Background
Large-head metal-on-metal total hips and many resurfacing systems are no longer used routinely because some designs produced excessive wear, metal-ion release and adverse local tissue reactions.
Presentation
Patients may be:
- asymptomatic with a recalled implant
- painful with otherwise normal radiographs
- painful with swelling, weakness, instability or a soft-tissue mass
Do not assume every painful metal-on-metal hip has metal-debris disease. Differential diagnoses include infection, loosening, iliopsoas pathology, spine disease and mechanical impingement.
Assessment
- implant identification and date of surgery
- symptom trajectory
- gait and abductor function
- plain radiographs
- whole-blood cobalt/chromium testing where indicated
- metal-artefact-reduction MRI or ultrasound for soft-tissue assessment
Follow current national regulator guidance for surveillance thresholds because recommendations can change.
Review:
- implant type
- head size
- component position
- symptoms
- serial radiographs
- blood metal-ion trends where indicated
- cross-sectional imaging
Metal artefact reduction MRI is useful for defining soft-tissue lesions and abductor damage.
ARMD
Adverse reaction to metal debris can produce:
- synovitis
- tissue necrosis
- fluid collections or pseudotumour
- abductor destruction
- osteolysis
Histology may show lymphocyte-predominant responses in some patients.
Revision principles
- rule out infection
- define soft-tissue damage before surgery
- remove the problematic bearing/components as required
- excise necrotic tissue while preserving viable structures
- reconstruct stability and offset
- use a non-metal-on-metal bearing
Extensive abductor damage increases the risk of postoperative instability.
Mechanisms of metal debris
Metal-on-metal hips can generate metal products through:
- bearing wear
- edge loading
- corrosion at modular junctions
- fretting at tapers
Cobalt and chromium species may enter local tissue and systemic circulation.
Adverse reaction to metal debris
ARMD describes a spectrum including:
- synovitis
- fluid collections
- pseudotumour
- necrosis
- muscle damage
- osteolysis
- loosening
Histology can show variable macrophage and lymphocyte-dominant responses.
Clinical presentation
Patients may report:
- groin pain
- swelling
- clicking
- instability
- weakness
- reduced function
Some substantial lesions are relatively asymptomatic, which is why surveillance programmes were developed for at-risk implants.
Interpretation of metal ions
A single ion result should not be used in isolation. Interpretation depends on:
- implant type
- trend
- symptoms
- imaging
- renal function
- other metal implants
Thresholds are surveillance tools rather than absolute revision indications.
Revision
Revision is considered when there is evidence of significant adverse tissue reaction, progressive damage, component failure or unacceptable symptoms.
Surgery may be difficult because tissue necrosis and abductor destruction can compromise stability.
At revision:
- remove the source of metal debris
- debride non-viable tissue
- assess component fixation
- address bone loss
- select an appropriate non-MoM bearing
FRCS synthesis
The key is that MoM failure can be a soft-tissue biological failure before radiographic loosening. Combine symptoms, ions, imaging and implant factors.