Total Hip Joint Dislocation

A dislocation occurs when the ball of the prosthetic implant is pushed out of the socket. The hip replacement performed through the anterior approach is more vulnerable to anterior dislocation. Similarly, the hip replacement performed utilizing the posterior approach is more vulnerable to posterior dislocation.

Total Hip Replacement and Dislocation

Total hip replacement is a surgery to replace the diseased parts of the hip joint with prosthetic parts. The surgery involves duplicating the function as well as balancing the muscular forces acting around the joint. Due to the unique biomechanics of the natural hip joint, the artificial joint needs to be placed in a proper position.

The hip joint is a large weight-bearing ball and socket joint. The ball is formed by the upper part of the thigh bone. The ball fits into the socket formed by the lower part of the pelvis. Hip replacement is commonly performed for arthritis after all conservative treatment options have been exhausted.

The joint is covered by a tough tissue known as a joint capsule, which seals the joint stabilizing it. The joint is also covered by ligaments and muscles which hold the joint in place. Together these structures keep the ball in the socket, preventing dislocation. The alignment of the natural hip further adds to its stability.

Hip replacement loosens the soft tissues around the joint, and patients are advised hip precautions to prevent dislocation. These are the movements and maneuvers the patients are advised to avoid. Certain positions may put the hip ball (femoral head) in a position where it may pop out of the socket.

Some of the common precautions are:

• Not to bend your thigh more than 90 degrees
• Not to sit in low chairs/sofas/toilet seats
• Not to cross legs
• Not to sleep on the side

These precautions are especially important in the traditionally performed posterior hip approach (from behind the hip). The hip precautions are practiced together with an abduction pillow for a few weeks after the surgery.

Symptoms and Causes

Often the patients report hip dislocation immediately as difficulty moving the Hip. Patients are unable to bear weight on the side involved without pain.

Various factors are involved in the incidence of hip dislocation after the surgery. The treated patients may feel anxious about hip movement as they are apprehensive about dislocation. They may patient factors or procedure factors.

Patient factors

Patient age plays an important factor in the risk of prosthetic hip dislocation. Older patients may sustain dislocation secondary to falls/trauma due to an increased risk of loss of balance. Patients in higher age groups are also less likely to be compliant with hip precautions.

Prior surgeries of the hip joint make it more vulnerable for dislocation after replacement. As with any surgical procedure, there may be a residual loosening of the tissues around the joint. Laxity of tissues supporting the hip joint makes it more vulnerable to dislocation.

Neuromuscular conditions such as Parkinson’s diseases, cerebral palsy, etc may make the patient prone to sustain a dislocation following surgery. Neuromuscular conditions render the muscle groups either spastic or paralytic contributing to instability and dislocation.

Developmental hip disorders such as hip dysplasia make the hip more prone to dislocation. The shallow acetabular cup and tightened or lax structures around the hip joint contribute to instability and dislocation.

Patients suffering from obesity and other medical conditions such as diabetes are more likely to sustain a dislocation after replacement surgery. The increased risk is attributed to the neurological deficit in limbs, loss of balance, and greater chances of infection.

Procedure factors

The posterior surgical approach (from behind) involves cutting the muscles and stabilizing structures. This makes the hip more vulnerable to posterior dislocation compared to the muscle-sparing anterior approach (from the front).

Implant position is an important factor in hip dislocation following replacement. The proper position of the components during surgery is vital for keeping the ball in the socket.

A constrained liner consists of a polyethylene liner containing the head of the femur. The head of femur while being contained in the socket does not dislocate. The constrained design may however increase the wear and tear of the implants.

The surgeon’s experience in performing the surgery through a particular approach reduces the risk of dislocation.

The choice of implant used reduces the incidence of dislocation episodes. During the past decade, the size of the head of the femur in implants has increased considerably. Larger head means a larger range of motion before dislocation.

Diagnosis and Management

Hip dislocation following replacement is evident with the symptoms experienced by the patients. Most patients report to the ER following an episode of dislocation. The patients are subjected to imaging such as X-Ray or CT scan (computed tomography).

Nonsurgical

A procedure called hip reduction is done which involves placing the ball back into the socket of the pelvis. It is generally done under moderate sedation or under anesthesia in the operating room. Once the reduction is achieved the hip is placed in a brace for several weeks. Subsequent risks of dislocation remain high in case the dislocation is due to unaligned implants.

Surgical

Multiple episodes of dislocation and dislocations due to implant alignment issues may require revision hip replacement surgery to place the components correctly. Special implants may be used to prevent future dislocations. The socket cup and femoral stem may be repositioned to achieve correct alignment.

Dual mobility head and liner utilizes a articulating small head held captive in the liner, the liner articulates inside the acetabular shell. The design ensures stability of the head while allowing greater movement of the prosthetic hip.

Specially designed acetabular liners with elevated lip or lateralization creates a deeper cavity for the head of the femur before it can pop out of the socket. The special design while allowing stability also does not restrict the range of motion of the head of the femur.

Recently specially designed implants are available to prevent the dislocations. Innovations in increasing head size and dual mobility of the implant cups are promising options for reducing the risks of dislocations. Robotic-assisted hip replacements promise accurate implant positioning. The choice of implant, the approach of surgery, and surgeon experience are factors important in reducing the risk of dislocation.