The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 88, Issue suppl 2

Workshop Articles | April 01, 2006

Test Protocols for Evaluation of Spinal Implants

Vijay K. Goel, PhD; Manohar M. Panjabi, PhD; Avinash G. Patwardhan, PhD; Andrew P. Dooris, PhD; Hassan Serhan, PhD

The authors did not receive grants or outside funding in support of their research for or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

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J Bone Joint Surg Am, 2006 Apr 01;88(suppl 2):103-109. doi: 10.2106/JBJS.E.01363

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Abstract

Prior to implantation, medical devices are subjected to rigorous testing to ensure safety and efficacy. A full battery of testing protocols for implantable spinal devices may include many steps. Testing for biocompatibility is a necessary first step. On selection of the material, evaluation protocols should address both the biomechanical and clinical performance of the device. Before and during mechanical testing, finite element modeling can be used to optimize the design, predict performance, and, to some extent, predict durability and efficacy of the device. Following bench-type evaluations, the biomechanical characteristics of the device (e.g., motion, load-sharing, and intradiscal pressure) can be evaluated with use of fresh human cadaveric spines. The information gained from cadaveric testing may be supplemented by the finite element model-based analyses. Upon the successful completion of these tests, studies that make use of an animal model are performed to assess the structure, function, histology, and biomechanics of the device in situ and as a final step before clinical investigations are initiated.

The protocols that are presently being used for the testing of spinal devices reflect the basic and applied research experience of the last three decades in the field of orthopaedic biomechanics in general and the spine in particular. The innovation within the spinal implant industry (e.g., fusion devices in the past versus motion-preservation devices at present) suggests that test protocols represent a dynamic process that must keep pace with changing expectations. Apart from randomized clinical trials, no single test can fully evaluate all of the characteristics of a device. Due to the inherent limitations of each test, data must be viewed in a proper context. Finally, a case is made for the medical community to converge toward standardized test protocols that will enable us to compare the vast number of currently available devices, whether on the market or still under development, in a systematic, laboratory-independent manner.

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Prior to implantation, medical devices are subjected to rigorous testing to demonstrate safety. Efficacy (i.e., the demonstrated and proven beneficial effects of the device on the involved pathology), is equally important in the modern and aggressive field of medical technology. As our understanding of the structure and function of the human body increases, the performance expectations for medical devices also rise. Consequently, test protocols, which probe the efficacy of medical devices, need to keep pace with the innovation in the medical device industry so that a particular revised characteristic of a device can be properly evaluated. Not long ago, the main purpose of implantable spinal devices was to provide rigid fixation to enhance fusion across a spinal segment; this characteristic is in direct contrast to the present drive for devices that will preserve or restore motion across the affected spinal segment. This philosophical reversal requires revised experimental techniques.

A full battery of testing protocols for implantable spinal devices includes many steps. Testing for biocompatibility is a necessary first step toward ensuring acceptance of the materials used in the manufacture of any medical device. On selection of the material, evaluation of the implant should address both the biomechanical and clinical performances. Initial mechanical tests should be designed to ensure safety and durability. Additional biomechanical studies can be performed by implantation of the device within human cadaveric spines. This aspect also can be supplemented with the finite element model-based analyses. When these steps have been successfully completed, studies using an animal model are suggested to assess the structure, function, histology, and biomechanics of the device in situ as a final step before clinical investigations.

A basic understanding of several aspects of the spine is necessary to fully appreciate this overview. These areas include spine anatomy (including the musculature), spinal biomechanics (including the contributions of various structures, such as the vertebral body, facet joints, intervertebral discs, and ligaments in supporting the axial, bending, and torsional loads on the skeleton), spinal kinematics (e.g., quality and quantity of motion and the instantaneous axis of rotation), associated pathologies that may impair active and passive structures (disc degeneration, facet arthritis, and spondylolisthesis), and the prevailing techniques to treat these pathologies. Presuming this background, the following sections briefly describe the testing strategies in use and/or under development for the evaluation of spinal instrumentation, under the two broad categories of safety and efficacy.

