Biomedical Engineering Reference
In-Depth Information
2.5.7.3 Summary
Abrasive wear can be reduced by smoother, and harder metallic or ceramic
surfaces, and by a reduction in contact pressures.
Adhesive wear can be reduced by smoother, and harder metallic or ceramic
surfaces, and by a reduction in contact pressures.
Third body wear can be reduced by harder metallic or ceramic surfaces, and by
a reduction in contact pressures.
Surface fatigue wear can be reduced by a reduction in contact pressures.
Wear in incongruent knee prostheses is much higher than in hip prostheses.
Reduction in contact pressures through use of mobile bearings is the principal
weapon in the fight against the degenerative effects of wear debris in knees.
Wear reduction is needed.
2.6 Biological Failure
Just as mechanical failure leads to poor function, biological failure of an artificial joint
replacement can lead to significantly worse complications or even death. The most
commonly encountered biological failure modes are: infection, osteolysis, progressive
osteoporosis, avascular necrosis, peri-prosthetic fracture and tumor formation.
2.6.1 Infection
Septic joint replacements occur in 1-2% of cases overall [33]. The gram positive
organisms of staph aureus and staph epidermides are most common and generally
thought to occur at the time of initial surgery or shortly afterwards if the skin
incision fails to heal in a timely fashion. Most studies prefer a delayed exchange
revision approach for infected joints [34],
but good success has been achieved in
primary exchange techniques if the nutritional status of the patient is satisfactory
and the wound heals without delay [35]. The danger of a septic joint replacement
is that generalized sepsis and death can occur if the organism is virulent and fails
to respond to surgical debridement and antibiotic management.
2.6.2 Osteolysis
Small (submicron) polyethylene or metallic wear particles incite an inflammatory
process, whereby macrophages and giant cells, phagocytose the particles and
attempt to digest them with lyzozymes and proteolytic enzymes. Unfortunately,
the wear particles persist in the cytoplasm of these cells and continue to stimulate
digestive enzyme production, which spills over into the surrounding bone and
begins to digest this host bone. Once enough bone is lost in this osteolytic
process, a cystic cavity filled with these macrophages and giant cells replaces the
normal bone and begins to expand if the threshold for particle volume is exceeded.
In hip replacements, the threshold for volumetric wear debris correlates with linear
wear rates greater than 0.1 mm per year in an UHMWPe acetabular cup [36].
