ASEPTIC LOOSENING IN THA - PARTICLE DISEASE
Wear Debris → Osteolysis → Implant Migration | Rule Out Infection | Paprosky Classification
GRUEN ZONES (FEMORAL)
Critical Must-Knows
- Aseptic loosening = implant failure due to particle-induced osteolysis, NOT infection
- Wear debris (polyethylene, metal, cement) triggers macrophage activation
- Gruen zones (1-7) for femur, DeLee/Charnley (I-III) for acetabulum
- Rule out infection with ESR, CRP, aspiration before labeling as aseptic
- Paprosky classification guides bone loss management in revision surgery
Examiner's Pearls
- "Aseptic loosening is a biologic process - particle disease
- "Progressive radiolucent lines, migration, subsidence on serial X-rays
- "Must exclude infection - aspirate if any doubt
- "Revision complexity depends on bone loss (Paprosky)
Critical Aseptic Loosening Exam Points
Particle Disease Pathway
Wear debris from bearing surfaces (PE most common) or cement triggers macrophage activation → cytokine release (TNF-α, IL-1, IL-6) → osteoclast recruitment → bone resorption → osteolysis → loosening. This is a biologic, not mechanical, process.
Infection Exclusion Critical
Never assume aseptic without workup. Check ESR, CRP (elevated in infection), hip aspiration (cell count, culture), and consider alpha-defensin or synoviasure. Infection has different management (2-stage vs 1-stage revision).
Radiographic Zones
Gruen zones 1-7 (femoral stem): Zone 1 (calcar resorption/subsidence), Zone 7 (stress shielding). DeLee/Charnley I-III (acetabular): Zone I superior, Zone II medial, Zone III inferior. Progressive lucent lines indicate loosening.
Migration vs. Subsidence
Migration = lateral shift, rotation, or tilt. Subsidence = distal vertical settling. More than 5mm migration in 2 years indicates loosening. Early settling under 2mm may be stable in uncemented stems.
Quick Decision Guide - Aseptic vs Septic Loosening
| Feature | Aseptic Loosening | Septic Loosening |
|---|---|---|
| Onset | Gradual (years), progressive pain | Acute or subacute, persistent pain |
| ESR/CRP | Normal or mildly elevated | Elevated (CRP more than 10, ESR more than 30) |
| Aspiration WCC | Less than 3000 cells/μL | More than 3000 cells/μL (PMN more than 80%) |
| Radiology | Progressive lucent lines, focal osteolysis | Rapid bone loss, periosteal reaction |
| Treatment | Single-stage revision | Two-stage revision (usually) |
PARTICLE - Mechanism of Aseptic Loosening
Memory Hook:PARTICLE describes the cascade from wear debris to implant loosening - it's a biologic process
GRUEN - Femoral Stem Zones (1-7)
Memory Hook:GRUEN zones: 1 and 7 are proximal (medial and lateral), 2-6 spiral down medial then lateral
DeLee - Acetabular Zones (I-III)
Memory Hook:DeLee zones: I (superior dome), II (medial), III (inferior) - think clockwise from top
ASPIRATION - When to Aspirate Hip
Memory Hook:ASPIRATION - When to aspirate: essentially, when ANY suspicion of infection exists
Overview and Epidemiology
Aseptic loosening is the most common indication for revision total hip arthroplasty in the late postoperative period (beyond 5 years). It represents failure of implant fixation due to particle-induced osteolysis rather than infection.
Definition:
- Loss of mechanical fixation between implant and bone
- Due to biologic reaction to wear debris (particle disease)
- Progressive bone resorption (osteolysis) around implant
- Results in pain, migration, and eventual mechanical failure
Epidemiology:
- Historical rates: 10-20% at 10 years (older implants)
- Modern rates: 1-2% per year after first 2 years (improved bearings)
- Most common between 10-15 years post-primary THA
- Higher rates with older polyethylene (non-cross-linked)
Modern Improvements
Highly cross-linked polyethylene (HXLPE) has dramatically reduced wear rates from 0.1-0.2mm/year (conventional PE) to under 0.05mm/year. This translates to significantly lower aseptic loosening rates in modern implants. Australian registry data (AOANJRR) confirms this trend.
