ORBICULARIS OCULI MUSCLE-FIBERS ARE RELATIVELY SHORT AND HETEROGENEOUS IN LENGTH

Purpose. The anatomy of individual myofibers within the orbicularis oc uli muscle was examined to determine individual myofiber lengths in th e different regions of the muscle. A wide variety of eyelid conditions require eyelid surgery or drug injections directly into the eyelid. K nowledge 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 neuromu scular junction location. Individual myofibers were dissected from the se muscle specimens and were measured to determine individual myofiber length and neuromuscular junction position. Additional eyelid specime ns of rabbits and humans were stained en bloc to Visualize neuromuscul ar junction location in the pretarsal and preseptal regions of the orb icularis oculi muscle. Results. The myofibers showed variable lengths, shorter in the pretarsal region of the muscle and longer in the prese ptal region. The average individual myofiber length in the pretarsal r egion was 36% as long as the entire length of the pretarsal muscle reg ion. In the preseptal region, the myofibers were slightly longer, cove ring 54% of the entire length of this region of the muscle. In both th e pretarsal and preseptal regions of the rabbit and human orbicularis oculi muscle, there were many clusters of neuromuscular junctions thro ughout the medial to lateral length of the muscle, with the majority o f the neuromuscular junctions in the medial and lateral canthal region s of the preseptal portion of the lid. This indicates that the muscle is composed of relatively short, overlapping myofibers, and that the s hortest myofibers reside in the medial and lateral canthal regions of the eyelid. Multiple innervation of one rabbit myofiber was observed a s a rare occurrence. Conclusions. Individual myofibers of the orbicula ris oculi muscle are relatively shore, end intrafascicularly, and are of heterogeneous lengths varying regionally within the muscle. Thus, f or drug injections into the eyelid, optimal drug effectiveness may req uire treatment of the entire lid from medial to lateral canthus to ove rcome the tissue barriers to diffusion. The existence of muscle fibers of heterogeneous lengths suggests that the complex organization of mu scle fibers may play previously unappreciated but important roles in n ormal function, pathophysiology, and age-related changes in the eyelid .