Magnetic resonance imaging after surgical transposition defines the anteroposterior location of the rectus muscle pulleys

Lateral rectus pulley concerning the orbital wall. Area of a stereotyped bony insertion

Purpose

To examine the lateral rectus muscle pulley and its bony insertion concerning the orbital rim and periorbita.

Design

Prospective. An observational anatomic study.

Methods

Study population: Twenty postmortem orbits (10 right, 10 left) of 10 Caucasian cadavers (8 females, 2 males; age range at death, 57-100 years; median age, 79.5 years) fixed by the Thiel method.Intervention: The floor of the temporal fossa was exposed, and a bone window on the lateral wall of the orbit, posterior to the sphenozygomatic suture, was created, keeping the periorbita intact. The lateral canthus and lateral palpebral ligament were isolated and opened, and the eyelids were folded back. The frontozygomatic suture was identified, and the orbital septum opened adjacent to the orbital rim. The conjunctiva was opened at the limbus, and the lateral rectus insertion was isolated. The bone pillar containing the frontozygomatic suture and the insertion of the periorbita and the pulley was isolated and removed en bloc. The lateral rectus muscle was isolated and excised.Main outcome measures: Position of the pulley ring on the lateral rectus muscle belly and its bony attachment area in the lateral wall of the orbit.

Results

The pulley bony attachment was roughly quadrilateral with an approximate area of 90 mm2, 3 mm (mean, range 1-5 mm) posteroinferior to the frontozygomatic suture and 1 mm posterior to the orbital rim. The anterior margin of the pulley sleeve was found at 21.0 mm (median, p25-75 20.0-22.8) from the scleral insertion.

Conclusions

The lateral rectus pulley is stereotyped in its position in the muscle belly and its bony insertion, coinciding with the point of greatest adhesion of the periorbita to the anterior part of the lateral wall of the orbit.

Dual-Augmented Transposition of Vertical Recti in Chronic Abducens Palsy

PURPOSE

To report the results of dual augmentation of vertical rectus muscle transposition (VRT) in the treatment of chronic sixth nerve palsy.

DESIGN

Retrospective case series.

METHODS

This is a retrospective review of medical records of patients with chronic sixth nerve palsy who underwent dual augmented VRT with or without medial rectus (MR) recession from 2013 to 2016. Data collection included sex, age, laterality, and duration of postoperative follow-up. Pre- and postoperative limitation of abduction and adduction were recorded using a 6-point scale. Improvement of esotropia in prism diopter (PD), head turn in degrees, and limitation of abduction and adduction were reported and analyzed.

RESULTS

Fourteen cases were identified. Mean patients' age at the time of surgery was 22.5 years. Postoperatively, esotropia and head turn were corrected by a mean of 31.3 PD and 18.2 degrees, respectively. Limited abduction was improved from -4.3 to -1.6, while in cases that underwent MR recession, adduction declined from 0.4 to -0.3. Postoperative induced small-amplitude hypertropia was reported in 3 cases.

CONCLUSION

Dual augmented VRT was effective in controlling esotropia, head turn, and limited abduction associated with chronic sixth nerve palsy with low rate of induced vertical deviation. Combined MR recession carries a risk of induced limitation of adduction.

Multiband diffusion-weighted MRI of the eye and orbit free of geometric distortions using a RARE-EPI hybrid

Diffusion-weighted imaging (DWI) provides information on tissue microstructure. Single-shot echo planar imaging (EPI) is the most common technique for DWI applications in the brain, but is prone to geometric distortions and signal voids. Rapid acquisition with relaxation enhancement [RARE, also known as fast spin echo (FSE)] imaging presents a valuable alternative to DWI with high anatomical accuracy. This work proposes a multi-shot diffusion-weighted RARE-EPI hybrid pulse sequence, combining the anatomical integrity of RARE with the imaging speed and radiofrequency (RF) power deposition advantage of EPI. The anatomical integrity of RARE-EPI was demonstrated and quantified by center of gravity analysis for both morphological images and diffusion-weighted acquisitions in phantom and in vivo experiments at 3.0 T and 7.0 T. The results indicate that half of the RARE echoes in the echo train can be replaced by EPI echoes whilst maintaining anatomical accuracy. The reduced RF power deposition of RARE-EPI enabled multiband RF pulses facilitating simultaneous multi-slice imaging. This study shows that diffusion-weighted RARE-EPI has the capability to acquire high fidelity, distortion-free images of the eye and the orbit. It is shown that RARE-EPI maintains the immunity to B₀ inhomogeneities reported for RARE imaging. This benefit can be exploited for the assessment of ocular masses and pathological changes of the eye and the orbit.

