This study examined the torsional and shear stiffness of Type II odontoid fractures that were stabilized with two 3.5-mm cannulated AO screws or a single 4.5-mm cannulated Herbert screw.
To determine the stability of a commonly used two-screw method versus a single-screw fixation method, Insertion of a single screw could make the procedure technically easier.
Biomechanical studies have shown the 4.5-mm Herbert screw to generate greater compressive forces than 3.5-mm cannulated screws, which are commonly 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.
Twelve cadaver C2 vertebrae were harvested and an osteotomy was performed to simulate a Type II fracture pattern. Six were stabilized with two 3.5-mm cannulated screws, and the remaining were stabilized with a single 4.5-mm cannulated Herbert screw. Each specimen then was tested in torsion through ±0.75°, ±1.25°, and ±1.75° for 10 cycles each. Shear forces then were applied from an anterior to posterior direction to a maximum load of 44.48 N.
The mean torsional stiffness for the Herbert screw was 1196 N-m/deg, which was significantly greater than the mean stiffness of the 3.5-mm screw fixation, which measured 434 N-m/deg. The mean shear stiffness for the Herbert screw fixation was 105.9 kN/m, compared to 86.1 kN/m for the 3.5-mm cannulated screw. This was not statistically significant.
The biomechanical properties of the 4.5-mm cannulated Herbert screw suggest it may lend itself for use in fixation of Type II odontoid fractures. With only a single screw to insert, this technically demanding procedure may be less daunting.
Department of Orthopaedic Surgery, Louisiana State University Medical Center, Shreveport, Louisiana.