HIP ARTHRODESIS
Salvage Procedure | Functional Limitation | Conversion to THA
INDICATIONS FOR HIP ARTHRODESIS
Critical Must-Knows
- Optimal fusion position: 25-30° flexion, 0-10° abduction, 15-20° external rotation
- Contralateral hip and ipsilateral knee MUST be normal for acceptable function
- Back pain develops in over 50% at 10 years due to increased lumbar stress
- Conversion to THA is technically demanding with unpredictable outcomes
- Leg length discrepancy of 3-5 cm is expected and must be counselled
Examiner's Pearls
- "Fusion position determines function: too much flexion impairs standing, too much abduction causes pelvic tilt
- "Ipsilateral knee develops degenerative changes in 30-40% at long-term follow-up
- "Conversion to THA has 40-50% complication rate: instability, leg length issues, abductor deficiency
- "Australian context: rarely performed now; most converted to THA within 20 years
Clinical Imaging
Imaging Gallery
Critical Hip Arthrodesis Exam Points
Indications Are Narrow
No longer first-line. Hip fusion is a salvage procedure reserved for young patients with failed arthroplasty, eradicated infection, or heavy manual labour requirements where revision THA would fail. Always consider modern revision techniques first.
Optimal Fusion Position
25-30° flexion, 0-10° abduction, 15-20° external rotation. This position allows sitting, standing, and walking with minimal compensatory pelvic tilt. Too much flexion impairs standing; too much abduction causes pelvic obliquity and gait abnormality.
Long-Term Sequelae
Adjacent joint disease is inevitable. Lumbar spine develops pain in over 50% at 10 years. Ipsilateral knee degenerates in 30-40%. Contralateral hip experiences increased stress. Patients must understand lifelong functional limitations.
Conversion to THA Challenges
Technically demanding with unpredictable results. Bone loss, leg length discrepancy, abductor deficiency, femoral deformity, and acetabular bone stock issues complicate conversion. Complication rate approaches 40-50% including instability and infection.
Quick Decision Guide: Hip Arthrodesis vs Alternatives
| Clinical Scenario | Patient Factors | Best Option | Key Pearl |
|---|---|---|---|
| Young manual labourer, failed THA | Age under 40, normal contralateral hip and knee | Consider arthrodesis after infection eradication | Counsel about functional limitations and adjacent joint disease |
| Eradicated septic arthritis | Young patient, high activity demands | Two-stage: resection arthrodesis then delayed fusion | Ensure infection markers normalized for at least 6 months |
| Multiple failed revisions | Poor bone stock, elderly, low demand | Resection arthroplasty or modern revision with augments | Arthrodesis rarely indicated in elderly; consider Girdlestone |
| Existing hip arthrodesis, disabling pain | Patient requesting improved function | Conversion to THA with extensive preoperative planning | CT for templating, plan for leg length and abductor reconstruction |
FEAROptimal Hip Arthrodesis Position
Memory Hook:FEAR the wrong position - it determines lifelong function! 30° Flexion, 20° External Rotation, 10° Abduction is the sweet spot.
FLIPIndications for Hip Arthrodesis (Historical)
Memory Hook:FLIP the decision tree - arthrodesis is a Last resort after Failed alternatives for young, high-demand patients!
BLANKSComplications After Hip Arthrodesis
Memory Hook:The hip fusion leaves BLANKS - compensatory stresses fill every adjacent joint with degenerative changes!
Overview and Epidemiology
Why Hip Arthrodesis Has Declined
Hip arthrodesis was commonly performed from the 1920s through 1980s for conditions that are now treated with total hip arthroplasty. The development of modern THA, improved infection management, and recognition of long-term sequelae (lumbar spine pain, adjacent joint degeneration) have made fusion a rare salvage procedure. Current indications are limited to young patients with failed arthroplasty or eradicated infection where revision THA has poor prognosis.
Historical Indications (Now Obsolete)
- Primary osteoarthritis (now THA)
- Post-traumatic arthritis (now THA)
- Ankylosing spondylitis (now THA)
- Tuberculosis (now medical treatment + THA)
- Primary hip sepsis (rarely fusion now)
These were common fusion indications before modern arthroplasty revolutionized hip reconstruction.
Current Indications (Rare)
- Recurrent infected THA after multiple failures
- Young manual labourer with destroyed hip
- Massive bone loss precluding revision THA
- Neurologic conditions requiring hip stability
- Failed salvage procedures (Girdlestone with pain)
Fusion is now a last resort when all reconstructive options have been exhausted.
Prerequisites for Hip Arthrodesis
Before considering hip fusion, confirm:
- Contralateral hip is NORMAL (or has well-functioning THA)
- Ipsilateral knee is NORMAL (degenerative changes are a contraindication)
- Lumbar spine is ASYMPTOMATIC (existing back pain will worsen)
- Patient understands functional limitations: no running, limited sitting, eventual conversion likely
- Alternative options (revision THA, resection arthroplasty) have been fully explored
Fusion with pre-existing knee or spine disease leads to early failure and poor outcomes.
Anatomy and Biomechanics
Biomechanical Principles
Force Transmission After Fusion
Normal Hip:
- Forces distributed through acetabulum and femoral head
- Motion occurs at hip joint
- Lumbar spine compensates minimally
Fused Hip:
- Forces transmitted directly through fusion mass
- Motion transferred to lumbar spine (flexion/extension)
- Contralateral hip compensates for abduction/adduction
- Ipsilateral knee compensates for rotation
This redistribution is inevitable and drives adjacent joint degeneration.
