In vitro testing of the spine provides valuable information to researchers and clinicians about how neurosurgical procedures affect spinal stability and motion (Crawford, 2002). The Spinal Biomechanics Laboratory has devised several novel techniques for experimentally testing cadaveric spines, enabling researchers at our institution to study spinal biomechanics in ways that were not possible at other institutions. Many of these techniques are incorporated in the custom software developed in the Spinal Biomechanics Laboratory. This software is now used not only at Barrow, but has also been provided by Barrow for use in other biomechanics laboratories at universities in the United States.
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Local Coordinate Systems
In the Spinal Biomechanics Laboratory, a technique was devised to enable each level of the spine to be tracked and studied independently (Crawford and Dickman, 1997). With this technique, the researcher points to specific vertebral landmarks with a probe in which optical markers are embedded. Custom testing software performs spatial transformations to align these landmarks with appropriate Cartesian coordinate system of the vertebra. Angular data can then be plotted in real time in individual local coordinate systems of multiple spinal levels during testing.
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Three-Dimensional Spinal Angle Calculation
Publications from the Spinal Biomechanics Laboratory have contributed to the understanding of how three-dimensional (3D) joint angles are best calculated. In Crawford et al, 1996, the differences and similarities of two methods for calculating 3D joint angles, projection angles and Euler angles, were described, and a method was proposed by which the most appropriate Euler angle sequence or projection angle set can be selected for the spine and other joints of the body. In Crawford et al, 1999, a new 3D angle technique, the "tilt/twist" method, which has advantages over both the projection and Euler methods, was developed. The custom software developed in the Spinal Biomechanics Lab uses this tilt/twist method technique to display spinal angles in real time during testing.
Illustrations reprinted from Human Movement Science, 15(1), Crawford NR, Yamaguchi GT, Dickman CA: Methods for determining spinal flexion/extension, lateral bending, and axial rotation from marker coordinate data: Analysis and refinement, pg.55-78, 1996, with permission from Elsevier.
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Axis of Rotation
In the Spinal Biomechanics Laboratory, the axis of rotation is calculated as a 3D vector from highly accurate optical position data (Optotrak 3020) and plotted in real time during testing. The axis of rotation provides useful information on the pattern of movement of the spine, which is especially important in studying artificial disks and other motion-sparing devices.
Dr. Crawford published a technical note on how to calculate and plot the axis of rotation on medical images, available for free download from the World Spine Journal.
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Pure Moment and Physiologic Loading Apparatuses
Two apparatuses for loading spines were devised in the Spinal Biomechanics Laboratory. The first apparatus, described in Crawford et al, 1995, applies pure moments to specimens through a system of strings and pulleys in conjunction with a MTS servohydraulic test frame (MTS Systems, Eden Prairie, MN). Pure moments are useful in causing all levels of the spine to bend or twist by a consistent and uniform angle to test motion limits.
The second apparatus applies a simplified flexion-compression load, simulating the anterior and posterior muscle straps of the neck or back, together with the weight of the head or torso and additional compressive force representing co-contraction of the muscle straps. The flexion-compression apparatus is useful for studying complex kinematics of the spine during flexion and extension and for studying how local alterations affect the distribution of muscle forces and hence global motion of the entire neck or back.
Spinal Laxity
The lax zone of movement is the portion of the spine's movement that can occur without much effort. For example, if you suddenly relax your head, your neck "flops" through a range before the muscles and ligaments start to stretch. An experimental technique that allows good understanding and reproducibility in measurements of spinal laxity was devised in the Spinal Biomechanics Lab and described in Crawford et al, 1998. This parameter is one of many biomechanical parameters reported in most publications generated by the laboratory.
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Facet Loading
When healthy patients bend and twist their spine, the facet joints, which are the posterior stabilizing elements of the spine, normally slide across each other. When pathology is present or after disk arthroplasty or other surgical procedures, however, the motion and loading at the facet joint can be altered. It is important to characterize loading of the facet joint after disk prostheses or posterior motion-sparing devices have been inserted because poor facet loading can lead to the eventual failure of the devices.
The Spinal Biomechanics Laboratory has made advances in a method for experimentally quantifying facet loads by gluing arrays of strain gauges to the lateral mass or lamina of the spine (Sawa and Crawford, 2008).
After testing is complete, the strain gauge arrays are calibrated by disarticulating the specimen and applying a series of point loads to the exposed articular surface.
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