Spinal biomechanics Presenters name Arial 24 pt Principles

Spinal biomechanics Presenter‘s name Arial 24 pt Principles course Presenter‘s title Arial 20 pt City, Month, Year Arial 20 pt

Learning outcomes • Define the mechanism and morphology of a spinal injury • Recognize the presence of instability • Describe the principles of spinal biomechanics

The spinal column and the motion segment • The spine is a multiarticular column of individual motion segments (functional spinal unit) Vertebra Facet joint Muscle and ligaments Intervertebral disc

Vertebral body • Between 70– 90% of static axial load is carried by the cancellous vertebral body Peripheral epiphyseal ring Spinal cord Intervertebral disc Nerve root Vertebra

Intervertebral disc • The intervertebral disc is composed of an outer annulus fibrosus and an inner nucleus pulposus The annulus (70% of the disc) is mostly type I collagen The nucleus is mostly water embedded in protein (proteoglycan) Anterior annulus has larger tensile modulus Degenerative disc type I collagen replaced by type II and have much lower moduli

Vertebral body • The stress-strain curve The strength and elastic modulus of cancellous bone are dependent on the density Compressive strength of the spinal column

Effect of posture on intradiscal pressure

Facet joint orientation and regional motion Coronal Cervical Thoracic Lumbar Sagittal

Facet joint and posterior ligamentous complex Supraspinal ligament Interspinal ligament Mean θx Posterior ligamentous complex Increase flexion and extension following removal of posterior element Ligamenta flava Facet joint and capsule Effect of partial and total facetectomy in ROM of the spine

Muscles and ligaments • Along with ligaments, muscles initiate and guide spinal movement • Spinal stability depends more on the damping effect of spinal musculature than stiffness of the passive elements • Spinal muscle damping protects the passive elements from exceeding their limits of shear and rotational resistance

Spinal motion • Vertebrae have six degrees of freedom, translation along and rotation about each of the three orthogonal axes, rotation, flexion and extension, and lateral bending

Basic nomenclature • Six degrees of freedom Transverse X-axis

Kinematics of the spine • Instantaneous axes of rotation (IAR) At every instant for a rigid body in plane motion, there is a hypothetical line in the body that does not move IAR and bending moment

Kinematics of the spine • Instantaneous axes of rotation (IAR) Changes with different magnitude of force and anatomical alternation of the structure Flexion/extension Lateral bending Thoracic spine Axial rotation Flexion/extension Lateral bending Lumbar spine Axial rotation

Kinematics of the spine • Instantaneous axes of rotation (IAR) Changes with pathological process and alternation of anatomy Degenerative spine Thoracolumbar fracture

Kinematics of the spine • Instantaneous axes of rotation (IAR) Understanding thoracolumbar fracture

Coupled motion • Two or more individual motions are said to be coupled when one motion is always accompanied by another motion Left lateral bending Left axial rotation

Coupled motion • More complex in the lumbar spine

Costovertebral articulation and the rib cage • Stabilization structure of the thoracic spine Costovertebral Articulation and capsule Costotransverse and intercostal Ligament complex

Take-home messages • Due to anatomical variation at various levels of the spine the biomechanical parameters also vary considerably • The spine is a complex structure of bone, synovial and fibrous joints, the intervertebral disc and muscles, all play a role in maintaining stability of the spine • Understanding the biomechanics helps us understand traumatic and degenerative conditions of the spine and how they should be managed or stabilized.

Excellence in Spine