Implant Wear, Degradation, and Breakage

Implant Wear

Implant wear caused by the sliding and rubbing between two bearing surfaces can lead to disruption of the atomic binding and electrorepulsive interactions at the contact points of the surfaces, resulting in the production of material debris and corrosion. These two processes can occur independently or as a result of each other. Wear debris is not limited to arthroplasty devices and can be generated in rigid spinal systems, usually observed at junctions where multiple components come into contact. The micromotion at these component interfaces can form local regions of wear and corrosion, which can result in local shedding of debris that can travel peripherally to the tissues, spinal fluids, and circulatory system. The debris reactivity is both local and systemic and can further augment tissue breakdown and degrade implant integrity. Corrosion of implants occurs as a result of implant material degradation in the presence of hostile electrolytic environments and is caused by chemical or electrochemical attack.

Wear Debris

Implant wear is a leading cause of aseptic loosening and osteolysis related to a host response to the excessive shed of particulate. The rate of failure at 7 years postimplantation for metal-on-metal total hip arthroplasties has been documented to be as high as 5%. There are three processes that contribute to wear: (1) abrasion, (2) adhesion, and (3) fatigue. Abrasion is the scraping effect between the two implant surfaces where there may be a difference in material hardness. Adhesion is the smearing of the softer material onto the surface of the harder material. Finally, fatigue is a result of the formation of subsurface cracking because of cyclical loading that creates a fluctuation of stresses, strains, or stress intensities to locations on structural components. In repetitive loading and unloading scenarios, these cracks will continue to propagate and eventually fail after multiple cycles, with the release of particulate upon cracking and failure. ^{, ,} Overall, the harder material used for a bearing surface that is in contact against a bearing surface with a lower hardness factor will cause the surface that is less hard to wear more rapidly than the harder surface.

Debris that is shed from implants because of wear can occur in two forms: particulate or insoluble ions (metallic). ^, Debris generated because of implant wear, fretting, or breakage is released into the tissues and body fluids and induces an inflammatory reaction. This initiates a foreign-body tissue response by inducing short-term fibrotic and histiocyctic reactions that can locally infiltrate the bone–implant interface and detrimentally affect the integrity of fixation at this interface. The cascade of debris production will continue throughout the course of tissue healing and alter the healing pathway.

Early phases of wear debris production occur because of the small peaks and irregularities on each surface after initial contact between the two surfaces. These asperities will wear down during this early phase of contact, and the wear debris production rate will taper to a steady state. Upon reaching steady state, the wear rate becomes more linear and is dependent on the sliding distance and the contact surface area. The linear steady-state volumetric wear rate can be calculated by the following equation: V = K × Fx, where V is volumetric wear (mm /year), K is the material constant, F is the contact force (N), and x is the relative travel distance (mm), thus providing insight toward predicting the wear potential of an implant.

Particulate Characterization

The particulate shape, size, distribution, number, area, and volume are essential parameters that provide insight into the causation and mechanisms of wear, the biocompatibility of the implant’s materials, and the potential reactions of local and peripheral tissue to wear debris.

A comprehensive profile of the debris provides insight into the potential risks that debris can pose to the host bone, fixation interface, soft tissue and surrounding neural structures of the spine. Therefore, preclinical benchtop testing, wear simulation studies, in vitro biomechanical assessments, and in vivo biocompatibility and safety studies are often conducted on motion-preserving spinal implants, and in many cases rigid spinal fixation systems.

