High-yield overview
Two Articulations | Increased Jump Distance | Reduced Dislocation | Beware IPD
2Articulation surfaces (dual)
0.5-2%Dislocation rate vs 2-5% standard
1950sBousquet original design (France)
0.5-1%Intraprosthetic dislocation (IPD) risk
KEY INDICATIONS
Primary THA High Risk
PatternNeuromuscular disease, abductor deficiency, cognitive impairment
TreatmentConsider dual mobility
Revision for Instability
PatternRecurrent dislocation after THA
TreatmentDual mobility standard choice
Hip Fracture in Elderly
PatternDisplaced femoral neck fracture, high fall risk
TreatmentDual mobility hemiarthroplasty or THA
Critical Must-Knows
- Two articulations: small head within mobile polyethylene liner, liner articulates with metal shell
- Increased jump distance from larger effective head size reduces impingement and dislocation
- Intraprosthetic dislocation (IPD) is unique complication - liner dissociates from head
- Primary indications: high-risk primary THA and revision for instability
- AOANJRR data shows excellent survivorship and low revision rates for instability
Clinical Pearls
- "Dual mobility has TWO bearing surfaces - not just a larger head
- "Jump distance is distance the head must travel to dislocate - larger with dual mobility
- "IPD occurs when mobile liner separates from femoral head (early vs late types)
- "Modern designs have improved liner retention reducing IPD risk
Critical Dual Mobility Exam Points
Biomechanical Principle
The dual mobility design creates two articulations: the small femoral head (typically 22-28mm) within a mobile polyethylene liner, and the liner articulating with the metal shell. This increases the effective head-to-neck ratio and jump distance.
Intraprosthetic Dislocation
IPD is the unique complication of dual mobility - the polyethylene liner dissociates from the femoral head. Early IPD (under 3 months) suggests technical error. Late IPD suggests liner wear or design issues.
Primary Indications
Use dual mobility for high-risk patients: neuromuscular disorders (Parkinson's, cerebral palsy), abductor deficiency, cognitive impairment, prior stroke, high BMI, or revision for recurrent instability.
AOANJRR Evidence
Australian registry data shows dual mobility has lower revision rates for dislocation compared to standard THA. Particularly beneficial in revision setting and femoral neck fracture treatment in elderly.
Dual Mobility vs Standard THA Design
| Feature | Standard THA | Dual Mobility THA |
|---|---|---|
| Number of articulations | One (head-liner) | Two (head-liner and liner-shell) |
| Effective head size | Actual head diameter (28-40mm) | Outer liner diameter (typically 40-55mm) |
| Head-to-neck ratio | Based on actual head/neck | Improved by larger effective head |
| Jump distance | Smaller (e.g., 5-8mm for 32mm head) | Larger (e.g., 12-18mm depending on liner) |
| Dislocation rate | 2-5% primary, 10-15% revision | 0.5-2% primary, 2-5% revision |
| Unique complication | None | Intraprosthetic dislocation (0.5-1%) |
| Liner constraint | None (standard) or constrained | Mobile polyethylene liner |
Dual Mobility THA At a Glance
| Category | Key Information |
|---|---|
| Design concept | Two articulations: small head (22-28mm) in mobile PE liner, liner articulates with metal shell |
| Key advantage | Increased jump distance (12-18mm) and effective head size (40-55mm) reduce dislocation |
| Primary indications | High-risk primary THA (neuromuscular, cognitive, abductor deficiency), revision for instability |
| Dislocation rates | 0.5-2% primary (vs 3-5% standard), 2-5% revision (vs 10-15% standard) |
| Unique complication | Intraprosthetic dislocation (IPD) 0.5-1% - liner dissociates from head |
| AOANJRR data | 70% reduction in dislocation revision, comparable overall survivorship (95-98% at 5 years) |
| Historical development | Bousquet (France, 1974), FDA approved USA 2009, modern HXLPE designs low IPD |
| Revision instability | Gold standard treatment - superior to constrained liner or large head revision |
Mnemonic
DUAL - CDUAL - Core Design Principles
| D | Double articulation Two bearing surfaces - head-liner and liner-shell |
| U | Unconstrained liner motion Polyethylene liner freely mobile within metal shell |
| A | Augmented head size Effective head size is outer liner diameter (40-55mm) |
| L | Larger jump distance Increased distance to dislocation reduces instability |
| D | Double articulation Two bearing surfaces - head-liner and liner-shell | A | Augmented head size Effective head size is outer liner diameter (40-55mm) |
| U | Unconstrained liner motion Polyethylene liner freely mobile within metal shell | L | Larger jump distance Increased distance to dislocation reduces instability |
Hook:DUAL reminds you this is about TWO articulations, not just a big head
Mnemonic
BOUSQUET - HBOUSQUET - Historical Development
| B | Bousquet original design French surgeon in 1970s |
| O | Original goal: reduce dislocation High dislocation rates with early THA |
| U | Used small head in large liner 22mm head in large polyethylene cup |
| S | Succeeded in reducing instability But early designs had high IPD rates |
| Q | Quality improved over time Modern designs with enhanced liner retention |
| U | Usage expanded worldwide Particularly Europe, now FDA approved in USA |
| E | Evidence growing Registry data supporting use |
| T | Two main indications High-risk primary and revision for instability |
| B | Bousquet original design French surgeon in 1970s | S | Succeeded in reducing instability But early designs had high IPD rates | E | Evidence growing Registry data supporting use |
| O | Original goal: reduce dislocation High dislocation rates with early THA | Q | Quality improved over time Modern designs with enhanced liner retention | T | Two main indications High-risk primary and revision for instability |
| U | Used small head in large liner 22mm head in large polyethylene cup | U | Usage expanded worldwide Particularly Europe, now FDA approved in USA |
Hook:BOUSQUET developed the concept in France - know the history for exam discussions
Mnemonic
THA IHIGH RISK - Primary THA Indications
| H | Hip fracture (elderly) Displaced femoral neck in frail elderly |
| I | Impaired cognition Dementia, Alzheimer's, post-stroke cognitive deficit |
| G | Gait abnormality Neuromuscular disorders (Parkinson's, CP) |
| H | High BMI/obesity Increased soft tissue laxity and dislocation risk |
| R | Recurrent dislocation history Prior THA dislocation, now revision |
| I | Inadequate abductors Abductor deficiency or tear |
| S | Spinal deformity Spinopelvic imbalance, flat lumbar spine |
| K | Keen to reduce dislocation Patient anxiety about instability |
| H | Hip fracture (elderly) Displaced femoral neck in frail elderly | H | High BMI/obesity Increased soft tissue laxity and dislocation risk | S | Spinal deformity Spinopelvic imbalance, flat lumbar spine |
| I | Impaired cognition Dementia, Alzheimer's, post-stroke cognitive deficit | R | Recurrent dislocation history Prior THA dislocation, now revision | K | Keen to reduce dislocation Patient anxiety about instability |
| G | Gait abnormality Neuromuscular disorders (Parkinson's, CP) | I | Inadequate abductors Abductor deficiency or tear |
Hook:HIGH RISK patients benefit most from dual mobility - know when to choose it
Mnemonic
IPD - IIPD - Intraprosthetic Dislocation Types
| I | Immediate/early (under 3 months) Usually technical error - impingement, malposition |
| P | Progressive liner wear Late IPD (over 3 months) from polyethylene wear |
| D | Dissociation of liner from head Liner separates from femoral head component |
| I | Immediate/early (under 3 months) Usually technical error - impingement, malposition |
| P | Progressive liner wear Late IPD (over 3 months) from polyethylene wear |
| D | Dissociation of liner from head Liner separates from femoral head component |
Hook:IPD is the unique dual mobility complication - early vs late has different causes
Overview and Historical Development
Dual mobility total hip arthroplasty uses a unique bearing design with two articulation surfaces to reduce the risk of hip dislocation. The concept was developed by Professor Gilles Bousquet in France in the 1970s in response to high dislocation rates with early THA designs.
