Intrinsic coordination of the fifth digit of the hand is largely controlled by the abductor digiti minimi (ADM), flexor digiti minimi brevis (FDMB), and opponens digiti minimi (ODM) muscles (i.e., the hypothenar muscles). Hypothenar muscle variations have been reported; however, biomechanical analyses are seldom performed and descriptions of rare variants such as the deep abductor-flexor of the fifth digit (DAF5) are especially scarce. This study aims to investigate an especially unique case involving the rare DAF5 among additional hypothenar muscle aberrations with biomechanical analysis and discussion of clinical implications.
An especially unique case of multiple unilateral (left) hypothenar muscle aberrations was discovered during routine human cadaver dissection. The aberrations were cleaned of extraneous fascia and photographed in situ. Mass and fascicle length were measured and mean postmortem fixed sarcomere states were determined via light microscopy to calculate a normalized maximal isometric force (Fmax) for each muscle.
The concurrent aberrations included two accessory ADM muscles, three accessory FDMB muscles, an accessory ODM muscle, and a bicipital DAF5. The curved long head of DAF5 (Fmax = 4.77 N) originated from the transverse carpal ligament (across midline), while the straight short head (Fmax = 1.23 N) originated from the pisiform. The two heads united to form a prominent 3 cm tendon which inserted on the medial aspect of the base of the fifth proximal phalanx. Notably, the long head separated the ulnar artery (superficial) from the ulnar nerve (deep).
The presence of consecutive hypothenar anomalies could influence fifth digit coordination for proper hand function, and the concomitant DAF5 could implicate the underlying median and ulnar nerves. Insights from this report may inform occupational and physical therapists, orthopedic surgeons, and medical anatomy educators when treating or deliberating related cases.
One variation of the atlanto-occipital joint contains a groove or crest dividing the superior articulating facet of C1 and/or the corresponding occipital condyle. This study aimed to estimate the prevalence of bipartition of the facets associated with the atlanto-occipital joint and to determine the relationships between the corresponding joint surfaces, bilateral and unilateral facets as well as demographic factors in a South African sample.
Data about shape and bipartition found on the superior articular facets of the atlas and the occipital condyles were collected from 109 skulls and atlases from the Pretoria Bone Collection, South Africa. Shape data was categorically classified according to existing criteria from literature.
The results revealed the occasional occurrence of a double facet at both the superior articular facet of the atlas (15.6 %), and the occipital condyle (7.34 %). Shape and facet number of the superior articulating facet and corresponding occipital condyle were significantly correlated. The prevalence of bipartition of neither the superior articular facets nor the occipital condyles were statistically significantly different between the left and right sides. Sex and age did not statistically significantly contribute to prevalence of bipartition in this population. However, population group statistically significantly contributed to prevalence of bipartition (p = 0.019), with the White South African sample having a much higher prevalence (88.24 % superior articulating facet, 100 % occipital condyle) of bipartition compared to the Black South African sample (11.76 % superior articular facet, 0 % occipital condyle).
Awareness of this joint variation and instances where it is more likely to occur may be valuable for neuro-vascular and orthopaedic surgeons when doing invasive treatments around the craniocervical junction, and chiropractors when assessing and treating patients.