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 the most critical parameter. Prevention strategies: head size 36mm or larger, meticulous posterior soft tissue repair (reduces risk 80%), 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) requires revision—liner exchange, constrained liner, or dual mobility depending on cause. AOANJRR data shows dislocation is the leading indication for early revision.

Definition and Significance

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

Key Epidemiological Data:

• Primary THA dislocation rate: 1-5% (varies by approach and surgeon experience)
• Revision THA dislocation rate: 10-25%
• Recurrence after first dislocation: 25-50%
• Leading cause of early revision in Australian registry data
• Peak incidence in first 3 months post-operatively (50% of all dislocations)

Clinical Impact:

• Severe pain and disability
• Emergency department presentations
• Need for closed reduction under sedation
• High risk of recurrence
• Psychological trauma and fear of movement
• Potential for subsequent revision surgery
• Reduced patient satisfaction scores

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 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 (larger heads, smaller necks) increases range before impingement
• Neck-cup impingement: Most common cause of mechanical dislocation
• Liner design: Elevated liners, lipped liners increase coverage
• Offset: Restores abductor moment arm and soft tissue tension

Positioning Factors:

• Acetabular inclination (abduction): Affects superior and medial coverage
• Acetabular anteversion: Affects anterior and posterior coverage
• Femoral anteversion: Works with acetabular version for combined stability
• Leg length: Adequate length provides soft tissue tension
• Offset: Lateral offset tensions abductors

Pathophysiology of Dislocation

Mechanism by Direction:

Posterior Dislocation (75-80%):

• Most common after posterior approach
• Mechanism: Hip flexion + internal rotation + adduction
• Common scenarios: Sitting to standing, getting out of low chair, tying shoes
• Posterior capsule disruption or inadequate repair
• Posterior wall deficiency (primary or iatrogenic)
• External rotator insufficiency

Anterior Dislocation (15-20%):

• More common after anterior or anterolateral approach
• Mechanism: Hip extension + external rotation + adduction
• Common scenarios: Stepping backward, arising from bed
• Anterior capsule disruption
• Excessive acetabular anteversion
• Psoas impingement on anterior neck

Superior Dislocation (Rare):

• Requires significant force or component failure
• Associated with severe abductor dysfunction
• May indicate component loosening or catastrophic wear

Component Malposition Leading to Dislocation:

Acetabular Cup Malposition:

• Excessive inclination (over 50°): Reduced superior coverage, posterior instability
• Insufficient inclination (under 30°): Medial impingement, reduced range of motion
• Excessive anteversion (over 25°): Anterior instability, posterior impingement
• Insufficient anteversion (under 5°): Posterior instability, anterior impingement
• Retroversion: High risk of posterior dislocation

Femoral Component Malposition:

• Excessive anteversion: Anterior instability
• Excessive retroversion: Posterior instability
• Varus positioning: Reduces offset, decreases soft tissue tension
• Undersized stem: Inadequate offset restoration

Combined Version Issues:

• Combined anteversion outside 25-45° increases risk
• Can compensate for individual component malposition
• Must consider both components together

Classification by Direction

Standard Directional Classification:

Posterior (75-80%):

• Most common type
• Associated with posterior approach
• Femoral head displaced posterosuperiorly
• May have posterior wall fracture

Anterior (15-20%):

• Associated with anterior/anterolateral approaches
• Femoral head displaced anteroinferiorly
• May have anterior column injury

Superior (Rare):

• Usually associated with component failure or severe abductor deficiency
• May indicate loosening or catastrophic wear

Classification by Timing:

Early (Under 3 months):

• Most common period (50% of dislocations)
• Usually related to component malposition or surgical technique
• Soft tissue healing incomplete
• Patient learning hip precautions

Intermediate (3-12 months):

• Soft tissues healed but patient compliance variable
• May indicate borderline component position
• Abductor dysfunction becoming apparent

Late (Over 1 year):

• Consider polyethylene wear
• Component loosening
• Soft tissue attenuation
• Progressive abductor dysfunction
• New neurological issues

Classification by Mechanism (Woo and Morrey)

Type I: Traumatic

• High-energy trauma overcomes well-positioned stable THA
• Normal component position
• Adequate soft tissue envelope
• Typically isolated event
• Good prognosis with closed reduction

Type II: Positional/Technical

• Component malposition as primary cause
• Outside safe zones
• Impingement as mechanism
• Recurrent unless components revised
• Poor prognosis with conservative treatment