Optimal Fusion Position Rationale
Flexion 25-30°:
- Allows sitting without excessive lumbar flexion
- Standing requires lumbar lordosis compensation
Abduction 0-10°:
- Minimizes pelvic obliquity during stance
- Excessive abduction causes limping
External Rotation 15-20°:
- Permits sitting with knees together
- Allows foot care and shoe donning
Position is determined by functional demands, not by anatomy.
Gait Analysis After Hip Arthrodesis
| Gait Phase | Normal Hip | Fused Hip | Compensation Strategy |
|---|---|---|---|
| Stance phase | Hip extends 10°, abducts 5° | No hip motion; pelvis tilts posteriorly | Lumbar spine hyperextends; contralateral hip abducts more |
| Swing phase | Hip flexes 30°, clears ground | No hip flexion; leg swings from pelvis | Lumbar flexion, pelvic hike, circumduction gait |
| Sitting | Hip flexes to 90° | Fusion at 30° flexion position | Lumbar spine flexes additional 60°; slouched posture |
Stress on Adjacent Joints
Inevitable Adjacent Joint Degeneration
Lumbar Spine:
- Increased flexion/extension range by 50-100%
- Back pain develops in over 50% at 10-year follow-up
- Disc degeneration accelerates at L4-L5 and L5-S1
Ipsilateral Knee:
- Increased rotational stress
- Symptomatic arthritis in 30-40% at 15-20 years
- May require TKA eventually
Contralateral Hip:
- Increased abduction loading
- Accelerated cartilage wear
- Typically remains asymptomatic if initially normal
This is NOT a complication - it is an expected consequence of altered biomechanics.
Classification Systems
Indications Classification
Hip arthrodesis is classified by indication rather than by fracture pattern. Understanding when fusion is appropriate versus when alternative treatments are superior is critical for exam success.
Modern Indications for Hip Arthrodesis
| Indication Category | Specific Scenarios | Prerequisites | Alternative Options |
|---|---|---|---|
| Failed Arthroplasty with Infection | Recurrent infected THA after multiple two-stage revisions; poor bone stock | Eradicated infection (CRP/ESR normal 6+ months, negative cultures) | Antibiotic spacer, resection arthroplasty (Girdlestone) |
| Young Heavy Manual Labourer | Age under 40, high-impact occupation, destroyed hip | Normal contralateral hip and ipsilateral knee; no spine disease | Modern revision THA with trabecular metal, dual mobility |
| Neurologic/Paralytic Hip | Flail hip from polio, spina bifida; requires stability for ambulation | Patient ambulatory with aids; adequate upper extremity function | Hip-knee-ankle-foot orthosis (HKAFO) for stability |
| Massive Bone Loss | Pelvic discontinuity, acetabular destruction precluding implant fixation | Young patient where revision THA would fail immediately | Custom triflange acetabular component, pelvic reconstruction cage |
Key Distinction: When Is Fusion Actually Better Than THA?
Fusion is RARELY superior to modern revision THA techniques. The only clear indication is eradicated infection in a young patient where multiple revision attempts have failed and bone stock is insufficient for further reconstruction. Even then, resection arthroplasty (Girdlestone) may provide acceptable function with less morbidity. Always exhaust revision THA options before offering fusion.
Current indications are extremely narrow and require careful patient selection.
Clinical Assessment and Patient Selection
Preoperative Evaluation
History Taking
Current Hip Symptoms:
- Pain severity and functional impact
- Previous hip surgeries (number of revisions, complications)
- Infection history (organism, treatment, duration of antibiotics)
- Current mobility (distance, aids required)
Medical History:
- Comorbidities (diabetes, immunosuppression affecting infection risk)
- Smoking status (major nonunion risk factor)
- Occupation and activity level (determines fusion appropriateness)
- Patient expectations (unrealistic expectations are a contraindication)
Red Flag Questions:
- "Do you have any back pain?" (lumbar disease will worsen)
- "Do you have any knee pain on the same side?" (knee arthritis will worsen)
- "Can you walk at all currently?" (if non-ambulatory, fusion unlikely to help)
Physical Examination
Affected Hip:
- Range of motion (typically stiff or ankylosed already)
- Pain on motion (indicates active pathology)
- Leg length discrepancy (document baseline)
- Scars from previous surgery (plan incision)
Contralateral Hip (CRITICAL):
- Full range of motion (must be normal to compensate)
- No pain or crepitus (early arthritis is a contraindication)
- Trendelenburg test (should be negative)
Ipsilateral Knee (CRITICAL):
- Full range of motion (any stiffness suggests arthritis)
- No effusion or crepitus (arthritis is contraindication)
- Ligament stability (instability will worsen with fusion stress)
Lumbar Spine:
- Range of motion (will need increased lumbar flexion after fusion)
- Tenderness (symptomatic spine disease is contraindication)
- Neurologic exam (rule out radiculopathy)
Neurovascular Exam:
- Pulses (PVD may worsen with fusion position)
- Sensation and motor function (baseline for sciatic nerve)
Assessment of Contralateral Hip and Ipsilateral Knee Is MANDATORY
This is the most important part of the clinical assessment:
- Obtain standing AP pelvis X-ray (assess BOTH hips)
- Obtain ipsilateral knee X-rays (AP, lateral, skyline)
- If ANY arthritis is present in contralateral hip or ipsilateral knee, arthrodesis is CONTRAINDICATED
- Document thorough examination findings - medicolegal risk is high if fusion proceeds despite diseased adjacent joints
Failure to assess adjacent joints is negligent and will lead to poor outcomes.
