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.