Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

The effect of progressive herniation on ovine lumbar functional spinal unit six degree of freedom mechanics (#31)

Michael P Russo 1 , Claire F Jones 2 , John J Costi 1
  1. Flinders University, Biomechanics & Implants Research Group, Medical Device Research Institute, College of Science and Engineering, Adelaide, SA, Australia
  2. The University of Adelaide, School of Electrical and Mechanical Engineering & Adelaide Spinal Research Group, Centre for Orthopaedics and Trauma Research, Adelaide, SA, Australia

Introduction. Studies in ovine lumbar segments have shown that rapid axial compression to 3 mm, superimposed on bending postures, causes disc herniation and lateral failure of the annulus fibrosus (AF)1,2. Furthermore, rapid axial compression to 1.5 mm (without herniation) leads to microstructural disorganisation of AF lamella, and reduced micro-tensile and shear moduli, and failure strength, of the interlamellar matrix1. However, the relationship between the compressive displacements that lead to herniation, and the six degree of freedom (6DOF) mechanical changes of the segment, have not been determined. The aim of this study was to investigate the effect of rapid axial compressive displacement magnitude on 6DOF segment mechanics during combined flexion and axial rotation motion.

 

Methods. Baseline 6DOF mechanics (stiffness and phase angle for +/- 6DOF except for tension) from 15 intact sheep lumbar functional spinal units (FSU) were measured in a hexapod robot. Specimens were then randomly assigned to one of four rapid compressive displacement groups: 0 mm (control; L4/5, N=4), 1 mm (L2/3, N=3), 2 mm (L2/3, N=3), or 3 mm (L2/3, N=4, L4/5, N=1). Specimens were postured at 13° flexion and 2° left axial rotation, relative to a neutral posture, then loaded at 400 mm/min to their respective compressive displacement groups. The 6DOF testing protocol was then repeated. Changes in stiffness and phase angle, in 6DOF, were assessed using a Bonferroni adjusted Repeated-Measures ANOVA, with a statistical significance of p<0.05, with marginal significance defined as 0.05<p<0.07. The presence of disc herniation was determined visually.      

 

Results. Herniation was observed for most specimens in the 2 mm (N=2) and 3 mm (N=4) groups. Mechanical changes were observed for all displacement groups, across the various DOF’s tested (Table 1). Posturing the specimen alone (0 mm group) significantly decreased stiffness in anterior shear and flexion (p<0.018). Stiffness for specimens in the 1 mm group significantly decreased in left lateral and anterior shear, flexion and right axial rotation (p<0.036). Phase for the 1 mm group significantly increased in flexion, right/left lateral bending (p<0.011). In the 2 mm group, stiffness significantly decreased for flexion and right axial rotation (p<0.003) and tended towards (marginal significance) decreasing in left lateral shear and axial rotation (p<0.063). Phase in the same group significantly increased for flexion, left lateral bending and right axial rotation (p<0.017). Specimens in the 3 mm group had significantly decreased stiffness for right lateral, anterior and posterior shear, flexion and left/right axial rotation (p<0.036). Phase in the 3 mm group significantly increased for all directions except anterior and posterior shear (p<0.019).

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Discussion. The 3 mm group had the greatest number of DOFs with stiffness and phase altered by rapid compression, while the control group had few (stiffness) or no (phase) altered DOFs after posturing. In axial rotation, increased rapid compressive displacement appeared to decrease stiffness and increase phase. Although the present sample size was small and visible herniation did not occur in all specimens, mechanical changes to the FSU were detected in some DOF. Rapid compressive displacement in all groups influenced FSU mechanics.

  1. Tavakoli, J., et al., The Biomechanics of the Inter-Lamellar Matrix and the Lamellae During Progression to Lumbar Disc Herniation: Which is the Weakest Structure? Ann Biomed Eng, 2018. 46(9): p. 1280-1291.
  2. van Heeswijk, V.M., et al., Posterolateral Disc Prolapse in Flexion Initiated by Lateral Inner Annular Failure: An Investigation of the Herniation Pathway. Spine (Phila Pa 1976), 2017. 42(21): p. 1604-1613.