Biomedical Engineering Reference
In-Depth Information
widely applied biomimetic treatment for metallic surfaces but several other
methods have been proposed (in some cases reaching clinical application)
like a bioactive glass coating or the anodic oxidation of titanium.
Despite composite materials having been studied as a possible replacement
for metals in weight-bearing applications, the convenient costs and easier
workability of metals support the view that they will keep their predominant
role, as long as a material for osteosynthesis or for artificial joint structure
has to be selected.
13.3.2 Non-biodegradable polymers
Polymethylmethacrylate (PMMA) and polyethylene (Pe) are polymers
which have ubiquitous applications in orthopaedic surgery. Despite their
physicochemical degradation with time, within the lifespan of an implant
inside the body, they are properly considered neither biodegradable nor
bioresorbable (meaning by hydrolytic degradation and by cell-mediated
degradation, respectively). Their applications in orthopaedic surgery are
different and they will be treated separately.
Recently, another non-biodegradable polymer has found a well-defined
application in orthopaedic surgery: it is polyether ether-ketone (PeeK). its
main application is in the intersomatic vertebral fusion when a so-called 'cage'
is interposed between two vertebral bodies. After the intervertebral disk and
the articular cartilage plates have been removed, these 'cages', filled with
bone chips, are used to promote bony fusion between the two vertebrae.
Polymethylmethacrylate
PMMA has become the material of reference when a cement is needed to
assemble an implant with the recipient bone, in a single unit; cementing the
stem of a joint prosthesis is the application of reference. PMMA has been,
probably, overused in the sense that sometimes the cement has been required
to cope with the mechanical mismatch between the implant and the recipient
bone, for example, when stability is needed for a stem too much solicited in
torsion, or when an osteoporotic bone, too weak to bear the load transmitted
at a joint, receives an implant (Fig. 13.4). Fracture and debris production
of the cement, which may occur in these cases, have been considered in the
past more a cause than an event in the mechanical mismatch, prompting the
possible avoidance of the use of PMMA cement, like it occurs in cementless
joint prostheses.
From the surgical point of view, the easy intraoperative workability of
PMMA cement favoured several other applications apart from cementing
joint stems, like filling large bone cavities produced by the surgical removal
of a tumour; in this application, PMMA often assured a long lasting
