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Proper injection of the posterior interosseous nerve (PIN) is important for both the therapeutic and diagnostic management of wrist pain. However, no anatomical study exists describing the site of injection based on individual wrist width. We sought to develop a reproducible anthropometric ratio utilizing external wrist surface anatomy to predict a safe and accurate injection site for the PIN. Fresh frozen cadaver forearms were dissected at the University of Louisville tissue lab. Several anthropometric measurements were obtained in order to develop a reproducible ratio to calculate location of injection. A total of 16 cadaver forearms of equal male to female ratio were obtained. On average, the male forearm had a greater mean wrist circumference obtained at the level of Lister's tubercle compared to female forearms, 17.1 cm vs. 13.5 cm. An injection given ulnar to proximal edge of Lister's tubercle at a length of one-fourth the distance between Lister's tubercle and radial aspect of ulnar styloid resulted in 100% accurate perineural injection without intraneural injection. An anthropometric ratio of one-fourth the distance from Lister's tubercle to the ulnar styloid was able to predict accurate injection sites for the distal PIN in cadaveric specimens of varying anatomical proportions. [ABSTRACT FROM AUTHOR]