Technical note
A continuous pure moment loading apparatus for biomechanical testing of multi-segment spine specimens

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Abstract

An apparatus is described that enables the application of continuous pure moment loads to multi-segment spine specimens. This loading apparatus allows continuous cycling of the spine between specified flexion and extension (or right and left lateral bending) maximum load endpoints. Using a six-degree-of-freedom load cell and three-dimensional optoelectronic stereophotogrammetry, characteristic displacement versus load hysteresis curves can be generated and analyzed for different spinal constructs of interest. Unlike quasi-static loading, the use of continuous loading permits the analysis of the spine's behaviour within the neutral zone. This information is of particular clinical significance given that the instability of a spinal segment is related to its flexibility within the neutral zone. Representative curves for the porcine lumbar spine in flexion–extension and lateral bending are presented to illustrate the capabilities of this system.

Introduction

The rationale for using the flexibility method with non-constraining pure moment loads for the in vitro biomechanical testing of multi-segment spine specimens was first presented more than a decade ago (Panjabi, 1988). The flexibility method of testing involves the controlled application of known loads to a specimen while measuring the resulting motion behaviour of the spine. The use of non-constraining pure moments ensures that the load experienced by a specimen remains constant along its length independent of its geometry, motion, or stiffness (Asazuma et al., 1990; Ashman et al., 1989; Crawford et al., 1995; Panjabi, 1988). This means that, throughout the loading cycle, the loading conditions at any two cross-sections in the spinal column are identical. A major advantage of pure moment loading, therefore, is that it allows for the unbiased comparison of the biomechanical properties of different spinal constructs (e.g. intact, injured, instrumented, or fused specimens) (Adams, 1995).

A number of devices designed to apply non-constraining pure moment loads to spine specimens have been described in the literature. The original concept of using deadweights and cables acting about pulleys to produce force couples (Goel et al., 1985) has been refined with the implementation of pneumatic actuators (Panjabi, 1994; Yamamoto et al., 1989) or a materials testing machine (Crawford et al., 1995) for load generation. More complicated systems based on the sophisticated control of torque motors (Kunz et al., 1994; Wilke et al., 1994) and stepper motors (Eggli et al., 1991) have also been described. Many of these designs apply incremental quasi-static loads to the spine. While the quasi-static approach yields useful information about the high-stiffness elastic zone, it does not provide data regarding the behaviour of the spine within the clinically important neutral zone (Panjabi, 1988).

The purpose of this technical note is to describe a relatively simple, inexpensive apparatus which can produce continuous angle-moment data for the entire range of motion of the spine — including the neutral zone. The apparatus applies pure moment loads to a multi-segment spine specimen and continuously cycles it between specified flexion and extension (or right and left lateral bending) maximum load endpoints. The main advantage of this approach is that it permits the analysis of the behaviour of the spine within the neutral zone. This is of particular interest given that the clinical instability of a spinal segment has been related to its behaviour within this zone (Panjabi et al., 1994).

Section snippets

Materials and methods

The loading apparatus described here is similar in principle to the one described by Crawford et al. (1995) with modifications to allow for continuous cyclic loading between specified flexion and extension (or right and left lateral bending) maximum load endpoints (Fig. 1). The mechanism is based on a modular stainless steel frame 600 mm wide×600 mm deep×1000 mm high (Item Products Inc., Houston TX). Mounted to the top of the frame is a load generating electromechanical linear actuator (Model

Results

The apparatus described in this paper was capable of applying continuous and bi-directional pure moments to multi-segment spine specimens in a highly reproducible fashion (Fig. 2a). The purity of the applied moments was evidenced by the small off-axis loads — the maximum off-axis moments remained less than 7% of the peak primary moment (Fig. 2a), and the off-axis forces were always less than ±8 N (Fig. 2b). Although small, these off-axis loads could be seen to have a noticeable effect on the

Discussion

The apparatus described in this paper applies pure moment loads to multi-segment spine specimens continuously and bi-directionally about the neutral position. The advantage of this approach is that it permits the analysis of the behaviour of the spine within the high-flexibility neutral zone region. This is of particular interest since the clinical instability of a spinal segment has been related to its behaviour within the neutral zone (Panjabi et al., 1994).

The limitations of this apparatus

Acknowledgements

The authors gratefully acknowledge: Gerald Saunders, David Siu, and Julie Howes of the Clinical Mechanics Group, Department of Surgery, Queen's University, Kingston, Ontario, for assistance in the development of the testing apparatus and associated software; Xunhua Yuan, Department of Orthopaedics, University Hospital, Lund, Sweden, for assistance with data collection and analysis; and Hye Won Yang, independent artist, for producing the graphical illustrations of the apparatus. This research

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