Dislocation after THA — Risk Factors & Management

Dislocation remains one of the most feared complications of total hip arthroplasty (THA), occurring in approximately 1–3% of primary THA and up to 10–15% of revision THA. It is the second most common indication for revision THA after aseptic loosening in major registries. Dislocation causes significant patient morbidity — pain, functional limitation, psychological distress, and the need for further intervention — and carries risks of recurrent instability, nerve injury during closed reduction, and ultimately revision surgery for recurrent dislocation. Understanding the modifiable and non-modifiable risk factors, the biomechanical principles governing hip stability, and the management algorithm from closed reduction to revision is essential.

• Biomechanics of hip stability — jump distance and arc of motion: stability of the hip after THA depends on two biomechanical principles: (1) jump distance — the lateral distance the femoral head must travel to escape the acetabular cup; determined by femoral head size (larger head = greater jump distance = more stable) and cup depth (deeper seating = more stable); a 36 mm head has ~50% greater jump distance than a 28 mm head; (2) arc of motion — the range of impingement-free motion before the femoral neck or trochanter impinges on the cup rim or liner; impingement is the mechanical precursor to dislocation — the neck levers against the cup rim and the head is levered out of the socket; impingement risk is reduced by optimal component positioning, larger head:neck ratio, trapezoidal neck designs, and appropriate offset restoration
• Direction of dislocation: determined by surgical approach and component position; posterior approach → posterior dislocation (the most common — head dislocates posteriorly and superiorly; hip internally rotated, adducted, and flexed beyond 90°); anterior or anterolateral approach → anterior dislocation (less common; hip in extension, external rotation, and abduction); the posterior approach has a historically higher dislocation rate than the anterolateral approach due to the posterior capsule and short external rotator division; modern posterior approach WITH posterior capsular repair and short external rotator repair has a dislocation rate comparable to the anterolateral approach

• Initial assessment: confirm dislocation on X-ray (AP pelvis + lateral hip); assess for periprosthetic fracture (AP + lateral of the full femur); assess neurovascular status (sciatic nerve — assess foot drop; femoral nerve — assess quadriceps power; vascular status of the limb); all patients with acute THA dislocation should be evaluated for cause to guide management
• Closed reduction: performed under appropriate sedation or general anaesthesia; the standard technique for posterior dislocation — longitudinal traction with hip in flexion, followed by internal rotation correction and extension (reversing the dislocation mechanism); fluoroscopy confirms reduction; after reduction, examine the hip through ROM to assess stability and identify the position(s) of impingement/re-dislocation; a single acute dislocation in a stable patient without clear component malposition is initially managed with closed reduction + bracing (hip abduction brace or hip orthosis restricting the provocative position) for 6–12 weeks; however, bracing compliance in elderly patients is poor and its efficacy is disputed
• Investigation after reduction: CT scan after reduction to assess component position (cup abduction and anteversion angles) and identify any periprosthetic fracture or component damage; MRI (MARS) if metal-on-metal bearing is present; serum CRP and ESR to screen for late PJI presenting as instability

• Recurrent dislocation (≥2 dislocations): surgical intervention is required when dislocation recurs; the surgical strategy must address the underlying cause — this requires identification of the cause pre-operatively with CT; causes of recurrent dislocation: (1) cup malposition (outside the Lewinnek safe zone or functional safe zone — most common correctable cause); (2) femoral stem malposition (excessive or insufficient anteversion); (3) leg length discrepancy / offset loss; (4) abductor deficiency; (5) large periprosthetic tissue damage; options — cup revision (repositioning within the safe zone); femoral stem revision (correct version); conversion to dual mobility cup; use of a larger femoral head; trochanteric advancement (for abductor deficiency)
• Dual mobility cup (DMC): a constrained-type cup designed to reduce dislocation risk without requiring cup revision for every malposition; the femoral head articulates within a mobile polyethylene liner, which in turn articulates within a fixed metal shell (two-bearing surfaces — hence `dual mobility`); the effective head size is the outer diameter of the mobile liner (equivalent to a 40–52 mm head) → much greater jump distance than a standard liner; dislocation requires the inner head to first dislocate from the PE liner (first dislocation — `intraprosthetic dislocation`) before the liner can dislocate from the shell; dramatically reduces dislocation rate in high-risk patients (posterior approach, neurological disease, revision THA, recurrent dislocators); the primary complication of DMC is intraprosthetic dislocation (the inner head dislocates within the PE liner — rare but requires liner exchange)

• THA dislocation incidence: 1–3% primary; 10–15% revision; 2nd most common indication for THA revision after aseptic loosening; posterior approach → posterior dislocation (most common); hip flexed >90° + IR + adduction = dislocation position
• Jump distance: determined by head size (larger = greater jump distance = more stable); 36 mm head has ~50% greater jump distance than 28 mm; impingement is the mechanical precursor to dislocation — neck levers against cup rim, ejects head
• Lewinnek safe zone: cup abduction 30–50°; anteversion 5–25°; outside safe zone → higher dislocation rate; functional safe zone (accounting for individual pelvic tilt/spinopelvic mobility) increasingly preferred over fixed radiographic targets
• Posterior approach: historically higher dislocation rate (3–5%); modern posterior approach WITH capsular repair + short external rotator repair → dislocation rate ~1% (comparable to anterolateral)
• Acute dislocation: closed reduction under sedation/GA; post-reduction CT to assess cup position and look for fracture; single episode + stable components → brace for 6–12 weeks; recurrent (≥2) → surgical intervention
• Dual mobility cup (DMC): mobile PE liner + fixed shell → effective head size = outer liner diameter (~40–52 mm); massive jump distance; reduces dislocation in high-risk patients (posterior approach, neuro disease, revision); intraprosthetic dislocation = rare complication
• Surgical management of recurrent dislocation: identify cause on CT first; cup revision (malposition); stem revision (malversion); larger head; DMC; trochanteric advancement (abductor deficiency)
• Abductor deficiency: trochanteric non-union or avulsion; denervation; produces chronic instability; trochanteric advancement or abductor reconstruction ± DMC; most difficult instability pattern to treat
• Risk factors to memorise: female sex; cognitive impairment/neuromuscular disease; femoral neck fracture indication; posterior approach without capsular repair; cup malposition; revision THA; LLD (shortened leg)