Anatomy, Shoulder and Upper Limb: Hand Anatomical Snuff Box

Structure and Function

The anatomical snuffbox is a broadly triangular, superficial depression in the skin that becomes apparent when the thumb is held in abduction and the wrist in ulnar deviation. The base of the anatomical snuffbox aligns with the radial styloid process, while the apex points distally toward the 1st metacarpophalangeal (MCP) joint.

Viewed from above, the medial (ulnar) boundary of the anatomical snuffbox is defined by the tendon of the EPL. The lateral (radial) boundary is formed by the parallel-running tendons of the EPB and the APL (see Image. Anatomical Snuff Box and Ulnar Styloid Process).

The EPL originates from the midshaft of the ulna and inserts into the base of the distal phalanx of the thumb, serving as the principal extensor of the terminal phalanx. This muscle receives innervation from the posterior interosseous nerve (PIN).[4]

The EPB and the APL both originate from the posterior surfaces of the radius and ulna. The EPB is more closely associated with the radius and the interosseous membrane.[5] The APL inserts via 2 tendinous slips: one at the base of the 1st metacarpal and another at the trapezium. The EPB inserts as a slender tendon at the base of the proximal phalanx of the thumb. Both the APL and EPB are innervated by the PIN. Functionally, both muscles extend the thumb at the carpometacarpal joint, and the EPB additionally extends the thumb at the MCP joint.

The base of the anatomical snuffbox contains several palpable bony landmarks. From proximal to distal, these landmarks include the radial styloid process, the scaphoid, the trapezium, and the base of the 1st metacarpal. These structures can be identified on examination with the hand placed in ulnar deviation and the thumb fully extended.[6]

Although the anatomical snuffbox is a surface anatomy feature, it gains clinical relevance through its spatial relationships with deeper structures. Accurate identification of its boundaries and contents allows the healthcare provider to formulate a differential diagnosis when evaluating localized pain. For instance, tenderness within the anatomical snuffbox following a fall on an outstretched hand suggests a scaphoid fracture, which requires further evaluation through radiographic imaging to confirm the diagnosis. Familiarity with the structures traversing this region also benefits vascular access procedures, particularly in patients with difficult peripheral access, because both the radial artery and the cephalic vein lie within the confines of the anatomical snuffbox.[7]

Blood Supply and Lymphatics

The anatomical snuffbox does not possess an independent blood supply, but several vessels traverse its boundaries. The cephalic vein and the radial artery both course through this region. Among these vessels, branches of the radial artery are most clinically significant due to their role in supplying the scaphoid bone. Approximately 80% of the proximal scaphoid receives blood via intraosseous retrograde flow from direct branches of the radial artery that enter through the articular foramina. The remaining perfusion is provided by volar scaphoid branches of the radial artery that enter at the distal pole. This retrograde vascular pattern predisposes the scaphoid to avascular necrosis, particularly following fractures that disrupt the arterial inflow within the anatomical snuffbox.[8]

Nerves

The superficial branches of the radial nerve that course through the anatomical snuffbox can be palpated and occasionally rolled over the tendon of the EPL, which defines the medial border of the snuffbox.[9] These sensory branches belong to the superficial branch of the radial nerve, which is this nerve's terminal division. The radial nerve arises from the posterior cord of the brachial plexus and receives contributions from nerve roots C5 to C8 and T1.

The radial nerve's course begins in the axilla and continues distally along the upper limb. The nerve passes between the long and medial heads of the triceps brachii, wraps around the posterior aspect of the humerus in a spiral trajectory, and pierces the lateral intermuscular septum approximately 1/3 of the way down the arm. As the nerve descends into the forearm, it gives rise to the PIN (C7, C8), which is its final motor branch.

The remaining fibers continue as the superficial branch of the radial nerve. This slender sensory branch travels along the lateral aspect of the forearm and enters the anatomical snuffbox, where it provides cutaneous innervation to the dorsum of the radial 2 1/2, as well as the adjacent dorsal surface of the hand (see Image. Cutaneous Innervation of the Right Upper Extremity).[10]

Muscles

The anatomical snuffbox is bordered medially by the tendon of the EPL and laterally by the tendons of the EPB and the APL. Both the EPB and the APL contribute to thumb extension, producing movement in the lateral horizontal plane.[11][12] The APL acts primarily at the carpometacarpal joint, while the EPB functions at both the carpometacarpal and MCP joints. The floor of the anatomical snuffbox is partially formed by the tendons of the extensor carpi radialis longus and the extensor carpi radialis brevis.[13]

