Dual Mobility Total Hip Arthroplasty

Historical evolution of dual mobility designs:

Key improvements over time:

• Liner retention mechanisms reduced IPD from 5-10% to under 1%
• HXLPE reduced volumetric wear rates significantly
• Uncemented options improved long-term fixation
• Modular designs allow revision flexibility

Evolution from first to third generation transformed dual mobility from experimental to mainstream.

Classification by bearing surface:

1. Standard dual mobility:

• Cobalt-chrome or stainless steel head
• HXLPE mobile liner
• Titanium shell with porous coating
• Most common configuration

2. Ceramic head dual mobility:

• Ceramic femoral head (alumina or delta ceramic)
• HXLPE mobile liner
• Theoretical reduced wear at primary articulation
• Less common, higher cost

3. Modular dual mobility:

• Can convert existing well-fixed shell
• Cemented dual mobility liner into retained shell
• Useful for revision scenarios
• Saves bone stock

Bearing selection considerations:

• Standard cobalt-chrome most common and cost-effective
• Ceramic head may reduce wear but limited long-term data for dual mobility
• Modular designs valuable for retaining stable shells in revision

Standard metal-on-HXLPE dual mobility is the workhorse for most applications.

Classification by fixation:

Cemented dual mobility:

• Metal-backed polyethylene cup cemented to bone
• Historical designs predominantly cemented
• Still used in elderly patients with poor bone quality
• Immediate fixation advantage
• Higher loosening risk long-term

Uncemented dual mobility:

• Porous coated titanium shell for bone ingrowth
• Modern standard for most patients
• Superior long-term fixation in good bone
• Requires adequate bone quality
• Biological fixation

Hybrid approach:

• Uncemented shell with cemented liner insert
• Revision-specific technique
• Allows retention of stable shell
• Cemented dual mobility liner into existing shell

Selection principles:

• Young patients with good bone: Uncemented preferred
• Elderly with osteoporotic bone: Cemented may be appropriate
• Revision with stable shell: Hybrid cemented liner technique

Modern practice favors uncemented fixation for most primary and revision cases.

High-risk patients for dislocation:

Neuromuscular disorders:

• Parkinson's disease (impaired proprioception, rigidity)
• Cerebral palsy (spasticity, muscle imbalance)
• Multiple sclerosis (weakness, spasticity)
• Prior stroke with residual weakness or spasticity
• Muscular dystrophy

Cognitive impairment:

• Dementia or Alzheimer's disease
• Inability to follow hip precautions
• Psychiatric disorders affecting compliance

Anatomical risk factors:

• Abductor muscle deficiency or tears
• Prior hip surgery with soft tissue damage
• Spinopelvic imbalance (flat lumbar spine, fixed sagittal imbalance)
• High BMI (over 35) with increased soft tissue laxity
• Developmental dysplasia of the hip (DDH) with abnormal anatomy

Medical risk factors:

• High fall risk (multiple comorbidities, frailty)
• Poor compliance anticipated
• Substance abuse affecting adherence

Femoral neck fracture in elderly:

• Displaced intracapsular fracture in frail elderly
• High dislocation risk (cognitive impairment, prior falls)
• Either hemiarthroplasty or THA with dual mobility

These indications reflect patients with elevated baseline dislocation risk where dual mobility offers protective benefit.

Recurrent instability:

• Gold standard indication: Revision for recurrent dislocation
• Previous THA with two or more dislocations
• Atraumatic dislocation pattern
• Failed prior stability measures (constrained liner, head size change)

Complex revision scenarios:

• Massive bone loss requiring structural grafts
• Abductor deficiency not amenable to repair
• Trochanteric nonunion or loss
• Radiation-treated pelvis (soft tissue compromise)

Infection treatment:

• Two-stage revision for chronic infection
• Higher dislocation risk with soft tissue damage from infection
• Dual mobility at reimplantation stage

Oncologic reconstruction:

• Proximal femoral replacement for tumor
• Acetabular reconstruction after tumor resection
• Significantly increased instability risk

Revision for instability is the strongest evidence-based indication for dual mobility with excellent outcomes.

