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To gain insight into a new technology, a novel facet arthroplasty device (TFAS) was compared to a rigid posterior fixation system (UCR). The axial and bending loads through the implants and at the bone-implant interfaces were evaluated using an ex vivo biomechanical study and matched finite element analysis. Kinematic behaviour has been reported for TFAS, but implant loads have not. Implant loads are important indicators of an implant's performance and safety. The rigid posterior fixation system is used for comparison due to the extensive information available about these systems.Unconstrained pure moments were applied to 13 L3-S1 cadaveric spine segments. Specimens were tested intact, following decompression, UCR fixation and TFAS implantation at L4-L5. UCR fixation was via standard pedicle screws and TFAS implantation was via PMMA-cemented transpedicular stems. Three-dimensional 10 Nm moments and a 600 N follower load were applied; L4-L5 disc pressures and implant loads were measured using a pressure sensor and strain gauges, respectively. A finite element model was used to calculate TFAS bone-implant interface loads.UCR experienced greater implant loads in extension (p0.004) and lateral bending (p0.02). Under flexion, TFAS was subject to greater implant moments (p0.04). At the bone-implant interface, flexion resulted in the smallest TFAS (average = 0.20 Nm) but greatest UCR (1.18 Nm) moment and axial rotation resulted in the greatest TFAS (3.10 Nm) and smallest UCR (0.40 Nm) moments. Disc pressures were similar to intact for TFAS but not for UCR (p0.04).These results are most applicable to the immediate post-operative period prior to remodelling of the bone-implant interface since the UCR and TFAS implants are intended for different service lives (UCR--until fusion, TFAS--indefinitely). TFAS reproduced intact-like anterior column load-sharing--as measured by disc pressure. The highest bone-implant moment of 3.1 Nm was measured in TFAS and for the same loading condition the UCR interface moment was considerably lower (0.4 Nm). For other loading conditions, the differences between TFAS and UCR were smaller, with the UCR sometimes having larger values and for others the TFAS was larger. The long-term physiological meaning of these findings is unknown and demonstrates the need for a better understanding of the relationship between spinal arthroplasty devices and the host tissue as development of next generation motion-preserving posterior devices that hope to more accurately replicate the natural functions of the native tissue continues.

Background context Polyethylene (PE) has been used in total disc replacements (TDRs) in Europe since the 1980s. However, the extent of surface damage of PE, including rim fracture and wear, after long-term implantation remains poorly understood. Purpose The purpose of this study was to evaluate the magnitude and rate of PE wear and surface damage in TDRs. Study design TDR components were retrieved from patients undergoing revision TDR surgery and conversion to fusion. Patient sample Twenty-one implants (SB Charite III; DePuy Spine, Raynham, MA) were analyzed from 18 patients (12 female, 6 male) undergoing TDR revision surgery. The components were implanted between 1.8 and 16.0 years (average: 7.8 years) at L2–L3 (n=1), L3–L4 (n=1), L4–L5 (n=11), and L5–S1 (n=8). They were removed due to pain (in all cases) and were associated with subsidence (n=6), anterior migration (n=2), core dislocation (n=2), lateral subluxation (n=1), wear with wire marker fracture (n=1), end plate loosening (n=2), and osteolysis (n=1). Outcome measures Clinical information was collected from medical records and radiographs. Retrieval analysis included dimensional measurements and assessment of the extent and severity of PE surface damage mechanisms. Methods MicroCT scanning was used to identify the presence of internal cracks in the PE core and to scan the geometry of the retrievals. Light microscopy, coupled with white light interferometry, was used to evaluate the surface damage mechanisms at the dome and rim. Results The dominant wear mechanism was adhesive/abrasive wear at both the dome and rim. End plate penetration (dome wear) ranged from 0.1 to 0.9 mm (average: 0.3 mm), and was correlated with implantation time (Spearman's rho=0.48, p=.03). There was also evidence of macroscopic rim damage, including radial and transverse cracking, fracture, plastic deformation, and third-body damage. End plate penetration measured at the rims ranged from 0.02 to 0.8 mm (average: 0.3 mm). Cracks in the core were oriented transversely in 11 of 21 implants (52%), and radially around the rim in 11 of 21 implants (52%). Radiographic wire marker fracture, observed in 9 of 21 implants (43%), was always associated with deformation, cracking, or fracture of the PE rim. In two cases, a fractured wire marker became lodged in the articulating surface between the PE and the metallic end plate. Conclusions This is the first study to quantitatively analyze the long-term PE damage mechanisms in contemporary TDRs. The TDRs displayed surface damage observed previously in both hip and knee replacements. Because of the evidence of increasing wear with implantation time, along with the demonstrated potential for osteolysis in the spine, regular long-term follow-up for patients undergoing TDRs is warranted.

