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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.

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 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.

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.

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.