The analysis of retrieved implants and tissues may involve a broad range of test methods, which are comprehensively described in ASTM Standard F561.⁴ This manual provides guidance for analysis of all implant components, including metallic, polymeric, and ceramic materials. Although a detailed summary of this standard is beyond the scope of this chapter, we highlight in subsequent sections specific test methods that have been particularly helpful in the characterization of retrieved metal-on-polyethylene and metal-on-metal disc replacements.

Wear and Damage Assessment

Because wear and damage of retrieved TDRs can occur at length scales that are invisible to the naked eye, microscopy may be necessary to identify damage modes. An optical stereomicroscope, with 10× to 40× magnification, is typically sufficient to identify worn regions of retrieved implants, but frequently it is helpful to analyze the wear surfaces using scanning electron microscopy (SEM), achieving magnifications of 5000× or greater. In addition to optical and SEM, micro-CT and white light interferometry (WLIR) are two methods that have proven particularly useful in our previous analyses of wear in retrieved TDR and dynamic stabilization components.^3,⁵^–⁷ These novel wear assessment methods are highlighted in this section.

Micro-CT Analysis

We have used a micro-CT to detect nondestructively surface and internal voids and cracks within retrieved polyethylene TDR components ⁵ and polyurethane motion preserving spinal implants.⁷ Depending on their thickness, CoCr alloy components produce substantial artifacts on micro-CT. Consequently, we have found it helpful to remove the wire marker before micro-CT analysis of rim wear in the Charité design. The removal of the wire marker for other analyses may be unnecessary because the artifact does not extend into the central core of the implant. For polymer components that do not incorporate wire markers (e.g., ProDisc [Synthes, Paoli, PA] or Dynesys [Zimmer Spine, Minneapolis, MN]), micro-CT artifacts are not an issue.

At the authors’ institution, polymer components of motion preserving spinal implants are scanned at 18 μm voxel resolution using a commercial micro-CT scanner (μCT80, Scanco Medical, Bassersdorf, Switzerland).⁵ The three-dimensional reconstructions of the component and two-dimensional sections taken through the component are evaluated for the presence of surface, through-thickness, and internal voids or cracks. Previously, we have observed the trajectory of cracks in polymer components of motion preserving spinal implants, including Dynesys.⁷ Using optical microscopy, we have also characterized permanent deformation and wear patterns of polymer components from motion preserving spinal devices. Using these methods, we have been able to distinguish these forms of surface and subsurface damage from iatrogenic damage that occurs during implant removal.

Because of attenuation artifacts encountered with metallic components, micro-CT is useful only for polymeric components from TDRs. To measure the macroscopic surface geometry of metallic components, tools employed by the research community include coordinate measurement machines, laser profilometers, and optical profilometers. To measure microscopic changes in the implant surface, WLIR may be used. As discussed next, interferometry is applicable to both metallic and polymeric components of disc replacements.

White Light Interferometry

We use WLIR to characterize the microscopic surface morphology of retrieved disc arthroplasty components. WLIR is capable of detecting surface height changes that are on the nanometer length scale by measuring the interference of white light reflected off the component within a specified field of view compared with the light from a reference beam. We have successfully analyzed the wear surfaces of polyethylene and CoCr alloy TDRs at our institution using a NewView 5000 equipped with advanced texture analysis software (Zygo, Middlefield, CT).⁶ We sample 5 to 10 square regions (typically 0.54 × 0.72 mm) of a component to obtain representative surface topography of the retrieved implant in both worn and unworn locations.

Figures 168-1A and B illustrate the surface topography obtained from unworn and worn surface regions of a retrieved polyethylene disc replacement component that was implanted for 12.7 years. The unworn polyethylene surface is dominated by machining marks that are on the order of several microns in amplitude (see Fig. 168-1A). Initially, microscopic evidence of adhesive and abrasive wear is detected by the erosion and removal of machining marks, along with the presence of fine scratches (see Fig. 168-1B).

FIGURE 168-1 White light interferometry images of polyethylene unworn (A) and worn (B) implant surfaces compared with cobalt chromium implant unworn (C) and worn (D) surfaces.

The surface topography of a retrieved CoCr alloy disc replacement is shown in Figures 168-1C and D. The unworn CoCr surface is usually relatively flat and featureless, aside from microscopic scratches generated during the final polishing stage of the manufacturing process (see Fig. 168-1C). In a region of wear, the CoCr surface has evidence of localized, microscopic scratches with a characteristic length scale that is larger than the residual features from polishing (Fig. 168-1D).

As shown in Figure 168-1, surface characterization using WLIR provides useful information about the wear mechanisms in TDRs. In addition, by quantitatively analyzing the surface data, the roughness and waviness can be quantified and compared with as-manufactured components, providing insight into the magnitude of surface changes or wear that occur in vivo.⁸^–¹⁰ Ultimately, quantitative measurements of implant surfaces are used to validate in vitro and computational models that seek to simulate in vivo wear processes.