Rectus Extraocular Muscle Size and Pulley Location in Concomitant and Pattern Exotropia

PURPOSE

To determine whether rectus extraocular muscle (EOM) sizes and pulley locations contribute to exotropia, we used magnetic resonance imaging (MRI) to measure these factors in normal control participants and in patients with concomitant and pattern exotropia.

DESIGN

Prospective case-control study.

PARTICIPANTS

Nine patients with concomitant exotropia, 6 patients with pattern exotropia, and 21 orthotropic normal control participants.

METHODS

High-resolution surface-coil MRI scans were obtained in contiguous, quasicoronal planes. Rectus pulley locations were determined in oculocentric coordinates for central gaze, supraduction, and infraduction. Cross sections in 4 contiguous image planes were summed and multiplied by the 2-mm slice thickness to obtain horizontal rectus posterior partial volumes (PPVs).

MAIN OUTCOME MEASURES

Rectus pulley locations and horizontal rectus PPVs.

RESULTS

Rectus pulleys were located differently in patients with A-pattern, versus V- and Y-pattern, exotropia. The lateral rectus (LR) pulleys were displaced significantly superiorly, the medial rectus (MR) pulleys were displaced inferiorly, and the inferior rectus pulleys were displaced laterally in A-pattern exotropia. However, the array of all rectus pulleys was excyclorotated in V- and Y-pattern exotropia. The PPV of the medial rectus muscle was statistically subnormal by approximately 29% in concomitant, but not pattern, exotropia (P < 0.05). The ratio of the PPV of the LR relative to the MR muscles in concomitant exotropia was significantly greater than in control participants and those with pattern exotropia (P < 0.05).

CONCLUSIONS

Abnormalities of EOMs and pulleys contribute differently in pattern versus concomitant exotropia. Abnormal rectus pulley locations derange EOM pulling directions that contribute to pattern exotropia, but in concomitant exotropia, pulley locations are normal, and relatively small medial rectus size reduces relative adducting force.

Recent advances clarifying the etiologies of strabismus

BACKGROUND

Strabismus is commonly encountered in neuro-ophthalmology practice. Adult patients may present with symptoms including disabling diplopia and decreased quality of life. Although presentation to the neuro-ophthalmologist often prompts a thorough workup for a neurologic basis of ocular misalignment, advances in orbital imaging and understanding of orbital mechanics have revealed novel mechanical causes. A goal of this review is to clarify mechanical mechanisms of strabismus that were formerly assumed be neurologic in origin.

EVIDENCE ACQUISITION

The authors combine their own research and clinical experience with a literature review using PubMed.

RESULTS

Aberrant paths of the extraocular muscles can lead to strabismus. The extraocular muscles have connective tissue pulleys that control muscle paths and are, in turn, influenced by the extraocular muscle orbital layers. Orbital connective tissues, including the pulleys, constrain extraocular muscle paths. Abnormalities of these tissues may lead to strabismus that is not due to neurologic pathology. Some extraocular muscles are divided into independent neuromuscular compartments, so that partial motor nerve lesions may manifest as selective denervation of only 1 compartment, complicating the presentation of neuropathic strabismus.

CONCLUSIONS

Strabismus in adults due to nonneurologic causes can result from recently described abnormalities of the orbital connective tissue pulley system. Advances in understanding of compartmental extraocular muscle anatomy and innervation can explain cyclovertical strabismus in partial nerve palsies. Recognition of the underlying pathogenesis of the strabismus can lead to improved treatments.

Vertical rectus transpositions in sixth nerve palsies

PURPOSE OF REVIEW

There are many surgical options to address a sixth nerve palsy including transpositions of the vertical recti to the lateral rectus. This review will summarize the results from variations on transpositions as they apply to sixth nerve palsies.

RECENT FINDINGS

Transposition of both vertical recti with posterior fixation creates the greatest correction of esotropia in primary position with largest field of single vision. Transposition of the superior rectus alone preserving anterior segment circulation achieves effects similar to transposition without posterior fixation. Augmentation with partial transposition also creates similar improvement in esotropia with possibilities for adjustable sutures and circulation preservation.

SUMMARY

These various options on transpositions allow options to specifically address surgeon preferences and patient needs to select the appropriate surgery.

