Intermuscular aponeuroses between the flexor muscles of the forearm and their relationships with the ulnar nerve

Introduction: The aim of this study was to clarify the morphological characteristics of the intermuscular aponeurosis between the flexor carpi ulnaris (FCU) and flexor digitorum superficialis (FDS; IMAS), and that between the FCU and flexor digitorum profundus (FDP; IMAP), and their topographic relationships with the ulnar nerve. Materials and methods: Fifty limbs of 38 adult cadavers were studied. Results: The IMAS extended along the deep surface of the FCU adjoining the FDS, having the appearance of a ladder, giving off “steps” that decreased in width from superficial to deep around the middle of the forearm. Its proximal part divided into two bands connected by a thin membrane, and was attached to the medial epicondyle and the tubercle (the most medial prominent part of the coronoid process of the ulna), respectively. The IMAP extended deep between the FCU and FDP from the antebrachial fascia, and its distal end was located on the posterior border of the FCU. The IMAP became broader toward its proximal part, and its proximal end was attached anterior and posterior to the tubercle and the olecranon, respectively. The ulnar nerve passed posterior to the medial epicondyle and then medial to the tubercle, and was crossed by the deep border of the IMAS at 58.3 ± 14.1 mm below the medial epicondyle. Conclusion: The deep border of the IMAS and aberrant tendinous structure passing across the ulnar nerve, or the parts of the IMAS and IMAP passing posterior to the ulnar nerve are potential causes of ulnar nerve compression.