Wear simulation studies are a conventional way to evaluate and predict the long-term extent of particulate that may be generated by an orthopedic or spinal implant. The simulation tests apply millions of cyclical supraphysiological motions along six degrees of freedom to motion-preserving spinal implants that are immersed in heated fluid baths (serum, synovial fluid, or phosphate buffered saline). The test is intended to simulate the human environment in an extreme situation, which provides a safety buffer toward wear assessment. The goal of this testing is to provide a means of quantifying and characterizing the debris generated by the device’s wear behavior under extreme long-term loading conditions to understand the potential risks to patients. A profile of the debris characteristics provides information related to the potential bioreactivity and adverse reactions that could occur in the local spinal and peripheral tissues, including the surrounding neural structures. More importantly, particulate debris is often a combination of nano- and micro-sized particles that can migrate to peripheral tissues throughout the body, with greater risk of detrimental influences to other organ systems. ^,

Measurement and characterization of the debris generated is essential to understanding how and where implants wear and to provide information that may predict the potential for an implant to generate debris. Morphological analysis of the wear particles can also provide insight into the biocompatibility of the material, with respect to the tissue reactions induced by the debris.

Many factors of the wear particulate can affect the biological response of the host and surrounding tissues. The number, shape, size, surface topography, material, and interaction of the multiple materials that may compose the particulate profile are a few of the many factors that can significantly influence the inflammatory response and tissue reactions to wear debris. Numerous parameters of the particulate, such as gravimetric mass to quantify wear loss, shape and size, size distribution, number, and volume for each implant material, will provide a more comprehensive analysis to predict the wear propensity.

The number of particles per specific tissue volume represents the “dosage” of particulate, which can influence the tissue’s inflammatory response. If the particulate is small but present in high numbers, it may have greater potential to provoke harmful effects in the tissue than a larger particulate that is present in smaller numbers in the same tissue. Although metallic debris is often only nanometers in size and thus small enough for cellular phagocytosis, if present in large numbers, these nano-sized metallic particulates can induce a greater proinflammatory response than a smaller dose or quantity of larger-sized particles in the same tissue. The shape of the particulate can also influence the inflammatory response. Studies have shown that round particles are less inflammatory than elongated elliptical particles, and particulate that is irregularly shaped or has a rough profile can provoke a greater inflammatory response compared with smooth particulate.

Debris reactivity is both local and systemic. The local inflammation associated with wear debris is attributed to macrophage ingestion of the debris and is differentially bioreactive. However, the particulate size and size distribution for each specific material will influence the biological response. Macrophages play a critical role in the immune system and rely on phagocytosis to regulate immune responses, and the size and shape of particulate can play a significant role in this process.

Phagocytosis is responsible for ensuring that the macrophage cells function properly. This process involves uptake of particles greater than 0.5 μm in size into the cells, where the particle shape and size can have different influences on phagocytosis. ^, Numerous studies have shown that eccentric particulate shapes with smaller surface areas have significantly slower uptake times during phagocytosis than larger spherical shapes. However, in similarly shaped particulates, size may be the determining factor for cellular uptake time. Larger particulates take longer to phagocytose. Additional studies have determined that the optimal particle size for maximum uptake and internalization via phagocytosis is 1 to 3 μm. Alternatively, phagocytosis of larger particles is dependent upon the surface geometry, as well as the size of the particle.

Multiple studies have shown that it is not solely the volume of wear generated that determines the biological response to the particulate debris, but also the concentration of wear particles between 0.2 and 0.8 μm within the tissue volume. Particles within this size range initiate macrophage activation, have been shown to perform favorably with respect to macrophage detection, and can be easily phagocytosed. Phagocytosis is relevant for particles with a maximum size of approximately 5 μm. Thus, macrophages provide similar responses to wear particles as they do to microorganisms of similar size (≥5 μm). Particles that cannot be engulfed because of their larger size tend to be encapsulated by surrounding tissue.

There are numerous additional factors related to particulate characterization that play a significant role in the initiation of an inflammatory response and are often the topic of debate. A combination of these many factors further compounds the difficulty in understanding the full scope of an implant’s wear potential and its biological risk. Continued research into the causation of wear and elimination of debris is vital to improving the longevity of spinal implants and the patient’s quality of life. This leads to further innovation of novel biomaterials and design features used for spinal implants that could possibly minimize the potential for wear debris altogether.