The Role of Extraocular Muscle Pulleys in Incomitant Non-Paralytic Strabismus

The rectus extraocular muscles (EOMs) and inferior oblique muscle have paths through the orbit constrained by connective tissue pulleys. These pulleys shift position during contraction and relaxation of the EOMs, dynamically changing the biomechanics of force transfer from the tendon onto the globe. The paths of the EOMs are tightly conserved in normal patients and disorders in the location and/or stability of the pulleys can create patterns of incomitant strabismus that may mimic oblique muscle dysfunction and cranial nerve paresis. Developmental disorders of pulley location can occur in conjunction with large, obvious abnormalities of orbital anatomy (e.g., craniosynostosis syndromes) or subtle, isolated abnormalities in the location of one or more pulleys. Acquired disorders of pulley location can be divided into four broad categories: Connective tissue disorders (e.g., Marfan syndrome), globe size disorders (e.g., high myopia), senile degeneration (e.g., sagging eye syndrome), and trauma (e.g., orbital fracture or postsurgical). Recognition of these disorders is important because abnormalities in pulley location and movement are often resistant to standard surgical approaches that involve strengthening or weakening the oblique muscles or changing the positions of the EOM insertions. Preoperative diagnosis is aided by: (1) Clinical history of predisposing risk factors, (2) observation of malpositioning of the medial canthus, lateral canthus, and globe, and (3) gaze-controlled orbital imaging using direct coronal slices. Finally, surgical correction frequently involves novel techniques that reposition and stabilize the pulley and posterior muscle belly within the orbit using permanent scleral sutures or silicone bands without changing the location of the muscle's insertion.

Vertical muscle transposition augmented with lateral fixation (Foster) suture for Duane syndrome and sixth nerve palsy

PURPOSE

To report the postoperative results of full-tendon vertical rectus transposition (VRT) augmented with lateral fixation suture for the treatment of type 1 Duane syndrome and sixth nerve palsy and to determine whether there was a decrease in the effect of the Foster suture over time.

METHODS

This retrospective, consecutive case series included patients who underwent a full-tendon VRT transposition with lateral fixation for type 1 Duane syndrome or sixth nerve palsy. The primary outcome measures included deviation, abnormal head posture(AHP), abduction deficiency, and postoperative binocular single visual field (BSVF).

RESULTS

Eighty-seven patients (87 eyes: 40 eyes with Duane syndrome and 47 eyes with sixth nerve palsy) were included in this study. In Duane syndrome patients, the deviation was reduced by a mean of 95%, the AHP was eliminated in 86% of patients, the abduction was improved by 42%, and a useful BSVF of ≈ 67% of normal was achieved at 1 year post operation. In sixth nerve palsy patients, the deviation was reduced by 99%, the abduction was improved by 59%, and a useful BSVF of ≈ 71% of normal was achieved at 1 year post operation. In both groups, the improvements in deviation angle and abduction were stable postoperatively. Sixteen patients needed reoperation for undercorrection.

CONCLUSION

VRT surgery with posterior fixation is an effective treatment method for complete sixth nerve palsy and Duane syndrome with esotropia, AHP, and abduction deficiency. The procedure carries a small risk of reoperation for undercorrection. The effect of the Foster suture did not decline over time.

Effects of oblique muscle surgery on the rectus muscle pulley

PURPOSE

To determine the position of rectus muscle pulleys in Japanese eyes and to evaluate the effect of oblique muscle surgery on rectus muscle pulleys.

METHODS

Quasi-coronal plane MRI was used to determine area centroids of the 4 rectus muscles. The area centroids of the rectus muscles were transformed to 2-dimensional coordinates to represent pulley positions. The effects of oblique muscle surgery on the rectus muscle pulley positions in the coronal plane were evaluated in 10 subjects with cyclovertical strabismus and, as a control, pulley locations in 7 normal Japanese subjects were calculated.

RESULTS

The mean positions of the rectus muscle pulleys in the coronal plane did not significantly differ from previous reports on normal populations, including Caucasians. There were significant positional shifts of the individual horizontal and vertical rectus muscle pulleys in 3 (100%) patients with inferior oblique advancement, but not in eyes with inferior oblique recession and superior oblique tendon advancement surgery. The surgical cyclorotatory effect was significantly correlated with the change in the angle of inclination formed by the line connecting the vertical rectus muscles (p = 0.0234), but weakly correlated with that of the horizontal rectus muscles.

CONCLUSIONS

The most important factor that affects the pulley position is the amount of ocular torsion, not the difference in surgical procedure induced by oblique muscle surgery.

Magnetic resonance imaging of the functional anatomy of the inferior rectus muscle in superior oblique muscle palsy

PURPOSE

Biomechanical modeling consistently indicates that superior oblique (SO) muscle weakness alone is insufficient to explain the large hypertropia often observed in SO muscle palsy. Magnetic resonance imaging (MRI) was used to investigate if any size or contractility changes in the inferior rectus (IR) muscle may contribute.

DESIGN

Prospective, case-control study.

PARTICIPANTS

Seventeen patients with unilateral SO muscle palsy and 18 orthotropic subjects.

METHODS

Surface coils were used to obtain sets of contiguous, 2-mm-thick, high-resolution, coronal MRI views in different gazes. Cross-sectional areas of the IR and SO muscles were determined in supraduction and infraduction for evaluation of size and contractility. Diagnosis of SO muscle palsy was based on clinical presentations, subnormal contractility, and SO muscle size less than the normal 95% confidence limit.

