Biomedical Engineering Reference
In-Depth Information
Figure 8.9
Total Metacarpophalangeal Replacement,
TMPR
. (Reproduced with kind permission from Fins-
bury Orthopaedics, Leatherhead, Surrey, UK.
by Finsbury
Orthopaedics.)
as with the Swanson, the neutral position of the hand at 30
◦
will induce a higher
strain in the material.
In vitro
testing of the Neuflex using a finger joint simulator
has shown that it lasts for much longer than the Swanson and Sutter implants,
with between 9 million and 20 million cycles of flexion/extension completed before
fracture was seen along the central hinge [48].
An alternative design of finger joint replacement implant to the use of silicone
implants is the use of articulating bearing surfaces. For theMCP joint, these designs
comprise metacarpal and phalangeal parts that form a ball and socket joint. How-
ever, many surgeons believe that this type of implant is inappropriate for patients
with rheumatoid arthritis, as the soft tissues are damaged by the disease and there
is inadequate support for the implant [44]. Examples of articulating bearing surface
implants include the Total Metacarpophalangeal Replacement, TMPR (Finsbury
Orthopaedics, Leatherhead, Surrey, UK) which has a cobalt chrome molybdenum
alloy against ultrahigh-molecular-weight polyethylene articulation (Figure 8.9) and
the SR
(Small Bone Innovations, Morrisville, PA, USA). The TMPR
design has
a mechanical fixation with a series of ultrahigh-molecular-weight polyethylene fins
that wedge against the inside of the bone.
Other designs of articulating finger joint replacements include the Pyrocarbon
Total Joint (Ascension Orthopaedics, Austin, TX, USA) which has an articulation of
pyrocarbon against itself. Current designs of articulating finger joint replacements
have been shown to operate with a boundary lubrication (metal against polymer)
or mixed lubrication regime (pyrocarbon against itself) [49].
