The anatomy of the orbicularis oris muscle in unilateral cleft lip based on a three-dimensional histologic reconstruction.

The midface of a full-term, stillborn infant with a right complete unilateral cleft lip and palate has been examined by serial histologic sections and a three-dimensional model has been created. The arrangement of the orbicularis oris muscle fibers observed differs markedly from previous descriptions. There is a chaotic arrangement of muscle fibers with no separate muscle layers distinguishable. Muscle fibers on both sides of the cleft insert into the dermis, although the amount of muscle on the medial side is quite sparse compared with the lateral side. There is no evidence of muscle bundles paralleling the cleft margins. An unexpected finding is a significant number of muscle fibers streaming over the cleft ala. In view of these findings, differing as they do from previous reports, there would seem to be strong justification for histologic investigation of the arrangement of muscle fibers in cleft lip to be repeated as further specimens become available.