Presented at NACOB 98: North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998

---

THE RESPONSE OF THE INTERVERTEBRAL DISC, THE PARS INTERARTICULARIS
AND THE POSTERIOR LIGAMENTS TO EXTERNAL ANTERIOR SHEAR LOADING

V.R. Yingling1, S.M. McGill2
1 Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, MO
2 Department of Kinesiology, University of Waterloo, Waterloo Ontario

INTRODUCTION

Research surrounding the etiology of spine injury has mainly focused on compressive loading, however, recent studies indicate the importance of shear loading in injury. For this reason, anterior shear loads were applied to spinal motion segments of three groups ( whole specimens, specimens with no posterior ligaments (NL) and specimens with no posterior ligaments and no facet joints (NFL)) in order to determine the function of each individual structure. An animal model was used (pig cervical spine), to obtain a homogeneous cohort of specimens and control for age, diet, physical activity and condition. The purpose of this study was to determine the function of each structure of the motion segment under an external anterior shear load, as well as to identify the injuries resulting from shear loading.

REVIEW AND THEORY

Motion segments are complex structures and their individual components have been investigated to understand their contribution in spine injury etiology. Farfan et. al (1970) investigated the contribution of each structure under torsional loading and Adams et. al. (1980) partitioned a flexion moment among the motion segment structures, however, the motion segment under shear loading has not been partitioned. A recent case-control study involving a large automotive plant identified shear loads as a strong predictor of low back pain (Norman et. al. , 1998).

PROCEDURES

Twenty-six porcine motion segments (C3-C4 orC5-C6) were separated into three groups. The first group were whole specimens, the second group had their posterior ligaments transected (NL), specifically interspinous and supraspinous, while the third group had the ligaments transected with the facet joints also removed (NFL). The specimens were potted in stainless steel cups and mounted in a custom designed jig which applied a pure anterior shear load to the specimens along the plane of the disc. A compressive load of 300 N was applied to the specimens before testing. The specimens were loaded at 100 N/s until a drop in the load signal of 6.25% was detected and then the test was stopped and the load removed. Load-deformation curves were collected at a sample rate of 100 Hz to obtain the ultimate shear load, the deformation at failure and the stiffness of the specimen. The load-deformation curves were averaged, normalized and modeled using a 2nd order polynomial constrained to a zero intercept. Injuries were determined through both dissection and radiography techniques.

RESULTS

The stiffness values were different between the three groups (whole, NL, NFL), post hoc tests showed that the NFL stiffness (237 N/mm) was lower than the whole (326 N/mm) and NL (364 N/mm) groups. The ultimate load at failure was significantly different between the groups, the load for the NFL group (1385 N) was lower that the whole (2215 N) and NL (2325 N) groups. The largest contributor of stiffness under anterior shear loading was found to be the disc complex, accounting for approximately 70 % of the overall stiffness of the motion segment. The pars complex contributed on average 30% of the overall stiffness. The average load to failure for the disc (NFL) was 1390 N compared with the pars complex at 830 N. Figure 1 illustrates the load-deformation curves of the intervertebral disc and the pars interarticularis under anterior shear loading. In order to obtain a load-deformation curve of the pars interarticularis, the load-deformation curve from the specimens with the intervertebral disc isolated was subtracted from the whole specimens (the posterior ligaments were found to have no significant effect on the resistance of an anterior shear load).

The resulting injuries were fractures to the pars interarticularis (Figure 2 & 3). Endplate avulsions were identified in the specimens with only the intervertebral disc resisting shear loading (Figure 2 & 3).

Figure 1: Load-deformation curves of the whole motion segment, a segment with no posterior ligaments or facet joints(NFL) representing the intervertebral disc, and the pars interarticularis.

Figure 2: Photograph of lateral view of a motion segment with bilateral facet fractures and an inferior endplate avulsion. (rt half).

Figure 3: An x-ray of a motion segment (lateral view) illustrating an endplate avulsion and a pars fracture.

DISCUSSION

The intervertebral disc was found to be the primary load bearing structure of the spinal motion segment (62.5% of the ultimate shear load), however, the pars interarticularis was the initial site of injury attaining an average max load of 830 N. This apparent inconsistency in the mechanical data and the injuries identified can be explained using results from the literature. Cripton et. al. (1995) showed that during an applied shear test on human specimens after facet failure, the intervertebral disc resisted 77% of the applied load. However, the disc pressure during the initial testing did not increase until after facet failure occurred suggesting that although the intervertebral disc is capable of sustaining a majority of the shear load it is only required to do so after an injury to the posterior bony elements. Furthermore, studies involving the mechanical testing of isolated pars interarticularis (Troup, 1976) found the pars to sustain up to 2000 n and to deform up to 8-10 mm before failure. The predominance of the disc in resisting shear loading along with the larger failure loads found in the literature for the pars and the occurrence of an injury to the pars prior to any disc injury may suggest that the motion segment is a redundant system. In-vivo the posterior elements sustain the initial failure even though the intervertebral disc is capable of sustaining the applied load. The disc may provide the strength and stiffness for the joint while the facets guide the motion.. Fracturing the pars interarticularis may not greatly weaken the joint but does appear to compromise its normal kinematics. Clinically, this may explain pain associated with the pars fracture, but not total disability.

REFERENCES

Cripton et. al. (1995) Symposium, Wayne State University

Farfan et. al. (1970) JBJS 54-A(3):492-510.

Norman et. al. (1998) In Press Clinical Biomech.

Troup (1976) Clinical Ortho and Related Res. 117: 59-67.