An Ultrasonographic Study of the Superficial Radial Nerve in Healthy Subjects: Suggesting a Safe Zone for Wrist Extensor Compartment Injections.

Background/Objectives: The superficial radial nerve (SRN) is highly susceptible to iatrogenic injury during wrist injection procedures. This study aimed to identify the anatomical trajectory of the SRN using high-resolution ultrasonography and to establish a reliable "safe zone" for wrist extensor compartment injections. Methods: Fifty-eight forearms from 29 healthy volunteers (15 males, 14 females) were evaluated. Four anatomical levels were defined: the proximal and distal ends of the extensor compartment I-II intersection area (Levels A and B), and the proximal and distal points of SRN crossing over the first compartment (Levels C and D). Longitudinal distances from the radial styloid, horizontal distances and depths of the SRN were measured. Generalized Estimating Equations (GEEs) were used to analyze the relationship between total forearm length and the longitudinal position of each landmark. Results: Total forearm length was significantly associated with proximal landmarks, La (B = 0.205, p < 0.001) and Lc (B = 0.105, p < 0.001). Although Lb also showed a significant association (B = 0.071, p = 0.019), its absolute variation was minimal. The most distal landmark Ld (B = -0.023, p = 0.610) exhibited no significant relationship. For intersection syndrome, a safe injection corridor was identified between 24.1% and 12.7% of forearm length (Level A to C), where a proximal-to-distal and dorsal-to-volar needle direction is recommended, as the SRN lies volar at this level. For De Quervain's tenosynovitis, a volar-to-dorsal needle direction at or distal to 0.8 cm from the radial styloid (Level D) minimizes nerve contact risk. Conclusions: This study suggests a differentiated, landmark-based approach for wrist injections: utilizing proportional ratios for proximal landmarks and fixed absolute distances for distal landmarks. This individualized guide is expected to enhance procedural safety and minimize the risk of iatrogenic SRN injury.