Type III: Component-Related

• Polyethylene wear
• Component loosening
• Liner dissociation
• Head-taper junction failure
• Requires revision surgery

Type IV: Patient-Related

• Neuromuscular disease (Parkinson's, stroke)
• Cognitive impairment (dementia, delirium)
• Non-compliance with precautions
• Abductor deficiency
• Challenging to manage, may need constrained/dual mobility

Patient Risk Factors

Demographic Factors:

• Age over 70 years: 2-3x increased risk
• Female sex: 1.5-2x increased risk (anatomical factors, lower muscle mass)
• BMI: Both extremes increase risk (under 20 or over 35)
• Frailty: Significant independent risk factor

Neuromuscular Factors (Highest Risk):

• Parkinson's disease: 5-10x increased risk
• Prior stroke with residual deficits: 3-5x increased risk
• Cerebral palsy or neuromuscular disorders: Very high risk
• Cognitive impairment or dementia: 3-4x increased risk
• Alcohol or substance abuse: Non-compliance with precautions
• Psychiatric disorders: Variable compliance

Medical Comorbidities:

• Previous ipsilateral hip surgery: 2-3x increased risk
• Developmental dysplasia (DDH): Anatomical factors
• Prior femoral fracture: Altered anatomy
• Inflammatory arthropathy: Soft tissue quality
• Abductor muscle damage or denervation: Critical stability loss

Surgical Risk Factors

Surgical Approach:

• Posterior approach: 2-5% (higher without soft tissue repair)
• Posterior approach with enhanced repair: 1-2% (comparable to other approaches)
• Anterolateral: 1-3%
• Direct anterior: 0.5-2% (lowest in experienced hands)
• Revision surgery: 10-25% regardless of approach

Component-Related Factors:

Acetabular Cup:

• Malposition outside Lewinnek safe zones: 2-4x increased risk
• Excessive inclination (over 50°): Posterior instability
• Excessive anteversion (over 25°): Anterior instability
• Retroversion: Significant posterior instability risk
• Insufficient anteversion (under 5°): Posterior instability

Femoral Component:

• Excessive anteversion: Anterior instability
• Excessive retroversion: Posterior instability
• Undersized stem: Inadequate offset and tension
• Varus positioning: Reduced offset

Bearing Surface and Head:

• Head size under 32mm: 2-3x increased risk vs 36mm
• 28mm heads: Highest risk (largely abandoned)
• Head-neck ratio: Low ratio (thick necks) increases impingement
• Modular necks: Potential for malposition if incorrectly assembled

Technical Factors:

• Inadequate soft tissue repair: Especially posterior capsule and external rotators
• Failure to restore offset: Reduces soft tissue tension
• Leg length discrepancy: Overlengthening or shortening both problematic
• Unrecognized intraoperative instability: Should test stability before closure
• Abductor muscle damage: Superior gluteal nerve injury, muscle detachment

Clinical Presentation

Acute Dislocation:

Patient History:

• Sudden onset of severe hip pain
• Precipitating movement or fall
• Sensation of "something went out"
• Inability to move the leg
• Inability to weight bear
• Previous dislocation episodes (if recurrent)

Mechanism History:

• Posterior: Sitting to standing, getting out of low chair, tying shoes, getting out of car
• Anterior: Stepping backward, arising from bed, external rotation movements
• Traumatic: Fall or significant impact

Physical Examination:

Posterior Dislocation (Most Common):

• Hip in flexion, adduction, and internal rotation
• Leg appears shortened (1-3cm)
• Greater trochanter prominent
• Severe pain with any attempted movement
• Inability to straight leg raise
• Leg cannot be placed in neutral position

Anterior Dislocation:

• Hip in extension, abduction, and external rotation
• Leg may appear lengthened or neutral
• Femoral head may be palpable in groin (rare)
• Severe pain with movement

Neurovascular Examination (CRITICAL):

• Sciatic nerve assessment: Ankle dorsiflexion (common peroneal), plantarflexion (tibial)
• Sensation: Dorsal foot (common peroneal), plantar foot (tibial)
• Document thoroughly before reduction attempt
• Sciatic nerve injury occurs in 10-20% of posterior dislocations
• Usually neuropraxia, but can be permanent
• Femoral nerve assessment in anterior dislocations (rare injury)
• Vascular injury extremely rare but must assess pulses