Absolute contraindications:

• Active infection: Must treat infection first
• Inadequate bone stock: Cannot achieve stable shell fixation
• Severe acetabular deficiency: May require alternative reconstruction

Relative contraindications:

• Young, high-demand patients: Wear concerns with higher activity
• Severe obesity preventing safe surgery: BMI over 50 (surgical challenge, not implant issue)
• Insufficient acetabular bone: May require augments or custom components

Technical considerations:

• Minimum acetabular bone for shell fixation required
• Adequate soft tissue coverage
• Ability to achieve stable component position

Patient factors:

• Very young patients (under 50) may prefer standard THA if low dislocation risk
• High activity level athletes (wear considerations)
• Patient preference for proven long-term designs

The main debate is around young, active patients where long-term wear and revision burden must be weighed against stability benefits.

Algorithm for dual mobility in primary THA:

Step 1: Assess dislocation risk

• Low risk: Young, active, no comorbidities → Standard THA appropriate
• Moderate risk: Elderly, high BMI, mild cognitive issues → Discuss options
• High risk: Neuromuscular, dementia, abductor deficiency → Recommend dual mobility
• Very high risk: Multiple risk factors combined → Strong dual mobility indication

Step 2: Patient counseling

• Explain dual mobility benefits (reduced dislocation 0.5-2% vs 3-5%)
• Discuss unique complication (IPD under 1%)
• Review recovery and precautions (may be relaxed with dual mobility)
• Shared decision-making for moderate risk patients

Step 3: Component selection

• Choose appropriate dual mobility system
• Size acetabular shell based on native acetabulum
• Select femoral stem based on canal geometry
• Plan combined anteversion (cup + stem = 25-40 degrees)

Step 4: Surgical approach

• Select approach based on surgeon experience
• Posterior most common for dual mobility
• Anterior or anterolateral also suitable
• Standard soft tissue protection regardless of approach

This structured approach ensures appropriate dual mobility use in primary THA.

Algorithm for revision THA instability:

Step 1: Confirm instability pattern

• Number of dislocations (two or more = recurrent instability)
• Direction (posterior, anterior, multidirectional)
• Timing (early under 90 days vs late)
• Associated factors (trauma vs atraumatic)

Step 2: Assess components

• Imaging: X-ray, CT for component position
• Lewinnek safe zone: Inclination 30-50 degrees, anteversion 5-25 degrees
• Combined anteversion: Sum of cup and stem 25-40 degrees
• Component wear or loosening
• Impingement sites

Step 3: Identify causes

• Component malposition (most common correctable cause)
• Abductor deficiency (examination, MRI)
• Spinopelvic imbalance (sitting/standing lateral radiographs)
• Patient factors (cognitive, neuromuscular, falls)
• Bone or soft tissue deficiency

Step 4: Treatment decision

• First choice: Dual mobility revision (gold standard)
• If well-fixed shell within 5 degrees of ideal: Retain shell + cement dual mobility liner
• If malpositioned or loose shell: Revise to new dual mobility shell
• If stem malpositioned or loose: Revise stem with appropriate version
• Address abductor deficiency if present (repair, trochanteric advancement)

Step 5: Alternative options (inferior to dual mobility)

• Constrained liner: Higher failure rate (10-15% vs 2-5% for dual mobility)
• Large head revision: Requires stem revision, less effective
• Repositioning components: May not address all causes

Evidence favors dual mobility as first-line treatment for recurrent instability.

