PURPOSE: The purpose of this study was to evaluate the influence of screw-to-canal diameter ratio on clinical and radiographic outcomes for patients with metacarpal shaft fractures treated with retrograde intramedullary headless compression screws (IMHS). METHODS: A retrospective analysis was conducted for all patients with metacarpal shaft fractures treated with retrograde IMHS between 2017 and 2022. All procedures were performed using retrograde IMHS through an ulnar sagittal band split. Patients initiated early range of motion and were not immobilized. The ratio of screw diameter to metacarpal canal diameter at the fracture site was calculated. Radiographic displacement, QuickDASH (Disabilities of the Arm, Shoulder, and Hand) scores, grip strength, and range of motion were recorded during a 6-month follow-up period. RESULTS: A total of 52 patients with 58 metacarpal shaft fractures treated with IMHS were included. All fractures achieved union, with 71% healed by 6 weeks and 100% healed by 12 weeks. Mean grip strength measured 86.0% relative to the contralateral side, and the average total active motion measured 248°. The mean QuickDASH score was 2.2, indicating minimal disability. No cases demonstrated extensor lag or required revision surgery. Mean screw-to-canal diameter ratio was 0.81. Oblique fractures were most likely to displace (50%), and there was a strong fit of the linear regression model for the ratio of canal fill and displacement (R CONCLUSIONS: Our findings suggest that the screw-to-canal diameter ratio influences radiographic displacement in oblique fracture patterns that have a greater propensity to displace. When the screw-to-canal ratio exceeded 80%, there were no cases of displacement regardless of fracture pattern. Despite the occurrence of radiographic displacement, QuickDASH scores, grip strength, and range of motion were similar between the groups. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.