Diagnostic Workup

Imaging Studies:

Radiographs (Essential):

Plain AP Pelvis and Lateral Hip:

• Confirm dislocation direction
• Assess component position (after reduction)
• Look for associated fractures (acetabular wall, femoral)
• Identify loose components or liner dissociation
• Compare to immediate post-operative radiographs

Post-Reduction Films:

• Confirm concentric reduction
• AP pelvis for cup inclination measurement
• Lateral view for cup anteversion assessment (cross-table or frog lateral)
• Assess for new fractures from reduction
• Shenton's line continuity

CT Scan (Often Essential):

Indications for CT:

• Recurrent dislocation (assess component position precisely)
• Planning for revision surgery
• Suspected acetabular or femoral fracture
• Assess for impingement sites
• Measure combined anteversion accurately
• 3D reconstruction helpful for surgical planning

CT Analysis:

• Precise acetabular inclination and anteversion
• Femoral version
• Combined anteversion calculation
• Impingement analysis (if software available)
• Bone stock assessment
• Occult fractures

Metal Artifact Reduction Sequence (MARS) MRI:

• Rarely needed for dislocation workup
• May be useful for assessing abductor integrity
• Soft tissue pathology (infection, tumor)
• Typically not first-line investigation

Assessment of Underlying Cause

Systematic Evaluation for Recurrent Instability:

Component Position Analysis:

1. Measure acetabular inclination (AP pelvis radiograph)
2. Estimate acetabular anteversion (lateral radiograph or CT)
3. Assess femoral version (CT if available)
4. Calculate combined anteversion
5. Compare to Lewinnek safe zones
6. Identify malpositioned component(s)

Impingement Analysis:

• Neck-cup impingement sites (CT 3D or fluoroscopy)
• Anterior impingement (psoas tendon, anterior acetabular rim)
• Posterior impingement (ischium, posterior wall)
• Component design factors (head-neck ratio)

Soft Tissue Factors:

• Abductor muscle function (Trendelenburg test, MRI if needed)
• Leg length and offset (compare to contralateral, pre-op templating)
• Soft tissue tension (clinical assessment, intraoperative if revision)

Patient Factors:

• Compliance with hip precautions
• New neurological symptoms or diagnosis
• Cognitive status assessment
• Falls risk assessment
• Social situation and support

Dislocation Risk Stratification:

• First dislocation, clear traumatic mechanism, good component position: Low recurrence risk
• First dislocation, component malposition: High recurrence risk, consider early revision
• Second dislocation: Very high recurrence risk, revision typically indicated
• Third or more dislocations: Revision mandatory

Plain Radiography

Standard Views:

AP Pelvis (Essential):

• Both hips for comparison
• Assess cup inclination (measure angle between cup opening plane and inter-teardrop line)
• Compare component position to immediate post-op films
• Look for component migration or loosening
• Assess for acetabular or femoral fractures
• Identify polyethylene wear (head position within cup)
• Check for heterotopic ossification

Lateral Hip View:

• Cross-table lateral or frog lateral
• Estimate cup anteversion (ellipse method, but CT more accurate)
• Assess anterior or posterior wall integrity
• Confirm concentric reduction post-reduction
• Femoral component profile

Post-Reduction Films (Mandatory):

• Confirm concentric reduction (no residual subluxation)
• Assess for new iatrogenic fractures
• Document final component relationship
• Baseline for future comparison

Radiographic Measurements:

Acetabular Inclination:

• Angle between cup opening plane and horizontal reference (inter-teardrop line)
• Normal: 40° ± 10° (30-50°)
• Over 50°: Increased posterior instability risk
• Under 30°: Medial impingement risk

Acetabular Anteversion (Radiographic Estimate):

• Less accurate on plain films
• Ellipse method on lateral view
• CT gold standard for precise measurement
• Normal: 15° ± 10° (5-25°)

Computed Tomography

CT Scan Protocol:

• Indication: Recurrent dislocation or planning revision surgery
• Coverage: Pelvis and proximal femur to below lesser trochanter
• Thin cuts: 1-2mm slices for accurate measurements
• 3D reconstruction: Helpful for surgical planning
• Bilateral if assessing version symmetry

CT Analysis:

Acetabular Measurements:

• Inclination: Most accurate on coronal reconstructions
• Anteversion: Axial cuts at superior dome level
• Version measurement: Angle between acetabular opening plane and AP axis of pelvis
• Bone stock assessment for revision planning

