Facet deflection and strain are dependent on axial compression and distraction in C5–C7 spinal segments under constrained flexion

• Published in: JOR-spine Vol. 7; no. 3; pp. e1360 - n/a
• Main Authors: Foroutan, Parham, Quarrington, Ryan D., Russo, Michael Pyrros, Ding, Boyin, Cripton, Peter A., Costi, John J., Jones, Claire F.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.09.2024; Wiley
• Subjects: cervical; cervical facet dislocation; Data acquisition systems; facet fracture; facet joint; Fractures; intervertebral; mechanics; Motion capture; Robots; rotation; spine; Strain gauges; Vertebrae; vertebral; Ontario Canada; Canada; United States > US; Germany; rotation; vertebral; cervical; cervical facet dislocation; facet joint; intervertebral; mechanics; spine; facet fracture
• ISSN: 2572-1143; 2572-1143
• DOI: 10.1002/jsp2.1360

Background Facet fractures are frequently associated with clinically observed cervical facet dislocations (CFDs); however, to date there has only been one experimental study, using functional spinal units (FSUs), which has systematically produced CFD with concomitant facet fracture. The role of axial compression and distraction on the mechanical response of the cervical facets under intervertebral motions associated with CFD in FSUs has previously been shown. The same has not been demonstrated in multi‐segment lower cervical spine specimens under flexion loading (postulated to be the local injury vector associated with CFD). Methods This study investigated the mechanical response of the bilateral inferior C6 facets of thirteen C5‐C7 specimens (67±13 yr, 6 male) during non‐destructive constrained flexion, superimposed with each of five axial conditions: (1) 50 N compression (simulating weight of the head); (2‐4) 300, 500, and 1000 N compression (simulating the spectrum of intervertebral compression resulting from neck muscle bracing prior to head‐first impact and/or externally applied compressive forces); and, (5) 2 mm of C6/C7 distraction (simulating the intervertebral distraction present during inertial loading of the cervical spine by the weight of the head). Linear mixed‐effects models (α = 0.05) assessed the effect of axial condition. Results Increasing amounts of intervertebral compression superimposed on flexion rotations, resulted in increased facet surface strains (range of estimated mean difference relative to Neutral: maximum principal = 77 to 110 με, minimum principal = 126 to 293 με, maximum shear = 203 to 375 με) and angular deflection of the bilateral inferior C6 facets relative to the C6 vertebral body (range of estimated mean difference relative to Neutral = 0.59° to 1.47°). Conclusions These findings suggest increased facet engagement and higher load transfer through the facet joint, and potentially a higher likelihood of facet fracture under the compressed axial conditions. Facet fractures are frequently associated with clinically observed cervical facet dislocations (CFDs); however, to date, CFDs with concomitant facet fractures have scarcely been reported in the experimental literature. The aim of this study was to investigate the sagittal deflections and surface strains of the bilateral inferior C6 facets of C5–C7 spinal segments under non‐destructive constrained flexion (postulated to be the local injury vector associated with CFDs), under a range of compression and distraction axial conditions. We observed increased facet surface strains and facet deflections with increasing amounts of intervertebral compression superimposed on flexion rotations. These data suggest increased facet engagement and higher load transfer through the facet joint, and therefore a higher likelihood of facet fracture, under axial compression conditions.