High Yield Overview
Arthroplasty Imaging: Assessment & Complications
—THA Revision Rate
5%Approximately at 10 years
—Loosening Sensitivity (X-ray)
90%Greater than
—Lucency Threshold
2Greater than mm progressive
—Cup Abduction
40° ± 10° optimal
Gruen Zones (Femoral Component)
Zone 1: Lateral proximal
Zone 2: Lateral mid
Zone 3: Lateral distal
Zone 4: Stem tip
Zone 5: Medial distal
Zone 6: Medial mid
Zone 7: Medial proximal
Key: Lucency in zones 1/7 (proximal) with cemented stem suggests high stress loosening
Critical Must-Knows
- Progressive lucency greater than 2mm suggests loosening
- DeLee-Charnley zones (acetabulum), Gruen zones (femur)
- Component migration is most specific sign of loosening
- Heterotopic ossification: Brooker classification (I-IV)
- Polyethylene wear: eccentric head position, osteolysis
Examiner's Pearls
- "Serial X-rays essential - single film cannot confirm loosening
- "All 3 acetabular zones or all femoral zones = definite loosening
- "PE wear precedes osteolysis by years
- "Varus/valgus stem: risk for periprosthetic fracture
- "CT with MARS for metal artefact reduction
Exam Warning
Arthroplasty imaging is commonly examined. You must know the zone systems (Gruen, DeLee-Charnley), how to measure component position (cup inclination/anteversion, stem alignment), and differentiate loosening from normal radiolucent lines.
Standard Radiographic Assessment
THA Radiographic Assessment Checklist
| Parameter | Measurement | Normal/Optimal Range |
|---|---|---|
| Cup inclination (abduction) | Angle between cup face and inter-teardrop line | 40° ± 10° (30-50°) |
| Cup anteversion | Lateral view or CT | 15° ± 10° (5-25°) |
| Cup coverage | Superolateral cup covered by bone | Greater than 80% |
| Leg length | Compare lesser trochanter to teardrop line | Within 10mm of contralateral |
| Femoral offset | Centre of head to femoral shaft axis | Restore native offset ± 5mm |
| Stem alignment | Varus/valgus relative to shaft | Neutral to 3° valgus |
| Stem subsidence | Distance from stem tip to fixed landmark | Less than 2-3mm stable |
Radiographic Examples
Click to expand
Click to expand
Click to expand
Click to expand
Click to expand
Zone Classification Systems
Gruen Zones - Femoral Component (THA)
| Zone | Location | Clinical Significance |
|---|---|---|
| Zone 1 | Lateral proximal (greater trochanter) | High stress area, loosening indicator |
| Zone 2 | Lateral middle | Cemented mantle assessment |
| Zone 3 | Lateral distal | Cement-bone interface |
| Zone 4 | Stem tip | End-bearing stress, pedestal formation |
| Zone 5 | Medial distal | Cement-bone interface |
| Zone 6 | Medial middle | Calcar region assessment |
| Zone 7 | Medial proximal (calcar) | High stress, loosening indicator |
Loosening Pattern
For cemented stems: Lucency in zones 1 and 7 (proximal medial and lateral) is the classic loosening pattern. Complete lucency in all zones = definite loosening. Zone 4 pedestal (distal cement) suggests load transfer distally with proximal loosening.
Loosening Assessment
Radiographic Signs of Loosening
| Sign | Description | Significance |
|---|---|---|
| Progressive lucency | Radiolucent line increasing over time | Greater than 2mm or progressive = loosening |
| Component migration | Change in position over serial films | Most specific sign of loosening |
| Cement fracture | Break in cement mantle | Cemented component failure |
| Particle disease/osteolysis | Focal lucent areas around component | Wear debris-induced resorption |
| Pedestal formation | Lucent line with distal sclerosis | Distal load transfer, proximal loosening |
| Subsidence | Distal migration of stem | Measure from tip to fixed landmark |
Mnemonic
Progressive = ProblemDifferentiating Loosening from Normal Lines
P
Progressive: Lucency increasing on serial films = loosening
P
Parallel: Thin line parallel to implant less than 2mm = often normal fibrous tissue
P
Partial: Incomplete lucency in 1-2 zones = monitor closely
P
Problem: Complete circumferential lucency or migration = definite loosening
Memory Hook:A single X-ray cannot confirm loosening - always compare with prior films and assess for progression
Cemented vs Uncemented Loosening
Cemented: Lucency at cement-bone interface, cement mantle fracture, component migration. Uncemented: Lucency at implant-bone interface, lack of bone ongrowth, shedding, stem subsidence greater than 3mm in first 2 years.