Risk factors for aseptic loosening:
- Young, active patients - higher wear rates
- Obesity - increased joint forces
- Poor implant positioning - edge loading, impingement
- Conventional polyethylene - higher wear
- Thin cement mantles - cement fracture
- Undersized components - inadequate fixation
- Osteolysis-prone patients - genetic factors (cytokine polymorphisms)
Pathophysiology - Particle Disease
The particle disease cascade:
Aseptic loosening is fundamentally a biologic process driven by wear particles, not a purely mechanical phenomenon.
Step 1: Particle Generation
- Polyethylene wear (most common source)
- Metal-on-metal wear (cobalt/chromium particles)
- Ceramic particles (if liner fracture)
- Cement particles (PMMA debris)
- Corrosion products (at modular junctions)
Step 2: Particle Distribution
- Particles migrate along effective joint space
- Enter periprosthetic tissues via joint fluid pumping
- Size matters: 0.1-10 micron particles most biologically active
- Smaller particles more phagocytosable, larger particles accumulate
Step 3: Macrophage Activation
- Macrophages phagocytose wear particles
- Frustrated phagocytosis - cannot digest polyethylene
- Macrophage activation and cytokine release
- Key cytokines: TNF-α, IL-1β, IL-6, RANKL
Step 4: Osteoclast Recruitment
- RANKL binds to RANK on osteoclast precursors
- OPG (osteoprotegerin) normally inhibits RANKL
- Imbalance: RANKL/OPG ratio increased
- Osteoclast activation and bone resorption
Step 5: Osteolysis and Membrane Formation
- Periprosthetic membrane forms (granulation tissue)
- Contains macrophages, giant cells, particles
- Progressive bone resorption creates cystic lesions
- Membrane extends along implant-bone interface
Step 6: Mechanical Loosening
- Loss of bone support
- Micromotion increases
- Further particle generation (vicious cycle)
- Implant migration or subsidence
Vicious Cycle
Loosening creates more micromotion → generates more particles → drives more osteolysis → creates more loosening. This is why early detection and intervention are important - the process accelerates once established.
Genetic factors:
- Polymorphisms in cytokine genes (TNF-α, IL-1)
- Some patients are high responders to particles
- Explains variability in osteolysis rates with similar wear
Classification Systems and Radiographic Assessment
Gruen Zones (Femoral Component)
The femur is divided into 7 zones around the stem on AP radiograph.
Proximal regions:
- Zone 1: Proximal medial (calcar region)
- Zone 7: Proximal lateral (greater trochanter region)
Mid-stem regions:
- Zone 2: Medial mid-stem (above Zone 3)
- Zone 6: Lateral mid-stem (above Zone 5)
Distal regions:
- Zone 3: Medial distal
- Zone 5: Lateral distal
- Zone 4: Tip of stem (below stem tip)
Clinical significance:
- Zone 1 lucency: Calcar resorption, subsidence risk
- Zone 7 lucency: Stress shielding (common, may be stable)
- Circumferential lucency (all zones): Definitely loose
- Progressive lucency (increasing on serial films): Loosening
- Lucency more than 2mm: Concerning for loosening
Zone 7 Stress Shielding
Stress shielding in Zone 7 (proximal lateral) is common with stiff stems and may be stable if non-progressive and stem otherwise well-fixed distally. This differs from true loosening which is progressive and often circumferential.
