Total Hip Arthroplasty: Dislocation

Definition and Significance

THA dislocation is complete loss of contact between the femoral head and the acetabular component — a devastating complication with significant impact on patient function, satisfaction, and healthcare costs.

Patient Risk Factors

Surgical Risk Factors

Hip Joint Stability Mechanisms

Understanding normal hip stability is essential for preventing and managing THA dislocation.

Osseous Factors:

• Acetabular coverage: native acetabulum covers 170-180° of the femoral head
• THA cup coverage: typically 150-170° (trade-off with range of motion)
• Femoral head-neck ratio: larger heads provide greater jump distance
• Impingement-free arc: determined by component design and positioning

Soft Tissue Stabilizers:

Static Stabilizers:

• Joint capsule: primary restraint to dislocation
• Iliofemoral ligament (Y-ligament of Bigelow): strongest, prevents hyperextension
• Ischiofemoral ligament: prevents internal rotation in extension
• Pubofemoral ligament: prevents abduction and external rotation
• Acetabular labrum: deepens socket (not present in THA)

Dynamic Stabilizers:

• Gluteus medius and minimus: anterior fibers prevent anterior dislocation
• Short external rotators: posterior capsule reinforcement (piriformis, obturators, gemelli)
• Iliopsoas: anterior stabilizer
• Adequate muscle tension and activation

THA-Specific Stability Factors:

Component Factors: head size (larger heads increase jump distance and impingement-free arc), head-neck ratio (lower ratio increases range before impingement), neck-cup impingement (most common cause of mechanical dislocation), liner design (elevated/lipped liners increase coverage), and offset (restores abductor moment arm and soft tissue tension).

Positioning Factors: acetabular inclination (affects superior/medial coverage), acetabular anteversion (affects anterior/posterior coverage), femoral anteversion (works with cup version for combined stability), leg length (provides soft tissue tension), and lateral offset (tensions abductors).

Pathophysiology by Direction

Posterior Dislocation (75-80%): most common after posterior approach. Mechanism: hip flexion + internal rotation + adduction (sitting to standing, low chairs, tying shoes). Caused by posterior capsule disruption or inadequate repair, posterior wall deficiency, or external rotator insufficiency.

Anterior Dislocation (15-20%): more common after anterior/anterolateral approach. Mechanism: hip extension + external rotation (stepping backward, arising from bed). Caused by anterior capsule disruption, excessive acetabular anteversion, or psoas impingement on the anterior neck.

Superior Dislocation (rare): requires significant force or component failure; associated with severe abductor dysfunction, loosening, or catastrophic wear.

Component Malposition Leading to Dislocation:

• Acetabular cup: excessive inclination over 50° (reduced superior coverage, posterior instability); insufficient inclination under 30° (medial impingement); excessive anteversion over 25° (anterior instability); insufficient anteversion under 5° or retroversion (posterior instability).
• Femoral component: excessive anteversion (anterior instability), retroversion (posterior instability), varus positioning or undersized stem (reduced offset and soft tissue tension).
• Combined version: combined anteversion outside 25-45° increases risk; appropriate femoral version can compensate for borderline cup position, so both components must be considered together.

Prosthetic Impingement: The Mechanism of Dislocation

The factors above name neck-on-cup impingement as "the most common cause of mechanical dislocation," but the mechanism deserves to be explicit, because it is the event that initiates most positional dislocations.

The two-stage mechanism. At the extreme of a movement one structure abuts another and creates a fulcrum; continued force then levers the head toward the opposite rim, and once the head translates further than its jump distance (below) it escapes the cup and dislocates. Impingement is the trigger; jump distance is the reserve that resists it.

Jump Distance: The Stability Reserve

The topic repeatedly credits larger heads and dual mobility with greater jump distance - the concept is worth defining, because it is the reserve that resists dislocation once impingement has levered the head toward the rim.

Definition. Jump distance is the lateral translation the femoral head must undergo to ride up and out of the acetabular component. For a hemispherical cup it approximately equals the head radius (roughly half the head diameter), modified by cup coverage and any liner lip - so a larger head has a proportionally larger jump distance.

What increases it:

• Head diameter: a 36mm head must travel farther than a 28mm head before it escapes - the geometric basis for the Howie RCT result (1.3% vs 5.4% dislocation with 36mm vs 28mm).
• Cup coverage / lipped liner: deeper coverage or an elevated rim increases the distance to escape (at the cost of arc to impingement on the opposite side - the same trade-off as above).
• Dual mobility: the large polyethylene "effective head" (around 50mm) gives a very large jump distance - the main reason dual mobility is so stable.

Limits. The benefit has diminishing returns and an upper ceiling: heads above about 40mm add little further jump distance but increase volumetric polyethylene wear and head-neck taper corrosion (trunnionosis), and a very large head needs a thin liner that may fracture. Hence the practical target of 36mm for most patients, 36-40mm or dual mobility for high-risk, and avoidance of heads under 32mm.

Impingement (the trigger) and jump distance (the reserve) together underpin every component-level stability decision in this topic.

