Total Hip Replacement Dislocation - Comprehensive Assessment and Management

The clinical problem. Dislocation after total hip arthroplasty is one of the most common complications and a leading indication for revision surgery. Management is dictated by the number of episodes, the direction, the position of the components and the integrity of the soft tissues — so the first job is a systematic assessment, not an operation. Step 1 — Rule out infection (mandatory). Before any revision for instability, exclude infection. Infection can present as recurrent dislocation (around 30 percent of "instability" cases in some series), and any revision will fail if infection is missed. Workup is ESR and CRP, plus joint aspiration as the gold standard — cell count (greater than 3000 cells per microlitre), differential (greater than 80 percent PMNs), Gram stain, and aerobic and anaerobic cultures held for 14 days. Step 2 — Image and measure component position. Standard radiographs (AP pelvis, lateral hip, shoot-through lateral femur) assess cup inclination, leg length and offset. A CT pelvis with metal-artifact reduction is the gold standard for component position: cup inclination (normal 35 to 45 degrees, safe zone 30 to 50 degrees), cup anteversion (normal 15 to 25 degrees, safe zone 5 to 25 degrees), stem version (CT femur, normal 10 to 15 degrees), impingement (anterior wall overhang, posterior offset deficiency), and spinopelvic parameters (pelvic tilt, sacral slope, lumbar lordosis). The Lewinnek safe zone — and its critical caveat. Lewinnek (1978) defined the safe zone as 40±10° inclination and 15±10° anteversion, with 1.5 percent dislocation inside versus 6.1 percent outside (roughly four-fold higher). The critical caveat is Abdel (2016): 58 percent of dislocated THAs had a cup WITHIN this zone. The safe zone is therefore necessary but not sufficient — instability is multifactorial, so read cup position together with stem version, combined anteversion, offset and spinopelvic mobility, never in isolation. Step 3 — Classify the direction. - Posterior (75 to 90 percent) — mechanism flexion + adduction + internal rotation (rising from a low chair, tying shoes, getting into a car). Component factors: cup or stem retroversion or insufficient combined anteversion, inadequate posterior offset, posterior capsular and short-rotator deficiency.

• Anterior (10 to 25 percent) — mechanism extension + external rotation + adduction (reaching back, pivoting, leg crossing). Component factors: excessive cup and/or stem anteversion (excessive combined anteversion), anterior impingement. Step 4 — Identify the correctable risk factors. Component malposition (the most common correctable cause); impingement (bony — anterior wall overhang, posterior offset deficiency, femoral neck osteophytes; component — neck-cup or liner-rim contact; soft-tissue — iliopsoas anteriorly, short external rotators posteriorly); soft-tissue deficiency (capsule deficient or absent, short-rotator attenuation, abductor insufficiency with a positive Trendelenburg); and patient factors (Parkinson's disease, dementia, CVA, cognitive or compliance problems, unrestored offset, leg-length inequality). Document the dislocation history: number of episodes (first versus recurrent), timing (early less than 90 days versus late), direction, energy (high versus low), reduction method (closed versus open), and any prior revisions for instability. The combined-anteversion concept. Stability depends on the sum of cup plus stem anteversion, not the cup alone. The commonly cited target is roughly 25 to 50 degrees (Dorr), with Widmer's optimum near 37 degrees. Excessive combined or stem anteversion (stem over 20 degrees) drives ANTERIOR instability; a retroverted stem (under 5 degrees) or low combined anteversion drives POSTERIOR instability. If the combined value is outside the zone, plan to address BOTH the cup and the stem.

The decision. - Non-operative — first dislocation with well-positioned components and correctable risk factors: closed reduction under sedation, a hip abduction orthosis for 6 to 12 weeks, and hip precautions for a minimum of 12 weeks. Success is roughly 60 to 70 percent after the first episode, falls to around 30 percent after the second, and is less than 20 percent after the third.

• Operative — recurrent instability (more than two episodes), or a first dislocation with component malposition or uncorrectable risk factors: correct the cause and add a stability-enhancing construct. The construct choice (in order of preference).

If the components are well-positioned, an isolated liner exchange to dual mobility is the least invasive option (it preserves well-fixed components). Add capsular reconstruction (Achilles allograft) for a deficient capsule and greater trochanteric advancement for abductor insufficiency. Consent specifically for re-dislocation, infection (2 to 5 percent in revision), sciatic nerve palsy (3 to 7 percent in revision versus 0.5 to 2 percent in primary), intraoperative fracture, leg-length or offset change, and the possibility of salvage (Girdlestone, arthrodesis) if multiple revisions fail. Setup. Lateral decubitus for a posterior approach (around 90 percent of primary THRs), supine if the primary was anterior; re-use the previous surgical approach to minimise further soft-tissue damage; fluoroscopy is essential throughout.

The goal is to expose the hip through the previous approach, identify and protect the sciatic nerve early, define the cause of instability (component position, impingement, soft-tissue deficiency), correct any malposition, and rebuild a stable construct — most often with a dual-mobility bearing, reinforced by soft-tissue repair or reconstruction. The exposure is laid out in full as the opening steps, because in the scarred, unstable hip the exposure and nerve protection are the whole game.