MAIN OUTCOME MEASURES

Cross-sectional areas of the IR and SO muscles.

RESULTS

Patients had 15.9+/-7.2 prism diopters (Delta; mean+/-standard deviation) of central gaze hypertropia and exhibited ipsilesional SO muscle atrophy and subnormal contractility. Mean ipsilesional, contralesional, and normal IR muscle cross-sections were 28.5+/-3.5 mm(2), 31.9+/-3.8 mm(2), and 31.8+/-5.8 mm(2), whereas mean contractility was 16.5+/-3.8 mm(2), 20.5+/-4.1 mm(2), and 16.6+/-4.8 mm(2), respectively. Ipsilesional IR muscle cross-section and contractility was significantly less than contralesional cross-section and contractility (P<0.01).

CONCLUSIONS

In SO muscle palsy, the contralesional IR muscle is larger and more contractile than the ipsilesional IR muscle, reflecting likely neurally mediated changes that augment the relatively small hypertropia resulting from SO muscle weakness alone. Recession of the hyperfunctioning contralesional IR muscle recession in SO muscle palsy is a physiologic therapy.

Evidence supporting extraocular muscle pulleys: refuting the platygean view of extraocular muscle mechanics

BACKGROUND

Late in the 20th century, it was recognized that connective tissue structures in the orbit influence the paths of the extraocular muscles and constitute their functional origins. Targeted investigations of these connective tissue "pulleys" led to the formulation of the active pulley hypothesis, which proposes that pulling directions of the rectus extraocular muscles are actively controlled via connective tissues.

PURPOSE

This review rebuts a series of criticisms of the active pulley hypothesis published by Jampel, and Jampel and Shi, in which these authors have disputed the existence and function of the pulleys.

METHODS

This article reviews published evidence for the existence of orbital pulleys, the active pulley hypothesis, and physiological tests of the active pulley hypothesis. Magnetic resonance imaging in a living subject and histological examination of a human cadaver directly illustrate the relationship of pulleys to extraocular muscles.

RESULTS

Strong scientific evidence is cited that supports the existence of orbital pulleys and their role in ocular motility. The criticisms of the hypothesis have ignored mathematical truisms and strong scientific evidence.

CONCLUSIONS

Actively control led orbital pulleys play a fundamental role in ocular motility. Pulleys profoundly influence the neural commands required to control eye movements and binocular alignment. Familiarity with the anatomy and physiology of the pulleys is requisite for a rational approach to diagnosing and treating strabismus using emerging methods. Conversely, approaches that deny or ignore the pulleys risk the sorts of errors that arise in geography and navigation from incorrect assumptions such as those of a flat ("platygean") earth.

Magnetic resonance imaging of the effects of horizontal rectus extraocular muscle surgery on pulley and globe positions and stability

PURPOSE

Magnetic resonance imaging (MRI) was used to determine the effect of recessions and resections on horizontal extraocular muscle (EOM) paths and globe position.

METHODS

Four adults with horizontal strabismus underwent contrast-enhanced, surface-coil MRI in central, secondary, and tertiary gazes, before and after horizontal EOM recessions and/or resections. EOM paths were determined from 2-mm thickness, quasicoronal MRI by analysis of cross-sectional area centroids in a normalized, oculocentric coordinate system. Globe displacement was determined by measuring the apparent shift of the bony orbit in eccentric gaze.

RESULTS

In all subjects, the anteroposterior positions of the horizontal rectus pulleys shifted by less than 2 mm after surgery, indistinguishable from zero within measurement precision. In three subjects who underwent medial rectus (MR) recession or resection, postoperative globe position was similar in central gaze, but globe translation during vertical gaze shift changed markedly. There was no effect on globe translation in the subject who underwent only lateral rectus (LR) resection.

CONCLUSIONS

Recessions and resections of horizontal EOMs have minimal effect on anteroposterior EOM pulley positions. Because the pulley does not shift appreciably despite large alterations in the EOM insertion, the proximity of a recessed EOM to its pulley would be expected to introduce torsional and vertical actions in tertiary gazes. Connective tissue dissection during MR surgery may destabilize the globe's vertical translational stability within the orbit, potentially changing the effective pulling directions of the rectus EOMs in vertical gazes. These changes may mimic oblique muscle dysfunction. LR surgery may avoid globe destabilization.

