High-yield overview
Systematic radiographic evaluation of joint replacements - component position, fixation, wear and the recognition of loosening, instability, periprosthetic fracture and adverse tissue reaction
—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
Clinical 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
Clinical 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.
Mnemonic
A-B-C-D-E-FSystematic Read of Any Arthroplasty Film
A
Alignment - component position (cup inclination/anteversion, stem varus/valgus, limb length, offset)
B
Bone - lucency at interfaces by zone (Gruen / DeLee-Charnley), osteolysis, stress shielding, fracture
C
Cement / Coating - cement mantle integrity and grade, or osseointegration of cementless implants
D
Device - implant type, modularity, bearing surface, size, evidence of breakage
E
Edges & soft tissues - heterotopic ossification, periprosthetic fracture, effusion, pseudotumour
F
Films over time - ALWAYS compare with serial radiographs; progression is the key to loosening
Hook:A single radiograph rarely confirms loosening - migration or progressive lucency on serial films is what counts. State this explicitly in the viva.
Mnemonic
The 4 P'sWhen a Radiolucent Line Means Loosening
P
Progressive - lucency thicker on the newer film than the older film
P
Pervasive - present in ALL zones (all 3 DeLee-Charnley or a complete Gruen envelope)
P
Plus migration - any change in component position is the most specific single sign
P
Painful - radiographic change that correlates with the patient's symptoms
Hook:A thin (under 2 mm), non-progressive, single-zone line is usually a stable fibrous membrane, not loosening. Width AND progression together raise concern.
Overview & Imaging Principles
Arthroplasty is among the highest-volume elective procedures in orthopaedics, and surveillance imaging is a core skill tested in every fellowship exam. The plain radiograph remains the first-line and most informative single investigation: it assesses component position, fixation, polyethylene wear, periprosthetic bone and fracture, and - through serial comparison - is the principal tool for diagnosing loosening. Cross-sectional imaging (CT with metal-artefact reduction, MRI with MARS, ultrasound and nuclear medicine) is reserved for specific questions that plain films cannot answer.
Why Serial Films Dominate
The diagnosis of aseptic loosening rests on demonstrating change over time - migration, subsidence, or progressive lucency. A single film captures a snapshot; the previous films supply the trajectory. Always request and compare priors before committing to a diagnosis of loosening.
Diagnostic Performance of Plain Films
In a meta-analysis of imaging for femoral aseptic loosening, plain radiography achieved sensitivity ~82% and specificity ~81% - comparable to arthrography and scintigraphy, with lower morbidity. Plain films and bone scintigraphy are the preferred initial work-up (Temmerman et al., JBJS Br 2005).
When to Escalate
Use CT-MARS for osteolysis volume, component version and bony defect mapping; MRI-MARS for adverse local tissue reaction, abductor/tendon integrity and soft-tissue masses; ultrasound for fluid collections; and nuclear medicine (WBC/marrow or three-phase bone scan) when prosthetic joint infection is suspected.
Clinical Imaging: Standard Radiographic Assessment
| 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
Systematic Approach: Zone Classification Systems
| 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
| 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
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
| 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
| 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
| 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
| 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
| 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.
Differential Diagnosis of the Painful Arthroplasty
A painful joint replacement is a common exam and clinic scenario. Imaging must be interpreted alongside the clinical picture and inflammatory markers, because the single most important diagnosis to exclude - prosthetic joint infection - can produce radiographs that look identical to aseptic loosening.
| Diagnosis | Typical Pain Pattern | Key Imaging Features | Confirmatory Test |
|---|---|---|---|
| Aseptic loosening | Start-up / activity-related, deep | Progressive lucency in all zones, migration, subsidence, no aggressive periostitis | Serial radiographs; intra-operative findings |
| Prosthetic joint infection | Constant rest pain, early onset, stiffness | Lucency that can be indistinguishable from aseptic loosening; periostitis or rapid osteolysis raise suspicion | Aspiration (cell count, culture), CRP/ESR, alpha-defensin |
| Polyethylene wear / osteolysis | Often painless until late | Eccentric head position, expansile balloon lucencies (particle disease) | Serial wear measurement, CT for osteolysis volume |
| Instability / recurrent dislocation | Mechanical, positional, giving way | Cup mal-orientation, eccentric head, impingement; combined version outside target | CT for version; dynamic/stress views |
| Periprosthetic fracture | Acute, post-fall, unable to weight-bear | Cortical break (Vancouver/Su); assess stem fixation | Orthogonal radiographs; CT if occult |
| Adverse local tissue reaction (MoM/taper) | Groin/thigh ache, swelling, clicking | Effusion, pseudotumour, muscle/tendon damage on MARS-MRI | MARS-MRI, serum cobalt/chromium ions |
| Stress shielding | Usually asymptomatic | Proximal femoral bone loss with cementless stems, no migration | Serial radiographs (not true loosening) |
| Extrinsic / referred | Variable, non-mechanical | Normal implant; look at spine, vessels, abdomen | Targeted imaging of source |
Caution
Never report progressive lucency around an implant as "aseptic loosening" without considering low-grade infection. Radiographs cannot reliably separate the two; the work-up of a painful arthroplasty must include inflammatory markers and a low threshold for joint aspiration.
