Notas / Notes - Active Congress.......

MARTES / TUESDAY
14
09.00 - 10.45 h
CADERA / HIP
Pares de fricción y nuevos materiales / Friction couple and new materials
Moderador: Victor M. Goldberg
STUDY OF WEAR BEHAVIOR IN A HIP
JOINT SIMULATOR OF DIFFERENT
CoCrMo COUNTERFACES ON UHMWPE
F. Javier Gil
CREB. Dept. Ciencia de Materiales e Ingeniería Metalúrgica.
ETSEIB. Universidad Politécnica de Cataluña.
Barcelona (Spain)
The objective in these wear tests was to study the effect of
different material counterfaces on the UHMWPE wear behaviour.
The materials used as counterfaces were based on
CoCrMo: forged with hand polished and mass finished,
CoCrMo coating applied on the forged CoCrMo alloy obtained
by Physical Vapour Deposition (PVD). A Hip Joint Simulator
have been designed and built for these studies. The worn
surfaces were observed by optical and scanning electron
microscopy. The results show that the hand polished CoCrMo
alloy caused the higher UHMWPE wear of the acetabular
cups. The CoCrMo coating causes the least UHMWPE wear,
while the mass finished CoCrMo alloy causes an intermediate
UHMWPE wear.
MATERIAL AND METHODS
The tests on the HJS wear test method were performed as
follows (Figure 1). The acetabular cups were mounted on a
home-made metal back support to transmit the load to each
of the three stations. The cups were then pressed against
the femoral heads. While the cups remains static, the heads
slides according to the biaxial rocking motion. A constant
load of 1000 N (102 Kg) per station was applied during the
test. The frequency of the motion was 1.23 Hz (810 ms/cycle).
In the HJS a cycle is considered as completion of one rotation
of the head. The wear of the UHMWPE cups was determined
by weight loss measurements every 333,333 cycles up to a
total test length of 3 million cycles. The test lubricant was
replaced with fresh solution after every weighing stop and
distilled water was added during the test for compensating
water evaporation. Each station has an environmental test
chamber made of a transparent polycarbonate wall. Each
chamber has been filled with 350 ml of lubricant. As test
lubricant, a solution consisting of bovine serum and distilled
water was used with a total protein concentration of 30 mg/ml.
The serum was purchased at Sigma-Aldrich SrI (Calf serum,
bovine donor; product No.C9676). The soak adsorption of
the UHMWPE cups was determined using an additional
control cup, which was loaded identically as the UHMWPE
cups in the RPOF machine, but no motion was applied. The
cleaning and drying of the UHMWPE cups was performed
according to the ASTM 1715 standard. Weighing was carried
out with a Mettler Toledo AT261DeltaRange® microbalance
with an accuracy of 10±µg.
Acetabular cups made of UHMWPE GUR1020 and previously
sterilised with 25 KGy (2.5 Mrad) gamma radiation were
used. The cups were supplied by the SAMO S.p.A. and
received with their packaging, as they are commercialised.
The cups are designed to be used with a metal back component
and in conjunction with 28 mm femoral heads. The
articulating counterfaces were therefore 28 mm femoral
heads. Three different femoral head materials were studied,
the standard material in this study was a hot-forged CoCrMo
alloy. Table shows the chemical composition of this material
and Table 2 the hardness and roughness of each material.
· CoCrMo forged (hand polished)
· CoCrMo forged (mass finished)
· CoCrMo forged with a CoCrMo coating obtained by means
of physical vapour deposition (PVD). The coating had the
same chemical composition as the substrate.
For each articulating counterface material three heads were
tested. A total of 18 wear tests were performed. The test
conditions and materials for the HJS wear tests are resumed
in Table 3.
RESULTS AND DISCUSSION
The wear of the UHMWPE specimens (cups) in the HJS
wear test method are shown in Figure 2, where the volumetric
wear (mm3) of the UHMWPE cups is represented as a
function of test duration in cycles for each the head material.
The volumetric wear results are calculated from the average
weight loss of three specimens.
The results shown above after 3 million cycles present a
high linearity and a low standard deviation. The standard
deviation after 3 million cycles, represents for the hand
polished alloy and the CoCrMo coating 10.20% and 4.66%
respectively, and 6.95% for the mass finished alloy.