Chapter 3: Summary of the Gross Anatomy of the Extraocular Muscles

More documents

Recommendations

Info

$Vision and Refraction (PDF 3.31 MB) - Cyber-Sight - Orbis$

Summary of the Gross Anatomy of the Extraocular Muscles 39FIGURE 3–2. Posterior aspect of orbit showing topographic relationship of muscle origins in theannulus of Zinn. N, cranial nerve.the rectus muscles). Here again, Tenon’s capsule,fastened to the orbital rim and the retrobulbartissue, must provide the necessary balance. 39Rectus MusclesThe rectus muscles are more or less flat narrowbands that attach themselves with broad, thin tendonsto the globe. There are four of these muscles:the medial (internal), the lateral (external), thesuperior, and the inferior.The extraocular muscles have delightful synonymsin the old anatomical texts, some of whichwe cannot refrain from quoting: medial rectus(bibitorius, the drinking, because the eyes arecrossed while looking at the bottom of the cup);lateral rectus (indignatorius, the angry); superiorrectus (superbus, the proud; pius, the pious, becausethe upward turning of the eyes expressesdevotion); inferior rectus (humilis, the humble).The superior oblique is also known as patheticus(the pathetic). 26 Powell 31 called the oblique musclesamatorii, quod sint velut in amore duces etfurtivum oculorum jactus promoveant (for they areas leaders in love and promote furtive glances ofthe eyes).The origins of the rectus muscles, the superioroblique muscle, and the levator muscle of theupper lid are at the tip of the orbital pyramid.There the origins of the muscles are arranged in amore or less circular fashion (the annulus of Zinn),surrounding the optic canal and in part the superiororbital fissure (Fig. 3–2). Through this ovalopening created by the origins of the muscles, theoptic nerve, the ophthalmic artery, and parts ofcranial nerves III and VI enter the muscle coneformed by the body of the rectus muscles.The interlocking of muscle and tendon fibersat the site of origin creates an extremely stronganchoring of the extraocular muscles. Avulsion ofa muscle at the origin is rare even in cases wheretraction or trauma is sufficiently severe to causeavulsion of the optic nerve. 33 Attachments existbetween the origins of the medial and superiorrecti and the dura of the optic nerve. This explainsthe pain occurring on eye movements in patientswith optic neuritis. 33The medial and lateral rectus muscles followthe corresponding walls of the orbit for a goodpart of their course, and the inferior rectus muscleremains in contact with the orbital floor for onlyabout half its length. The superior rectus muscleis separated from the roof of the orbit by thelevator muscle of the upper lid.If the rectus muscles were to continue theircourse in their original direction, they would nottouch the globe; but about 10 mm posterior to theequator, the muscle paths curve toward the globerather abruptly and eventually insert on the scleraat varying distances from the corneal limbus. Thereason for this change in course is musculo-orbitaltissue connections (the muscle pulleys; see below).Charpy, 5 quoting Motais, 28 describes howrecurrent fibers may detach themselves from thebulbar side of the rectus muscles near their inser-
40 Physiology of the Sensorimotor Cooperation of the EyesFIGURE 3–3. Insertions of rectusmuscles. Average measurements arein millimeters. (Data from Apt L: Ananatomical evaluation of rectus muscleinsertions. Trans Am OphthalmolSoc 78:365, 1980.)tion, attaching themselves to the sclera 1 to 5mm behind the insertions. Scobee 32 called theseattachments footplates and attributed considerableimportance to them in the etiology of esotropia(see Chapter 9).Because the insertions of the rectus musclesare not equidistant from the corneal limbus, theydo not lie on a circle that is concentric with it butrather on a spiral (the spiral of Tillaux). The insertionof the medial rectus muscle is closest to thecorneal limbus, followed by the inferior, lateral,and superior rectus insertions, with the superiorrectus insertion being the most distant (Fig. 3–3).The lines of insertion are generally not straight,but are more or less curved and sometimes evenwavy. The straightest ones are the insertions ofthe medial and lateral rectus muscles, but thesetoo are often slightly convex toward the corneallimbus. Fuchs 15 found in 50 cadaver eyes thatin half the cases the horizontal meridian cut theinsertions symmetrically. For the rest of the cases,up to two thirds of the width of the tendon of themedial rectus muscle was above the horizontalmeridian and that of the lateral rectus muscle wasbelow it. Fuchs found also that the insertion line ofthese muscles was perpendicular to the horizontalmeridian in less than half the eyes. In the othersthe insertion lines ran obliquely up and in, in thecase of the medial rectus, and up and out, in thecase of the lateral rectus.The lines of insertion of the superior and inferiorrectus muscles are markedly convex towardthe corneal limbus and run obliquely upward andlaterally. The rounded, temporal ends of their insertionstherefore are more distant from the corneallimbus than their nasal ends. The amount ofobliquity varies in different eyes but is usuallymarked and, according to Fuchs, 15 is usually ofthe same degree for the two muscles of the sameeye. The lines of insertion are cut asymmetricallyby the vertical meridian. The greater part of thetendon (two thirds of its width according to Fuchs)of the superior rectus lies temporal to the meridian.In one third of the eyes, Fuchs found thatthe meridian bisected the inferior rectus tendon;otherwise, the larger segment of the insertion linewas found to lie lateral to it.The normal distance between muscle insertionand limbus is of importance during operationsand reoperations on the extraocular muscles. Databased on measurements taken by Apt 1 from cadavereyes of adult subjects (mean age, 60.3 years)are shown in Figure 3–3 and Table 3–1. Theanterior limbus was defined by Apt as the transitionfrom clear cornea to gray and the posteriorlimbus as the transition from gray cornea to whitesclera. While the means are similar to those ofanother recent study, 23 the range of variations betweendata reported elsewhere in the literature isremarkable. 15, 16, 22, 24, 35 The experienced surgeon isaware how often differences of several millimetersfrom the norms shown in Figure 3–3 can be found.
Page 1: CHAPTER3Summary of the GrossAnatomy
Page 5 and 6: 42 Physiology of the Sensorimotor C

Chapter 3: Summary of the Gross Anatomy of the Extraocular Muscles

Create successful ePaper yourself

Delete template?

Save as template?