COMPONENT TYPES
• metal on polyethylene
Earliest implants
Issue of polyethylene wear and wear particle disease
Wear shown as reduced distance between head and acetabular margin
• ceramic on polyethylene
Low wear ratio
Ceramic head can fracture
• ceramic on ceramic
Ceramic fracture risk
• metal on metal
Low wear
Metal ions released
ARMD Reaction to metal ions
Also called ALVAL or Pseudotumor
FIXATION
• Cemented
• Uncemented
• Hybrid (cemented stem uncemented cup)
• Reverse hybrid
REPORT SHOULD CONSIDER
ACETABULAR COMPONENT
• Abduction or Inclination angle
30-50 degrees See figure adjacent.
Angle varies according to desired mobility.
Angle proportional to dislocation risk
• Anterversion
10-30 degrees See figure above.
Simple calculation of angle theta useful for quick assessment.
May require 'guestimate' of margin of cup if obscured by metal head
Ackland method more accurate if cup partially obscured but requires tables (below)
CT better if accurate assessment required
FEMORAL COMPONENT
• Limb length
Various methods but teardrop line to lesser troc easy
Less than 1cm difference acceptable
• Offset (centre of rotation)
Compare with contraletaral side
Quick method: compare distance from head center to teardrop
cal center of rotation is distance from head center to interischial line
• Tip position. Valgus/Neutral/Varus
Is tip in centre of medulla?
Center (Neutral) or Medial (Valgus) are optimal
• Concentric to acetabular component
Circle on Femoral head, insert & Cup ..
Should be isocentric
• Femoral anteversion
RESURFACING
• Stem 5-10 degrees valgus wrt the femoral neck
ie Stem lies below axis of femoral neck
• Cup 30-50 degrees abduction 15 to 25 degrees ante version
COMPLICATIONS TO LOOK FOR
• Aseptic loosening
Use Gruen's zones to describe
1-7 from lateral on AP
8-14 from posterior on lateral
1-3 from superior for acetabular cup
more than 2mm significant
• Infection (septic loosening)
• Wear
• Fracture
• Metal fatigue (Trunnionosis)
• Heterotopic bone formation
• Failure of incorporation of uncemmented components
ACKLAND ANTEVERSION TABLES
