A biomechanical evaluation of three revision screw strategies for failed lateral mass fixation.

STUDY DESIGN

This is a biomechanical study evaluating 3 revision strategies for failed cervical lateral mass screw fixation.

OBJECTIVE

Our primary objective was to compare, following a Magerl trajectory screw failure in the subaxial cervical spine, the pullout strength of (1) a revision screw in the same trajectory, (2) a Roy-Camille trajectory, and (3) pedicle screw fixation. We additionally analyzed the contributions of bone mineral density (BMD) and peak insertional torque to pullout strength.

SUMMARY OF BACKGROUND DATA

Biomechanical studies that have examined revision screw strategies for lateral mass fixation have found either unsatisfactory or highly variable performance.

METHODS

Fresh frozen cervical spinal segments were harvested and BMD testing performed. Bicortical (3.5-mm Vertex) lateral mass screws were placed in a Magerl trajectory in 57 fresh frozen human subaxial cervical vertebrae. All screws were then stripped and revision screws (4.0-mm Vertex) placed using either the same screw path or conversion to a Roy-Camille trajectory. In line pullout testing was performed on each of the revision screws (57 in Magerl revision group, 55 in Roy-Camille). Specimens that had not fractured during testing then had cervical pedicle screws (3.5-mm Vertex) placed and in-line pullout testing repeated (64 pedicles were instrumented) The pullout failure results of the Magerl revision, Roy-Camille revision, and pedicle screw revision groups were compared.

RESULTS

No significant difference was noted in insertional torque (0.28-Nm Magerl, 0.35 Nm Roy-Camille, P > 0.05) or pullout (382-N Magerl, 351 N Roy-Camille, P > 0.05) between the Magerl and Roy-Camille revision groups. Pedicle screw revision had greater pullout strength (566 N) when compared with either the Magerl (382 N) or Roy-Camille (351 N) revision groups (P < 0.01) but also had a 20% pedicle wall breech rate by visual inspection. Insertional torque and pullout strength increased with increased BMD and were significantly correlated in all 3 revision groups (P < 0.05). Similarly, increased BMD was associated with increased pullout strength as demonstrated by the significant positive correlation (P < 0.05).

CONCLUSION

Conversion of a stripped lateral mass screw to an alternate trajectory appears to offer no biomechanical advantage over placement of an increased diameter salvage screw using the same trajectory. Pedicle screw fixation provides superior biomechanical fixation but was associated with a significant breech rate.