Purpose: The aim of this study is to correlate the ultrasound (US) appearance of the persistent double or bifid distal tendon of the biceps brachii muscle with anatomical and histological data. This will provide a new model to study the pathological distal biceps brachii tendon (DBBT).
Methods: The DBBT of 20 cadaveric elbows were examined with linear array broadband US transducers (frequency band 14-6 MHz) using an anterior approach. Trypan blue dye was injected underneath the paratenon under US guidance in 16 specimens. After they were dissected, five of them were processed to obtain histological slices stained with hematoxylin-eosin and antiserum to protein S100.
Results: At US, the DBBT is a tendon in which the fascicles are organized in two different hyperechoic components separated by a hyperechoic septum related to the endotenon. The endotenon is lax, flexible, and makes folding and gliding of the two portions feasible. The DBBT is surrounded by a hyperechoic paratenon adjacent to the tendon surface, which is only differentiable by US when dye is interposed between such structures.
Conclusions: The connective septum of endotenon located between the two main components of the DBBT is responsible for the US image of two separate tendons and functionally enables it to work as two separate entities, thus allowing respective folding and gliding. The paratenon surrounding the lacertus fibrosus and the DBBT plays an important stabilization role, enabling them to change shape and arrangement during joint motion. It is also an important conduit for nerves and blood vessels.