The spine is the most complex mechanical system in the human body. With the University of Minnesota in Minneapolis, Instron (Norwood, MA) is utilizing the BioPuls™ Multi-Axial Spine Testing System to characterize the multi-axial dynamism of the backbone. Such testing will help further the understanding of spine kinetics, simulating precisely the complex, coupled motions typical of the spine and impact on spinal procedures. The system can be used to help evaluate the design of new instruments and implants that allow a greater physiological range of motion.
The system, which enables six degrees of freedom motion, uses a “free-end model” approach to the testing of spinal segments and to calculate the finite helical axis of rotation (FHA). Vertebrae harvested from a pig, sheep, or human cadaver are loaded into the device and then subjected to various forces and movements. A set of specimen cups holds the test “segment” (two vertebrae with a disc in between) during testing, and the system crosshead can be adjusted vertically using hydraulic lifts to accommodate different specimen sizes and different fixturing. A flexion head controls the three rotational moments of torsion, lateral bending, and flexion-extension. Dynamism is applied at the top on the specimen by a flexion head. The lower end, where a six-axis load cell is used to ensure that the demand input loads and moments are met, is monitored for reaction. The centers of rotation are located along the length of the specimen and as a result, the application of pure flexion-extension and lateral bending movement can be made without inducing excessive shear forces.
The unit tests for axial compression, axial torsion, flexion/extension, lateral bend, anterior/posterior shear, and lateral sheer, mimicking the full range of motion of which the spine is capable. The main goal of the device is to evaluate fixation (such as fusion or a spinal plate) and disc replacement, although information about other spinal conditions, such as scoliosis, can also be gathered.
A common experiment would be to test a segment with its original disc intact, and then replace the disc with an artificial one typical in disc replacement surgery, and then test the segment again in the same protocol. The two sets of data would then be compared, using programs such as Excel or Metlab, in the hopes of creating more effective therapies. That information can be bolstered using conventional methods such as X-rays or MRI scans.
Additionally, the BioPuls can be used for adjacent mechanical studies; how the altering of one part of the spine impacts the rest of the system. While a full spine can be fitted into the BioPuls, five or less segments are used for effective testing, with multiple configurations allowing physiologically relevant results.