Metal on metal hip

Question 1

metal on metal - risk factors

Answer 1

Implant factors
- > 36mm stemmed head
- < 48mm resurfacing head
- ASR implant

Patient factors
- renal insuffiency
- malpositioning of prothesis
- obesity BMI> 30
- metal sensitivity
- immunosupressed/ steroids
- sex - female
- bilateral resurfacings/ metal on metal THR
- pseudotumours

High risk group
Resurfacing
- female
- ASR
- male with <48mm head

Stemmed
- >36mm head - volumetric wear

Low risk group
Resurfacing
- male
- head>48mm
- non-ASR

Stemmed <36mm head

Question 2

Assessment of low risk metal on metal

Answer 2

Based on MHRA guidelines

Routine
- bloods - cobalt and chrome - abnormal if >7parts per billion
- Xray
- Oxford hip score

Symptomatic
- MARS MRI scan - to look for muscle and bone damage, and fluid around hip

Frequence
based on ODEP rating of the implant
- 10a - 1yr, 7yrs and 10yrs, then 3yrly
- no 10a rating - annually for 5yrs, then two yearly till year 10, then 3yrly

Question 3

Assessment of high risk patients (metal on metal)

Answer 3

Annual assessment
- bloods - cobalt chrome >7ppb
- OHS
- xray
- MARS MRI scan - if deterioration in OHS or elevated bloods
- rule out infection if first presentation

Question 4

Management of metal on metals hips?

Answer 4

Low risk group
- continued observation

Moderate risk group
- moderate pain
- cystic tumour - MHRA suggests these can be observed
- moderate bloods
- Refer to specialist MDT

Severe risk group
- pain
- solid pseudotumour
- muscle damage
- REVISE

Question 5

surgical considerations and surgical plan for MOM hip revision

Answer 5

surgical considerations:
- soft tissue destruction
- possible need for greater constraint - dual mobility/ constrained liner
- increased constraint causes greater wear

Surgical plan
- rule out infection
- discussed in MDT
- experienced surgeon
- aggressive debridement of the metalosis
- samples for infection
- THR with adequate constraint

Question 6

contraindications for MOM resurfacing?

Answer 6

Contraindications:
- Bone loss - severe osteoporosis, insufficient bone stock, extensive osteonecrosis, large bone cysts (rheumatoid)
- Patient factors - obesity, metal hypersensitivity, chronic renal disease, hip dysplasia, young female

Question 7

advantages of resurfacing

Answer 7

• retain bone stock
• improved proprioception
• improved ROM
• improved wear
• no PE induced osteolysis
• better restoration of hip biomechanics

Question 8

causes of revision of resurfacing

Answer 8

decreasing order of frequency:
- fracture - most common
- loosening/ lysis
- infection
- ARMD
- pain
- dislocation

Question 9

ALVAL, ARMD and pseudotumour

Answer 9

ARMD
adverse reaction of metal debris
term to describe joint failure associated with pain, sterile effusion and metallosis

ALVAL
adverse lymphocytic vasculitis associated lesion
- delayed hypersensitivity like reaction
- found in soft tissues at revision show immunological response
- can lead to periprosthetic osteolysis

Pseudotumour
- causes extensive collateral damage
- synovial like biomembrane forms leading to bone resorption and osteolysis - production of IL1, collagenase and TNF