Femoral Measurements:

• Femoral neck version: Angle between femoral neck axis and posterior femoral condyles
• Assess on axial cuts
• Stem version: Stem axis relative to femoral anatomic axis

Combined Anteversion:

• Combined AV = Acetabular Anteversion + Femoral Anteversion
• Target range: 25-45°
• McKibbin's formula (traditional): Acetabular AV + Femoral AV = 37° ± 10°
• More important than individual component positions

Impingement Assessment:

• Software-based impingement detection (if available)
• Manual assessment of bony proximity
• Anterior impingement: Psoas tendon, anterior acetabular rim
• Posterior impingement: Ischium, posterior wall, component-component

3D Reconstruction:

• Visualize complex anatomy (DDH, revision cases)
• Surgical planning for component removal and reimplantation
• Patient education and consent discussions
• Identification of bone defects

Additional Investigations

Laboratory Studies:

• Generally not indicated for acute dislocation
• If revision planned or infection suspected:
  • ESR and CRP (baseline, trend if elevated)
  • White cell count
  • Aspiration if infection concern (cell count, culture)

Aspiration:

• Not routine for dislocation
• Indications: Suspected infection, unexplained pain, elevated inflammatory markers
• Cell count and differential
• Culture (aerobic, anaerobic, fungal if high suspicion)
• Alpha-defensin or other biomarkers

Neuromuscular Assessment:

• Formal neurology consult if persistent nerve deficit post-reduction
• EMG/NCS if sciatic nerve injury (typically delayed 3-4 weeks)
• Physical therapy assessment of abductor function
• Gait analysis if available (research/tertiary centers)

Principles of Revision for Instability

Pre-Operative Planning:

Imaging Analysis:

• CT scan with 3D reconstruction
• Precise measurement of cup inclination and version
• Femoral version measurement
• Combined anteversion calculation
• Identify malpositioned component(s)
• Assess bone stock
• Plan target positions for revision components

Implant Selection:

• Have full revision system available
• Dual mobility components (if converting)
• Constrained liners (backup option)
• Larger head sizes available
• Augments for bone defects
• Consider patient-specific factors (age, activity, neuromuscular status)

Surgical Approach:

• Typically use same approach as index surgery
• May use alternative approach if index approach contributed to instability
• Posterior approach most common for revision
• Extensile approaches if extensive revision needed

Recurrent Dislocation

Definition and Incidence:

• Two or more dislocation episodes
• Occurs in 25-50% of patients after first dislocation
• Risk increases dramatically with each subsequent dislocation (60-75% after second event)

Risk Factors for Recurrence:

Patient Factors:

• Non-compliance with hip precautions (most common)
• Neuromuscular disease progression
• Cognitive decline
• Persistent high-risk behaviors

Surgical Factors:

• Unrecognized component malposition
• Failure to address underlying cause
• Inadequate soft tissue repair
• Inappropriate head size selection
• Impingement not corrected

Management:

• Systematic workup to identify cause (CT scan mandatory)
• Non-operative treatment rarely successful
• Revision surgery typically indicated after second dislocation
• Address underlying cause (component malposition, soft tissue deficiency)
• Consider dual mobility or constrained liner
• Counsel patient on high risk of further recurrence if cause not corrected

Prevention Strategies:

• Correct identification of dislocation direction and mechanism
• CT imaging after first or second dislocation
• Early revision if clear malposition identified
• Enhanced rehabilitation and precautions education

Sciatic Nerve Injury

Incidence:

• Acute dislocation: 10-20% have some degree of sciatic nerve dysfunction
• Usually neuropraxia (stretching injury)
• Permanent deficit: 1-2%

Mechanism:

• Nerve stretched during dislocation (especially posterior)
• May be worsened by closed reduction
• Rarely, nerve becomes entrapped in joint
• Pre-existing nerve compromise from index surgery

Clinical Presentation:

Common Peroneal Division (Most Common):

• Foot drop (ankle dorsiflexion weakness)
• Numbness dorsal foot and web space between 1st and 2nd toes
• High steppage gait

Tibial Division:

• Plantarflexion weakness (less functionally limiting)
• Numbness plantar foot
• Loss of intrinsic foot function

Assessment:

