Strain energy based equilibrium model of lumbar spine

Abstract

In this study we have constructed an equilibrium model simulating a manual load handling situation based on experimental data. The load carrying capacity of the low back has been determined based on the concept of strain energy absorption characteristcs. Fresh human male cadaveric lumbar intervertebral joints were subjected to axial compressive loading in an MTS testing apparatus. Load and deformations were recorded as a function of time using a digital oscilloscope. The load deflection response indicated non-linear and sigmoidal characteristics. Based on the instantaneous stiffness (ability to offer resistance) of the structure, the initiation of trauma was defined as the point at which the stiffness begins to decrease for the first time. The total strain energy absorbed by the structure up to the initiation of trauma and up to the maximum load carrying capacity (point of zero stiffness) was computed using the integration principle. To assess the manual load carrying capacity of the lumbar spine, a static equilibrium analysis was performed to determine the normal and shear forces acting on the lumbosacral joint due to a load held in hand. The model accounted for the intra-abdominal pressure, and the force in the erector spinae muscle acting at eccentricities of 200 mm and 5 mm respectively from the center of gravity (C.G.) of the L5-S1 disc. The internal strain energy capacities due to normal and shear forces were computed by incorporating the geometry, and Young's and shear modulus of elasticity for both normal and degenerative spines estimated from finite element analyses. Strength of the spine was assessed by correlating the theoretical limiting strain energy absorbing capabilities with the experimentally determined strain energy absorption capacities at the initiation of trauma. Results indicated that normal spines can resist higher compressive load, trauma initiation load, and stiffness compared to degenerate spines. In addition, they have higher strain energy absorbing capacities. However, the deflection at failure was approximately the same for both types of structures. The range of manual load carrying capacity (load held at a distance of 450 mm from the C.G. of the structure) was found to be 200–500 N. Based on the strain energy criteria, this study has provided experimental limits of injury to an intervertebral joint, and assessed the range of loads that can be held in a manual material handling situation using a midsagittal equilibrium model of the lumbar spine. However, to specify injury tolerances to the low back, the analysis should extend to include the three-dimensional features of the spinal response.