METHODS Lateral dual-energy x-ray absorptiometry was used to examine isolated changes in vertebral body mineral density in humans after instrumented posterolateral lumbar fusion. OBJECTIVE To determine if device-related osteopenia will occur in humans who undergo spinal fusion. Device-related osteopenia is known to occur as a result of local stress shielding after instrumentation in the appendicular skeleton. This effect has not been observed, however, in humans after spine fusion. To evaluate such changes, the vertebral body mineral density was measured in eight patients who had instrumented lumbar fusion and in eight matched control patients who had lumbar surgery with no fusion. BACKGROUND In previous studies of dogs, vertebral body osteopenia occurred as a result of instrumented spine fusions. Previous studies in humans, however, have been limited by the relative insensitivity of conventional photon absorptiometry to isolated changes in the vertebral body because of overlying posterior elements. METHODS Absorptiometry was performed an average of 31.9 months after posterolateral fusion that bridged at least one level in the region of L2-L4. To reduce the effects of individual variations in mineral metabolism, the vertebral values were standardized by using the ratio of vertebral body to femoral neck density for each patient. RESULTS The mean density ratio for the group of patients who underwent spine fusion was 0.733. This value was significantly lower than the control ratio of 0.879 (P = 0.048). CONCLUSIONS Patients who have undergone instrumented posterolateral lumbar fusions have decreased vertebral body bone mineral density at the level of fusion compared with that of matched controls.