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.     

Incomitant strabismus associated with instability of rectus pulleys

PURPOSE: Connective tissue pulleys serve as functional mechanical origins of the extraocular muscles (EOMs) and are normally stable relative to the orbit during gaze shifts. This study evaluated pulley stability in incomitant strabismus. METHODS: Contiguous 2- or 3-mm thick magnetic resonance images (MRIs) perpendicular to the orbital axis spanned the anteroposterior extents of 12 orbits of six patients with incomitant strabismus. Imaging was performed in central gaze, supraduction, infraduction, abduction, and adduction. Rectus EOM paths were defined by their area centroids and plotted in a normalized, oculocentric coordinate system. Paths of EOMs ran toward the pulleys. Sharp EOM path inflections in secondary gaze indicated pulley locations in three dimensions. RESULTS: MRI revealed substantial inferior shift of the lateral rectus (LR) pulley of up to 1 mm during vertical gaze shifts in patients with axial high myopia and a posterior shift from abduction to adduction in simulated Brown syndrome. There was substantial LR pulley shift opposite the direction of vertical gaze in a subject with X-pattern exotropia who had undergone repeated LR surgery. The medial rectus (MR) pulley shifted inferiorly with gaze elevation in Marfan syndrome. Pulley instability was associated with significantly increased globe translation during gaze shifts. CONCLUSIONS: Pulley instability, resulting in EOM sideslip during ductions, occurs in some cases of incomitant strabismus. Resultant patterns of strabismus may depend on static pulley positions, pulley instability, and coexisting globe translation that alters pulley locations relative to the globe. Translational instability of pulleys and the globe could produce abnormalities in actions of otherwise normal EOMs, and connective tissue disorders causing these instabilities should be considered as potential causes of strabismus.     

Active pulleys: magnetic resonance imaging of rectus muscle paths in tertiary gazes

PURPOSE: The orbital layer of each rectus extraocular muscle (EOM) inserts on connective tissue, and the global layer inserts on the eyeball. The active-pulley hypothesis (APH) proposes that a condensation of this connective tissue constitutes a pulley serving as the functional origin of the rectus EOM, and that this pulley makes coordinated, gaze-related translations along the EOM axis to implement a linear ocular motor plant. This study was designed to measure gaze-related shifts in EOM pulley locations. METHODS: Magnetic resonance imaging (MRI) was performed in eight normal volunteers in 2-mm thickness coronal planes perpendicular to the orbital axis for nine cardinal gaze directions. Intravenous gadodiamide contrast was administered to define EOM tendons anterior to the globe equator. Paths of EOMs, defined by their area centroids, were transformed into an oculocentric coordinate system. Sharp inflections in EOM paths in secondary and tertiary gaze positions defined pulley locations which were then correlated with gaze direction and compared with theoretical predictions. RESULTS: Rectus pulley positions were consistent with a central primary position. In tertiary gaze positions, each of the four rectus pulleys translated posteriorly with EOM contraction and anteriorly with EOM relaxation by a significant (P < 0.02) amount predicted by the APH, but more than 100 times greater than the translation predicted by a passive pulley model. CONCLUSIONS: The APH prediction of coordinated anteroposterior shifting of EOM pulleys with gaze is quantitatively supported by changes in EOM path inflections among tertiary-gaze positions. Human rectus pulleys move to shift the ocular rotational axis to attain commutative behavior of the ocular motor plant.     

Evidence for active control of rectus extraocular muscle pulleys

PURPOSE: Connective tissue structures constrain paths of the rectus extraocular muscles (EOMs), acting as pulleys and serving as functional EOM origins. This study was conducted to investigate the relationship of orbital and global EOM layers to pulleys and kinematic implications of this anatomy. METHODS: High-resolution magnetic resonance imaging (MRI) was used to define the anterior paths of rectus EOMs, as influenced by gaze direction in living subjects. Pulley tissues were examined at cadaveric dissections and surgical exposures. Human and monkey orbits were step and serially sectioned for histologic staining to distinguish EOM fiber layers in relationship to pulleys. RESULTS: MRI consistently demonstrated gaze-related shifts in the anteroposterior locations of human EOM path inflections, as well as shifts in components of the pulleys themselves. Histologic studies of human and monkey orbits confirmed gross examinations and surgical exposures to indicate that the orbital layer of each rectus EOM inserts on its corresponding pulley, rather than on the globe. Only the global layer of the EOM inserts on the sclera. This dual insertion was visualized in vivo by MRI in human horizontal rectus EOMs. CONCLUSIONS: The authors propose the active-pulley hypothesis: By dual insertions the global layer of each rectus EOM rotates the globe while the orbital layer inserts on its pulley to position it linearly and thus influence the EOM's rotational axis. Pulley locations may also be altered in convergence. This overall arrangement is parsimoniously suited to account for numerous aspects of ocular dynamics and kinematics, including Listing's law.     

