In vitro lumbar spine testing with simulated muscular preloads

Lower back pain is a significant disability in the United States and worldwide. It is estimated that 70-80% of people will experience some type of lower back pain at some point in their lives. Annually over one million people are hospitalized with back conditions. Intervertebral disc disorders account for one third of the cases. The most common injury involving the intervertebral disc (IVD) is a disc herniation. Since the IVD will not naturally regenerate due to the low blood supply, surgery is the most common treatment. Laminectomy, spinal fusion, and disc replacements are the typical surgical techniques to alleviate pain from disc herniations. Several studies have identified a need to develop in vitro studies for multiple motion segment spinal testing. There is a clinical need to evaluate current surgical instrumentation devices. Multiple motion segment cadaver specimens tend to fail in buckling prior to in vivo loading conditions. Lack of surrounding musculature, which stabilizes the spine and provides stiffness, has been identified as the cause of the increased buckling. Patwardhan has developed a follower load method that applies the vertical load along the center of rotation to minimize internal shear forces and bending moments loading the specimen purely in compression. Although the follower load provides stiffness to the spine, the anatomical evidence supporting the theory has been in question. This study used anthropometric data and data from in vitro and in vivo tests to develop biomechanical models that simulate the pressure within the IVD and force exerted by surrounding stabilizing musculature. Calculated pressure within the IVD between the L3 and L4 vertebrae was 0.39 MPa, 1.91 MPa, and 1.05 MPa for standing, picking up a 20 kg box, and standing while holding a 20 kg box respectively. In vivo testing in a clinical study by Wilke et al. found IVD pressures of 0.50 MPa, 2.30 MPa, and 1.0 MPa for the particular cases. The erector spinae was identified as the primary muscle group resisting flexion up the upper torso. Recent in vitro and in vivo studies involve the use of sheep cadavers for anatomical and biomechanical comparison to humans for different regions of the spine. An in vitro study was performed using the Purdue University Spine Simulator, a five axis degree of freedom hydraulic system previously validated for spine testing. Lumbar spines from 3 sheep that were divided into L2/L5 motion segments. Tests were performed simulating erector spinae muscle activation and anatomically correct vertical preloads. The 44.28 in-lb flexion/extension bending moment was applied for 13 cycles at 10 second periods, with the first 3 cycles preconditioning the specimens. Stiffness calculations for the lumbar specimens with and without muscle loads were between 250-300 in/°. Stiffness calculations without vertical preloads were less than 250-300 in/°. The results did not show that simulated posterior muscle forces had significantly affected the stiffness of the specimens. However, the Purdue University Spine Simulator was successful in simulating muscle loads along with testing a multi-motion segment lumbar sheep spine. Higher vertical preloads were found to significantly increase the stiffness of the spines