• Document detailed motor and sensory examination before reduction
• Repeat examination after reduction (critical to document changes)
• Compare to pre-operative baseline if available
• EMG/NCS at 3-4 weeks if deficit persists (allows wallerian degeneration)

Management:

Acute:

• Gentle reduction technique to minimize further injury
• If deficit worsens after reduction, consider nerve entrapment (urgent MRI/exploration)
• Ankle-foot orthosis (AFO) for foot drop
• Physical therapy to prevent contractures
• Protect anesthetic areas from pressure injuries

Chronic:

• Most recover over 6-12 months if neuropraxia
• Serial clinical examinations and EMG/NCS
• Continue AFO until recovery
• Tendon transfer if no recovery by 12-18 months (posterior tibial tendon transfer for foot drop)
• Patient education on permanent deficit possibility

Prognosis:

• Neuropraxia (stretch): 80-90% full recovery over months
• Axonotmesis: Partial recovery common
• Neurotmesis: No recovery, need reconstruction
• Earlier recovery onset predicts better outcome

Acetabular or Femoral Fracture

Mechanisms:

• Dislocation event itself (posterior wall fracture most common)
• Closed reduction attempt (can fracture osteoporotic bone)
• Open reduction with excessive force

Acetabular Fractures:

Posterior Wall Fracture:

• Most common fracture with posterior dislocation
• May be occult on initial radiographs
• CT scan often needed to identify
• Small fragments may not affect stability
• Large fragments require fixation (over 25-30% of wall)

Management:

• Small fragment, stable after reduction → Non-operative
• Large fragment, unstable → Open reduction and internal fixation
• May need revision to larger head or dual mobility for stability
• Standard posterior wall fixation techniques (lag screws, buttress plate)

Femoral Fractures:

• Periprosthetic femoral fracture during dislocation (rare)
• Vancouver classification applies
• Treatment based on fracture pattern and stem stability
• May occur during closed reduction attempt (gentle technique essential)

Iatrogenic Fractures:

• Can occur during revision surgery
• Greater trochanter fracture during exposure
• Acetabular fracture during cup removal or insertion
• Femoral fracture during stem removal
• Intraoperative recognition and fixation critical

Component Wear and Loosening

Accelerated Polyethylene Wear:

• Recurrent dislocation causes repetitive trauma to bearing surface
• Increased wear compared to stable THA
• May lead to osteolysis
• Late dislocations may be due to wear and laxity

Component Loosening:

• Constrained liners increase forces at cup-bone interface
• Can accelerate cup loosening
• Multiple dislocations may damage fixation
• Progressive radiolucent lines on serial radiographs
• May present as late dislocation when component migrates

Assessment:

• Serial radiographs (compare to immediate post-op films)
• Look for progressive radiolucent lines
• Component migration
• Osteolysis
• Changing component position

Management:

• Aseptic loosening requires revision
• Osteolysis may need bone grafting
• Correct component malposition at time of revision
• Address wear (larger heads, dual mobility, highly cross-linked polyethylene)

Psychological Impact

Patient Concerns:

• Fear of recurrent dislocation
• Anxiety about movement
• Depression
• Reduced quality of life
• Fear of falling
• Avoidance of activities
• Dissatisfaction with surgery outcome

Impact on Function:

• Self-imposed activity restriction
• Reduced independence
• Need for ongoing assistive devices
• Home modifications
• Caregiver burden

Management:

• Patient education and counseling
• Realistic expectations about recurrence risk
• Psychological support if needed
• Gradual return to activities with supervision
• Support groups or peer counseling
• Emphasis on what patient CAN do safely

Pre-Operative Risk Stratification

Identify High-Risk Patients:

Very High Risk (Consider Enhanced Stability Implants):

• Prior THA dislocation (same hip)
• Neuromuscular disease: Parkinson's, stroke with residual deficit, cerebral palsy
• Severe cognitive impairment or dementia
• Abductor deficiency or damage from previous surgery
• Revision THA for instability
• Tumor resection with massive soft tissue loss
• Multiple previous hip surgeries
• Age over 80 with frailty

Moderately High Risk (Modify Surgical Technique):

• Age 70-80
• Revision THA (non-instability indication)
• Previous hip surgery (fracture fixation, prior arthroplasty)
• Developmental dysplasia with anatomical abnormalities
• Inflammatory arthropathy
• Morbid obesity
• Neuromuscular disease (milder forms)

Risk Stratification Impact on Surgical Plan:

Very High Risk Patients:

• Dual mobility prosthesis (primary or revision)
• Consider direct anterior approach (if experienced)
• Larger head size if standard bearing (36-40mm)
• Extended rehabilitation with precautions
• Consider constrained liner in salvage situations

Moderately High Risk:

• Head size ≥36mm
• Meticulous soft tissue repair (especially posterior approach)
• Extended hip precautions (12 weeks vs 6 weeks)
• Hip abduction brace
• Enhanced patient education

Surgical Technique Optimization

Component Positioning (Most Important):

Acetabular Component:

• Target: 40° inclination, 15° anteversion (Lewinnek safe zones)
• More important: Combined anteversion 25-45°
• Individualize based on femoral version and patient anatomy
• Use navigation if available (improved accuracy, reduced outliers)
• Mechanical alignment guides (bubble levels, alignment rods)
• Intraoperative fluoroscopy
• Avoid excessive inclination (over 50°) - high dislocation risk
• Avoid retroversion - very high posterior dislocation risk

Femoral Component:

• Restore native femoral version (typically 10-15° anteversion)
• Combined with acetabular version for total 25-45°
• Adequate anteversion for anterior approach (reduce posterior risk)
• Avoid excessive retroversion
• Restore offset (lateral offset tensions abductors)
• Appropriate leg length (adequate soft tissue tension without overlengthening)

Head Size Selection:

• Minimum 32mm for all primary THA (modern standard)
• 36mm for most patients (good balance of stability and wear)
• 40mm for high-risk patients if acetabulum large enough
• Larger heads: Increased jump distance, increased impingement-free arc
• Diminishing returns above 40mm, and concerns about taper corrosion
• Never use 28mm heads in modern practice (high dislocation risk)

Soft Tissue Management:

Posterior Approach:

• Meticulous posterior soft tissue repair (critical)
• Repair short external rotators to greater trochanter (transosseous or suture anchors)
• Capsular repair with heavy braided suture
• Posterior soft tissue repair reduces dislocation from 5% to 1-2%
• Consider capsular plication if lax
• Trochanteric slide osteotomy in complex revisions (better healing than muscle detachment)

Anterolateral Approach:

• Minimize gluteus medius damage
• Anatomic repair of gluteus medius to greater trochanter
• Avoid superior gluteal nerve injury
• Restore abductor tension

Direct Anterior Approach:

• Preserve anterior capsule (reduces anterior dislocation risk)
• Avoid excessive external rotation during exposure
• Femoral neck cut adequate but not excessive (prevent anterior notching)
• Capsular closure if disrupted

Offset and Leg Length:

• Restore native offset (±5mm)
• Lateral offset tensions abductors (dynamic stability)
• Adequate leg length for soft tissue tension
• Avoid overlengthening (patient dissatisfaction, nerve injury)
• Avoid shortening (reduces soft tissue tension, instability)
• Use intraoperative measurement techniques (pelvic reference, trial components)

Intraoperative Stability Testing (Essential):

1. Before final component insertion, test with trial components
2. Hip should be stable in:
   • Flexion 90° + internal rotation + adduction (posterior stability)
   • Extension + external rotation + adduction (anterior stability)
3. Assess impingement-free arc
4. If unstable with trials, modify component position or increase head size
5. NEVER accept instability at closure - address immediately

Enhanced Recovery and Patient Education

Pre-Operative Education:

• Hip precautions teaching before surgery
• Video education or physical demonstration
• Written materials for reference
• Setting realistic expectations about recovery and restrictions
• Identify home hazards (low chairs, low toilet, pets)
• Plan for assistive devices and home modifications

Post-Operative Hip Precautions:

Posterior Approach Precautions (Most Common):

1. No hip flexion beyond 90° (for 6-12 weeks)
2. No hip adduction past midline
3. No internal rotation
4. No combined flexion + adduction + internal rotation
5. No low chairs, couches, or toilet seats
6. Use elevated toilet seat
7. No bending to tie shoes (use sock aid, long shoe horn)
8. Sleep supine or on non-operative side with pillow between legs

Anterior Approach Precautions:

1. No hip extension beyond neutral
2. No external rotation
3. No combined extension + external rotation
4. No reaching backward while standing
5. Safe to sit in low chairs (advantage of anterior approach)

Duration:

• Minimum 6 weeks for soft tissue healing
• High-risk patients: 12 weeks
• Some surgeons advocate lifelong precautions for high-risk movements
• Dual mobility patients: May have reduced restrictions (surgeon-dependent)

Hip Abduction Brace:

• High-risk patients (first dislocation or high-risk primary)
• Fitted in neutral to 15-20° abduction
• Wear 24 hours/day initially (including sleep)
• May remove for hygiene and supervised therapy
• Duration: 6-12 weeks
• Wean gradually (day use first, then discontinue)

Physical Therapy Protocol:

• Early mobilization with assistive device (walker or crutches)
• Precautions reinforcement
• Abductor strengthening (critical for dynamic stability)
• Progressive return to activities
• Gait training
• Home safety assessment

Long-Term Patient Education:

• Lifelong awareness of high-risk positions
• Certain activities may always carry risk (contact sports, extreme yoga)
• Teach patients to recognize instability symptoms
• Immediate medical attention if dislocation suspected
• Annual follow-up for surveillance

Immediate Postoperative Period (0-6 weeks)

Day 0-1 (Hospital):

• Hip precautions initiated immediately
• Hip abduction pillow or brace if high risk
• Neurovascular checks (especially if closed reduction performed)
• Pain management (multimodal analgesia)
• DVT prophylaxis (per institutional protocol)
• Physical therapy evaluation and mobilization

Day 1-3 (Hospital Discharge):

• Mobilization with assistive device (walker initially)
• Stair training if needed
• Hip precautions education reinforcement
• Home safety assessment and planning
• Assistive devices arranged (elevated toilet seat, reacher, sock aid, long shoe horn)
• Discharge when safe mobilization achieved

Week 1-2:

• First post-operative visit
• Wound check (if primary surgery)
• Radiographs (AP pelvis, lateral hip)
• Assess for early dislocation or complications
• Hip precautions reinforcement
• Physical therapy prescription
• Pain management optimization

Week 2-6:

• Progressive mobilization
• Wean assistive device (walker → cane → none) as strength improves
• Abductor strengthening exercises
• Hip precautions continued strictly
• Avoid prolonged sitting or high-risk positions
• Hip abduction brace (if used) continued 24 hours/day

Intermediate Period (6-12 weeks)

Week 6 Visit:

• Clinical examination
• Radiographs (assess component position, no migration)
• Assess abductor strength (Trendelenburg test)
• Begin weaning hip abduction brace (if used)
• Liberalize activity restrictions gradually
• May discontinue assistive device if gait stable
• Continue hip precautions (especially for high-risk movements)

Week 6-12:

• Progressive strengthening
• Return to low-impact activities (walking, swimming, cycling)
• Hip precautions continued but can be more liberal
• Most patients independent with mobility
• Driving (if right hip and off opioids, surgeon-dependent)
• Light work or activities

Week 12 Visit:

• Final short-term follow-up
• Clinical examination (range of motion, strength, gait)
• Radiographs (component position stable)
• Discontinue formal hip precautions if low risk and stable
• Educate on lifelong awareness of extreme positions
• Return to most activities of daily living
• Gradual return to higher-impact activities over next 3-6 months

Long-Term Follow-Up

3-6 Months:

• Return to unrestricted activities (if stable and no complications)
• High-impact activities at surgeon discretion
• Abductor strengthening maintenance program
• Radiographs (if revision surgery, otherwise may defer to 1 year)

1 Year:

• Annual visit (standard for all THA)
• Clinical examination
• Radiographs (AP pelvis, lateral hip)
• Assess for late complications (wear, loosening, late dislocation)
• Patient education on warning signs

Long-Term (Annual):

• Annual clinical and radiographic surveillance
• Earlier if symptoms (pain, instability sensation, reduced function)
• Monitor for late dislocation (may indicate wear or loosening)
• Serial radiographs to assess wear and component position
• Lifelong follow-up recommended for all THA

Red Flags for Urgent Evaluation:

• Sensation of instability or near-dislocation
• Acute dislocation (immediate ED presentation)
• New onset of severe pain
• Leg length change
• Inability to weight bear
• New neurological symptoms

Outcomes by Treatment Strategy

Non-Operative Treatment (First Dislocation):

• Success rate (no recurrence): 50-75%
• Better outcomes if: Well-positioned components, traumatic mechanism, good patient compliance
• Worse outcomes if: Malpositioned components, atraumatic dislocation, neuromuscular disease, cognitive impairment
• Hip precautions and brace critical for 6-12 weeks