History and Examination

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Plain Radiography

Computed Tomography

CT is often essential for recurrent dislocation or revision planning. Protocol: thin (1-2mm) cuts of pelvis and proximal femur to below the lesser trochanter, with 3D reconstruction.

• Acetabular measurements: inclination (coronal reconstructions), anteversion (axial cuts at the superior dome; angle between acetabular opening plane and the AP pelvic axis), bone stock assessment.
• Femoral measurements: femoral neck/stem version (angle between neck axis and posterior femoral condyles).
• Combined anteversion: acetabular AV + femoral AV; target 25-45°. McKibbin's combined index (traditional) is acetabular AV + femoral AV = 37° ± 10°.
• Impingement assessment: anterior (psoas tendon, anterior rim), posterior (ischium, posterior wall, component-component), with software-based detection where available.
• 3D reconstruction: useful in complex anatomy (DDH, revision), surgical planning, bone defect identification, and consent.

Additional Investigations

• Laboratory/aspiration: not routine; obtain ESR/CRP and aspirate (cell count, culture, alpha-defensin) if infection is suspected before revision.
• MARS MRI: rarely needed; useful to assess abductor integrity or soft tissue pathology.
• Neuromuscular: formal neurology consult and EMG/NCS (at 3-4 weeks) for persistent sciatic deficit; physiotherapy assessment of abductor function.

Principles of Revision for Instability

Pre-Operative Planning: CT with 3D reconstruction; precise cup inclination/version and femoral version; combined anteversion calculation; identify malpositioned component(s); assess bone stock; plan target positions. Have a full revision system available — dual mobility components, constrained liners (backup), larger heads, and augments for bone defects. Approach is usually the same as the index surgery (most commonly posterior), with extensile exposure if needed.

Recurrent Dislocation

Two or more episodes; occurs in 25-50% after a first dislocation, rising to 60-75% after a second. Drivers include non-compliance, neuromuscular/cognitive decline, unrecognized component malposition, failure to address the underlying cause, inadequate soft tissue repair, inappropriate head size, and uncorrected impingement. Management: systematic CT-based workup, revision surgery typically after the second dislocation, correction of the underlying cause, and an enhanced stability construct (dual mobility preferred). Non-operative treatment rarely succeeds.

Sciatic Nerve Injury

Occurs in 10-20% of acute dislocations (usually neuropraxia; permanent deficit 1-2%). The common peroneal division is most affected — foot drop, numbness over the dorsal foot and first web space, high-steppage gait; the tibial division causes plantarflexion weakness and plantar numbness. Document detailed motor/sensory exam before and after reduction; if a deficit worsens after reduction, consider nerve entrapment (urgent MRI/exploration). Manage with gentle reduction, AFO for foot drop, and physiotherapy. EMG/NCS at 3-4 weeks if persistent. Neuropraxia recovers in 80-90% over 6-12 months; consider tendon transfer (posterior tibial for foot drop) if no recovery by 12-18 months.

Periprosthetic Fracture

Posterior wall acetabular fracture is the most common fracture with posterior dislocation — may be occult on plain films, often needs CT. Small stable fragments are treated non-operatively; large fragments (over 25-30% of the wall) require ORIF and may need a larger head or dual mobility for stability. Femoral periprosthetic fractures (Vancouver classification) are rare and may occur during reduction — use gentle technique. Iatrogenic fractures during revision (greater trochanter, acetabulum, femur) require intraoperative recognition and fixation.

Component Wear and Loosening

Recurrent dislocation causes repetitive bearing trauma, accelerating polyethylene wear and potential osteolysis; constrained liners increase cup-bone interface forces and can accelerate loosening. Assess with serial radiographs (progressive radiolucent lines, migration, osteolysis, changing position). Late dislocation may reflect wear and laxity or component migration. Aseptic loosening requires revision; correct malposition and address wear (larger heads, dual mobility, highly cross-linked polyethylene) at the same time.

Psychological Impact

Fear of recurrence, anxiety about movement, depression, reduced quality of life, and self-imposed activity restriction are common. Manage with education, realistic expectations, psychological support where needed, supervised graded return to activity, and emphasis on what the patient can do safely.

Immediate Period (0-6 weeks)

Hip precautions from day 0 with an abduction pillow/brace if high risk; neurovascular checks (especially after closed reduction); multimodal analgesia; VTE prophylaxis per protocol; early mobilization with a walker. Discharge once safe mobilization is achieved, with assistive devices (elevated toilet seat, reacher, sock aid, long shoe horn). First review at 1-2 weeks with wound check and radiographs to exclude early dislocation. Continue strict precautions and wean the assistive device as strength improves.

Intermediate Period (6-12 weeks)

Week 6: clinical and radiographic review, Trendelenburg assessment, begin weaning the abduction brace, and gradually liberalise activity. Weeks 6-12: progressive strengthening, low-impact activity (walking, swimming, cycling), and driving when safe. Week 12: final short-term review — discontinue formal precautions if low risk and stable, with lifelong awareness of extreme positions.

Long-Term Follow-Up

3-6 months: return to unrestricted activity if stable; maintenance abductor strengthening. 1 year and annually thereafter: clinical and radiographic surveillance for late dislocation, wear, and loosening; lifelong follow-up is recommended for all THA.

