Experimental Characterization of the Anatomical Structures of the Lumbar Spine Under Dynamic Sagittal Bending.

Underbody blast (UBB) events transmit high-rate vertical loads through the seated occupant’s lumbar spine and have a high probability of inducing severe injury. While previous studies have characterized the lumbar spine under quasi-static loading, additional work should focus on the complex kinetic and kinematic response under high loading rates. To discern the biomechanical influence of the lumbar spine’s anatomical structures during dynamic loading, the axial force, flexion-extension moments and range of motion for lumbar motion segments (n=18) were measured during different states of progressive dissection. Pre-compression was applied using a static mass while dynamic bending was applied using an offset drop mass. Dynamic loading resulted in peak axial loads of 4,224±133 N, while maximum peak extension and flexion moments were 19.6±12.5 and -44.8±8.6 Nm in the pre-dissected state, respectively. Upon dissection, transection of the interspinous ligament, ligamentum flavum and facet capsules resulted in significantly larger flexion angles, while the removal of the posterior elements increased the total peak angular displacement in extension from 3.3±1.5 to 5.0±1.7 degrees (p=0.002). This study provides insight on the contribution of individual anatomical components on overall lumbar response under high-rate loading, as well as validation data for numerical models.

Material Parameter Determination of an L4-L5 Motion Segment Finite Element Model Under High Loading Rates.

Underbody blast (UBB) events impart vertical loads through a victim’s lumbar spine, resulting in fracture, paralysis, and disc rupture. Validated biofidelic lumbar models allow characterization of injury mechanisms and development of personal protective equipment. Previous studies have focused on lumbar mechanics under quasi-static loading. However, it is unclear how the role and response of individual spinal components of the lumbar spine change under dynamic loading. The present study leverages high-rate impacts of progressively dissected two-vertebra lumbar motion segments and Split-Hopkinson pressure bar tissue characterization to identify and validate material properties of a high-fidelity lumbar spine finite element model for UBB. The annulus fibrosus was modeled as a fiber-reinforced Mooney-Rivlin material, while ligaments were represented by nonlinear spring elements. Optimization and evaluation of material parameters was achieved by minimizing the root-mean-square (RMS) of compressive displacement and sagittal rotation for selected experimental conditions. Applying dynamic based material models and parameters resulted in a 0.42% difference between predicted and experiment axial compression during impact loading. This dynamically optimized lumbar model is suited for cross validation against whole-lumbar loading scenarios, and prediction of injury during UBB and other dynamic events.

Development and calibration of a load sensing cervical distractor capable of withstanding autoclave sterilization.

In surgery of the cervical spine, a Caspar pin distractor is often used to apply a tensile load to the spine in order to open up the disc space. This is often done in order to place a graft or other interbody fusion device in the spine. Ideally a tight interference fit is achieved. If the spine is over distracted, allowing for a large graft, there is an increased risk of subsidence into the endplate. If there is too little distraction, there is an increased risk of graft dislodgement or pseudoarthrosis. Generally, graft height is selected from preoperative measurements and observed distraction without knowing the intraoperative compressive load. This device was designed to give the surgeon an assessment of this applied load. Instrumentation of the device involved the application of strain gauges and the selection of materials that would survive standard autoclave sterilization. The device was calibrated, sterilized and once again calibrated to demonstrate its suitability for surgical use. Results demonstrate excellent linearity in the calibration, and no difference was detected in the pre- and post-sterilization calibrations.

Strain-rate dependent material properties of the porcine and human kidney capsule.

This study was performed to characterize the mechanical properties of the kidney capsular membrane at strain-rates associated with blunt abdominal trauma. Uniaxial quasi-static and dynamic tensile experiments were performed on fresh, unfrozen porcine and human renal capsules at deformation rates ranging from 0.0001 to 7 m/s (strain-rates of 0.005-250 s(-1)). Single stroke, dynamic tests were performed on samples of porcine renal capsule at strain-rates of 0.005 s(-1) (n = 33), 0.05 s(-1) (n = 17), 0.5 s(-1) (n = 38), 2 s(-1) (n = 10), 4 s(-1) (n = 10), 50 s(-1) (n = 21), 100 s(-1) (n = 18), 150 s(-1) (n = 17), 200 s(-1) (n = 10), and 250 s(-1) (n = 17). Due to limited availability of human tissues, only quasi-static tests were performed (0.005 s(-1), n = 25). Porcine renal capsule properties were found to match the material properties of human capsular tissue sufficiently well such that porcine tissue material can be used as a human test surrogate. The apparent elastic modulus and breaking stress of the porcine renal capsule were observed to increase significantly with increasing strain-rate (p < 0.01). Breaking strain was inversely related to strain-rate (p < 0.01). The effect of increasing strain-rate on material properties diminished appreciably at rates exceeding 150 s(-1). Empirically derived mathematical models of constitutive behavior were developed using a hyperelastic/viscoelastic Ogden formulation, as well as a Cowper-Symonds law material curve multiplication.

The relationship between loading conditions and fracture patterns of the proximal femur.

In an attempt to test the hypothesis of spontaneous hip fracture, seven pairs of femurs, with ages ranging from 59 to 90, were tested under two loading conditions designed to simulate muscular contraction. Simulated iliopsoas contraction produced femoral neck fractures at an average normalized ultimate load of 5.2 +/- 0.8 times body weight. Simulated gluteus medius contraction produced sub-/inter-trochanteric fractures at an average normalized ultimate load of 4.1 +/- 0.6 times body weight. The average ultimate load for all specimens was 3040 +/- 720 N. Fracture patterns produced by both loading conditions were clinically relevant. The results from this study suggest that abnormal contraction produced by major rotator muscles could induce hip fracture.