Stanley A. Brown

Distribution of cobalt chromium wear and corrosion products and biologic reactions.

Replacement hip arthroplasty with the use of ultrahigh molecular weight polyethylene for the cup articulating with a metal head has provided a low friction arthroplasty with years of success. However, the search for improved materials and designs for articulating surfaces continues. The use of metallic heads articulating with metallic cups is now being reconsidered for total hip replacements. Success will be enhanced if wear and corrosion of the articulating surfaces can be kept below that of the metal on ultrahigh molecular weight polyethylene couple. Concern has been raised about the release, and biologic fate, of metal species from corrosion and wear. Titanium alloys have been shown to have limitations as an articulating surface showing significant wear, and the alloy per se should not be considered for wear couples in total hip replacements. The cobalt chromium alloys are known to have reasonable wear and corrosion properties and continue to be evaluated. The issue of cobalt chromium wear and corrosion products and how this relates to the biologic performance of total hip replacement devices is reviewed. Under the condition of wear as currently experienced at the articulating surfaces of cobalt chromium alloys and ultrahigh molecular weight polyethylene, the amount of metallic products transferred to the tissues is sufficiently low to be well tolerated by the biologic system. Nickel and cobalt ions arc, rapidly transported from the implant site and eliminated in the urine. Chromium is stored in the tissue and eliminated more slowly. The issue of host hypersensitivity to these elements remains of concern. All 3 elements, in ionic form, are known to cause contact dermatitis. Untoward biologic reactions, including hypersensitivity, should be minimized if wear and corrosion phenomena are minimized.

Electrochemical studies on the influence of proteins on the corrosion of implant alloys

The effect of proteins on corrosion rates of 316L stainless steel, commercially pure titanium and titanium 6-aluminium 4-vanadium was studied in the static and fretting modes. The static mode was studied using cylindrical specimens as per ASTM F-746, and static fracture fixation plates. The fretting mode was studied using a two-hole plate fretting machine which caused a cyclic rocking motion between the plate and the screws, as per ASTM F-897. Electrochemical techniques of polarization resistance and Tafel slope measurements were used to study effects of proteins on the anodic and cathodic corrosion reactions. It was found that proteins increased the corrosion rate of the stainless steel and C.P. titanium cylindrical specimens, but did not have an effect on the Ti-6AI-4V cylinders. In the fretting mode proteins decreased the corrosion rate of the stainless steel plates, but did not have an appreciable effect on either of the titanium alloys. The presence of proteins appeared to cause an increase in the anodic Tafel constant and a decrease in the cathodic Tafel constant of stainless steel specimens. Significant differences in the shapes of the cathodic Tafel slopes were also seen with cylinders with different surface conditions, and static versus fretting plates.

Metal allergy and the surgical patient

Allergy to metal implants is under study at the Dartmouth-Hitchcock Medical Center using in vitro examination of white blood cell migration. Retrospective data from 121 patients confirm that allergic responses do occur. Prospective data are being gathered to investigate the incidence, prevalence and relation to morbidity.

TISSUE REACTION AND METAL SENSITIVITY An Animal Study

An animal study is presented in which nickel sensitivity as determined by an in vitro test for leukocyte migration inhibition (LIF) is correlated with results of skin tests with NiCl2 and with the degree of adverse tissue reaction to implanted stainless steel screws. Screws were implanted in the humeri of New Zealand white rabbits, one group of which received repeated injections of nickel chloride following an initial injection of nickel chloride in Freun dˆs adjuvant. All the injected rabbits became skin test positive to nickel and demonstrated LIF production in the presence of nickel chloride. Some of the injected rabbits, 6–9 weeks after implantation of the screws, developed an inability for leukocyte migration even when the cells were incubated only with saline and serum. the tissue reaction to screws in the nickel sensitive animals showed a significant increase in inflammatory cells and foreign body giant cells when compared with the reaction in nonsensitive animals. the most severe reactions, some o...

