Anatomy of the orbicularis oris muscle in incomplete unilateral cleft lip based on histological examination.

Histological sections of tissue removed from three incomplete cleft lips at the time of repair, a nature's scar line, and a noncleft lip were examined serially in the horizontal and saggital planes. The findings confirm previous data from electrical stimulation studies of clefts and a three-dimensional histological model of the orbicularis oris in a complete cleft lip. There is a disorganized mass of muscle fibers with numerous dermal insertions and no evidence of well-defined muscle bundles running parallel to the cleft margins. The fibers are somewhat hypoplastic and less abundant than those present in noncleft lips.