CIM Case: Metal-on-Metal Hip Failure

Clinical Scenario

Patient: 58-year-old woman Presentation: Right hip and groin pain 7 years after metal-on-metal THR, worsening over 6 months, difficulty walking and ascending stairs Relevant history: MoM THR in 2017 for hip dysplasia (36mm head), no prior hip surgery, under annual surveillance, asymptomatic until recently, non-smoker, no other joint replacements Examination findings:

Antalgic gait
Tender over greater trochanter and groin
Range of motion limited: flexion 90° (painful), IR 10°, ER 30°
Positive anterior impingement sign
Mild Trendelenburg gait (subtle abductor weakness)
Well-healed lateral wound
No distal neurovascular deficit
No palpable lymphadenopathy
Cardiovascular examination normal

Investigations Provided

Laboratory Results

Test	Result	Normal Range	Interpretation
Cobalt	12 ppb	<7 ppb	↑ Elevated (concerning)
Chromium	9 ppb	<7 ppb	↑ Elevated
Hb	125 g/L	120-150 g/L	Normal
WCC	7.2 ×10⁹/L	4-11 ×10⁹/L	Normal
CRP	5 mg/L	<5 mg/L	Normal
ESR	12 mm/hr	<20 mm/hr	Normal
Creatinine	68 μmol/L	45-90 μmol/L	Normal (renal function)
LFTs	Normal	-	Normal (hepatic function)

Imaging

Image 1: AP Pelvis and Lateral Hip Radiograph

Radiological features:

Large head metal-on-metal THR in situ (36mm)
Cup inclination 55° (steep - outside safe zone of 35-45°)
Cup anteversion appears normal
No obvious osteolysis on plain film
Stem appears well-fixed
No loosening features

Image 2: MARS MRI (Metal Artifact Reduction Sequence)

MRI findings:

Large fluid collection posterior to hip joint (5cm pseudotumour)
Significant soft tissue oedema in periarticular region
Gluteal muscle atrophy with fatty infiltration
No definite abductor tendon tear
Marked inflammatory changes around implant
Features consistent with ARMD (Adverse Reaction to Metal Debris)

Questions & Model Answers

What is the diagnosis and what investigations would you perform?

How do you risk-stratify MoM patients and what surveillance is recommended?

What are the risk factors for MoM failure and the pathophysiology of ARMD?

What are the local and systemic effects of metal poisoning?

The MRI shows pseudotumour. What is your management plan?

What are the expected outcomes and how do you counsel this patient?

Key Teaching Points

Pattern Recognition

This pattern suggests MoM Failure (ARMD):

Patient with MoM bearing and new/progressive hip symptoms
Elevated metal ion levels (>7 ppb cobalt or chromium)
MARS MRI showing pseudotumour or soft tissue destruction
Risk factors: female, steep cup (>50°), larger head size

ARMD vs Periprosthetic Joint Infection:

Feature	ARMD	Infection
CRP/ESR	Usually normal	Elevated
Aspirate	Metallic debris, lymphocytes	Elevated WCC, positive culture
Histology	ALVAL (lymphocyte-dominated)	Neutrophil infiltration
Onset	Gradual (years)	May be acute or chronic

Critical Management Points

All MoM patients need lifelong surveillance - annual clinical review and metal ion testing
Metal ions >7 ppb = concerning - warrant cross-sectional imaging
Edge loading is key mechanism - cup inclination >50° dramatically increases wear
MARS MRI is investigation of choice - detects pseudotumour and soft tissue damage
Early revision = better outcomes - before extensive tissue destruction
Never use MoM at revision - change to ceramic-on-polyethylene or ceramic-on-ceramic

Common Examiner Follow-ups

Q: "What is ARMD and ALVAL?"

Term	Definition
ARMD	Adverse Reaction to Metal Debris - umbrella term for all local adverse reactions to MoM debris (pseudotumour, necrosis, osteolysis)
ALVAL	Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion - specific histological pattern with perivascular lymphocytic infiltrate and tissue necrosis

Key difference: ARMD is clinical/radiological diagnosis; ALVAL is histological diagnosis.

ALVAL represents a type IV hypersensitivity reaction (delayed cell-mediated) to metal ions, unlike polyethylene wear which causes macrophage-dominated osteolysis.

Q: "What is edge loading and why does it matter?"

Edge loading occurs when:

Head articulates at rim of cup rather than centrally
Caused by: steep cup inclination (>50°), inadequate cup coverage, impingement

Consequences:

Friction and wear massively increased (100-1000×)
Rapid metal debris generation
Accelerated ARMD development

Prevention: Cup inclination 35-45° ("safe zone"), adequate anteversion (15-25°).

Q: "What bearing surface would you use at revision?"

Scenario	Recommended Bearing
Standard revision	Ceramic-on-XLPE
Young, active patient	Ceramic-on-ceramic
Abductor deficiency	Dual mobility (stability)
Major bone loss	Ceramic-on-XLPE with reconstruction

Never use metal-on-metal again. The offending bearing must be completely removed.

Q: "What is the DePuy ASR recall and why was it important?"

The DePuy ASR (Articular Surface Replacement) was recalled in 2010:

Hip resurfacing and MoM THR system
Unacceptably high revision rates (5-year: 12-13%)
Design issues causing edge loading
Widespread ARMD

Implications:

Led to increased surveillance of all MoM hips
Established metal ion thresholds (>7 ppb)
Demonstrated importance of arthroplasty registries (AOANJRR)
Litigation and regulatory changes

AOANJRR Hip Registry Data
Bearing Surfaces in Arthroplasty
Metal Hypersensitivity
Revision Hip Arthroplasty
Pseudotumour of Hip
Hip Resurfacing

Metal-on-Metal Hip Failure