Red flags for urgent evaluation: sensation of instability or near-dislocation, acute dislocation, new severe pain, leg-length change, inability to weight bear, or new neurological symptoms.

Outcomes by Treatment Strategy

• Non-operative (first dislocation): no-recurrence rate 50-75%; better with well-positioned components, traumatic mechanism, and good compliance; worse with malposition, atraumatic mechanism, neuromuscular disease, or cognitive impairment.
• Isolated liner exchange: 60-80% success if the cup is well positioned; better with larger heads and dual mobility liners; fails if underlying malposition is not addressed.
• Acetabular revision: 80-90% success with dual mobility, 70-85% with a standard large head; achieving combined anteversion 25-45° is critical.
• Dual mobility: dislocation 0.5-2% (primary use), 2-5% (revision); intraprosthetic dislocation 0.5-1%; best outcomes for recurrent instability in current evidence.
• Constrained liners: mechanical prevention 80-90% but reoperation 15-25% for locking ring failure/loosening — less favourable than dual mobility; salvage option.

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Surgical Technique Optimization

Pre-Operative Risk Stratification

• Very high risk (consider enhanced stability implants): prior THA dislocation, neuromuscular disease, severe cognitive impairment/dementia, abductor deficiency, revision for instability, tumor resection with soft tissue loss, multiple prior hip surgeries, age over 80 with frailty. Plan: dual mobility (primary or revision), consider direct anterior if experienced, larger head if standard bearing, extended rehabilitation.
• Moderately high risk (modify technique): age 70-80, non-instability revision, prior hip surgery, DDH, inflammatory arthropathy, morbid obesity, milder neuromuscular disease. Plan: head ≥36mm, meticulous soft tissue repair, extended precautions (12 weeks), abduction brace, enhanced education.

Patient Education and Enhanced Recovery

Pre-operative precautions teaching (video/demonstration, written materials), realistic expectations, and home-hazard planning. Post-operatively, deliver direction-specific precautions, early mobilization with an assistive device, abductor strengthening (critical for dynamic stability), and home safety assessment. High-risk patients use a hip abduction brace 6-12 weeks. Maintain lifelong awareness of high-risk positions and the need for urgent review if instability is suspected.

Global Epidemiology and Registry Evidence

Instability/dislocation is consistently among the leading early indications for revision THA across major national registries (AOANJRR, NJR for England/Wales, AJRR, Swedish/Nordic registries). Registry data converge on several practice-changing signals:

• Approach: posterior approach carries a higher early dislocation/revision rate than direct anterior in most registries; the gap narrows substantially with enhanced posterior soft tissue repair.
• Head size: heads ≥36mm have lower dislocation than 28-32mm; 28mm is now rarely used in modern primary THA.
• Dual mobility: lowers revision for dislocation specifically, but international registry meta-analysis (Farey 2022, including AOANJRR, NJR, and four others) found no reduction in all-cause revision for hip-fracture THA and a possible infection trade-off — supporting selective rather than universal use.
• Re-revision burden: revision for instability carries a high re-revision rate, underlining that prevention at the index operation is paramount.

Side-by-Side Society Guidance

High- vs Limited-Resource Variation

• High-resource settings: CT and navigation/robotics for version planning; routine availability of dual mobility, constrained liners, augments, and modular revision systems; spinopelvic imaging for at-risk patients.
• Limited-resource settings: reliance on plain radiographs and mechanical alignment guides; emphasis on fundamentals — optimal positioning, ≥36mm heads where available, meticulous posterior soft tissue repair, and robust patient education/precautions, which deliver most of the achievable risk reduction at low cost.

Peri-operative Pharmacology (Global Principles)

• VTE prophylaxis: LMWH or oral anticoagulant (e.g. rivaroxaban, apixaban) with mechanical prophylaxis; duration commonly 10-14 days, extended up to ~35 days in higher-risk patients (follow local/society guidance).
• Antibiotic prophylaxis: a first-generation cephalosporin (e.g. cefazolin, weight-adjusted) within 60 minutes of incision, re-dosed for long cases or major blood loss; vancomycin/teicoplanin if MRSA risk or beta-lactam allergy; no proven benefit beyond 24 hours.

THA dislocation occurs in 1-5% of primary cases, predominantly posterior direction (75-80%) with hip flexed, adducted, and internally rotated. Risk factors follow the 4 P's: Patient (age over 70, cognitive impairment, neuromuscular disease), Procedure (posterior approach 2-3× higher risk, revision), Position (component malposition), Power (inadequate soft tissue tension). Lewinnek safe zones: inclination 40±10°, anteversion 15±10°; combined anteversion 25-45° (cup + stem) is widely regarded as the most critical parameter. Prevention strategies: head size 36mm or larger (RCT-supported), meticulous posterior soft tissue repair (reduces risk substantially), and dual mobility cups for high-risk patients. First dislocation: closed reduction, identify cause with CT for component version, hip brace. Recurrent dislocation (25-50% recurrence after first event) usually requires revision — liner exchange, dual mobility, constrained liner, or component revision depending on cause. Registry data show dislocation/instability is among the leading indications for early revision.