Evidence Base & Landmark Classifications
The radiographic frameworks used at the viva table - Gruen and DeLee-Charnley zones, the Lewinnek safe zone, the Brooker grade and the Vancouver classification - each originate from a specific paper. Knowing the source, the cohort and the modern caveats demonstrates depth.
Evidence
Gruen Zones - 'Modes of Failure' of Cemented Femoral Stems
Gruen, McNeice & Amstutz. Clin Orthop Relat Res 1979;(141):17-27
Key Findings:
- Defined the 7-zone femoral analysis still used today
- Loosening is a zonal, mode-specific process - not random
- Proximal (calcar) and distal (tip) patterns reflect different failure mechanics
- Serial radiographs distinguished progressive from static lucency
Clinical implication: Report femoral lucency zone by zone; a calcar (zone 7) or proximal pattern suggests proximal pivot loosening, while a distal pedestal with proximal lucency reflects cantilever-type failure.
Evidence
DeLee-Charnley Zones - Acetabular Radiolucency
DeLee & Charnley. Clin Orthop Relat Res 1976;(121):20-32
Key Findings:
- Established the 3-zone (I superior, II medial, III inferior) acetabular system
- Demarcation is common and usually benign; migration is the worrying sign
- Progressive migration - not the mere presence of a line - predicts failure
- Technical/surgical factors and low-grade sepsis explained most migrating cases
Clinical implication: A thin acetabular demarcation line is frequently a stable finding; reserve the diagnosis of cup loosening for lucency in all three zones or definite migration on serial films.
Evidence
Lewinnek Safe Zone - Cup Orientation and Dislocation
Lewinnek et al. J Bone Joint Surg Am 1978;60(2):217-20
Key Findings:
- Origin of the classic 40 deg plus or minus 10 inclination and 15 deg plus or minus 10 anteversion targets
- Roughly four-fold higher dislocation outside the zone
- Greatest dislocation risk in the first 30 days and after prior surgery
- Defined cup orientation as a measurable, modifiable radiographic target
Clinical implication: Measure cup inclination and anteversion on every post-operative film; the Lewinnek targets remain a useful checklist, but see the contemporary caveat below.
Evidence
Contemporary Caveat - The Lewinnek 'Safe Zone' Is Not Truly Safe
Abdel et al. Clin Orthop Relat Res 2016;474(2):386-91
Key Findings:
- The majority of dislocating THAs were inside the 'safe zone'
- Cup position is necessary but not sufficient for stability
- Spinopelvic mobility, soft tissues, head size and approach all contribute
- Targets are a guide, not a guarantee
Clinical implication: Use the Lewinnek targets as a starting checklist but recognise that a cup 'within range' does not exclude instability - integrate spinopelvic factors and patient-specific risk in the exam answer.
Evidence
Brooker Classification - Heterotopic Ossification after THA
Brooker, Bowerman, Robinson & Riley. J Bone Joint Surg Am 1973;55(8):1629-32
Key Findings:
- Standard radiographic grading of HO around the hip
- Higher grades correlate with reduced range of motion
- Grade IV (ankylosis) may warrant excision after maturation
- Provides a reproducible language for follow-up films
Clinical implication: Grade heterotopic ossification with the Brooker system on AP pelvis films; reserve excision for symptomatic Grade III-IV disease after maturation (typically 12-18 months) with prophylaxis to prevent recurrence.
Evidence
Imaging Accuracy for Aseptic Femoral Loosening (Meta-analysis)
Temmerman et al. J Bone Joint Surg Br 2005;87(6):781-5
Key Findings:
- Plain radiography performs as well as more invasive tests
- No single modality is clearly superior for femoral loosening
- Plain films plus bone scintigraphy are the preferred first-line work-up
- Specificity is imperfect - correlate with symptoms and serial films
Clinical implication: Justify plain radiography as the default investigation for suspected loosening; escalate to scintigraphy or cross-sectional imaging only when films are equivocal or infection is suspected.
Evidence
Vancouver Classification - Periprosthetic Femoral Fractures
Greidanus, Mitchell, Masri, Garbuz & Duncan. Instr Course Lect 2003;52:309-22
Key Findings:
- Treatment hinges on whether the stem is well-fixed or loose
- B1 - retain stem with ORIF; B2 - revise the stem; B3 - revise plus reconstruct bone
- Radiographic assessment of pre-existing lucency is essential to subtype
- Fractures around loose/malaligned stems do poorly with fixation alone
Clinical implication: When reading a periprosthetic femoral fracture, the decisive radiographic question is stem fixation - obtain prior films and assess lucency/subsidence before classifying B1 versus B2/B3.
Areas of Uncertainty & Controversy
What Is a 'Safe' Cup Position?
The Lewinnek zone is being superseded by functional, spinopelvic-aware targets. Patients with a stiff or hypermobile lumbosacral junction may dislocate despite a textbook static cup angle. Functional anteversion (assessed on standing/sitting lateral imaging or EOS) is increasingly emphasised, but no single replacement target is universally agreed.