Historical context:
- 1974: Bousquet introduced the concept at the University Hospital of Saint-Etienne, France
- Initial design: 22mm cobalt-chrome head within large mobile polyethylene liner
- Goal: Increase effective head size and range of motion before impingement
- European adoption: Widely used in France and Europe from 1980s onwards
- USA adoption: FDA approval in 2009, increasing use since then
- Modern designs: Multiple manufacturers with improved liner retention mechanisms
Why Dual Mobility Works
The biomechanical principle is that hip dislocation requires the femoral head to "jump" over the acetabular rim. Dual mobility increases this jump distance by creating a larger effective head size (the outer diameter of the mobile liner, typically 40-55mm) compared to the actual femoral head (22-28mm). This larger effective head increases the range of motion before impingement and makes dislocation mechanically more difficult.
Current usage patterns:
- Europe: 10-25% of primary THA in some countries (France highest)
- Australia: 15-20% of primary THA (AOANJRR data)
- USA: Increasing adoption, particularly for high-risk cases
- Indications: High-risk primary THA and revision for instability
Anatomy and Biomechanics
Component design:
1. Acetabular component:
- Metal shell: Typically titanium with porous coating for bone ingrowth
- Mobile polyethylene liner: Highly cross-linked polyethylene (HXLPE)
- Liner retention mechanism: Enhanced design features to prevent dissociation
- Shell sizes: Typically 44mm to 66mm diameter
2. Femoral component:
- Standard femoral stem: Same as conventional THA (cemented or uncemented)
- Small femoral head: Typically 22mm or 28mm diameter
- Materials: Cobalt-chrome or ceramic (ceramic less common)
3. Two articulations:
- Primary articulation: Small head rotating within mobile liner (intraprosthetic)
- Secondary articulation: Mobile liner rotating within metal shell (extraprosthetic)
Biomechanical advantages:
Biomechanical Comparison
| Parameter | Standard 32mm Head | Dual Mobility (28mm head, 50mm liner) |
|---|---|---|
| Effective head diameter | 32mm | 50mm (outer liner) |
| Head-to-neck ratio | Approximately 1.8:1 | Approximately 3.5:1 |
| Jump distance | Approximately 7mm | Approximately 15mm |
| Range of motion before impingement | Standard (limited by neck) | Increased (improved ratio) |
| Volumetric wear | Lower (smaller bearing) | Higher (larger polyethylene surface) |
Jump distance concept:
- Definition: Distance the femoral head center must travel perpendicular to the acetabular opening to dislocate
- Formula: Jump distance = (effective head radius) - (neck radius) × sin(inclination angle)
- Larger jump distance = more stable: Requires greater force and displacement to dislocate
- Clinical relevance: Dual mobility jump distance typically 12-18mm vs 5-8mm for standard THA
Which Articulation Moves?
Both articulations can move, but the distribution of motion varies. Initially, the intraprosthetic articulation (head within liner) predominates. Over time, as the liner seats into the shell, the extraprosthetic articulation (liner within shell) increases. The ratio typically stabilizes at 60-70% intraprosthetic and 30-40% extraprosthetic motion.
Wear considerations:
- Two bearing surfaces: Polyethylene wear at both articulations
- Volumetric wear: Greater total polyethylene surface area than standard THA
- HXLPE: Highly cross-linked polyethylene reduces wear rate
- Modern evidence: Acceptable wear rates with current generation designs
- Long-term concern: More polyethylene debris than metal-on-HXLPE standard THA
Classification Systems
Historical evolution of dual mobility designs:
Dual Mobility Design Generations
| Generation | Era | Key Features | Limitations |
|---|---|---|---|
| First Generation | 1970s-1990s | Bousquet design, cemented metal-backed cup, standard polyethylene | High wear rates, frequent IPD (5-10%) |
| Second Generation | 1990s-2000s | Improved liner retention, cementless options, enhanced polyethylene | Reduced IPD but still concerns about wear |
| Third Generation (Modern) | 2000s-present | HXLPE, advanced retention mechanisms, multiple bearing options | Low IPD (under 1%), acceptable wear rates |
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.
