The influence of torsion on disc herniation when combined with flexion

Abstract

The role of torsion in the mechanical derangement of intervertebral discs remains largely undefined. The current study sought to investigate if torsion, when applied in combination with flexion, affects the internal failure mechanics of the disc wall when exposed to high nuclear pressure. Thirty ovine lumbar motion segments were each positioned in 2°axial rotation plus 7°fl exion. Whilst maintained in this posture, the nucleus of each segment was gradually injected with a viscous radioopaque gel, via an injection screw placed longitudinally within the inferior vertebra, until failure occurred. Segments were then inspected using micro-CT and optical microscopy in tandem. Five motion segments failed to pressurize correctly. Of the remaining 25 successfully tested motion segments, 17 suffered vertebral endplate rupture and 8 suffered disc failure. Disc failure occurred in mature motion segments significantly more often than immature segments. The most common mode of disc failure was a central posterior radial tear involving a systematic annulus-endplate-annulus failure pattern. The endplate portion of these radial tears often propagated contralateral to the direction of applied axial rotation, and, at the lateral margin, only those fibres inclined in the direction of the applied torque were affected. Apart from the 2°of applied axial rotation, the methods employed in this study replicated those used in a previously published study. Consequently, the different outcome obtained in this study can be directly attributed to the applied axial rotation. These inter-study differences show that when combined with flexion, torsion markedly reduces the nuclear pressure required to form clinically relevant radial tears that involve cartilaginous endplate failure. Conversely, torsion appears to increase the disc wall's resistance to radial tears that do not involve cartilaginous endplate failure, effectively halving the disc wall's overall risk of rupture.

The intradiscal failure pressure on porcine lumbar intervertebral discs: an experimental approach

“…The comparison between these results with the failure pressure of the annular fibers reported in previous studies for several MS models (Table 2) reveals that the values documented on this report are appreciably lower than the more recent studies, performed by Schechtman et al (2006) and Veres et al (2010). In fact, several studies reported the magnitude of the rupture values obtained in this study as within the normal range of physiological IDP for human samples (Claus et al, 2008;Dennison et al, 2008;Sato et al, 1999;Wilke et al, 1999).…”

“…Few studies include the determination of the failure pressure of the IVDs (Iencean, 2000;Menkowitz et al, 2005;Schechtman et al, 2006;Veres et al, 2010). Schechtman et al (2006) investigated the intrinsic failure strength of the intact bovine caudal disc using an hydraulic inflation actuator.…”

“…However, it does not give information about the microstructural behavior of inner annular fibers after the inflation. Later, Veres et al (2010) used the same technique performed by Schechtman et al (2006) to investigate the role of high IDP in the disruption of the annular fibers of the ovine lumbar IVDs. This team included the analysis of the AF damage after pressure insertion by a microstructural investigation.…”

Association of facet tropism with lumbar disc herniation

“…Whilst torsion alone may not pose a threat to the structural integrity of lumbar intervertebral discs, it appears to have a marked effect when applied in conjunction with flexion [31].…”

“…Using finite element analysis, Schmidt et al [33] found that under this same loading combination, flexion plus left axial rotation, the fibres of the inner right posterolateral annulus experience the largest shear strain. Various biomechanical studies have demonstrated that the combination of flexion and torsion is capable of producing injury to the posteriolateral annulus [31,34].…”