Loosening versus Infection on Plain Films
There is no radiographic sign that reliably separates aseptic loosening from low-grade prosthetic joint infection. Rapid or aggressive osteolysis and periostitis raise suspicion, but a normal-looking lucency does not exclude infection - hence the mandatory role of aspiration and inflammatory markers.
Surveillance of Metal-on-Metal Bearings
Thresholds for cross-sectional imaging and revision in metal-on-metal and modular-taper hips remain debated. Regulators advise risk-stratified follow-up using symptoms, blood metal ions and MARS-MRI, but exact ion cut-offs and imaging intervals differ between national agencies.
Guidelines, Registries & Global Practice
Arthroplasty surveillance is informed by national joint registries and society guidance worldwide. Imaging recommendations converge on serial plain radiographs as the backbone, with risk-stratified cross-sectional imaging for specific failure modes.
| Body (Region) | Focus | Imaging-Relevant Guidance |
|---|---|---|
| AAOS (US) | THA/TKA surveillance & PJI | Plain radiographs first-line; standardised work-up for the painful arthroplasty including ESR/CRP and aspiration before attributing pain to aseptic loosening |
| BOA / NICE (UK) | Joint replacement follow-up | Risk-stratified radiographic follow-up (e.g. at defined intervals for higher-risk implants); registry-linked surveillance |
| MHRA / FDA (UK / US regulators) | Metal-on-metal & modular tapers | Risk-based surveillance using symptoms, serum cobalt/chromium ions and cross-sectional imaging (MARS-MRI or ultrasound) for adverse local tissue reaction |
| EFORT / national societies (Europe) | Outcome reporting & follow-up | Harmonised radiographic outcome measures; emphasis on registry data for implant performance |
| ISO 5832 / implant standards | Implant identification | Underpins component traceability that aids radiographic implant recognition |
| Registry (Region) | Contribution | Imaging Relevance |
|---|---|---|
| NJR (UK) | Large primary & revision volumes | Implant- and bearing-specific revision rates flag designs warranting closer radiographic surveillance |
| AOANJRR (Australia) | Detailed revision-by-diagnosis data | Early identification of poorly performing implants (e.g. certain MoM hips) prompting targeted imaging |
| AJRR (US) | Growing national dataset | Benchmarking revision causes including loosening, instability and PJI |
| SHAR / Nordic registries | Long-term implant survival | Decades-long survivorship informing follow-up intervals and revision-risk imaging |
High-Resource Settings
Access to CT-MARS, MARS-MRI, serum metal ions and registry-linked recall enables risk-stratified, implant-specific surveillance and earlier detection of failure modes.
Limited-Resource Settings
Plain radiography - cheap, available and diagnostically robust - carries most of the surveillance load. A disciplined systematic read with serial comparison and clinical/biochemical correlation remains the highest-yield approach where advanced imaging is scarce.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Clinical prompt
“A 72-year-old presents with thigh pain 8 years after cemented total hip arthroplasty. You are shown an AP pelvis X-ray.”
Practical approach
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 clinical points
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 pitfalls
Diagnosing loosening on single X-ray
Confusing stress shielding with loosening
Missing PE wear (eccentric head position)
Further questions
“What investigations would you request if you suspected infection rather than aseptic loosening?”
Viva scenarioChallenging
Clinical prompt
“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.”
Practical approach
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 clinical points
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 pitfalls
Not assessing stem fixation before planning treatment
Missing pre-existing loosening
Treating B2/B3 as B1 (will fail with retained loose stem)
Further questions
“How would you fix a Vancouver B1 fracture? What implants would you use for B3?”
Viva scenarioStandard
Clinical prompt
“A patient 2 years post TKR presents with anterior knee pain and difficulty with stairs. X-rays including skyline view are shown.”
Practical approach
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 clinical points
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 pitfalls
Not obtaining skyline view
Missing subtle maltracking
Ignoring joint line height changes
Further questions
“What are the treatment options for a loose patellar component? When would you leave the patella unresurfaced?”
MCQ & Exam Practice Points
Key Point
Q: A radiolucent line is seen in only DeLee-Charnley zone III on a single post-operative pelvis film. Does this confirm acetabular loosening? A: No. A thin, non-progressive line in one zone is usually a stable fibrous membrane. Loosening requires lucency in all three zones, progression on serial films, or definite cup migration - a single film cannot confirm it.
Key Point
Q: A dislocated THA has a cup measured at 42 degrees inclination and 14 degrees anteversion. The cup is 'within the Lewinnek safe zone' - so cup position cannot be the problem, correct? A: Incorrect. Abdel et al. (CORR 2016) showed 58% of dislocating THAs had cups inside the Lewinnek zone. Stability is multifactorial - consider spinopelvic mobility, head size, soft-tissue tension, offset and approach.
Key Point
Q: Which single radiographic sign is the most specific for component loosening? A: Component migration (a change in position on serial films). Progressive lucency over 2 mm is supportive, but migration is the most specific single sign - which is why prior films are essential.
Exam day cheat sheet
Arthroplasty Imaging Quick Reference
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