Safety

Safety | Efficacy | Summary | References

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Figs. 1-A and 1-B: The screw is mounted in a foam block and is being pulled out along its axis in a servohydraulic materials testing machine (model #810; MTS Systems, Eden Prairie, Minnesota). View showing the pneumatic system for applying compression load to the device held between foam blocks or vertebrae while being pulled or pushed out. (Clockwise from right: actuator, load cell, readout display, pneumatic regulator, and lever).

Safety testing may be largely divided into two categories—tests to ensure that the device tolerates the body, and tests to ensure that the body tolerates the device.

Biocompatibility

One of the causes and/or results of medical implant failure is the release of particulate material debris from the device because of wear, physical deterioration, or chemical attack by the harsh physiologic environment. Biocompatibility testing may be categorized according to bulk material biocompatibility or particulate biocompatibility. In its bulk form, the implant's surface may be leached or corroded. This would be applicable to devices subject to fatigue failure such as from fretting at screw junctions, corrosion, and elastomeric fatigue. Although some materials may be biocompatible in bulk form, as particles they may incite adverse reactions from cells. The presence of particles or any evidence of acute inflammation, chronic inflammation, granulation tissue, fibrosis of the meninges, foreign body reaction, arachnoiditis, or transdural migration of new material particles should be assessed. Because of the proximity of the disc replacement devices to sensitive neural tissues, the effects of the new material particles on the dura and neural tissue should also be evaluated. To evaluate biocompatibility, a twelve-week rat subcutaneous pouch model might be used. This test is customarily recommended when a new material without a previous clinical history is introduced. Typically, the cellular response of the test material is compared with the response to high-density polyethylene, a biocompatible material used in many joint prosthetic devices. Neurotoxicity might be evaluated with use of a sheep or a rabbit model¹.

Implant-Tissue Interface

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Figs. 2-A and 2-B: Schematic showing follower setup to apply preloads during testing of multisegment specimens. Radiograph of a specimen showing follower preload path during testing of an actual specimen.

In addition to the chemical and biologic reactions occurring at the interface between the device and the body, potential failure modes of device interfaces may also be mechanical in nature. As with biocompatibility testing, this testing focuses on the device (and procedure) safety rather than efficacy. Depending on the spinal instrumentation, the implant-tissue interface may include interactions of the implant with any of the functional spinal unit elements, including the laminae, pedicles, spinous processes, vertebral body, vertebral end plates, anulus layers, and nucleus. Quasistatic and cyclic pullout/bending tests of screws placed in vertebrae or foam blocks are fairly common (Figs. 1-A and 1-B)². Although the pullout test is simple and popular, it does not represent the complex in vivo loads at the bone-screw interface. Interlaminar hooks are also used to secure the device to the spine, similarly to screws. A hook's ability to hold on to the lamina without slippage has been investigated in a manner similar to pullout strength testing of screws.

Performance of an interbody device also depends to some degree on the interaction with the adjoining vertebral end plate. An evaluation of this interface may include compression of the device between vertebrae or foam blocks followed by push or pullout of the device in the direction perpendicular to the axial compression (Fig. 1-B). Although device migration is prevented by the surface shear strength of the end plate, subsidence is resisted by surface compression³. For devices with osteoconductive coatings, which may increase the interface strength over time due to bone-device integration, evaluation of implant-bone interfaces over time is best performed with the use of animal models, which allow for tissue ingrowth.

Mechanical Testing

Because the primary function of most orthopaedic and/or spinal implants is structural, tests to investigate whether the device tolerates the in situ environment are often mechanical in nature. Generally, the device should be capable of withstanding maximum expected static loads and average daily dynamic loads throughout its predicted life without functional compromise or major permanent deformation. The goal of this characterization is to provide sufficient evidence to support the use of the device in clinical trial applications. Quasistatic mechanical tests can be used to characterize the ultimate strength and deformation of the device as well as failure modes in axial compression, compressive shear, torsion, lateral bending, flexion, and extension⁴. Dynamic test results can be used to establish endurance limits and associated long-term performance of the device under difficult, yet realistic, conditions.

