Sacral screw loads in lumbosacral fixation for spinal deformity.

Fixation to the sacrum and pelvis is a problem in the operative treatment of spinal deformity. Previous testing of pedicle screws address axial pull-out strength, yet how screws are loaded in vivo remains unknown. The goals of this study were to determine the loads experienced by sacral screws when loaded as part of Cotrel-Dubousset (CD) sacral instrumentation and whether different anterior grafting methods would effect screw loads. Sacral screws were modified to become transducers capable of measuring axial and bending loads. The screw transducers were incorporated into the sacral fixation of CD instrumentation in seven calf spines. Specimens were loaded to simulate flexion. The sacral screws carried axial loads (1.1 N/[Nm of load]) and bending moments (1.1 Nm/[Nm of load]). The results suggest bending of the sacral screws may be important in their failure, and screw loading was not dependent on graft types used.