Infection Assessment (If Indication Is Post-Septic)
Confirming Eradicated Infection
- CRP and ESR must be NORMAL for at least 6 months after antibiotics stopped
- If elevated, infection is NOT eradicated; do NOT proceed with fusion
- Ultrasound-guided hip aspiration
- Send fluid for culture (aerobic, anaerobic, fungal, mycobacterial)
- Cell count (WBC under 3000, PMN under 80% suggests eradicated infection)
- If positive culture, infection is NOT eradicated; treat with antibiotics
- PET-CT or labeled WBC scan to assess for active infection
- Useful if aspiration not possible (ankylosis) or equivocal results
Infection Eradication Pearl
Proceeding with fusion in the presence of active infection will result in infected nonunion - a devastating complication requiring resection arthroplasty. The infection MUST be eradicated with at least 6 months of normal inflammatory markers after antibiotics stopped. If in doubt, delay fusion and repeat inflammatory markers in 3 months.
Thorough clinical assessment prevents inappropriate patient selection and poor outcomes.
Investigations
Imaging Protocol
Preoperative Imaging Workup
Affected Hip:
- AP pelvis and lateral hip
- Assess bone stock (acetabulum and proximal femur)
- Identify retained hardware from previous surgeries
- Measure leg length discrepancy (will increase by 3-5 cm)
Contralateral Hip:
- AP pelvis includes both hips (assess for arthritis)
- If ANY joint space narrowing, osteophytes, or sclerosis: CONTRAINDICATED
Ipsilateral Knee:
- AP, lateral, and skyline views
- Assess for arthritis (joint space narrowing, osteophytes)
- If ANY arthritis present: CONTRAINDICATED
Lumbar Spine:
- AP and lateral lumbar spine
- Assess for degenerative disc disease, spondylolisthesis
- Symptomatic spine disease is a relative contraindication
Indications for CT:
- Multiple previous surgeries (assess bone stock accurately)
- Pelvic discontinuity or massive acetabular bone loss
- Retained hardware (plan removal strategy)
- Conversion cases (assess fusion mass and plan osteotomy)
CT with 3D reconstruction:
- Provides excellent visualization of bone stock
- Helps plan fixation strategy (plate vs screws)
- Essential for conversion THA planning
Indications for MRI:
- Rule out active infection (fluid collections, bone marrow edema)
- Assess soft tissue (muscle atrophy, abductor quality)
- Evaluate lumbar spine if symptomatic (disc herniation, stenosis)
Findings:
- Active infection: rim-enhancing fluid collection, bone marrow edema
- Chronic changes: muscle atrophy, fatty infiltration of abductors
PET-CT:
- High sensitivity for active infection (FDG uptake at fusion site)
- Useful if aspiration not possible or equivocal
Labeled WBC Scan:
- Specific for infection (WBC accumulation at site)
- Combine with bone marrow scan to improve specificity
Laboratory Investigations
If Infection History
Essential Tests:
- CRP and ESR (must be normal for 6+ months off antibiotics)
- Complete blood count (WBC, differential)
- Hip aspiration culture (if joint accessible)
Interpretation:
- CRP over 10 mg/L → likely active infection
- ESR over 30 mm/hr → likely active infection
- Positive culture → active infection; do NOT proceed with fusion
Routine Preoperative
Standard Tests:
- Full blood count (anemia, infection)
- Renal function (contrast CT, antibiotic dosing)
- Coagulation studies (bleeding risk)
- Blood group and save (large operation, potential blood loss)
If Comorbidities:
- HbA1c if diabetic (glycemic control affects healing)
- Liver function if alcohol history
- ECG and echocardiogram if cardiac disease
Do Not Rely on Inflammatory Markers Alone
Inflammatory markers can be normal despite persistent infection:
- Low-virulence organisms (e.g., Cutibacterium acnes) may not elevate CRP/ESR
- Biofilm infections may have low inflammatory response
- If clinical suspicion for infection, proceed to aspiration or biopsy even if markers normal
The gold standard is culture - not inflammatory markers.
Templating for Fusion Position
Preoperative Planning Steps
- Use AP and lateral radiographs of pelvis
- Mark 25-30° flexion relative to pelvis on lateral view
- Mark 0-10° abduction on AP view (measure pelvic obliquity)
- Plan external rotation 15-20° (measured clinically, not radiographically)
- Measure from ilium to mid-femoral shaft (plate length 20-30 cm)
- Assess bone quality (osteoporosis may require longer plate, more screws)
- Identify screw trajectories (avoid previous screw holes if possible)
- Fusion typically adds 3-5 cm shortening
- Measure current LLD and add 3-5 cm for expected postoperative LLD
- Plan shoe lift requirement (counsel patient preoperatively)
Comprehensive preoperative imaging and planning are essential for successful fusion.
