Friction and wear - Orthopaedic Biomaterials in Research and Practice - page 265

Hydrostatic

P P

We eping

Squeeze film

FIGUre 11.6 (See color insert.) alternative mechanisms of lubrication.

Squeeze film . Structured surfaces may be used to hinder the lateral
flow of lubricant out from the sliding interface under the influence
of the applied pressure. This produces a dynamic version of hydro-
static lubrication, without the need for an external pressure source.
Squeeze film lubrication may be assisted in the natural joint by
deformation of the cartilage, which will restrict fluid movement
out of the contact region.

Wear

Normal wear

Because of the contact of surface asperities during relative motion, it
is usual for one or more of the materials to be eroded or worn away.
This has unfortunate consequences both to the material, since its origi-
nal shape and size are changed, and to the surrounding biologic system
through the production of wear debris.
Such debris are produced by all material pairs used in total joint
replacement (TJR) articulations; they may also be produced in other
multipart devices. After an initial high wear rate period, called the wear-
ing in ( or running in ) period, the quantity of wear debris produced, usu-
ally measured as a volume, is given by the relationship (Figure 11.7)

V = k ∙ F ∙ x

where k is a constant characteristic of the material pair making up the
articulation and its local environment, F is the force across the articu-
lation, and x is the distance of relative travel between the articulating
faces. It should be noted with care that wear depends on interfacial force ,
not on interfacial stress ; this is a result of the true contact area increasing
with load while the true stress (greater than the geometric stress) actu-
ally stays nearly constant (≈σ y ).