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.
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ABSTRACT: Injury, instrumentation, or surgery may change the functional biomechanics of the spine. Adverse changes at one level may affect the adjacent levels. Modeling these changes can increase the understanding of adjacent-level effects and may help in the creation of devices that minimize adverse outcomes. The current modeling techniques (e.g., animal models, in vitro testing, and finite element analysis) used to analyze these effects are costly and are not readily accessible to the clinician. It is proposed that the pseudo-rigid-body model(PRBM) may be used to accurately predict adjacent level effects in a quick and cost effective manner that may lend itself to a clinically relevant tool for identifying the adjacent-level effects of various treatment options for patients with complex surgical indications. A PRBM of the lumbar spine (lower back) was developed using a compliant mechanism analysis approach. The global moment-rotation response, relative motion, and local moment-rotation response of a cadaveric specimen were determined through experimental testing under three conditions: intact, fused, and implanted with a prototype total disc replacement. The spine was modeled using the PRBM and compared with the values obtained through in-vitro testing for the three cases. The PRBM accurately predicted the moment-rotation response of the entire specimen. Additionally, the PRBM predicted changes in relative motion patterns of the specimen. The resulting models show particular promise in evaluating various procedures and implants in a clinical setting and in the early stage design process. [DOI: 10.1115/1.4004896]Journal of Mechanisms and Robotics 11/2011; 3(4):041008. DOI:10.1115/1.4004896 · 0.86 Impact Factor