Load-carrying capacity of the human cervical spine in compression is increased under a follower load.
ABSTRACT An experimental approach was used to test human cadaveric cervical spine specimens.
To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine.
The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained.
A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression.
A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05).
The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.
- Spine 11/1989; 14(10):1040-5. · 2.16 Impact Factor
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ABSTRACT: In the field of spinal fixation devices, there is a profusion of new instrumentations. Often, the biomechanical evaluation is done in a nonstandardized manner, which makes it difficult to compare the results of one researcher with those of another, for the same device or for different devices. There is a need for a conceptual framework under which guidelines may be suggested for the evaluation of these devices in some uniform and comprehensive manner. There are three basic biomechanical tests: strength, fatigue, and stability. The strength test evaluates the failure load of the device, determines its weak points, and is helpful in the initial development of the device. The fatigue test provides a measure of longevity of the device, either alone or as part of the spinal construct, by testing the device to failure using cyclically varying loads. In contrast, the stability test measures the capability of the device to provide multi-directional stability to the injured spine. There is no failure of the device, and the results of this test are clinically important, as they characterize the potential for early fracture healing and early fusion. A conceptual framework for the evaluation of multi-direction stability of spinal fixation devices and guidelines for designing the necessary experiments are described.Spine 11/1988; 13(10):1129-34. · 2.16 Impact Factor
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ABSTRACT: Facetectomy, either unilateral or bilateral, significantly altered the capacity of cervical spine functional units to withstand increasing compression-flexion loads applied in a constant mode to different specimen configurations. Unilateral facetectomy resulted in an average 31.6 +/- 9.7 percent decrease in strength whereas bilateral disruption caused an average 53.1 +/- 11 percent decrease in strength. Motion analysis in a two-dimensional plane after facetectomy indicated an anterior displacement of the instantaneous axis of rotation (IAR) with a resultant increased load on the vertebral bodies and disc. This anterior shift of the IAR in the horizontal plane was significantly but not completely resolved by wire fixation of the facet joints. These fixation techniques, consisting of either facet to facet or facet to spinous process wiring, demonstrated a similar capability to restore strength to the functional units as well as reducing excessive motion in the vertical and anterior axes induced by the facetectomies.Spine 08/1988; 13(7):808-12. · 2.16 Impact Factor