Biomedical Engineering Reference
In-Depth Information
seen on X-rays) to form a paste. Additionally, an antibacterial agent may also be
added. The cement is mixed in the operating theater by either hand or machine, and
may sometimes be mixed in a vacuum. Mixing these components together causes
an exothermic reaction and the temperature of the cement is typically in the range
67-124
◦
C. Once the cement has been mixed, it is injected or applied manually
into the bone cavity. It is important that pressure is applied to the cement so that
it can be forced into the interstices of the bone, which will increase the strength at
the bone/cement interface. In some cases, such as the acetabular component of a
hip replacement, holes may be drilled into the bone so that the cement can enter
deeper into the bone [39].
Implant stems for use in cement are generally designed with a tapered shape
so that as the stem is pushed into the bone cement it will cause pressurization
of the cement. Once the implant is in place, the cement will cure and harden in
around 6-15min, depending on the type of cement used. Bone cement is not an
adhesive, but acts as a filler between the bone and the implant stem and therefore
relies on mechanical interlock. The main advantage of using bone cement is that
after it has cured, there is immediate fixation of the implant within the bone and
the patient can weight-bear soon after surgery. Disadvantages can include bone
necrosis from the temperature of the exothermic reaction, blood contamination,
and fatigue failure.
Fixation of implants can also be achieved through bone ingrowth. The implant
stem is manufactured so that it is roughened or a porous surface is applied. The
bone does not attach itself to the implant, but instead grows into the roughened
or porous surface. There is a variety of techniques for producing a porous surface
such as sintered titanium alloy beads or the use of hydroxyapatite, which is applied
by a plasma spray [40, 41]. Bone ingrowth techniques do require the patient to have
good bone stock and the ultimate fixation is not immediate as time is required for
thebonetogrowintotheporouscoating.
Mechanical fixation includes the use of an interference fit and screws. An
interference, or press fit, involves a component being pushed into the bone. This
can be used for hip and knee joint replacement implants, but generally there is
a roughened or porous surface for bone ingrowth as well. Some implants rely
completely on a mechanical fixation. Screw fixation is used in the Hintegra total
ankle prosthesis (Newdeal, Lyon, France) and is also used for additional fixation in
hip replacement surgery where the acetabular part is revised.
8.4
Joint Replacement Implants for Joints of the Hand and Wrist
8.4.1
Introduction
Joint replacement in the hand and wrist is not as common as replacement of the hip
and knee. Some of this can be attributed to the implants not matching the clinical
