Effect of specimen length: are the mechanics of individual motion segments comparable in functional spinal units and multisegment specimens?

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Abstract

Functional spinal units (FSUs) are frequently used for in vitro mechanical testing. This approach assumes that the mechanical behavior of the FSUs is equivalent to the mechanics of these segments within the intact spine. The purpose of this study was to determine whether normal spinal mechanics are compromised in FSU preparations. Flexion-extension pure-moment flexibility testing was performed on 13 L2–L5 porcine specimens. The moment–angle relationship of the L3/L4 segment was recorded, and then the multisegment specimens were cut down to L3/L4 FSUs and retested. Comparisons of stiffness, range of motion, and laxity zone were made between conditions. The neutral zone and range of motion parameters were significantly larger in the L3/L4 motion segment compared to the L3/L4 segment tested within the multisegment specimen; the stiffness was not significantly different. These differences were attributed to cutting the supraspinous ligament as this ligament spans several vertebral levels. Flexion mechanical tests performed on FSUs should be interpreted cautiously as the biomechanics of FSUs is altered from normal. Although the choice of spine length depends on the experimental purpose, spinal flexion studies should be performed on multisegment specimens to appropriately represent the anatomical boundary conditions.

Introduction

In vitro testing is an essential approach for studying spinal mechanics [1], however, individual researchers perform tests on specimens incorporating different numbers of vertebral levels. The choice of specimen length depends in part on the particular experimental question [2]. Many researchers have performed mechanical testing on functional spinal units (FSUs; two adjacent vertebrae and all intervening tissue) [3], [4], [5], [6], [7], while other researchers have used multisegment spinal specimens [8], [9], [10], [11], [12], [13].

Anatomic studies show that structures such as the supraspinous and longitudinal ligaments span several spinal levels [14]. This implies that the mechanics of FSUs may be compromised, as these ligaments are cut when the FSUs are prepared. Few studies have compared the mechanics of FSUs and multisegment spinal specimens. Kettler et al. observed that FSUs had significantly reduced neutral zones and coupled motions, and significantly larger range of motions and hysteresis areas compared to multisegment specimens [15]. The purpose of Kettler’s study was to assess the effects of specimen length for evaluating spinal implants, and therefore they compared the mechanical properties of the end level within the multisegment specimens and FSUs. The anatomical boundary conditions for this end level were compromised as the supraspinous ligament was cut immediately adjacent to this level, and accordingly this study does not directly assess whether the mechanics of FSUs are compromised compared to multisegment specimens (note that this was not their intention). The purpose of the current study was to directly compare the effect of specimen length (number of vertebral levels) on in vitro spinal mechanics. As the supraspinous ligament spans several vertebral levels [14], it is cut when FSUs are prepared, and contributes to resist spinal flexion [16], this study evaluated spinal flexion. Cycles of flexion-extension driven by pure-moments were applied to multisegment porcine lumbar specimens (L2–L5). The moment–angle relationship for the L3/L4 segment within the multisegment specimens was recorded, and then the multisegment specimens were cut down to L3/L4 FSUs and retested. The L3/L4 stiffness, laxity zone and range of motion parameters were compared between the multisegment and FSU specimens. The hypotheses for this experiment were that the stiffness, range of motion and laxity zone measures would be equal in the FSU and multisegment preparations.

Section snippets

Specimens

A local abattoir provided 13 spines from immature domestic pigs. The thoracolumbar spines including the lowest two thoracic levels (T10–T12) the entire lumbar spine (L1–L7) and the sacrum were harvested in one piece, including a protective layer of soft tissue, and were stored frozen at −20 °C. Before testing, the specimens were thawed and all musculature was carefully dissected. The spines were transected through the L1/L2 and L5/L6 discs and wrapped with plastic film to prevent dehydration

Results

Typical motion segment flexibility curves for the three experimental conditions are shown in Fig. 2. We observed systematic differences in the mechanics of the L3/L4 motion segment when it was tested as part of a multisegment spine with supraspinous and interspinous ligaments cut, and as a FSU.

Discussion

This study establishes that the flexion mechanics of FSUs are significantly different from the mechanics of the same segments within multisegment specimens based on the significantly different flexion range of motion and neutral zone parameters in the two conditions. This infers that pure-moment flexion mechanical tests performed on FSUs should be interpreted cautiously as the biomechanics of FSUs is altered from normal.

A number of researchers have stated that the mechanical integrity of the

Acknowledgements

This work was funded by the Natural Sciences and Engineering Research Council of Canada. Special thanks to Drs Jack Callaghan and John Runciman for helpful advice, and Mr Kevin Gillespie for assistance with this project.

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