Clinical Examination of the Extensor Pollicis Brevis: Anatomical Study and Description of a Novel Clinical Sign

The Extensor Pollicis Brevis (EPB) is an extrinsic thumb muscle whose main function is extension of the first metacarpophalangeal joint (MCPJ). It is subject to significant anatomical variation and may be absent, vestigial or have an anomalous distal attachment. Clinical examination of EPB is notoriously difficult and no reliable test has yet been described. We propose a novel test for the accurate examination of EPB. We sought to clarify the anatomical variations of EPB and to validate our clinical test using human cadaveric anatomical tests.

A structured literature review of all human cadaveric anatomical studies describing the attachments of EPB was performed using MEDLINE and Embase with the key words "Extensor Pollicis Brevis". A cadaveric anatomical study was performed using 18 unembalmed upper limbs. Positive and negative tests were simulated by manipulating the tendons of EPB, Extensor Pollicis Longus (EPL) and Flexor Pollicis Longus (FPL). Changes in tendon tension and joint position were measured and recorded. The EPB anatomy was then determined by dissection.

Anatomical variations were present in the majority of wrists, with only 35% of EPB tendons having a distal attachment to the proximal phalanx alone. EPB was absent in 5% of specimens. There was a significant difference between the change in MCPJ position between a positive (36 degrees; 95% CI 25 to 47 degrees) and negative (19 degrees; 95% CI 14 to 25 degrees) clinical test (p = 0.002).

The functional importance of EPB depends on its congenital architecture in addition to the functional demands of the patient. We report a novel clinical test which is effective in demonstrating the integrity of the EPB. A positive test result is observed when a change in MCPJ position that occurs while the interphalangeal joint is brought into flexion from full thumb extension is 25 degrees or more.