Vd. Good et al., WATER AND BOVINE SERUM LUBRICATION COMPARED IN SIMULATOR PTFE COCR WEAR MODEL/, Journal of biomedical materials research, 33(4), 1996, pp. 275-283
Controversy surrounds wear data from laboratory hip simulator studies,
whether derived from water-based or serum-based studies or whether a
major design parameter such as the size of the femoral head has an eff
ect on the volume of wear particulate released. To investigate these r
elationships, we studied cup wear in water- and serum-based lubricants
using as our standard the polytetrafluoroethylene (PTFE) data derived
by Charnley. To model Charnley's clinical experience, PTFE acetabular
cups were used in sets of three each of four sizes of CoCr femoral he
ads: 22.25-, 28-, 32-, and 42-mm diameters. Six criteria were used to
evaluate the performance of the lubricants against clinical accuracy a
nd scientific methods. The PTFE wear data from the serum-based tests w
as consistently linear with the duration of the test, exhibited a prec
ision within +/- 3% about the average for each set of three cups, and
copious amounts of wear debris were clearly seen circulating and settl
ing to the bottom of the wear chambers. The wear data clearly demonstr
ated Charnley's thesis that volume of wear increases with regard to th
e size of the femoral head in a linear manner. This increase was consi
dered satisfactory at 9%/mm. However, in terms of clinical accuracy, t
he simulator wear rates averaged 3 to 4 times greater than the compara
ble clinical data for wear magnitude. Thus, the serum-based tests sati
sfied three of the six criteria used. The PTFE wear data from the wate
r-based tests was generally nonlinear, continually increasing with tes
t duration. These wear trends were examined in three discrete phases t
o estimate the changing wear rates. By the end of the tests, the wear
rates had increased from 1.3 to 3.9 times, with the 42-mm heads showin
g the greatest change. The resulting precision was never better than /- 26% and deteriorated to +/- 70%. In terms of clinical accuracy, the
water-based wear rates varied from 2 to 7 times less than the Charnle
y PTFE wear magnitudes, averaging 4 times less. The water-based data d
id not satisfactorily model the relationship between increased wear wi
th increased head size. Minimal PTFE wear debris was observed, and wha
t did emerge after thousands of wear cycles appeared as streamers up t
o 30 mm long and up to 5 mm wide. When these detached, they floated up
to the surface where they could be separated into smaller particulate
s. A similar phenomenon was noted for polyethylene wear tests conducte
d with water lubrication. Thus the water-based tests satisfied none of
the six validation criteria evaluated. These data raise serious doubt
s as to the validity of testing implant and material combinations in w
ater as a predictor of clinical performance. Bovine serum was not tota
lly satisfactory, but the wear data did model some of the important cl
inical characteristics of hip joint behavior. (C) 1996 John Wiley & So
ns, Inc.