Static and dynamic mechanical tests for interbody fusion devices and artificial discs are very similar, with two key exceptions: (1) artificial discs are intended to last for the life of the patient, and thus the total number of cycles performed in the fatigue test is larger; and (2) artificial discs are intended to move and/or rotate and/or deform (depending on the nature of the implant) for the life of the patient, and so should resist tearing, denting, cracking, or wearing. Wear tests for artificial discs have particularly benefited from three decades of experience with hip replacements. The artificial disc wear-testing has been recommended to run ten million cycles, which is twice that of hips. Typical outcomes of this testing are gravimetric wear rate and particulate analysis. Particulate analysis is complicated because the effects of debris in a nonsynovial joint have not been well characterized. The presumption is that, barring neurotoxic effects, achievement of wear rates lower than those of hips provides a sufficient safety factor for artificial discs.

Although the American Society for Testing and Materials test standards (Table I) represent to a large extent the mechanical characterization requested by the United States Food and Drug Administration, it may be necessary to perform many additional tests to assess safety and durability⁵. In all motion preservation systems, including artificial discs, nuclei, dynamic posterior screws, and interspinous spacers, the test loading modalities include compression, shear, flexion and extension, lateral bending, and torsion; however, they all have different failure loads, failure criteria, and necessary fatigue strengths. Nucleus replacements may require the performance of many material characterization tests that are not associated with spine geometry but that are necessary to interpret the effects of body loads over time. A number of researchers and companies have proposed testing nucleus devices inside an artificial anulus fixture or within a fixture mimicking vertebral end plates. This proposal brings with it many questions concerning the implant-fixture interface, the load-sharing between the nucleus and anulus, and ideal disc space geometries. Much research is needed in those areas and in posterior dynamic stabilization systems, such as Dynesys (Zimmer Spine, Minneapolis, Minnesota), DIAM (Device for Intervertebral Assisted Motion; Medtronic, Minneapolis Minnesota), and X STOP (St. Francis Medical Technologies, Alameda, California). The complicated biomechanical loading environment of the Dynesys system and the potential failure modes of the DIAM and X STOP systems have not yet been fully elucidated and require additional research. Artificial facet systems are even more complicated due to complex in vivo loads and potential implant wear issues.

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TABLE I ASTM Proposed Standards⁵That May Be Adapted for Device Evaluation^*

Standard or Guide	Status	Focus	Use
ASTM F 1582-98 "Standard Terminology Relating to Spinal Implants"	Reapproved 2003	General spinal device testing	Defines basic terms and considerations for spinal implant devices and their mechanical analyses
ASTM F 2193-02 "Standard Specifications and Test Methods for Components Used in the Surgical Fixation of the Spinal Skeletal System"	Approved 2002	Fusion devices	Static and fatigue testing of components—screws, rods, and plates
ASTM F 1717-96 "Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model"	Reapproved 2004	Fusion devices	Materials and methods for static and fatigue testing of constructs (implants in UHMWPE blocks). Focus on estimation of short-term stability while arthrodesis takes place
ASTM F 2077-01 "Test Methods For Intervertebral Body Fusion Devices"	Approved 2001	Fusion devices	Axial compression, compressive shear, and torsion of interbody devices in static and fatigue testing
ASTM F 2267-04 "Standard Test Method for Measuring Load Induced Subsidence of an Intervertebral Body Fusion Device Under Static Axial Compression"	Approved 2004	Fusion devices	Materials and methods for the axial compressive subsidence testing of nonbiologic intervertebral body fusion devices; spinal implants designed to promote arthrodesis at a given spinal motion segment
ASTM Draft Method WK455 "Draft Standard Test Methods for Occipital-Cervical and Occipital-Cervical-Thoracic Spinal Implant Constructs in a Vertebrectomy Model"	Draft	Fusion	Materials and methods for the static and fatigue testing of OC and OCT spinal implant assemblies in a vertebrectomy model (UHMWPE). OCT load and geometry specific. (May be incorporated into F 1717 in future)
ASTM Method F 2346-05 "Standard Test Methods for Static and Dynamic Characterization of Spinal Artificial Discs"	Approved 2005	Artificial discs	Materials and methods for static and dynamic testing of artificial discs. Compression, compressive shear, and torsion are included. Similar to its fusion device counterpart
ASTM WK7479 "Draft Standard Test Method for the Functional, Kinematic, and Wear Assessment of Extra-Discal Spinal Motion Preserving Implants"	Draft	Artificial discs	For wear assessment of lumbar and cervical disc replacements, including methods for loads, angles, moments, test environment, and data analysis
ISO ISO/TC 150 / SC 5 Implants for surgery — Wear of total intervertebral spinal disc prostheses — Part 1: Loading and displacement parameters for wear testing and corresponding environmental conditions for tests	En route to approval 2005	Artificial discs	For wear assessment of lumbar and cervical disc replacements, including methods for loads, angles, moments, test environment, and data analysis, more applicable to sliding implants (nonelastomeric)
ASTM WK4863 "Draft Standard Practice/Guide for the Mechanical Characterization of Lumbar Nucleus Devices"	Draft	Nucleus replacements	This guide gives general guidance on testing methods for various forms of nucleus replacement and nucleus augmentation devices, outlining the types of testing that are recommended