Clinical Presentation and Assessment
History:
- Onset: Gradual, progressive (years not weeks)
- Pain: Groin pain (acetabular) or thigh pain (femoral)
- Start-up pain: Worse after rest, improves with walking
- Activity limitation: Progressive difficulty with activities
- Previous function: Often initially well-functioning THA
- Time since surgery: Typically more than 5-10 years
Symptoms suggesting aseptic (not septic):
- Gradual onset over months to years
- No systemic symptoms (no fever, malaise)
- Pain improving with activity (mechanical)
- No night sweats or constitutional symptoms
Symptoms raising suspicion for infection:
- Acute or subacute onset
- Persistent pain despite rest
- Systemic symptoms (fever, malaise)
- Night pain, rest pain
- History of wound problems
Physical examination:
Physical Examination Findings
| Finding | Significance | Interpretation |
|---|---|---|
| Antalgic gait | Pain with weight-bearing | Mechanical pain from loosening |
| Groin pain on axial load | Acetabular component loosening | Positive impingement test |
| Thigh pain on axial load | Femoral component loosening | Start-up pain typical |
| Leg length discrepancy | Component migration | Measure and compare to prior |
| Normal wound, no warmth | Suggests aseptic | Infection would show inflammation |
| Erythema, warmth, sinus | Suggests septic | Requires infection workup |
Range of motion:
- Usually preserved unless severe migration
- Pain at end range if impingement
- Stiffness suggests other pathology (infection, heterotopic ossification)
Neurovascular examination:
- Document distal pulses, sensation
- Sciatic nerve (prior posterior approach)
- Femoral nerve (prior anterior approach)
Investigations and Differential Diagnosis
Radiographic workup:
Standard X-rays (AP pelvis, AP hip, lateral hip):
- Compare to prior films (serial assessment)
- Evaluate Gruen zones (femur) and DeLee/Charnley zones (acetabulum)
- Measure lucent lines (width and extent)
- Assess for migration (quantify displacement)
- Look for osteolysis (focal bone loss)
- Evaluate cement mantle (if cemented)
CT scan (if planning revision):
- Better assessment of bone loss (Paprosky classification)
- Identify osteolysis not visible on X-ray
- Evaluate pelvic discontinuity
- Plan reconstruction strategy
- 3D reconstruction for templating
Laboratory investigations:
Laboratory Investigations - Infection Screening
| Test | Aseptic Loosening | Infection | Action |
|---|---|---|---|
| ESR | Normal or mildly elevated (under 30) | Elevated (more than 30) | First-line screening |
| CRP | Normal (under 10 mg/L) | Elevated (more than 10 mg/L) | More specific than ESR |
| WBC | Normal | May be elevated | Less specific for PJI |
| IL-6 | Normal or mildly elevated | Elevated | Research tool mostly |
Hip aspiration (when to perform):
Indications for aspiration:
- Any elevated inflammatory markers (ESR more than 30, CRP more than 10)
- Clinical suspicion of infection
- All revisions (rule out infection before any revision)
- Atypical presentation
- Rapid bone loss
Aspiration technique:
- Fluoroscopic or ultrasound guidance
- Lateral approach (avoid contamination)
- Send for: cell count with differential, culture (hold 14 days), alpha-defensin
- Stop antibiotics 2 weeks before aspiration
Aspiration interpretation:
Synovial Fluid Analysis
| Parameter | Aseptic | Infected | Threshold |
|---|---|---|---|
| WBC count | Less than 3000 cells/μL | More than 3000 cells/μL | 3000 cells/μL |
| PMN percentage | Less than 80% | More than 80% | 80% PMNs |
| Alpha-defensin | Negative | Positive | High specificity |
| Culture | Negative | Positive | Gold standard but 7% false negative |
MSIS Criteria
Major criteria for PJI (any one = infected):
- Sinus tract communicating with prosthesis
- Two positive cultures of same organism
Minor criteria (need threshold score):
- Elevated ESR (more than 30) or CRP (more than 10)
- Elevated synovial WBC (more than 3000) or PMN (more than 80%)
- Positive alpha-defensin
- Positive histology (more than 5 PMNs per high-power field)
If major criteria absent, use scoring system with minor criteria. Always rule out infection before labeling as aseptic.
