An experimental study of a combination method using a pedicle screw and laminar hook for the osteoporotic spine

Study Design. Using human cadaver spines, the authors compared the effect of using a combination of pedicle screw and laminar hook on the same vertebra with that of using a pedicle screw alone in reference to bone mineral density of the vertebra under nondestructive cyclic loading. Objectives. To quantify stiffness obtained by pedicle screw alone and by the combination method and to clarify a relationship between stiffness obtained by each instrumentation method and bone mineral density of the vertebra. Summary of Background Data. The use of pedicle screws apparently improves the union rate of spinal fusion. Instrumentation failures sometimes occur, however, such as loosening or loss of correction of the spine, especially in patients with osteoporosis. Some augmentation method in instrumentation is necessary to overcome bone fragility in the osteoporotic spine. Methods. Thirteen cadaver lumbar vertebrae were used for this study. Bone mineral density was measured by dual-energy X-ray absorptiometer. After separating each vertebrae, the pedicle screw was screwed into a vertebra. Five cycles of cephalocaudal loading were per formed to the shank of the screw with a cross-head speed of 3 mm/min under a maximum load control of 29.4 N using an Instron type testing machine, and the stiffness obtained with the pedicle screw (Kj) was calculated from the load-deformation curve. Then, a laminar hook was set and connected to the screw via a rod. Mechanical testing was performed in the same way, and the stiffness obtained with the combination method (Kf) was determined. Kj and Kf were compared using the paired t test. The relationship between Kj, Kf, or the stiffness improvement ratio ([Kf - Kj]/Kj) by the combination method and bone mineral density was analyzed by linear regression analysis. Results. Stiffness obtained by the combination method was significantly greater than that obtained by pedicle screw alone (89.8 +/- 35.0 N/mm by the combination method, 60.2 +/- 19.6 N/mm by pedicle screwing alone; P < 0.0001). Stiffness, whether obtained by pedicle screw alone or by the combination method, was positively correlated with bone mineral density (with pedicle screw alone, R-2 = 0.614, P<0.0001; with the combination method, R-2 = 0.645, P<0.0001). However, there was no significant correlation between stiffness improvement ratio and bone mineral density. Conclusion. Instrumentation stiffness obtained by the combination method was significantly greater than that obtained by the use of pedicle screw alone. There was no significant correlation between the improvement ratio by the combination method and bone mineral density. These results suggest that the combination method is valuable irrespective of the presence of spinal osteoporosis.