An Improved Biomechanical Testing Protocol
Object: An experimental study was performed to determine the biomechanical end-mounting configurations that replicate in vivo physiological motion of the cervical spine in a multiple-level human cadaveric model. The vertebral motion response for the modified testing protocol was compared to in vivo motion data and traditional pure-moment testing methods.
Methods: Biomechanical tests were performed on fresh human cadaveric cervical spines (C2–T1) mounted in a programmable testing apparatus. Three different end-mounting conditions were studied: pinned–pinned, pinned–fixed, and translational/pinned–fixed. The motion response of the individual segmental vertebral rotations was statistically compared using one-way analysis of variance and Student-Newman-Keuls tests (p < 0.05 unless otherwise stated) to determine differences in the motion responses for different testing methods.
Conclusions: A translational/pinned–fixed mounting configuration induced a bending-moment distribution across the cervical spine, resulting in a motion response that closely matched the in vivo case. In contrast, application of pure-moment loading did not reproduce the physiological response and is less suitable for studying disc arthroplasty and nonfusion devices.

Spinal instrumentation is commonly used to provide immediate stabilization and to promote anterior cervical fusion. Unfortunately, fusion surgery has been reported to increase the biomechanical stresses at the adjacent segments, which could lead to further degenerative symptomatic disc disease at the adjacent levels. An alternative approach to cervical fusion surgery is to restore motion to the diseased segment with disc arthroplasty. The goal of the disc prosthesis is to replace the diseased disc while preserving and/or restoring the motion at the treated spinal level. Limited biomechanical data exist for analysis of the effects of disc arthroplasty on cervical spine biomechanics; the most suitable method for evaluating these mobile spinal devices in vitro remains unclear.

Biomechanical testing on human cadaveric tissue offers a practical means for evaluation and ranking of different surgical techniques; however, there are no standard tissue-based testing protocols for evaluating spinal devices. Furthermore, in vitro tests can be performed under load control, displacement control, or more recently, hybrid control (combinations of load and displacement) through the use of multiple-axis, robotic controllers.

The cervical spine consists of a series of free VBs that exhibit complex, coupled motions and loading behaviors. For the subaxial cervical spine, in vivo motion is greatest in the sagittal plane, with more rotation occurring in extension than in flexion. Only small amounts of muscle activity are needed to maintain the head's orientation in an erect (neutral) position. Thus, muscle-induced compression is small and head weight is the typical physiological force that acts on the cervical spine. Flexion or extension of the head induces a bending-moment distribution through out the cervical spine that increases caudally and acts in combination with the compressive (head weight) force (Fig. 1). In vitro testing methods should replicate this motion response.



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Computer model showing in vivo sagittal motion and mechanics of the cervical spine. This illustration depicts the caudally increasing bending-moment distribution (2–5 Nm), which is greatest at the C5–6 level, and is created by continuous rotations with like polarity at each vertebral level (40° flexion or extension) with an axial compressive load (50-N head weight).





The objective of this study was to identify the appropriate loading conditions that would replicate the in vivo motion response of the cervical spine, and thus to develop a biomechanical testing protocol for evaluation of disc arthroplasty or motion preservation devices. The motion response for the improved testing method was compared with in vivo motion data obtained from the literature and with traditional in vitro pure-moment methods.