Additional investigations:
Nuclear medicine (if diagnosis uncertain):
- Bone scan (Tc-99m): Sensitive but not specific
- Labeled WBC scan (In-111 or Tc-99m): More specific for infection
- FDG-PET: High sensitivity for infection but expensive
Other tests:
- D-dimer: Elevated in infection (but not specific)
- Synoviasure: Newer biomarker panel
- PCR for organisms: If culture-negative but high suspicion
Management Algorithm
Step 1: Confirm diagnosis of loosening
- Serial radiographs showing progression
- Definite radiographic criteria (migration, lucency)
- Exclude other causes of pain (referred, spine)
Step 2: Rule out infection
- ESR, CRP (if elevated → aspirate)
- Hip aspiration if any suspicion
- MSIS criteria to classify as aseptic vs septic
Step 3: Assess symptoms and function
- Pain severity and functional limitation
- Impact on activities of daily living
- Patient expectations and goals
Step 4: Determine treatment approach
- Symptomatic + radiographic progression → Surgery
- Asymptomatic + early changes → Observation with serial X-rays
- Minimal symptoms + stable → Conservative management
This systematic assessment guides appropriate management decisions for each patient.
Surgical Technique - Revision for Aseptic Loosening
Surgical Approach Selection
Posterior approach (most common):
- Good exposure to acetabulum and femur
- Extensile if needed (trochanteric slide/osteotomy)
- Familiar to most surgeons
- Risk: Dislocation (especially if prior posterior approach)
Direct anterior approach:
- If prior anterior primary
- Good acetabular exposure
- Difficult femoral exposure for revision
- Limited extensile options
Lateral approach:
- Less common for revisions
- Risk to abductors
- Transtrochanteric extension possible
Extended trochanteric osteotomy (ETO):
- For difficult femoral extraction
- Well-fixed uncemented stems
- Cement column removal
- Allows distal access to femur
- Must repair and protect postoperatively
Key principles:
- Adequate exposure is critical
- Protect neurovascular structures
- Plan extensile approach if needed
- Don't fight through small exposure
Extended approaches allow safer component removal and better access for reconstruction.
Complications of Revision THA
Complications of Revision for Aseptic Loosening
| Complication | Incidence | Prevention/Management |
|---|---|---|
| Dislocation | 5-15% (higher than primary) | Restore offset, tension; constrained liner if needed |
| Intraoperative fracture | 5-10% | Careful extraction; ETO for well-fixed stems |
| Periprosthetic fracture (postop) | 2-5% | Protected weight-bearing; adequate fixation |
| Nerve injury | 1-3% | Careful retraction; document preop exam |
| Infection | 2-5% | Antibiotic prophylaxis; minimize operative time |
| Leg length discrepancy | Common | Templating; intraoperative measurement |
| Re-revision | 10-15% at 10 years | Address bone loss adequately; stable fixation |
| Thromboembolic disease | 1-2% | Chemical and mechanical prophylaxis |
Dislocation:
- Higher risk than primary THA (abductor damage, soft tissue laxity)
- Restore offset and leg length
- Consider constrained liner or dual-mobility
- Posterior repair if posterior approach
Intraoperative fracture:
- Femoral fractures during stem removal (most common)
- Treat with cerclage cables and/or revision to longer stem
- Acetabular fractures (medial wall blow-out)
- May need cage or column plating
Nerve injury:
- Sciatic nerve most at risk (posterior approach, retraction)
- Femoral nerve (anterior approach)
- Document preoperative exam
- Avoid excessive retraction and leg lengthening
Infection:
- Higher risk in revision than primary
- Meticulous debridement
- Antibiotic prophylaxis
- Consider vancomycin powder locally (controversial)
Dislocation Risk
Dislocation rates are higher after revision (5-15%) than primary (1-3%). Contributing factors: soft tissue damage, abductor insufficiency, poor bone stock affecting component position. Consider dual-mobility bearings or constrained liners in high-risk patients.
