Anatomy of the Normal Spinal Disk and the Degenerative Process

The intervertebral disk is a weight-bearing and stress-absorbing structure located between the vertebral bodies of the spine. The disk itself is composed of three separate components: the nucleus pulposus, the anulus fibrosus, and the cartilaginous end plates. The central nucleus pulposus is a hydrated gel of proteoglycans with sulfated glycosaminoglycan side chains. The anulus fibrosus has 12 coaxial layers of collagen fibers that insert on the adjacent end plates at an oblique angle. These collagen fibers consist of both type I and type II collagen. The cartilage of the end plates is called hyaline cartilage. It is primarily a proteoglycan gel similar to the nucleus, but it is reinforced by collagen fibers. As wear occurs within the intervertebral disk, there is a progressive loss of cellularity, as well as hydration. With loss of hydration, the proteoglycans begin to lose their function. The amount of type I collagen gradually decreases, whereas the amount of type II collagen gradually increases. Annular fissures or tears begin to develop, loss of mechanical competence occurs, fibrovascular changes begin to develop in the subchondral bone, and as segmental instability develops, pain and radicular irritation may occur. Commonly seen in the later phases of degeneration are sclerotic changes in subchondral bone. End plate sclerosis as a result of aging and degeneration basically blocks the pores that allow diffusion of nutrients across the vertebral body end plate into the disk space. Because the disk space itself has relatively few cells to begin with, it does not require a tremendous amount of blood flow to maintain health and integrity. Conversely, the lack of blood flow to the disk associated with the aging process clearly speeds degenerative destruction of the individual disk components.

Arthroplasty Biomaterials

Spinal disk arthroplasty designs have clearly been influenced by progress made in the field of hip and knee joint arthroplasty. A number of the designs use a combination of metals and polymers. The polymers or plastics essentially provide some measure of shock absorption for the joint while also providing a low-friction surface for joint articulation. Polymers themselves are not strong enough to tolerate the stress of normal joints and are therefore always supported by a metal base. Articular surfaces may involve a metal-on-metal, metal-on-polymer, ceramic-on-polymer, or ceramic-on-ceramic interface. Two polymers that have been used successfully are polyurethane and the derived ultra-high-molecular-weight polyethylene (UHMWPE). Metallic devices have been constructed from solitary metals such as stainless steel, titanium, and cobalt. Newer devices take advantage of metallic alloys. Alloys are homogeneous mixtures or solid solutions of two or more metals. For example, cobalt-chromium alloy, cobalt-chromium-molybdenum alloy, and titanium-aluminum-vanadium⁷ have characteristics that make them uniquely suited for use in arthroplasty. The alloys seem to wear more slowly than polymers and may resist corrosion better than single-metal implants.

Regardless of whether the prosthesis is metal on metal or metal on polymer, the choice of material used in creating the device must be governed by three principles: articular surface wear, generation of wear debris, and host inflammatory response. The first is the extent of articular surface wear of the device based on repetitive motion cycles. A motion cycle is a typical bending motion of the spine such as flexion-extension or lateral bending. If multiplied by the number of times that the patient bends or twists in a day or a year, the number of motion cycles is considerable. The extent of weight bearing and joint range of motion (ROM) will also determine the amount of friction on the articulating surfaces and therefore the amount of destruction of the device over time. The second factor is generation of wear debris. Because there is no frictionless surface, load on a device combined with motion will generate wear debris. Wear debris, when trapped within the joint, can lead to rapid destruction of the articular surfaces, generation of toxic metal ions, and a profound inflammatory response, depending on the materials involved. The third principle is generation of the host inflammatory response. The inflammatory response can lead to resorption of the bone surrounding the prosthesis, which can result in loosening of the implant and ultimately failure. In general, the greater the degree of wear debris production, the greater the degree of inflammation, and the greater the extent of periprosthetic bone resorption.