Clinical Assessment and Patient Evaluation
Preoperative assessment for dual mobility consideration:
History:
- Primary diagnosis: Osteoarthritis, AVN, femoral neck fracture, inflammatory arthritis
- Dislocation risk factors: Prior dislocation, neuromuscular disorder, cognitive status, fall history
- Medical comorbidities: Parkinson's disease, stroke, dementia, epilepsy, substance use
- Functional status: Mobility aids, independence, living situation
- Prior hip surgery: Previous THA, osteotomy, fracture fixation, arthroscopy
- Medications: Medications affecting balance, anticoagulation
Physical examination:
- Gait assessment: Trendelenburg sign (abductor weakness), neuromuscular abnormalities
- Hip range of motion: Flexion, extension, abduction, adduction, rotation
- Abductor strength: Resisted abduction testing, single leg stance
- Leg length: Measure discrepancy if present
- Neurovascular: Document baseline function
- Cognitive assessment: Orientation, ability to follow instructions
- Spinal alignment: Sagittal balance, lumbar lordosis, flexibility
Spinopelvic assessment:
- Standing lateral spine radiograph: Measure lumbar lordosis, sacral slope, pelvic tilt
- Sitting lateral radiograph: Assess spinopelvic mobility
- Flat lumbar spine: High risk for posterior dislocation when sitting (pelvic retroversion)
- Stiff spine: Limited compensatory motion increases hip demands
Risk stratification:
Dislocation Risk Assessment
| Risk Level | Patient Factors | Consider Dual Mobility? |
|---|---|---|
| Low Risk | Young, active, no risk factors, primary OA, good bone/soft tissue | Optional - standard THA acceptable |
| Moderate Risk | Elderly, high BMI, mild cognitive impairment, prior hip surgery | Consider dual mobility - discuss risks/benefits |
| High Risk | Neuromuscular disease, dementia, recurrent dislocation, abductor deficiency | Strong indication for dual mobility |
| Very High Risk | Revision for instability, multiple prior dislocations, combined risk factors | Dual mobility gold standard |
Decision-making framework:
- Low risk primary: Standard THA reasonable, dual mobility optional
- Moderate risk: Shared decision-making with patient about dual mobility benefits
- High risk: Strong recommendation for dual mobility
- Revision instability: Dual mobility is standard of care
Proper patient assessment and risk stratification guide appropriate dual mobility use.
Investigations and Imaging
Preoperative imaging:
Plain radiographs (essential):
- AP pelvis: Assess both hips, bone quality, dysplasia, prior hardware
- Lateral hip: Assess femoral offset, version estimate
- Assess for: Bone loss, dysplasia, protrusio, prior surgery, acetabular defects
CT scan (selective indications):
- Revision cases: Assess bone stock, component position, osteolysis
- Complex primary: Severe dysplasia, prior fracture, bone loss
- Version assessment: Acetabular and femoral version measurement
- 3D reconstruction: Surgical planning for complex anatomy
MRI (selective indications):
- Abductor assessment: Suspected gluteus medius/minimus tear
- AVN staging: MRI is gold standard for early AVN
- Soft tissue evaluation: Infection suspicion, tumor
Spinopelvic imaging (for high-risk patients):
- Standing lateral spine radiograph: Lumbar lordosis, pelvic incidence, sacral slope
- Sitting lateral radiograph: Assess pelvic tilt change (spinopelvic mobility)
- Indications: Prior dislocation, spinal fusion, flat back syndrome, elderly
Laboratory investigations:
Routine preoperative:
- FBC, UEC, coagulation profile
- Blood group and antibody screen
- ECG, CXR as per anesthetic assessment
Infection workup (if suspected):
- ESR, CRP (elevated suggests infection)
- Aspiration with culture if revision or concern for infection
- PJI workup if revision case
Special investigations:
- Bone density scan if concern for osteoporosis (affects fixation choice)
- Cardiac/respiratory workup as needed for comorbidities
Templating:
- Digital templating: Plan component sizes (acetabular shell, femoral stem)
- Offset and leg length: Plan restoration of hip biomechanics
- Component position: Plan target inclination and anteversion
Appropriate imaging guides surgical planning and dual mobility component selection.
Indications and Patient Selection
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.
Management Algorithm
Algorithm
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.
Surgical Technique
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.
Complications
Dual Mobility Specific Complications
| Complication | Incidence | Management |
|---|---|---|
| Intraprosthetic dislocation (IPD) | 0.5-1% (modern designs) | Closed reduction possible if early, open reduction and liner exchange if recurrent |
| True dislocation (liner and head from shell) | 0.5-2% primary, 2-5% revision | Closed reduction, assess for malposition or impingement |
| Squeaking | 1-3% (more with ceramic heads) | Usually benign, rarely requires revision |
| Accelerated polyethylene wear | Theoretical concern, rare clinically | Surveillance radiographs, revision if osteolysis |
| Liner dissociation from shell | Very rare with modern designs (under 0.5%) | Revision with liner exchange, assess shell stability |
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)
Differential diagnosis of the painful or "unstable-feeling" dual-mobility hip:
Differential Diagnosis - Dual-Mobility Hip Presenting with Pain or Instability
| Diagnosis | Distinguishing features | Key investigation / next step |
|---|---|---|
| Intraprosthetic dislocation (IPD) | Liner separates from head; less deformity than true dislocation; head appears medialised/eccentric in shell, double-density sign | AP + lateral radiograph; closed reduction attempt, open reduction + liner exchange if recurrent |
| True (extra-articular) dislocation | Whole liner-and-head construct out of shell; classic shortened/rotated limb, frank deformity | Radiograph; closed reduction, assess component position and impingement |
| Component malposition / impingement | Recurrent instability despite reduction; cup outside safe zone or abnormal combined anteversion | CT for version; revise malpositioned component |
| Periprosthetic joint infection | Rest/night pain, warmth, effusion, raised inflammatory markers; can mimic loosening | ESR/CRP, aspiration with culture and cell count |
| Aseptic loosening / osteolysis | Activity-related start-up pain, progressive radiolucent lines, component migration | Serial radiographs; revision if symptomatic or progressive |
| Periprosthetic fracture | Acute pain after fall/load, inability to weight-bear, fracture on imaging | Radiograph (Vancouver classification); fixation versus revision |
| Iliopsoas / soft-tissue impingement | Groin pain on active hip flexion / resisted straight-leg raise; prominent cup rim | Examination, image-guided injection; rarely revision/release |
Postoperative Care and Rehabilitation
Postoperative protocol for dual mobility THA:
Day 0-1 (Immediate Postop)
- Standard THA postoperative care
- Mobilization day of surgery or day 1 (per institutional protocol)
- Hip precautions may be relaxed compared to standard THA
- Weight bearing as tolerated (unless femoral/acetabular bone concerns)
- DVT prophylaxis per guidelines
Day 2-Discharge (Typically Day 2-3)
- Progressive mobilization with physiotherapy
- Stairs training before discharge
- Hip precautions: Many surgeons use reduced or no formal precautions with dual mobility
- Wound care education
- Discharge planning for home support if needed
Week 2-6
- Wound check and suture/staple removal (day 10-14)
- Progressive walking distance and activities
- Weaning from walking aids as tolerated
- Return to driving (4-6 weeks, when off opioids and good control)
- Light activities of daily living
Week 6-12
- X-ray at 6 weeks (AP pelvis and lateral hip)
- Assess component position and bone ingrowth
- Progressive strengthening exercises
- Return to sedentary work
- Recreational activities as tolerated
3-12 Months
- Full functional recovery expected by 3-6 months
- Return to full activities including sports (discuss with surgeon)
- Annual follow-up with X-rays for surveillance
- Watch for late complications or wear
Hip precautions debate:
- Traditional approach: Standard THA precautions (no flexion over 90 degrees, no adduction past midline, no internal rotation)
- Dual mobility advantage: Increased stability may allow reduced or no formal precautions
- Current trend: Many surgeons reduce or eliminate precautions for dual mobility patients
- Patient-specific: Consider patient's cognitive status, compliance, fall risk
Precautions with Dual Mobility
The benefit of dual mobility is that many surgeons feel comfortable eliminating or significantly reducing hip precautions. The increased jump distance and range of motion before impingement provide inherent stability. However, this remains surgeon-dependent, and high-risk patients (cognitive impairment, prior dislocation) may still benefit from precautions.