Postoperative Care and Rehabilitation
Immediate postoperative care:
- ICU/HDU if complex or prolonged case
- DVT prophylaxis (chemical and mechanical)
- Pain management (multimodal, avoid excessive opioids)
- Monitor hemoglobin (transfuse if needed)
- Foley catheter typically removed day 1
- Early mobilization to chair
- Physiotherapy with walking frame
- Partial weight-bearing (if uncemented femur)
- Toe-touch weight-bearing (if ETO)
- Hip precautions (if posterior approach)
- Wound inspection
- Progress to crutches or walker
- Progressive weight-bearing (if stable fixation)
- Continue hip precautions for 6-12 weeks
- Outpatient physiotherapy
- Wound check at 2 weeks (remove sutures/staples)
- First follow-up X-rays at 6 weeks
- Advance to full weight-bearing (if healing well)
- Wean from walking aids
- ETO protected until union (usually 12 weeks)
- Progressive strengthening exercises
- Monitor for complications
- Return to normal activities gradually
- Avoid high-impact sports indefinitely
- Serial X-rays to monitor fixation (6 weeks, 3 months, 1 year)
- Long-term follow-up annually
Weight-bearing restrictions:
- Uncemented acetabulum: Partial weight-bearing for 6 weeks
- Extensively coated femoral stem: May allow immediate weight-bearing
- Extended trochanteric osteotomy: Toe-touch for 6 weeks, partial for 6-12 weeks
- Bone grafting: Protected weight-bearing until graft incorporation
Hip precautions (if posterior approach):
- No hip flexion more than 90 degrees
- No adduction past midline
- No internal rotation
- Duration: 6-12 weeks (surgeon preference)
Monitoring for complications:
- Wound infection signs (erythema, drainage)
- DVT/PE symptoms (leg swelling, chest pain)
- Neurovascular status
- Pain out of proportion (compartment syndrome rare)
Long-term follow-up:
- Radiographs: 6 weeks, 3 months, 1 year, then annually
- Monitor for loosening, wear, osteolysis
- Document stable fixation (osseointegration)
- Check for heterotopic ossification
Outcomes and Prognosis
Revision THA outcomes:
Modern techniques and implants have improved revision THA outcomes substantially.
Survivorship (freedom from re-revision):
- 10-year survivorship: 80-90% (modern series)
- 15-year survivorship: 70-80%
- Worse outcomes with severe bone loss (Paprosky IIIB/IV)
Factors affecting outcomes:
Patient factors:
- Age (younger patients higher revision risk)
- Activity level
- BMI (obesity increases failure risk)
- Bone quality
Surgical factors:
- Severity of bone loss (Paprosky grade)
- Surgeon experience with revisions
- Achieving stable fixation
- Managing soft tissue/abductor integrity
Implant factors:
- Extensively coated stems (better outcomes than cemented in Paprosky III)
- Tantalum augments/cups (good outcomes in deficient acetabulum)
- Constrained liners (higher dislocation rate but necessary in some)
Functional outcomes:
- Most patients achieve pain relief
- Functional scores improve (Harris Hip Score, WOMAC)
- Return to low-impact activities
- Avoid high-impact sports
Australian context (AOANJRR data):
- Revision for aseptic loosening accounts for approximately 25-30% of all revisions
- Uncemented femoral stems have lower re-revision rates than cemented (in Paprosky III)
- Trabecular metal cups show good outcomes in deficient acetabulum
- Dual-mobility bearings reduce dislocation in revision setting
Registry Data
AOANJRR shows that aseptic loosening revision rates have decreased with modern implants (HXLPE, improved surfaces). However, when loosening occurs, revision is complex and outcomes are inferior to primary THA. Prevention through optimal primary technique is key.
Evidence Base
- Analysis of 159,000 primary THAs showed aseptic loosening rate decreased from 10% at 10 years (1979-1985) to under 2% (2000-2006) with improved polyethylene and surfaces. Highly cross-linked PE reduced wear by 80-90%.
- Aseptic loosening accounts for 25% of all revisions. Uncemented stems in revision have lower re-revision rates than cemented (8.5% vs 11.2% at 10 years). Trabecular metal cups show good fixation in bone loss.
- Paprosky classification reliably predicts revision complexity and outcomes. Type I-II have 90-95% success with standard cups. Type IIIB requires advanced reconstruction (augments, cages) with 75-80% success.
- Polyethylene particles 0.1-10 microns activate macrophages. RANKL/OPG imbalance drives osteoclastogenesis. Cytokine polymorphisms (TNF-α, IL-1) affect osteolysis susceptibility. Cross-linked PE reduces particle generation by 90%.
