Orbicularis oculi muscle fibers are relatively short and heterogeneous in length.

PURPOSE The anatomy of individual myofibers within the orbicularis oculi muscle was examined to determine individual myofiber lengths in the different regions of the muscle. A wide variety of eyelid conditions require eyelid surgery or drug injections directly into the eyelid. Knowledge of regional myofiber anatomy and physiology is important for accurate treatment of these conditions. METHODS Eyelid specimens from rabbits were treated with collagenase, fixed, and stained for neuromuscular junction location. Individual myofibers were dissected from these muscle specimens and were measured to determine individual myofiber length and neuromuscular junction position. Additional eyelid specimens of rabbits and humans were stained en bloc to visualize neuromuscular junction location in the pretarsal and preseptal regions of the orbicularis oculi muscle. RESULTS The myofibers showed variable lengths, shorter in the pretarsal region of the muscle and longer in the preseptal region. The average individual myofiber length in the pretarsal region was 36% as long as the entire length of the pretarsal muscle region. In the preseptal region, the myofibers were slightly longer, covering 54% of the entire length of this region of the muscle. In both the pretarsal and preseptal regions of the rabbit and human orbicularis oculi muscle, there were many clusters of neuromuscular junctions throughout the medial to lateral length of the muscle, with the majority of the neuromuscular junctions in the medial and lateral canthal regions of the preseptal portion of the lid. This indicates that the muscle is composed of relatively short, overlapping myofibers, and that the shortest myofibers reside in the medial and lateral canthal regions of the eyelid. Multiple innervation of one rabbit myofiber was observed as a rare occurrence. CONCLUSIONS Individual myofibers of the orbicularis oculi muscle are relatively short, end intrafascicularly, and are of heterogeneous lengths varying regionally within the muscle. Thus, for drug injections into the eyelid, optimal drug effectiveness may require treatment of the entire lid from medial to lateral canthus to overcome the tissue barriers to diffusion. The existence of muscle fibers of heterogeneous lengths suggests that the complex organization of muscle fibers may play previously unappreciated but important roles in normal function, pathophysiology, and age-related changes in the eyelid.