Previous observations of reduced uniaxial elongation, fracture resistance, and crack propagation resistance of highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have contributed to concern that the technology may not be appropriate for systems undergoing cyclic fatigue loading. Using a “total life” approach, we examined the influence of radiation crosslinking on the fatigue response of UHMWPE under cyclic loading via the small punch test. Our goal in this study was to evaluate the suitability of the small punch test for conducting miniature-specimen, cyclic loading, and fatigue experiments of conventional and highly crosslinked UHMWPE. We subjected four types of conventional and highly crosslinked UHMWPE to cyclic loading at 200 N/s and at body temperature in a small punch test apparatus. After failure, the fracture surfaces were characterized with the use of field emission scanning electron microscopy to evaluate the fatigue mechanisms. Cyclic small punch testing under load control was found to be an effective and repeatable method for relative assessment of the fatigue resistance of conventional and highly crosslinked UHMWPE specimens under multiaxial loading conditions. For each of the four conventional and highly crosslinked UHMWPE materials evaluated in this study, fatigue failures were consistently produced according to a power law relationship in the low cycle regimen, corresponding to failures below 10,000 cycles. The fatigue failures were all found to be consistent with a single source of initiation and propagation to failure. Our long-term goal in this research is to develop miniature-specimen fatigue testing techniques for characterization of retrieved UHMWPE inserts. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 66A: 298–309, 2003

The study design is a computer visualization model that simulated flexion deformities about the lumbar spine for evaluation of the naked facet sign (NFS). The objectives were to ascertain the angles of rotation required for NFS to occur in the lumbar spine with various centers of rotation about the vertebral body and to assess whether NFS correlates with unstable flexion-distraction injuries in the lumbar spine. The presence of the NFS on axial computed tomography (CT) images occurs when the inferior articulating facet of the cephalad vertebra is not paired with an adjacent superior articulating facet of the caudal vertebra. This sign, when evidenced in the lumbar spine, is suggestive of significant injury secondary to a flexion-distraction force. A previous study using a computer-generated spine model challenged the utility of the NFS in the thoracolumbar spine. The NFS may prove to be more diagnostic of an unstable injury in the lumbar spine because of its normal lordotic resting position. A commercial spine computer visualization model was used to simulate various degrees of flexion injury in the lumbar spine. Lumbar functional spinal units (FSU) L2-L5 were each examined separately. The model simulated two CT scan slices (each 2 mm thick), which were created parallel to the inferior endplate of the cephalad vertebra of each FSU. The cephalad vertebra was rotated in 0.5 degrees increments until NFS was produced. The appearance of NFS required >/=11 degrees kyphotic angulation in more than two thirds of simulated centers of rotation about the lumbar vertebral bodies. The NFS was produced between a range of 8-24.5 degrees. For rotations about a point located 3 cm anterior to the vertebral body (to simulate seat-belt-type flexion-distraction injuries), the minimum angle required for NFS was 7.5 degrees. Our data correlate well with previously published results from in vitro and cadaveric studies. As opposed to the thoracolumbar spine, which normally rests in a neutral position, the lumbar spine normally rests in a lordotic position. Therefore, NFS in the lumbar spine may be more suggestive of an unstable injury and would warrant closer examination of the patient and additional radiographic studies.

Study Design. Epidemiological study using national administrative data. Objective. To evaluate the temporal trends in on-label and off-label bone morphogenetic protein (BMP) usage in primary and revision spine fusion by spine region and surgical approach, and nonspine applications in the United States from 2002 to 2007. Summary of Background Data. The prevalence of BMP usage for spine fusion has been on the rise, but its use has not been stratified by surgical approach, particularly for lumbar fusion where it has only been Food And Drug Administration-approved for anterior lumbar interbody fusion (ALIF). Methods. The prevalence of BMP usage in the United States was evaluated using the Nationwide Inpatient Sample between October 1, 2002 and December 31, 2007. The Nationwide Inpatient Sample is the single largest all-payer inpatient care database in the United States. The principal procedure associated with BMP use was determined, and the prevalence of BMP use was calculated for various population subgroups. Results. A total of 340,251 inpatient procedures with BMP usage were identified. Between 2003 and 2007, the annual number of procedures involving BMP increased by 4.3-fold from 23,900 to 103,194. Spine fusion accounted for the vast majority (92.8%) of principal procedures with BMP. The predominant use of BMP was in primary posterior lumbar interbody fusion or transforaminal lumbar interbody fusion (PLIF/TLIF) (30.0%), followed by primary posterolateral spine fusion (20.4%), primary ALIF (16.6%), primary cervical fusions (13.6%), and primary thoracolumbar fusions (3.9%). Of primary ALIF with BMP, 19.3% did not involve the implantation of an interbody device. Conclusion. At least 85% of principal procedures using BMP were for off-label applications. With uncertainty regarding the risks of using BMP in certain off-labelapplications, further research will be needed to better define the appropriate indications. Our study also demonstrates that disparities in the differential rates of BMP use exist in the spine fusion population.

