Objectives To evaluate the effect of sagittal fixation angle on the stresses of adjacent segments in lumbar fusion in vitro.Methods Lumbar spine(L1-S2) specimens from nine fresh human cadavers were used.After intact tests of the original normal spines,fixations with three different sagittal angles(1) in situ(lordosis 10 degrees),(2) kyphotic(kyphosis 5 degrees) and(3) hyper-lordotic(lordosis 25 degrees) at L3,4 and 5 were randomly conducted using pedicle screws on each specimen.Subsequently,biomechanical tests were performed under flexion,extension and lateral bending modes.Stresses of the facet joints and intervertebral disks at L2/3 and L5/S1 were measured and compared.Results In flexion tests,all the three fixation strategies resulted in higher stresses within the adjacent segments than that of the intact spine,the highest being the kyphotic fixation group.In extension tests,stresses of the kyphotic group were significantly higher than that of the other two fixation groups.However,there was no statistical difference among the in situ and hyper-lordotic fixation group.The results of bilateral bending were similar to that in extension.Conclusions Spinal fixation can cause stress increase within adjacent segments.Lumbar fixations with abnormal angles may further deteriorate the associated biomechanical condition.Therefore,for lumbar reconstruction,it is crucial to keep the lumbar alignment to its normal orientation.