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European Spine Journal

, Volume 24, Issue 9, pp 1882–1892 | Cite as

Limitations of current in vitro test protocols for investigation of instrumented adjacent segment biomechanics: critical analysis of the literature

  • David Volkheimer
  • Masoud Malakoutian
  • Thomas R. Oxland
  • Hans-Joachim Wilke
Review Article

Abstract

Purpose

Accelerated degenerative changes at intervertebral levels adjacent to a spinal fusion, the so-called adjacent segment degeneration (ASD), have been reported in many clinical studies. Even though the pathogenesis of ASD is still widely unknown, biomechanical in vitro approaches have often been used to investigate the impact of spinal instrumentation on the adjacent segments. The goal of this review is (1) to summarize the results of these studies with respect to the applied protocol and loads and (2) to discuss if the assumptions made for the different protocols match the patients’ postoperative situation.

Methods

A systematic MEDLINE search was performed using the keywords “adjacent”, “in vitro” and “spine” in combination. This revealed a total of 247 articles of which 33 met the inclusion criteria. In addition, a mechanical model was developed to evaluate the effects of the current in vitro biomechanical test protocols on the changes in the adjacent segments resulting from different stiffnesses of the “treated” segment.

Results

The surgical treatments reported in biomechanical in vitro studies investigating ASD can be categorized into fusion procedures, total disc replacement (TDR), and dynamic implants. Three different test protocols (i.e. flexibility, stiffness, hybrid) with different loading scenarios (e.g. pure moment or eccentric load) are used in current biomechanical in vitro studies investigating ASD. According to the findings with the mechanical model, we found that the results for fusion procedures highly depend on the test protocol and method of load application, whereas for TDR and dynamic implants, most studies did not find significant changes in the adjacent segments, independent of which test protocol was used.

Conclusions

The three test protocols mainly differ in the assumption on the postoperative motion behavior of the patients, which is the main reason for the conflicting findings. However, the protocols have never been validated using in vivo kinematic data. In a parallel review on in vivo kinematics by Malakoutian et al., it was found that the assumption that the patients move exactly the same after fusion implemented with the stiffness- and hybrid protocol does not match the patients’ behavior. They showed that the motion of the whole lumbar spine rather tends to decrease in most studies, which could be predicted by the flexibility protocol. However, when the flexibility protocol is used with the “gold standard” pure moment, the difference in the kinematic changes between the cranial and caudal adjacent segment cannot be reproduced, putting the validity of current in vitro protocols into question.

Keywords

In vitro experiments Kinematics Range of motion Adjacent segment degeneration Biomechanics 

Notes

Acknowledgments

We wish to thank the Alexander von Humboldt Foundation for their generous support of this research through a Research Award to Thomas R. Oxland during his sabbatical leave at the University of Ulm.

Conflict of interest

The authors affirm that they have no financial affiliation or involvement with any commercial organization that has direct financial interest in any matter included in this article.

Supplementary material

586_2015_4040_MOESM1_ESM.pdf (78 kb)
Supplementary material Table 1 Summary of the biomechanical articles addressing ALEs after spinal fusion (PDF 77.8 kb)
586_2015_4040_MOESM2_ESM.pdf (28 kb)
Supplementary material Table 2 Summary of the biomechanical articles addressing ALEs after TDR (PDF 28 kb)
586_2015_4040_MOESM3_ESM.pdf (42 kb)
Supplementary material Table 3 Summary of the biomechanical articles addressing ALEs after dynamic instrumentation (PDF 41.9 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • David Volkheimer
    • 1
  • Masoud Malakoutian
    • 2
  • Thomas R. Oxland
    • 2
    • 3
  • Hans-Joachim Wilke
    • 1
  1. 1.Institute of Orthopaedic Research and Biomechanics, Centre of Musculoskeletal ResearchUlm UniversityUlmGermany
  2. 2.Department of OrthopaedicsUniversity of British ColumbiaVancouverCanada
  3. 3.Department of Mechanical EngineeringUniversity of British ColumbiaVancouverCanada

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