The dorsum of the wrist is subdivided into six compartments. The PIN is the only nerve found in the dorsal compartments. It is consistently found on the base of the fourth compartment. Anatomic structures within each compartment are:
Compartment 1: Abductor pollicus longus, extensor pollicus brevis.
Compartment 2: Extensor carpi radialis longus and brevis
Compartment 3: Extensor pollicus longus
Compartment 4: Extensor digiti communis, extensor indicis propius, posterior interosseous nerve
Compartment 5: Extensor digiti minimi
Compartment 6: Extensor carpi ulnaris
Iwamoto et al performed a cadaveric study of the six compartments and septa. They determined that the 1/2 compartment had the thickest radial surface while the 3/4 septa was the thinnest. The fifth dorsal compartment had the lowest resistance to failure.
Palmer et al also performed a cadaveric study on the extensor retinaculum. They concluded that the retinaculum provides a check rein to bowstringing and should be repaired during surgery.
Grutter et al injected the PIN and AIN in fresh-frozen cadavers. In the first group, the AIN and PIN were injected via a single injection 1cm ulnar and 3cm proximal to Lister's tubercle. For group 2 (PIN alone), the injection site was 1cm ulnar to the proximal aspect of Lister's tubercle. Their technique led to accurate delivery of anesthetic to the PIN and AIN in 100% of the cadavers.
Illustration A shows the 6 dorsal wrist compartments. Illustration B shows the injection site in the Grutter article.
Iwamoto A, Morris RP, Andersen C, Patterson RM, Viegas SF. An anatomic and biomechanic study of the wrist extensor retinaculum septa and tendon compartments. J Hand Surg Am. 2006 Jul-Aug;31(6):896-903.
PMID:16843147 (Link to Abstract)
Palmer AK, Skahen JR, Werner FW, Glisson RR. The extensor retinaculum of the wrist: an anatomical and biomechanical study. J Hand Surg Br. 1985 Feb;10(1):11-6.
PMID:3998587 (Link to Abstract)
Grutter PW, Desilva GL, Meehan RE, Desilva SP. The accuracy of distal posterior interosseous and anterior interosseous nerve injection. J Hand Surg Am. 2004 Sep;29(5):865-70.
PMID:15465236. (Link to Abstract)