Biomedical Engineering Reference
In-Depth Information
grommets, which fit over the stems of the implant, were introduced to act as a shield
between the silicone and the bone. However, fractures have still been observed. The
Avanta Soft Skeletal Implant (Avanta Orthopaedics, San Diego, California, USA)
was introduced as the Sutter implant. It is a similar design to the Swanson and the
problems of fracture have also been seen clinically. At 48 months follow-up, 26%
of Sutter implants had fractured in one study [45].
The Swanson implant has been subjected to
in vitro
mechanical testing using a
finger joint simulator and the implants have also been found to develop a crack at
the junction between the barrel and the distal stem. The crack was found to grow
as the implant was subjected to flexion/extension in the simulator until fracture
occurred after about 3 million cycles of flexion/extension [46]. Similar results have
also been found when the Sutter implant was mechanically tested using a finger
joint simulator, with fracture occurring at the junction of the distal stem and the
hinge between 5 million and 10 million cycles of flexion/extension [47].
Newer designs of single-piece silicone implants that have been designed for the
finger joint include the Preflex (Avanta Orthopaedics, San Diego, CA, USA) and
Neuflex (DePuy, Leeds, UK) [44]. The Neuflex is shown in Figure 8.8. These newer
designs have the neutral position of the implant with one stem at an angle of 30
◦
to
the other. The design philosophy for this type of implant is that the natural resting
position of the hand has the MCP joint at about 30
◦
. If the implant is straight,
Figure 8.8
Neuflex finger joint. (Reproduced with kind
permission from DePuy International Ltd, Leeds, UK.
by DePuy International Ltd.)
