Objective: To clarify the clinical features of the microform cleft lip and to establish the ultrastructural characteristics of the orbicularis muscle.

Design: Clinical observations of the characteristic deformities and associated anomalies were made. Muscle biopsies were harvested for histologic and ultrastructural analyses.

Patients: Seventy-one consecutive patients with microform cleft lip were included in the study. Muscle biopsies were investigated in 11 patients among them.

Results: Nasal deformity, a ridge or a groove from the vermilion to the nostril sill, and interruption of the "white roll" were present in all patients. Lack of a philtral column and a free border notch was observed in over 97% of patients. The orbicularis muscle demonstrated hypoplastic myofibers with nonneurogenic atrophy and focal accumulation of subsarcolemmal mitochondria.

Conclusion: The typical gross morphology of the microform cleft lip is a surface manifestation of muscular defect, and the disruption of the muscle further extends down to the ultrastructural level. The clinical features, taken together with the ultrastructural defects of the musculature, might help with a more precise delineation of the microform cleft lip, and provide better understanding of cleft lip in general.