Biomedical Engineering Reference
In-Depth Information
success of hip and knee joint replacement implants. However, rheumatoid arthritis
which commonly affects the joints of the hand and wrist causes extensive damage
to the surrounding soft tissues and bone, leading to severe deformities. This is
an added complication when it comes to the design of a successful replacement
implant for these joints. In this section, the designs of replacement implants for
the joints of the hand and wrist are discussed.
8.4.2
Finger Joint Replacement
The finger joints in the human hand include the metacarpophalangeal (MCP),
proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints and are
essential for hand function. The MCP is the finger joint most commonly replaced
with an implant [42]. For the MCP joint, the surgical procedure involves cutting to
remove the bone ends of the joint and broaching the shaft of the bone to create a
cavity for the implant stems. There is a variety of designs of finger joints available
and they can be broadly divided into single-piece silicone joints and articulating
bearing surface joints [43, 44].
The most common design of joint replacement implant for the fingers is a
single-piece silicone spacer. The Swanson finger implant (Wright Medical Tech-
nology, Arlington, Tennessee, USA) has been in use since the 1960s and comprises
two stems that are joined to a central barrel (Figure 8.7). Since its introduction,
the implant has been manufactured from different grades of silicone (including
conventional and high-performance silicone); it is currently manufactured from
Flexspan
.
The stems of the Swanson implant are not actually fixed within the bone;
during finger movement, the implant pistons in and out of the bone cavity. The
Swanson implant has been found to have a poor survivorship, compared with
joint replacement implants for the hip and knee. Failure of the Swanson implant
has typically involved fractures occurring between the barrel and the distal stem.
Studies have shown that 34% of implants had fractured at 48 months follow-up
[45]. The crack is believed to be initiated from contact with the sharp bone edges
or from abrasion against the bone during finger movement as the implant pistons
in and out of the bone. To protect the Swanson implant from the bone, titanium
Stem
Barrel
Stem
Figure 8.7
Swanson finger implant.
