ANTERIOR SCREW FIXATION OF TYPE-II ODONTOID FRACTURES - A BIOMECHANICAL STUDY

Study Design. This study examined the torsional and shear stiffness of Type II odontoid fractures that were stabilized with two 3.5-mm cannu lated AO screws or a single 4.5-mm cannulated Herbert screw. Objective . To determine the stability of a commonly used two-screw method versu s a single-screw fixation method. Insertion of a single screw could ma ke the procedure technically easier. Summary of Background Data. Biome chanical studies have shown the 4.5-mm Herbert screw to generate great er compressive forces than 3.5-mm cannulated screws, which are commonl y used to anteriorly stabilize Type II odontoid fractures. No previous biomechanical studies have compared the shear or torsional stiffness of Type II odontoid fractures stabilized with these screws. Methods. T welve cadaver C2 vertebrae were harvested and an osteotomy was perform ed to simulate a Type II fracture pattern. Six were stabilized with tw o 3.5-mm cannulated screws, and the remaining were stabilized with a s ingle 4.5-mm cannulated Herbert screw. Each specimen then was tested i n torsion through +/-0.75 degrees, +/-1.25 degrees, and +/-1.75 degree s for 10 cycles each. Shear forces then were applied from an anterior to posterior direction to a maximum load of 44.48 N. Results. The mean torsional stiffness for the Herbert screw was 1196 N . m/deg, which w as significantly greater than the mean stiffness of the 3.5-mm screw f ixation, which measured 434 N . m/deg. The mean shear stiffness for th e Herbert screw fixation was 106.9 kN/m, compared to 86.1 kN/m for the 3.5-mm cannulated screw. This was not statistically significant. Conc lusions. The biomechanical properties of the 4.5-mm cannulated Herbert screw suggest it may lend ;itself for use in fixation of Type II odon toid fractures. With only a single screw to insert, this technically d emanding procedure may be less daunting.