Management Algorithm
Decision Tree for Hip Pathology Requiring Salvage
When to Choose Hip Arthrodesis vs Alternative Salvage Options
| Clinical Scenario | Fusion Appropriate? | Alternative Options | Recommended Approach |
|---|---|---|---|
| Young (under 40), failed THA, eradicated infection, manual labourer | YES - if contralateral hip and knee normal | Resection arthroplasty, antibiotic spacer, modern revision THA | Fusion likely best option; provides stability for high-impact work |
| Young (under 40), failed THA, eradicated infection, sedentary occupation | MAYBE - consider alternatives first | Modern revision THA with trabecular metal, dual mobility, antibiotic cement | Attempt revision THA first; fusion only if multiple revision failures |
| Elderly (over 60), failed THA, eradicated infection, low demand | NO - resection arthroplasty better | Resection arthroplasty (Girdlestone), antibiotic spacer with chronic suppression | Resection arthroplasty provides pain relief with less morbidity than fusion |
| Any age, ipsilateral knee arthritis or lumbar spine disease | NO - absolute contraindication | Address knee/spine first, or accept current hip status | Fusion will worsen knee/spine disease; do NOT proceed |
| Neurologic condition (flail hip), young, ambulatory with aids | MAYBE - consider orthotic first | Hip-knee-ankle-foot orthosis (HKAFO) for stability during gait | Trial orthotic first; fusion if orthotic fails and patient motivated |
Algorithm Pearl: Fusion Is Last Resort
Hip arthrodesis should be the LAST option considered after all alternatives have been exhausted. The algorithm should be: (1) Attempt modern revision THA with advanced techniques, (2) If infection cannot be eradicated or bone stock insufficient, consider resection arthroplasty, (3) If patient young, high-demand, and resection unacceptable, THEN consider fusion. Jumping straight to fusion without exhausting alternatives is poor practice.
Fusion is rarely the first choice - consider all alternatives first.
Surgical Technique
Patient Selection and Counseling
Ideal Candidate (Rare)
- Age: Under 40 years
- Occupation: Heavy manual labour
- Activity: High impact demands
- Contralateral hip: Normal
- Ipsilateral knee: Normal
- Spine: Asymptomatic
- Expectations: Realistic about limitations
Even ideal candidates should be counseled about eventual conversion to THA.
Contraindications
- Ipsilateral knee arthritis (will worsen)
- Symptomatic lumbar spine (will worsen)
- Contralateral hip disease (cannot compensate)
- Obesity (poor functional outcomes)
- Elderly patients (resection arthroplasty better)
- Unrealistic expectations (expects normal function)
These are ABSOLUTE contraindications - do not proceed.
Imaging and Templating
Preoperative Assessment
- Assess acetabular bone stock (for fixation)
- Measure leg length discrepancy (will increase by 3-5 cm)
- Evaluate femoral shaft anatomy (for intramedullary fixation)
- Document baseline status of contralateral hip (must be normal)
- Assess ipsilateral knee for early degenerative changes
- If arthritis present, arthrodesis is contraindicated
- Assess for pre-existing degenerative disease
- Symptomatic spine disease is a relative contraindication
- Counsel patient about inevitable progression
- For conversion cases: assess acetabular bone loss
- Identify retained hardware
- Plan fixation strategy (plate vs intramedullary device)
Consent Points
Critical Counseling Points
Functional Limitations:
- Cannot run or perform high-impact activities
- Sitting requires lumbar flexion; may be uncomfortable after 30-60 minutes
- Leg length discrepancy of 3-5 cm; requires shoe lift
- Gait abnormality with circumduction
Long-Term Sequelae:
- Back pain develops in over 50% within 10 years
- Ipsilateral knee arthritis in 30-40% at 15-20 years
- May require eventual conversion to THA (challenging procedure)
Complications:
- Nonunion: 5-10% (requires revision surgery)
- Infection: 2-5% (may require resection)
- Nerve injury: under 1% (sciatic nerve)
- Conversion to THA if fusion fails or function unacceptable
Document this discussion extensively - medicolegal risk is high for this procedure.
Ensure the patient understands that hip fusion provides pain relief and stability at the cost of permanent functional limitation.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Nonunion | 5-10% (plate), 10-15% (screws only) | Poor surface preparation, inadequate fixation, smoking, infection | Revision arthrodesis with bone grafting and improved fixation; consider conversion to THA if revision fails |
| Infection (deep) | 2-5% (higher if prior infection) | Diabetes, immunosuppression, prolonged operative time, prior septic arthritis | Irrigation and debridement with hardware retention (if fusion solid); consider resection arthroplasty if fusion incomplete |
| Lumbar back pain | Over 50% at 10 years | Pre-existing spine disease, suboptimal fusion position, young age (longer follow-up) | Physiotherapy, NSAIDs, activity modification; may require spinal fusion or conversion to THA if disabling |
| Ipsilateral knee arthritis | 30-40% at 15-20 years | Pre-existing knee disease, excessive fusion flexion (increases knee stress), high activity level | Conservative management initially; may require TKA if symptomatic; consider conversion to THA before knee replacement |
| Leg length discrepancy | 100% (expected outcome) | Shortening of 3-5 cm is typical due to loss of hip motion and fusion position | Shoe lift (add 2-3 cm to affected side); counsel preoperatively that this is expected |
| Sciatic nerve palsy | Under 1% | Excessive retraction, screw penetration into sciatic notch, positioning injury | Exploration if immediate postoperative deficit; nerve recovery unpredictable (may take 12-18 months) |
| Hardware prominence/pain | 10-20% with plate fixation | Thin soft tissue coverage, prominent iliac screws, patient sensitivity | Hardware removal after fusion is solid (at least 12-18 months); avoid early removal (nonunion risk) |
| Malposition (poor fusion angle) | 5-10% | Inadequate intraoperative position verification, hardware failure, loss of fixation | Revision arthrodesis with osteotomy to correct position if symptomatic and nonunion; very difficult procedure |
Managing Nonunion
Diagnosis:
- Persistent pain at fusion site beyond 6 months
- Radiographic lucency around screws or at fusion interface
- CT scan: lack of bridging bone across acetabulum-femoral head junction
Treatment:
- Revision arthrodesis with bone grafting (autograft from iliac crest or femoral head allograft)
- Upgrade fixation (add compression screws or convert to plate if screws only)
- Address infection (send cultures at revision)
- If revision fails or patient unwilling: consider conversion to THA (challenging but may restore function)
Nonunion is the most common cause of revision surgery after hip arthrodesis.
