Concept
Femur
- removal of femoral head cartilage
- resurfacing with metal
- cemented / uncemented
Acetabulum
- standard technique
Bearing surface
- metal on metal
Indications
Relatively young man (40 - 50)
OA
Absolute Contra-indications
Severe bone loss femoral head
Large femoral neck cyst
Small acetabulum
Relative Contra-indications
Osteoporosis
Age > 65
BMI > 35
Caution
RA
Female
AVN femoral head
Advantages (many theoretical)
1. Bone preservation
- preserves femoral bone stock
- however makes acetabular preparation more difficult
2. Improved stress transfer to proximal femur
- less proximal stress shielding
- improved proximal bone density
3. Reduced dislocation rates
- heads 36-54 have reduced rates compared with 22-32
- can occur though if poor technique or component loosening
4. Better kinetics
- faster walking speeds
- may be better ROM
- possible better proprioception
- may be element of selection bias (i.e. is done in younger, fitter patients)
4. Easier revision of femoral component
- better bone stock
- simply recut and use stem
6. Possible improved longetivity
- very low wear rates metal on metal
Disadvantages
1. Poor modularity
- difficult to adjust LLD
- difficult to adjust offset
- patients with very abnormal abnormality better off with conventional THA
2. Not suitable for elderly / poor bone stock
- increased risk femoral neck fracture
3. Femoral neck fracture
4. Metal ions
- in serum, RBC, urine
Problems
- risk metal sensitivity
- risk carcinogenesis / teratogenesis
- CI in woman of child bearing age
5. Loosening
Complications
Femoral Neck Fracture
Incidence 0-4%
- 1.5% in a study of 3500 BHR in Australia (JBJS Br 2005)
- early in learning curve
- early in prosthesis life
Risk Factors
- decreased bone mass / osteoporosis
- elderly
- inflammatory arthritis
- females (risk x2)(AJR)
- femoral head and neck cysts
- femoral neck notching
- varus femoral component (< 130o neck shaft angle)
- cup impingement on neck
- improper implant seating
- AVN femoral Head
Revision
- relatively simple
- recut neck
- femoral implant with large metal head
Early loosening
Machining
Initially due to poor early manufacturing
- decreased clearance
- inadequate polar bearing
- increased peripheral bearing, seizing, cold welding and loosening
Modern machining
- small surface asperities
- improved fluid film lubrication
- polar bearing with small clearances
- very low wear and little particle production
Causes
A. Oversized heads / notching
B. Varus Femoral Component
C. Femoral head AVN
Due to extensive releases required to expose / surgically dislocate femoral head
D. Open Acetabular Component
Theorised to cause point loading
- increased metal wear
- best to close cup
E. Other
Australian Joint Registry 2023
15387 procedures
Best outcome
- Male < 65 with OA
- Able to get 55mm head or larger
Revision rate
Cumulative
- 11% @ 20y for OA
Reason for revision
- Loosening (25%) - most common after 10y
- Metal disease (22%) - also late, after 10y
- Fracture (20%) - peaks early
Revision by diagnosis at 20y
- OA 11%
- DDH 19%
- AVN 16%
Revision by prosthesis
- Adept 6.5% @ 15y
- BHR 9.5% @ 15y, 12% @ 20y
Revision by age at 20y
- < 55 = 11%
- 55-64 = 12%
- > 65 = 9%
Revision by sex at 20y
- Male 8%
- Female 22%
Revision by head size at 20y
- < 45 = 31%
- 45-49 = 15%
- 50-54 = 8%
- > 55 = 6%