Biomedical Engineering Reference
In-Depth Information
For convex side of the artifi cial joint, metals (titanium or CoCr alloys) or ceramic (aluminum
ceramic) components are used. These components are equipped with short or long stems, which
can be fi xed to the trabecular bone by means of bone ingrowth to a porous surface with or without
porous hydroxyapatite (HA) coating or by means of poly(methyl methacrylate) (PMMA) cement
[67]. To fi xate the prosthetic components, a surgeon has to remove not only affected and painful
joint surfaces but also much of the healthy bone [68].
Although millions of TJRs are performed annually to improve quality of life of the patients,
despite the early and mid-range follow-up good results of the human load-bearing joint replace-
ment, the complications of artifi cial joint are inevitable, with an incidence of approximately 14%
in 12-15 years after shoulder replacement and up to 70% of the total hip replacements (THR) after
10 years or less. The complications result in pain, reduction of the range of joint movements, loosen-
ing of components, and fi nally in a revision operation [33].
One of the main reasons of these complications is the failure or degradation of the implant bio-
materials. The biomaterials such as metals and polymers (synthetic or natural), ceramics and their
composites degrade and lose their original properties due to exposure to
in vivo
conditions [67]. The
implant biomaterials subjected to highly demanding conditions such as high stresses and high cyclic
loadings, coupled with aggressive body environment, degrade in time, losing their properties such
as strength and wear or corrosion resistance. The undesirable degradation takes place in the form
of wear, corrosion, deformation, creep, fatigue, fractures, and oxidation of the biomaterials. Despite
all the progresses made in regenerative medicine, these phenomena are recognized as major factors
limiting the success of the TJR.
Abrasion, burnishing, pitting, erosion, and delamination were found to be the most predominant
modes of
in vivo
degradation (wear and cold fl ow) of polyethylene in TJR. From the scanning elec-
tron micrographs of the exposed surfaces of the retrievals, it was found that fi ne multidirectional
scratches were dominant (Figure 21.16a). In addition to the scratches, fl akes and rim erosion are also
observed. Two implants revealed pitting areas and surface microcracks, which most likely resulted
from subsurface fatigue. Polyethylene delamination was observed for metal-backed component.
Some of the implants were completely worn out, in some places, to the metal backing.
In vivo
degradation products such as particulate and ionic wear and corrosion debris cause
aggressive biologic response that can lead to synovitis, periprosthetic bone loss, and aseptic loosen-
ing of the implants [67]. The polyethylene wear particles migrate into the periprosthetic spaces and
stimulate the activity of the macrophages by the release of cytokines, which activate the osteoclasts,
(b)
(a)
FIGURE 21.16
Atomic force microscope (AFM) image of (a) the polyethylene surface with multidirectional
scratches and (b) degradation and wear of the glenoid component.
