The instant axis of rotation influences facet forces at L5/S1 during flexion/extension and lateral bending

Abstract

Because the disc and facets work together to constrain spinal kinematics, changes in the instant axis of rotation associated with disc degeneration or disc replacement may adversely influence risk for facet overloading and arthritis. The relationships between L5/S1 segmental kinematics and facet forces are not well defined, since previous studies have separated investigations of spinal motion and facet force. The goal of this cadaveric biomechanical study was to report and correlate a measure of intervertebral kinematics (the centrode, or the path of the instant axis of rotation) and the facet forces at the L5/S1 motion segment while under a physiologic combination of compression and anterior shear loading. Twelve fresh-frozen human cadaveric L5/S1 joints (age range 50-64 years) were tested biomechanically under semi-constrained conditions by applying compression plus shear forces in several postures: neutral, and 3 degrees and 6 degrees of flexion, extension and lateral bending. The experimental boundary conditions imposed compression and shear representative of in vivo conditions during upright stance. The 3-D instantaneous axis of rotation (IAR) was calculated between two consecutive postures. The facet joint force was simultaneously measured using thin-film sensors placed between both facet surfaces. Variations of IAR location and facet force during motion were analyzed. During flexion and extension, the IAR was oriented laterally. The IAR intersection with the mid-sagittal plane moved cephalad relative to S1 endplate during flexion (P=0.010), and posterior during extension (P=0.001). The facet force did not correlate with posture (P=0.844). However, changes in the facet force between postures did correlate with IAR position: higher IAR's during flexion correlated with lower facet forces and vice versa (P=0.04). During lateral bending, the IAR was oblique relative to the main plane of motion and translated parallel to S1 endplate, toward the side of the bending. Overall, the facet force was increased on the ipsilateral side of bending (P=0.002). The IAR positions demonstrate that the L5 vertebral body primarily rotates forward during flexion (IAR close to vertebral body center) and rotates/translates backward during extension (IAR at or below the L5/S1 intervertebral disc). In lateral bending, the IAR obliquity demonstrated coupling with axial torsion due to resistance of the ipsilateral facet.

Sagittal plane lumbar intervertebral motion during seated flexion-extension radiographs of 658 asymptomatic nondegenerated levels

“…Work by Rousseau et al showed that forces exerted on the facet joints increased with variable positions of the COR. 29 We know that after surgical procedures, as well as with idiopathic degeneration, the COR changes. 16 In both cases, changes in the COR set up a perpetual cycle of further stress and degeneration.…”

Influence of lumbar intervertebral disc degeneration on the outcome of total lumbar disc replacement: a prospective clinical, histological, X-ray and MRI investigation

“…Thereafter, the degenerative cascade will result in a loss of intervertebral disc space height (DSH) which from then on can be observed on X-ray images. Depending on the degree of DDD, these degenerative changes have been shown to have a significant influence on the biomechanics and kinematics of a lumbar motion segment [38,39,41,45,[65][66][67][68][69][70][71].…”

Investigation of coupled bending of the lumbar spine during dynamic axial rotation of the body

“…Altered vertebral motion has been widely assumed as a biomechanical factor causing spinal pathology [1][2][3][4][5]. Therefore, numerous studies have been conducted to understand spinal kinematics using both in vitro and in vivo approaches [2,[6][7][8][9][10][11]. Most cadaveric studies tested motion segments by applying a flexionextension, bending, or axial rotation torque, with or without a compressive load to measure the vertebral motion using various techniques [8,[11][12][13][14][15].…”

“…Therefore, numerous studies have been conducted to understand spinal kinematics using both in vitro and in vivo approaches [2,[6][7][8][9][10][11]. Most cadaveric studies tested motion segments by applying a flexionextension, bending, or axial rotation torque, with or without a compressive load to measure the vertebral motion using various techniques [8,[11][12][13][14][15]. Most of the in vivo studies have used skin markers or plain radiographs to measure the vertebral motion during various dynamic motion or static postures [16][17][18].…”