| THE
PROBLEM OF CORROSION IN ORTHOPAEDIC IMPLANTS |
D.
Sharan
ABSTRACT
Corrosion, the gradual degradation of materials
by electrochemical attack, is a concern particularly when
a metallic implant is placed in the hostile electrolytic environment
provided by the human body (Litsky and Spector, 1994). Even
though the freely corroding implant materials used in the
past have been replaced with modern corrosion resistant superalloys,
deleterious corrosion processes have been observed in certain
clinical settings (Jacob et al, 1998)
i) Corrosion can severely limit the fatigue
life and ultimate strength of the material, leading to the
mechanical failure of implant. There is a low but finite prevalence
of corrosion - related fracture of the implant (Jacobs, 1998),
ii) The release of corrosion products may elicit an adverse
biological reaction in the host, and several authors have
reported increased concentrations of local and systemic trace
metals in association with metal implants ( Pazzaglia et al,
1986, Dorr et al, 1990 ). Although there is no specific histological
evidence of the slow release of metal species that is thought
to occur in association with all metal implants, accelerated
corrosion and a tissue response ( e.g., discolouration, foreign
body response ) that can be related directly to identifiable
corrosion products have been demonstrated in the tissues surrounding
multiple - part devices (Urban et al., 1994).
iii) Corrosion products have been implicated in causing local
pain and swelling in the region of the implant, in the absence
of infection ( Park and Lakes, 1992).
iv) The presence of particulate corrosion and wear products
in the tissue surrounding the implant may ultimately result
in a cascade of events leading to periprosthetic bone loss
( Urban et al, 1994).
v) Excretion of excess metal ions (especially chromium, cobalt
and nickel) and their suspected role in induction of tumours
e.g. malignant fibrous histiocytoma (Black, 1985). It remains
to be proven whether the reports of tumours developing in
the vicinity of metal implants are coincidental or otherwise.
When the litany of documented toxicities (metabolic, bacteriological,
immunological or carcinogenic) of these elements is considered,
it should be emphasised that they generally apply to soluble
forms of the elements and may not apply to the degradation
products of prosthetic implants (Jacob et al, 1998)
It is
important to realise that corrosion of orthopaedic blomaterials
is not just an exercise in physics and chemistry. It is a
pertinent clinical issue confronting all orthopaedic surgeons,
irrespective of the location or the level of sophistication
of their practice. While the need for stringent quality control
on the part of indigenous implant manufacturers remains of
paramount importance in the context of the Indian scenario,
orthopaedic surgeons and other theatre personnel need to be
made more aware to ensure that implants do not corrode and
fail due to carelessness or ignorance.
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