How Cyclical Spinal Flexion and Extension Independently Alter Mechanical Integrity of the Intervertebral Disc

Introduction:\nIntervertebral disc (IVD) herniation is a cause of low back pain and creates significant health care and financial burden in Canada. It has been observed that with moderate compression, combined cyclical flexion and extension are sufficient to cause herniation in functional spinal units (Callaghan & McGill, 2001). Furthermore, it has been observed that combined cyclical flexion and extension leads to more damage to the posterior annulus fibrosus (AF) when compared to cyclical flexion alone, quantified by increased nuclear migration (Balkovec & McGill, 2012). A recent study by Briar & Gregory (2023) found that static flexion negatively influenced the adhesion strength of the interlamellar matrix of the AF, regardless of sample location. It has been shown that when flexing an IVD, the anterior annulus fibrosus is compressed. This compression creates an increase in intradiscal pressure (Ekström et al., 2004), and the increased pressure applies tension to the posterior AF. It is hypothesized that the increased damage seen in combined flexion/extension loading protocols is due to the alternation of tension and compression on the AF.\nAims:\n The aim of this study was to investigate how cyclical flexion and extension of a functional spinal unit independently influences the adhesive properties of the interlamellar matrix.\nMethodology:\n Cervical porcine spines were used; specifically, the C3/4 and C5/6 were excised from the cervical spines and placed into one of three experimental conditions: 1) cyclical spinal flexion, 2) cyclical spinal extension, and 3) static compression. All groups experienced 900N of compressive load for 6000 cycles at 1hz. Following mechanical loading, the intervertebral disc was dissected out and multi-layer samples of both the posterior and anterior AF were extracted. The multi-layer samples were loaded onto a UStretch device (Cellscale Inc., Ontario, Canada) and underwent a 180° peel test to determine the adhesion properties of the Interlamellar matrix. Full height samples of the AF, approximately 2-4 layers in width were tested using a standardized bilayer protocol in a Biotester 5000 (Cellscale Inc., Ontario, Canada). These “bilayers” are thought to holistically quantify tensile properties of the AF. The outcome measures were compared using two-way mixed model ANOVAs with the within factor being location (anterior, posterior; repeated), and the between factor being loading condition (flexion, extension, control; non-repeated).\nResults:\n The results of this experiment suggest that isolated cyclical flexion and isolated cyclical extension do not significantly influence the mechanical properties of the annulus fibrosus when compared to static compression alone. There were no significant main effects or interactions observed for any of the bilayer tests. Two significant findings were observed in the 180° peel tests. A main effect of location was observed for peel strength and peel stiffness. The anterior average peel stiffness (M=0.92 N/mm2) was significantly (p=0.028) lower than the average posterior peel stiffness M=1.21 N/mm2. Likewise, the anterior average peel strength (M=2.24 N/mm) was significantly (p=0.028) weaker than the posterior average peel strength(M=3.09 N/mm). While there were no significant main effects of loading for any of the outcome measures, there was a trend for peel stiffness. This trend supports the concept of compression having a detrimental impact on peel stiffness.\nDiscussion/Conclusion:\nThe findings of the current study suggest that isolated cyclical flexion and isolated cyclical extension of functional spinal units do not appear to mechanically compromise them when compared to a statically compressed control. However, a trend present for peel stiffness appears to provide evidence for compression being more detrimental to interlamellar adhesive properties when compared to tension. This was indicated by decreased anterior peel stiffness for the cyclical flexion group when compared to the control, and posterior peel stiffness being decreased in the cyclical extension group when compared to the control. A novel finding of the current experiment was the effect of location, with the posterior peel strength and stiffness being significantly (p<0.05) stronger than the anterior peel stiffness and strength. This novel finding is incongruent with previous literature, which in many instances provides evidence of the anterior annulus being stronger than the posterior annulus (Tavakoli et al., 2018; Gregory et al., 2014; Tsuji et al., 1993). However, given the torque-controlled loading parameters, and the effects of the posterior elements shielding the posterior AF from applied compression, these results make sense and provide further support for compression having a more detrimental effect on interlamellar adhesive properties.

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