The variable screw placement spinal fixation system : report of a prospective study of 250 patients enrolled in food and drug administration clinical trials

The variable screw placement (VSP) system uses transpedicular screws and slotted plates to achieve constrained segmental fixation of the spine. A prospective multicenter study of 250 patients was performed under a Food and Drug Administration approved protocol to test the safety and effectiveness of this system. Complete 2-year follow-up data are reported for 169 of 223 (73%) patients who had fixation of three spinal levels or fewer. "Clinical success" was defined as an excellent or good result or improvement of two grades in a 10-point combined pain and function scale. "Fusion success" was defined by presence of continuous trabecular bone bridging the fusion area. For 86 patients with postsurgical failed back syndrome, clinical success was achieved in 69 (80.2%) and fusion success in 76 of 83 (91.6%) patients. These success rates were statistically higher than previous literature reports (P = 0.01 for clinical success and P = 0.02 for fusion success). For spondylolisthesis, clinical success was achieved in 44 of 57 patients (86.3%) and fusion success in 43 of 47 (91.5%) patients. For spinal stenosis, clinical success was achieved in 25 of 32 patients (78.1%) and fusion success in 30 of 31 (96.8%) patients. Overall fusion success was achieved in 186 of 200 (93%) patients with no statistical difference when comparing the numbers of levels fused. There were 21 device-related complications (8.4%) and 16 non-device-related complications (6.4%).