Effects of Different Disinfection and Sterilization Methods on Tensile Strength of Materials Used for Single-Use Devices

Driven by economic and time constraints, some medical centers and third parties are resterilizing single-use devices (SUDs) for reuse. The steam autoclave is quick, but most plastics used in SUDs cannot survive the temperature. Thus, a number of new methods of cleaning, disinfecting, and sterilizing these complex devices are being introduced on the market. The present study investigated the effects of a range of methods on the tensile strength of latex rubber, silicone elastomer, 2 different formulations of polyurethane, nylon, and high-density polyethylene (HDPE) specimens. The methods used were sodium hypochlorite bleach (Clorox), peracetic acid + hydrogen peroxide (Steris), formaldehyde gas (Chemiclave), low-temperature peracetic acid and gas plasma (Plazlyte), and low-temperature hydrogen peroxide gas plasma (Sterrad). The results showed that silicone elastomer was minimally affected, whereas the strengths of nylon, polyethylene, and latex were reduced by some of the methods. Depending on the formulation, the strength of polyurethane either increased or decreased. The data demonstrated that disinfection and sterilization can affect the tensile strength of certain materials used in medical devices.

XPS Analysis of 316 LVM Corroded in Serum and Saline

Surface chemical analysis by x-ray photoelectron spectroscopy (XPS) was undertaken on 316 LVM stainless steel in the attempt to better understand corrosion occurring in vivo. Samples were dipped in saline or in blood serum, corroded in serum or saline by the application of a 5 volt anodic potential, or corroded by fretting. The products produced by fretting corrosion were also examined. XPS analysis revealed rapid protein coating of the stainless steel surfaces exposed to serum, changes in the oxidation state of the surfaces, and changes in the chlorine on the surface. In addition it was demonstrated that the corrosion products generated by fretting in saline had an oxidation state similar to that of chronic chloride whereas the corrosion products generated in serum had an oxidation state similar to that of potassium dichromate. These findings may have important implications since the chromium in dichromate is more biologically active than that in chronic chloride.

Effects of Metal Particles and Ions on the Biological System

Summary: Metallic devices have been used successfully by the orthopaedic community for stabilization of fractures and more recently for total joint replacement. The metals used are alloys involving several elements. These alloys were not developed by the medical industry but have been adopted by them because of strength, corrosion resistance, and biocompatibility. In general, the materials are well accepted. However, there is concern because of the recognition that metal is released from the implant into the peri-implant tissue and then is transported to distant sites in the body. Some of the metals are known to be causes of contact dermatitis in the normal human population, and some of them in special forms have been associated with carcinogenicity. Nevertheless, the devices have functioned well without major consequences. It remains important to use the best materials available, to insert them correctly, and to minimize situations that will lead to wear, corrosion, and deposition of particulate debris or ions into the tissue.

Healing of Femoral Osteotomies with Plastic Plate Fixation

Osteotomies of the canine femur were internally stabilized with four-hole plates made of thermoplastic polyacetal and polyester. The plates were strong enough to stabilize the bony fragments without additional protection while being flexible enough to permit healing with periosteal callus and subsequent remodeling without stress protection effects.

IN VIVO AND IN VITRO CONSIDERATIONS OF CORROSION TESTING

In vitro experiments were conducted in which the fretting corrosion rate of stainless steel plates and screws in 0.9% saline was compared with the rate in solutions of 10% calf serum in saline. The results demonstrated a ten-fold decrease in the fretting corrosion rate with the addition of serum to saline. However, it also demonstrated that the lower concentration of nickel in the serum solutions was more biologically active than the higher concentration in saline when the solutions were used to skin test rabbits made allergic to nickel by injection.

Effects of mechanical testing device variables on polymer composite femoral stem strains

Polymer composite femoral stems do not have a well-established in vitro mechanical testing method. The objective of this study was to examine mechanical testing devices for pressfit composite stems, using finite element analysis. The goals were to examine the effects of testing device design variables (geometry, material, interface friction, embedding height and applied load angle) and to reproduce the maximum strains of the stem implanted in a femur. The stem strains were affected by design changes to the testing device. The maximum normal and interlaminar shear strains of the composite stem in the femur were not as well reproduced by the testing device as were the maximum in-plane tensile strains. Decreasing the embedding height increased the stem strains and shifted the stem failure location from the neck to the embedding height. Testing a femoral stem using a testing device with a low embedding height may be inappropriate when trying to induce neck failure, since failure may occur at the embedding height instead of in the neck. A single-material testing device of birchwood, an orthotropic material with a longitudinal stiffness in the range of bone; best simulated a femur in this study.