Dislocated Total Hip Replacement
Although uncommon, it is possible for a total hip replacement to pop out of the joint (“dislocate”). When this occurs, patients usually have pain and an inability to bear any weight on the affected leg. The hip joint will typically dislocate “out from the back” (posteriorly) or through the front (anteriorly) and usually occurs when the patient positions the hip beyond the range of motion that the hip joint can accommodate. Risk factors for dislocation of a total hip replacement include a history of previous dislocations, age greater than 70, female sex, and a history of revision total hip replacement surgery. Most dislocated total hip replacements can be treated successfully with a closed reduction (putting the hip back in place) in the emergency room. Recurrent hip dislocations or dislocations secondary to a problem with the prosthesis may require surgery to replace or reposition the prothesis.
A hip joint can dislocate when a force is applied to the joint that overcomes the soft-tissues (ligaments and muscles) that stabilize the joint and causes the two sides of the joint to lose contact. In a normal hip, this requires a lot of force, such as that caused by a car accident. A dislocated normal hip joint is very painful and can cause a lot of damage to the surrounding tissues such as nerves and blood vessels, especially in light of the mechanism of injury and force required to dislocate the hip. The patient often loses partial or total function of the limb due to a combination of pain, misalignment of the bones, and in severe cases nerve damage.
In contrast to a normal hip, a dislocated total hip replacement is much more common and requires much less force for a dislocation to occur. Risk factors for a dislocated prosthetic hip includes a previous history of a dislocation, female sex, age greater than 70 years, and a history of a previous surgical revision of the hip replacement.
A patient with a dislocated hip replacement will usually complain of immediate pain and an inability to stand or walk. They will present to the emergency room with pain and difficulty moving the limb. Some patients will have symptoms from nerve damage including numbness and/or tingling in the affected leg. A patient with a dislocated prosthetic hip often gives a history of performing an activity or motion that places the hip in a vulnerable position.
Posterior hip dislocation
A posterior dislocation refers to dislocation of the femoral head backwards out of the acetabulum. Excessive flexion of the hip joint places the hip at risk of a posterior dislocation. Posterior dislocations account for approximately 80-90% of all dislocated total hip replacements. They tend to be more common in patients whose hip replacement was performed via a posterior approach -a surgical approach which exposes the hip joint by dissecting through the soft-tissues at the posterior part of the hip joint. Examples of activities that may cause a posterior hip dislocation include bending down to tie a shoe, sitting down on a low seat or toilet and getting in and out of a low-lying car.
Anterior hip dislocation
The activities that cause a dislocation of a hip replacement tend to be lower-force activities. Occasionally the prosthetic hip will only partially dislocate (subluxate) or even spontaneously “pop” back into place after the dislocation.
A physical exam is an integral part of determining whether a dislocation has occurred as well as the extent of damage that may have occurred.
Observing the patient and the position of the affected leg is an important first step in assessing a potential hip dislocation. In a posterior dislocation, the patient will tend to be in a position that caused the dislocation in the first place with the hip joint flexed, internally rotated, and abducted.
In an anterior hip dislocation, the leg will typically lie in an extended position with the foot rotated externally and leg abducted. It may also be possible to feel the head of the femur under the skin near the groin or the upper thigh. For both posterior and anterior hip dislocations, the involved leg is often visibly shorter than the unaffected leg.
Signs of injury to nerves include an inability or decreased ability to move the knee or foot, loss of reflexes at the knee or ankle, numbness, loss of sensation, or tingling in areas of skin below the hip. Signs of injury to blood vessels include hematomas, loss of the pulses in the foot and skin paleness.
X-rays are the most useful imaging for confirming a suspected dislocation. The typical views used include a view of the hip from the front known as an “AP” or anterior-posterior view, and an x-ray from the side of the hip, known as a lateral view. The x-rays will demonstrate the abnormal dislocated position of the prosthetic hip joint.
Prevention is the best treatment. Every patient who undergoes a total hip replacement receives instructions in regards to what activities should be avoided. These activities place the hip in a vulnerable position for dislocation. A few examples of motions to be avoided include bringing the knee up beyond the level of the hip (such as sitting in a low chair), and crossing the legs at the knees. Additionally, an extensive physical therapy regimen to strengthen the muscles and soft tissues around the hip joint is an integral part of post-surgical recovery in order to avoid dislocation.
Once a dislocation has occurred, is can most often be treated effectively without surgery by performing a closed reduction of the dislocation (putting it back in place) and protecting the hip with some form of immobilization. This may be performed under light sedation, known as conscious sedation, and can occur in an emergency department. It can also be performed under a general anesthetic in the operating room. Once the reduction is complete, a brace may be applied to immobilize the hip temporarily to let the surrounding soft tissue heal and strengthen.
Dislocations that may not be appropriate for a closed reduction include those occurring long after the initial operation and those that have dislocated multiple times. Any dislocation that fails an attempted closed reduction multiple times also falls into this category. In these cases, surgery is required to resolve the problem.
Prosthetic hip dislocations considered inappropriate for non-operative management often require surgical intervention. Surgery that aims to prevent further hip dislocations from occurring is known as a revision total hip replacement. Revision surgery involves one or more of a variety of procedures that aim to realign the head of the femur with the acetabulum, and or improve the stability of the hip joint. Revision surgery may include:
- Removing impinging structures that serve to push or pull the hip out of joint
- Replacing the liner of the acetabular component to better hold the prosthetic femoral head
- Replacing and realigning the acetabular component of the prosthesis
- Replacing and realigning the femoral component of the prosthesis