ASTM = American Society for Testing and Materials, UHMWPE = ultra-high molecular weight polyethylene

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TABLE I ASTM Proposed Standards⁵That May Be Adapted for Device Evaluation^*

Standard or Guide	Status	Focus	Use
ASTM F 1582-98 "Standard Terminology Relating to Spinal Implants"	Reapproved 2003	General spinal device testing	Defines basic terms and considerations for spinal implant devices and their mechanical analyses
ASTM F 2193-02 "Standard Specifications and Test Methods for Components Used in the Surgical Fixation of the Spinal Skeletal System"	Approved 2002	Fusion devices	Static and fatigue testing of components—screws, rods, and plates
ASTM F 1717-96 "Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model"	Reapproved 2004	Fusion devices	Materials and methods for static and fatigue testing of constructs (implants in UHMWPE blocks). Focus on estimation of short-term stability while arthrodesis takes place
ASTM F 2077-01 "Test Methods For Intervertebral Body Fusion Devices"	Approved 2001	Fusion devices	Axial compression, compressive shear, and torsion of interbody devices in static and fatigue testing
ASTM F 2267-04 "Standard Test Method for Measuring Load Induced Subsidence of an Intervertebral Body Fusion Device Under Static Axial Compression"	Approved 2004	Fusion devices	Materials and methods for the axial compressive subsidence testing of nonbiologic intervertebral body fusion devices; spinal implants designed to promote arthrodesis at a given spinal motion segment
ASTM Draft Method WK455 "Draft Standard Test Methods for Occipital-Cervical and Occipital-Cervical-Thoracic Spinal Implant Constructs in a Vertebrectomy Model"	Draft	Fusion	Materials and methods for the static and fatigue testing of OC and OCT spinal implant assemblies in a vertebrectomy model (UHMWPE). OCT load and geometry specific. (May be incorporated into F 1717 in future)
ASTM Method F 2346-05 "Standard Test Methods for Static and Dynamic Characterization of Spinal Artificial Discs"	Approved 2005	Artificial discs	Materials and methods for static and dynamic testing of artificial discs. Compression, compressive shear, and torsion are included. Similar to its fusion device counterpart
ASTM WK7479 "Draft Standard Test Method for the Functional, Kinematic, and Wear Assessment of Extra-Discal Spinal Motion Preserving Implants"	Draft	Artificial discs	For wear assessment of lumbar and cervical disc replacements, including methods for loads, angles, moments, test environment, and data analysis
ISO ISO/TC 150 / SC 5 Implants for surgery — Wear of total intervertebral spinal disc prostheses — Part 1: Loading and displacement parameters for wear testing and corresponding environmental conditions for tests	En route to approval 2005	Artificial discs	For wear assessment of lumbar and cervical disc replacements, including methods for loads, angles, moments, test environment, and data analysis, more applicable to sliding implants (nonelastomeric)
ASTM WK4863 "Draft Standard Practice/Guide for the Mechanical Characterization of Lumbar Nucleus Devices"	Draft	Nucleus replacements	This guide gives general guidance on testing methods for various forms of nucleus replacement and nucleus augmentation devices, outlining the types of testing that are recommended

ASTM = American Society for Testing and Materials, UHMWPE = ultra-high molecular weight polyethylene

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Fig. 3: Three-dimensional finite element model of the ligamentous L3-S1 segment. (A): Intact. (B): Modified to simulate placement of a Charité artificial disc (DePuy Spine, Raynham, Massachusetts) across the L5-S1 segment.

