(B-44) Lumbar Spine Orientation Affects Compressive Fracture Characteristics

Introduction:: Military members experience more lumbar spine injuries compared to the general population due to forces they experience during underbody blasts, aircraft ejections, and helicopter crashes. This analysis was conducted to determine whether there is a relationship between lumbar spine orientation at the time of compressive loading and resulting fracture characteristics.

Materials and Methods:: Twenty-three human lumbar spines (levels T12-L5) were isolated and anchored at T12 and L5 to polymethylmethacrylate (PMMA), while maintaining natural lordotic curvature. The spine was attached to two platforms with 5 Nm pre-flexion (Stemper et al., 2011). The platforms were held together by a cable. Mass on the upper platform simulated the torso, head-neck, and upper extremities. The entire apparatus (platforms, mass, spine) was accelerated down a drop tower until impacting pulse-shaping foam at the bottom. Acceleration characteristics were controlled by drop height and the foam’s mechanical characteristics. Drop height and corresponding acceleration/force were increased for each specimen until vertebral fracture occurred. Pre and post x-rays were taken for each of the 23 lumbar spines and the resulting fractures.

Fractures were classified by the affected columns (anterior, middle, posterior). Pre-test x-rays were used to quantify spinal orientation. The Cobb angle of each spine was measured between T12 inferior vertebral body to L5 superior vertebral body. The lumbar spine angle was determined by connecting the centroids of T12 and L5 and measuring the angle from horizontal to quantify the degree of sagittal tilt. The horizontal distance between the application of loading on T12 and the centroid of T12 was measured.


Results, Conclusions, and Discussions:: The loading resulted in 23 fractures, classified into six categories (Wedge: n=8, Burst: n=7, Hyperextension: n=5, Spinous process: n=2, Hyperflexion: n=1, None: n=3). Spinous process and hyperflexion fractures were excluded due to low sample size. Orientation measurements were compared between fracture type using a single-factor ANOVA. Spines that sustained burst fracture had a Cobb angle of 11.4±8.7° and a lumbar spine angle of 77.7±4.3°. Spines that sustained wedge fracture had a Cobb angle of 22.0±10.6° and a lumbar spine angle of 86.5±5.5°. Spines that sustained hyperextension fracture had a Cobb angle of 31.4±14.7° and a lumbar spine angle of 74.4±6.2°. Spines that sustained wedge fracture and burst fracture had an average horizontal distance from load application to T12 centroid of 0.53±0.14 inches and 0.37±0.13 inches, respectively.

Spines that sustained hyperextension fracture had significantly larger Cobb angles compared to spines that sustained burst fracture (p=0.03). Spines that sustained wedge fracture had significantly larger lumbar spine angles (i.e., closer to vertical orientation) compared to spines that sustained burst (p=0.03) and hyperextension (p=0.02) fracture. Spines that sustained wedge fracture also had a nearly significant longer horizontal distance between the T12 centroid and the load location compared to spines that sustained burst fracture (p=0.07).

Several key orientations distinguished fracture types. Hyperextension fractures occurred in spines with enhanced lordosis. Burst fractures occurred in straight spines with load application closer to the centroid of T12. Wedge fractures occurred in spines that were aligned vertically with the load application oriented more anteriorly.

This study demonstrated that orientation of the lumbar spine affected the injury (Figure 1). This is important for military members because it shows that their position at time of injury may result in different fracture types, including some fractures that maintain column stability (i.e., wedge fracture). These findings can be applied to potential training of military members and safety enhancements to reduce the risk of severe lumbar spine injury. Posture influences injuries, injury mechanisms, and human tolerance. The present results can be used as validation data for hierarchical models of the spine and human body for improved biofidelity and applicability to military loading scenarios.


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