summary THA Dislocation is a complication following THA and may occur due to patient noncomplicance with post-operative restrictions, implant malposition, or soft-tissue deficiency. Diagnosis can be made with plain radiographs of the hip. CT of the pelvis can assist with assessing for implant malpositioning. Treatment is closed reduction of the hip. Surgical management with possible revision THA is indicated for irreducible dislocations, recurrent instability, and implant malposition. Epidemiology Incidence 1-3% 70% occur within first month 75-90% posterior Etiology Mechanism anterior extension and external rotation of hip posterior flexion, internal rotation, adduction of hip Risk factors prior hip surgery (greatest risk factor) conflicting evidence regarding patient gender higher comorbidity burden BMI < 20 or > 35 use of meta-on-poly or metal-on-metal bearing surfaces use of cemented components posterior surgical approach repairing capsule and reconstructing external rotators brings dislocation rate close to anterior approach malpositioning of components ideal positioning of acetabular component is 40 degrees of abduction and 15 degrees anteversion in general, excessive anteversion increases risk of anterior hip dislocation; excessive retroversion increases risk of posterior hip dislocation spastic or neuromuscular disease (Parkinson's) drug or alcohol abuse decreased femoral offset (decreases tissue tension and stability) decreased femoral head to neck ratio prior spinal fusion or fixed spinopelvic alignment polyethylene wear common cause of late instability occuring >5 years after procedure Presentation History often reports activity that puts patient in a position that provokes dislocation (hip flexion, adduction, internal rotation) shoe tying sitting in low seat or toilet Imaging Radiographs recommended views AP cross-table lateral findings increased acetabular inclination > 60° increased acetabular anteversion > 20° aceabular retroversion look for eccentric position of femoral head as an indication of polyethylene wear and risk for impending dislocation CT scan indications to assess for implant malposition Treatment Nonoperative closed reduction and immobilization indications two-thirds of early dislocations can be treated with closed reduction and immobilization technique immobilize with hip spica cast, hip abduction brace, or knee immobilizer Operative polyethylene exchange indications stable well-aligned implants with extensive polyethylene wear thought to be sole reason for dislocation revision THA indications indicated if 2 or more dislocations with evidence of implant malalignment vertical acetabular component acetabular retroversion implant failure polyethylene wear techniques see below conversion to hemiarthroplasty with larger femoral head indications for soft tissue deficiency or dysfunction contraindicated if acetabular bone is compromised older technique rarely used with development of dual mobility implants resection arthroplasty indications when all options have been exhausted significant bone loss and soft tissue deficiency psychiatric patients who are dislocating for secondary gain Technique Revision THA techniques to prevent future dislocation during THA include realign components indicated if malalignment explains dislocation retroverted acetabulum vertical acetabulum short femoral neck lack of femoral neck offset retroverted femoral component head enlargement optimize head-neck ratio utilization of a lateralized liner increases femoral offset trochanteric osteotomy and advancement places abductor complex under tension which increases hip compression force conversion to a constrained acetabular component indications recurrent instability with a well positioned acetabular component due to soft tissue deficiency or dysfunction conversion to dual mobility (DM) implant consists of a small femoral head captive and mobile within a polyethylene liner Due to a more favorable (effective) head-neck ratio, dual mobility cups permit a larger range of motion compared to fixed-liner implants DM have unique risk for intraprosthetic dislocation conversion to tripolar construct