Ryan Siskey

In vivo deformation, surface damage, and biostability of retrieved Dynesys systems.

Study Design. Retrospective retrieval analysis. Objective. To evaluate wear, deformation and biodegradation within retrieved polycarbonate urethane (PCU) components of Dynesys systems. Summary of Background Data. The Dynesys Dynamic Stabilization System (Zimmer Spine) consists of pedicle screws (Ti alloy), polycarbonate urethane (PCU) spacers, and a polyethylene-terephthalate cord. Methods. Seventeen retrieved (mean implantation: 2.5 years, range: 0.7–7.0 years) and 2 exemplar implant systems were available. Reasons for revision were persistent pain (16/17), infection (1/17), and/or screw loosening (11/17), with 1/17 case of implant migration. Optical microscopy, microCT, and scanning electron microscopy were conducted to evaluate PCU spacer wear and deformation. Attenuated total reflectance Fourier transform infrared spectroscopy was used to assess spacer surface chemical composition. Results. Retrieved spacer components exhibited permanent bending deformation (mean: 4.3°, range: 0.0°–15.8°). We observed evidence of PCU spacer contact with pedicle screws, cords, and surrounding bony structures (74/75, 69/75, and 51/75 spacers, respectively). Relatively infrequent damage modes included PCU fracture (1/75 spacers) or cracking (2/75 spacers), as well as pedicle screw fracture (3/103 screws). PCU degradation products were identified in 10/75 spacers, which represented retrievals having significantly longer implantation times (mean: 4.3 years, range: 1.0–7.0 years). Of these spacers, 8/10 had degradation peaks identified along the side of the spacer where the material would have been in contact with bodily fluid. Conclusion. PCU spacers from retrieved Dynesys systems exhibited permanent deformation, focal regions of in vivo wear and surface damage. Chemical changes associated with PCU biodegradation were associated with longer-term retrievals. The most frequently observed complication was pedicle screw loosening, with 3 incidences of screw breakage in 2 patients. These retrieval data provide a crucial basis for developing in vitro tests to simulate in vivo damage and degradation of posterior dynamic motion preservation implants. Longer-term retrievals, as well as retrievals that include more recent design features (e.g., HA coating), will be useful to provide a greater context for the clinical implications of our short-term observations.

Retrieval Analysis of a ProDisc-L Total Disc Replacement

Study Design We retrieved a functioning ProDisc-L total disc replacement and associated tissues at 16 months of service life. Objective To analyze a previously unreported mode of implant malpositioning, wear mechanisms, and polyethylene locking mechanism, and to study retrieved periprosthetic tissues. Summary of Background Data The clinical performance of polyethylene in the context of total disc replacements remains poorly understood. In the ProDisc-L, the polyethylene core is fixed to the inferior metal endplate through a mechanical interference locking mechanism similar to those used in tibial total knee components. This case represents the third report of an explanted ProDisc-L prosthesis, and the first reported case of posterior malpositioning with this device. Methods The implant was removed via a transperitoneal approach. Its polyethylene core was evaluated for burnishing, fracture, third-body abrasion, and permanent deformation. An identical, never-implanted set of polyethylene and endplate components served as controls for the microscopic evaluation of wear. Two tissue samples were collected from a region adjacent to the failed implant to evaluate tissue morphology and inflammation. Hematoxylin and eosin-stained tissue sections were also evaluated for the presence of polyethylene debris by polarized light microscopy. Results The implant was removed without serious incident, although there were incidental venotomies. The patient went on to solid arthrodesis. We found minimal wear, oxidation, and periprosthetic tissue reaction, as might be expected given the short-term duration of implantation and its reason for revision. No evidence was found of malfunction or improper deployment of the locking mechanism. Burnishing seemed to be the result of short-term impingement. Some areas of the tissue matrix showed evidence of early cell degeneration, and some of these areas contained polyethylene particles identified by polarized light microscopy. Conclusions A larger series of implant retrievals will be needed to investigate possible wear and the biologic response to increased particle generation.