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Post-operative protocol | Phase | Timing | Brace / weight-bearing | Activity | |-------|--------|------------------------|----------| | 1 | 0 to 6 weeks | Hip abduction orthosis day and night; WBAT (touch-down weight-bearing 12 weeks if GT advancement) | Strict hip precautions | | 2 | 6 to 12 weeks | Orthosis night-time only; advance weight-bearing as able | Wean brace, advance activity | | 3 | 3 months and beyond | None | Precautions lifted if stable; graded return to function | Hip precautions apply for a minimum of 12 weeks (consider lifelong if high risk). Posterior approach: no flexion more than 90°, no adduction past midline, no internal rotation. Anterior approach: no extension, no external rotation, no adduction. DVT prophylaxis is extended 4 to 6 weeks in the high-risk patient (LMWH or DOAC per protocol). Follow-up schedule | Interval | Review | |----------|--------| | 2 weeks | Wound check | | 6 weeks | Radiographs, advance activity | | 3 months | Radiographs, wean brace | | 6 months | Radiographs and function | | 1 year | Radiographs | | Annually for life | Radiographs and function (Oxford Hip Score, HOOS) | Surveillance watches for late dislocation, accelerated wear (dual-mobility metal-on-poly, constrained poly), component loosening (especially the torque on a constrained liner), and neuromuscular progression. Outcomes. Non-operative management of a first dislocation keeps roughly 60 to 70 percent stable, but success falls steeply with each further episode (weak evidence base). Dual mobility delivers low re-dislocation in revision series — Philippot 2009 (3.7 percent overall, 0 percent in the recurrent-instability subgroup, 7-year cup survival 96.1 percent) and Guyen 2009 (54 unstable hips restored to stability) — and registries report lower revision-for-dislocation than single bearings. The constrained liner carries a recognised catastrophic mode (locking-ring or liner dissociation). Worse prognosis attends neuromuscular or cognitive disorders (a relative risk only — Meek 2006 found no increased dislocation with Parkinson's or stroke in their cohort), severe abductor deficiency, multiple prior revisions, and a stiff or fused spine.

Salvage options for failed revision (multiple recurrent dislocations despite optimal revision, unreconstructable bone loss, or chronic infection): - Girdlestone resection arthroplasty — remove all components and accept a shortened, painful limb. Best for the elderly, low-demand, walker-dependent patient; significant functional limitation but pain relief.

• Hip arthrodesis — for the young (under 40), unilateral, high-demand patient without hip or spine arthritis. Position in 20 to 25° flexion, 0° adduction or abduction, neutral rotation. Stable and painless but causes back and knee pain; a rare last resort.
• Bipolar hemiarthroplasty — when severe acetabular bone loss precludes a stable cup; accepts a higher dislocation rate but avoids a complex acetabular reconstruction.

Epidemiology. Dislocation is consistently one of the leading indications for revision THA across the major registries (AOANJRR, the National Joint Registry for England, Wales, Northern Ireland and the Isle of Man, the Nordic Arthroplasty Register Association, and AJRR). Posterior dislocation predominates (75 to 90 percent), and the cumulative dislocation risk after a primary THA rises over time. Why the safe zone is necessary but not sufficient. Lewinnek (1978) established the 40±10° inclination and 15±10° anteversion target from 300 THAs (9 dislocations, 3 percent), finding 1.5 percent dislocation inside versus 6.1 percent outside. Abdel (2016) re-examined 206 dislocated THAs from 9784 primaries and found 58 percent sat WITHIN the zone — instability is multifactorial, and for some patients the ideal cup position lies outside the historical zone. The exam message: quote Lewinnek for the numbers, then Abdel for the nuance. Why dual mobility works — the head-size rationale. Berry (2005) showed in 21,047 primary THAs that larger femoral heads reduce dislocation (relative risk 1.7 for 22 mm versus 32 mm heads), with the greatest benefit in the posterolateral approach (10-year dislocation 12.1 percent for 22 mm versus 3.8 percent for 32 mm heads). Dual mobility exploits this by providing a large effective head (the 52 to 60 mm outer bearing) that lengthens jump distance and delays impingement, while the small inner bearing preserves the head-to-neck ratio. Spinopelvic stiffness — the 'well-positioned but unstable' hip. Pelvic tilt changes the functional cup orientation between standing and sitting. A stiff or fused spine (a long lumbar fusion, flatback deformity, or loss of lordosis in the elderly) that cannot posteriorly tilt on sitting markedly raises posterior dislocation risk. Assess with lateral standing and sitting radiographs (the pelvic tilt change) and individualise cup anteversion accordingly (more anteversion if anterior tilt, less if posterior tilt). Registries and global practice. The major registries report lower revision-for-dislocation rates with dual mobility versus conventional single bearings, with rising dual-mobility use in high-risk primary and revision-for-instability settings; the trade-off is polyethylene wear and rare intraprosthetic (inner-bearing) dissociation, mandating lifelong radiographic surveillance. For surgical antibiotic prophylaxis the globally concordant principle is a weight-adjusted first-generation cephalosporin (e.g. cefazolin) within 60 minutes of incision (AAOS, NICE and SIGN, WHO surgical site infection guidance), with vancomycin or clindamycin for true beta-lactam allergy and a single pre-incision dose (routine continuation beyond 24 hours is not supported). Name the principle rather than country-specific brand names, codes or billing items.