Publisher Summary This chapter explains ultra-high molecular weight polyethylene (UHMWPE) that is used in implants to treat chronic neck and back pain, which represent leading causes of disability in the United States and around the world. Neck and back pain may manifest acutely or, more uncommonly, as a chronic condition. Acute back pain typically resolves within weeks, but it is also possible for low-grade symptoms to persist for years following an initial episode. Although less common, chronic and intractable back pain can lead to serious, permanent disability. Patients with neck or back pain are usually initially treated by conservative therapies, which include antidepressants, massage, exercise, spinal manipulation, combined physical and cognitive behavior training, and corticosteroid injections. When degenerative disc disease is the underlying cause of pain, conservative therapy may not alleviate the patient's symptoms. Total disc arthroplasty is conceived as an alternative treatment to fusion, in an effort to avert adjacent segment degeneration. Researchers adapted successful biomaterials and design principles from hip and knee replacements for total disc replacements (TDR). Disc replacement occupies a unique and important position at one end of the pathological spectrum in the treatment continuum spanning early degenerative disc disease, with spinal fusion at the other end. Proponents of TDR have hypothesized that by maintaining the mobility of the treated disc, long-term adjacent segment degeneration may be forestalled or possibly averted.

Background Motion restoring implants function in a dynamic environment that encompasses the full range of spinal kinematics. Accurate assessment of the in situ performance of these devices using numerical techniques requires model verification and validation against the well-established nonlinear quality of motion of the spine, as opposed to the previous norm of matching kinematic endpoint metrics such as range of motion and intervertebral disc pressure measurements at a single kinematic reference point. Methods Experimental data was obtained during cadaveric testing of nine three-functional spinal unit (L3–S1) lumbar spine segments. Each specimen was tested from 8 Nm of applied flexion moment to 6 Nm of applied extension moment with an applied 400 N compressive follower preload. A nonlinear kinematic curve representing the spinal quality of motion (applied moment versus angular rotation) for the index finite element model was constructed and compared to the kinematic responses of the experimental specimens. The effect of spinal soft tissue structure mechanical behaviors on the fidelity of the model’s quality of motion to experimental data was assessed by iteratively modifying the material representations of annulus fibrosus, nucleus pulposus, and ligaments. Findings The present work demonstrated that for this model, the annulus fibrosus played a small role in the nonlinear quality of motion of the model, whereas changes in ligament representations had a large effect, as validated against the full kinematic range of motion. An anisotropic continuum representation of the annulus fibrosus was used, along with nonlinear fabric representations of the ligaments and a hyperelastic representation of the nucleus pulposus. Interpretation Our results suggest that improvements in current methodologies broadly used in numerical simulations of the lumbar spine are needed to fully describe the highly nonlinear motion of the spine.

