1Biomechanics Laboratory, University of California School of Medicine, San Francisco
2Department of Mechanical Engineering, College of Engineering, University of California, Berkeley
Static loading tests on fresh human spinal segments obtained at autopsy were performed to measure the deformation of thoracic and lumbar intervertebral joints in response to lateral bending moment, flexion moment, extension moment, torsional moment, anteroposterior and mediolateral shear force, axial tension force, and axial compression force. The findings in these tests were as follows:
The mechanical resistance (stiffness) of the intervertebral joint to lateral bending, flexion, extension, and torsion increases with continued deformation, that is, the moment-rotation curves for these modes of deformation are non-linear. The initial rotatory stiffness (the slope of the initial part of the moment-rotation) was for lateral bending (T8 to L4), 0.7 to 4.1; for flexion (T8 to L4), 0.7 to 4.1; for extension (T8 to L4), 1.4 to 4.7; and for torsion (T8 to T11), 1.4 to 2.7 and (T11 to L4), 5.4 to 14.9 newton-meters per degree.
The transverse force necessary to produce pure shearing displacements at an intervertebral disc increased linearly with deformation as small shear forces were applied. The initial static shear stiffness for a single lumbar disc was 1,050 to 5,100 newtons per centimeter for both anteroposterior and lateral shear.
For axial tension and compression, the load-deformation behavior of an intervertebral disc was non-linear, the stiffness increasing with the load. The intervertebral disc was 1.5 to 3.0 times stiffer in compression than in tension. The values for the axial stiffness in newtons per centimeter for loads of 220 to 670 newtons were 7,000 to 15,800 for compression and 12,300 to 33,200 for tension.
The articular processes and related ligaments contributed to the mechanical rigidity of the intervertebral joint during bending and torsion. These processes are of particular mechanical significance in the extension stiffness of the thoracic and lumbar joints and in the torsional stiffness of the lumbar joints.
Although the spinal column is non-uniform in size and the discs are of varying height and area, the initial bending stiffnesses and axial compression stiffnesses showed very little variation in the thoracic and lumbar discs. The torsional stiffness, however, showed a marked change at the eleventh and twelfth thoracic vertebrae, and it is hypothesized that this discontinuity represents a site of structural weakness for torsional stresses to the spinal column.