Biomechanical Comparison of the Load-Sharing Capacity of High and Low Implant Density Constructs With Three Types of Pedicle Screws for the Instrumentation of Adolescent Idiopathic Scoliosis

Study Design: Biomechanical numerical simulation analysis of implant design and density in adolescent idiopathic scoliosis posterior instrumentation. Objectives: To evaluate the combined effect of pedicle screw design and density on deformity correction and construct load-sharing capacity. Summary of Background Data: Screw density is an area of popular study because of the impact of cost and potential patient morbidity of higher-density constructs. Using fewer screws raises concern about reduced correction and greater forces on each screw. Methods: Personalized spinal numerical models were created for five patients. The correction techniques from five spine surgeons using both a high- and a low-density implant pattern (2 vs. 1.4 ± 0.22 screws/level) with uniaxial, multiaxial, and favored angle screws were simulated. The predicted correction and forces sustained by the implants were compared. The postoperative load-sharing capacity of a high- and a low-density construct, with or without crosslinks, was compared by simulating daily activities motions. Results: The major coronal curve correction was similar with high- and low-density constructs (73% ± 10% vs. 72% ± 10%; p >.05) but was higher when using uniaxial (77% ± 8%) compared to multiaxial (69% ± 11%) and favored angle screws (71% ± 10%; p =.009). High- and low-density constructs sustained similar intraoperative peak forces (305 ± 61 N vs. 301 ± 73 N; p =.23) regardless of screw design (all p >.05). Multiaxial and favored angle screws reduced the peak axial force by 23% and 38% compared to uniaxial screws (p =.007). The high-density construct reduced the postoperative loads sustained by each implant by 31% (p =.006). Crosslinks had no effect on load sharing (p =.23). Conclusion: High- and low-density implant patterns achieved similar coronal correction with equivalent capacity to share corrective forces regardless of the screw design. Increased degrees of freedom of the screw head reduces the capacity to correct coronal deformity but generates lower bone-screw forces. The reduced number of screws increased the postoperative forces sustained by each screw, but its effect on potential complications requires further investigations. Level of Evidence: Level 4.