Isolated Liner Exchange:

• Success rate: 60-80% if well-positioned cup
• Better with larger head size (36-40mm)
• Better with dual mobility liner vs standard liner
• Fails if underlying malposition not addressed

Acetabular Revision:

• Success rate: 80-90% with dual mobility
• 70-85% with standard bearing (large head)
• Higher failure if multiple previous dislocations
• Critical to achieve target combined anteversion 25-45°
• Navigation may improve component positioning accuracy

Dual Mobility:

• Dislocation rate: 0.5-2% (primary use)
• Dislocation rate: 2-5% (revision setting)
• Intraprosthetic dislocation: 0.5-1%
• Re-revision rate: 5-10% at 5 years
• Best outcomes for recurrent instability in current evidence

Constrained Liners:

• Mechanical dislocation prevention: 80-90%
• But reoperation rate: 15-25% for locking ring failure, loosening
• Less favorable than dual mobility in most comparative studies
• Should be salvage option

AOANJRR Data on Dislocation

Key Registry Findings:

Dislocation as Revision Indication:

• Leading cause of early revision (within 1 year): 22.3% of all revisions
• Remains significant at 1-5 years: 15.1% of revisions
• Decreases at 5+ years: 8.3% of revisions (late dislocations often due to wear/loosening)
• Higher burden than infection in early period

Approach Differences:

• Posterior approach: Highest dislocation revision rate
• Anterolateral approach: Intermediate rate
• Direct anterior approach: Lowest dislocation revision rate (hazard ratio 0.7 vs posterior)
• Posterior approach with enhanced soft tissue repair: Approaching anterior approach rates

Bearing Surface Impact:

• Dual mobility: Lowest dislocation revision rate (0.5-1% cumulative)
• Large heads (≥36mm): Lower than smaller heads
• 28mm heads: Significantly higher (rarely used in modern practice)

Re-Revision After Instability Revision:

• Cumulative re-revision rate at 5 years: 25%
• Higher than revisions for other indications (15% average)
• Highlights difficulty of managing recurrent instability

Implications for Practice:

• Dislocation prevention should be priority in all THAs
• Consider anterior approach if experienced (lower dislocation rate)
• Enhanced posterior repair techniques if using posterior approach
• Head size ≥36mm as standard
• Dual mobility for high-risk patients
• Address malposition early rather than multiple closed reductions

PBS and Medicare Considerations

PBS Medication Access:

• Analgesia: Standard post-operative pain medications available
• DVT prophylaxis: LMWH or oral anticoagulants (rivaroxaban, apixaban)
• No specific PBS restrictions for THA complications

Private Health Insurance:

• Waiting periods for joint replacement: 12 months (pre-existing condition)
• Acute dislocation management: Immediate coverage (emergency)
• Revision surgery: Covered if acute complication of covered procedure
• Gap payments vary by insurer and surgeon

Public Hospital Access:

• Category 2 urgency for primary THA (within 90 days)
• Category 1 urgency for acute dislocation (within 24 hours)
• Revision for recurrent instability: Category 2-3 depending on functional impact
• Waiting times vary significantly by state and hospital

Australian Guidelines and Standards

ANZHFR (Australian and New Zealand Hip Fracture Registry):

• Does not specifically track THA dislocation (focuses on fragility fractures)
• Relevant for periprosthetic fractures associated with dislocation

Therapeutic Guidelines (eTG):

DVT Prophylaxis Post-THA:

• LMWH (enoxaparin 40mg SC daily) or
• Rivaroxaban 10mg PO daily or
• Apixaban 2.5mg PO BD
• Duration: 10-14 days minimum, up to 35 days for high-risk patients
• Mechanical prophylaxis (compression stockings, intermittent pneumatic compression)

Antibiotic Prophylaxis:

• Cefazolin 2g IV (or 3g if over 120kg) within 60 minutes of incision
• Repeat dosing if surgery over 4 hours or blood loss over 1500mL
• Alternatives: Vancomycin if MRSA risk or beta-lactam allergy
• No proven benefit to extending beyond 24 hours

Imaging Guidelines:

• Radiographs mandatory for acute dislocation diagnosis
• CT recommended for recurrent dislocation or revision planning
• MRI rarely indicated (abductor assessment in specific cases)
• No specific Medicare restrictions on imaging for THA complications