AJR 2005; 184:1219-1222
© American Roentgen Ray Society
Electrochemical Corrosion of Metal Implants
Scott P. Patterson1,
Richard H. Daffner1 and
Robert A. Gallo2
1 Department of Diagnostic Radiology, Allegheny General Hospital, 320 E North
Ave., Pittsburgh, PA 15212-4772.
2 Department of Orthopaedic Surgery, Allegheny General Hospital, Pittsburgh, PA
15212-4722.
Received July 15, 2004;
accepted after revision September 25, 2004.
Address correspondence to R. H. Daffner
(rhdaffner{at}netscape.net).
Abstract
OBJECTIVE. The objective of our study was to show the radiographic
changes that result from electrochemical corrosion of implanted metal in the
body.
CONCLUSION. Corrosion of metal implants is not rare. Radiologists
should become familiar with the changes this process produces.
Introduction
Under macroscopic observation, human tissue may appear to be
chemically inert; however, at the molecular level, human tissue is a dynamic
environment for immersed metals. Metals implanted into this saline milieu
inevitably undergo corrosion. The degradation of these metals can produce
detrimental effects both locally and systemically within the human body. A
brief explanation of corrosion is offered, but the main focus of this article
is to show the radiographic findings associated with this process.
Theory of Electrochemical Corrosion
Corrosion of metals is a complex phenomenon that depends on geometric,
mechanical, and chemical solution parameters. Although a comprehensive
explanation of corrosion is beyond the scope of this article, a basic
understanding is required to elucidate the radiologic findings of corroded
metal objects in human tissue.
The human body depends on a large number of chemical reactions occurring
continuously to sustain its viability. These chemical reactions produce an
abundance of oxidizing agents, which creates an unfriendly environment for
metals and alloys. Even the most corrosion-resistant materials are not immune
to the forces of nature and undergo some degree of corrosion
[1]. Some metals like stainless
steel may decay at a finite rate, whereas others like gold and platinum are
extremely corrosion-resistant
[2-4].
During the corrosion process, a coupled oxidation-reduction reaction takes
place, in which one species gains electrons (oxidizing agent) while the other
donates electrons (reducing agent). This reaction occurs spontaneously when
energy is released by the reaction. Most implanted metals, such as titanium,
cobalt-chromium, and stainless steels, have a tendency to lose electrons in
solution, and as a result, they have a high potential to corrode
[2-4].
The result is dissolution of the metal and formation of metallic ions.
Multiple factors affect these spontaneous reactions and determine the rate
at which they occur. All metals used for human implantation initially corrode
and form a thin barrier film. The barrier film, formed on the surface of the
newly implanted metal, offers a chemical barrier to corrosion and prevents the
degradation of deeper metal atoms. Without this barrier, these metals would
react violently with the surrounding chemical environment and eventually
dissolve [3,
4]. Mechanical forces can
disrupt this layer, which then leaves reactive metal atoms susceptible to
corrosion
[1-3].
Radiographic Findings of Corrosion
Although electrochemical corrosion typically is clinically silent, certain
radiographic features show that corrosion is occurring. One of the earliest
findings is the transformation of the metal surface margin from sharp to
irregular and smudgy (Figs. 1A,
1B,
1C,
2,
3A, and
3B). As corrosion progresses,
dissolution of the metal eventually leads to erosion of the entire metal
surface (Figs. 4,
5A, and
5B). Subsequently, the
deteriorating metal becomes brittle and may fracture (Figs.
6A,
6B,
7,
8A, and
8B). After the metal
fractures, the rate of corrosion drastically increases due to an increased
amount of exposed surface area and the loss of a protective passive layer. If
the metal fragment is not surgically extracted, further dissolution and
fragmentation can occur, which may cause inflammation of the surrounding
tissues. Corrosion can be seen as increased activity on skeletal scintigraphy
(Fig. 1C). This finding on a
bone scan can be particularly troublesome in cases of patients with known
malignancy. However, comparison with radiographs will show that the increased
activity was due to corrosion.
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Fig. 1A. —68-year-old woman with history of breast carcinoma and
positive findings on bone scan in right femur. History revealed cerclage wires
placed 20 years earlier for femur fracture. Frontal (A) and lateral
(B) radiographs show fuzziness of margins of wires indicating
electrolytic corrosion. Radionuclide bone scan (C) shows increased
tracer activity in left femur. This corresponds to site of corrosion.
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Fig. 1B. —68-year-old woman with history of breast carcinoma and
positive findings on bone scan in right femur. History revealed cerclage wires
placed 20 years earlier for femur fracture. Frontal (A) and lateral
(B) radiographs show fuzziness of margins of wires indicating
electrolytic corrosion. Radionuclide bone scan (C) shows increased
tracer activity in left femur. This corresponds to site of corrosion.