Background context Kyphoplasty is used to treat vertebral compression fractures (VCFs) by inflating a balloon within the vertebral body (VB) to create a void, thereby reducing the fracture, and then depositing polymethylmethacrylate (PMMA) into that void to augment the VB. Calcium phosphate (CaP) may be preferable to PMMA because it is resorbable and nontoxic, although there are concerns about its compressive strength during the setting process. Purpose To evaluate the ability of a particular self-setting CaP cement to restore the structural integrity of a VCF in a 2-functional spinal unit (2FSU) cadaver model under physiologically relevant loading. Study design/setting Repeated-measures compressive testing on a cadaver thoracolumbar 2FSU VCF model. Methods Ten 2FSU thoracolumbar specimens were tested to evaluate structural integrity under compressive loading during initial anterior VCF creation (in the central VB), after fracture, and after kyphoplasty treatment. Bipedicular kyphoplasty treatment was performed in a 37°C chamber to reduce the fracture and create a void, which was filled with CaP (n=5) or PMMA (n=5) and allowed to cure for at least 15 minutes. Using fluoroscopic imaging, the sagittal area of the VB (SAVB), the minimum central VB height (MCVBH), and the wedge angle were measured on the central VB for each condition at a 1,000-N compressive load. A repeated-measures linear model was used to determine if the differences in these parameters among the various experimental conditions were statistically significant (p Results Compared with the fractured condition, there was a significant improvement in the SAVB, MCVBH, and wedge angle under a physiologically relevant 1,000-N compressive load applied after kyphoplasty. There was no statistically significant difference between treatment with CaP or PMMA. Conclusions The structural properties of CaP-augmented VBs are similar to those of PMMA-augmented VBs. Our study indicated that, after at least 15 minutes of setting, a fractured 2FSU specimen treated with kyphoplasty with PMMA or CaP could withstand physiologically relevant loading.

We investigated the histological and compressive properties of three different calcium phosphate cements (CPCs) using a sheep vertebral bone void model. One of the CPCs contained barium sulfate to enhance its radiopacity. Bone voids were surgically created in the lumbar region of 23 ovine spines—L3, L4, and L5 (n = 69 total vertebral bodies)—and the voids were filled with one of the three CPCs. A fourth group consisted of whole intact vertebrae. Histologic evaluation was performed for 30 of the 69 vertebrae 2 or 4 months after surgery along with radiographic evaluation. Compressive testing was performed on 39 vertebrae 4 months after surgery along with micro-CT analysis. All three CPCs were biocompatible and extremely osteoconductive. Osteoclasts associated with adjacent bone formation suggest that each cement can undergo slow resorption and replacement by bone and bone marrow. Compressive testing did not reveal a significant difference in the ultimate strength, ultimate strain, and structural modulus, among the three CPCs and intact whole vertebrae. Micro-CT analysis revealed good osseointegration between all three CPCs and adjacent bone. The barium sulfate did not affect the CPCs biocompatibility or mechanical properties. These results suggest that CPC might be a good alternative to polymethylmethacrylate for selected indications. © 2007 Wiley Periodicals, Inc. J Biomed Mater Res 2007

STUDY DESIGN Posterior thoracolumbar spine implants retrieved as part of routine clinical practice over a 2-year period were analyzed to identify wear and corrosion. OBJECTIVE Engineering analyses of retrieved posterior instrumentation for indications of performance and failure and correlation of this information with clinical factors. SUMMARY OF BACKGROUND DATA Recent studies have reported spinal instrumentation particulate wear debris and have noted the importance of design considerations at implant connector interfaces. METHODS A total of 57 implants were analyzed from patients (39 female, 18 male) whose average age at implantation was 43.9 years (range, 13.7-77.4 years). Time of implantation ranged from 2 months to 13.5 years. The top 3 implantation diagnoses were radiculopathy (33%), scoliosis (30%), and back pain (25%). Metallurgical analyses were performed to characterize the wear and/or corrosion, and fractures of the implants. RESULTS Wear was present in 75%, corrosion in 39%, and fractures in 7% of the retrieved implants. Wear and/or corrosion was more prevalent, with respect to the total number of implants retrieved, in implants that had been in service at least 1 year. There was no evidence of corrosion in any of the Ti implants, whereas corrosion was present (with wear) in 58% of the stainless steel (SS) implants. Wear and corrosion were more frequently observed in long rods than in short rods. Implantation times were longer for SS implants than for Ti implants. CONCLUSIONS Retrieved rods exhibited corrosion, wear, and fracture, with wear and corrosion mainly located at the interfaces with hooks, screws, or cross-connectors. The mechanisms causing this material loss in situ, as well as what local or systemic responses it may stimulate are of clinical significance and should be studied further.

This chapter evaluates the clinical performance of rods, plates, and cages used for spine stabilization and fusion. It covers the common damage modes for spine implants, including corrosion, ion release, fatigue, and fracture. Successful clinical performance of spine instrumentation is the goal of the original clinical intervention. Sometimes patient or surgical related factors place the implants in situations that lead to failure.. Proper training and use of implants in the hands of experienced surgeons can help minimize complications. An additional complication due to the presence of implanted devices in the spine is the development of pathology at adjacent segments to the treated ones, which is also known as adjacent segment disease. Clinical indications accompanying those retrieved implants provide valuable information to both improve implants and enhance clinical practices for the use of spinal instrumentation. Therefore, the number of choices of spinal instrumentation for treatment of various indications has increased dramatically over the last two decades. In addition to understanding indications and proper surgical techniques, clinicians and design engineers can also learn from the clinical performance of spine implants to design the next generation of implants. Successful procedures with positive patient outcomes not requiring re-operation are great examples from which to learn. In addition, damage modes or indications for retrieval can also provide valuable information for both the clinicians and design engineers as the next generation of implants continues to be developed.

