The origins and insertions of the extraocular muscles: development, histologic features, and clinical significance.

The tendinous origins and insertions of the extraocular muscles were studied embryologically by macroscopic and microscopic methods. It is concluded from this investigation that these tendons of origin and insertion arise from mesenchymal tissue similar to that of their respective muscles. These tendon-muscle groups have developed from superior and inferior mesenchymal complexes. The origins of the extraocular muscles are attached to the periorbita by an interlocking of the tendinous and muscular fibers, which allows for mobility of the extraocular muscles in all extreme directions of gaze and also results in a strong mechanical mooring for these muscles. Avulsion at the origins of the extraocular muscles following severe traction or trauma is rare. The additional origin of the superior and medial rectus muscles to the dura of the optic nerve explains the pain that may occur on movement of the eye in optic neuritis. Optic nerve compression and thyroid myopathy is explained by mucopolysaccharide and inflammatory cell infiltration of the muscular interdigitations that extend up to the site of origin of the rectus muscles. Findings of this investigation suggest that the association of ptosis and superior rectus muscle underaction may be due to a persistence of fibrous tissue that has endured from embryologic development between the superior rectus and levator palpebrae superioris muscles. Superior oblique tendon sheath syndrome is explained by embryologic strands remaining between the tendon of the superior oblique muscle and the trochlea. The insertions of the rectus muscles extend from the equator of the eye to the limbus early on in development. By processes of differential degeneration between the sclera and the rectus tendon, posterior recession of the tendon from the limbus, and contemporaneous growth of the anterior segment of the eye, these tendons reach their adult location only between the ages of 18 months and 2 years. In strabismus surgery, measurements for muscle adjustments should be assessed from the limbus rather than from the sites of insertion of these tendons. In the series of patients with esotropia, no mechanical abnormalities were noted in relationship to the insertions of the medial or lateral recti muscles. Furthermore, no correlation was found between the site of insertion of the medial rectus muscle and the degree of esotropia.