Sagittal Spine Balance,Segmental Load Bearing and Curvature Transition Zones

Dear Editor,

This author reads with great interest the article published in the GSJ titled: Prevalence and Risk Factors of Degenerative Spondylolisthesis and Retrolisthesis in the Thoracolumbar and Lumbar Spine – An EOS Study Using Updated Radiographic Parameters. Global Spine Journal. 2023;0 (0) doi:10.1177/21925682221134044 by Hey et al. In this article the authors describe the use of EOS technology to investigate potentially etiological, and biomechanically mutual relationship between spondylolisthesis and retrolisthesis (in the lumbar and thoracolumbar spine, respectively) and to highlight the need for simultaneous risk evaluation of both the conditions in a patient in context of segmental instability, associated back pain and/or spinal deformity. The authors deserve congratulations - for bringing to fore certain biomechanical concepts of sagittal imbalance and compensatory mechanisms in a clinical context.

To continue this discussion further, it would be appropriate to state that concept of structural and functional sagittal balance in the spine has been there for a while.^1-4 The biomechanical explanations of the spine curvatures and their implications in terms of intra- and inter-segmental load bearing have been extensively studied. ² These studies very precisely elucidate specific mechanisms of load transfer and switch of loading trajectories between the anterior and posterior elements (both within and between vertebrae) in different segments of the spine.^1,2 Additionally, considering the arguments put forward by Hey et al focusing on the vulnerability of junctional segments of the spine to translational deformities, these arguments could be further substantiated by the evidence demonstrated in mechanisms involving the change in facet orientation from the thoracic to the lumbar patterns as seen in normal spines, as well as in context of numerical anomalies associated with transitional vertebrae. ⁵ Further, in terms of the role played by the center of gravity in regulating corresponding compensatory mechanisms (as discussed in the study), these reports have quantitatively demonstrated the contribution of gravity in anatomical and biomechanical structuring of the body and neural arch elements of curvature-specific vertebrae in the spinal column.

In agreement to the Hey et al on the need for further studies investigating both spondylolisthesis and retrolisthesis within a biomechanical and clinical spectrum, specifically in terms of testing their hypothesis with differential loading conditions (and to address a major limitation pointed out in their study by the authors), this author believes that the use of finite element modeling could be an important investigative approach to consider. ⁶ The association of retrolisthesis with “smaller L4S (P = .011), as well as a smaller SS (P = .033)” hypothesized due postural changes in sitting related pelvic retroversion may be tested with such modeling. Moreover, longitudinal studies involving participants with structural and/or functional risks for vertebral ante- and retro-listhesis such as atypical facets transitions at the curvatural junctions could be an interesting idea to pursue.^7,8 Lastly, the role of ligamentous and neuro-muscular control on mechanisms regulating segmental balance cannot be overemphasized. Therefore, their association with the age-related degree of segmental degeneration as pointed out in this study, is crucial. As such, use of dynamic positional imaging may improve detection of the true behavior of vertebral translation, spine instability and to provide additional information useful for clinical decision making, as previously pointed out in the literature.^9-11 This author congratulates Hey et al for the publication of their study.