Exponent’s capabilities include a state-of-the-art spinal loading simulator (Figure 1), used to study the three-dimensional load-displacement characteristics of in vitro human spines. The loading frame can accommodate spinal motion segments of full intact human cervical, thoracic, or lumbosacral spine segments. This spine simulator is capable of transmitting pure bending about a single axis, or combined loading about multiple axes at quasistatic to dynamic rates, replicating typical to extreme physiologic loading of the spine by imparting pure bending moments along the spine’s three orthogonal axes. The caudal end of the spine specimen is rigidly fixed to a six-axis load cell and collects force and moment data transmitted through the specimen. The simulator’s loading profile is computer controlled, with feedback from electric encoders on the motors that can deliver a maximum torque to the spine of up to 64-Nm about all axes.
Data Acquisition and Processing
Segmental kinematic data are collected with a Vicon infrared camera system. The optical motion capture system includes four cameras positioned around the specimen to track a set of reflective markers that are rigidly attached to each vertebral body. Vicon BodyBuilder™ software is used to compute relative angles between the vertebrae and is capable of computing translations of each vertebral body as well. Load-cell data are acquired with a high-speed multifunction data acquisition board. Data acquisition is controlled by a custom-designed LabVIEW™ virtual instrument (VI). The kinematic and kinetic data are combined to produce load-displacement curves (Figure 2).
Figure 1. Cadaveric lumbar spine specimen
mounted in Spine Simulator
Figure 2. Sample kinematic signatures of natural, fused, and
dynamically-stabilized lumbar spine