Pedicle or lateral mass screws in Goel-Harms construct? A biomechanical analysis.

BACKGROUND

The use of the posterior arch of C1 as pedicle has shown beneficial stability regarding screw loosening, however, the C1 pedicle screw placement is challenging. Therefore, the study aimed to analyse the bending forces of the Harms construct used in fixation of C1/C2 when using pedicle screws compared to lateral mass screws.

METHODS

Five cadaveric specimens with a mean age of 72 years at death and bone mineral density measuring for 512.4 Hounsfield Units (HU) on average were used. A custom-made biomechanical setup was used to test the specimens with a C1/C2 Harms construct each with the use of lateral mass screws and pedicle screws in sequence. Strain gauges were used to analyse the bending forces from C1 to C2 in cyclic axial compression (μm/m). All underwent cyclic biomechanical testing using 50, 75 and 100 N.

FINDINGS

In all specimens, placement of lateral mass screws and pedicle screws was feasible. All underwent cyclic biomechanical testing. For the lateral mass screw, a bending of 142.04 µm/m at 50 N, 166.56 µm/m at 75 N and 188.54 µm/m at 100 N was measured. For the pedicle screws, bending force was slightly elevated with 165.98 µm/m at 50 N, 190.58 µm/m at 75 N and 195.95 µm/m at 100 N. However, bending forces did not vary significantly. In all measurements, no statistical significance was found when comparing pedicle screws and lateral mass screws.

INTERPRETATION

The lateral mass screw used in the Harms Construct to stabilize C1/2 showed less bending forces, therefore the construct with lateral mass screws appears more stable in axial compression compared to the one with pedicle screws. However, bending forces did not vary significantly.