70 The Biochemistry of Spinal Implants
Short- and Long-Term Considerations
KEY POINTS
Historical Background
In 1892, Sir William Aruthnot Lane began to fix tibia fractures with ordinary steel (Figure 70-1). He was successful in treating a large number of patients, but noted that the steel plates he used became corroded after time. Fortunately, and unbeknown to him, the rust that formed acted as a pseudoinsulator (oxide layer), and prevented further degradation and, likely, failure of the plate. If he had used a dissimilar metal, this layer would not have formed and a severe electrolyte reaction would have ensued, leading to the destruction of the metal and inflammation of the tissues. Though metals had previously been implanted in patients, it was with this advancement that the use of metal implants for fracture stabilization became a practical procedure.1
Tissue Response to Biomaterials
The biocompatibility of a material is directly related to the tissue response generated by the material. These are time-dependent processes and can be viewed in two different but interconnected ways: first, the bulk properties of a material, and, second, the physiochemical surface properties of the material, both of which contribute to the initial incorporation and long-term survival of biologic prostheses (Table 70-1).
Implant-Tissue Reaction | Consequence |
---|---|
Toxic | Tissue necrosis |
Biologically inert—smooth surface | Implant is encapsulated without bonding |
Biologically inert—porous surface | Tissue grows into pores and forms mechanical bonds |
Bioactive | Tissue forms interfacial bond with implant (bioactive fixation) |
Dissolution of implant | Implant resorption and replacement with soft tissue or bone |
Metals
Current implantable metal alloys with wide use in orthopedics are 316L stainless steel, cobalt-chromium alloys, titanium alloys, and tantalum (Table 70-2). In general, metals are used routinely for weight-bearing or load-bearing implants such as plates, nails, stems, and screws. Though biocompatibility is good with metals, there are issues of concern. Corrosion, metallic toxicity, hypersensitivity, genotoxicity, and carcinogenesis all have been described in the literature with the use of metallic implants.
Metal Types
Tantalum
Tantalum is a gray, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below 150° C, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. At high temperatures, tantalum becomes much more reactive. Tantalum is used to make a variety of alloys with desirable properties such as high melting point, high strength, and good ductility. Tantalum readily forms oxides and is most stable as +5 tantalum pentoxide. Elemental tantalum unites strength and corrosion resistance with excellent biocompatibility. Tantalum is the metal used in the construction of Trabecular Metal (Zimmer). The cellular structure of Trabecular Metal resembles bone and approximates its physical and mechanical properties more closely than any other prosthetic material. Its unique, highly porous, trabecular configuration is conducive to bone formation, enabling rapid and extensive tissue infiltration and strong attachment.
Corrosion
Most fluids in the human body are of similar chloride content and pH to sea water (20 g/L and 7.4); therefore many metals used in orthopedic implants have been those most resistant to corrosion in sea water. Corrosion is, simply, the dissolution of metallic ions in aqueous solution. Electrochemical cells are produced in the body when these metallic implants are used and equilibria of metallic ions in solution are achieved within body fluids over time (Figure 70-2).
Generally three types of corrosion exist with the use of metallic implants and include (1) galvanic, (2) crevice or pitting, and (3) fretting corrosion. Galvanic corrosion is corrosion due to the use of dissimilar metals in contact with one another or electrochemical dissolution. Pitting corrosion is a form of localized corrosion that leads to the creation of small holes or defects in the metal (Figure 70-3). The driving power for pitting corrosion is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions, further increasing the localized lack of oxygen. The mechanism of pitting corrosion is probably the same as crevice corrosion. Finally, fretting corrosion, as defined by the ASM Handbook on Fatigue and Fracture, is: “A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force.” The relative small motion causes mechanical wear and material transfer at the surface of the metals, followed by oxidation of that debris and the freshly exposed surface. This debris then acts as an additional abrasive product that is often harder than the original metal and perpetuates the process.