Friction and wear - Orthopaedic Biomaterials in Research and Practice - page 272

of wear debris in hard-on-hard joints is one to two orders of magnitude
lower than traditional metal-on-polyethylene bearings. (Also of note:
dimensions of hard bearing wear particles are also much smaller.) The
past 15 years have seen further development of hard-on-hard bearings to
directly address the issue of implant wear.
Wear in newer generations of dense, polycrystalline alumina is on
the order of a few microns per year, though there are exceptions. Some
alumina bearings show a patch of localized surface damage, including
loss of grains, commonly termed striped wear . The initiation of striped
wear is suspected to occur when the femoral head dislocates slightly
from the bearing and impinges upon the edge at the beginning of the gait
cycle. This process is variously termed lateralization or microseparation.
Zirconia has improved fracture toughness properties compared to alu-
mina, but the amount of wear in zirconia ceramic on ceramic bearings is
somewhat controversial. Some studies have reported severe wear, while
others report wear to be very low. Still, because of the typically low wear
rate of ceramic bearings, osteolysis is very rare.
MOM bearings have also gained significant market share, though
the tenure of this bearing has been more controversial. MOM bearings
exhibit a linear wear rate that is on the order of a few microns per year,
though wear tends to be very dependent on finding an optimum clear-
ance. A smaller clearance in MOM bearings increases the capability
to generate fluid film thickness. Unfortunately, if the tolerance of the
sphericity is larger than that of the clearance, binding or excessive wear
may occur. On the other hand, too large a clearance will not facilitate
fluid film lubrication, lead to a smaller contact area, and ultimately result
in higher contact stresses. Carbon content and heat treatment are also
important and can affect this distribution of surface carbides, which
directly affect wear. Also notable, it has been proposed that MOM bear-
ings can self-polish, moderating surface scratches in vivo .
As a gross approximation, total volume of wear is equal to the wear
depth multiplied by the contact area. Increasing the diameter of the
head will increase the contact area and the sliding distance. In metal-
on-polyethylene total hip arthroplasty articulations, the volume of wear
per step has been noted to increase with increasing ball diameter. This
relationship does not hold for metal-on-metal bearings. This is likely due
to the ability of larger-diameter metal articulations to achieve fluid film
lubrication. Because of this, larger-diameter MOM components were con-
sidered advantageous in that they could reduce the risk for dislocation, and
the thinner acetabular cup component facilitated the conservation of more
bone stock in the acetabulum. There have been noted potential biological
adverse effects of metal ions, which is discussed in detail in Chapter 14.
Wear rates for any material pair increase with higher patient weight and
activity. Larger femoral head sizes tend to result in lower surface recession
rates due to the lower contact stress but may be associated with greater vol-
umes of wear debris due to the greater sliding distance. This may be par-
ticularly obvious with surface replacement devices for the hip, for which it
has been estimated that wear rates (as measured by rate of debris produc-
tion) are 1.5 to 2 times those for conventional THR designs.