cadera / hip - Active Congress.......

MARTES / TUESDAY
30
B. The Solution
- Optimization/Development of Bearing
Surfaces to Reduce Wear 25,83,91,137,169
• Metal on Metal
• Ceramic on Ceramic
• Improved Polyethylene (Highly Cross-
Linked)
II. Metal on Metal Bearing Surface
A.History
• First metal on metal total hip performed by
Philip Wiles in 1938
• Initial thrust of development in England
in 1960’s
• First widely used metal on metal articulation
featured Cobalt-Chromium alloy
against itself (McKee-Farrar, Stanmore,
Muller, Ring and Sivash)
• Metal on metal articulations abandoned
by the early 1970’s secondary to relatively
high short-term clinical failure (early
loosening) and growing success of the
Charnley prosthesis
• Disregarding early failures, long-term survivorship
of the early designs was noted to
be comparable to the Charnley prosthesis
and wear noted to be magnitudes less than
the metal-on-polyethylene designs 98
• Revival and development of second generation
metal-on-metal bearings in the
1980’s as wear and osteolysis became a
foremost concern as complications of total
hip arthroplasty
• Early and mid-term clinical follow-up of
second generation metal on metal implants
comparable to conventional metal
on polyethylene and better than that of fi rst
generation metal on metal THA 52,53
• Biologic response to metal particles/ions
comes under scrutiny
B. Design Issues
• First generation metal on metal articulations
• Early clinical failures secondary to poor
design features
• Equatorial bearing contact 105,205
• Poor surface fi nish and irregular geometry130
• Thin & fl exible metal → bearing lock-up
• Poor stem design → premature loosening128,129,130
• Second generation metal on metal articulations
• Advances in engineering/manufacturing
• Material 26,172
• Cobalt chromium molybdenum favored
• Hardness (> 40 R ) c
• Processing: Cast vs. Forged/High vs.
Low carbon
• Scientifi c data unclear
• Appears no signifi cant effect on wear
with today’s higher-quality initial surface
fi nish and sphericity 26
• Clearance: Desire polar bearing, not
equatorial
- Wear decreases with decreasing diam-
26, 28,29,59,138,139
etral clearance
- Volumetric wear halved by reducing
clearance from 90 to 45 um
- Manufacturing diffi culties controlling
dimensional tolerances below 20 um
- Desired diametral clearance dependent
on femoral head size
· Ideally less than 90 um for 28 mm
femoral head
· Acceptable clearance higher as femoral
head size increases
• Surface Roughness
- Wear decreases with decreasing surface
roughness (linear regression, R2 =
0.91) 28
26, 27
• Fluid Film Lubrication
- Low clearance and low surface roughness…favorable
parameters for fl uid
fi lm lubrication and decreased wear
C. Results
• First generation metal on metal articulations
• Many fi rst generation metal on metal failures
were design related and unassociated with
the metal bearing material itself
• McKee-Farrar