Samuel Howarth

CMCC, Graduate Education and Research, Faculty Member

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I have received a B.Sc. in Kinesiology with a Joint Honours in Mathematics (2004) as well as an M.Sc. (2006) and Ph.D. (2011) in Biomechanics from the University of Waterloo. During my research career, I have conducted biomechanical research using both experimental (in vivo and in vitro) and theoretical (computational modeling) approaches. My doctoral research investigated how passive tissue damage in the lumbar spine under acute as well as repetitive shear loading is influenced by modulating factors such as morphology, bone density, posture, compressive load and shear load rate. Findings from this research will be used to enhance our understanding of injury mechanisms so that future interventions may be developed to reduce the incidence of shear related low-back injury.
Supervisors: Dr. Jack Callaghan

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Postural influence on the neutral zone of the porcine cervical spine under anterior–posterior shear load

Medical Engineering & Physics, Jan 1, 2012

Segmental instability, characterized by excessive or aberrant movement of the vertebrae can be as... more Segmental instability, characterized by excessive or aberrant movement of the vertebrae can be assessed quantitatively using mechanical characteristics within a region of minimal resistance called the neutral zone. The diagnosis of instability is often used to decide whether or not to surgically fuse the vertebrae. Alterations in flexion/extension posture cause changes in both contact area and spacing between articulating facets that may lead to changes in the mechanical response of the functional spinal unit (FSU) within the neutral zone. This investigation quantified neutral zone (NZ) length under anterior and posterior shear loading and the influence of posture on the shear NZ characteristics of the vertebral joint. Thirty porcine cervical FSUs (15 C34 and 15 C56) were tested. Endplate area was calculated from measurements of the exposed endplates while facet angles were measured from X-rays taken in the transverse plane. Specimens were exposed to a 300 N compressive preload followed by a test to determine flexion/extension NZ limits. These limits were used as target angles during shear passive tests performed in extended and flexed postures. Displacement rate during shear passive tests was 0.2mm/s and five cycles of anterior-posterior shear were performed to a target of ±400 N in a randomized order of extended, neutral and flexed postures. Shear NZ length and average stiffness were quantified. Stiffness within the shear NZ was 67 N/mm in the neutral posture. Extended postures produced a 37% (p&amp;amp;amp;amp;lt;0.0001) increase in shear stiffness within the NZ compared to both flexed and neutral postures. Posture did not influence shear NZ length. Therefore, a true region of zero stiffness does not exist during shear loading with a baseline compressive load. Neutral zone length for the porcine FSU exposed to shear load was not influenced, despite known changes in facet articulation, by changing posture. Average stiffness increased likely as a result of increased contact area and force in extension. The results from this investigation demonstrate that postural deviation of the vertebral joint is not likely a significant confounding factor when assessing segmental stability.

The impact of posture and prolonged cyclic compressive loading on vertebral joint mechanics

Biomechanical properties of the transverse carpal ligament under biaxial strain

The transverse carpal ligament (TCL) influences carpal stability and carpal tunnel mechanics, yet... more The transverse carpal ligament (TCL) influences carpal stability and carpal tunnel mechanics, yet little is known about its mechanical properties. We investigated the tissue properties of TCLs extracted from eight cadaver arms and divided into six tissue samples from the distal radial, distal middle, distal ulnar, proximal radial, proximal middle, and proximal ulnar regions. The 5% and 15% strains were applied biaxially to each sample at rates of 0.1, 0.25, 0.5, and 1%/s. Ligament thickness ranged from 1.22 to 2.90 mm. Samples from the middle of the TCL were thicker proximally than distally (p < 0.013). Tissue location significantly affected elastic modulus (p < 0.001). Modulus was greatest in the proximal radial samples (mean 2.8 MPa), which were 64% and 44% greater than the distal radial and proximal ulnar samples, respectively. Samples from the middle had a modulus that was 20-39% greater in the proximal versus more distal samples. The TCL exhibited different properties within different locations and in particular greater moduli were found near the carpal bone attachments. These properties contribute to the understanding of carpal tunnel mechanics that is critical to understanding disorders of the wrist. ß

Samuel Howarth

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