- Validated criteria for PJI diagnosis. Major criteria (sinus tract, positive cultures) have 100% specificity. Minor criteria scored: synovial WBC more than 3000 or PMN more than 80% highly predictive. Alpha-defensin has 97% sensitivity and specificity.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Presentation of Aseptic Loosening
"A 68-year-old man presents with progressive groin pain 12 years after cemented THA. He had good function until 18 months ago when pain gradually worsened. X-rays show progressive lucent lines in DeLee/Charnley zones I and II around the acetabular component, measuring 3mm. Gruen zone 1 and 7 show 2mm lucent lines on the femoral side. ESR is 18, CRP is 6. How do you assess and manage this patient?"
Scenario 2: Differentiating Aseptic from Septic Loosening
"A 72-year-old woman presents with 6 months of progressive hip pain, 8 years after uncemented THA. X-rays show lucent lines around the femoral stem in all Gruen zones. ESR is 45, CRP is 18. She has no fever or systemic symptoms. How do you proceed?"
Scenario 3: Severe Bone Loss - Paprosky IIIB
"You are planning revision THA for aseptic loosening. The CT scan shows Paprosky Type IIIB acetabular bone loss with superior migration more than 4cm and destruction of the ischium and teardrop. There is also complete loss of the anterior and posterior columns. How do you manage this acetabular defect?"
MCQ Practice Points
Mechanism Question
Q: What is the primary mechanism by which polyethylene wear debris causes osteolysis in aseptic loosening?
A: Macrophage activation leading to cytokine release (TNF-α, IL-1, IL-6, RANKL). These cytokines recruit and activate osteoclasts, causing bone resorption. This is a biologic process (particle disease), not purely mechanical loosening.
Zone Question
Q: A patient has a 3mm lucent line in Gruen zone 1. What does this indicate?
A: Gruen zone 1 is the proximal medial (calcar) region. A 3mm lucent line (threshold is 2mm) indicates calcar resorption and suggests femoral component loosening or impending subsidence. This is concerning and warrants close follow-up or consideration of revision.
Infection Differentiation
Q: What is the threshold synovial fluid white blood cell count that suggests periprosthetic joint infection rather than aseptic loosening?
A: More than 3000 cells/μL OR more than 80% polymorphonuclear cells (PMNs). Either of these thresholds is a minor criterion in the MSIS criteria for PJI. Aseptic loosening typically has synovial WBC less than 3000 with less than 80% PMNs.
Classification Question
Q: A revision THA CT shows acetabular superior migration of 4cm with destruction of the ischium and teardrop. What Paprosky classification is this?
A: Paprosky Type IIIB. Type III is defined by superior migration more than 3cm. Type IIIB specifically has ischial and teardrop destruction (loss of Köhler's line), indicating severe bone loss. This requires complex reconstruction (cup-cage or triflange).
Treatment Question
Q: What is the primary advantage of highly cross-linked polyethylene (HXLPE) over conventional polyethylene in THA?
A: Reduced wear rate by 80-90% (from 0.1-0.2mm/year to under 0.05mm/year). This reduces particle generation, decreasing osteolysis and aseptic loosening rates. Registry data shows HXLPE has significantly improved long-term implant survivorship.
Radiology Question
Q: What radiographic finding is definitive for component loosening?
A: Migration of the component (change in position more than 5mm in 2 years) or component/cement fracture. Progressive lucent lines suggest loosening but are not definitive until migration occurs or lucency becomes circumferential (more than 2mm).
Australian Context
AOANJRR (Australian Orthopaedic Association National Joint Replacement Registry) Data:
The AOANJRR provides world-leading data on THA outcomes in Australia.
Key findings relevant to aseptic loosening:
Primary THA:
- Aseptic loosening rate: Decreased from 10% at 10 years (1990s) to under 2% per year (2010s)
- HXLPE impact: Introduction of highly cross-linked polyethylene in 2000s correlated with decreased loosening rates
- Uncemented vs cemented: Similar loosening rates in modern implants (under 65 years: uncemented preferred; over 65: either acceptable)
Revision THA:
- Aseptic loosening accounts for 25-30% of all revision indications
- Uncemented stems in revision have lower re-revision rates than cemented (8.5% vs 11.2% at 10 years)
- Trabecular metal cups show good fixation in acetabular bone loss scenarios
- Dual-mobility bearings reduce dislocation risk in revision setting
Australian clinical practice:
The management of aseptic loosening in Australia follows a multidisciplinary approach with arthroplasty surgeons, infectious disease specialists (when infection suspected), and specialized anesthesia teams. Complex revisions are typically managed in major public hospitals with subspecialty expertise and access to advanced implants.
