The spondylolytic vertebra and its adjacent segment. Mobility measured before and after posterolateral fusion

STUDY DESIGN: By using roentgen stereophotogrammetric analysis in six patients having tantalum indicators implanted at a preoperative external fixation test, the mobility in the spondylolytic lumbosacral level and its adjacent segment could be studied before fusion and during the course of postoperative fusion consolidation. OBJECTIVE: To study the mobility effects on the segment adjacent to a lumbar fusion over time from the preoperative situation until fusion healing as defined by roentgen stereophotogrammetric analysis. SUMMARY OF BACKGROUND DATA: In vitro studies indicate that the altered biomechanical situation after lumbar fusion increases the intradiscal pressure and changes the kinematics in the juxtafused segment. METHODS: Six patients with low grade spondylolysisolisthesis were scheduled for fusion of the spondylolytic lumbosacral segment after a preoperative external fixation test. The latter procedure also included implantation of tantalum markers for spinal roentgen stereophotogrammetric analysis. Each patient was examined by roentgen stereophotogrammetric analysis at four separate occasions: before fusion (2 months after removal of the external frame) and 3, 6, and 12 months after surgery. The translatory movements of the L5 vertebra in relation to sacrum and of the L4 vertebra in relation to the L5 vertebra were calculated at each examination. RESULTS: For the juxtafused L4-L5 level, increased and decreased mobility patterns could be identified. Transformation of the preoperative mobility in the lumbosacral segment to the adjacent segment during fusion consolidation was verified in two patients but was not a general phenomenon. CONCLUSION: Fusion of the lumbosacral segment can alter the kinematics of the adjacent segment, redistributing the mobility toward relative hypermobility in the juxtafused segment.