Biomechanical study of parallel versus cross placement of anterior cervical plate screws

Objective To compare the stresses on the screw tail between parallel and cross placements of anterior cervical plate screws using pulling out and fatigue tests so as to provide a theoretical basis for screw placement in surgery. Methods Eighty-eight fresh specimens of goat cervical vertebras (C4-C6) were prepared and randomly divided into 4 even groups (n = 22). Internal fixation with anterior cervical plate and screws was conducted in each specimen of C4-C6. Parallel placement of 4 screws was performed in groups A and B while cross placement of 4 screws in groups C and D. A pulling out test was performed on a universal testing machine to compare the maximum axial strength needed to pull out the plate between groups A and B while a fatigue test was performed to compare fatigue life and strength of the plate-screw system between groups C and D. loosening of the plate-screw system was also observed in groups C and D. Results The maximum axial pullout strength was 1, 101. 7 ± 238. 2 N in group A and 1, 190. 7 ± 247.6 N in group B, showing no statistical difference ( t = -1.214, P = 0. 232). Loosening of plate-screw connections was observed in both groups C and D but no breakage of the plate or screw occurred in either group. There were no significant differences between groups C and D regarding fatigue life (0. 056 × 107 times versus 0. 054 × 107 times) or fatigue strength (469. 3 ±83. 8 MPa versus 472. 5 ±53. 8 MPa) (P > 0.05) . Conclusions There is no obvious statistical difference in biodynamics between parallel and cross placements of anterior cervical plate screws. We believe that there is no requirement for absolute screw symmetry when parallel nailing cannot be achieved under some special circumstances. Key words: Cervical vertebrae; Bone plate; Fracture fixation, internal; Angles of nailing; Biomechanics