Study Design.
The stability of motion segments of human cervical spines was sequentially tested as portions of the vertebral anatomy were removed or cut. Isolated, individual facet joints were then similarly studied.
Objectives.
To define the laxity of isolated cervical facet joints and the relative contribution of the different components of the vertebral anatomy to the overall stability of the cervical spine.
Summary of Background Data.
Facet joints are known to be important in determining cervical stiffness and mobility. This is the first known study in which the biomechanical behavior of isolated cervical facet joints has been documented.
Methods.
From five fresh frozen human cervical spines, three C3-C4 and five C5-C6 motion segments were dissected and potted. Rotations and translations in response to 10 bending or twisting moments were recorded by tracking the motion of a testing plate fixed to the superior vertebrae using an articulated arm digitizer. Each motion segment was tested five times, with sequential dissections performed as follows: intact; after removal of the anterior longitudinal ligament, intervertebral disc, and posterior longitudinal ligament; after cutting the interspinous ligament; after isolation of the left facet joint; and after isolation of the right facet joint. Each testing sequence involved applying low and high forces 10 cm from the center of the testing plate in each of 10 testing directions. After completion of rotational testing, landmarks on the superior vertebral body and facet joints were digitized to calculate vertebral translations.
Results.
Isolated facet joints allowed up to 19° of flexion, 14° of extension, 28° of lateral bending, and 17° of rotation. Coupled motions were less in isolated facet joints compared with those in intact vertebral bodies. Isolated facet joints allowed up to 9 mm of translation between superior and inferior surfaces.
Conclusions.
Isolated cervical facet joints are highly mobile in comparison with their motions within the constraints of intact motion segments; gliding motions of the isolated facet to near dislocation is possible before the facet capsule constrains motion. Cervical coupled motions are a result of an intact vertebral ring and a combination of the two facet joints. The vertebral ring with facet joints and capsules all intact is necessary for lateral bending stability and rotational stability in the cervical spine.