Magnetic Resonance Imaging of Human Extraocular Muscles During Static Ocular Counter-Rolling

The rectus extraocular muscle (EOM) pulleys constrain EOM paths. During visual fixation with head immobile, actively controlled pulleys are known to maintain positions causing EOM pulling directions to change by one-half the change in eye position. This pulley behavior is consistent with Listing's law (LL) of ocular torsion as observed during fixation, saccades, and pursuit. However, pulley behavior during the vestibulo-ocular reflex (VOR) has been unstudied. This experiment studied ocular counter-rolling (OCR), a static torsional VOR that violates LL but can be evoked during MRI. Tri-planar MRI was performed in 10 adult humans during central target fixation while positioned in right and left side down positions known to evoke static OCR. EOM cross-sections and paths were determined from area centroids. Paths were used to locate pulleys in three dimensions. Significant ( P < 0.025) counter-rotational repositioning of the rectus pulley arrays of both orbits was observed in the coronal plane averaging 4.1° (maximum, 8.7°) from right to left side down positions for the inferior, medial, and superior rectus pulleys. There was a trend for the lateral rectus averaging 1.4°. Torsional shift of the rectus pulley array was associated with significant contractile cross-section changes in the superior and inferior oblique muscles. Torsional rectus pulley shift during OCR, which changes pulling directions of the rectus EOMs, correlates with known insertions of the oblique EOM orbital layers on rectus pulleys. The amount of pulley reconfiguration is roughly one-half of published values of ocular torsion during static OCR, an arrangement that would cause rectus pulling directions to change by less than one-half the amount of ocular torsion.

Structural details of rat extraocular muscles and three-dimensional reconstruction of the rat inferior rectus muscle and muscle-pulley interface

Light microscopy, electron microscopy and morphometry revealed structural details and allowed generation of a three-dimensional reconstruction of the pulley and muscle-pulley interface of extraocular muscle. The inferior rectus orbital layer was bifurcate in shape and extended anterior to the pulley. The putative pulley structure itself was asymmetric; loosely attached at the orbital aspect it adhered tightly to the global aspect of muscle. Orbital multiply innervated fiber proportion increased anterior to the pulley insertion site. Additionally longitudinal variation in juxtaposition of orbital and global layers was noted. These newly described structural details provide novel mechanistic insight for extraocular muscle function in rats.

Orbital imaging demonstrates occult blow out fracture in complex strabismus

BACKGROUND

While strabismologists are familiar with diagnostic evaluation of suspected blow out fractures, unsuspected blow out fractures may further complicate difficult cases of strabismus not clinically supposed to be related to orbital trauma.

METHODS

According to a prospective protocol, we studied five adults presenting with diplopia, and one with convergence-related asthenopia. No patient recalled or had any clinical suspicion of orbital fracture at initial evaluation. Surface coil magnetic resonance imaging of the orbits was performed at 312 microm resolution, slice thickness 2 mm. Quasicoronal images in central gaze were supplemented with eccentric gaze positions, and sagittal and axial images as indicated.

RESULTS

Five patients had incomitant hypertropia, and one had abducens paralysis. Magnetic resonance imaging disclosed previously unsuspected blow out fractures in all six patients. Three patients had medial wall fracture, one bilaterally. Two patients had inferior wall fractures, and one inferomedial. Although only one patient had an extraocular muscle displaced into a sinus, all had evidence of orbital connective tissue distortion in the region of the rectus extraocular muscle pulleys influencing muscle paths. These effects altered the presentations of more familiar pathologies such as superior oblique palsy. After learning of the MRI findings, most patients then recalled orbital trauma from as early as childhood.

CONCLUSION

Unsuspected blow out fractures occur and may confound the usual findings in complex strabismus. High-resolution orbital imaging can detect blow out fractures and clarify the pathophysiology, enabling appropriate surgical management.

Partial rectus muscle-augmented transpositions in abduction deficiency

PURPOSE

Lateral posterior fixation sutures increase the effect of full rectus extraocular muscle transpositions. Partial rectus muscle transposition may be indicated to minimize the risk of anterior ischemia when multiple rectus muscles require surgery to achieve ocular alignment.

PURPOSE

To report a modification of full vertical rectus muscle transposition with lateral posterior fixation sutures for use in patients at risk for anterior segment ischemia.

METHODS

Ten cases of unilateral split rectus muscle transposition augmented with lateral posterior fixation sutures were analyzed. Five patients had Duane's syndrome with esotropia in primary position, and five patients had sixth-nerve palsy.

RESULTS

Seven patients had a history of ipsilateral rectus muscle surgery, and three patients underwent simultaneous surgery on ipsilateral horizontal rectus muscles. In Duane's syndrome patients, the preoperative angle of deviation at distance was 15.8 +/- 5.8 prism diopters (PD) (range, 10 to 25) compared with 3.2 +/- 4.4 PD (range, 0 to 8) postoperatively (P =.005). In patients with sixth-nerve palsy, the preoperative angle of deviation at distance was 45.2 +/- 23.9 PD (range, 16 to 80) compared with -5 +/- 14.1 PD (range, -30 to 5) postoperatively (P =.004). Postoperative binocular single visual fields enlarged in seven of seven patients.