Efficacy

Safety | Efficacy | Summary | References

Although an understanding of the device function and design are necessary for safety testing, a clear description of the device purpose and the associated pathology are critical in the testing of device efficacy.

In Vitro Spinal Segment Testing

Multisegment testing is performed to assess the function of a spinal construct (the spine segment with the implant in place)⁶. Change in motion due to instrumentation with respect to the intac case can be described in terms of three rotations of the vertebra and three translations of a point on the vertebra, the instantaneous axis of rotation of the segment (or its equivalent helical axis of motion in three-dimensional space), the quality of the motion (nonlinearity, neutral zone, and coupled motions), and the quantity of the motion (e.g., range of motion)⁷. Several other parameters of interest are changes during testing of the construct in disc height, disc bulge, intradiscal pressure, spinal alignment, vertebral body strains, creep and viscoelastic behavior of the construct, and bone-implant interface interactions. Another focus of this testing is quantification of load-sharing between the device and functional spinal unit structures. Besides the use of a motion measurement system, additional transducers, such as the strain gauges applied to vertebrae and pressure transducers inserted into the nucleus, may be needed to document these parameters in response to the applied loads.

For such studies, fresh osteoligamentous spines of appropriate length are procured, screened (e.g., bone mineral density above a certain value), and prepared for testing⁶. Test data from single functional spinal units and multispinal segments have been reported in the literature. A consensus seems to be emerging that the multispinal segments should be used for testing and should include at least one free functional spinal unit on either side of the construct length essential for evaluation of the device. The total segment length should also account for assessment of the effects of the device on the adjacent segments, if needed. Loads are applied to a loading frame attached to an end (usually the cephalad) vertebra, and the resulting motion of each vertebra is recorded with the help of transducers. The data are obtained sequentially for the intact case and treatment cases, such as the placement of an artificial disc.

Role of Muscles

In vitro tests cannot incorporate the effect of the musculature. Furthermore, the biomechanical role of muscles in general, and especially following surgery and until healing occurs, is not well documented. During testing, an application of compressive preload with accompanying moments in clinically relevant loading modes may simulate the effect of musculature. Data in the earlier literature were based on tests undertaken with no preload. The current consensus is that, in the lumbar region, a preload of 400 N (about 60% of the body weight above the L3-L4 disc level of an average person) with a maximum of 1000 N should be included in the tests, using the follower load concept⁸; the test setup used by Patwardhan et al.⁸ for applying follower-type preload is shown in Figure 2. The effect of the preload on the outcome is highly dependent on its location with respect to the center of rotation of the individual segments. For the cervical region, the follower preload may range from 50 to 100 N (one to two times the weight of the head). How will this concept apply during sequential tests such as decompression and instrumentation of the segment? Additionally, at present, the application of the follower load does not yield satisfactory results in modes other than flexion and extension. The quality of the measured data with and without preloads is similar, and thus tests without preload may also be acceptable for comparative evaluation of spinal instrumentation⁷. However, construct testing should be conducted with a follower load whenever possible.

Load Types and Magnitudes

Load testing may be the most controversial issue facing the industry and its research. Some tests apply a known displacement at the free end and quantify the resulting loads and motions across various segments (displacement control or stiffness protocol). Others measure the motion in response to a known load (load control or flexibility protocol). In the load control testing protocol, one could apply forces such as anterior and posterior shear, pure moments, or a complex load. Application of forces results in a varying amount of bending moment across the segment, depending on the location within the cantilever specimen and the deflection of the specimen itself. Furthermore, the deflection of the specimen following stabilization changes significantly compared with the intact case. These could present difficulties in computing the load displacement curves of the specimen for different treatment simulations (e.g., intact, decompression, and fusion). This issue is mitigated if one applies pure moments and the resulting unconstrained motions are measured^7,9,10. Consensus seems to be forming that investigators should provide data that have included the use of pure moments in which the specimens are free to move in an unconstrained manner. The moments may range from 6 to 10 Nm for the lumbar region and 1.5 to 3 Nm for the cervical region. These moments may be applied in a quasistatic manner with three load-unload cycles. The data are recorded on the third load cycle.

