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

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Summary of the Gross Anatomy of the Extraocular Muscles 43FIGURE 3–5. Two-mm-thick, 320-m resolution axial MRI scan of normal left orbit in primary andsecondary gaze positions, showing near constancy of the positions of the rectus muscles posteriorto the pulleys. IR, inferior rectus; SR, superior rectus; MR, medial rectus; LR, lateral rectus; ON,optic nerve. Note stability of the coronal sections of the rectus muscles but movement of the sectionof the optic nerve in secondary gaze postions. (Courtesy of Dr. J.L. Demer, Los Angeles.)The trochlea is a tube 4 to 6 mm long formed inits medial aspect by bone (the trochlear fossa ofthe frontal bone). The rest of the circumference iscomposed of connective tissue that may containcartilaginous or bony elements. After passing thetrochlea, the superior oblique muscle turns in laterodorsally,forming an angle of about 54 withthe pretrochlear or direct portion of the muscle.A fibrillar, vascular sheath surrounds the intratrochlearsuperior oblique tendon. This portion ofthe tendon consists of discrete fibers with fewinterfibrillar connections, as reported by Helvestonand coworkers. 21 Each fiber of the tendon movesthrough the trochlea in a sliding, telescoping fashionwith the central fibers undergoing maximaland the peripheral fibers the least excursion. Thetotal travel of the central fibers appears to be 8mm in either direction. 20Helveston and coworkers 21 also described abursa-like structure lying between the trochlear‘‘saddle’’ and the vascular sheath of the superioroblique tendon and postulated that pathologic alterationsof the bursa may be a factor in theetiology of Brown syndrome (see Chapter 21).At about the distal third of the direct portion(10 mm behind the trochlea), the muscle becomestendinous and remains tendinous in its entire posttrochlearor reflected part. The tendon passes underthe superior rectus muscle, fans out, andmerges laterally with the sclera to the vertical
44 Physiology of the Sensorimotor Cooperation of the Eyesinsertion forms a curved concave line toward theorigin of the muscle. Its anterior margin is about10 mm behind the lower edge of the insertion ofthe lateral rectus muscle; its posterior end is 1 mmbelow and 1 to 2 mm in front of the macula (Fig.3–7). Near its insertion the posterior border of themuscle is related to the inferior vortex vein.Unlike the other extraocular muscles, especiallythe superior obliques, which have both muscularand tendinous components, the inferioroblique is almost wholly muscular. It forms anangle of about 51 with the vertical plane of theglobe.FIGURE 3–6. Relationships of tendons of superioroblique muscle. Measurements are in millimeters. (Modifiedfrom Fink WH: Surgery of the Vertical Muscles ofthe Eyes, ed 2. Springfield, IL, Charles C Thomas, 1962.)meridian, forming a concave curved line towardthe trochlea (Fig. 3–6). The anterior end of theinsertion lies 3.0 to 4.5 mm behind the lateral endof the insertion of the superior rectus muscle and13.8 mm behind the corneal limbus. The posteriorend of the insertion lies 13.6 mm behind themedial end of the insertion of the superior rectusmuscle and 18.8 mm behind the corneal limbus.The width of the insertion of the superior obliquemuscle varies greatly (from 7 to 18 mm, Fink 14 )but is 11 mm on average. The medial end of theinsertion lies about 8 mm from the posterior poleof the globe. Near its insertion the posterior borderof the muscle is related to the superior vortex vein.The length of the direct part of the superioroblique muscle is about 40 mm and that of thereflected tendon is about 19.5 mm. From a physiologicand kinematic standpoint, the trochlea is theorigin of the muscle.The inferior oblique muscle is the shortest ofall the eye muscles, being only 37 mm long. Itarises in the anteroinferior angle of the bony orbitin a shallow depression in the orbital plate of themaxilla near the lateral edge of the entrance intothe nasolacrimal canal. The origin is readily locatedby drawing a perpendicular line from thesupraorbital notch to the lower orbital margin.The muscle continues from its origin backward,upward, and laterally, passing between the floorof the orbit and the inferior rectus muscle. Itinserts by a short tendon (1 to 2 mm) in theposterior and external aspect of the sclera. Thewidth of the insertion varies widely (5 to 14 mm,Fink 14 ) and may be around 9 mm on average. TheFascial SystemTenon’s CapsuleThe eyeball is suspended within the orbit by asystem of fasciae. The way in which this isachieved represents an ideal solution to the problemof suspending a spheroid body in a coneshapedcavity. The bulk of the system is made upof Tenon’s capsule, which is a condensation offibrous tissue that covers the eyeball from theentrance of the optic nerve to near the corneallimbus, where it is firmly fused with the conjunctiva.Except for this area of fusion, the two structuresare separated by the subconjunctival space.Tenon’s capsule is also separated from the sclera.Between the two is the episcleral space (Tenon’sspace), which can be readily injected (Fig. 3–8).On its outer aspect the capsule is intimately relatedto the orbital reticular tissue. Its posterior edge isFIGURE 3–7. Course of inferior oblique muscle and therelationships of its tendon. Measurements are in millimeters.(Modified from Fink WH: Surgery of the VerticalMuscles of the Eyes, ed 2. Springfield, IL, Charles CThomas, 1962.)
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Chapter 3: Summary of the Gross Anatomy of the Extraocular Muscles

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