CONCLUSION

Partial rectus muscle-augmented transposition allows surgery on multiple ipsilateral rectus muscles in (1) Duane's syndrome patients with esotropia, marked cocontraction, and/or limitation to both horizontal rotations and in (2) sixth-nerve palsy patients with ipsilateral medial rectus tightness. Augmented partial rectus muscle transpositions improve ocular alignment and may enlarge binocular single fields in patients with persistent deviations despite previous muscle surgery.

Magnetic resonance imaging of the functional anatomy of the inferior oblique muscle in superior oblique palsy

PURPOSE

To study size and contractility of the normal inferior oblique (IO) muscle using high-resolution magnetic resonance imaging (MRI) and to evaluate abnormalities of the superior oblique (SO) and IO muscles in chronic SO palsy.

DESIGN

Prospective, case control study.

PARTICIPANTS

Thirteen patients with SO palsy and 17 orthotropic subjects.

METHODS

High-resolution, surface coil MRI was used to obtain sets of contiguous, 2-mm thick coronal and sagittal images repeated in multiple gaze directions. Digital image analysis was used to measure IO and SO muscle cross-sectional areas for evaluation of size and contractility. Diagnosis of SO palsy in one bilateral and 12 unilateral cases was based on subnormal contractility and SO size less than the normal 95% confidence limit. Ipsilesional and contralesional oblique muscles were compared with controls and correlated with clinical characteristics.

RESULTS

In all subjects, anterior movement and contractile thickening of the IO were observed in supraduction, with posterior movement and relaxational thinning in infraduction. The mean (+/- standard deviation) cross-sectional area of 15 normal control IO muscles was 13.4 +/- 3.9 mm(2), with mean contractile increase from infraduction to supraduction of 5.7 +/- 2.6 mm(2). Subjects with SO palsy had incomitant hypertropia with a wide range of overelevation and underelevation in adduction (i.e., upshoot, downshoot). SO atrophy correlated with underdepression in adduction (P < 0.0001). Contralesional SO cross-section was slightly greater than normal (P = 0.004). The IO cross-section ipsilesional and contralesional to SO palsy did not, however, differ significantly from normal and did not correlate with elevation in adduction (P > 0.2).

CONCLUSIONS

Quantitative morphometry by MRI can demonstrate IO size and contractility. Even in cases of unequivocal SO palsy associated with ipsilesional SO atrophy and deficient contractility, the degree of elevation in adduction was not correlated with IO size. This finding suggests that the associated overelevation in adduction, commonly termed "inferior oblique overaction," actually arises from some other mechanism than IO hypertrophy or excess contractility. Revision of clinical terminology seems warranted.

Magnetic resonance imaging of human extraocular muscles in convergence

Extraocular muscle (EOM) paths during asymmetrical convergence were evaluated by tri-planar, contrast-enhanced magnetic resonance imaging of the orbits of eight young adults during binocular fixation of a target aligned to one eye at 800 and 15 cm distance. Cross sections and paths of EOMs were determined from area centroids. In convergence, the aligned eye rotated and translated negligibly, while its inferior oblique (IO) muscle exhibited significant contractile thickening. There were no significant contractile changes in the cross sections of aligned eye rectus or superior oblique (SO) muscles in convergence. The converging eye rotated nasally 22.4 degrees but translated negligibly. The converging eye medial (MR) and lateral rectus (LR) muscles exhibited large contractile cross-section changes, and the IO showed significant contractile thickening, while the vertical rectus muscles and the SO did not. Anterior paths of three aligned eye rectus EOMs could be determined in convergence and shifted consistent with a 1.9 degrees extorsion of the rectus pulley array. Such extorsional reconfiguration of the rectus pulleys would move the pulleys in coordination with globe extorsion and avoid imparting torsional action to these EOMs. Extorsional rectus pulley shift in convergence is inconsistent with the reconfiguration predicted to explain the temporal tilting of Listing's planes, instead suggesting that this temporal tilting is due to variations in oblique EOM innervation. Absence of globe translation in convergence argues against overall EOM co-contraction. The reconfiguration of EOM geometry in convergence has important implications for single-unit studies of neural control.

Effect of aging on human rectus extraocular muscle paths demonstrated by magnetic resonance imaging

PURPOSE

To compare normal functional anatomy of rectus extraocular muscles (EOMs) and pulleys in normal older humans with previously reported findings in younger subjects.

DESIGN

Experimental study of the orbits of normal healthy older volunteers by magnetic resonance imaging (MRI).

METHODS

In planes perpendicular to the orbital axis, contiguous MRI images spanned the anteroposterior extents of 22 orbits in 12 older adults with an average age of 65.2 years (range, 56-74). Images were obtained in central gaze in all subjects and repeated in supraduction, infraduction, abduction, and adduction in some subjects. Mean EOM cross-sectional area centroids were normalized to an oculocentric coordinate system and plotted over the length of each EOM to determine paths.