Testing of Adjacent Level Effects

The flexibility protocol using pure moments may be modified to address the effects of a device on the adjacent segments. For example, it would be advantageous to use a protocol that would achieve the same overall range of motion for the intact specimen as well as for the instrumented construct by applying pure moments that distribute evenly down the column. This is the hybrid protocol^7,9,10.

Functional Animal Studies

Although spinal segment testing can be an effective tool, it is limited by the lack of bone and soft-tissue remodeling (that may include osteophyte generation, end-plate settling, and scar-tissue formation), the biochemical response, and by limited replication of in vivo loads. These restrictions limit functional spinal unit testing results to immediate postoperative conditions. To account for such shortcomings and for the biologic effects, spectrum, magnitude, and mode of in vivo loads, animal models are necessary. Animal models provide a dynamic biologic and mechanical environment in which the implant can be evaluated. Temporal changes in both the host biologic tissue and the instrumentation can be assessed with selective incremental sacrificing of the animals. Common limitations of animal studies include dissimilarities between human and animal spines with respect to the spinal loads, spinal motions, anatomy, and the difficulties in adjusting the device to properly fit the animal spine. Animal models can range from rats to rabbits, sheep, goats, dogs, pigs, and baboons. Functional evaluation of a disc replacement requires use of an animal model that simulates the structure, function, and biomechanical properties of the human spine. Use of comparable surgical techniques and approaches as in humans is also desirable. For evaluating interbody devices, the sheep and baboon are often used for the cervical and lumbar regions, respectively¹¹. Smaller primates can be used to approximate load modes, but larger primates such as baboons are necessary to simulate both load magnitude and direction. Another complication of using functional animal models may be in appropriately sizing or shaping the device to fit the anatomy of the animal. Not all device features may be reducible to animal-sized discs; some mechanisms (such as diffusion across hydrogels) are size-dependent. Likewise, a sheep cervical disc displays a very narrow posterior disc height and a caudally sloping anterior that pose potential issues for interbody devices. Thus, animal studies have limitations for evaluating the function of a device.

Both implant and implant-tissue interface characteristics may be evaluated in the animal model. Implant characteristics include such features as resorption (such as with polylactic acid and/or polyglycolic acid devices), static compressive strength, wear, cracking, and deterioration. The interface may be investigated for subsidence (osseous end-plate deterioration), fixation (migration of the device), ingrowth (into osteoconductive coatings), and possible wear-debris effects on neural elements and surrounding tissues. In most animal studies, a systemic analysis is also commonly performed, including the histopathologic response in local and systemic tissues to device material and possibly wear debris generated in nonfailure and failure modes (if applicable). Pathologic assessment for all tissues should include but not be limited to comments on the architecture of the tissues and the presence of wear debris, as well as any signs of foreign-body giant-cell and/or granuloma inflammatory reactions, degenerative changes, or autolysis. For motion-preserving devices, the segmental stiffness properties through the normal range of motion may be investigated.

Finite Element Models

In vitro investigations and in vivo animal studies contain numerous limitations, including that they are both time-consuming and expensive. The most important limitations of in vitro studies are that muscle contributions to loading are not usually incorporated; in addition, the quality of the cadaveric specimens is highly variable. In vivo animal studies involve quadruped animals, and the implant sizes usually need to be scaled according to the animal size. In an attempt to complement the above protocols, several experimentally validated finite element models of the ligamentous spine have been developed (Fig. 3)¹⁰. Such studies can be used to help design the device itself or study the effects of the device on the load-sharing, stresses, and strains in the spinal structures under simple and complex loading scenarios. Finite element modeling has been coupled with adaptive bone remodeling algorithms to investigate the temporal changes associated with interbody fusion devices such as cages.

