Recovery of single muscle transfers in the rat hindlimb: Interaction between transfer mass and requirements of the recipient site.

Abstract

Reconstructive surgeons frequently transfer skeletal muscles to replace diseased or damaged muscles. Although numerous studies in animal models have documented the recovery of muscles transferred in the presence of synergistic muscles, the replacement of an entire muscle group with a single donor muscle is more relevant clinically. The purpose of this study was to determine the influence of transfer mass and the functional requirements of the recipient site on the structural and functional recovery of single muscle transfers. In 12 month old male Fisher 344 rats, individual plantarflexor or dorsiflexor muscles were transferred orthotopically after resection of all other plantarflexor or dorsiflexor muscles, respectively. For each transfer, the tendons and the nerve were sectioned and repaired, while the vasculature remained intact. After 120 days, maximum isometric tetanic force (F$\rm\sb{o})$ was measured in vivo. We hypothesized that, compared with their individual control values for muscle mass and F$\rm\sb{o},$ transfers representing 20% to 80% of the mass of the plantarflexor group would display a hierarchical array of deficits, proportional to their initial mass and, consequently, inversely proportional to the relative load on the transfers. This hypothesis was rejected. While transfers representing 20% of the mass of the total group recovered 100% of their individual control values for these variables, each larger transfer recovered only 60% of its individual control values. Reportedly, a maximum of 33% of total plantarflexor group F$\rm\sb{o}$ is required during normal activities, which might explain the complete recovery of transfers representing 20% of the mass of the total group, and conversely, the incomplete recovery of the larger transfers. For the dorsiflexor group, the normal force requirements are less than 33% of total group F$\rm\sb{o}.$ Therefore, we hypothesized that, compared with their individual control values for muscle mass and F$\rm\sb{o},$ transfers representing 20% of the mass of dorsiflexor group would exhibit greater deficits than transfers representing 20% of the plantarflexor group, whereas the deficits displayed by plantarflexor and dorsiflexor transfers representing 80% of the mass of their respective muscle groups would not be different. This hypothesis was supported. Each of the 80% transfers recovered 60% of their individual control values for muscle mass and F$\rm\sb{o},$ whereas the only 20% transfers to exhibit deficits were those in the dorsiflexor site. Since the recovery of each of the 80% transfers restored only 45% of their original group F$\rm\sb{o},$ conditioning programs will be required to improve the recovery of single muscle transfers in clinical situations.