RESULTS

Compared with images obtained using identical technique in 12 younger subjects (average age, 28.5 years, range 21-33), the horizontal rectus EOMs in the 12 older subjects were significantly displaced inferiorly throughout the anteroposterior extent of the orbit. The vertical rectus EOM was positioned identically to those of younger subjects. Inflections in EOM paths produced by the connective tissue pulleys could not be determined in most older subjects, because of difficulties in maintaining extreme eccentric gaze. For one subject who was able to do this, the anteroposterior location of the medial rectus pulley inferred from path inflection was similar to that of younger subjects.

CONCLUSIONS

The horizontal rectus EOMs are displaced inferiorly in the elderly relative to the globe center. This displacement presumably reflects an inferior location of the corresponding pulleys, partially converting horizontal rectus EOM force to depression. This may contribute to the observed impairment of elevation in older people and predispose them to a characteristic pattern of incomitant strabismus.

The orbital pulley system: a revolution in concepts of orbital anatomy

Magnetic resonance imaging (MRI) now enables precise visualization of the mechanical state of the living human orbit. Resulting insights have motivated histological re-examination of human and simian orbits, providing abundant consistent evidence for the active pulley hypothesis, a re-formulation of ocular motor physiology. Each extraocular muscle (EOM) consists of a global layer (GL) contiguous with the tendon and inserting on the eyeball, and a similar-sized orbital layer (OL) inserting on a connective tissue ring forming the EOM pulley. The pulley controls the EOM path and serves as the EOM's functional origin. Activity of the OL positions the pulley along each rectus EOM to assure that its pulling direction shifts by half the change in ocular orientation, the half-angle behavior characteristic of a linear ocular motor plant. Half-angle behavior is equivalent to Listing's law of ocular torsion, and makes 3-D ocular rotations effectively commutative. Pulleys are configured to maintain oblique EOM paths orthogonal to half-angle behavior, and violate Listing's law during the vestibulo-ocular reflex. Rectus pulley positions shift during convergence, facilitating stereopsis. Innervations, fiber types, and metabolism of the OL and GL differ, consistent with the elastic loading of the former, and viscous loading of the latter. Disorders of the location and stability of rectus pulleys are associated with predictable patterns of incomitant strabismus that may mimic cranial nerve palsies. Surgical interventions improve defective pulley function. Understanding of ocular motor control requires characterization of the behavior of the EOM pulleys as well as knowledge of angular eye orientation.

Rectus extraocular muscle pulley displacement after surgical transposition and posterior fixation for treatment of paralytic strabismus

PURPOSE

To determine the effect of rectus extraocular muscle (EOM) transposition with posterior fixation (PF), we employed magnetic resonance imaging (MRI) to demonstrate pulley inflections in EOM paths before and after surgery in patients with paralytic strabismus.

DESIGN

Consecutive interventional case series.

METHODS

Five consecutive patients (three males and two females with a mean age 52 years, range 33 to 77 years) with paralytic strabismus were studied prospectively before and more than 6 weeks after EOM transposition and PF by means of contiguous cross-sectional MRI obtained in planes perpendicular to the long axis of the orbit. Muscle paths were determined in three dimensions (3-D) for each EOM by analysis of cross-sectional area centroids in normalized, oculocentric coordinate systems.

RESULTS

Four patients underwent full tendon transposition with PF of the vertical rectus EOMs. One other patient underwent full tendon transposition without PF of the horizontal rectus EOMs superiorly. For transpositions with PF, there was a large displacement of EOM path in central (straight ahead) gaze beginning in the posterior orbit. After surgical transposition, clear inflections representing pulley locations of the superior, medial, and lateral rectus paths occurred in central gaze. There was no clear path inflection for the inferior rectus in central gaze, but there was a small inflection in adduction. After all transpositions, the globe center shifted away from the transposed insertions.

CONCLUSIONS

Rectus EOM transpositions with PF shift EOM pulleys posteriorly and in the directions of the transposed EOM tendons, while translating the globe center. These changes may explain the superior effectiveness of PF in increasing duction towards the transposition.

Vertical rectus muscle augmented transposition in Duane syndrome

INTRODUCTION

Reduction or elimination of face turn and esotropia in the primary position while maintaining the largest possible diplopia-free field are the major surgical goals in Duane syndrome with esotropia. Unsatisfactory postoperative results may occur because of limitation in adduction, poor abduction, or induced vertical deviations. Recent reports have shown enhanced results from rectus muscle transposition techniques when a lateral posterior augmentation fixation is placed.

METHODS

Preoperative and postoperative data of 2 groups of subjects who had Duane syndrome with esotropia in primary position and markedly reduced abduction were comparatively analyzed. Group A consisted of subjects who had transposition of both vertical rectus muscles to the lateral rectus muscle with a posterior lateral augmentation suture placed in each transposed muscle. Group B subjects had transposition of both vertical rectus muscles to the lateral rectus muscle without the posterior lateral augmentation suture.