Registry participation through the AOANJRR is mandatory for all joint replacements in Australia. This world-leading registry provides comprehensive data showing that aseptic loosening rates have decreased significantly with modern highly cross-linked polyethylene. Uncemented stems are preferred in revision surgery based on registry evidence of superior outcomes, and trabecular metal components show good results for acetabular bone loss reconstruction.
Implant availability is excellent in Australia with both public and private sectors having access to advanced revision systems including modular stems, trabecular metal cups, reconstruction cages, and dual-mobility bearings. The choice of implants is increasingly guided by registry data demonstrating long-term outcomes.
Registry Importance
AOANJRR is the world's largest and most comprehensive joint replacement registry. For the Orthopaedic exam, be familiar with: (1) Aseptic loosening decreased with HXLPE, (2) Uncemented stems preferred in revision, (3) Trabecular metal useful for bone loss, (4) Dual-mobility reduces dislocation in revision.
ASEPTIC LOOSENING IN THA
High-Yield Exam Summary
DEFINITION AND MECHANISM
- •Aseptic loosening = implant failure due to particle-induced osteolysis (NOT infection)
- •Particle disease: PE wear → macrophage activation → cytokines (TNF-α, IL-1, RANKL) → osteoclasts → bone resorption
- •Vicious cycle: loosening → micromotion → more particles → more osteolysis
- •Most common late cause of THA revision (10-15 years post-primary)
RADIOGRAPHIC ZONES
- •Gruen zones 1-7 (femur): Zone 1 (calcar), Zone 7 (lateral proximal)
- •DeLee/Charnley I-III (acetabulum): Zone I (superior dome), Zone II (medial), Zone III (inferior)
- •Lucent line more than 2mm = concerning for loosening
- •Migration more than 5mm in 2 years = definite loosening
- •Serial X-rays essential - progression is key finding
INFECTION EXCLUSION (CRITICAL)
- •NEVER assume aseptic without ruling out infection
- •Check ESR (threshold 30), CRP (threshold 10)
- •If elevated markers: MUST perform hip aspiration
- •Aspiration: WBC more than 3000 or PMN more than 80% suggests infection
- •MSIS criteria: major (sinus, 2+ cultures) or minor (scored)
- •Alpha-defensin: high sensitivity/specificity for PJI
PAPROSKY CLASSIFICATION
- •Acetabular Type I: minimal bone loss (under 25%)
- •Type IIA-IIB: 25-50% loss, migration under 3cm
- •Type IIIA: migration more than 3cm, intact Köhler's line
- •Type IIIB: migration more than 3cm + ischium/teardrop destruction
- •Femoral Type I-II: metaphyseal loss, intact diaphysis
- •Type IIIA-IIIB: diaphyseal involvement
- •Type IV: non-supportive diaphysis
REVISION STRATEGY
- •Rule out infection first (aspiration if any doubt)
- •CT scan for bone loss assessment (Paprosky classification)
- •Type I-IIA acetabulum: standard hemispheric cup
- •Type IIB-IIIA: jumbo cup, augments, trabecular metal
- •Type IIIB: cup-cage construct or custom triflange
- •Femoral Type I-II: cemented or proximally coated
- •Type IIIA-IIIB: extensively coated diaphyseal fit
KEY EXAM POINTS
- •Aseptic loosening is biologic process (particle disease), not mechanical
- •HXLPE reduced wear by 80-90% (modern standard)
- •Always aspirate if ESR more than 30 or CRP more than 10 before revision
- •Serial X-rays show progression (single film cannot diagnose)
- •Outcomes: 80-90% survivorship at 10 years (worse than primary)
- •AOANJRR shows uncemented stems better in revision than cemented