Morphometrical study of arteries and veins in the human sheet-like muscles (pectoralis major, latissimus dorsi, gluteus maximus and trapezius) with special reference to a paradoxical venous merging pattern of the trapezius

The trapezius is one of the critical targets of physical therapy for shoulder functional disorder because this muscle plays a great role in fixation and elevation of the shoulder. Trapezius (n=63), latissimus dorsi (n=5), pectoralis major (n=7) and gluteus maximus (n=7) muscles were obtained from 43 donated cadavers, and vascular morphologies were compared. An artery without a concomitant vein was found in 35 of 63 trapezius muscles. Moreover, twice as many venous merging sites as arterial branching sites were present in the trapezius. Notably, peripheral or distal venous tributaries were larger in caliber than the proximally located vein in 6.2% of all 404 venous merging sites in the trapezius. Moreover, similar paradoxical venous merging, in which tributaries were relatively thicker than drainage capacity of the venous trunk after merging according to Poisueille's law (peripheral or distal thickness >0.85×proximal thickness), was observed in 39.1% (158 of 404 merging sites) in the trapezius, compared to 8-9% in the other 3 muscles. Given this vascular morphology, trapezius muscle seems likely to frequently display venous blood retention. Interestingly, venous valves were not observed in the trapezius, compared to 1-7 valves per one of the other 3 muscles. Abundant venous mergings without valves might form a venous network acting as a reservoir. Therefore, for the trapezius, collateral venous routes such as the external vertebral plexus are essential to provide additional peripheral drainage. However, sufficient muscle-pump function does not seem to be expected for trapezius muscle, given the venous courses parallel to muscle fibers.