Long-term surveillance:
- Annual X-rays: AP pelvis and lateral hip for first 2-5 years
- Watch for: Osteolysis, component migration, wear
- Symptomatic review: Pain, clicking, instability sensation
- After 5 years: Consider X-rays every 2-3 years if asymptomatic
Outcomes and Evidence Synthesis
Pooled clinical outcomes (PubMed-verified):
Primary THA:
- Dislocation: roughly six-fold lower than fixed-bearing cups (dislocation risk ratio 0.16) (Romagnoli et al. Int Orthop 2018; DOI).
- Intraprosthetic dislocation: about 1.1% across 10,783 primary dual-mobility THAs, mostly first-generation designs (Darrith et al. Bone Joint J 2018; DOI).
- Survivorship: approximately 98% at mean 8.5 years pooled, and approximately 98% at 10 years with no IPD/osteolysis for a modern cementless cup at 10-16 years (Gaillard/Batailler et al. J Arthroplasty 2019; DOI).
Revision THA for instability:
- Versus constrained liners: higher survivorship (94.7% vs 81.0%) and lower dislocation (2.6% vs 11.0%) (Van Eecke et al. Hip Pelvis 2020; DOI).
- Versus large femoral heads: no significant dislocation difference in the only RCT to date, both strategies giving low rates (Weintraub/Darrith et al. J Arthroplasty 2023; DOI).
Femoral neck fracture:
- Dislocation roughly halved (RR 0.47) and revision reduced (RR 0.77) versus conventional THA, with a small increase in heterotopic ossification (Santiago et al. J Orthop Surg Res 2025; DOI).
Real-world joint registries (AOANJRR, NJR, AJRR and the Nordic registries) corroborate these findings - lower revision for instability with comparable overall survivorship - and are summarised in the Guidelines, Registries & Global Practice section below.
Wear performance:
- Volumetric wear: Higher than standard metal-on-HXLPE THA (larger surface area)
- Linear wear rates: Acceptable with modern HXLPE (0.05-0.1mm/year)
- Osteolysis: Rare with modern HXLPE dual mobility (under 2% at 10 years)
- Revision for wear: Very low (under 1% at 10 years)
Patient-reported outcomes:
- Function scores: Comparable to standard THA (Oxford Hip Score, HOOS)
- Satisfaction: High satisfaction rates (85-95%)
- Return to activity: Similar to standard THA
- Quality of life: Improved from preoperative baseline, similar to standard THA
Specific indication outcomes:
Femoral neck fracture:
- Dual mobility reduces dislocation vs standard hemiarthroplasty or THA
- Particularly beneficial in frail elderly with cognitive impairment
- AOANJRR shows lower revision rate for instability
Revision for instability:
- Re-dislocation reduced from 10-15% to 2-5%
- Patient satisfaction high when instability resolved
- Most cost-effective approach for recurrent dislocation
Evidence Base
Level III
Darrith, Courtney & Della Valle. Outcomes of dual mobility components in THA: systematic review
Key Findings:
- Systematic review of 54 studies. For 10,783 PRIMARY dual mobility THAs: intraprosthetic dislocation (IPD) 1.1%, extra-articular dislocation 0.46%, aseptic loosening 1.3%, overall survivorship 98.0% at mean 8.5 years.
- For 3008 REVISION dual mobility THAs: IPD 0.3%, extra-articular dislocation 2.2%, survivorship 96.6% at 5.4 years.
- IPD was low and largely limited to earlier (first-generation) designs.
Clinical Implication: Largest pooled dataset confirming dual mobility gives low instability and good survivorship in primary, revision and femoral-neck-fracture THA. IPD is rare with modern bearings - this is the benchmark systematic review for the topic.
Source: Bone Joint J 2018;100-B:11-19
Level III
Romagnoli et al. Efficacy of dual-mobility cup in preventing dislocation after THA: meta-analysis of comparative studies
Key Findings:
- Meta-analysis of 15 comparative studies, 2408 THAs (50.6% dual-mobility, 49.4% fixed-bearing).
- Dislocation risk ratio 0.16 (95% CI 0.09-0.28) favouring dual mobility - roughly a six-fold reduction in dislocation risk.
- Benefit held across primary, revision, trauma and elective subgroups and in high-risk patients.
Clinical Implication: High-quality pooled comparative evidence that dual mobility substantially lowers dislocation versus fixed-bearing cups, including in high-risk patients. Supports use whenever instability risk is elevated.
Source: Int Orthop 2018;43:1071-1082
Registry (Level II)
AOANJRR - Australian Orthopaedic Association National Joint Replacement Registry
Key Findings:
- National registry capturing essentially all hip arthroplasties performed in Australia, used here as a representative example of registry evidence.
- Dual-mobility bearings are associated with lower revision for dislocation/instability than fixed-bearing THA, particularly in revision and femoral-neck-fracture settings, with comparable overall cumulative revision.
- Registry use of dual mobility has risen over the last decade, reflecting growing surgeon confidence.
Clinical Implication: Registry data (AOANJRR, and comparably NJR, AJRR, Nordic registries) provide the real-world denominator confirming reduced instability revision with dual mobility. Cite the current annual report for exact figures as they update yearly.
