Spine

Nucleotomy Alters Internal Strain Distribution of the Human Lumbar Intervertebral Disc

Nucleotomy is a surgical procedure following herniation and also simulates the reduced nucleus pulpousus (NP) pressure that occurs with disc degeneration. Internal disc strains are an important factor in disc function, yet it is unclear how internal strains are affected by nucleotomy. Grade II L3-L4 human cadaveric discs (n=6) were analyzed intact and after a partial nucleotomy that removed 30-50% of the NP through a left posterolateral incision (incision) while the contralateral side remained intact (uninjured). Two cycles of stress-relaxation testing were performed for reference (50N) and loaded (0.70MPa) configurations. After each 8hour equilibration period, the reference and loaded discs were imaged separately in a 7T MRI scanner (0.3mm isotropic resolution). The reference and loaded images were registered to calculate internal strain within the annulus fibrosus (AF) lamellae and discs were averaged to create anatomical templates. Circumferential, radial, and axial strains for each disc were transformed to the average templates, effectively normalizing the strains. Five circumferential regions were defined within the mid-third of the templates. Nucleotomy altered disc strains on both the incision and uninjured sides from the intact state. Strain fields were inhomogeneous through the five regions. Mean circumferential strain was unaffected by nucleotomy on the uninjured side, but decreased with incision, showing hoop strains through the AF were disrupted. Mean compressive axial strains were higher after nucleotomy, effectively reducing AF stiffness, and mean radial strains were unaltered after partial nucleotomy. These findings are important to address etiology and progression of degeneration, and to develop and evaluate therapeutic interventions.
Listed In: Biomechanical Engineering, Biomechanics, Orthopedic Research


Human cadaveric bi-Segment impact experiments at different postures

Victims of improvised explosive devices (IEDs) that have presented spinal injury in recent conflicts have been shown to have a high incidence of lumbar spine fractures. Previous studies have shown that the initial positioning of spinal bone-disc-bone complexes affects their biomechanical response when loaded quasi-statically; such a correlation, however, has not been explored at appropriate high loading rate scenarios that simulate injury. This study aims to investigate the response of lumbar spine cadaveric segments in different postures under axial impact conditions. Three T11-L1 bi-segments were dissected and tested destructively in a drop tower under flexed/neutral/extended postures. Strains were measured on the vertebral body and the spinous process of T12. Forces were measured cranially using a 6-axis load cell, and a high-speed camera was used to capture displacements and fracture. The impacted specimens were CT-scanned to identify the fracture pattern. Whilst axial force to failure was similar for flexed and extended postures, the non-axial forces and the bending moments, however, were dissimilar between postures. Although all specimens showed a burst fracture pattern, the extended posture failed more posteriorly. This suggests that axial force alone is not adequate to predict injury severity in the lumbar spine. This insight would not have been possible without the use of the 6-axis load cell. As metrics for spinal injury in surrogates take into account only the axial force, this programme of work may provide data for a better injury criterion and allow for a mechanistic understanding of the effects of posture on injury risk.
Listed In: Biomechanical Engineering, Biomechanics, Mechanical Engineering, Orthopedic Research