Biomechanical comparison of the angle of inserted screws and the length of anterior cervical plate systems with allograft spacers

Background: Comparative studies of the biomechanical effects of plates of varying lengths and different screw insertion angles on allograft spacers are lacking. Methods: Finite element model analysis of a previously validated, three-dimensional, intact cervical spinal segment model of C3-6 was conducted in the present study. On the C5-6 segment, anterior discectomy and fusion were performed using allograft spacers and different combinations of anterior plates and screws. The biomechanical characteristics of combinations of short, medium, and maximal length plates with screw insertion angles of 0 degrees, 8 degrees, 16 degrees, and 32 degrees were analyzed. Findings: In flexion and extension, the risk of allograft spacer subsidence decreased as screw angles increased. Short plates with a screw insertion angle of 32 degrees posed the lowest subsidence risk, similar to medium length plates with a screw insertion angle of 16 degrees, in all motion conditions. The risk of bone yielding increased as plate length increased, but decreased as the screw insertion angle increased. Interpretation: Short plates with a large screw insertion angle (32 degrees) showed the highest mechanical stability and load sharing of allograft spacers and the lowest risk of screw loosening. Accordingly, we recommend the use of a short plate and large screw insertion angle for anterior cervical discectomy and fusion.