Source: AOANJRR Annual Report (Hip, Knee & Shoulder Arthroplasty)
Level II
Santiago et al. Dual mobility versus conventional THA for femoral neck fractures: systematic review and meta-analysis including registry data
Key Findings:
- 3 RCTs plus 10 cohort studies, 21,585 patients (4887 dual mobility, 16,698 conventional THA).
- Dual mobility lowered dislocation (RR 0.47, 95% CI 0.34-0.65) and revision (RR 0.77, 95% CI 0.67-0.89) versus conventional THA.
- Trade-offs: higher heterotopic ossification (RR 1.98) and slightly worse early functional scores at 6-9 months; posterior approach did not alter the dislocation benefit.
Clinical Implication: In femoral neck fracture THA, dual mobility roughly halves dislocation and reduces revision, supporting its use in this high-falls, often cognitively impaired population - balanced against a small increase in heterotopic ossification.
Source: J Orthop Surg Res 2025;20:405
Level III
Gaillard, Batailler, Lustig et al. Ten- to 16-year results of a modern cementless dual-mobility cup in primary THA
Key Findings:
- 310 primary cementless dual-mobility THAs (Saturne cup), mean follow-up 152 months (minimum 10 years).
- No prosthetic dislocation, no intraprosthetic dislocation and no acetabular osteolysis or cup loosening at final review.
- Acetabular component survivorship approximately 98% at 10 years - only stem revisions were for periprosthetic femoral fracture.
Clinical Implication: Best available long-term (10-16 year) evidence for a modern-generation cementless dual-mobility cup: durable fixation, no osteolysis and no IPD, refuting the historical wear/IPD concerns of first-generation Bousquet designs.
Source: J Arthroplasty 2019;34:2704-2710
Level III
Van Eecke et al. Constrained acetabular components versus dual-mobility cups in revision THA: literature review
Key Findings:
- Review of 46 reports, 5617 revision hips: 31 dual-mobility series, 15 constrained-liner (CAL) series.
- Dual mobility outperformed constrained liners: overall survivorship 94.7% vs 81.0%, dislocation 2.6% vs 11.0%, acetabular loosening 1.0% vs 2.0%.
- Mean IPD rate 0.6%; residual concern is polyethylene wear in younger, high-demand patients.
Clinical Implication: For revision THA instability, dual mobility is superior to constrained liners on survivorship, dislocation and loosening - underpinning its role as the preferred construct for recurrent instability rather than a constrained liner.
Source: Hip Pelvis 2020;32:59-69
Level I (RCT, interim)
Weintraub, Darrith, Della Valle et al. Dual mobility versus large femoral heads in revision THA (interim RCT analysis)
Key Findings:
- Multicentre randomized controlled trial, 146 patients undergoing posterior-approach revision THA (76 dual mobility, 70 large head greater than or equal to 36 mm).
- At mean 18 months, dislocation was 2.6% (dual mobility) versus 4.3% (large head) - no statistically significant difference (p = 0.67), with lower-than-anticipated event rates.
- Both modern strategies markedly reduce instability; longer follow-up and full recruitment are needed to detect a difference.
Clinical Implication: The only randomized comparison to date: in revision THA both dual mobility and large femoral heads give low dislocation rates. Dual mobility remains favoured where a large head is constrained by bone loss or where intraprosthetic head-neck ratio cannot be optimised.
Source: J Arthroplasty 2023;38(7S):S206-S210
Clinical Decision Scenarios
Use these scenarios to practise clinical reasoning and management decisions
CLINICAL SCENARIOStandard
Scenario 1: Primary THA Indication Decision
CLINICAL PROMPT
"A 72-year-old woman with severe hip osteoarthritis is scheduled for primary THA. She has a history of Parkinson's disease with moderate tremor and rigidity, but is cognitively intact and independently mobile with a walking stick. Her daughter asks if there's anything you can do to reduce the risk of hip dislocation, which happened to a friend. How would you counsel this patient about dual mobility THA?"
PRACTICAL APPROACH
This patient has **Parkinson's disease which is a recognized risk factor for THA dislocation** due to impaired proprioception, rigidity, and increased fall risk. This makes her an excellent candidate for dual mobility THA. **Why she is high-risk for dislocation:** Parkinson's disease patients have 2-3 times higher dislocation risk than standard THA patients. The mechanisms include impaired motor control, rigidity preventing protective reflexes, falls, and difficulty following hip precautions due to movement disorders. **Benefits of dual mobility for this patient:** I would explain that dual mobility THA uses a special cup design with **two articulation surfaces** rather than one. This creates a larger effective ball size (similar to having a 50mm head instead of 32mm) which makes the hip mechanically more stable. The dislocation rate drops from approximately 3-5% with standard THA to 0.5-2% with dual mobility in patients like her. **How it works:** The small femoral head sits inside a mobile polyethylene liner, and that liner rotates within the metal shell. This dual articulation increases the "jump distance" - the distance the hip must displace to dislocate - from approximately 7mm to 15mm. This makes everyday activities much safer without restricting her function. **Evidence base:** I would mention that dual mobility is widely used internationally, particularly in Europe and Australia. The Australian Joint Registry shows excellent results with lower dislocation rates and comparable long-term survivorship to standard THA. **Other benefits for her:** Many surgeons, including myself, use relaxed or no formal hip precautions with dual mobility, which can be beneficial for patients with Parkinson's who may struggle with complex activity restrictions. She can resume normal activities more quickly and safely. **Risks specific to dual mobility:** I would counsel about intraprosthetic dislocation (IPD) - a unique complication where the liner separates from the head - occurring in under 1% with modern designs. The surgical approach, recovery, and other standard THA risks are identical. **Recommendation:** Given her Parkinson's disease and consequent elevated dislocation risk, I would strongly recommend dual mobility THA. The evidence supports this approach for patients with neuromuscular disorders, and the benefits of reduced dislocation risk outweigh the minimal additional risks.
KEY CLINICAL POINTS
Parkinson's disease increases THA dislocation risk 2-3 fold
Dual mobility reduces dislocation from 3-5% to 0.5-2%
Mechanism: Two articulations create larger effective head size
Jump distance increased from approximately 7mm to 15mm
AOANJRR and international registry data support use
May allow reduced or no hip precautions
IPD is unique complication, occurs in under 1% with modern designs
Surgical approach and standard THA risks unchanged
Strong evidence for neuromuscular disorder patients
Recommend dual mobility for this high-risk patient
COMMON PITFALLS
Not recognizing Parkinson's as high-risk for dislocation
Dismissing dual mobility as unnecessary for primary THA
Not explaining the biomechanical principle of how it works
Overemphasizing IPD risk (it is rare with modern designs)
Not mentioning registry data and evidence base
FURTHER QUESTIONS
"What is intraprosthetic dislocation and how do you prevent it?"
