INTRODUCTION

When removed from the surrounding tissues, the lumbar spine buckles at relatively small compressive loads (~90N). These loads are substantially lower than those measured in-vivo and, in the majority of adults, are lower than the weight of the torso. Patwardhan et al. showed that the load carrying capacity of the lumbar spine significantly increased when the load was placed in the posterior section of the vertebra, passing through the instantaneous axis of rotation (IAR) and acting tangential to the curvature of the spine. This condition minimized the internal bending and shear forces on the spine. The physiological rational behind the follower load was presented that the load path followed roughly that prescribed by the muscle architecture. We hypothesize that the location of the aggregate abdominal center of mass also plays a critical role in the load bearing capacity of the lumbar spine.

METHODS

The mass and center of mass of the abdominal cross section of the thoracic and lumbar regions (T1-L5) of four adults were obtained from the literature. The height/mass percentiles were 40/45 and 90/45 (males) and 95/70 and 35/50 (females). The aggregate mass at each level was computed as the mass of that level in addition to the superior levels. The aggregate center at each level was defined as the mass center of the aggregate mass. Starting with T1 the mass of the level (T1) was applied at the center of mass to the lever model. The location of the IAR, represented by the lever’s fulcrum, was calculated so as to put the level in static equilibrium with the aggregate mass and aggregate center of mass. This process was then repeated for levels T2-L5.

RESULTS

The location of the aggregate center of mass (T2-L5) was posterior to the vertebral center for all of the patients. The anterior-posterior position correlates with the reported anterior-posterior location of the IAR for the thoracic and lumbar vertebrae.

DISCUSSION

As the center of mass passes through or near the IAR, the internal bending moment required to maintain the spine in an upright position is small. Thus the spine is “balanced†around the IAR and can withstand a high compressive force without buckling and is energetically favorable to the neutral posture. Small perturbations of the center of mass can efficiently be controlled by the surrounding soft tissue.