The objective of this chapter is to trace the history and development of implant technologies that have led to techniques currently used to stabilize and fuse the spine. This chapter covers the history of spine instrumentation starting with posterior applications for the thoraco lumbar and lumbo-sacral spine, including Harrington and Luque, and then continuing with anterior instrumentation for the thoraco lumbar spine. Then a historical review of intervertebral cages is presented. The development of spine instrumentation increased the armamentarium of tools available to spine surgeons as they treat patients with numerous pathologies with the goal of improving their quality of life. Even with more instrumentation choices available, success of their use in patients also depends on proper clinical evaluation of patient indications, as well as a thorough understanding of the biomechanical principles for their successful performance.Finally, a good understanding of the development of spinal instrumentation since the early 1900s lays the proper foundation for a better understanding of the clinical performance of these implants and of future developments.

It remains unclear whether adjacent vertebral body fractures are related to the natural progression of osteoporosis or if adjacent fractures are a consequence of augmentation with bone cement. Experimental or computational studies have not completely addressed the biomechanical effects of kyphoplasty on adjacent levels immediately following augmentation. This study presents a validated two-functional spinal unit (FSU) T12-L2 finite element model with a simulated kyphoplasty augmentation in L1 to predict stresses and strains within the bone cement and bone of the treated and adjacent nontreated vertebral bodies. The findings from this multiple-FSU study and a recent retrospective clinical study suggest that changes in stresses and strains in levels adjacent to a kyphoplasty-treated level are minimal. Furthermore, the stress and strain levels found in the treated levels are less than injury tolerance limits of cancellous and cortical bone. Therefore, subsequent adjacent level fractures may be related to the underlying etiology (weakening of the bone) rather than the surgical intervention.

Publisher Summary This chapter discusses the clinical wear performance of ultra-high molecular weight polyethylene (UHMWPE) in spine. UHMWPE is the polymeric bearing material of choice in artificial total disc replacements (TDR). There are currently two designs, namely, SB Charite™ III and PRODISC, of total artificial discs that include an UHMWPE component as a bearing surface for their implant. The SB Charite™ artificial disc is made of three components, two of which are metal endplates that attach to the upper and lower vertebral body endplates by means of anchor teeth, and an UHMWPE sliding core that moves in between these two metal endplates. On the other hand, PRODISC's modular design allows the surgeon to customize it to the patient's anatomy. It comes in two sizes (M and L), with each size having two available angulations of the superior plate component (6 degrees and 11 degrees), and three different heights of the polyethylene inlay (10 mm, 12 mm, and 14 mm). However, the ultimate clinical performance of UHMWPE in the spine in total artificial discs can be comprehensively evaluated only after long-term follow-up of patients is made available. The examination of retrieved implants, when available, will also provide the scientific community with a better understanding of the performance of UHMWPE in vivo in the spine anatomical setting.

A spine implant retrieval program has recently been established at the Implant Research Center at Drexel University in collaboration with the Rothman Institute at Thomas Jefferson University. Engineering failure analysis of the retrieved implants, tissue analysis and medical record evaluations are performed and synthesized. The relevant failure mechanisms can then be definitively identified. The results of these evaluations will ultimately lead to improvements in the longterm clinical performance of spinal implants.

In glaucoma, retinal axons are damaged as they pass through the connective tissue beams (the lamina cribrosa) which span the scleral canal. We created a set of idealized finite element models (FEMs) of the posterior scleral shell of the human eye to study stresses within the lamina cribrosa and surrounding sclera. These models incorporate a range of human dimensions for scleral wall thickness (SWT), axial length (AL), and size and shape of the scleral canal. Stresses within the laminar tissues of circular canals ranged from 48/spl times/IOP (intraocular pressure) to 163/spl times/ IOP. In regions where laminar tissues inserted into the surrounding scleral shell, stresses were highest on the anterior (inner) side of the laminar beams. For elliptically shaped canals, the ratio of vertical to horizontal diameters had an effect on the maximum laminar stress and maximum stress at the insertion zones, and on the distance from the canal where the sclera withstands increased stresses. Decreasing SWT led to significant increases in laminar, insertion zone, and peripapillary scleral stress. Changes in AL of the hemisphere had little effect on stresses within the tissues.