"What does the AOANJRR data show about dual mobility outcomes?"
"Would you use hip precautions with dual mobility in this patient?"
CLINICAL SCENARIOChallenging
Scenario 2: Revision for Recurrent Instability
CLINICAL PROMPT
"A 68-year-old man underwent primary THA via posterior approach 3 years ago for osteoarthritis. He has had four posterior dislocations, all reduced in the emergency department. The last dislocation was 2 weeks ago. CT scan shows well-positioned components (cup 40 degrees inclination, 20 degrees anteversion, stem in 15 degrees anteversion). What are your options for managing his recurrent instability and why would you choose dual mobility?"
PRACTICAL APPROACH
This patient has **recurrent posterior instability after primary THA** despite apparently well-positioned components. After four dislocations, non-operative management has clearly failed and he requires surgical intervention. Dual mobility revision is the gold standard approach. **Assessment:** First, I would confirm the CT findings showing components are within the Lewinnek safe zone (inclination 30-50 degrees, anteversion 5-25 degrees) and assess combined anteversion (sum of cup and stem anteversion should be 25-40 degrees per Ranawat). His combined anteversion is 35 degrees which is appropriate. I would also assess for: (1) component loosening on X-ray, (2) abductor muscle integrity on examination and MRI if uncertain, (3) spinopelvic mobility with standing and sitting lateral radiographs, (4) any cognitive or neuromuscular factors. **Why is he dislocating despite good component position?** Multiple factors can cause instability with well-positioned components: abductor muscle insufficiency, spinopelvic imbalance (particularly stiff lumbar spine causing pelvic tilt on sitting), component wear creating increased laxity, posterior capsular insufficiency, patient factors (high BMI, cognitive issues), or subtle malposition not apparent on static CT. **Treatment options:** **1. Dual mobility acetabular revision (my preference):** This is the gold standard for recurrent instability. I would revise the acetabular component to a dual mobility construct. Technique involves removing the existing liner, assessing the shell - if stable and well-positioned, I can retain it and cement a dual mobility liner into the existing shell. If the shell is loose or malpositioned, I would revise to a new dual mobility shell. **2. Constrained liner:** Alternative option but inferior outcomes. Constrained liners have 10-15% failure rates due to increased torque at bone-implant interface and risk of liner dissociation. Meta-analyses show dual mobility has lower re-dislocation rates (4.5% vs 10%) and lower revision rates than constrained liners. **3. Large head revision:** Could revise to 36mm or 40mm head, but this requires stem revision and doesn't address underlying soft tissue or spinopelvic issues. Less effective than dual mobility. **Why dual mobility is best:** The evidence is compelling - dual mobility reduces re-dislocation from 10-15% to 2-5% in revision instability. The Australian Registry shows dual mobility has the lowest revision rate for recurrent instability. The biomechanical advantage of increased jump distance addresses the instability regardless of the underlying cause (abductor weakness, spinopelvic issues, soft tissue laxity). **Surgical plan:** I would use a posterior approach (same as his primary). After exposure, I would assess the acetabular shell - if stable and within 5 degrees of optimal position, I would retain it and cement a dual mobility liner into the existing shell (saves bone stock). If loose or malpositioned, I would revise to new uncemented dual mobility shell. I would assess stem stability - if stable, retain stem and place appropriate size dual mobility femoral head. Meticulous posterior soft tissue repair with capsular plication. Intraoperative stability testing through full ROM. **Post-operative:** Hip precautions for 6 weeks given his dislocation history (despite dual mobility stability, psychological benefit and tissue healing). Abductor strengthening. Address any modifiable risk factors. Long-term surveillance for IPD or wear. The evidence strongly supports dual mobility as the treatment of choice for recurrent THA instability.
KEY CLINICAL POINTS
Recurrent instability with four dislocations requires surgical intervention
Components appear well-positioned on CT (within safe zones)
Must assess for causes: abductor deficiency, spinopelvic imbalance, loosening
Dual mobility revision is gold standard for recurrent instability
Can retain well-fixed shell and cement dual mobility liner if appropriate
Dual mobility re-dislocation rate 2-5% vs 10-15% with other options
Constrained liner inferior - higher failure and revision rates
Meta-analyses show dual mobility superior to constrained liner
AOANJRR data supports dual mobility for revision instability
Posterior approach with meticulous soft tissue repair
COMMON PITFALLS
Suggesting continued non-operative management after four dislocations
Not recognizing dual mobility as gold standard for revision instability
Recommending constrained liner without discussing dual mobility
Not assessing for underlying causes of instability
Not mentioning option to retain stable shell with cemented dual mobility liner
Focusing only on component position without considering soft tissue/spinopelvic factors
FURTHER QUESTIONS
"How would you retain the existing shell and use a dual mobility liner?"
"What are the advantages and disadvantages of constrained liners?"
"How does spinopelvic mobility affect hip stability?"
CLINICAL SCENARIOCritical
Scenario 3: Intraprosthetic Dislocation Management
CLINICAL PROMPT
"You performed a dual mobility THA 6 weeks ago using a posterior approach. The patient presents to ED reporting the hip 'went out' while bending to tie shoes. On examination, the leg is slightly shortened and internally rotated, but less deformity than a typical dislocation. AP pelvis X-ray shows the femoral head appears eccentric within the acetabular component, medialized relative to the shell. What has happened and how do you manage this?"