Wear and Damage Mechanisms

As discussed in the previous section, metal and polymeric components from retrieved motion preserving spinal devices should be evaluated macroscopically and microscopically for the presence of damage modes typically observed in large joint arthroplasty components (e.g., burnishing, abrasion, scratching, pitting, plastic deformation, fracture, fatigue damage, and embedded debris). A detailed description of wear and wear mechanisms is beyond the scope of this chapter and can be found in books dedicated to this subject.¹¹ A generic guide for the analysis of retrieved components is summarized in the Appendices for ASTM Standard F561.⁴ This section provides a concise summary of the most relevant wear and fatigue damage modes that may be encountered when inspecting retrieved motion preserving spinal implant components.

Abrasion and Scratching

Abrasive wear is evidenced by scratching and is common to both metallic and polymeric components for TDR. Abrasion may be apparent macroscopically by the naked eye, or it may be apparent only when viewed using microscopy. Abrasive wear occurs when microscopic surface irregularities (also referred to as “asperities”) in one implant scratch the surface of the opposing counterface. In the case of metal-on-polyethylene, the asperities on the metallic implant produce scratches in the softer polymeric implant. In the case of CoCr alloy metal-on-metal implants, abrasive wear is produced by locally stiffer asperities, such as carbides, plowing through the relatively softer cobalt alloy matrix. Abrasive wear can also occur when softer polymeric components of the implant contact surrounding bony structures (Fig. 168-2).

FIGURE 168-2 Abrasive wear observed on polymeric components from retrieved Dynesys systems, implanted 1.1 years.

During retrieval analysis, the pattern of scratches on an implant, whether macroscopic or microscopic, provides clues to the kinematics (motion) of the surfaces while they were contact in vivo. The microscopic multidirectional scratches and crisscrossing wear paths at the dome of a retrieved polyethylene TDR (Fig. 168-3) are consistent with the type of microscopic abrasive wear mechanisms previously observed in retrieved hip replacement components.^12,¹³ By matching comparable regions of damage on two opposing bearing surfaces, it is further possible to infer the orientation of the components while they were in contact.

FIGURE 168-3 Microscopic, multidirectional scratches and crisscrossing wear paths at the dome of a retrieved polyethylene total disc replacement that was implanted 6.2 years.

Burnishing

Typically encountered with polyethylene disc components, burnishing gives the polymer surface a polished, glossy appearance (Fig. 168-4). At a microscopic length scale, burnishing is associated with an adhesive wear mechanism, whereby the polyethylene surface wear occurs by adhesion to the metallic counterface. Magnified images of a burnished wear zone from a retrieved TDR are shown in Figure 168-4 and as noted also show evidence of scratching, which denotes the presence of abrasion. For this reason, the dominant wear mechanism in metal-on-polyethylene articulations is considered to be a combination of adhesion and abrasion, as seen in total joint replacements.^12,¹³

FIGURE 168-4 Burnishing observed on the dome of the polyethylene component of a retrieved total disc replacement.

Regions of burnishing on polyethylene retrievals may be appreciated with the naked eye under the proper lighting conditions. In contrast, the bearing surfaces of retrieved metallic components are typically highly polished after removal from the body. Burnishing on a metal-on-metal disc replacement that occurred in vivo is very difficult to discern without the aid of SEM.

Surface Deformation

Surface deformation, sometimes referred to as plastic deformation or creep, corresponds to permanent changes in the shape or geometry of a TDR, without the loss of material. Although surface deformation is not considered a wear mechanism, it could represent an undesirable damage mode. When permanent changes in the geometry of a device compromise the in vivo function or kinematics of the device, surface deformation is considered a failure mode for the implant.

In motion preserving spinal devices, macroscopic surface deformation has been observed in components that undergo compression in vivo. Polycarbonate urethane (PCU) spacers used in the Dynesys system undergo deformation owing to cold flow of the material, the compressive load applied during the surgery, and subsequent loading in vivo, which results in permanent bending of the implant and indentations from the supporting polyaxial screws (Fig. 168-5). Additional deformation of soft polymer components may occur from interaction with other components from the implant, for example, the cord component that passes through the center of the polyurethane spacer in the Dynesys system (see Fig. 168-5).

FIGURE 168-5 Polycarbonate urethane component of a retrieved Dynesys system that was implanted for 1 year, showing deformation from the polyester cord and the supporting pedicle screw.

Fatigue Wear and Fracture

Fatigue wear and fracture, especially of the rim, are a concern with polyethylene TDRs. David et al.¹⁴ reported a case in which the entire rim of a disc replacement fractured from the central body of the core after 9.5 years in vivo. This case of rim failure was attributed to severe oxidation degradation after gamma sterilization in air.

The severity and clinical manifestation of fatigue-related rim damage in the Charité design varies widely, ranging from full-thickness transverse rim fracture (Fig. 168-6) to more benign radial crack formation (Fig. 168-7). In our retrieval studies of the Charité, radial cracks have been observed in 27 of 53 implants examined so far.¹⁵ Similarly, transverse cracks have been observed in 17 of 53 retrieved implants.¹⁵ In most cases, fatigue fracture is related to impingement by the metallic end plates. Fractures have also been observed in polymer components of dorsal devices such as the Dynesys (Fig. 168-8).