TO THE EDITOR:
One of the issues relating to the corrosion of orthopaedic implants that was not mentioned in "Current Concepts Review. Corrosion of Metal Orthopaedic Implants" (80-A: 268—282, Feb. 1998), by Jacobs et al., is the profound difference between the corrosion that occurs when implants are in contact with aqueous solutions in vitro and the corrosion that occurs in vivo. The crux of the difference has to do with how the energy released by the chemical reactions affects the elements in proximity to the implant, such as the film of fluid (exudate or transudate) between the implant and the fibrous, osseous, or marrow tissue of the host. The energy from well tolerated implants may be insufficient to incite substantial inflammation (which perhaps defines the term tolerated). The energy may well be cumulative and, over months or years, may be responsible for the failure of the implant due to corrosion.
In addition, the authors did not consider in detail the occurrence and character of corrosion products that separate from the parent metal. Many authors have described the particles produced by corrosion, and some authors3,6,7 have measured the particles, which are found intracellularly as well as extracellularly. Often, these particles are submicroscopic in size. The hyperreactivity of tissues or any aqueous medium to very small particles should be mentioned, as was emphasized previously5.
The persistence of metal particles (presumed to be corrosion products) in daughter cells after fifteen reseedings of tissue cultures4 also deserves mention, as does the applicability of this concept of hyperreactivity of small particles to polymers as well as to metals. The relevance of this matter to the problem of resorption of bone around orthopaedic implants is obvious. However, it must also be apparent that research to clarify the nature and extent of the tolerance of tissues to corrosion products and to particles poses major difficulties, including sizing of the particles, identification of the chemistry of corrosion particles, and energy considerations. In particular, in a research project in which particles are investigated in vitro or in vivo, the manner of production of the particles, the sampling of sizes of the particles, and several other details that affect the reactions at the surface of the particles must be considered.
Jonathan Cohen, M.D.: Franciscan Children's Hospital and Rehabilitation Center, 30 Warren Street, Boston, Massachusetts 02135-3680
Dr. Jacobs, Dr. Gilbert, and Mr. Urban reply:
We concur that the classical kinetic and thermodynamic descriptions of corrosion in aqueous solution may not be fully representative of the much more complex in vivo situation. Dr. Cohen is correct in his assertion that the electrochemical corrosion reactions are spontaneous and that, as a result, free energy is released into the surroundings. The magnitude and time-course of released energy is linked to the magnitude and time-course of the oxidation processes. Some energy is consumed in the reduction reaction, some is dissipated as electrical energy, and some is dissipated as thermal energy. Preliminary unpublished laboratory experiments performed by one of us (J. L. G.) have demonstrated that transient electrical fields are generated in response to mechanical abrasion of self-passivating alloys immersed in saline solution. While the effects of these transient fields on the surrounding tissues are not known, the resulting potential drop increases the local consumption of oxygen at the surface of the alloy and thus affects the availability of oxygen to the surrounding cells, which may affect their metabolic activity. Very little is known in this arena, which is certainly worthy of additional study.
With regard to the second point, space limitations precluded us from providing a detailed discussion of the extensive literature dedicated to the in vitro bioreactivity of particulate degradation products from metallic implants. Nonetheless, we do have an intense interest in this subject and have discussed it elsewhere1,2,7. We acknowledge Dr. Cohen's pioneering work in this area4,5, which began more than three decades ago and served as a forerunner of an extensive body of literature dealing with the modeling of biological reactions to corrosion and wear products with the use of tissue-culture techniques. With the advent of powerful molecular biological tools, additional insight is being gained into the interaction between the particle and the cell, including the elucidation of the role of certain signal transduction pathways. This has generated considerable interest in the orthopaedic community and has provided strategies for pharmacological manipulation of the host response. We have been keenly interested in the role of corrosion products in the mediation of periprosthetic osteolysis2; however, the situation is quite complex, as Dr. Cohen pointed out.
We echo Dr. Cohen's final point concerning the issue of particle size, chemistry, and surface energy, and we urge future investigators in this field to consider these factors when constructing in vitro models that attempt to simulate the interactions at the bone-implant interface.
Joshua J. Jacobs, M.D.; Robert M. Urban: Department of Orthopaedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, Rush Medical College, 1725 West Harrison Street, Suite 1063, Chicago, Illinois 60612
Jeremy L. Gilbert, Ph.D.: Division of Biological Materials, Northwestern University, 311 East Chicago Avenue, Chicago, Illinois 60611