Muscle fiber diameter and muscle type distribution following free microvascular muscle transfers: a prospective study

The histology of free microvascular muscle flaps in 19 patients was studied prospectively. Biopsies were taken during operation, and after 2 and 6 weeks, as well as 3, 6, and 9 months, postoperatively, fixed and stained using the van Gieson method. Fiber diameters were analyzed morphometrically and fiber types defined immunohistochemically using myosin fast antibody. During the nine-month follow-up period, mean muscle fiber diameter decreased significantly (P < 0.01), type-1 fibers atrophied significantly (P < 0.05) compared to type-2 fibers, and the percentage of type-2 fibers increased from a mean of 56% intraoperatively to 73% at 9 months. Fatty change and fibrosis were already present 2 weeks after operation and increased with the duration of follow-up. The decrease in muscle fiber diameter 9 months after free flap transfer correlated with clinical factors such as the delay of reconstructive surgery, recipient site, postoperative infection, and postoperative immobilization. The present results confirm that type-specific atrophy related to denervation appears and indicates that clinical events other than denervation influence the muscle atrophy seen in human free muscle flaps. These findings focus attention on the role of muscle regeneration, reinnervation, and revascularization taking place after free flap transfer.