Summary of Key Points
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Wear and wear-related inflammatory reaction is likely to be the cause of the long-term failure of total disc replacements.
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Tribology is the science examining wear, friction, and lubrication and is essential when designing and testing prosthetic devices.
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Wear is related to the bearing couples (opposing surfaces), load, and total amount of motion.
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Modes of wear define how the bearing couples interact. Normal type 1 wear is from movement between bearing surfaces. Other modes such as edge impingement, third body, and backside wear are pathologic and are not characterized in preclinical testing explant analyses.
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Total disc replacements vary in ability to reproduce normal spinal kinematics. These differences may affect long-term but have not shown any differences on short-term performance.
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Explant analysis is essential to the understanding of in vivo total disc replacement performance and to the host response to the implant and any wear debris.
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CoCrMo alloy total disc replacements are associated with inflammatory reactions as seen in total hip arthroplasty including large pseudotumor causing visceral and neurologic obstruction.
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Comparisons of simulator tested and explanted devices show similar patterns of normal wear, thus the simulators are reproducing normal patterns of motion.
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Explant analysis also shows that pathologic modes of wear are frequently seen and are likely associated with device failure.
Motion sparing technology in the spine aims to treat painful conditions such as degenerative disc disease or radiculopathy by decompression and reconstruction without arthrodesis. The techniques are designed to reestablish kinematics thereby theoretically decreasing the tendency for adjacent segment degeneration. In addition, this technology attempts to avoid changes in facet loading, which could cause pain and degeneration. Although there have been many approaches to motion sparing technology, only total disc replacements have been approved and are in common use in the United States. The currently available total disc replacements have mobile bearing surfaces that move in relation to each other and thus will be subject to processes of wear and the potential for biologically induced harm from wear particles. In vivo reactions to wear particles can result in prosthetic failure and production of inflammatory masses that can compress neural elements. Less commonly, the device can degrade and ultimately fail in response to in vivo conditions and repetitive loading.
Tribology
Tribology is the body of science involving the interaction of moving surfaces and the study of the principles of friction, lubrication, and wear. Tribology is a mature science in appendicular arthroplasty but remains at a nascent stage in the spine. An important component is to understand the host’s reaction to the implant and wear debris, as well as the changes that occur to the implant after implantation. This chapter reviews basic principles of tribology applied to spinal arthroplasty and examines known wear-related problems. Understanding the principles of tribology is important as it is likely that late failures of spinal disc replacements will occur from wear and wear-related debris.
The rubbing of two surfaces produces wear due to the interaction of asperities, which are projections of material from the machined surfaces ( Fig. 189-1 ). Asperities exist no matter how well surfaces have been machined. During movement, opposing asperities contact each other and, with increased loading, the asperities deform and friction is increased. Wear occurs as a result of chemical and mechanical processes and results in removal of surface material. It is obligatory in all arthroplasty devices despite the hardness of materials and the smoothness of the machined surfaces. Chemical processes can lead to wear from corrosion, which is a common problem at the trunnion-head junction in total hip arthroplasty, for instance. A high degree of wear is rare in disc replacements as far as we know, although it is present in many spinal fixation devices.
Mechanical wear results in permanent changes in the implant surface. Several mechanisms of wear occur. Abrasive wear occurs when a harder material’s asperities remove or plastically deform those of the softer materials, creating scratching of the surface. Adhesive wear is the most common wear mechanism seen in arthroplasty and occurs when the interfacial asperities are cleaved and form a permanent attachment to the opposing interfacial surface. The newly attached asperities can increase surface area of contact and thus reduce contact stresses. A consequence of wear can be the thinning of the bearing surface and potential failure by fatigue fracture or change in geometry, which could accelerate further wear. Third body wear is when material is interposed between bearing surfaces, which creates accelerated abrasive wear. Fatigue wear occurs when surfaces are thinned so that the fatigue limits are reached and surface delamination or component fracture results.
Lubrication reduces friction by decreasing the contact of interfacial asperities. Three types of lubrication occur. Fluid film or hydrodynamic lubrication occurs when the surfaces are completely separated so contact of asperities is not possible ( Fig. 189-2A ). This occurs with higher velocities and has a low coefficient of friction. In boundary lubrication, contact of interfacial asperities occurs, thus increasing friction ( Fig. 189-2B ). This takes place during high loads and low velocities. Mixed lubrication is a transition between fluid film and boundary lubrication.
Normal lubrication in synovial joints is from the synovial fluid produced by the synovial cells. This produces excellent lubrication with low friction despite the relatively high surface roughness of human articular surfaces. However, normal synovial cells are not present after arthroplasty, especially in the spine. Lubrication is generated by genetically altered synovial cells, which produce fluid that is two to three times less viscous than synovial fluid. The physical and chemical characteristics of the fluid in spine arthroplasty are unknown and may be different than joint arthroplasty.
Modes of Wear
McKellop proposed that arthroplasty devices (including spinal devices) perform under five different modes of contact during movement ( Fig. 189-3 ). Mode 1 is the normal intended sliding movement along bearing surfaces. Modes 2 through 4 are abnormal modes and, although occurring frequently in all arthroplasty devices, are considered abnormal and can lead to excess wear. Mode 2 wear occurs when bearing surfaces contact nonbearing surfaces such as pinching in extension of mobile components of disc replacements. Mode 3 is third body wear, where material is interspersed between the normal-bearing joint surfaces, which can lead to abrasive wear. This material can be from fracture components, cement debris, bone, porous surface coatings, or wear particles. Mode 4A results when two load-bearing surfaces make contact. Mode 4A is the most common and occurs from edge impingement at extremes of motion in disc replacements. This is commonly seen in over 50% of retrieved explants. Mode 4B is backside wear between a nonbearing surface and its supporting tray.
