The anatomical snuff box has one of the most unique name origins among anatomical structures. This shallow depression on the posterolateral aspect of the hand and wrist junction is named after the historical practice of having ground tobacco, otherwise known as “snuff”, placed in the depression and then inhaled through the nose. The depression is deepest and most noticeable when the thumb is fully extended and abducted. Much like the femoral triangle in the supero-anterior aspect of the thigh, the anatomical snuffbox is known for, and used mostly as a way of identifying structures that define its borders and those structures that pass through it.
Anatomical Snuffbox Location, Anatomy, and Borders
From anatomical position, the snuffbox is located distal to the end of the radius posterolaterally. The base of this triangular shaped depression is located just distal to the end of the radius with the triangle’s apex pointing towards the thumb. The floor of the snuffbox is made up of the scaphoid and trapezium carpal bones, which are located between the radial styloid process proximally and the base of the 1st metacarpal distally.
The medial and lateral borders of the snuffbox are made up of three muscles that act on the thumb:
Unlike the long extensors of the posterior forearm, the outcropping muscles arise from the middle portion of the posterior radius and ulna. As these muscles run distally towards the thumb, they produce a palpable bulge, especially when the thumb is circumducted. The tendons of two of the outcropping muscles make up the lateral border of the anatomical snuffbox; they are the abductor pollicis longus and extensor pollicis brevis. The medial border of the snuffbox is made up of the remaining outcropping muscle, the extensor pollicis longus.
Contents of the Anatomical Snuffbox
When considering the contents of the anatomical snuffbox, it is helpful to divide the structures into two groups: those that lie superficial to the extensor retinaculum and the tendons of the outcropping muscles, and those that lie deep to these structures. The extensor retinaculum is a thin band of fibrous connective tissue that runs across the posterior aspect of the distal forearm. Its function is to keep the tendons of the extensor and outcropping muscles located below it, from bowing out when the muscles are contracted and tension on the tendons occurs.
Structures Superficial to the Extensor Retinaculum and Outcropping Muscle Tendons
- Dorsal digital branches of the superficial radial nerve: The radial nerve descends from the posterior compartment of the arm and crosses into the cubital region by passing anterior to the lateral epicondyle of the humerus. It then divides into deep and superficial branches. The cutaneous superficial branch then runs down the forearm under the brachioradialis muscle. As it enters the distal part of the forearm, it enters the roof of the anatomical snuffbox and branches into a number of dorsal digital branches. One of these branches then continues through the snuffbox.
- Cephalic vein: Arising medial to the digital nerve is the cephalic vein. The cephalic vein forms from the dorsal venous network which drains blood from dorsal aspect of the hand. It ascends from the lateral aspect of the forearm and arm, running superficially in the subcutaneous tissue.
Structures Deep to the Extensor Retinaculum and Outcropping Muscle Tendons
- Radial artery: The radial artery forms as a branch of the brachial artery as it passes through the cubital fossa. It runs down the lateral aspect of the forearm, deep to the brachioradialis muscle, and at the wrist is lateral to the flexor carpi radialis tendon. It then leaves the forearm by passing posterolaterally to run obliquely along the floor of the anatomical snuffbox.
- Tendons of extensor carpi radialis longus and brevis: As the outcropping muscles emerge from under the extensor digitorum, they run superficial to the tendons of extensor carpi radialis longus and brevis. The tendons of the extensor carpi radialis longus and brevis run medial to the radial artery along the floor of the anatomical snuffbox to insert onto the base of the 2nd and 3rd metacarpals. (Figure 3).
Below is a summary of the borders of the anatomical snuffbox, and the structures that pass through the snuffbox.
- Proximal = radial styloid process
- Distal = base of 1st metacarpal
- Floor = scaphoid and trapezium
- Medial = extensor pollicis longus
- Lateral = abductor pollicis longus and extensor pollicis brevis
- Superficial = dorsal digital branches of the radial nerve, cephalic vein
- Deep = radial artery, tendons of extensor carpi radialis longus and brevis
Other than locating the structures that pass through this region, the anatomical snuffbox is clinically noteworthy for two reasons. First, it is one of six locations in the upper limb at which a pulse can be felt. It should not be mistaken however, for the radial pulse, which is located anterolaterally in the distal forearm and not within the anatomical snuffbox!
Secondly, with the hand deviated towards the ulna, the scaphoid becomes palpable on the floor of the snuffbox. The scaphoid can be fractured due to falls onto an outstretched hand and can result in pain and swelling within the region of the anatomical snuffbox. In ulnar deviation, the scaphoid can be assessed for fractures.