Joint Replacement
Summary
For advanced arthritis, joints can be replaced with synthetic
materials. Metals like cobalt-chrome and titanium alloys are
used in the larger joints. Polymers (long-chained molecules) such
as silicone are used in the smaller joints, such as in the
fingers.
The devices must be durable and must not stimulate attack by the
already overactive immune system, interfere with healing, or push
surrounding structures out of their normal position.
Before the advent of implants, surgeons would remove joint
surfaces, hoping that the scar tissue filling in the area would
allow more mobility than the arthritic joint.
This type of surgery often failed. Implants proved far more
successful. They were pioneered by an army surgeon from Grand
Rapids, Mich., Alfred Swanson, M.D. He fashioned the first such
devices in the late 1950s out of silicone elastomers, polymers
made from the element silicone, which is found in quartz.
Research to fine-tune the implants continued in the 1960s, and in
1969, the first silicone-based joint implants came on the market.
These implants provided a flexible hinge for the joints of the
fingers, wrists and toes. Since then, more than two dozen models
have been developed, several by Swanson.
More than a million people have received joint replacements
(mostly in the hip). During implant surgery, technically called
"implant resection arthroplasty," the surgeon first removes the
surface of the joint bones as well as excess cartilage.
The centers of the tips of abutting bones are hollowed out, and
the stems of the implant are inserted there. Between the bones
lies the hinge part of the implant, which both aligns the bones
and allows them to bend at the joint.
The implant is "fixed," or held in place, with bone cement and,
finally, the tendons, muscles and ligaments are repaired. As the
site heals, the patient must exercise, but it can take a year of
physical therapy to achieve maximum rehabilitation.
A new type of hand surgery replaces joints and realigns fingers
at the same time. Rheumatoid arthritis patients who have a
severe deviation of their fingers away from the thumb may be
candidates for this procedure.
Recently introduced is an operation that repositions the tendon
at the base of the little finger. Holding the finger in place
forces the other fingers to align properly. Many patients who
choose this surgery start moving their hands the night after the
operation, and wear a splint-like support device for the next
three
weeks.
By six weeks after the operation, individuals are able to do most
of their usual activities, especially the simple tasks that most
of us take for granted, but were virtually impossible for these
individuals.
Newer joint replacements use materials that resemble body
components. "Recent hip implants have been coated with calcium
phosphate materials, like hydroxylapatite, which interact with
bone. The aim is to enhance the attachment of the implant to the
bone with a biologically active material," says an expert at the
FDA's Center for Devices and Radiological Health.
Rather than filling in the spaces with cement, investigators are testing a variety of porous coatings that allow "biological fixation," in which bone can grow into the implant area.