A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane.
METHODS
An in vitro biomechanical study of lumbar spine segments.


OBJECTIVE
To study the characteristics of the stiffness test protocol.


BACKGROUND
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.
Panjabi
M M
MM
Biomechanics Laboratory, Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut 06520-8071, USA.
Kato
Y
Y
Hoffman
H
H
Cholewicki
J
J
Krag
M
M
D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D006801:Humans; D066298:In Vitro Techniques; D017846:Longitudinal Ligaments / Q000502:physiology; D008159:Lumbar Vertebrae / Q000293:injuries*; D008875:Middle Aged; D016059:Range of Motion, Articular; D012399:Rotation; D016103:Spinal Fractures / Q000503:physiopathology*; D014102:Torsion Abnormality; D016474:Weight-Bearing / Q000502:physiology*