Dynamic biochemical comparison of single versus dual threaded pedicle screws.

摘要

Under imposed stress/strain, an anisotropic orientation of the bone is generated so as to minimize the loading effects. Bone may be classified into two main categories. Cancellous or soft bone may be found in the epiphysial regions of long bones and within the main body of vertebrae. Cortical or hard bone is located within the diaphysial or midshaft region of long bones and in the outer shell of the vertebral body. Though in use since the 1960's it was not until the 1980s that three-dimensional instrumentation such as screws, rods, and hooks were manufactured to enable specific contouring of spinal instrumentation systems for deformity correction and stability. The use of pedicle screws forms the anchor for these spinal constructs. Until recently, these screws were of a single thread. The advantages associated with the development of dual threaded screws are that threads may be optimized for the engagement of both cortical and cancellous bone. It was hypothesized that dual threaded screws could provide mechanical performance via reduced deflection as compared to single threaded screws. While simple axial pullout is a material and current clinical standard for evaluation of pedicle screw design, it is unsuitable for mechanical evaluation of performance as bone fails in shear. Such a mechanism of failure makes such a method for evaluation insensitive to design features. As well, such a failure mechanism is not documented clinically. Dynamic toggle has been applied to vertebral bodies in a manner that continuously records deformation and load data. Using nonlinear analysis of the deformation versus cycle number, characteristic curves representative of screw performance were computed and displayed statistically significant differences between dual threaded pedicle screws and single threaded screws from various manufacturers. The mathematical representation of performance under cyclic loading provided an unambiguous measure of the interaction between the screw, bone and rod connector. The series of experiments not only characterized the performance of various commercial pedicle screws but has generated a useful mathematical method for determination of screw performance under clinically relevant loading conditions.