From Adult Morphology to Developmental Hypothesis: Variation of the Adult Lateral Wrist Extensors-A Developmental Viewpoint.

Background: Anatomical variations are inevitable part of studying the human body. Very often, muscles of the limbs may show atypical attachments, extra or fewer muscle bellies. These variations are likely rooted in limb development. Our goal was to thoroughly study and describe the variations in the lateral wrist extensors. Our initial goal was to attempt to explain the developmental processes that occur before the formation of these variations, with a focus on the interconnecting tendons. Methods: We used a standard dissection technique, paying extra attention to the space between the two radial wrist extensors to properly visualize interconnecting tendons. Taking advantage of the chi square test, we compared the observed vs the expected random distribution of interconnecting tendons. Results: In this article, we systematically studied the variations in the interconnecting tendons of the lateral carpal extensors in 58 upper limbs of our cadaver donors used for the education of medical students. The main variation we found is interconnecting tendons between the extensor carpi radialis longus and extensor carpi radialis brevis. The insertion and origin of the interconnecting tendons were consistent: it either originates from the middle of the ECRB body and inserts medial to ECRL tendon or it originates from the ventral side ECRL and inserts ventral to the ECRB tendon. We supplemented them with two dissections of fetal upper limbs (at GW 12 and GW 17). Statistical analysis of the distribution of single vs double interconnecting tendons suggests that they are dependent events, consistent with literature data. Conclusions: Based on our observations and the literature, we propose that oblique muscle division and a mismatch between the muscle fission plane and the initial distal tendon fission plane may result in the observed phenotype. We also suggest that the origin of the extra numerary tenons form ECRL body may play a role when choosing which one to mobilize for tendon transfer.