Background

In the second half of the 20th century, the research area of biomechanics encompassed a problem that, at one time, was of interest to Leonardo da Vinci—the human spine. In the 1970s and 1980s, in particular, there was a rapid increase in the biomechanical analysis and quantitative understanding of the anatomy of the spine and clinical issues related to its treatment. This new insight enabled researchers to design and develop devices that aimed to restore normal physiologic movement. However, one of the consequences that became obvious from this flurry of scientific activity was a need to develop standards. Depending on the laboratory and the application, devices were being evaluated under different conditions, making comparison difficult. Limitations related to the peculiar nature of the spinal anatomy and testing made standardization difficult. Eventually, consensus was achieved and standards evolved. From the economic point of view, spine biomechanics seems to have delivered on its promise. According to Decision Resources Group, the United States spinal implant market will expand modestly through 2022 to reach a value of nearly $7.1 billion. Longstanding factors such as the aging population, rising rates of obesity, paradigm shift from open surgery to surgeries through minimal exposure, and potential adoption of nonfusion devices will be primary market drivers, although the market will continue to be shaped by changing regulations.

Another avenue of inquiry has been aimed at resolving clinical issues without the use of any mechanical devices. This field, tissue engineering, has started to show great promise in the area of spine biomechanics. With U.S. federal funding available for stem cell research, a global market of $4.8 billion, and about 200 companies working on designing newer and better orthopaedic biomaterials, the future is bound to offer a growing influence of tissue engineering in spine deformity correction. Although challenges exist, our understanding of issues related to the regeneration of the nucleus pulposus and annulus fibrosus has increased manifold. As a case in point, much interest is being paid to scaffolding. Interested readers are advised to review a classic publication by Lanza and colleagues.

Low back pain is the single leading cause of disability worldwide, according to the Global Burden of Disease 2010. According to American Chiropractic Society, Americans spend at least $50 billion each year on back pain—and that is just for the more easily identified costs; experts estimate that as many as 80% of the population will experience a back problem at some time in their lives. In the same decade, an increase of nearly 50% was found in the number of patients seeking spine-related health care expenditure. In parallel, a 65% increase in health care expenditure in general was measured. Numerous types of surgical procedures are performed on the spine to prevent further deterioration of spinal components or escalation of pain, and various devices are being conceived, designed, tested, and implanted to aid in these treatments. Most of this instrumentation—for example, interlaminar hooks, transpedicular screws, interbody spacers, and cages—is relevant to spinal fusion. The goal of such instrumentation is to fuse two or more vertebrae together to eliminate pain and allow the patient to return to normal activities. Alternatives to fusion include the hydrogel-based prosthetic nucleus, the liquid polymer-based nucleus, motion preservation devices, and artificial discs.

Almost everything that is done in biomechanical testing flows from an existing spinal disorder and the perspective of the individual researcher ( Fig. 34-1 ). We do not claim that this algorithm is comprehensive, a case in point being the regulatory part of the process. Other variables include the types of perspectives (e.g., material science), concepts, and tests. Based on one’s perspective and the clinical objective, a researcher may come up with a concept of a solution—for example, a tissue-engineered nucleus for a damaged intervertebral disc. This concept is tested, proving or disproving a predefined hypothesis. The nature of the specific test may be purely mechanical, biomechanical, or based on biocompatibility. In the case of an engineered nucleus, for example, a test could be any of these (except in vivo–limited clinical trials). For the nucleus, such a test could involve measurement of motion after surgical implantation in a cadaveric spine model or, perhaps, a purely mechanical study assessing its compressive modulus (i.e., a bench-type test). On the other hand, if the clinical objective is being met from a mechanical perspective, resulting in a mechanical device, the range of tests would include pure mechanical tests such as fatigue and wear tests. Determination of the chemical composition following corrosion and wear testing complements these mechanical tests. At some stage, testing using animal spine models, cadaveric spines, analog spines, or computer simulations (i.e., in silico), and, eventually, clinical trials on human subjects will follow.

A testing algorithm for spinal implants.

Although all types of testing modalities are important in the process of concept evaluation and assessment as shown in the algorithm, this chapter focuses on three: bench type; in vitro or, more appropriately, cadaveric; and in silico testing of devices and engineered tissues under the overarching term of biomechanical testing. Moreover, we differentiate between construct testing and implant testing. The terminology, testing procedures, apparatuses, and protocols that have evolved over the years in testing of spinal implants are reviewed as well. We also speculate regarding future prospects for biomechanical devices and note the areas that may need more attention from the spine biomechanics community.

Bench-Type Tests for Approval by the U.S. Food and Drug Administration

To evaluate the endurance and strength of orthopaedic implants, various mechanical and materials testing protocols have been proposed by ASTM International (formerly known as the American Society for Testing and Materials) as standards for testing of such devices under different dynamic and static loading profiles ( Table 34-1 ). These protocols allow researchers to estimate the static strength and fatigue limits of an implant assembly and its individual components in a consistent way, thereby enabling a fair comparison of results. ASTM and the International Organization for Standardization (ISO) have also proposed guidelines for evaluating fixation of the parts and the loosening effect at the interface of implant components.

Data from these standardized tests are used, as part of the design file, by the medical device industry to seek approval for commercial distribution of devices from the U.S. Food and Drug Administration (FDA). Medical devices are categorized by the FDA in classes—class I, class II, and class III—based on the degree of regulatory control. Most class II devices, such as the pedicle screw–based instrumentation systems, require submission of a Premarket Notification 510(k), whereas class III devices—devices that pose a significant risk of illness or injury—require premarket approval (PMA). Motion preservation systems, for instance, are categorized as class III devices. The test protocols listed in Table 34-1 pertain to class II devices. Class III devices also may be assessed using these protocols, but approval for commercial distribution of such devices requires submission of clinical data in support of the manufacturers’ claims.

