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:
| 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) |
Image 1: AP Pelvis and Lateral Hip Radiograph
Radiological features:
Image 2: MARS MRI (Metal Artifact Reduction Sequence)
MRI findings:
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?
This pattern suggests MoM Failure (ARMD):
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 |
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:
Consequences:
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:
Implications: