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STUDY DESIGN: Human cadaveric cervical spines were subjected to nondestructive repeated-measures flexibility tests before, during, and after unilateral facet dislocation. OBJECTIVES: To assess the relative stability and kinematics of the spine in the normal condition, after unilateral facet lock without bony injury and after reducing (unlocking) the facet dislocation. SUMMARY OF BACKGROUND DATA: Experimental unilateral facet dislocation has been studied only when associated with extensive damage to disc, bone, or both. The relative stability of the normal, locked, and unlocked cervical spine has not been studied. METHODS: Unilateral facet dislocation with minimal associated injury was created by slowly applying axial torque while specimens were bent laterally and flexed. Before and after injury, nondestructive torques were applied about each anatomic axis, while three-dimensional specimen motion was recorded stereophotogrammetrically. RESULTS: Unilateral facet dislocation was created with a mean axial torque of 10.2 +/- 2.7 Nm. After locking, range of motion, lax zone, and stiff zone were significantly reduced relative to normal (P < 0.05). After unlocking, the same parameters were significantly increased relative to normal. The position of the axis of rotation shifted when the facet was locked and the centrode elongated after the facet was unlocked. CONCLUSIONS: Unilateral facet dislocation without fracture can be created with moderate loads when axial torque is applied while the facet is distracted. The spine is stable while the articular masses are locked unilaterally; however, the motion segment becomes overtly unstable after the facet dislocation is reduced.

The center of rotation of the spinal motion segment is an indicator of spinal instability. However, the motion segment does not move about a single fixed axis of rotation, but rather a locus of instantaneous axes of rotation, the centrode. This paper studies centrodes in various stages of degenerative disc disease comparing them with normal spines. Computer analysis is undertaken in 47 cadaveric spines, 22 of which also were evaluated with axial loading. The normal centrode fell within the posterior half of the disc space and averaged 21 mm in ten specimens. In the earliest stages of degenerative disc disease, the centrode lengths increased significantly (average 116 mm). Specimens with moderate disc degeneration also migrated inferiorly into the L5 vertebra. Axial loading did not appear to influence the centrode lengths or position. This technique is highly sensitive, detecting 94% of the abnormal spines as compared with only 25% detected by means of measuring an excessive range of motion on flexion and extension radiographs. This method provides a highly reliable and quantifiable method of detecting early changes in spinal motion in degenerative disc disease prior to the well recognized radiographic abnormalities.

Moiré fringes were used to determine centrode patterns in cadaveric spines with degenerative disc disease. The normal centrodes were compared with those with minor, mild, moderate, and severe degenerative disc disease. The most complex loci noted were those discs that showed minimal radiographic evidence of degeneration, that is, the minor and mild groups. The loci lengthened significantly (P less than 0.001) when compared with normal controls. The position of the centrode shifted downward into the body of L5 in the moderate group.

The kinematic technique of instant center analysis of the knee is described. Determination of the instant center or centrode permits identification of the type of motion of the joint surfaces.Twenty-five normal knees were studied and the centrodes were located for the range of motion between full ex

A cadaveric study provides a quantitative measurement of early degenerative disc disease before significant radiographic changes are present. Accurate assessment of spinal motion is hampered by the inaccessibility of the lumbar joints and because of the small range of motion at each motion segment. Using Moire fringes and, more recently, computerized digitization of spinal motion, the authors have developed a technique of measuring centers of rotation for small ranges of motion. The centers of rotation when joined form a locus that has characteristics that allow one to identify spines with degenerative disc disease. These loci, or centrodes, are longest in the earliest stages of degeneration, but maintain their length through moderate degenerative disc disease. Furthermore, radiographic changes consistent with moderate disc disease are associated with inferior migration of the centrode.