springs from the compact medial wall of the shaft, and extends into the spongy bone of the neck. This is called the calcar femorale.

Blood supply of the femoral head comes from three main sources : the medullary vessels from the neck; the retinacular vessels entering from the lateral side of the head; and the foveal vessel from the ligamentum teres. The most important of these, the medullary vessels from the neck; the retinacular vessels are generally cut off following a fracture of the neck of the femur, and sometimes result in avascular necrosis of the head.

When a person stands on one leg, the body weight tends to tilt the pelvis down on the other side. The ipsilateral hip acts as a fulcrum in this. The abductors of the hip on the side on which one is standing, contract to counter this. This helps in keeping the pelvis horizontal. This abductor mechanism (hip joint-neck of the femur-abductor muscles) is called abductor lever arm, as it acts like a lever. This has great clinical relevance in biomechanics of the hip, and is disrupted in conditions like dislocation of the hip, fracture of the neck of the femur.

The major ligaments stabilizing the joint from directional forces include the iliofemoral ligament located anteriorly and the ischiofemoral ligament located posteriorly. Because the anterior ligaments are stronger, trauma to the hip commonly presents as a posterior dislocation when discovered (90% of cases). Dynamic muscular support includes the rectus 3 femoris, gluteal muscles, and short external rotators. An understanding of the vasculature is important because trauma to the hip can displace the femoral head and interrupt the blood supply, leading to avascular necrosis (AVN).

Isolated hip or knee dislocations are not rare injuries; however, simultaneous ipsilateral dislocation of the hip and the knee is an unusual event. When they occur together, they represent a challenging problem because of the need for urgent reduction, which is balanced by the need to avoid iatrogenic injury that may be caused by reduction attempts.

Hip dislocations may occur in either an anterior or posterior fashion that is dependent upon the inciting mechanism. In posterior dislocations, the femoral head is displaced posteriorly in relation to the acetabulum. Hip flexion, adduction, and internal rotation will produce posterior dislocations, whereas hyper-abduction with the extension will produce an anterior dislocation, with the large majority of atypical and axial-loading injury patterns producing posterior dislocation. 

Age, race, and gender are important risk factors for these types of injuries with the incidence being two times greater in women than in men. Posterior hip dislocations (90%) are much more common than anterior hip dislocations; additionally, there is significant morbidity and mortality associated with posterior hip dislocations if there are any associated fractures. In addition to severe pain, other associated injuries include acetabular fracture, hip/femur fracture, osteonecrosis, sciatic nerve damage, recurrent dislocations, bone bruise (33%), ipsilateral knee meniscal tears (30%), knee effusion (37%) and labral tear (30% rate). In addition, thoracic aortic injury has been reported to be associated with posterior hip dislocation in 8% of cases due to abrupt deceleration injuries.

The injury is sustained by violence directed along the shaft of the femur, with the hip flexed. It requires a moderately severe force to dislocate a hip, as often occurs in motor accidents. The occupant of the car is thrown forwards and his knee strikes against the dashboard. The force is transmitted up the femoral shaft, resulting in posterior dislocation of the hip. It is, therefore, also known as dashboard injury.

An isolated posterior dislocation of the hip is easy to diagnose. The patient presents with a history of severe trauma followed by pain, swelling and deformity (flexion, adduction and internal rotation). This is associated with a shortening of the leg. One may be able to feel the head of the femur in the gluteal region. The injury is sometimes missed, especially when associated with other more obvious injuries such as fracture of the shaft of the femur. It may go unnoticed in an unconscious patient. It is wise to X-ray the pelvis in all patients with fracture of the femur to avoid this mistake.

The femoral head is out of the acetabulum. The thigh is internally rotated so that the lesser trochanter is not seen. Shenton's line is broken. One must look for any bony chip from the posterior lip of the acetabulum or from the head. A comparison from the opposite, normal side may be useful. CT scan may be necessary, in cases where an associated fracture is suspected.

Reduction of a dislocated hip is an emergency, since longer the head remains out, more the chances of it becoming avascular. In most cases it is possible to reduce the hip by manipulation under general anaesthesia. The chip fracture of the acetabulum, if present, usually falls in place as the head is reduced. Open reduction may be required in cases where: (i) closed reduction fails, usually in those presenting late; (ii) if there is intra-articular loose fragment not allowing accurate reduction; and (iii) if the acetabular fragment is large and is from the weight bearing part of the acetabulum.