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A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane
M M Panjabi1, Y Kato, H Hoffman
1Biomechanics Laboratory, Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut 06520-8071, USA.
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 protocol, forces are applied and motions are measured; no center of rotation needs to be specified. In a study using a stiffness protocol, the forces are measured and the motions are applied. This does require the center of rotation to be specified. Many biomechanical studies of the spine are available, but 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 flexion and extension rotations around five centers of rotations located on anteroposterior line through the middle of L1. Maximum moment of 4 Nm was applied.
Results:
The authors found load-displacement curves, ranges of motion, and neutral zones obtained at the five centers of rotations to be markedly different. The center of rotation located at the posterior longitudinal ligament produced large range of motion and neutral zones in comparison to the centers of rotation located at the anterior longitudinal ligament and the spinous process tip (P<0.01).
Conclusions:
The stiffness protocol requires that a center of rotation be specified. Shown here is the significant variability in the load-displacement curves, depending on the choice of the location of the center of rotation. Certain center of rotation locations may block the natural motions of the spine and may result in tissue damage.