Early Complications (0-6 weeks)
- Infection: Wound drainage, fever, elevated CRP
- Nerve injury: Sciatic palsy (foot drop)
- DVT/PE: Lower limb swelling, chest pain
- Hardware failure: Loss of fixation (rare if technique sound)
Most complications present within the first 6 weeks and require prompt recognition.
Late Complications (months to years)
- Nonunion: Persistent pain, motion at fusion site
- Lumbar back pain: Develops gradually over 5-10 years
- Knee arthritis: Progressive over 10-20 years
- Patient dissatisfaction: Functional limitations worse than expected
Late complications are EXPECTED and should be discussed preoperatively.
Long-term complications are inevitable and drive eventual conversion to THA in many patients.
Postoperative Care and Rehabilitation
Immediate Postoperative Management
- Pain control: Epidural or PCA; transition to oral opioids by day 2
- DVT prophylaxis: LMWH (enoxaparin 40 mg daily) for 35 days (high risk)
- Antibiotic prophylaxis: 24 hours (unless infection history; extend to 48 hours)
- Wound check: Inspect drain output (expect 100-200 mL in first 24 hours)
- Mobilization: Bed rest on day 0; sit in chair on day 1 (if patient tolerates)
- Drain removal: When drainage under 30 mL per 8 hours (typically day 2-3)
- Mobilization: Stand and pivot transfer with physiotherapy
- Gait training: Touch weight-bearing (toe touch only) with crutches or walker
- Hip orthosis: Fit hip abduction orthosis (if not using spica cast)
- Discharge planning: Home with walking aids; ensure safe mobility before discharge
- Wound care: Keep dry; remove staples at 14 days
- Weight-bearing: Touch weight-bearing with crutches (10-20 kg maximum)
- Mobilization: Short walks (50-100 m) multiple times per day
- X-ray: AP pelvis at 2 weeks to check hardware position
- Weight-bearing: Gradually increase to partial (25-50 kg) if no pain
- Radiographs: AP pelvis and lateral hip at 6 weeks
- Goal: Independent mobility with crutches by 6 weeks
If radiographs show early fusion (bridging bone), advance weight-bearing more aggressively.
Red Flags in Early Postoperative Period
Wound concerns:
- Persistent drainage beyond 5 days (concern for deep infection)
- Erythema extending beyond 2 cm from incision (cellulitis vs deep infection)
- Fever over 38.5°C beyond day 3
Hardware concerns:
- Sudden increase in pain (hardware loosening or fracture)
- Inability to bear any weight (fixation failure)
Investigate promptly with inflammatory markers (CRP, ESR) and radiographs.
The first 6 weeks focus on wound healing and protected weight-bearing.
Conversion of Hip Arthrodesis to THA
Why Conversion Is Challenging
Conversion Is High-Risk Surgery
Technical Challenges:
- Bone loss: Acetabular bone deficiency from prior fusion reaming
- Leg length discrepancy: 3-5 cm shortening; restoring length risks nerve injury
- Abductor deficiency: No functional gluteus medius/minimus (atrophied during fusion)
- Femoral deformity: Proximal femur may have healed in varus or external rotation
- Heterotopic ossification: Bridging bone around fusion site limits motion
Complication Rate: 40-50% (infection, dislocation, leg length issues, nerve injury).
Preoperative Planning for Conversion
Conversion THA Planning
- Assess acetabular bone stock (often deficient from prior reaming)
- Identify retained hardware (screws may be buried in bone)
- Measure leg length discrepancy (guide for femoral osteotomy)
- Plan cup size and need for augments/structural graft
- Template on non-fused side (mirror image for acetabulum)
- Plan femoral component size (often undersized due to canal narrowing)
- Determine safe amount of leg length restoration (maximum 3-4 cm to avoid nerve injury)
- Plan acetabular reconstruction (jumbo cup, augments, or structural allograft)
- Complications: 40-50% rate (dislocation, infection, nerve injury)
- Leg length: Cannot fully restore (nerve stretch injury risk if over 4 cm lengthening)
- Instability: High dislocation rate (20-30%) due to abductor deficiency
- Reoperation: May require constrained liner or abductor reconstruction
- Alternative: Accept current function (avoid high-risk surgery)
Document extensive discussion - medicolegal risk is very high.
