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Chapter 2: Surgical Anatomy :
Surgical anatomy of the rectus muscles
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Each rectus muscle inserts at a different distance from the limbus. The insertions of these muscles are the prime surgical landmarks in extraocular muscle surgery. The medial rectus is said to insert in the normal eye 5.5 mm from the limbus. This figure presumably was arrived at from study of otherwise normal eyes. Since no specific mention is made of whether the measurements were taken from specimens with strabismus, it is assumed they were not. The average distance between the limbus and the medial rectus insertion of 112 medial rectus muscles in 66 esotropic patients was 4.4 mm with a range of 3.0 to 6.0 mm. Eight patients had unequal medial rectus insertion to limbus measurements. There was no correlation found between the angle of esodeviation and the distance of the medial rectus insertion from the limbus. The variability of this insertion along with its lack of correlation with the angle of esotropia begs the question, “Is the insertion the best landmark for measurement of a medial rectus recession?” Since the answer is obviously no, it is preferable to use the limbus, a more consistent anatomical point, as the reference for recession of the medial rectus muscle. When measuring from the limbus, the amount of muscle retroplacement from the muscle's actual insertion can be noted by those surgeons accustomed to the ‘traditional’ medial rectus recession ‘numbers’ used as guidelines for recession. For example, if a 5.5 mm recession of the medial rectus is done in a patient whose medial rectus inserts 4.5 mm from the limbus (not noticed by the surgeon) and a 5.5 mm recession is done, the new insertion site is located 9.0 mm from the limbus in a normal sized eye. This could result in an undercorrection, and in all likelihood, this occurred not infrequently when 5.5 mm was considered the maximum medial rectus recession. On the other hand, if in this same patient the medial rectus were recessed 10.0 mm from the limbus, the resultant recession measured from the insertion would actually be 6.5 mm, a number perhaps considered too large for the deviation but one which would be required because of the medial rectus insertion site being closer to the limbus.
Use of the limbus as the point of reference for medial rectus recession allows the surgeon to perform larger recessions safely by not exceeding the landmark of the equator. The equatorial landmark has been shown to be reliable because in patients with refractive errors between + or - 4.00 diopters, the axial length of the eye is predictable for the age of the patient. This has been confirmed by simple to perform axial length measurement with the A-scan device. At the same time, the corneal dimension is also reliable. If it appears to be other than the normal dimension, this is obvious and measurement for confirmation is simple. Whether or not discovery of the pulleys will alter this thinking is not clear now. It is known, however, that successful realignment of congenital esotropia occurs more frequently when measurement is carried out from the limbus compared to the prior upper limit of recession of 5.5 mm. With larger medial rectus recession measured from the insertion now being done, first surgery alignment in congenital esotropia is improved, but the incidence of overcorrection is not known.
The inferior rectus inserts 6.5 mm from the limbus; the lateral rectus inserts 6.9 mm from the limbus (range: 4.5 to 8.0 mm);* and the superior rectus inserts 7.7 mm from the limbus. Beginning with the medial rectus and moving inferiorly and temporally, each rectus muscle inserts farther from the limbus. The line connecting these insertions is called the spiral of Tillaux (Figure 27). The circumference of the ring formed by closing the spiral is approximately 80 mm. The width of the insertion of each of the rectus muscles is approximately 10 mm. The distance between the adjacent insertion borders is approximately 10 mm (Figure 28).
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Figure 27
The spiral of Tillaux and the relationship of the rectus muscle insertions. |
Figure 28
Width of the rectus muscle insertions |
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The issue can be summed up as follows: the insertion of the medial rectus muscle in esotropia tends to be closer to the limbus than the 5.5 mm stated for the normal. Therefore, recession measured from the limbus, a more reliable landmark, allows larger recessions to be done safely thus reducing the likelihood of undercorrection. |
The insertion of the rectus muscles can be seen relatively easily through the intact conjunctiva. This means that the muscles’ location can be confirmed when the eye is rotated and the conjunctiva is brought tightly over the insertion of any of the rectus muscles. Close observation reveals the line of insertion of the muscle, with the muscle appearing as a slightly darker and faintly raised structure beneath conjunctiva (Figure 29). By confirming the rectus muscle’s insertion in this manner, the surgeon can locate each of the rectus muscles accurately in roughly the 3, 6, 9, and 12 o'clock positions of the globe. This maneuver leads to proper traction suture or traction forceps placement and allows strategic placement of the incision through conjunctiva leading to accurate localization of the muscle to be operated upon. This maneuver to establish the location of the rectus muscles should be done routinely at the outset of each eye muscle surgical procedure.
The rectus muscles are all approximately 40 mm long and each receives innervation from the undersurface (intraconal space) at the junction of the middle and posterior thirds of the muscle or 26 mm from the insertion. The six pairs of extraocular muscles are characterized in Table 1.
* Although the lateral rectus insertion site is variable, it is not common to measure from the limbus for recession of this muscle.
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Figure 29
A The superior rectus muscle seen through the intact conjunctiva and anterior Tenon’s capsule.
B The insertion of the inferior rectus muscle seen through the intact conjunctiva. Note fat pad.
C The insertion of the lateral rectus muscle seen through the intact conjunctiva.
D The insertion of the medial rectus muscle seen through the intact conjunctiva.
E The insertion of the lateral and inferior rectus muscles seen through the intact conjunctiva with the inferior temporal orbital fat pad seen just inside the lower lid margin. The site of the incision for inferior oblique exposure is shown. This view is shown from above.
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Muscle |
Length(mm) |
Nerve |
Point of Innervation |
Tendon* (mm) |
Muscle action |
Medial
rectus (MR) |
40 |
III Inferior division |
26 mm from insertion |
L: 3.7
W: 10.3 |
Adduction |
Inferior
rectus (IR) |
40 |
III Inferior division |
26 mm from insertion |
L: 5.5
W: 9.8 |
Depression
Excycloduction
Adduction |
Lateral
rectus (LR) |
40 |
VI |
26 mm from insertion |
L: 8.8
W: 9.2 |
Abduction |
Superior
rectus (SR) |
40 |
III Superior division |
26 mm from insertion |
L: 5.8
W: 10.8 |
Elevation
Incycloduction
Adduction |
Inferior
oblique (IO) |
36 |
III Inferior division |
12 mm posterior to insertion of inferior rectus at its lateral border |
L: < 1
W: 9.4 |
Elevation
Excycloduction
Abduction |
Superior
oblique (SO) |
60 |
IV |
26 mm from trochlea |
L: 30
W: 10.7 |
Depression
Incycloduction
Abduction |
* L - length; W - width at insertion
Table 1 Extraocular Muscles |
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