The effect of muscle transplantation after unilateral partial facial paralysis on craniofacial growth and development: Relationship between muscle and nerve histomorphometric findings

Muscle transplantation has become an indispensable tool to restore the smile in patients with long-standing or congenital facial paralysis. However, little is known of the effect of this surgical intervention on craniofacial growth and development or of the adaptation of the transplant to its recipient site under circumstances of growth. The present study investigates these phenomena in the rabbit model. Twelve-day-old New Zealand White rabbits were randomly assigned to three experimental groups. The control group was used to study normal craniofacial growth and development (n = 15). In the nerve ablation group, unilateral paralysis of the buccal branches of the facial nerve was surgically induced (n = 15). In the transplant group, the surgically induced unilateral paralysis of the buccal branches was immediately followed by a neuromuscular graft (n = 12). All animals were operated on at the age of 12 days, and follow-up evaluations were performed at the ages of 2 months and 6 months. Computerized dorsoventral roentgencephalometric and computed tomography investigations were performed at both ages. Nerve and muscle histomorphometric measurements were performed at the age Of 6 months to relate the quality of nerve and muscle regeneration to the growth parameters. The roentgencephalometric measurements revealed that analogous disturbed parameters were present in the nerve ablation and the transplant groups. However, in the transplant group, an additional significant effect of time between 2 and 6 months was seen for some parameters. This resulted in significant differences between the nerve ablation and transplant groups at 6 months for these parameters.' Computed tomography measurements showed no significant differences in maxillary or mandibular volume in the transplant group compared with the control or nerve ablation groups. However, a significantly diminished increase in bone volume existed in the transplant group for the time period between 2 and 6 months in comparison with the control and nerve ablation groups. Muscle histomorphometric findings revealed a significant change in muscle fiber composition in the graft compared with the normal latissimus dorsi muscle; this was due to a major decrease in type IIB fibers, with an increase in type I and type IIA fibers. Compared with the normal zygomaticoauricular muscle, the amount of type I fibers was significantly increased. No fiber atrophy was found. Macroscopically, the transplanted muscle failed to increase its length during growth. Nerve histomorphometric findings demonstrated a normal amount of nerve fibers; however, they had significantly decreased diameters and reduced myelin areas. The nerve histomorphometric parameters were related to the muscle histomorphometric findings, which in turn were related to craniofacial growth disturbances. These findings suggested that the main growth differences between the transplant group and the control group may have been due to altered nerve function influencing muscle function. Scar tissue formation and the development of more intense muscle activity later are suggested as the causes of the additional effect of time between 2 and 6 months for the several parameters in the transplant group. Reasons for the failure of complete conversion of the graft to a fast muscle and the failure of the transplant to elongate during growth are discussed.