Wear processes in polymer implants - Durability and Reliability of Medical Polymers - page 142

higher Young's modulus than that of the natural articular cartilage that is
being replaced. It has been proposed that future designs of joint replacement
implants may benefit from the use of a soft layer, which has mechanical
properties similar to the natural articular cartilage (Fig. 7.3). This type of
implant is known variously as compliant, cushion or soft-layer bearings.
Over the years there has been much development in this area with designs
proposed for replacement of the hip and knee (Dowson et al. , 1991; auger
et al. , 1995; Scholes et al. , 2007). although there are promising results from
laboratory studies, this type of implant has yet to be used in patients.

7.2.3 Single-piece elastomer joint replacement
Replacement of the finger and wrist joints commonly involves the use of
single-piece implants manufactured from silicone. These implants do not
have articulating bearing surfaces, as with most joint replacement implants,
but consist of two stems joined to a central barrel (Fig. 7.4). The original
design of implants for the finger and wrist were the Swanson designs. For
the Swanson wrist implant, the proximal stem fits into a cavity created in
the radius and the distal stem fits into a cavity created in the carpal bones
and into the third metacarpal (Costi et al. , 1998; Shepherd and Johnstone,
2002). The implant stems are not fixed in position and the stems can move
like a piston in and out of the bone during motion of the wrist. The Swanson
finger implant is similar in design and there are a number of other designs
of single-piece finger replacement joints such as the NeuFlex and Preflex
(Joyce, 2004).

Ball

￿ ￿ ￿ ￿ ￿ ￿

Soft layer

Rigid backing material

7.3 Cushion form joint for the hip.