• WEAR SINMULATION COMPARISON OF A ZIRCONIA AND A COBALT CHROME FEMORAL KNEE IMPLANT
+*Alberts, L Russell (A-Encore Orthopedics, Inc.); *Neff, J R. (A-Encore Orthopedics, Inc.); **Webb, J D. (E-Encore Orthopedics, Inc.)
+*University of Nebraska Medical Center, Omaha, NE. 4426 So. 187th Street, Omaha, NE, 402-5590-5608, Fax: 402-559-5511, [email protected]
Introduction
In recent years the major cause of long-term failure of hip and knee
total arthroplasties has been identified as originating with wear particles
produced at the interface in the synthetic articulating surfaces. The purpose
of this study was to test the hypothesis that a zirconia (zirconium oxide) femur
would produce less wear of the counterfacing ultra-high molecular weight
polyethylene (UHMWPE) insert than a standard cobalt chrome molybdenum
femur of similar design.
Methods
Eight total knee implants, four with cobalt-chrome femurs and four
with zirconia femurs, were tested on two Instron-Stanmore knee wear
simulators for five million cycles. Norton Desmarquest, Evereux, France,
manufactured the Zirconia femoral components. Encore Orthopedics, Inc.
provided metal femoral components (Foundation Knee, size 6, left) and all of
the tibial components. Two implants of each design were mounted on each
simulator machine and run at a cycle rate of 1.± .1 cycles per second. To
avoid station-to-station variability in the simulators, after 2.5 million cycles
the total knees combinations were rotated
All of the UHMWPE inserts had been sterilized with 25 kGy of
gamma radiation in air. One UHMWPE insert was used as a soak control.
During the test, the tibial components were kept immersed in a lubricating
solution at 37° C. The lubricating solution was composed of 50% filtersterilized bovine serum, 50% deionized water with .02% sodium azide added
to slow bacterial growth and 20 mM (7.45 g/L) EDTA (ethylene
diaminetetraacetic acid) added to reduce calcium precipitation. The load
profiles specified by the International Standards Organization, ISO were used
as guidelines for the flexion/extension, the axial force, the anterior-posterior
force and the rotational torque. When linear trends in the soak control
adjusted weight loss data were noted, least squares regression with 95%
confidence intervals were calculated for the linear region using Table-Curve
2D software (SPSS inc.) for each insert and for the average weight loss of the
cobalt-chrome and zirconia insert groups.
Results
The loading cycles were maintained for the full 5 million cycles with no
gross breakage or damage to either the tibial inserts or the femurs. The
variability of the peak axial load was less then five percent for all stations.
For the inserts under the zirconia femurs, the average wear rate reached to
24.4 mg/million cycles at zero to 300,000 cycles; then declined to 3.9
mg/million cycles at 1.0 to 1.3 million cycles. For the inserts under the
cobalt-chrome alloy femurs, the average wear rate increased to 34.5
mg/million cycles at 300,000 to 600,000 cycles then declined to 12.6
mg/million cycles at 1.0 to 1.3 million cycles.
After an initial wear in period of one million cycles the average
steady state UHMWPE (ultra-high molecular weight polyethylene) wear rate
determined by least squares regression for the inserts interfacing the zirconia
femurs was 4.2 mg per million cycles (95% confidence interval, 3.4 to 4.8)
and the average steady state wear rate of the inserts facing the cobalt-chrome
femur implants was 11.5 mg per million cycles (95% confidence interval, 10.6
to 12.4).
Scratches visible to the naked eye appeared deeper and more
numerous on the cobalt-chrome femurs than on the zirconia knees. All of the
UHMWPE inserts showed signs of burnishing but no cracking or
delamination were seen.
Discussion
Our results show a definite reduction in the average steady-state
wear rate and the total wear in UHMWPE inserts articulating with the zirconia
femurs compared to those articulating with the cobalt-chrome femurs. We
speculate that this reduction was due to the increased hardness, scratch
resistance and smoothness of the zirconia femurs
Figure 1. Zirconia femur after five million cycles
Figure 2. Cobalt-chrome-molybdenum femur after five million cycles.
Our UHMWPE wear results are consistent with hip simulator
results that have compared zirconia and metal heads interfacing with
UHMWPE cups. Saikko (1995) found that after three million cycles zirconia
produced a significant reduction in the wear of polyethylene when used as the
head instead of cobalt-chrome or ion-implanted cobalt chrome. Kumar et al
(1991) found that zirconia heads reduced polyethylene wear by 40% to 60%
than alumina and a five to ten fold reduction of wear over SUS316L stainless
steel heads. Derbyshire et al (1994) stated that compared to stainless steel
heads, zirconia heads consistently reduced the volume change in UHMWPE
cups. They also noted and quantified an increased roughness of the metal
femoral heads.
References
Derbyshire B et al. 1994 Med Eng Phys 16(3):229-36.
Kumar P et al. 1991 J Biomed Mater Res 25(7):813-28.
Saikko VO 1995 Acta Orthop Scand 66(6):501-6.
Acknowledgement
We would like to thank Hani Haider for invaluable assistance in and
advise on the operation of the Instron-Sranmore simulators.
**Encore Orthopedics, Inc., Austin, TX.
Poster Session - Knee Arthroplasty - Hall E
47th Annual Meeting, Orthopaedic Research Society, February 25 - 28, 2001, San Francisco, California
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