Total Hip Arthroplasty (THA), commonly known as hip replacement surgery, is a highly effective procedure designed to relieve pain and restore mobility. The procedure involves removing the damaged hip joint and replacing it with prosthetic components. Hip dislocation occurs when the prosthetic ball comes out of the artificial socket, causing sudden, sharp pain and inability to bear weight. While THA is routinely successful, dislocation remains a primary concern for patients. Understanding the risk and necessary protective steps is important during recovery.
Dislocation Rates Following Hip Replacement
The likelihood of hip dislocation after a primary THA is low, ranging from 1% to 4%. This rate represents the risk over several years following the operation.
A majority of dislocations (70% to 75%) occur within the first year after surgery. The first two to three months are the period of highest risk while the surrounding soft tissues, which provide stability, are still healing. Dislocation rates vary based on the specific surgical approach and the patient population studied.
Factors Increasing the Likelihood of Dislocation
Patient Factors
Variables related to the patient can elevate the risk of dislocation. Advanced age, particularly over 70 years old, is associated with higher instability. Patients with neurological conditions, such as Parkinson’s disease, or cognitive impairment face increased risk due to impaired muscle control and difficulty following post-operative instructions.
Previous hip surgery on the same joint, including prior fractures or failed procedures, significantly increases the chance of dislocation. Other factors contributing to instability include a higher body mass index (BMI over 30 kg/m\(^2\)) and conditions affecting tissue quality, such as rheumatoid arthritis. Adherence to physical therapy and movement restrictions is important in mitigating this risk.
Technical Factors
Technical aspects of the surgery also play a substantial role in joint stability. The placement of prosthetic components is highly sensitive; if the acetabular cup or femoral stem is positioned outside of ideal angles, the joint may be less stable. For example, excessive cup abduction can predispose the hip to instability.
The choice of surgical approach influences immediate post-operative risk before soft tissues heal. The posterior approach is associated with a slightly higher dislocation rate compared to the anterior or anterolateral approaches. Surgeons mitigate this by carefully repairing the posterior soft tissues and joint capsule. Additionally, using a smaller femoral head size increases the risk of instability compared to larger head sizes.
Post-Surgical Precautions and Movement Guidelines
Preventing dislocation relies on strictly following specific movement restrictions, known as “hip precautions,” during the initial recovery period. These guidelines focus on avoiding movements that destabilize the new joint.
Patients must avoid bending the hip past 90 degrees, meaning the upper body and thigh should not come closer than a right angle. This restriction means patients should not lean forward to pick up objects or sit on low chairs or toilets. Using a raised toilet seat or firm cushions helps keep the hip joint extended. Patients should use long-handled reaching aids for dressing and picking up items.
The second major precaution involves avoiding specific rotation and adduction movements. Patients must not cross the operated leg over the midline of the body, including crossing the ankles or knees. They also need to avoid turning the foot and knee inward (internal rotation), which can destabilize the joint.
When turning, patients should take small steps rather than twisting the body over the operated leg. Sleeping positions require attention; patients are advised to sleep on their back and use a pillow between their knees when lying on the unoperated side to prevent crossing the midline. These precautions are maintained until the surgeon confirms the soft tissues have healed, typically six to twelve weeks.
Managing a Dislocation
If a dislocation occurs, patients experience severe, immediate pain and an inability to move the leg or bear weight. This event requires prompt medical attention, usually in an emergency room setting. The initial treatment for a first-time dislocation is closed reduction, a non-surgical procedure. A physician manually manipulates the leg to guide the prosthetic ball back into the socket while the patient is under sedation. Imaging confirms component positions and stability after relocation.
Recurrent dislocations indicate an underlying issue with component positioning or soft tissue deficiency. Revision surgery may be necessary, involving repositioning the implants or using specialized devices, such as constrained liners or dual mobility implants, to increase joint stability.