RESULTS

A total of 32 subjects in group A and 22 subjects in group B were analyzed. In group A, anomalous head position improved 19.1 degrees +/- 10.3 degrees compared with group B subjects who improved 10.6 degrees +/- 5.8 degrees (P <.05). In group A, esotropia in primary position improved 16.4 +/- 9.2 PD compared with group B subjects who improved 8.5 +/- 6.9 PD (P <.05).

CONCLUSIONS

Subjects with Duane syndrome and esotropia in primary position who had undergone augmented transposition of the vertical rectus muscles obtained improved head position and better alignment in primary position and had a reduction in the incidence of reoperation for undercorrection when compared with similar patients who had undergone vertical rectus muscle transposition without posterior lateral augmentation sutures.

Role of muscle pulleys in producing eye position-dependence in the angular vestibuloocular reflex: a model-based study

It is well established that the head and eye velocity axes do not always align during compensatory vestibular slow phases. It has been shown that the eye velocity axis systematically tilts away from the head velocity axis in a manner that is dependent on eye-in-head position. The mechanisms responsible for producing these axis tilts are unclear. In this model-based study, we aimed to determine whether muscle pulleys could be involved in bringing about these phenomena. The model presented incorporates semicircular canals, central vestibular pathways, and an ocular motor plant with pulleys. The pulleys were modeled so that they brought about a rotation of the torque axes of the extraocular muscles that was a fraction of the angle of eye deviation from primary position. The degree to which the pulleys rotated the torque axes was altered by means of a pulley coefficient. Model input was head velocity and initial eye position data from passive and active yaw head impulses with fixation at 0 degrees, 20 degrees up and 20 degrees down, obtained from a previous experiment. The optimal pulley coefficient required to fit the data was determined by calculating the mean square error between data and model predictions of torsional eye velocity. For active head impulses, the optimal pulley coefficient varied considerably between subjects. The median optimal pulley coefficient was found to be 0.5, the pulley coefficient required for producing saccades that perfectly obey Listing's law when using a two-dimensional saccadic pulse signal. The model predicted the direction of the axis tilts observed in response to passive head impulses from 50 ms after onset. During passive head impulses, the median optimal pulley coefficient was found to be 0.21, when roll gain was fixed at 0.7. The model did not accurately predict the alignment of the eye and head velocity axes that was observed early in the response to passive head impulses. We found that this alignment could be well predicted if the roll gain of the angular vestibuloocular reflex was modified during the initial period of the response, while pulley coefficient was maintained at 0.5. Hence a roll gain modification allows stabilization of the retinal image without requiring a change in the pulley effect. Our results therefore indicate that the eye position-dependent velocity axis tilts could arise due to the effects of the pulleys and that a roll gain modification in the central vestibular structures may be responsible for countering the pulley effect.

Posterior fixation sutures: a revised mechanical explanation for the fadenoperation based on rectus extraocular muscle pulleys

PURPOSE

To determine the effect of the rectus extraocular muscle pulleys on the fadenoperation, an operation designed to fixate the posterior muscle belly to the underlying retroequatorial sclera.

METHODS

First, duction into the field of action of the operated-on muscle was quantified retrospectively after fadenoperation. Magnetic resonance imaging was then performed prospectively after surgery to verify anatomic changes. Forced duction testing was performed prospectively during surgery before and after faden placement. Finally, computed tomography in a cadaver containing radiographic markers was performed prospectively to determine the effect of fadenoperation on the position of the medial rectus insertion relative to its pulley.

RESULTS

Mean maximum adduction after medial rectus fadenoperation was 18 degrees (range, 10 to 25 degrees; 13 eyes). Fadenoperations combined with large medial rectus recessions restricted adduction more than fadenoperations combined with smaller recessions (P = .019), but even fadenoperations without recessions substantially restricted adduction. Mean maximum abduction after lateral rectus fadenoperation was 40 degrees (range, 25 to 45 degrees; four eyes). Axial magnetic resonance imaging in two eyes demonstrated a smaller loss of muscle tangency to the globe during contraction than predicted by geometric models. Forced ductions in nine patients performed immediately after faden placement demonstrated a new mechanical restriction to duction toward the operated-on muscle. Cadaveric computed tomographic scans demonstrated posterior displacement of the medial rectus pulley during adduction after fadenoperation.

CONCLUSIONS

Posterior fixation sutures do not significantly decrease muscle torque during contraction. Because posterior fixation sutures posteriorly displace the pulley sleeve during duction toward the operated-on muscle, the mechanical restriction after surgery probably represents the force deforming the pulley. This mechanical restriction may account for the limitation in duction seen after fadenoperation.