PRACTICAL APPROACH
This clinical and radiographic picture is classic for **intraprosthetic dislocation (IPD)** - the unique complication of dual mobility THA where the polyethylene liner dissociates from the femoral head while the liner remains within the metal shell. **Diagnosis:** The key radiographic findings that confirm IPD are: (1) femoral head appears medialized or eccentric within the acetabular component, (2) head is not concentrically positioned within the liner on AP view, (3) "double density" sign where the shadows of the head and liner rim overlap, (4) on lateral view, the head is not centered in the cup. The patient's symptoms are less dramatic than a true dislocation because the hip is not completely out - the liner remains in the shell, but the head has come out of the liner. **Classification:** This is an **early IPD** (6 weeks postoperative). Early IPD (under 3 months) is usually due to technical factors: incomplete liner seating at the time of surgery, impingement causing the liner to lever off the head, or component malposition causing neck-on-liner contact. Late IPD (over 3 months) is more commonly due to polyethylene wear or liner deformation. **Immediate management:** **Step 1: Attempt closed reduction:** Under conscious sedation in ED, I would attempt gentle closed reduction using standard technique (traction, flexion, internal rotation for posterior IPD). IPD can sometimes reduce closed if the liner is not damaged and the impingement has resolved with hip position change. The reduction is into the liner (not into the shell). **Step 2: Post-reduction imaging:** If closed reduction is successful, obtain AP and lateral X-rays to confirm concentric reduction of the head within the liner. The head should be centered in the acetabular component on both views. **Step 3: Assess stability:** If reduced, assess clinical stability and range of motion. If stable and concentric, can trial non-operative management with close monitoring. **Definitive management options:** **If closed reduction successful and first IPD:** - Trial of non-operative management with hip precautions - Close radiographic surveillance (X-ray at 2 weeks, 6 weeks, 3 months) - If remains stable and concentric, can observe - High suspicion this was due to technical issue (incomplete liner seating or impingement) **If closed reduction fails OR this is recurrent IPD:** - Surgical intervention required - Open reduction via posterior approach - Remove components and inspect liner for damage - **Liner exchange mandatory** - the dissociation indicates the liner retention mechanism has failed - Assess for impingement points (neck on liner, especially posteriorly) - Check component position - if malpositioned (excessive anteversion, inclination), may need shell revision - Reduce with new liner and test stability through full ROM **Intraoperative findings to look for:** - Evidence of neck-on-liner impingement (wear marks on liner) - Incomplete liner seating (gap between liner and shell) - Liner damage or deformation - Component malposition **Preventing recurrence:** - Meticulous liner seating with new liner (clean and dry shell taper, ensure audible/tactile click) - Address any impingement points - Consider component revision if malpositioned - Modern dual mobility designs have improved liner retention mechanisms **Patient counseling:** I would explain that IPD is a recognized complication of dual mobility (occurs in under 1% with modern designs), that it's different from a standard dislocation, and that with liner exchange and ensuring no impingement, the risk of recurrence is low. The overall benefit of dual mobility in terms of stability still outweighs this risk. Given this is his first IPD at 6 weeks, I would attempt closed reduction, and if successful, trial non-operative management with close surveillance. If it recurs or closed reduction fails, I would proceed with open reduction and liner exchange.
KEY CLINICAL POINTS
This is intraprosthetic dislocation (IPD) - unique dual mobility complication
Radiographic signs: eccentric head, medialized position, double density sign
Early IPD (under 3 months) usually due to technical factors
Attempt closed reduction first if presenting acutely
Post-reduction imaging to confirm concentric position
If successful closed reduction and first episode, trial non-operative with surveillance
If failed closed reduction or recurrent, need open reduction and liner exchange
Assess for impingement points and component malposition at surgery
Ensure meticulous liner seating with new liner
IPD rate under 1% with modern designs
Overall dual mobility benefits still outweigh IPD risk
COMMON PITFALLS
Not recognizing this as IPD rather than true dislocation
Not attempting closed reduction (it can work for IPD)
Thinking you can observe without reduction
Not understanding the difference between early and late IPD
Not explaining that liner exchange is required if surgical
Not assessing for impingement as cause
Not knowing the radiographic signs of IPD
FURTHER QUESTIONS
"What is the difference between early and late IPD?"
"What are the radiographic signs that confirm IPD?"
"If you revise this patient, would you retain the shell or revise it?"
"What improvements have modern dual mobility designs made to prevent IPD?"
MCQ Practice Points
Biomechanics Question
Q: What is the primary biomechanical advantage of dual mobility THA that reduces dislocation risk? A: Increased jump distance due to larger effective head size. The outer diameter of the mobile polyethylene liner (typically 40-55mm) acts as the effective head, creating a larger head-to-neck ratio and requiring greater displacement to dislocate (12-18mm jump distance vs 5-8mm for standard 32mm head).
Indication Question
Q: What is the strongest evidence-based indication for dual mobility THA? A: Revision THA for recurrent instability. Meta-analyses and registry data show dual mobility reduces re-dislocation from 10-15% to 2-5% and has superior outcomes compared to constrained liners or large head revision. This is the gold standard approach for recurrent THA dislocation.
Complication Question
Q: A patient presents 4 weeks after dual mobility THA with hip pain and X-ray showing the femoral head medialized and eccentric within the acetabular component. What is the diagnosis? A: Intraprosthetic dislocation (IPD). The polyethylene liner has dissociated from the femoral head while remaining in the shell. Early IPD (under 3 months) suggests technical error such as incomplete liner seating or impingement. Management includes attempted closed reduction and if unsuccessful or recurrent, open reduction with liner exchange.
Design Question
Q: How many articulation surfaces does a dual mobility THA have and what are they? A: Two articulations: (1) Primary/intraprosthetic articulation - small femoral head (22-28mm) rotating within mobile polyethylene liner, (2) Secondary/extraprosthetic articulation - mobile liner rotating within metal shell. Both articulations contribute to overall motion with typically 60-70% occurring at the intraprosthetic and 30-40% at the extraprosthetic surface.
Registry Data Question
Q: According to AOANJRR data, what is the revision rate for dislocation with dual mobility THA compared to standard THA? A: Dual mobility has significantly lower revision for dislocation - approximately 0.3% at 5 years compared to 1.1% for standard THA. This represents a 70% reduction in dislocation revision risk. Overall survivorship is comparable between dual mobility and standard THA.
High-Risk Patient Question
Q: Which patient populations are considered high-risk for THA dislocation and may benefit from dual mobility? A: High-risk populations include: (1) Neuromuscular disorders (Parkinson's, cerebral palsy, prior stroke, MS), (2) Cognitive impairment (dementia, psychiatric disorders), (3) Anatomical factors (abductor deficiency, spinopelvic imbalance, DDH), (4) Medical factors (high fall risk, obesity, prior dislocation history), (5) Femoral neck fracture in frail elderly.
Guidelines, Registries & Global Practice
Hip instability is the leading early cause of revision THA worldwide, and dual mobility has become a globally accepted strategy to address it. The picture below is evidence-led rather than tied to any single health system.