Stress Shielding
Reduced bone density in proximal femur with uncemented stems. Due to load bypass. Not true loosening but indicates altered biomechanics. More common with extensively coated stems.
Polyethylene Wear
Signs of Polyethylene Wear
| Finding | Description | Clinical Implication |
|---|---|---|
| Eccentric head position | Femoral head not centred in cup | Direct wear measurement possible |
| Superior head migration | Head moves toward dome of cup | Most common wear direction |
| Linear wear rate | Measure head-cup distance on serial films | Greater than 0.2mm/year = excessive |
| Osteolysis | Lucent areas around components | Wear debris-induced bone resorption |
| Particle disease | Balloon lesions expanding from joint | Requires revision to halt progression |
Wear Measurement Technique
Compare femoral head position relative to cup centre. Measure minimum distance from head to cup margin. Serial films allow calculation of wear rate. CT can assess volumetric wear more accurately. Greater than 0.2mm/year linear wear concerning for conventional PE.
Heterotopic Ossification
Brooker Classification of Heterotopic Ossification
| Grade | Description | Clinical Significance |
|---|---|---|
| Grade I | Islands of bone within soft tissue | Usually asymptomatic |
| Grade II | Bone spurs from pelvis or proximal femur, greater than 1cm gap | Mild limitation |
| Grade III | Bone spurs with less than 1cm gap | Moderate limitation |
| Grade IV | Apparent bony ankylosis | Severe limitation, may need excision |
HO Prevention
Risk factors: Male, post-traumatic OA, hypertrophic OA, previous HO, ankylosing spondylitis. Prevention: NSAIDs (indomethacin 75mg daily x 6 weeks) or radiation (single dose 700cGy within 72 hours). Excision: Wait 12-18 months for maturation before surgical excision.
Instability and Dislocation
Risk Factors for THA Dislocation
| Factor | Risk Contribution | Radiographic Assessment |
|---|---|---|
| Cup malposition | Abduction greater than 55° or less than 30° | Measure cup inclination on AP |
| Anteversion | Combined less than 25° or greater than 50° | CT for accurate measurement |
| Femoral offset | Reduced offset decreases stability | Compare to contralateral |
| Head size | Smaller heads higher dislocation risk | Less than 32mm higher risk |
| Impingement | Bone or soft tissue blocking motion | Look for osteophytes, HO |
Periprosthetic Fracture
Vancouver Classification of Periprosthetic Hip Fractures
| Type | Location | Subtype | Treatment Principle |
|---|---|---|---|
| A | Trochanteric region | AG: Greater troch, AL: Lesser troch | Often non-operative if stable |
| B1 | Around/below stem | Stem WELL FIXED | ORIF, keep stem |
| B2 | Around/below stem | Stem LOOSE | Revision stem + ORIF |
| B3 | Around/below stem | Stem LOOSE + poor bone | Revision with structural allograft |
| C | Below stem tip | Stem unaffected | Treat as standard fracture |
Key Decision Point
B1 vs B2/B3: Is the stem loose? This determines whether to keep or revise the stem. X-ray assessment: Look for pre-existing lucencies, component position change, subsidence. If uncertain, obtain prior films for comparison or proceed to intraoperative assessment.
Metal Artefact Reduction
Imaging Options for Metal Artefact
| Modality | Technique | Indication |
|---|---|---|
| X-ray | Standard technique | First-line, least artefact |
| CT MARS | Metal artefact reduction sequences | Bone detail, osteolysis assessment |
| MRI MARS | Metal artefact reduction, STIR | Soft tissue, ALTR assessment |
| Ultrasound | No metal artefact | Fluid collections, tendons |
| Nuclear medicine | WBC/marrow scan | PJI assessment |
ALTR Imaging (Metal-on-Metal)
Adverse Local Tissue Reaction in MoM bearings. MRI with MARS (metal artefact reduction) is gold standard. Look for: pseudotumour, solid or cystic masses, fluid collections, muscle atrophy, tendon damage. Ultrasound useful for fluid-predominant lesions. Blood metal ion levels guide surveillance frequency.
Special Circumstances
Unicompartmental Knee
Assess: Component alignment, progression of opposite compartment OA, overhang (medial tibial component may cause MCL irritation). Common failure: progression of OA in unreplaced compartments, tibial loosening.
Reverse Shoulder Arthroplasty
Key measurements: Glenosphere position relative to inferior glenoid (should be low), baseplate inclination. Watch for: scapular notching (inferior glenoid wear), acromial fractures, component loosening.
Revision Arthroplasty
Classify bone loss (Paprosky for THA, AORI for TKR). Assess remaining bone stock. Look for previous cement/hardware. Plan for longer stems, augments, or structural grafts.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 72-year-old presents with thigh pain 8 years after cemented total hip arthroplasty. You are shown an AP pelvis X-ray."