Accelerated aging, natural aging, and small punch testing of gamma-air sterilized polycarbonate urethane acetabular components

The objectives of this study were three-fold: (1) to determine the applicability of the small punch test to characterize Bionate 80A polycarbonate urethane (PCU) acetabular implants; (2) to evaluate the susceptibility of PCU acetabular implants to exhibit degradation of mechanical behavior following gamma irradiation in air and accelerated aging; and (3) to compare the oxidation of gamma-air sterilized PCU following accelerated aging and 5 years of natural shelf aging. In addition to attenuated total reflectance-Fourier transform infrared spectroscopy, we also adapted a miniature specimen mechanical test, the small punch test, for the deformable PCU cups. Accelerated aging was performed using ASTM F2003, a standard test that represents a severe oxidative challenge. The results of this study suggest that the small punch test is sufficiently sensitive and reproducible to discriminate slight differences in the large-deformation mechanical behavior of Bionate 80A following accelerated aging. The gamma-air sterilized PCU had a reduction of 9% in ultimate load after aging. Five years of shelf aging had little effect on the mechanical properties of the PCU. Overall, our findings suggest that the Bionate 80A material has greater oxidative stability than ultra-high molecular weight polyethylene following gamma irradiation in air and exposure to a severe oxidative challenge.

Accelerated aqueous aging simulation of in vivo oxidation for gamma-sterilized UHMWPE

In vivo oxidation of gamma air-sterilized ultrahigh-molecular-weight polyethylene (UHMWPE) has been observed when joint replacement hip and knee components are explanted during revision surgery. The purpose of the present study was to extend a previously published accelerated aging protocol for gamma-sterilized UHMWPE. Unsterilized and gamma-sterilized GUR 1150 resin samples were aged in phosphate-buffered saline (PBS) at 40 or 50 degrees C for up to 52 weeks. Under these conditions, slower changes in oxidation index (OI) occurred than those previously observed by aging at 60 degrees C. Reduction of aging temperature below 60 degrees C also changed the kinetics of oxidation such that the aldehyde peak (1732 cm(-1)) present at higher temperature was eliminated making the ketone/carboxylic acid region (1713-1718 cm(-1)) the primary region contributing to the calculation of the OIs for each group. The oxidation profiles obtained after 52 weeks at 40 and 50 degrees C were consistent with retrievals that have undergone low oxidation, associated with maximum OI values of less than 1. Aging at 50 degrees C represents a compromise between the slower oxidation rate of in vivo temperatures and the nonphysiological kinetics of elevated temperatures in an aqueous environment. However, even at 50 degrees C over a year of in vitro aqueous aging will be necessary to reproduce the oxidation levels observed in long-term implanted acetabular retrievals. (

Comparison of in vivo and simulator-retrieved metal-on-metal cervical disc replacements

Background Cervical disc arthroplasty is regarded as a promising treatment for myelopathy and radiculopathy as an alternative to cervical spine fusion. On the basis of 2-year clinical data for the PRESTIGE® Cervical Disc (Medtronic, Memphis, Tennessee), the Food and Drug Administration recommended conditional approval in September 2006 and final approval in July 2007; however, relatively little is known about its wear and damage modes in vivo. The main objective was to analyze the tribological findings of the PRESTIGE® Cervical Disc. This study characterized the in vivo wear patterns of retrieved cervical discs and tested the hypothesis that the total disc replacements exhibited similar surface morphology and wear patterns in vitro as in vivo. Methods Ten explanted total disc replacements (PRESTIGE®, PRESTIGE® I, and PRESTIGE® II) from 10 patients retrieved after a mean of 1.8 years (range, 0.3–4.1 years) were analyzed. Wear testing included coupled lateral bending ( ±4.7°) and axial rotation ( ±3.8°) with a 49 N axial load for 5 million cycles followed by 10 million cycles of flexion-extension ( ±9.7°) with 148 N. Implant surfaces were characterized by the use of white-light interferometry, scanning electron microscopy, and energy dispersive spectroscopy. Results The explants generally exhibited a slightly discolored, elliptic wear region of varying dimension centered in the bearing center, with the long axis oriented in the medial-lateral direction. Abrasive wear was the dominant in vivo wear mechanism, with microscopic scratches generally oriented in the medial-lateral direction. Wear testing resulted in severe abrasive wear in a curvilinear fashion oriented primarily in the medial-lateral direction. All retrievals showed evidence of an abrasive wear mechanism. Conclusions This study documented important similarity between the wear mechanisms of components tested in vitro and explanted PRESTIGE® Cervical Discs; however, the severity of wear was much greater during the in vitro test compared with the retrievals.