Algorithm for femoral neck fracture in elderly:

Step 1: Confirm indication for arthroplasty

• Displaced intracapsular femoral neck fracture (Garden III/IV)
• Elderly patient (typically over 65-70 years)
• Pre-injury ambulatory status
• Life expectancy allowing benefit from arthroplasty

Step 2: Choose arthroplasty type

• Hemiarthroplasty: Less active, limited life expectancy, significant comorbidity
• Total hip arthroplasty: More active, longer life expectancy, acetabular arthritis
• Dual mobility consideration: High dislocation risk patients in either group

Step 3: Assess dislocation risk

• High risk indicators: Cognitive impairment, prior falls, Parkinson's, institutionalized
• Standard risk: Cognitively intact, community dwelling, no neuromuscular disease
• AOANJRR data: Dual mobility reduces dislocation in femoral neck fracture

Step 4: Dual mobility decision

• High risk patients: Strong dual mobility indication (hemi or THA)
• Standard risk: Consider dual mobility vs standard based on activity level and preference
• Growing trend toward dual mobility for all femoral neck fracture arthroplasty

Step 5: Surgical timing and approach

• Surgery within 24-48 hours (fracture urgency)
• Any standard approach suitable
• Cemented femoral stem often preferred in elderly fragile bone
• Standard postoperative care with early mobilization

Dual mobility reduces dislocation burden in this frail, high-risk population.

Algorithm for dual mobility complications:

Intraprosthetic Dislocation (IPD):

Acute presentation:

• Clinical assessment: Leg position, range of motion
• X-ray: Eccentric head, medialized position, double density sign
• Attempt closed reduction under sedation
• Post-reduction X-ray: Confirm concentric reduction

After successful closed reduction:

• First IPD: Trial non-operative with precautions and surveillance
• X-ray follow-up: 2 weeks, 6 weeks, 3 months
• If stable and concentric: Continue observation
• If recurrent IPD: Proceed to surgery

After failed closed reduction or recurrent IPD:

• Open reduction via surgical approach
• Inspect liner for damage or wear
• Exchange liner (mandatory for recurrent IPD)
• Assess for impingement (neck-on-liner)
• Check component position (may need shell revision if malpositioned)
• Reduce with new liner and test stability

True Dislocation (liner and head from shell):

• Same management as standard THA dislocation
• Closed reduction attempt
• Assess for malposition or impingement
• If recurrent: May need revision with component repositioning
• Consider increasing constraints (though already dual mobility)

Wear and osteolysis:

• Surveillance radiographs: Annual for first 5 years
• If progressive osteolysis: Revision with bone grafting and new components
• Modern HXLPE makes this rare

Prompt recognition and appropriate management of complications optimizes outcomes.

Dual mobility can be used with any standard THA approach:

Posterior approach:

• Most commonly used approach for dual mobility
• Excellent acetabular exposure for shell positioning
• Capsular repair important for stability (as with all THA)
• Standard posterior soft tissue repair techniques

Anterolateral approach:

• Good visualization of acetabulum
• Preserves posterior capsule
• Abductor split or detachment techniques

Direct anterior approach:

• Increasingly popular for dual mobility
• Intermuscular interval, no muscle cutting
• Good component positioning
• May be more challenging for larger shells

Key principle: Approach selection based on surgeon experience and patient anatomy, not determined by dual mobility choice.

Acetabular reaming:

• Standard reaming technique: Spherical reaming to bleeding bone
• Ream to native bone: 1-2mm undersize relative to final shell
• Assess bone quality: Ensure adequate fixation for uncemented shell
• Hemostasis: Control bleeding for optimal shell seating

Shell sizing:

• Measure reamed diameter: Final reamer size guides shell selection
• Consider shell outer diameter: Dual mobility shells slightly larger than standard
• Peripheral fit: Ensure rim contact for stability
• Trial shell: Confirm fit and stability before final implant

Shell positioning:

• Target inclination: 35-45 degrees (Lewinnek safe zone)
• Target anteversion: 15-25 degrees
• Combined anteversion: Consider femoral and cup anteversion (Ranawat safe zone)
• Avoid excessive inclination: Increases wear and edge loading risk

Shell fixation:

• Press-fit technique: 1-2mm press-fit for initial stability
• Supplemental screw fixation: As needed for stability (peripheral holes)
• Assess stability: Ensure no micromotion before liner insertion

Proper shell position is critical - malposition increases impingement and IPD risk.

