A unique bilateral accessory forearm flexor muscle

Muscular and neurovascular variations in the upper extremity are of utmost clinical significance. Here we report a unique bilateral accessory muscle in the forearm and palm of an 89-year-old male cadaver. The accessory muscle presented two bellies on the right side, one in the forearm, innervated by the anterior interosseous nerve, and the other in the palm, innervated by a branch of the median nerve. A long tendon interconnected the two bellies. On the left side, the muscle had a single belly in the palm, which began at the end of a long tendon that extended from the forearm. However, on both sides, the muscle originated from the posterior surface of the flexor digitorum superficialis belly and inserted along with the first lumbrical muscle into the dorsal digital expansion of the index finger. The proximal parts of the variant muscles were sandwiched between the flexor digitorum muscles. The palmar bellies coursed distally through the carpal canal and lay deep to the superficial palmar arch, and superficial to the first lumbrical, between the thenar muscles and the lateral-most tendon of the flexor digitorum superficialis. Arguably, the accessory muscle might be a variant of a lumbrical muscle, as reported before, but innervation of the proximal belly by the anterior interosseous nerve suggests that the muscle may well be a deep accessory muscle at the forearm, probably appeared as a diverted part of the flexor digitorum profundus. Its space-occupying course through the forearm and palm, especially through the carpal canal, might be clinically significant as it might contribute to nerve compression pathologies in the upper extremity. This accessory muscle also indicates the complex nature of individual muscle formation and evolution of the upper extremity with constant changes in the morphology of muscles based on their changing functions.

Mapping of finger fascicles within the flexor digitorum superficialis and profundus muscles

INTRODUCTION

In this study we aimed to systematically investigate and map localization of the individual finger fascicles within the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP) muscles.

METHODS

Using B-mode ultrasonographic assessments, the right forearms of ten healthy participants (five males and five females) were examined during active and passive finger movements.

RESULTS

A topographical map indicating clear core areas of fascicle activation within the FDS and FDP muscles was created. Borders that could, to some degree, define individual differences were also indicated.

DISCUSSION

Our findings offer a detailed image of fascicle distributions within the FDS and FDP muscles. Various challenges, such as managing individual muscular profiles or the existence of the palmaris longus muscle, are discussed and should always be taken into consideration. The current map can serve as a general orientation for future measurements or injection therapies.

Development of digastric muscles in human foetuses: a review and findings in the flexor digitorum superficialis muscle

The digastricus and omohyoideus muscles are digastric muscles with two muscle bellies. An insertion tendon of the posterior belly becomes an intermediate tendon in digastricus muscles, whereas a single band-like muscle in omohyoideus muscles may later be interrupted by an intermediate tendon, possibly due to muscle cell death caused by mechanical stress. In human foetuses, an intermediate tendon provides the temporal origins of the tensor veli palatini and tensor tympani muscles. Some reptiles, including snakes, carry multiple series of digastric-like axial muscles, in which each intersegmental septum is likely to become an intermediate tendon. These findings indicate that many pathways are involved in the development of digastric muscles. A review of these morphologies suggested that the flexor digi-torum superficialis (FDS) muscle was a digastric muscle, although the intermediate tendon may not be visible in the surface view in adults. The present observations support the hypothesis that the proximal anlage at the elbow develops into a deep muscle slip to a limited finger, while the distal anlage at the wrist develops into the other slips. The findings suggest that, in the FDS muscle, the proximal and distal bellies of the embryonic digastric muscle fuse together to form a laminar structure, in which muscle slips accumulate from the palmar to the deep side of the forearm. (Folia Morphol 2018; 77, 2: 362-370).