Morphology and Crystalline Architecture of Polyaryletherketones

Publisher Summary Polyaryletherketones (PAEKs) are linear aromatic polymers exhibiting desirable physical properties and durability against environmental challenge. These properties are the direct result of the chemistry and architecture of the polymer backbone, the manner and extent to which the molecules organize when the polymer is solidified, and any morphological changes that occur during subsequent thermal exposure. It provides a discussion of the semicrystalline nature of the PAEKs and the techniques traditionally used to characterize their structures, including density, X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and microscopy. Importantly, the crystallization behavior of the PAEKs does not allow for the degree of crystallinity at room temperature to be obtained readily from DSC. This chapter also contains a description of the effects of thermal history, including annealing and quenching, on structure and physical properties. A brief review of the connection between composition, morphology, and basic physical properties such as stiffness, strength, elongation, and fatigue behavior of neat resins as well as PEEK composites is also given in the chapter. FTIR spectroscopy provides information related to the structure and mobility of dipoles and is commonly used to characterize polymeric materials. DSC provides a relative measure of heat flow that provides an insight into molecular motion. DSC is readily used to determine the heat capacity of a polymer, the glass transition temperature, and the melting point. Depending on the desired magnification, optical and electron microscopy may be useful to examine PAEK polymers. Both techniques can be used in reflection or transmission to provide insight into polymer structure.

Technical Note: Is Corrosion a Threat to the Strength of the Taper Connection in Femoral Components of Total Hip Replacements?

Taper corrosion has been suggested as a possible contributor to in vivo disassociation of modular connections in total hip arthroplasty (THA) systems, but this relationship has not been explored ex...

Structure–property relationships for 3D-printed PEEK intervertebral lumbar cages produced using fused filament fabrication

Recent advances in additive manufacturing technology now enable fused filament fabrication (FFF) of Polyetheretherketone (PEEK). A standardized lumbar fusion cage design was 3D printed with different speeds of the print head nozzle to investigate whether 3D printed PEEK cages exhibit sufficient material properties for lumbar fusion applications. It was observed that the compressive and shear strength of the 3D printed cages were 63-71% of the machined cages, whereas the torsion strength was 92%. Printing speed is an important printing parameter for 3D printed PEEK, which resulted in up to 20% porosity at the highest speed of 3000 mm/min, leading to reduced cage strength. Printing speeds below 1500 mm/min can be chosen as the optimal printing speed for this printer to reduce the printing time while maintaining strength. The crystallinity of printed PEEK did not differ significantly from as-machined PEEK cages from extruded rods, indicating that the processing provides similar microstructure.

Development of a clinically relevant impingement test method for a mobile bearing lumbar total disc replacement

BACKGROUND CONTEXTnTotal disc arthroplasty is an alternative therapy to spinal fusion for the treatment of neck or low back pain and is hypothesized to reduce the risk of disease progression to the adjacent spinal levels. Radiographic and retrieval analyses of various total disc replacements (TDRs) have shown evidence of impingement damage. Impingement of TDRs can occur when the device reaches the limits of its functional range of motion, causing contact between peripheral regions of the device.nnnPURPOSEnImpingement can be associated with increased wear and mechanical damage; however, impingement conditions are not simulated in current standardized mechanical bench test methods. This study explored the test conditions necessary to apply clinically relevant impingement loading to a lumbar TDR in vitro.nnnSTUDY DESIGNnAn experimental protocol was developed and evaluated using in vivo retrievals for qualitative and quantitative validation.nnnMETHODSnRetrieval analysis was conducted on a set of 11 size 3 retrieved Charité devices using American Society for Testing and Materials F561 as a guide. The impingement range of motion was determined using a combination of modeling and experiments, and was used as an input in vitro testing. A 1-million cycle in vitro test was then conducted, and the in vitro samples were characterized using methods similar to the retreived devices.nnnRESULTSnAll in vitro tested samples exhibited impingement regions and damage patterns consistent with retrieved devices. Consistent with the retrievals, the impingement damage on the rim was a combination of abrasive wear and plastic deformation. Micro computed tomography (microCT) was used to quantitatively assess rim damage due to impingement. Rim penetration was statistically lower in the retrievals when compared with both in vitro groups. Rim elongation was comparable among all groups. The simulated-facet group had statistically greater angular rim deformations than the retrieval group and the no-facet group.nnnCONCLUSIONSnResults demonstrate that clinically relevant impingement seen on mobile bearings of lumbar TDRs can be replicated on the bench.