Surgical Technique for Conversion
Approach Selection
- Preferred: Posterior approach (preserves any remaining anterior soft tissue)
- Alternative: Extensile lateral if prior fusion was via lateral approach (utilize old scar)
Exposure
Exposure Steps
- Remove all previous fusion hardware (cobra plate, screws)
- May require extended exposure to access buried screws
- Save removed hardware for reference
- Use oscillating saw to divide fusion mass
- Create plane between femoral head and acetabulum
- Remove bridging bone and heterotopic ossification
- Goal: Separate femur from pelvis to allow dislocation
- Perform femoral neck osteotomy at planned level (based on templating)
- Remove femoral head and any residual acetabular bone
- Assess femoral canal (often narrowed or deformed)
Sciatic Nerve at High Risk
The sciatic nerve is at EXTREME risk during conversion THA:
- Nerve is scarred and adherent from prior surgery
- Leg lengthening over 3-4 cm risks traction injury
- Posterior dissection can directly injure nerve
Protective measures:
- Identify and protect nerve early
- Use intraoperative neuromonitoring
- Limit leg lengthening (accept residual LLD if necessary)
- Consider staged lengthening (femoral osteotomy first, THA later)
Exposure is technically demanding and time-consuming - allow 1-2 hours for this step alone.
Outcomes of Conversion THA
| Outcome Measure | Expected Result | Comparison to Primary THA | Notes |
|---|---|---|---|
| Complication rate | 40-50% | Primary THA: 5-10% | Infection, dislocation, nerve injury, leg length issues |
| Dislocation rate | 20-30% | Primary THA: 2-5% | Abductor deficiency is primary cause; dual mobility reduces risk |
| Infection | 5-10% | Primary THA: 1-2% | Higher if prior septic arthritis was indication for fusion |
| Nerve injury (sciatic) | 5-10% | Primary THA: under 1% | Excessive leg lengthening is main cause; monitor intraoperatively |
| Patient satisfaction | 60-70% | Primary THA: over 90% | Improved function but residual gait abnormality and LLD |
| Reoperation rate (within 5 years) | 30-40% | Primary THA: 5-10% | For instability, infection, or component loosening |
Conversion THA Exam Pearl
Examiner will ask: "A patient with a hip arthrodesis from 20 years ago requests conversion to THA to improve function. What do you counsel?"
Model Answer: "Thank you. I would counsel that conversion THA is a high-risk procedure with a complication rate of 40-50%, significantly higher than primary THA. Key risks include instability (20-30% dislocation rate due to abductor deficiency), sciatic nerve injury (5-10% from leg lengthening), and infection (5-10%). I would emphasize that while function typically improves, a residual limp and leg length discrepancy are expected. The reoperation rate is 30-40% within 5 years. I would recommend extensive preoperative planning with CT scan, templating for bone loss, and consideration of dual mobility components. I would also discuss the alternative of accepting current function rather than undergoing high-risk surgery. If the patient proceeds, I would use intraoperative neuromonitoring, limit leg lengthening to 3-4 cm maximum, and employ a dual mobility cup to reduce dislocation risk."
Conversion THA is technically demanding with unpredictable results - thorough preoperative counseling is essential.
Outcomes and Prognosis
Functional Outcomes
| Functional Domain | Typical Outcome | Limiting Factor | Patient Expectation |
|---|---|---|---|
| Walking distance | Unlimited on level ground | Lumbar fatigue, circumduction gait | Good - most can walk several kilometers |
| Sitting tolerance | 30-60 minutes | Lumbar flexion requirement, discomfort | Fair - limited by back pain and position |
| Stair climbing | Possible with handrail | Ipsilateral knee stress, leg length discrepancy | Fair - can manage stairs but requires effort |
| Manual labor | Light to moderate work | Cannot perform high-impact or heavy lifting | Variable - some return to work, others cannot |
| Driving | Automatic transmission only | Cannot operate clutch (requires hip flexion range) | Good - most can drive automatic vehicles |
| Sports/recreation | Low-impact only (swimming, cycling) | No running, jumping, or pivoting sports | Poor - major lifestyle limitation for active patients |
Predictors of Poor Outcome
Factors Associated with Poor Satisfaction
Patient Factors:
- Young age (longer follow-up → more adjacent joint disease)
- High activity expectations (fusion cannot meet demands)
- Pre-existing knee or spine disease (will worsen)
- Obesity (poor gait mechanics, increased stress on adjacent joints)
Technical Factors:
- Malposition (excessive flexion or abduction → gait dysfunction)
- Nonunion (persistent pain, instability)
- Leg length discrepancy over 5 cm (severe gait abnormality)
Social Factors:
- Manual labor occupation (may not be able to return to work)
- Lack of social support (difficulty with mobility aids)
Identifying these factors preoperatively helps select appropriate candidates.
Long-Term Natural History
Expected Course After Hip Arthrodesis
- Fusion consolidates
- Patient adapts to gait abnormality
- Satisfaction typically high (pain relief achieved)
- Lumbar back pain develops in 30-50%
- Ipsilateral knee symptoms begin (early arthritis)
- Satisfaction declines as functional limitations become apparent
- Lumbar back pain present in over 50%
- Ipsilateral knee arthritis in 30-40%
- Many patients request conversion to THA for improved function
- Majority have been converted to THA
- Those with retained fusion have disabling adjacent joint disease
- Functional decline accelerates with age
Natural History Pearl
Hip arthrodesis is NOT a definitive long-term solution. Most patients eventually require conversion to THA within 15-20 years due to adjacent joint disease or patient dissatisfaction with function. The fusion should be viewed as a TEMPORARY solution that provides pain relief and stability while deferring arthroplasty in young patients or those with contraindications to THA (e.g., active infection).
