The rectus muscles are more or less flat narrow bands that attach themselves with broad, thin tendons to the globe. There are four of these muscles: the medial (internal), the lateral (external), the superior, and the inferior.
The extraocular muscles have delightful synonyms in the old anatomical texts, some of which we cannot refrain from quoting: medial rectus (bibitorius, the drinking, because the eyes are crossed while looking at the bottom of the cup); lateral rectus (indignatorius, the angry); superior rectus (superbus, the proud; pius, the pious, because the upward turning of the eyes expresses devotion); inferior rectus (humilis, the humble). The superior oblique is also known as patheticus (the pathetic).26 Powell31 called the oblique muscles amatorii, quod sint velut in amore duces et furtivum oculorum jactus promoveant (for they are as leaders in love and promote furtive glances of the eyes).
The origins of the rectus muscles, the superior oblique muscle, and the levator muscle of the upper lid are at the tip of the orbital pyramid. There the origins of the muscles are arranged in a more or less circular fashion (the annulus of Zinn), surrounding the optic canal and in part the superior orbital fissure (Fig. 3–2). Through this oval opening created by the origins of the muscles, the opticnerve, the ophthalmic artery, and parts of cranial nerves III and VI enter the muscle cone formed by the body of the rectus muscles.
FIGURE 3–2. Posterior aspect of orbit showing topographic relationship of muscle origins in the annulus of Zinn. N, cranial nerve.
The interlocking of muscle and tendon fibers at the site of origin creates an extremely strong anchoring of the extraocular muscles. Avulsion of a muscle at the origin is rare even in cases where traction or trauma is sufficiently severe to cause avulsion of the optic nerve.33Attachments exist between the origins of the medial and superior recti and the dura of the optic nerve. This explains the pain occurring on eye movements in patients with optic neuritis.33
The medial and lateral rectus muscles follow the corresponding walls of the orbit for a good part of their course, and the inferior rectus muscle remains in contact with the orbital floor for only about half its length. The superior rectus muscle is separated from the roof of the orbit by the levator muscle of the upper lid.
If the rectus muscles were to continue their course in their original direction, they would not touch the globe; but about 10 mm posterior to the equator, the muscle paths curve toward the globe rather abruptly and eventually insert on the sclera at varying distances from the corneal limbus. The reason for this change in course is musculo-orbital tissue connections (the muscle pulleys; see below). Charpy,5quoting Motais,28describes how recurrent fibers may detach themselves from the bulbar side of the rectus muscles near their insertion, attaching themselves to the sclera 1 to 5 mm behind the insertions. Scobee32called these attachments footplates and attributed considerable importance to them in the etiology of esotropia (see Chapter 9).
Because the insertions of the rectus muscles are not equidistant from the corneal limbus, they do not lie on a circle that is concentric with it but rather on a spiral (the spiral of Tillaux). The insertion of the medial rectus muscle is closest to the corneal limbus, followed by the inferior, lateral, and superior rectus insertions, with the superior rectus insertion being the most distant (Fig. 3–3).
FIGURE 3–3. Insertions of rectus muscles. Average measurements are in millimeters. (Data from Apt L: An anatomical evaluation of rectus muscle insertions. Trans Am Ophthalmol Soc 78:365, 1980.)
The lines of insertion are generally not straight, but are more or less curved and sometimes even wavy. The straightest ones are the insertions of the medial and lateral rectus muscles, but these too are often slightly convex toward the corneal limbus. Fuchs15 found in 50 cadaver eyes that in half the cases the horizontal meridian cut the insertions symmetrically. For the rest of the cases, up to two thirds of the width of the tendon of the medial rectus muscle was above the horizontal meridian and that of the lateral rectus muscle was below it. Fuchs found also that the insertion line of these muscles was perpendicular to the horizontal meridian in less than half the eyes. In the others the insertion lines ran obliquely up and in, in the case of the medial rectus, and up and out, in the case of the lateral rectus.
The lines of insertion of the superior and inferior rectus muscles are markedly convex toward the corneal limbus and run obliquely upward and laterally. The rounded, temporal ends of their insertions therefore are more distant from the corneal limbus than their nasal ends. The amount of obliquity varies in different eyes but is usually marked and, according to Fuchs,15 is usually of the same degree for the two muscles of the same eye. The lines of insertion are cut asymmetrically by the vertical meridian. The greater part of the tendon (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 that the meridian bisected the inferior rectus tendon; otherwise, the larger segment of the insertion line was found to lie lateral to it.
The normal distance between muscle insertion and limbus is of importance during operations and reoperations on the extraocular muscles. Data based on measurements taken by Apt1 from cadaver eyes of adult subjects (mean age, 60.3 years) are shown in Figure 3–3 and Table 3–1. The anterior limbus was defined by Apt as the transition from clear cornea to gray and the posterior limbus as the transition from gray cornea to white sclera. While the means are similar to those of another recent study,23 the range of variations between data reported elsewhere in the literature is remarkable.15, 16, 22, 24, 35 The experienced surgeon is aware how often differences of several millimeters from the norms shown in Figure 3–3 can be found.
TABLE 3–1. Distance from Limbus to Rectus Muscle Insertions
Since a topographic correlation exists between the location of the tendon insertion and the ora serrata and since the distance of the ora from the limbus depends on the anteroposterior diameter of the globe,36, 37the distance of the tendon from the limbus may be influenced by age and axial refractive errors of the eye.15 If these variations in the location of the insertion are not taken into account (see Table 3–1), the value of geometric calculations in predicting the results of surgery on the action of the extraocular muscles is limited. For instance, the effect of a 4-mm muscle recession will vary significantly with the distance of the anatomical insertion from the limbus. These considerations apply especially when considering the effect of muscle surgery in infants. Table 3–2 shows a substantial difference in mean anatomical data obtained from adult and newborn eyes. According to Souza-Dias and coworkers,35 age differences in the distance between limbus and insertion can be neglected in strabismus operations in children older than 6 months. In view of the fact that the longitudinal growth of the eye is not completed by that age, we take a more conservative view and would put the age at which adult dosages of strabismus surgery may be applied at 2 years and older.
TABLE 3–2. Comparative Measurements of Medial and Lateral Rectus Muscles in Adults* and Newborns†
The length of the rectus muscles exclusive of tendon is fairly constant, but there are variations between the width of the insertion and the length of tendon of the different muscles (Table 3–3). Other anatomical data of importance to the kinematics of the eye are discussed in Chapter 4.
TABLE 3–3. Means and Range (in parentheses) of Measurements of Rectus Muscles (mm)*