Summary

Safety | Efficacy | Summary | References

Arigorous testing methodology has been presented that can be used to evaluate the function of a spinal device. Because of the complex role of a device, the sensitivity of the surrounding structures, and the unique biomechanical role of the spine, testing requires considerable planning and resources. Care must be taken to consider the limitations of each test while evaluating the test results. Although no test except for pilot clinical studies can sufficiently account for all variables or thoroughly evaluate efficacy, it is imperative to produce a full battery of test results that can reasonably predict safety to the candidate patient. This review also underscores the fact that additional innovative test protocols are needed to address the newer designs that are about to come on the market, such as artificial facets and nucleus replacements. The orthopaedic community must work hard to agree on standardized test protocols that will enable the clinician to compare the vast number of devices currently available on the market as well as those under development on a laboratory-independent basis. ?

References

Safety | Efficacy | Summary | References

Cunningham BW, Orbegoso CM, Dmitriev AE, Hallab NJ, Sefter JC, Asdourian P, McAfee PC. The effect of spinal instrumentation particulate wear debris. An in vivo rabbit model and applied clinical study of retrieved instrumentation cases. Spine J. 2003;3: 19-32.319 2003 [PubMed][CrossRef]

Lim TH, An HS, Hasegawa T, McGrady L, Hasanoglu KY, Wilson CR. Prediction of fatigue screw loosening in anterior spinal fixation using dual energy x-ray absorptiometry. Spine. 1995;20: 2565-9.202565 1995 [PubMed][CrossRef]

Steffen T, Tsantrizos A, Aebi M. Effect of implant design and endplate preparation on the compressive strength of interbody fusion constructs. Spine. 2000;25: 1077-84.251077 2000 [PubMed][CrossRef]

Spenciner DB, Paiva JA, Crisco JJ. Testing of human cadaveric functional spinal units to the ASTM draft standard, "Static and fatigue methods for spinal artificial disc". In: Melkerson MN, Kirkpatrick J, Griffith S, editors. Spinal implants: are we evaluating them appropriately? West Conshohocken, PA: ASTM International; 2003. p 114-26.114 2003

Annual book of ASTM standards. Volume 13.01: Medical and surgical materials and devices; anesthetic and respiratory equipment; pharmaceutical application of process analytical technology. West Conshohocken, PA: ASTM International; 2005. 2005

Panjabi MM. Biomechanical evaluation of spinal fixation devices: I. A conceptual framework. Spine. 1988;13: 1129-34.131129 1988 [PubMed][CrossRef]

Goel VK, Panjabi MM, editors. Roundtables in spine surgery; spine biomechanics; evaluation of motion preservation devices and relevant terminology. St. Louis: Quality Medical Publishing; 2005. 2005

Patwardhan AG, Carandang G, Ghanayem AJ, Havey RM, Cunningham B, Voronov LI, Phillips FM. Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs. J Bone Joint Surg Am. 2003;85: 1749-56.851749 2003

Panjabi MM. Biomechanical testing to quantify adjacent-level effects. World Congress of Biomechanics; 2002 Aug 4-9; Calgary, Alberta, Canada. Abstract 5079. 2002

Goel VK, Grauer JN, Patel TC, Biyani A, Sairyo K, Vishnubhotla S, Matyas A, Cowgill I, Shaw M, Long R, Dick D, Panjabi MM, Serhan H. Effects of Charite artificial disc on the implanted and adjacent spinal segments mechanics using a hybrid testing protocol. Spine. 2005;30: 2755-64.302755 2005 [PubMed][CrossRef]

Wilke HJ, Kettler A, Claes LE. Are sheep spines a valid biomechanical model for human spines? Spine. 1997;22: 2365-74.222365 1997 [PubMed][CrossRef]

Topics

fatigue ; biomechanics ; bone plates ; bone screws ; cell nucleus ; animal model ; range of motion ; safety ; histology ; spine

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Vijay K. Goel, Manohar M. Panjabi, Avinash G. Patwardhan, Andrew P. Dooris, Hassan Serhan; Test Protocols for Evaluation of Spinal Implants. The Journal of Bone & Joint Surgery. 2006 Apr;88(suppl_2):103-109.

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