Outcomes are acceptable in the short to medium term but decline over time due to adjacent joint disease.
Evidence Base and Key Studies
Long-Term Results of Hip Arthrodesis
- 37 patients with hip arthrodesis followed for average 30 years
- Back pain developed in 60% at long-term follow-up
- Ipsilateral knee arthritis in 38% (symptomatic)
- 27% were converted to THA for improved function
- Patient satisfaction declined significantly after 15-20 years
Conversion of Hip Arthrodesis to Total Hip Arthroplasty
- 24 conversions from hip fusion to THA
- Complication rate 46% (instability, infection, nerve injury)
- Dislocation rate 29% (abductor deficiency primary cause)
- Patient satisfaction 67% despite high complication rate
- Functional improvement in all patients who did not have complications
Hip Fusion in Young Patients: Long-Term Functional and Radiographic Results
- 41 patients with hip arthrodesis followed for mean 23 years
- Fusion rate 93% (plate fixation superior to screws alone)
- Lumbar back pain in 52% at final follow-up
- 30% had ipsilateral knee arthritis requiring TKA
- 22% were converted to THA
Dual Mobility Cups in Conversion THA After Hip Arthrodesis
- 18 conversion THA cases using dual mobility cups
- Dislocation rate 11% (vs historical 20-30% with standard cups)
- No nerve injuries (careful leg lengthening protocol)
- Patient satisfaction 78%
- Reoperation rate 22% at 5-year follow-up
Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) Data on Hip Fusion
- Hip arthrodesis represents under 0.1% of hip procedures in Australia
- Most common indication: failed revision THA with infection
- Conversion to THA accounts for 15% of complex revision cases
- Complication rate for conversion THA: 35-40% (infection, dislocation)
- Use of dual mobility cups in conversion cases increasing (now 60% of conversions)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Indications and Counseling for Hip Arthrodesis
"A 35-year-old male manual labourer presents with severe hip pain and disability after multiple failed revisions of an infected total hip arthroplasty. The infection has been eradicated with two-stage revision, but he has had recurrent dislocations and poor bone stock precludes further arthroplasty. He asks about hip fusion. What are the indications for hip arthrodesis, and how would you counsel this patient?"
Scenario 2: Surgical Technique for Hip Arthrodesis
"You have decided to proceed with hip arthrodesis using cobra plate fixation for a young patient with a failed revision THA. Walk me through your surgical technique, focusing on positioning, surface preparation, and fixation."
Scenario 3: Conversion of Hip Arthrodesis to THA
"A 55-year-old woman had a hip arthrodesis 20 years ago for post-septic arthritis. She now has disabling lumbar back pain and requests conversion to total hip arthroplasty to improve her function. On examination, she has a 4 cm leg length discrepancy and a Trendelenburg gait (from the contralateral hip compensating). Radiographs show a solid fusion with a cobra plate in situ. How would you approach this case?"
MCQ Practice Points
Fusion Position Question
Q: What is the optimal position for hip arthrodesis? A: 25-30° flexion, 0-10° abduction, 15-20° external rotation. This position allows sitting (flexion), standing without excessive lumbar compensation (not too much flexion), walking without pelvic obliquity (minimal abduction), and sitting with knees together plus foot hygiene (external rotation). Too much flexion impairs standing; too much abduction causes limping.
Long-Term Complications Question
Q: What percentage of patients develop lumbar back pain after hip arthrodesis, and why? A: Over 50% develop lumbar back pain within 10 years. The fused hip transfers motion to the lumbar spine, which must compensate for the loss of hip flexion/extension. This increases lumbar range of motion by 50-100%, accelerating disc degeneration at L4-L5 and L5-S1. Back pain is NOT a complication - it is an expected consequence of altered biomechanics.
Fixation Technique Question
Q: What is the preferred fixation method for hip arthrodesis, and what fusion rate does it achieve? A: Cobra (arthrodesis) plate fixation is preferred, spanning from the ilium to the mid-femoral shaft with at least 6-8 bicortical screws. This achieves a fusion rate of 90-95%. Screw fixation alone (transfixion screws from ilium through acetabulum to femoral head) achieves only 85-90% fusion and has less rotational stability. Compression across the fusion site is critical regardless of fixation method.
Conversion THA Question
Q: What is the most common complication after conversion of hip arthrodesis to THA, and how can it be reduced? A: Dislocation (20-30% incidence) due to abductor deficiency. The gluteus medius and minimus have atrophied during years of fusion and cannot be restored. Risk can be reduced by using dual mobility cups (reduces dislocation to 10-15%), large femoral heads (36-40 mm), and careful soft tissue repair. Despite these measures, instability remains the most common complication requiring reoperation.
Contraindications Question
Q: What are absolute contraindications to hip arthrodesis? A: Ipsilateral knee arthritis (will worsen due to compensatory stress), symptomatic lumbar spine disease (will worsen from increased motion demands), and contralateral hip disease (cannot compensate for the fused hip). The contralateral hip and ipsilateral knee MUST be normal, and the lumbar spine should be asymptomatic. If these joints are diseased, hip fusion will fail due to progressive adjacent joint degeneration and patient dissatisfaction.