To describe the complex interaction of biomaterials in motion preservation devices, a unique discipline known as tribology has developed. Tribology is the science and technology of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication, and wear. Typically, in a normal joint the cartilage in contact with the adjacent articulating surface will wear. That cartilage does cause an inflammatory response in the periarticular tissues, known as arthritis. In the case of disk arthroplasty, wear debris clearly causes an inflammatory reaction. In certain designs, such as those using stainless steel, the extent of the inflammatory response is proportional to the amount of wear debris generated. This inflammatory response is often manifested not only as the cardinal signs of inflammation (pain, heat, redness, and swelling) but also at the microscopic level, where macrophages are actually seen having engulfed metal or plastic fragments. On the molecular level, measurable quantities of potentially toxic metal ions are released into the circulation. The reaction of the body to the inevitable generation of wear debris is the third point to consider when choosing the appropriate biomaterial from which to fabricate an arthroplasty device. It is thought that the inflammatory response associated with wear debris is what leads to osteolysis surrounding the implant. Osteolysis can result in loosening of the implant, abnormal motion of the implant, and eventually misalignment.

Corrosion is an important principle to consider when selecting metallic biomaterials. The chemical reactions that are part of normal human metabolism produce an abundance of oxidizing agents, which creates a destructive environment for metals and alloys. Even the most corrosion-resistant biomaterials will undergo some degree of corrosion. Some metals such as stainless steel decay at a predictable rate, whereas others such as gold and platinum are extremely corrosion resistant. The process entails a coupled oxidation-reduction reaction in which one element gains electrons (oxidizing agent) and the other donates electrons (reducing agent). Most implanted metals, such as titanium, cobalt-chromium, and stainless steel, have a tendency to lose electrons in solution, and as a result, they have a high potential to corrode. The result is dissolution of the metal and the formation of metallic ions. All metals used for human implantation initially corrode and form a thin barrier film. This surface oxidative film offers both a chemical barrier to corrosion and prevents degradation of the deeper metal. If mechanical forces disrupt this layer, the underlying reactive metal atoms become susceptible to corrosion.

Use of a metallic alloy such as cobalt-chromium-molybdenum to minimize the effects of oxidative corrosion unfortunately does not eliminate the concern for wear of the articular surface. Any articulating surface will generate debris secondary to friction. The greater the physiologic load on the device, as well as the greater the ROM, the greater the generation of wear debris. In particular, Hellier and coauthors published the results of determining the absolute and relative wear volume rates of various metal alloys through simulation of an intervertebral disk prosthesis.⁸ It was found that among all the available alloys, a cobalt-chromium-molybdenum (CoCrMo) alloy generated the least amount of wear debris. It had an average wear volume rate of 0.093 mm³ per million cycles from an arthroplasty device, whereas a titanium alloy containing 6% aluminum and 4% vanadium (Ti6Al4V) had an average wear volume rate of 2.9 mm³ per million cycles. Schmiedberg and colleagues in 1994 used scanning electron microscopy to further define the size and shape of the wear debris fragments generated from an arthroplasty articular surface.⁷ The fragments from a titanium-aluminum-vanadium surface are extremely rough and irregularly shaped. The size of the fragments ranges from less than 1.0 µm to greater than 30 µm. Fragments from the cobalt-chromium-molybdenum alloy have an irregular polyhedral shape when the alloy was formed from a forged process, but the fragments have a spherical shape ranging between 5.0 and 30 µm when the alloy was formed from a hot isostatically pressed process. Catelas and associates published the results of their study investigating wear debris in metal-on-metal total hip arthroplasty devices.⁹ The study, published in 2003, demonstrated a significant number of wear debris particles composed predominantly of chromium oxide particles, with estimated loss rates as high as 100 mm³/yr for hip arthroplasty.

Basic Terms for Biomaterials

When selecting materials to use for the manufacture of arthroplasty devices, a number of basic terms should be defined.