Femoral preparation identical to standard THA:

Broaching/reaming:

• Follow standard technique for chosen stem design
• Cemented or uncemented stem as per surgeon preference
• Ensure appropriate size and fill

Stem positioning:

• Version control: Aim for 10-15 degrees anteversion
• Combined anteversion: Sum of cup and stem anteversion 25-40 degrees (Ranawat)
• Avoid retroversion: Increases posterior impingement risk
• Height: Restore leg length and offset

Trial reduction:

• Trial femoral component with trial head and liner
• Assess stability: Range of motion without impingement
• Check combined version: Ensure no impingement in functional positions
• Leg length and offset: Confirm restoration or appropriate adjustment

Head size selection:

• Typically 22mm or 28mm: Based on stem taper and system design
• Smaller head reduces intraprosthetic friction: May allow more motion
• Larger head increases constraint: Some systems offer choice

Standard femoral technique applies - the dual mobility design is on the acetabular side.

Liner insertion critical step:

1. Liner selection:

• Size based on metal shell inner diameter
• Ensure correct liner-shell matching (manufacturer specific)
• Confirm liner orientation markings

2. Liner insertion technique:

• Clean and dry shell: Remove all blood and debris from shell taper
• Inspect liner: Ensure no damage or debris on liner
• Align liner: Match orientation marks (if present)
• Seated insertion: Press liner into shell ensuring full seating
• Audible/tactile click: Modern designs have locking mechanism confirmation
• Confirm seating: Visual inspection - no gap between liner rim and shell

3. Head-liner assembly:

• Reduce head into liner: May be done on back table or in situ
• Concentric reduction: Ensure head fully seated in liner
• Assess capture: Liner should fully encompass head

4. Trial reduction (if not done):

• Reduce hip with assembled dual mobility construct
• Assess stability through full range of motion
• Check for impingement (neck-on-liner, liner-on-shell)
• Confirm no subluxation or maltracking

5. Final implant:

• Disassemble trial (if used)
• Insert final components
• Reduce final head-liner construct
• Final stability assessment

Common technical errors to avoid:

• Incomplete liner seating (may lead to early IPD)
• Blood or debris in shell taper (prevents proper locking)
• Excessive force during reduction (may damage liner)
• Malpositioned components (increases impingement)

Proper liner insertion technique is critical to prevent early IPD.

Intraprosthetic dislocation (IPD) - unique to dual mobility:

Definition: Dissociation of the polyethylene liner from the femoral head, while the liner remains in the metal shell.

Types:

• Early IPD (under 3 months postop):

  • Usually due to technical error
  • Incomplete liner seating during surgery
  • Impingement (neck-on-liner) levering liner off head
  • Component malposition

• Late IPD (over 3 months postop):

  • Polyethylene liner wear
  • Liner deformation over time
  • Older generation liner retention designs

Clinical presentation:

• Patient feels hip "go out" but different sensation than typical dislocation
• Hip may appear reduced on X-ray (liner still in shell, head medial)
• Eccentric position of femoral head within acetabulum on X-ray
• May have pain or clicking sensation

Radiographic diagnosis:

• AP pelvis X-ray: Femoral head appears medialized or eccentric in cup
• Lateral X-ray: Head not concentrically positioned in liner
• "Double-density" sign: Overlapping shadows of head and liner rim

Management of IPD:

• Acute presentation: Attempt closed reduction (may succeed if liner not damaged)
• Recurrent IPD: Open reduction, liner exchange, assess for impingement or malposition
• Late IPD with wear: Liner exchange, consider shell revision if worn or malpositioned

Prevention:

• Meticulous surgical technique ensuring complete liner seating
• Proper component positioning to avoid impingement
• Modern liner designs with enhanced retention mechanisms

Standard THA complications also apply:

• Infection (same risk as standard THA)
• Periprosthetic fracture (same risk)
• Loosening (potentially higher polyethylene wear load)
• Neurovascular injury (same risk)
• Leg length discrepancy (same risk)