To frame the study of the modes of wear of explanted spinal devices, modification of the McKellop modes of wear has been proposed to characterize observed findings on explant analysis (e.g., for a semiconstrained metal-on-polyethylene [MOP] total disc replacement [TDR]; Table 189-1 ).
| Wear Mode | McKellop Modes of Wear for Joint Replacements | Surfaces Generating Wear in CTDR | |
|---|---|---|---|
| Superior Component | Inferior Component | ||
| 1 | Two bearing surfaces articulate in manner intended by the implant designer | CoCrMo “socket” | PE dome |
| 2 | Bearing surface articulates against a nonbearing surface | CoCrMo end plate | PE dome |
| 3 | “Third-body” abrasive particles entrapped between two bearing surfaces | CoCrMo “socket” + third-body particle | PE dome + third-body particle |
| 4 | (A) Two nonbearing surfaces articulate against each other—End plate–End plate impingement | CoCrMo end plate | CoCrMo end plate |
| (B) Two nonbearing surfaces articulate against each other—“Backside wear” | — | PE insert undersurface and CoCrMo “tray” | |
Factors Involved with Wear
Wear is determined by magnitude of motion, load, materials, surface conditions, geometry, lubrication, and alignment. The amount of prosthetic wear can be predicted under known engineering principles. The Archard relationship ( Equation 1 ) shows that for adhesive wear the amount of wear is linearly related to the load, a constant, and the total amount of angular displacement and inversely to surface hardness, calculated as follows:
Q = k W L / H
Given the projected long survivability of patients having disc replacement, the most important factor to determine ultimate wear will be cumulative total angular movement. This has been estimated to be 1 million annual cycles for joint arthroplasty, which has been adapted without evidence for the spine.
Mechanical Testing
Mechanical testing of medical devices is essential to assure a minimum performance prior to approval of a device. This does not warrant that all devices will perform satisfactorily in vivo. To allow comparison between devices, testing standards have been developed and are currently used in all motion sparing spinal devices. The American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO) produce testing standards that define test methods but do not provide performance criteria. It is expected that the results will be compared to those from a like device that has a satisfactory clinical record. Standards are always evolving as more clinical information becomes available that may dictate changes in protocols to better mimic in vivo conditions.
Intervertebral disc replacement devices are tested for static and dynamic strength—that is, how much load the device can withstand and how is it affected by cyclical loading. Further wear of the bearing surface is tested by simulation using complex testing methods under physiologic conditions. In wear simulators, the device is displaced a specified range and direction of movements under specific loads for 10 million to 40 million cycles of motion. Movement directions can be tested independently or by coupled movements, which is preferable. At various intervals, the bearing surfaces are examined visually, gravimetrically, and any wear debris is analyzed. Final reports are presented as a volumetric or gravimetric wear rate per million cycles and the size and shape of wear particles.
Given that wear is directly related to total movement, knowledge of how often and to what degree we move each motion segment is important. Using goniometers and accelerometers, Cobian determined that the cervical and lumbar spine moves up to 5.3 million and 3.4 million cycles per year in flexion and extension, respectively.
However, the mean motion at a single segment is relatively small averaging only 2.6 and 2.2 degrees in flexion-extension for a single motion segment in the lumbar and cervical spine. The overall predicted annual motion compares favorably to what is specified in the ASTM wear testing standards.
Materials
The material choice for the bearing surface or bearing couple is paramount to the design and long-term function of an arthroplasty. Spine total disc replacements have utilized both traditional arthroplasty materials with proved efficacy and, in the cervical spine, new materials ( Table 189-2 ). As of 2014, cobalt-chrome-molybdenum alloy (CoCrMo) coupled with ultrahigh-molecular-weight polyethylene (UHMWPE) is the standard for joint arthroplasty. This bearing couple is used in all approved lumbar disc arthroplasties and for the majority of cervical arthroplasties. In addition, approved cervical devices have titanium-polyurethane, titanium carbide on titanium carbide, and stainless steel on stainless steel bearing couples.
| Bearing | Type | Materials | Number of Bearing Components | |
|---|---|---|---|---|
| L umbar | ||||
| Charitie | Mobile | MOP | CoCrMo-UHMWPE | 3 |
| Prodisc L | Fixed ball and socket | MOP | CoCrMo-UHMWPE | 2 |
| C ervical | ||||
| Prestige ST | Ball and trough | MOM | Stainless steel-Stainless steel | 2 |
| Prodisc C | Fixed ball and socket | MOP | CoCrMo-UHMWPE | 2 |
| Bryan | Mobile core | MOP | Ti-PCU | 3 |
| PCM | Fixed ball and socket | MOP | CoCrMo-UHMWPE | 2 |
| SecureC | Mobile core | MOP | CoCrMo-UHMWPE | 3 |
| MobiC | Mobile core | MOP | CoCrMo-UHMWPE | 3 |
| Prestige LP | Ball and trough | MOM | TiCarbide-TiCarbide | 2 |
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