Investigators: Scott A. Yerby, PhD and Eugene J. Carragee, MD Project Staff: Janet Y. Sun, BS and Daniel Kim, MD Project Categories: Spinal Cord Injury / Osteoporosis - 2000 Pedicle screw systems are widely used to stabilize the posterior spine in the case of acute injury, congenital deformity or disease. It is not uncommon, however, for these systems to fail at the bone-implant interface. The goal of this study is to investigate the effect of vertebral bone mineral density on implant life (cycles to failure) for a given loading amplitude in an in vitro model. Seven fresh human cadaver spine segments (T11-L3) were procured and stored at -20 deg C until tested. On the day of testing, each specimen was scanned laterally using a dual energy x-ray absorptiometry (DXA) bone densitometer. Bone mineral density (BMD) of the T12 and L2 vertebral bodies were measured in the sagittal plane using a lumbar spine analysis protocol. Following scanning, each specimen was instrumented with 7 mm x 45 mm pedicle screws and 6.5 mm rods at T12 and L2. A vertebral body corpectomy was created at the L1 level by removing the L1 vertebral body, while maintaining the thin posterior cortex and all of the posterior elements. The T11 and L3 vertebrae were secured with PMMA and each specimen was mounted in a materials testing machine and loaded from the neutral, unloaded position to 7.5 Nm of flexion under load control at 0.5 Hz. Pedicle screw angles were measured by mounting an extension arm from the posterior end of each pedicle screw to a position parallel to the pedicle and 1 cm lateral. There continues to be a lack of meaningful predictors for the risk of spine fixation failure. Most assessments of the holding potential of spinal implants include pullout force and insertional torque, which cannot be determined preoperatively. However, bone mineral density (BMD), assessed through dual energy x-ray absorptiometry can be determined prior to surgery. The current study describes the relationship between BMD and cycles to failure in a corpectomy at a given load. For this given load (7.5 Nm), we have established a strong, non-linear relationship between the two parameters. In addition, we have established a threshold BMD value of 0.50 g/cm2, above which a large number of cycles is needed to produce substantial screw toggle, and below which the construct fails in less than 50,000 cycles. This threshold value may be useful to surgeons when preoperatively planning surgeries for patients with poor bone quality, and may indicate that supplemental fixation is required. Publications: Sun J, Yerby SA, Carragee E, Kim D: Cyclic Failure of Multi-Segment Pedicle Screw Systems. Trans Orthop Res Soc 25:270, 2000. Funding Source: North American Spine Society |
