OBJECTIVESTo compare the stability of an enhanced load sharing dynamic pedicle screw fixation device with its equivalent rigid device and to evaluate biomechanical roles of the dynamic fixation.

METHODSA model of L(1) body fracture was produced on seven specimens of fresh adult cadaver spine from T(10) to L(4). Both dynamic and rigid devices were applied in the specimens to strength the injured level. Ranges of three dimensional movements and stiffness under flexion-compression were measured in intact, injured and stabilized specimens.

RESULTSBoth dynamic and rigid devices were found to provide significant stability for injured segment in flexion-extension and lateral bending. In axial rotation, the devices could restore the stability to levels similar to those in an intact spine. Results indicated 40% increase in range of motion in flexion-extension and 24.1 Nmm reduction in stiffness of flexion-compression for dynamic device, compared with the rigid device.

CONCLUSIONThe dynamic device offers a design that may enhance load sharing without sacrificing the stability and will decrease stress-shielding and stress concentration.

Biomechanical Phenomena ; Bone Screws ; Fracture Fixation, Internal ; instrumentation ; Humans ; Male ; Spinal Injuries ; surgery

Objective:To develop a specialized clival-cervical plate fixation (CCPF) for anterior surgery to treat craniovertebral instability, and to compare it with a posterior occipitocervical fixation (POCF) in biomechanical validation.Methods:Based on the measurement of 40 adult dry bones and 30 volunteers CT images, the clival-cervical plate was designed and manufactured. 8 cadaveric specimens (occiput-C 3) were tested in five conditions including the intact status, the intact+CCPF status, the injury status, the injury+CCPF status, and the injury+POCF status. Specimens were applied a pure moment of 1.5 N·m in flexion, extension, lateral bending, and axial rotation. Calculating and comparing the range of motion (ROM) and neutral zone (NZ) for the occiput to C 2. The effects of different fixation methods on the distribution of ROMs at the occipitocervical region were compared. Results:The injury+CCPF status constrained ROMs to 1.7° in flexion ( q=4.68, P=0.055) , 1.2° in extension ( q=0.39, P=0.9922) , 2.8° in lateral bending ( q=1.25, P=0.814) , and 4.3° in axial rotation ( q=5.08, P=0.035) , resulted in larger ROM in axial rotation but similar ROMs in other directions ( P>0.05) when compared with the injury+POCF status. There were no significant differences between the above two fixation methods in flexion-extension ( q=1.94, P=0.554) , lateral bending ( q=1.79, P=0.611) and axial rotation ( q=2.14, P=0.478) for the NZs. For the flexion, extension,lateral bendingand axial rotation direction, the proportion of the C 1, 2 ROM to the overall ROM was 28%, 25%, 34% and 56% respectively in the injury+CCPF status, and it was 59%, 53%, 42% and 71% respectively in the injury+POCF status. Conclusion:CCPF is a biomechanically effective alternative or supplemental method of POCF for the craniocervical instability.