Biomedical Engineering Reference
In-Depth Information
8.4.3
Wrist Joint Replacement
The human wrist joint is made up of the radius, the ulna, and the eight carpal
bones. Wrist joint replacement involves cutting away the diseased bone and using
a broach to create a cavity within the radius and carpal bones for insertion
of the implant stems. A single-piece silicone Swanson implant is available for
the wrist, with the design along similar lines to the Swanson finger implant,
with stems joined to a central barrel (Figure 8.10). Titanium grommets are also
available to shield the silicone from the edges of the bone. The Swanson wrist
implant also suffers from the same problems as the Swanson finger implant,
with fractures occurring at the junction between the barrel and the distal stem.
Fracture rates of up to 65% have been reported at follow-up studies of 6 years
[50, 51].
Articulating surface implants, with a cobalt chrome molybdenum alloy against
ultrahigh-molecular-weight polyethylene articulation, are also available for wrist
replacement implants (Figure 8.11). Designs include the Biaxial (DePuy Interna-
tional Ltd, Leeds, UK), Universal 2 (Integra, Plainsboro, NJ, USA), and Re-motion
(Small Bone innovations, Morrisville, PA, USA) [51]. These cobalt chromemolybde-
num alloys against ultrahigh-molecular-weight polyethylene articulating implants
operate with a boundary lubrication regime [52] and therefore wear is likely.
However, owing to the early failure of these wrist implants, wear problems have
only been detailed in a few studies. Revision surgery is more often required for
loosening, dislocation, and subsidence of the stems into the soft rheumatoid bone.
In many cases, a wrist fusion is performed rather than attempting to implant
another wrist replacement.
Stem
Stem
Grommet
Grommet
Central barrel
Figure 8.10
Swanson wrist implant. Lower part of the figure shows grommets fitted.
