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Will my new hip dislocate?
What is a "total hip dislocation?"
Total hip replacement dislocation is a painful condition in which the prosthetic femoral head, or the "ball" on the proximal end of the femur or thigh bone, no longer articulates, or "comes out of joint," with the socket in the acetabular cup of the pelvis.
How often does it happen?
The incidence of dislocation can vary and is as high as 4%. The incidence at the Rothman Institute is less than 0.5%. Dislocation is the second most common reason for revision surgery after prosthetic loosening. Multiple revision surgeries can increase the rate of dislocation to 25%.
Timing from surgery: Up to one third of dislocations occur within 6 weeks after surgery (early dislocations), and the rest happen after 6 weeks (late dislocations).
Why is it a problem?
Total hip dislocation is very painful and distressing to the patient, preventing ambulation.
Initial management and reduction (relocating the ball of the hip joint back into the socket of the acetabular component of the pelvis) of dislocation require an emergency room visit.
Reduction requires the use of anesthesia with its inherent health risks, whether reduction takes place in the emergency room or in the operating room.
Reoperation may become necessary if closed methods of reduction are not successful.
Why does it happen?
There are many reasons why a total hip dislocation can occur:
Surgical Approach: The posterolateral approach has the highest associated incidence of dislocation, and this is believed to be due to straightening of the spine and flexing of the pelvis in the side-lying position used during this approach on the operating table. This may lead to an inappropriately positioned socket. Use of this approach following a hip fracture is also associated with an increased risk of dislocation. The rate of dislocation with this approach and others decreases inversely with the experience of the surgeon.
Orientation of the components: Proper component orientation is the most important factor in preventing future dislocations. The prosthetic stem position depends upon the surgical approach, but in general it should be anteverted, or angulated forward, by about 10 degrees. Proper socket positioning requires more experience and most authors have reported component stability in 30 45 degrees of abduction (lateral angulation) and 10 degrees of anteversion (forward angulation). A malpositioned component may be protected from dislocation after the early post-operative period due to soft-tissue and capsular healing. Long-term dislocation rates (late dislocations occurring 5 to 10 years after surgery) of 0.4% suggest soft tissues cannot indefinitely compensate for component malposition. Such long-term dislocations may also be due to component wear over time.
Prosthetic component design: Dislocation can result from problems with component design and/or selection. If the components are not properly selected, the prosthetic femoral neck may lever against bony prominences around the pelvis. This "impingement" must be avoided as the prosthesis can lever out of the acetabulum in certain positions: extension/external rotation and flexion/internal rotation.
Inadequate restoration of soft tissue tension/failure of the abductor mechanism: Restoring the appropriate degree of soft tissue tension (tightness) is one of the most important factors in preventing dislocation following a total hip replacement. 75% of patients with dislocation have poor soft tissue tension due to a variety of reasons. Previous hip surgery and/or revision arthroplasty surgery may predispose to problems with soft tissue tension and have been associated with increased incidences of dislocation. Detachment of the insertion of the abductor muscles into the greater trochanter of the femur or trochanteric nonunion following an approach using a trochanteric osteotomy leads to inadequate soft tissue tension and is one of the major causes of dislocation.
Patient cooperation: A patient's noncompliance with total hip precautions (proper body positioning) increases the likelihood that a dislocation will occur.
How can you prevent it?
Proper body positioning is the key factor in preventing dislocation. In the early post-operative period, learning total hip precautions, or positions of potential instability to avoid, are critical. As surgical incisional pain decreases beyond the immediate post-operative period, it is important for the patient to remember the presence of the prosthetic joint and not to become too careless about their activities.
Can it be treated without surgery?
Fortunately, most dislocations can be treated without surgery. Options include bracing, casting, or watchful waiting following a period of bedrest. These methods of treatment are most appropriate for dislocations occuring within the first six weeks to three months following the initial surgery. Such early dislocations are generally believed to be due to relaxed soft tissues and immature scar tissue. Immobilization of the hip allows time for sufficient scar tissue solidation so that recurrent dislocations and reoperation are less likely.
In the absence of component malposition or soft tissue imbalance, patient re-education and use of a hip brace or cast can be successful if worn continuously for six weeks. Abduction bracing and spica casting have been successful in preventing recurrent dislocation in up to 95% of cases studied.
When is surgery necessary?
The rule of "three strikes and you're out" applies: most surgeons recommend revision arthroplasty following the third dislocation. Systematic evaluation can guide surgical correction to the specific problem or problems causing the total hip replacement to dislocate recurrently.
Radiographic evaluation with a combination of plain x-ray views of the pelvis and affected hip can be used to assess the orientation of the patient's components, to evaluate the trochanteric mechanism if an osteotomy was used, and to evaluate for polyethylene liner wear. Component angulation too far laterally (excessive abduction) or too far posteriorly (excessive retroversion), trochanteric nonunion, and acetabular wear are all problems which may contribute to instability and are amenable to operative correction.
CT scanning can also be used to assess the orientation of the prosthetic components if this not evident from plain radiographs.
If gross component malposition is discovered during the radiographic work-up, studies have shown that operative intervention should be undertaken sooner rather than later. Such malpositioned components generally fail conservative treatment.
Studies have shown that surgical management is most successful in preventing further dislocations when a specific component problem, soft tissue problem, or both are found and therefore corrected with revision surgery.
What can be done?
Change the orientation of malpositioned components: Removal of malaligned components and placement of new components in the proper orientation as described above should by definition prevent further dislocations since stability is inherent in a properly positioned prosthetic joint.
"Tighten up the soft tissues" through trochanteric advancement: Trochanteric advancement involves removing the greater trochanter from the femur along with its attached abductor muscles and reattaching them distally down the femur to "tighten up" the muscles on the lateral side of the leg and to reposition the trochanter so that it is less likely to impinge upon the pelvis. This procedure has a variable success rate in management of chronic dislocation with soft tissue laxity with no other identifiable causes.
Removal of impinging cement, bone spurs, or soft tissues: In one series impingement was believed to be the primary cause of instability in 10% of patients; however, its surgical correction led to only a 50% success rate in preventing further dislocations. Impingement is more often an important secondary cause of instability, and removal of its sources is an important step at the time of revision surgery for other primary causes.
Use a constrained acetabular socket: Constrained acetabular sockets are a new alternative to traditional components and are indicated in patients who continue to dislocate despite appropriate revision surgery or in whom no correctable cause of instability can be found. They are designed to decrease the likelihood of dislocation by preventing the extremes of hip motion which can lead to instability. Various models of constrained sockets have been used successfully in such patients up to 96% of patients having no subsequent dislocations. |
