A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane

Study Design. An in vitro biomechanical study of lumbar spine segments. Objective. To study the characteristics of the stiffness test protocol. Summary of Background Data. In an in vitro study using a flexibility protoc ol, forces are applied and motions are measured; no center of rotation need s to be specified. In a study using a stiffness protocol, the forces are me asured and the motions are applied. This does require the center of rotatio n to be specified. Many biomechanical studies of the spine are available, b ut there is lack of clarity concerning which of these two test protocols is appropriate to achieve a certain study goal. Methods. Five-vertebrae lumbar spine specimens with burst fractures in the middle vertebrae (L1) were used. Specially designed apparatus applied flexi on and extension rotations around five centers of rotations located on ante roposterior line through the middle of L1. Maximum moment of 4 Nm was appli ed. Results. The authors found load-displacement curves, ranges of motion, and neutral zones obtained at the five centers of rotations to be markedly diff erent. The center of rotation located at the posterior longitudinal ligamen t produced large range of motion and neutral zones in comparison to the cen ters of rotation located at the anterior longitudinal ligament and the spin ous process tip (P < 0.01). Conclusions. The stiffness protocol requires that a center of rotation be s pecified. Shown here is the significant variability in the load-displacemen t curves, depending on the choice of the location of the center of relation . Certain center of rotation locations may block the natural motions of tbe spine and may result in tissue damage.