On transverse computed tomographic (CT) scan cuts of the thoracolumbar spine, the naked facet sign occurs when the inferior articular facets of the cephalad vertebra do not appear adjacent to the superior facets of the subjacent caudal vertebra. The objective of this study was to determine the angles of rotation required for the naked facet sign to occur in the thoracolumbar spine, with the center of rotation located at various points in or anterior to the vertebral body. A commercial spinal model and visualization software were used to simulate various flexion injuries. Each functional spinal unit (FSU; T11-T12, T12-L1, and L1-L2) was examined separately. In the model, two CT scan slices (each 2 mm thick) were created parallel to the inferior end plate of the cephalad vertebra of each FSU. The cephalad vertebra was rotated in 0.5 degrees increments, and after rotation both modeled CT slices were examined for the presence of the naked facet sign. If the sign did not occur, the process was repeated, rotating the cephalad vertebra an additional 0.5 degrees until the naked facet sign occurred. The angle of rotation necessary for the sign to occur increases as the point of rotation of the vertebra moves from anterior to posterior and from superior to inferior. The naked facet sign occurred at a minimum rotation angle of 5 degrees (with respect to the anterior-superior point on T11) and at a maximum rotation angle of 16.5 degrees (with respect to the posterior-inferior point on L1). For rotations about a point located 3 cm anterior to the vertebral body, the minimum angles required for the sign decreased only 1 degrees for each FSU. These results suggest that the naked facet sign does not consistently imply the presence of posterior column vertebral instability. This will help clinicians to relate the mechanism of injury, radiographic findings (including the naked facet sign), and the implied injury pattern to the determination of stability, and ultimately the management options for the injury.

The use of Ti-6Al-4V for spinal implants has increased due to its advantageous mechanical properties, biocompatibility, corrosion resistance, and compatibility with MRI procedures. The release of metal ions or particulates, which can result from mechanical loads imposed on the implant, can affect the degree of biocompatibility of spinal implants, possibly influencing their clinical performance. In this project, the titanium metal release from Ti-6Al-4V posterior cervical spine plates was examined using a computational and experimental approach in the canine model.

The sections in this article are 1 Paper-Based PIMs 2 Software-Based PIMs and PIMS 3 Personal Digital Assistants 4 Living with a PIMS Keywords: integrated software; software packages; time management; integrated software tools; personal information manager; group calendar; Rolodex-type Infodex database; PC network; PDA; PIM, PIMS; palmtop computer; pocket computer; information overload team-work

Erratum to ‘‘Static and fatigue mechanical behaviour of bone cement with elevated barium sulfate content for treatment of vertebral compression fractures’’ [Biomaterials 26 (2005) 3699–3712] S.M. Kurtz , M.L. Villarraga, K. Zhao, A.A. Edidin Exponent, Inc., 3401 Market Street, Suite 300, Philadelphia, PA 19104, USA School of Biomedical Engineering, Science, and Health Systems, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA Exponent, Inc., 149 Commonwealth Drive, Menlo Park, CA 94025, USA Kyphon Inc., 1221 Crossman Avenue, Sunnyvale, CA 94089, USA

We hypothesized that oxidation would influence the resistance to fatigue crack initiation and propagation of Ultra-High molecular weight polyethylene (UHMWPE). We subjected tibial insert surrogates (ram extruded GUR 1050) to accelerated aging protocols following ASTM F 2003-00 (14, 21 and 28 days). Subsurface disc specimens from the control and aged materials at each time period were subjected to cyclic small punch loading to failure (modification of ASTM F 2183-02). A significant decrease in fatigue loading was observed, relative to the un-aged controls, starting at three weeks of accelerated aging. Furthermore, SEM examination of the failed aged specimens revealed a network of multiple secondary initiation sites, which was also confirmed by observation with endoscopy, and microCT. Thus, in contrast to the unoxidized highly crosslinked conventional materials evaluated previously, the oxidized materials failed by the initiation and propagation of cracks from numerous initiation sites with the brittle appearance increasing with oxidation time. These results suggest that oxidized UHMWPE exhibits a different fatigue crack initiation and propagation behavior compared to unoxidized virgin, and crosslinked UHMWPE. Future studies will be needed to increase our understanding of the clinically acceptable fatigue properties for new tibial bearing materials, such as highly crosslinked UHMWPEs.