A MOTION ANALYSIS OF THE CERVICAL FACET JOINT

Study Design; The stability of motion segments of human cervical spine s was sequentially tested as portions of the vertebral anatomy were re moved or cut. isolated individual facet joints were then similarly stu died. Objectives. Tb define the:laxity of isolated cervical facet join ts and the relative contribution:of the different components of the ve rtebral anatomy to the overall stability of the cervical spine. Summar y of Background Data. Facet joints are known to be important in determ ining 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 sp ines, three C3-C4 and five C5-C6 motion segments were dissected and po tted. Rotations and:translations in: response to 10 bending or twistin g moments were recorded by tracking the motion of a testing plate fixe d to the superior vertebrae using an articulated arm digitizer. Each m otion segment was tested five times, with sequential dissections perfo rmed as follows intact; after removal of the anterior longitudinal lig ament; intervertebral disc and posterior longitudinal ligament after c utting the interspinous ligament after isolation of the left facet joi nt and after isolation of the right facet joint. Each: testing sequenc e:involved applying low and high forces 10 cm from the center of the t esting plate in each of 10 testing directions. After completion of rot ational testing, landmarks on the superior vertebral body and facet jo ints were digitized to calculate vertebral translations. Results. isol ated facet joints allowed up to 19 degrees of flexion, 14 degrees of e xtension, 28 degrees of lateral bending; and 17 degrees of rotation. C oupled motions were less in isolated facet joints compared with those in intact vertebral bodies. Isolated facet joints allowed up to 9 mm o f translation between-superior and inferior surfaces. Conclusions. iso lated cervical facet joints are highly ; mobile in::comparison with-th eir motions:within the constraints of intact motion segments; gliding motions bf the isolated facet to near dislocation is possible before t he-facet capsule-constrains motion; Cervical coupled motions are a;res ult of an intact vertebral ring and a combination of the-two facet joi nts. the vertebral ring with facet joints and capsules all intact is n ecessary for lateral bending Stability and:rotational stability in the cervical spine.