Nerve Injury in Conversion Question
Q: What is the maximum safe leg lengthening during conversion of hip arthrodesis to THA, and why? A: 3-4 cm maximum. The sciatic nerve is at extreme risk during conversion THA due to scarring from prior surgery and the need to restore leg length (typical LLD after fusion is 3-5 cm). Lengthening over 4 cm causes traction injury to the sciatic nerve, resulting in permanent foot drop and sensory loss. If preoperative LLD is over 5 cm, consider staged lengthening (femoral osteotomy weeks before THA) or accept residual LLD rather than risk nerve injury. Intraoperative neuromonitoring is recommended.
Australian Context and Medicolegal Considerations
AOANJRR Data
- Hip arthrodesis: Under 0.1% of hip procedures in Australia (extremely rare)
- Main indication: Failed infected revision THA after staged treatment
- Conversion to THA: Accounts for 15% of complex revision hip procedures
- Dual mobility use: Increasing in conversion cases (now 60% vs 20% historically)
- Complication rate: Conversion THA has 35-40% complication rate per registry data
Hip fusion is now a salvage procedure of last resort in Australia.
Australian Guidelines
- ACSQHC surgical site infection guidelines: Extended antibiotic prophylaxis (48 hours) recommended for conversion THA after infected fusion
- PBS listings: No specific implant subsidies for hip arthrodesis (rarely performed)
- DVT prophylaxis: Extended LMWH for 35 days (high-risk surgery per NHMRC guidelines)
- Consent standards: Document extensive discussion of functional limitations and adjacent joint disease
Follow ACSQHC and NHMRC guidelines for perioperative management.
Medicolegal Considerations
High-Risk Procedure - Document Extensively:
Informed Consent Must Include:
- Functional limitations: no running, limited sitting (30-60 min), gait abnormality, 3-5 cm LLD
- Adjacent joint disease: over 50% develop back pain, 30-40% develop knee arthritis by 15-20 years
- Eventual conversion: most patients request THA within 20 years; conversion is high-risk (40-50% complications)
- Complications: nonunion (5-10%), infection (2-5%), nerve injury (under 1%), hardware prominence
- Alternatives: revision THA with modern techniques (augments, antibiotic cement), resection arthroplasty
For Conversion THA:
- Dislocation risk 20-30% (despite dual mobility cups)
- Nerve injury risk 5-10% from leg lengthening
- Cannot fully restore leg length (residual LLD expected)
- Reoperation rate 30-40% within 5 years
- Alternative: accept current function and manage back pain conservatively
Documentation:
- Record preoperative assessment of contralateral hip, ipsilateral knee, lumbar spine (must be normal)
- Document fusion position planning (goniometer measurements, fluoroscopy images)
- Photograph intraoperative fusion position for medicolegal record
- Document postoperative counseling about weight-bearing and rehabilitation timeline
Common Litigation Issues:
- Malposition (wrong fusion angle → lifelong gait dysfunction) - PREVENT with meticulous position verification
- Nerve injury in conversion THA (excessive leg lengthening) - PREVENT by limiting lengthening to 3-4 cm
- Failure to warn about functional limitations and adjacent joint disease - PREVENT with extensive documented consent
The medicolegal risk is HIGH for this procedure - document everything.
Hospital System Considerations
- Pre-approval required: Most private insurers require pre-authorization for hip fusion (high cost, rare procedure)
- Implant availability: Cobra plates may not be stocked; order in advance
- Extended LOS: Expect 5-7 days for arthrodesis, 7-10 days for conversion THA (longer than primary THA)
- Rehabilitation facility: Arrange transfer to rehab hospital if patient cannot mobilize safely
Plan logistics in advance - this is not a routine procedure.
Australian context emphasizes that hip fusion is extremely rare and conversion THA is high-risk with unpredictable outcomes.
Hip Arthrodesis
High-Yield Exam Summary
Optimal Fusion Position
- •Flexion: 25-30° (allows sitting without excessive lumbar flexion)
- •Abduction: 0-10° (neutral to slight abduction minimizes pelvic tilt)
- •External rotation: 15-20° (permits sitting with knees together)
- •Verify position with goniometer and fluoroscopy BEFORE draping
Indications (Rare)
- •Failed revision THA with eradicated infection and poor bone stock
- •Young manual labourer (under 40) with destroyed hip
- •Massive bone loss precluding arthroplasty
- •Neurologic conditions requiring hip stability (flail hip)
Surgical Technique
- •Lateral approach with complete capsulectomy
- •Remove ALL cartilage; maximize bone-to-bone contact (70-80% surface area)
- •Cobra plate fixation: ilium to femoral shaft, at least 6-8 screws
- •Compression across fusion site is critical for union
Long-Term Complications (Expected)
- •Lumbar back pain: over 50% at 10 years (increased spinal motion)
- •Ipsilateral knee arthritis: 30-40% at 15-20 years (rotational stress)
- •Leg length discrepancy: 3-5 cm shortening (counsel preoperatively)
- •Nonunion: 5-10% with plate fixation; requires revision with bone graft
Conversion to THA Challenges
- •Complication rate: 40-50% (instability, nerve injury, infection)
- •Dislocation risk: 20-30% (abductor deficiency); use dual mobility cups
- •Limit leg lengthening to 3-4 cm maximum (sciatic nerve stretch injury risk)
- •Acetabular reconstruction: often Paprosky IIB-IIIA bone loss; plan for augments