Similar tests sometimes are carried out on ligamentous motion segments. These tests include subsidence tests, pullout or pushout testing of pedicle screw systems and cages, respectively, and fatigue tests. Subsidence is a phenomenon in which one or both vertebral end plates adjacent to the implant collapse and allow the implant to move in, increasing the probability of deformity progression and worsening of the fusion. Static, quasi-static, or dynamic tests such as pullout tests also are performed on pedicle screws to measure bone-implant interface strength under such forces. New ASTM guidelines are available for the assessment of facet replacement technologies, wear characterization, and motion preservation systems such as artificial discs.

Wear testing is carried out on a wear simulator ( Fig. 34-2A ). One such simulator (MTS Bionix, MTS Systems Corp., Eden Prairie, MN) consists of six active stations (test stations) and one control station.

A, MTS spine wear simulator (MTS, Eden Prairie, MN). Test stations have six degrees of freedom, whereas the control station is under compressive load only. B, As the load is applied, deformation of a specimen follows a typical curve that reveals hysteresis. The unloading part of the curve does not retrace the loading part of the curve. EZ, elastic zone; EZS, EZ stiffness; NZ, neutral zone; NZS, NZ stiffness; ROM, range of motion.

( A, Adapted from Bhattacharya S, Nayak A, Goel VK, et al: Gravimetric wear analysis and particulate characterization of a dynamic posterior system, PercuDyn™. Presented at the 55th Annual Meeting of the Orthopaedic Research Society, February 22–24, 2009, Las Vegas, NV, Orthopaedic Research Society, Rosemont, IL. B, Adapted from Wilke HJ, Wenger K, Claes L: Testing criteria for spinal implants: recommendations for the standardization of in vitro stability testing of spinal implants. Eur Spine J 7:148–154, 1998.)

Polymeric components in a disc replacement device are soaked in a bath for a week before the test. These are then cleaned and dried in accordance with ASTM F2423-05 (see Table 34-1 ). Flexion-extension, lateral bending, and rotations are simulated under a constant preload as per ASTM standards. Mass measurements are performed both before and after testing to assess the wear rate. Particulate characterization and element contributions are evaluated using computer-controlled scanning electron microscopy.

Analysis

Three-dimensional (3D) marker placement data are converted to evaluate the Cardan or the Euler angles. To determine the motion of the specimen, the data are entered onto the global coordinate system. Relative motion of a component of the construct also may be determined with respect to a static fixture—for example, the mounting platform. Appropriate statistical analysis is performed to assess the impact of a surgical procedure. In most cases, a two-tailed t test, a Tukey test, or a one-way analysis of variance (ANOVA) turns out to be sufficient.

Some of the terminology and parameters associated with the analysis of load-displacement data from a typical in vitro test are as follows:

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  Elastic zone. The amount of total deformation that offers resistance to the applied load. It is measured by evaluating the tangent to the curve at the load that causes maximum deformation ( Fig. 34-2B , points 5 and 6).

  
  
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  Elastic zone stiffness. The stiffness that characterizes the amount of elastic (or recoverable) deformation of the specimen.

  
  
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  Energy dissipation. To characterize the viscoelasticity or plasticity of the specimen being loaded, the area enclosed by the load-displacement curve is evaluated. This quantity provides a measure of the dissipated energy.

  
  
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  Neutral zone. The amount of unrecovered deformation once the specimen is under no load. In cycle 3 shown in Figure 34-2B , NZ is the neutral zone. It also may be defined as the part of the range of motion wherein the specimen offers the least resistance to the applied deformation.

  
  
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  Neutral zone stiffness. The stiffness of the specimen in the neutral zone, determined by the slope of the load-displacement curve at the point of no deformation.

  
  
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  Preconditioning. Cycles of load applied to the specimen—intact or otherwise—to mitigate the impact of the viscoelastic nature of the tissues. From Figure 34-2B , cycles 1 and 2 are the preconditioning cycles.

  
  
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  Range of motion (ROM). The linear or the angular distance that a specimen (intact, injured, or construct) travels in a plane with the application of load in that plane. From the load-displacement curve of Figure 34-2B , the ROM can be calculated as (+ROM) − (−ROM).

  
  
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  Relative range of motion (RROM). The relative motion for the entire spine or a segment or even a vertebral body with respect to the static mounting platform.

  
  
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  Sigmoidity. A measure of the nonlinearity present in the mechanical behavior of the specimen, calculated as the ratio of the neutral zone stiffness and elastic zone stiffness.

  
  
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  Stiffness. The mechanical resistance of a specimen to an applied load, measured by the slope of the load-deformation or load-displacement curve along a linear region or regions in a nonlinear curve.

In Vitro Testing

The human spine is a complex structure composed of hard and soft, active and passive tissue. This structure has multiple degrees of freedom at each one of several joints formed by intervertebral discs. Ideally, from a biomechanical and biochemical point of view, the most physiologically relevant model for testing the efficacy of a device, surgical technique, or engineered tissue is the human patient. However, this is not a practical option. In vitro testing offers significant advantages, even though factors such as intra-abdominal pressure and muscular forces are hard to replicate. In vitro studies offer the possibility of standardization and make it easier to estimate the impact of a surgical procedure, or a simulated injury or stabilization using an implant, because the loads can be varied with relative ease. Such protocols enable researchers to compare different devices designed and developed for the same clinical requirement. Once the device components have been tested using protocols cited in Table 34-1 , in vitro testing brings their performance evaluation closer to in vivo use in patients.