The effects of spinal fusion on fused segment and the adjacent, unfused segments play a significant role in the clinical effectiveness of spinal fusion for low-back pain with or without sciatica. Much of the information on this important subject is derived from clinical impressions. The purpose of this biomechanical study is to investigate the altered kinematics and biomechanics of the three different types of spinal fusion (posterior, bilateral-lateral, and anterior) on the adjacent, unfused segment as well as within the fused segment and to investigate their clinical implications. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The test specimens included three motion segments, L3-4, L4-5, and L5-S1. To study the mechanics of the lumbar spine under combined compression and bending loads, a special apparatus was designed. These loads were applied by an MTS machine through two sets of pulley systems. The loads, as well as displacement data from both actuators, were recorded. A video camera system was utilized to record the kinematics of the spinal motion segment. The unfused specimen was tested first, and the fused specimen then was retested under the identical loading conditions. A total of 16 spine specimens were tested and evaluated--five posterior, four bilateral-lateral, and seven anterior fusions. All types of fusion resulted in increased bending and axial stiffnesses. Overall, anterior fusion provided the largest increase in stiffness, followed by bilateral-lateral fusion and posterior fusion.(ABSTRACT TRUNCATED AT 250 WORDS)