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Fig. 1C. —68-year-old woman with history of breast carcinoma and
positive findings on bone scan in right femur. History revealed cerclage wires
placed 20 years earlier for femur fracture. Frontal (A) and lateral
(B) radiographs show fuzziness of margins of wires indicating
electrolytic corrosion. Radionuclide bone scan (C) shows increased
tracer activity in left femur. This corresponds to site of corrosion.
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Fig. 3A. —78-year-old woman with distal humeral fracture. Lateral
(A) and frontal (B) radiographs show corrosion of retained
surgical screw placed in radius 12 years earlier. Note fuzziness along surface
of screw.
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Fig. 3B. —78-year-old woman with distal humeral fracture. Lateral
(A) and frontal (B) radiographs show corrosion of retained
surgical screw placed in radius 12 years earlier. Note fuzziness along surface
of screw.
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Fig. 5A. —68-year-old woman with thigh pain after fall. Frontal
(A) and lateral (B) radiographs show corrosion and dissolution
of retained plate and screws, placed 60 years earlier when patient was
child.
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Fig. 5B. —68-year-old woman with thigh pain after fall. Frontal
(A) and lateral (B) radiographs show corrosion and dissolution
of retained plate and screws, placed 60 years earlier when patient was
child.
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Fig. 6A. —73-year-old woman with foot pain. She gave history of having
stepped on needle 5 years earlier. Frontal (A) and lateral (B)
radiographs show corrosion and fracture of sewing needle in plantar aspect of
foot.
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Fig. 6B. —73-year-old woman with foot pain. She gave history of having
stepped on needle 5 years earlier. Frontal (A) and lateral (B)
radiographs show corrosion and fracture of sewing needle in plantar aspect of
foot.
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Fig. 7. —33-year-old male professional hockey player. Radiograph of
heel shows that corrosion and fractures of broken hypodermic needles from
self-injections. Patient had been performing such injections for years.
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Fig. 8A. —54-year-old woman with foot pain. Patient suffered puncture
wound while walking barefoot on lawn 4 years earlier. Frontal (A) and
lateral (B) radiographs show corrosion and fractures of broken needle
in plantar aspect of foot.
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Fig. 8B. —54-year-old woman with foot pain. Patient suffered puncture
wound while walking barefoot on lawn 4 years earlier. Frontal (A) and
lateral (B) radiographs show corrosion and fractures of broken needle
in plantar aspect of foot.
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Clinical Implications
Technologic advances have allowed the increased use of metallic implants
such as screws, pins, plates, artificial joints, and pacemakers. New alloys
and better techniques of insertion have been developed, yet no implant is
completely immune from the corrosion that transpires within the human body.
Therefore, any time an implant is introduced into the human body, the
individual is subject to the adverse effects of corrosion. Although corrosion
is rarely clinically significant, one should address two issues: implant
failure and inflammation caused by degradation products.
Corrosion resistance is a crucial determinant in the selection of
orthopedic appliances. Corrosion can weaken an implant so that the metal can
no longer withstand normal stresses before failing. For example, stainless
steel, which is particularly prone to corrosion and subsequent implant
failure, is generally restricted to the fixation of fractures
[5]. In this setting, the
implant needs to be functional only until the bone heals. In contrast,
cobalt-chromium and titanium are used for artificial joint replacements in
part because of their increased resistance to corrosion. As a result, failure
of the implant due to corrosion is extremely rare.
Degradation products of corrosion can cause a local inflammatory response.
Locally, these products have been linked to cessation of bone formation,
synovitis, and loosening of artificial joint implants
[3,
6,
7]. Systemically, several
reports have suggested that metallic degradation products may cause the
formation of neoplasms. Most of these reports, however, are confined to animal
models [3]. Much research
remains to be performed regarding the long-term systemic effects of metallic
corrosion.
Not all metal is implanted into the body purposefully. Puncture wounds are
common from needles and other metallic objects (Figs.
6A,
6B,
7,
8A, and
8B). In addition, most
acupuncture and self-administered needles are temporarily inserted into
subcutaneous tissues of the body, but they may accidentally be left in place
[8]
(Fig. 7). These metallic
objects are subject to the same degradative forces in the same manner as
surgically implanted metals.
Conclusion
Despite the best efforts of metallurgists, failures through broken
connections in pacemakers, fracture of weight-bearing orthopedic devices, and
inflammation caused by corrosion products in the tissue around the implant
will continue to occur [2,
3]. As our images show,
corrosion is not an infrequent finding among implanted devices. Typically,
this finding has little impact on clinical decision-making. However, in
certain circumstances, corrosion of a metal implant may contribute to the
clinical condition. Therefore, the challenge for the radiologist is to
determine which of these findings is clinically relevant and which is
incidental.
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Abstract
Introduction