Physiologic Variants

Anatomical research has identified multiple variations in the tendons associated with the anatomical snuffbox. Both the EPL and the APL may present with supernumerary tendon slips and atypical insertion sites. These variations hold clinical relevance, as certain configurations may predispose patients to tendinopathy involving either muscle. Tendon variability also has implications for surgical planning, particularly when operative intervention is considered for tendon pathology or trauma.[14]

Cadaveric studies have similarly documented variations in the course of the cephalic vein and the radial artery within the anatomical snuffbox. These findings are clinically significant in the context of intravenous cannulation or vascular access, which may be attempted in this region. Anomalous vascular anatomy increases the risk of inadvertent arterial puncture, potentially resulting in pseudoaneurysm formation, arterial occlusion, or localized hematoma.[15]

Surgical Considerations

The superficial branch of the radial nerve is the 3rd most frequently injured peripheral nerve. Damage to this branch produces paresthesia over the dorsum of the 1st webspace of the hand. Awareness of the nerve's variable course within the anatomical snuffbox is essential during surgical procedures to prevent iatrogenic injury.

The anatomical snuffbox also serves as an alternative access point for percutaneous arterial entry during coronary intervention.[16][17] This approach offers several advantages over the conventional radial artery puncture. Most notably, collateral perfusion from the proximal radial artery remains intact in the event of complete arterial occlusion by the angiography catheter, thereby minimizing the risk of hand ischemia. Bleeding complications or localized subcutaneous hematomas arising from this access point are often controlled directly within the snuffbox, without the need for circumferential wrist compression. In contrast to traditional radial access, this technique preserves wrist mobility and improves patient comfort during the postprocedural period.

The conventional radial artery approach remains a reliable alternative if arterial access via the anatomical snuffbox is not feasible, or complications such as vasospasm or hemorrhage arise following successful cannulation. A limitation of snuffbox access is the relatively small caliber of the radial artery in this region. Despite this anatomical constraint, a cohort study reported successful cannulation using a 6 Fr catheter in 132 of 150 patients. Based on these findings, the authors concluded that anatomical snuffbox access is both feasible and safe for coronary angiography and percutaneous coronary intervention.[18]

Clinical Significance

The anatomical snuffbox holds clinical relevance when palpation within its boundaries elicits pain. Scaphoid fractures represent approximately 2/3 of all carpal bone fractures and are frequently misdiagnosed. The most common mechanism of injury involves a fall onto an outstretched hand positioned in pronation and ulnar deviation.

Tenderness within the anatomical snuffbox is a hallmark finding and demonstrates high sensitivity for scaphoid fractures, although specificity is limited. Patients with snuffbox tenderness should undergo radiographic evaluation of the wrist because of this limitation. In cases where initial imaging is negative, management may include immobilization with a short-arm thumb spica splint or further evaluation using advanced imaging tools, such as magnetic resonance imaging or computed tomography, to confirm or exclude fracture. Due to the scaphoid’s retrograde blood supply, the fracture location significantly influences treatment decisions and the risk of avascular necrosis.[19]

Accurate diagnosis of scaphoid fractures is essential, as up to 32% of cases progress to nonunion, ultimately resulting in osteoarthritis.[20] Failure to identify and treat this fracture can have significant and lasting consequences for a patient's quality of life. In cases of nondisplaced or minimally displaced scaphoid fractures, a systematic review and meta-analysis of available studies found no significant difference in long-term pain or disability outcomes between operative and nonoperative treatment strategies.[21]

De Quervain tenosynovitis is another clinically relevant condition associated with the anatomical snuffbox. This stenosing tendinopathy affects the 1st dorsal compartment, which contains the APL and the EPB, both of which form the lateral border of the anatomical snuffbox. Diagnosis is based on clinical history and physical examination.

Affected individuals often report repetitive wrist motion, particularly involving repeated radioulnar deviation. Common clinical findings include pain and localized swelling over the dorsolateral aspect of the wrist, with symptom provocation during resisted thumb extension or abduction. Initial management involves conservative therapy, including nonsteroidal anti-inflammatory drugs, corticosteroid injections, physical therapy, or immobilization using a thumb spica splint. Surgical release is reserved for cases with persistent symptoms despite conservative treatment.[22]