<BR><BR>Pulleys<BR><BR>


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Around 1990, Joseph Demer and associates began study of the anatomy and actions of the extraocular muscles using high resolution magnetic resonance imaging on clinical patients and normal subjects aided in some cases by the use of paramagnetic MRI contrast agents (Figure 7, 8). This work, along with detailed histologic and histochemical study of human and monkey orbital tissue in the laboratory, led to the following summation by the authors in 2002. "The resulting reexamination of EOM (extraocular muscle) anatomy and physiology has been so revealing as to suggest a fundamental paradigm shift having broad basic and clinical implications."

SAGGITAL

Figure 7
The �pulley zone� is roughly at the junction of the middle and posterior third of the globe, similar to Listing�s plane (see page 32).
A Trochlea of the superior oblique - inflection of the superior oblique.
B Lockwood ligament - the �pulley� of inferior rectus -- the functional origin of the inferior oblique (the functional insertion of the inferior oblique after distal myectomy).
C The pulley of the horizontal

CORONAL

Figure 8
A Trochlea
B Confluence of superior oblique tendon and superior
rectus sheath
C Lockwood�s ligament
D Pulley of the horizontal recti
E Whitnall�s ligament
F Levator palpebri

Demer and associates� conclusions are summarized as follows:

Orbital structures called �pulleys� are associated with each of the rectus muscles and the inferior oblique.
The �pulleys� receive the contractile force of the extraocular muscles and inflect the paths of the muscles in a "qualitatively similar manner to the inflection of the superior oblique (SO) tendon path by the trochlea" (Figure 9).
The paths of the extraocular muscles posterior to the pulleys (between the pulleys and the annulus of Zinn) remain constant regardless of the position of the globe. There is no �side slip� of the rectus muscles, (except in the case of an abnormality of the pulley).
The functional origin of the extraocular muscles is at their pulleys (Figure 10).
The orbital one-half of the extraocular muscle fibers insert into the pulley and the bulbar one-half of extraocular muscle fibers pass forward to attach to the globe at the muscle�s insertion (Figure 11).
Only that portion of the extraocular muscle anterior to the pulleys moves in the direction of the globe�s movement (Figure 12).
Upward displacement of the lateral rectus pulleys and downward displacement of the medial rectus pulleys are associated with �A� pattern. (Figure 13) Downward displacement of the lateral rectus pulleys and upward displacement of the medial rectus pulleys are associated with �V� pattern (Figure 14).
Pulleys made up of collagen, elastin, and richly innervated smooth muscle are situated in the orbit in the area previously called check ligaments. They are not readily distinguishable clinically and require special techniques to be seen in the laboratory.
Several other pulley abnormalities could be associated with strabismus entities including:

Figure 9
The pulleys which �inflect the paths of the muscle.�
A Medial rectus pulley
B Lateral rectus pulley

Figure 10
The functional origin of the rectus muscles is at the pulleys.

Figure 11
The orbital fibers insert into the pulleys of the horizontal recti and the global fibers insert into sclera.

fig. 12a fig. 12b fig. 12c

Figure 12
The muscle - tendon anterior to the pulley
A Passes straight in primary position
B Courses upward in upgaze
C Courses downward in downgaze
D The direction of the muscle posterior to the pulley does not change during up and downgaze

fig. 13

Figure 13
Some combination of:
Upward displacement of the lateral recti
Downward displacement leads to �A� pattern

fig. 14

Figure 14
Some combination of:
Downward displacement of the lateral recti
Upward displacement of the medial recti leads to �V� pattern

�Y� pattern exotropia (pulley instability of the superior rectus, inferior rectus and lateral rectus); incomitant strabismus (abnormal sideslip of rectus extraocular muscle paths in certain gaze positions); and Brown syndrome (downward shift of the lateral rectus pulley in adduction or supraduction).

The strabismus surgeon should be aware of the existence of pulleys and of their significance when undertaking the management of strabismus. Although the surgeon will neither observe nor manipulate these structures except in special cases, they are nonetheless an important factor in establishing a proper diagnosis and in designing the best surgical procedure in many cases. An appreciation of the function of the pulleys contributes to understanding the behavior of the eye movements in the strabismus patient as well as in the normal. Diagnosis, plan for treatment, and assessment of the outcomes of treatment of strabismus are enhanced by knowledge of the extraocular muscle pulleys.

The reasons that the strabismus surgeon is not likely to see the pulleys are several. First, surgery of the extraocular muscles is carried out beneath anterior Tenon's capsule and in the plane of posterior Tenon's capsule. It is done anterior to the origin of anterior Tenon's capsule which itself is just anterior to the location of the pulleys. Second, dissection carried posterior to the origin of anterior Tenon's capsule (where it can be seen fusing with the muscle sheath or posterior Tenon's capsule) will expose extraconal fat, which both complicates surgery and obscures the surrounding anatomy, including the pulleys. Third, although the pulleys are located in the orbital fat just behind the insertion of anterior Tenon's capsule, they are virtually impossible to identify for what they are. This should be obvious since these structures escaped detection for nearly 200 years, in spite of extensive study of the contents of the human orbit carried out by many competent investigators.

Prior to the studies of Demer and associates, the closest anyone could come to identifying these structures was a description of the �check ligaments� of the horizontal recti, Whitnall's ligament superiorly, the trochlea, and Lockwood's ligament. By asking the right questions and using advanced techniques for imaging and histochemical analysis, along with meticulous dissection and histologic study, Demer and associates accomplished the difficult task of describing newly-recognized anatomy. But, as with so many other discoveries in science, this new revelation came after the earlier work of others which gave hints of what would be. In this case, the observation of Urrets-Zavalia, which called attention to the relationship of palpebral fissure configuration and vertical incomitance, is certainly a precursor to the revelation of pulley displacement. The long known association of �V� pattern with the pronounced antimon goloid fissures of Crouzon and the individuals with true mongoloid fissures who demonstrate an �A� pattern also provided clues. Limon in Mexico was particularly instrumental in correlating orbital anatomy with a variety of strabismus patterns which were no doubt influenced by the as yet undiscovered pulleys.