Biomechanical analysis of corrective forces in spinal instrumentation for scoliosis treatment

Abstract

Study design: Computer modeling and simulations to analyze correction forces at the bone-screw interface in scoliosis instrumentation.

Objective: To derive the minimum corrective forces applied on vertebrae through pedicle screws to achieve desired scoliosis corrections and evaluate the actual bone-screw forces associated with 3 types of pedicle screws (monoaxial, polyaxial, and dorsoaxial).

Summary of background data: The optimum screw pattern has not been established in the literature. The final bone-screw forces in scoliosis instrumentation consist of "true corrective forces" (i.e., the minimum forces required to achieve the desired corrections without considering adequate rod seating at all pedicle screws) and "extra forces" (EF) (i.e., supplementary forces applied to ensure proper rod seating when the attachment of some screws is not in compliance with the attachment of their neighboring screws; they have no benefit to overall corrections). METHODS.: Using patient-specific computer models, true corrective forces were estimated for 10 spinal instrumentation cases. EF were computed by simulating the instrumentations of the 10 cases using respectively monoaxial, polyaxial, and dorsoaxial screws.

Results: The average true corrective forces were 50 ± 30 N. The average bone-screw forces were 229 ± 140 N, 141 ± 99 N, and 103 ± 42 N, respectively, for monoaxial, polyaxial, and dorsoaxial screws; the averages of the EF magnitudes were 205 ± 136 N, 125 ± 93 N, and 65 ± 39 N, respectively.

Conclusion: Bone-screw forces to achieve desired corrections can be minimized. However, EF are inevitable to secure the locking of all screws. Higher EF were associated with pedicle screws, with less degrees of freedom for connecting screw body to rod, that is, monoaxial followed by polyaxial and then by dorsoaxial screws.