Coupled motions of the spine under standardized in vitro conditions: A systematic review and meta-analysis

Christian Liebsch^*Hans-Joachim Wilke

• Ulm University Medical Center, Ulm, Germany

Introduction: Coupled motions, defined as motions outside the primary motion plane, are used for in vivo kinematic measurements and the validation of experimental and numerical models of the spine. Due to the interindividual movement comfort of participants and imprecise measurement methods, previous in vivo studies have not been able to determine distinct coupled motion patterns. The aim of this meta-analysis was to identify reproducible coupled motion patterns from in vitro studies with standardized loading conditions for each spinal section of the spine. Methods: A systematic literature search in the PubMed, Web of Science and Embase databases was conducted in accordance with PRISMA guidelines to identify any in vitro studies on coupled motions of human specimens (n=120). In a three-stage procedure, all studies were filtered out that allowed quantitative comparability of coupled motions in the individual loading directions (n=20). Inclusion criteria were testing the intact state, quasi-static flexibility measurement using pure moments, and specification of the analyzed levels. Coupled motions were calculated as relative values of the primary range of motion and quantitatively evaluated using meta-analysis. One-sample Wilcoxon signed rank test was performed in SPSS to determine the reproducibility of coupled motions for each segmental level. Results: Overall, no relevant coupled motions were identified in primary flexion and extension (p > 0.05). In primary lateral bending, evidence was particularly found for low extension and moderate to high ipsilateral axial rotation in the thoracic spine as well as moderate ipsilateral axial rotation in the subaxial cervical spine (p < 0.05). In primary axial rotation, low to moderate contralateral lateral bending in the thoracic spine and high to dominant ipsilateral lateral bending in the subaxial cervical spine were particularly detected (p < 0.05). Discussion: This meta-analysis of in vitro studies identified characteristic coupled motion patterns of the spine, specifically a strong motion coupling interrelationship between lateral bending and axial rotation. More studies are required to extend and substantiate the findings of this meta-analysis. Nevertheless, this data set is valuable for the validation of experimental and numerical studies of the spine and the interpretation of the coupled motion behavior of the passive spinal structures.