Total hip arthroplasty (THA) dislocation is the disengagement of contact between the femoral head and the acetabular cup implant. Patients typically present with acute hip / back pain, malrotation of the affected leg, shortening of the affected leg, and inability to move the affected leg or bear weight.

Causes / typical injury mechanism: THA dislocation is usually secondary to placing the hip in a position that provokes a dislocation. Anterior dislocations are usually caused by abduction, extension, and external rotation, while posterior dislocations are usually the result of adduction, internal rotation, and flexion of the hip. Compared to native hip dislocation, artificial hip dislocation can be caused by relatively common motions, such as getting up from a chair or the toilet, with deep hip flexion or a misstep resulting in eccentric loading of the hip joint in a compromised position. Dislocations in the postoperative period may be due to not following posterior hip precautions (ie, no flexion past 90 degrees, no leg adduction, and no internal rotation). THA dislocation can also be seen in high-energy trauma such as motor vehicle crashes and falls from a height. Dislocation is the most common cause of revision hip surgery in the United States.

Prevalence: The incidence of dislocation is approximately 1%-3%. Posterior dislocations are predominant (75%-90%) and occur most often in the first 30 days after THA (70%).

Risk factors: The greatest risk factor for dislocation is previous hip surgery or revision THA. Other risk factors include patient age older than 70 years, female sex, neurologic conditions affecting the mobility of the patient or the strength of supporting muscles (eg, Parkinson disease), spinopelvic alignment and history of previous spinal fusion surgery, decreased femoral offset, decreased head-to-neck ratio (smaller femoral head component), polyethylene wear and resultant instability, and a history of recurrent dislocations suggestive of capsular and abductor insufficiency.