EXCEPTIONAL ANSWER
I would systematically assess: (1) Acetabular component - cup inclination (optimal 40° ± 10°), DeLee-Charnley zone lucencies, migration or change in version. (2) Femoral component - Gruen zone lucencies (especially zones 1 and 7 for cemented stems), stem alignment, subsidence compared to prior films, cement mantle integrity, distal pedestal formation. (3) Polyethylene wear - eccentric head position, osteolysis. Features suggesting loosening: progressive lucency greater than 2mm, lucency in all zones, component migration, cement fracture, subsidence. I would compare with prior films as a single X-ray cannot confirm loosening.
KEY POINTS TO SCORE
Systematic: cup then stem, zone by zone
Compare with prior films - progression is key
Gruen zones 1 and 7 = high stress, early loosening
Greater than 2mm lucency or complete circumferential = loosening
Migration is most specific sign
COMMON TRAPS
✗Diagnosing loosening on single X-ray
✗Confusing stress shielding with loosening
✗Missing PE wear (eccentric head position)
VIVA SCENARIOStandard
EXAMINER
"A 60-year-old falls at home 3 years after cementless THA. X-ray shows a periprosthetic femoral fracture at the level of the stem."
EXCEPTIONAL ANSWER
This is a Vancouver B fracture (around or below the stem). The critical question is whether the stem is well-fixed or loose, which determines B1, B2, or B3 classification. I would assess: pre-existing lucencies at the stem-bone interface, any change in stem position compared to prior films, quality of bone stock. Vancouver B1 (stem well-fixed): ORIF with plate, retain stem. Vancouver B2 (stem loose, adequate bone): revision long stem plus fracture fixation. Vancouver B3 (stem loose, poor bone): revision with structural allograft or tumour prosthesis. The key decision is stem fixation status - this dictates whether to revise or retain.
KEY POINTS TO SCORE
Vancouver B1/B2/B3 depends on stem fixation status
B1: stem well-fixed = ORIF, keep stem
B2: stem loose, good bone = revision stem
B3: stem loose, poor bone = revision + augmentation
Compare with prior films for pre-existing lucency
COMMON TRAPS
✗Not assessing stem fixation before planning treatment
✗Missing pre-existing loosening
✗Treating B2/B3 as B1 (will fail with retained loose stem)
VIVA SCENARIOStandard
EXAMINER
"A patient 2 years post TKR presents with anterior knee pain and difficulty with stairs. X-rays including skyline view are shown."
EXCEPTIONAL ANSWER
On the skyline view, I would assess: (1) Patellar component position - central tracking, any lateral tilt or subluxation. (2) Component overhang - resurfaced patella should not overstuff the patellofemoral joint. (3) Patellar thickness - should be greater than 12mm to reduce fracture risk. (4) Lucency around pegs - suggesting loosening. (5) Evidence of fracture - transverse or vertical patterns. (6) Patellar clunk syndrome - soft tissue in intercondylar notch. On lateral view: joint line height (patella baja or alta), component size relative to native patella. For an unresurfaced patella: assess for progressive OA, osteophyte formation, bone loss.
KEY POINTS TO SCORE
Skyline view essential for patellar assessment
Lateral tilt/subluxation = maltracking
Patella thickness less than 12mm = fracture risk
Lucency around pegs = loosening
Joint line height affects patellofemoral mechanics
COMMON TRAPS
✗Not obtaining skyline view
✗Missing subtle maltracking
✗Ignoring joint line height changes
Arthroplasty Imaging Quick Reference
High-Yield Exam Summary
Optimal Component Position
- •Cup inclination: 40° ± 10°
- •Cup anteversion: 15° ± 10°
- •Combined anteversion: 25-50°
- •Stem: Neutral to 3° valgus
- •Leg length: Within 10mm
Zone Systems
- •Gruen (femur): 7 zones (1-3 lateral, 4 tip, 5-7 medial)
- •DeLee-Charnley (acetabulum): 3 zones (I superior, II medial, III inferior)
- •All zones lucent = definite loosening
- •Zones 1 and 7 = high stress loosening pattern
Loosening Signs
- •Progressive lucency greater than 2mm
- •Component migration (most specific)
- •Cement fracture
- •Subsidence greater than 3mm
- •Serial films essential
Vancouver Classification (PPF)
- •A: Trochanteric (AG/AL) - often non-op
- •B1: Stem fixed - ORIF, keep stem
- •B2: Stem loose, good bone - revision stem
- •B3: Stem loose, poor bone - revision + graft
- •C: Below stem - treat as standard fracture