The distal biceps tendon (DBT) is a strong duplicated extrasynovial tendon that connects the distally variably interlinked long and short head of the biceps brachii muscle to the bicipital tuberosity of the proximal radius. Besides the distal biceps tendon, the biceps muscle is distally attached to the forearm by an aponeurotic expansion called lacertus fibrosus.
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Gross Anatomy
The distal musculotendinous unit of the biceps muscle is covered by a paratenon and consists of two tendons. It can be subdivided into three zones: the musculotendinous junction proximally, the middle zone where the lacertus fibrosus, an aponeurotic structure encircles the two tendons and the distal zone 1-3.
The muscle bellies of the long and short head of the biceps muscle are distally variably interlinked at the level of the musculotendinous junction and their appearance has been compared to a ‘goose quill’ 3. The long head muscle belly of the biceps muscle is laterally and the short head medially positioned. The distal biceps tendon unit undergoes an external rotation of approximately 90° from the myotendinous junction to the distal insertion at the radial tuberosity 1,2.
The distal tendon of the long head features a more crescentic and the distal tendon of the short head muscle a more oval shape 1.
Attachments
The distal tendon of the long head biceps muscle belly features an oval-shaped insertion onto the large proximal part of the bicipital tuberosity a bit more posteriorly, whereas the distal tendon originating from the short head of the biceps muscle attaches onto the distal portion of the bicipital tuberosity with a fan-shaped insertion extending even further distally 1,2.
The insertional anatomy of the two biceps tendons implicates that the long head provides an increased supination power and more flexion strength on the supinated forearm, whereas the short head provides higher flexion power especially in a neutral position and pronation 1,2.
Relations
The distal tendon insertions of both tendons are surrounded by the bicipitoradial bursa 1.
Arterial supply
The proximal zone with the myotendinous junction is supplied from branches of the brachial artery, the distal portions of the tendons receive blood from branches of the posterior interosseous recurrent artery and the middle zone is supplied from both arteries.
Variant Anatomy
Up to 40-50% of individuals show significant interdigitation of the two muscle bellies in the distal third of the muscle. However, in the majority of cases, the two distal tendons can move or slide separately from each other 1.
There are sexual differences and variations in respect to the size of the footprint of the distal biceps tendons 2.
Radiographic features
Ultrasound
The distal biceps tendon can be easily assessed from a medial longitudinal position with the forearm supinated and the elbow in a 20°-30° flexed position. The biceps tendon is seen enveloped by its hyperechoic paratenon inserting at the radial tuberosity 4. The anterior approach in the antecubital fossa is more prone to be influenced by anisotropy due to the oblique course of the tendon 6.
MRI
On MRI the distal biceps tendon can be conveniently evaluated in the FABS position, where it can be visualized in two longitudinal views 5. A possible alternative is the acquisition of oblique coronal images besides normal sagittal planes on an exam with the elbow close to the body.
The distal biceps tendon is typically hypointense like all tendons and the short head tendon can be seen fanning out distally onto the proximal radius.
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