Defining the Optimal Entry Point for Antegrade Humeral Nailing: A CT-Based Proportional Ratio Derived from Medullary Canal Anatomy

Background Antegrade humeral nailing requires selection of an entry point near on the proximal humerus. The purpose of this study was to evaluate the canal axis difference between the proximal and distal humerus, and to derive a clinically applicable entry point ratio that can guide straight nail start point selection for both short and long nail constructs. Methods CT scans from 40 normal adult humeri were analyzed by two independent observers using calibrated coronal and sagittal reconstructions. A circle-fitting technique identified the medullary canal center at proximal and distal levels. At each level and plane, distances from the greater tuberosity tip (GT) and humeral head apex (Head) to the canal center were recorded. An entry point ratio was defined as GT / (GT + Head), representing the proportional distance from the GT tip to the canal center as a fraction of the total GT-to-head width. Inter-observer reliability was assessed using the intraclass correlation coefficient (ICC), standard error of measurement, and minimal detectable change. Paired t-tests compared proximal and distal ratios, with Bonferroni correction applied for multiple comparisons. Results Inter-observer reliability was good for all GT distance measurements (ICC 0.824–0.882) and moderate-to-good for head distance measurements (ICC 0.579–0.882). Using averaged measurements, the coronal entry point ratio was 0.730 ± 0.145 at the proximal level and 0.698 ± 0.153 at the distal level, a difference of +0.032 (p = 0.011). In the sagittal plane, the proximal ratio was 0.608 ± 0.180 versus 0.581 ± 0.192 distally (difference +0.027, p=0.044). All four raw paired comparisons (GT and head distances in both planes) reached statistical significance, with the proximal canal axis consistently more medial than the distal axis. Conclusion The optimal entry point for an antegrade humeral nail can be estimated as a proportion of the total GT-to-humeral-head distance on pre-operative coronal CT: approximately two-thirds (ratio ≈ 0.70) for a long nail and slightly more medial (ratio ≈ 0.73) for a short nail. These patient-specific, anatomically derived ratios allow surgeons to restore native alignment by providing a more rigorous basis for nail entry point planning than fixed landmark descriptions and account for nail length.