Marta L. Villarraga

Off-Label Use of Bone Morphogenetic Proteins in the United States Using Administrative Data

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.

The biomechanical effects of kyphoplasty on treated and adjacent nontreated vertebral bodies.

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.

Wear and corrosion in retrieved thoracolumbar posterior internal fixation.

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.

Quality of motion considerations in numerical analysis of motion restoring implants of the spine

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 models 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.

Contact Analysis of a Posterior Cervical Spine Plate Using a Three-Dimensional Canine Finite Element Model

The development of a three-dimensional finite element model of a posteriorly plated canine cervical spine (C3-C6) including contact nonlinearities is described. The model was created from axial CT scans and the material properties were derived from the literature. The model demonstrated sufficient accuracy from the results of a mesh convergence test. Significant steps were taken toward establishing model validation by comparison of plate surface strains with a posteriorly plated canine cervical spine under three-point bending. This model was developed to better characterize the contact pressures at the various interfaces under average physiologic canine loading. The analysis showed that the screw-plate interfaces had the highest values of all the mechanical parameters evaluated.

Multiaxial Fatigue Behavior of Oxidized and Unoxidized UHMWPE During Cyclic Small Punch Testing at Body Temperature

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.

The Clinical Performance of UHMWPE in the Spine

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 patients 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.

Chapter 12 – The Clinical Performance of UHMWPE in the Spine

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 patients 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.

Clinical Performance of Rods, Plates, Screws, and Cages

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.

Chapter 7 – Historical Review of Spinal Instrumentation for Fusion: Rods, Plates, Screws, and Cages

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.Publisher Summary 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.