Hybrid multidirectional test method to evaluate spinal adjacent-level effects
Introduction
Clinical studies have documented long-term accelerated degeneration at adjacent-levels due to spinal fusion surgery (Axelsson et al., 1997, Frymoyer et al., 1979, Goffin et al., 2004, Hilibrand et al., 1999, Kellgren and Lawrence, 1958, Lee, 1988, Lunsford et al., 1980, Schlegel et al., 1996). These spinal adjacent-level effects (ALE) may be explained on the basis of biomechanical hypothesis of stress concentration and subsequent motion re-distribution. In response to the altered biomechanical environment due to the fusion, overtime adaptive changes follow post-surgery. As a result, the free (unfused) intervertebral levels are subjected to additional stresses and acquire increasing motions overtime. In vitro biomechanical testing mimicking the in vivo behavior may help to evaluate the ALE.
There have been several biomechanical studies attempting to document changes in spine due to simulated fusion. Inappropriate moment-input and ill-defined displacement-input methods have been used (explained later). Consequently, the results in the form of intervertebral rotations and disc pressures have varied considerably between the studies: large increases (Chow et al., 1996, DiAngelo et al., 2003, Eck et al., 2002, Fuller et al., 1998), small increases (Eck et al., 1999, Fuller et al., 1998, Shono et al., 1998), minimal changes, (Lindsey et al., 2003, Rohlmann et al., 2001, Schmoelz et al., 2003) and even decreases (Chow et al., 1996, Shono et al., 1998) have been reported.
Non-fusion motion preservation devices, including artificial discs and flexible stabilizers, are increasingly coming into use and are being tested biomechanically (Bertagnoli et al., 2005, Blumenthal et al., 2002, Buttermann and Beaubien, 2004, Delamarter et al., 2003, DiAngelo et al., 2004, Goffin et al., 2002, Guyer and Ohnmeiss, 2003, Klara and Ray, 2002, Kotani et al., 2002, Kotani et al., 2005, Link, 2002, McAfee et al., 2003, Minns and Walsh, 1997, O’Leary et al., 2005, Schlegel et al., 1996, Senegas, 2002, Sengupta, 2004, Valdevit and Errico, 2004). As these devices are designed to avoid or minimize the ALE, there is an urgent need to develop an appropriate biomechanical test method to evaluate the ALE, so that the new devices are tested in the laboratory to document their effectiveness. The main purpose of the paper is to present a new biomechanical test method designed especially and appropriately to evaluate the ALE in multidirectional testing of fusion and non-fusion devices. Relevant terminology, definitions, and important concepts in biomechanical testing are provided, and the previous studies and their methods are critically analyzed.
Section snippets
Well-defined test method
By a well-defined test method it is meant that the end-conditions of the specimen during every instance of testing are completely defined. In other words, the loads and motions at the ends of the specimen, and, consequently at each intervertebral level, are precisely known or can be calculated. This makes the test method reproducible and transportable across biomechanics laboratories, helping generate reliable, comparable, and laboratory-independent data.
Test protocols
Two well-defined test protocols for
Discussion
Adjacent-level accelerated degeneration subsequent to a fusion has been documented in many clinical studies. The non-fusion motion preservation devices are designed to eliminate or slow down such adverse effects. These devices are gaining popularity. Thus the biomechanical evaluation of the spinal adjacent-level effects (ALE) is important and timely. Although several biomechanical studies are available, the results have large variation and are conflicting, mostly due to the use of either an
Conclusions
The well-defined Hybrid method is designed to evaluate the adjacent-level effects due to the fusion and non-fusion devices. It is a multidirectional method, and uses unconstrained pure moments of the popular Flexibility method to produce the displacement-inputs to the specimen. It is recommended that the specimen length chosen be the entire mobile region of the spine so that all free spinal levels within the mobile region could participate in the motion re-distribution. Many flexibility test
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