Immunohistochemical analysis of orbicularis oris muscle fiber distribution at the philtrum in healthy infants

ObjectiveTo characterize the fiber-type distribution of the orbicularis oris muscle at the philtrum in healthy infants by immunohistochemistry and examine the relationship between orbicularis oris and philtrum structure.MethodsSamples of the upper lip were obtained from two infant cadavers. Serial sagittal sections were obtained at the midline of the philtral dimple, unilateral philtral ridge, and the lateral side. Three sections from each site were prepared for immunohistochemical staining using myosin heavy chain fast fiber (MHCf) and myosin heavy chain slow fiber (MHCs) antibodies to determine the ratio of fast to slow skeletal muscle fibers.ResultsThe ratio of fast to slow muscle fibers differed significantly among the superficial orbicularis oris muscle (98.30%:1.13%), deep pars peripheralis (95.30%:3.14%), and deep pars marginalis (91.31%:5.74%), with a significantly higher percentage of slow fibers in the pars marginalis compared to pars peripheralis (P = 0.002) and fast fibers in the superficial muscle compared to pars marginalis and peripheralis (both P = 0.000). Similarly, the fast:slow fiber ratio differed among the superficial philtral dimple (95.88%:2.41%), superficial philtral ridge (98.52%:1.11%), and superficial midlateral philtral ridge (99.07%:0.66%), with a higher percentage of fast fibers higher on the lateral side of the superficial philtral ridge than at the philtral ridge (P = 0.030) and higher at the philtral ridge than the philtral dimple (P = 0.001). The fast:slow fiber ratio did not differ within the pars peripheralis at the philtral dimple (93.94%:4.19%), philtral ridge (94.49%:3.84%), and lateral philtral ridge (95.79%:2.70%) (all P > 0.05).ConclusionsPhiltum structure is likely determined in part by the distribution of muscle fiber types among philtral dimple, ridge, and lateral side. These differences should be considered in cleft lip repair.