Measurement Study of the “Optimal” Screw Axis Location and Its Angles with the First and Second Metacarpals Based on X Imaging of the Scaphoid Bone

Background: Accurate screw placement is critical for successful internal fixation of scaphoid fractures. However, during percutaneous Herbert screw fixation via the volar approach, there is currently no standardized anatomical reference for determining the entry point and screw axis. Traditional methods often rely on experience or intraoperative fluoroscopy, which can increase radiation exposure and reduce accuracy. This study aims to propose a novel reference method based on the angular relationship between the scaphoid screw axis and the first and second metacarpals, and to assess its anatomical consistency and potential clinical utility. Methods: A retrospective analysis was conducted on radiographs of healthy adults from 2021 to 2023, including anteroposterior and lateral wrist views. Two senior trainees and two experienced surgeons measured the imaging data. The measurements included the screw axis length, angles between the screw axis and metacarpal axes, skin thickness(the thickness beneath the skin of the scaphoid tubercle), and entry point distance(the vertical distance from the intersection of the extended screw axis with the palmar skin to the apex of the scaphoid tubercle). The influences of the thumb position, gender, and skin thickness on the screw axis orientation and entry point location were analyzed. Results: A total of 151 adults were analyzed, including 87 females (57.6%) and 64 males (42.4%) with an average age of 43.2 ± 15.9 years. In the lateral view, the mean angle between the screw axis and the second metacarpal axis was 71.5° ± 6.7°, with minimal variance, indicating high consistency across individuals. In the PA view, the angle between the screw axis and the first metacarpal axis varied with the thumb position: 4.8° ± 2.8° in abduction versus 18.7° ± 5.7° at rest. In some cases, the axes were nearly parallel in abduction. The screw axis length measured in the lateral view was closely aligned with that reported in previous anatomical studies(male 2.5 ± 0.3 cm vs female 2.2 ± 0.2 cm, p < 0.005). No significant correlation was found between subcutaneous thickness and entry point distance (r = 0.048, p = 0.559). Additionally, males had greater entry point distances (males 1.5 ± 0.3 cm vs females 1.3 ± 0.3 cm, p < 0.005). Conclusions: In this study, we find a defined anatomical relationship between the scaphoid screw axis and the axes of the first and second metacarpals. Additionally, the guidewire entry point on the skin should be located distal to the scaphoid tubercle, with the distance varying by gender. The second metacarpal is recommended as a lateral view guide because of its stability and consistency, whereas the first metacarpal can be used in the PA view, particularly when the thumb is in abduction. Further clinical validation through prospective studies is warranted.