A “Toolbox” for Forensic Engineers

296 Forensic Materials Engineering: Case Studies
of the stem was changed from matte to polished. During 1992 the product
range was extended to include the option of a polished conventional stainless
steel stem, and a polished high nitrogen stainless steel stem was added to the
range in 2001.
In the U.K., the Medical Devices Agency (MDA) had become aware of
28 revisions, due to recipient pain, where corrosion was confirmed in the
polished titanium alloy femoral stem. The incidence of these revisions was
spread across nine different centers, and represented a confirmed failure rate
of 0.14%. In addition, the MDA became aware of eight additional possible
cases, of which five were considered by the clinicians involved to have been
resolved without the need for further surgery. Since 1999 the number of
reported cases of corrosion of the polished titanium stem has continued to
increase.
The most common clinical symptom signaling potential problems was
severe atypical pain at 2 to 5 years post-implantation, along with an absence
of overt infection or femoral loosening. However, it would appear that pain
relief could be achieved upon movement. Additionally, corrosion has been
seen along the full length of the explanted stems, but in some cases it is
focused on the distal half of the stem.
A most interesting aspect of the problem is that extensive testing in five
independent biomaterials laboratories has failed to establish categorically
why the polished titanium alloy modular femoral stem sometimes corrodes
in this manner. Furthermore, the MDA has no information of any case of
corrosion in the original matte-finished titanium alloy stem or in either of
the stainless steel variants. Although presenting manufacturing difficulties,
titanium alloy is generally considered to be highly corrosion resistant in vivo.
One probable answer is that the matte surface would bond better with the
bone and resist any physical actions that would damage the TiO 2 surface film.
It is this film that gives titanium its corrosion resistance, as the metal itself
is highly reactive chemically.
In addition, the elastic modulus of titanium is a closer match to that of
bone than the range of alternative bio-alloy systems. Consequently, from a
purely mechanistic view, titanium alloys are gaining popularity for implant
device fabrication. Nevertheless, this particular case does raise questions as
to the performance attributes of polished titanium prosthetic devices.
A review of the published literature 2–4 describes sporadic cases of corrosion
of other models of polished and unpolished cemented titanium alloy
hip implants. However, the polished stem in question is currently the only
titanium alloy hip implant repeatedly exhibiting this corrosion problem in
the U.K. Accordingly, the MDA issued a device alert –— MDA DA2002 (04),
dated May 2002 — for the device in question.