Global epidemiology and burden of instability
- Dislocation is among the commonest indications for early revision THA; pooled comparative data show dual-mobility cups cut dislocation roughly six-fold versus fixed-bearing cups (dislocation risk ratio 0.16) (Romagnoli et al. Int Orthop 2018; DOI).
- Dual mobility was conceived by Gilles Bousquet in Saint-Etienne, France (early 1970s); long-term modern-generation cementless cups now show no intraprosthetic dislocation or osteolysis at 10-16 years (Gaillard/Batailler et al. J Arthroplasty 2019; DOI).
- Use is highest in France and continental Europe, rising in Australasia and North America (FDA clearance for modular dual mobility, 2009).
Major guidance, side by side
Guideline / Society Positions on Dual Mobility
| Body (region) | Position on dual mobility | Evidence level / note |
|---|---|---|
| AAOS (USA) | Recognises dual mobility as an option to reduce instability, especially in revision and high-risk primary THA; emphasises individualised selection | Consensus / moderate-quality evidence |
| NICE / BOA (UK) | Implant choice guided by ODEP benchmarking and NJR outcome data; dual mobility used for instability-risk and recurrent dislocation rather than as a default primary bearing | Registry-anchored guidance |
| AO Foundation | Supports dual mobility (and large heads) as constructs that raise jump distance and head-neck ratio for unstable or revision hips | Expert / principle-based |
| EFORT / European consensus | Endorses dual mobility for high-dislocation-risk primary THA, femoral neck fracture in the elderly and revision for instability | Consensus, Level III evidence base |
Registry evidence (instability and survivorship)
Joint Registry Signals for Dual Mobility
| Registry | Signal | Practice point |
|---|---|---|
| AOANJRR (Australia) | Lower revision for dislocation than fixed-bearing THA; comparable overall cumulative revision; rising utilisation | Favoured for revision instability and frail FNF |
| NJR (England & Wales) | Used with ODEP benchmarking; instability/revision indications predominate over routine primary use | Bearing choice tied to outcome data |
| AJRR (USA) | Steadily increasing dual-mobility use in primary and revision THA | Adoption reflects instability-prevention strategy |
| Nordic registries (SHAR, Norwegian, Danish) | Support dual mobility for elderly femoral neck fracture and instability-risk hips | Strong trauma-arthroplasty signal |
Global practice variation
- High-resource settings: dual mobility increasingly routine for revision instability and elderly femoral neck fracture; debate persists about routine use in young, high-demand primary THA owing to polyethylene wear concerns.
- Limited-resource settings: higher implant cost and supply constraints limit uptake; standard fixed-bearing THA with optimised component position and head size remains the mainstay, with dual mobility reserved for highest-risk hips.
- Bearing trends: modern HXLPE liners and improved retention mechanisms have largely resolved the first-generation IPD/wear problem; ceramic-head dual mobility is under evaluation but carries limited long-term data.
Registry/Guideline Key Message
For any board, the world standard is consistent: dual mobility is the construct of choice for revision instability and a strong option for high-dislocation-risk primary THA and elderly femoral neck fracture. Registries (AOANJRR, NJR, AJRR, Nordic) show reduced instability revision with comparable overall survivorship; the main unresolved question is wear in young, high-demand patients.
DUAL MOBILITY THA
Clinical summary
CORE DESIGN PRINCIPLES
- •Two articulations: head-in-liner (intraprosthetic) + liner-in-shell (extraprosthetic)
- •Small head (22-28mm) in mobile polyethylene liner
- •Effective head size = outer liner diameter (40-55mm)
- •Increased jump distance (12-18mm vs 5-8mm standard)
- •Larger head-to-neck ratio reduces impingement
- •Both articulations contribute to motion (60% intra, 40% extra)
KEY INDICATIONS
- •Primary THA: Neuromuscular (Parkinson's, CP, stroke), cognitive impairment, abductor deficiency
- •Primary THA: High fall risk, femoral neck fracture in elderly, prior hip surgery
- •Revision THA: Recurrent instability (GOLD STANDARD), complex revision with bone loss
- •Other: Oncologic reconstruction, radiation pelvis, prior infection with soft tissue compromise
- •Relative: BMI over 35, spinopelvic pathology, hypermobility syndromes
OUTCOMES AND EVIDENCE
- •Dislocation rate: 0.5-2% primary, 2-5% revision (vs 3-5% and 10-15% standard)
- •AOANJRR: 0.3% revision for dislocation at 5 years (vs 1.1% standard)
- •Survivorship: 95-98% at 5 years (comparable to standard THA)
- •Re-dislocation in revision: 2-5% dual mobility vs 10-15% other options
- •Meta-analyses: Dual mobility superior to constrained liner for revision instability
INTRAPROSTHETIC DISLOCATION (IPD)
- •Unique complication: Liner dissociates from head (head medializes, liner stays in shell)
- •Incidence: 0.5-1% with modern designs (higher with older designs)
- •Early IPD (under 3 months): Technical error (incomplete seating, impingement)
- •Late IPD (over 3 months): Polyethylene wear, liner deformation
- •X-ray signs: Eccentric head, medialized position, double density sign
- •Management: Attempt closed reduction; if fails or recurrent, open reduction + liner exchange
SURGICAL TECHNIQUE
- •Any standard approach (posterior most common)
- •Acetabular shell: Standard positioning (40 degrees inclination, 15-25 degrees anteversion)
- •Critical: Meticulous liner insertion (clean/dry shell, ensure full seating, audible click)
- •Femoral component: Standard technique, 10-15 degrees stem anteversion
- •Combined anteversion: 25-40 degrees (Ranawat safe zone)
- •Intraoperative stability test: Full ROM, no impingement
AOANJRR KEY DATA
- •Usage: 18% of primary THA in Australia (2023), up from 5% in 2010
- •Dislocation revision: 70% reduction vs standard THA
- •Femoral neck fracture: Lower revision than hemiarthroplasty or standard THA
- •Revision instability: Lowest re-revision rate among all options
- •Overall survivorship: Comparable to standard THA (no increase in other failures)
EXAM TRAPS AND PEARLS
- •Don't confuse dual mobility with just a large head - it has TWO articulations
- •IPD is specific to dual mobility - know diagnosis (X-ray signs) and management
- •Revision for instability: Dual mobility is gold standard (superior to constrained)
- •Can often eliminate or reduce hip precautions with dual mobility
- •Modern HXLPE has acceptable wear rates despite larger surface area
- •Bousquet developed in France 1970s - know the history