正常人眼球直肌Pulley功能位置的动态磁共振成像研究

目的研究正常人眼球运动动态磁共振成像（MRI）4条直肌Pulley（滑车）的功能性位置。方法采用西门子公司Sonata1.5T超导型MRI扫描仪,应用眼球运动动态MRI技术,获取20名正常人（20个眼眶）眼球原在位及上转、下转、内转、外转20度时垂直于眶轴的眼眶冠状位MRI图像。以眼球中心为原点建立三维坐标系,应用ScionImage医学图像测量软件分别测量各层面眼球垂直转动时水平直肌、眼球水平转动时垂直直肌的横截面质心。根据各层面直肌横截面质心的坐标值建立直线回归方程,分别求得眼球垂直转动时内、外直肌径路及眼球水平转动时上、下直肌径路直线回归曲线斜率变化最大的一点,即为该直肌Pulley的功能性位置。对4条直肌Pulley相对于眼球中心的坐标值（X、Y）进行统计。结果内直肌Pulley位于眼球中心后4mm,内14.7mm,下0.3mm;外直肌Pulley位于眼球中心后8mm,外9.8mm,下0.3mm;上直肌Pulley位于眼球中心后6mm,内1.6mm,上11.5mm;下直肌Pulley位于眼球中心后6mm,内4.4mm,下12.7mm。结论应用眼球运动动态MRI技术,分析眼球转动时直肌径路的变化,可证实4条直肌Pulley的存在并确定其功能位置。     

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.     

Quantitative analysis of the structure of the human extraocular muscle pulley system

PURPOSE: Extraocular muscle (EOM) paths are constrained by connective tissue pulleys serving as functional origins. The quantitative structural features of pulleys and their intercouplings and orbital suspensions remain undetermined. This study was designed to quantify the composition of EOM pulleys and suspensory tissues. METHODS: Five human orbits, ages 33 weeks gestation to 93 years, were imaged intact by magnetic resonance (MRI), serially sectioned at 10 micro m thickness, and stained for collagen, elastin, and smooth muscle (SM). With MRI used as a reference, digital images of sections were geometrically corrected for shrinkage and processing deformations, and normalized to standard normal adult globe diameter. EOM pulleys, interconnections, suspensory tissues, and entheses were quantitatively analyzed for collagen, elastin, and SM thickness and density. RESULTS: Rectus and inferior oblique pulleys had uniform structural features in all specimens, comprising a dense EOM encirclement by collagen 1 to 2 mm thick. Elastin distribution varied, but was greatest in the orbital suspension of the medial rectus pulley and in a band from it to the inferior rectus pulley. This region corresponded to maximum SM density. Structural features of pulleys, intercouplings, and entheses were similar among specimens. The major mechanical couplings to the osseous orbit were near the medial and lateral rectus pulleys. CONCLUSIONS: Quantitative analysis of structure and composition of EOM pulleys and their suspensions is consistent with in vivo MRI observations showing discrete inflections in EOM paths that shift predictably with gaze. Focal SM distributions in the suspensions suggest distinct roles in stiffening as well as shifting rectus pulleys.     

Displacement of the rectus muscle pulleys simulating superior oblique palsy

Purpose To investigate the structural basis of three cases of apparent superior oblique (SO) palsy caused by extraocular muscle (EOM) pulley heterotopy. Methods Three subjects were diagnosed as having decompensated idiopathic left SO palsy on the basis of misalignment in diagnostic gaze positions, response to the head tilt test, and results of the Hess screen test. Magnetic resonance imaging of the orbits in coronal planes was used to determine SO muscle size and contractility and to define the rectus EOM pulley locations. Orbit 1.8 computer simulation was performed for each subject by using measured rectus pulley locations. Simulated binocular alignment was compared with the measurements. Results The maximal SO cross sections of both eyes of each subject were similar, and exhibited similar contractile thickening from supraduction to infraduction. The superior rectus muscle pulleys in three eyes exhibited significant temporal displacement, while the lateral rectus muscle pulleys in five eyes and the medial rectus muscle pulleys in two eyes were displaced significantly inferiorly compared with published norms. Simulations based on observed pulley position abnormalities alone predicted measured Hess screen data better than did simulations incorporating SO weakness, either alone or combined with other structural abnormalities. Conclusions Heterotopy of the rectus EOM pulleys may be associated with cyclovertical strabismus that simulates SO palsy.     

人眼直肌滑车的计算机三维重建

目的建立数字化可视人眼直肌滑车的三维立体模型。方法经Masson染色的全眼眶冠状位连续组织切片图像用数码相机摄像后逐一输入计算机内,经PhotoShop7．0软件除噪处理,应用3D—Doctor软件通过对图像的定位、分割进行直肌滑车系统与眼球、4条直肌及眼眶壁的三维立体模型的重建。同时应用3D—Doctor软件的测量工具对直肌滑车及相关组织结构进行测量和分析。结果人眼直肌滑车三维立体模型直观的展示了直肌滑车系统与眼球、直肌及眼眶壁的三维立体关系。该三维立体模型可绕任意轴旋转,方便多角度观察。可对重建的三维结构选择性演示,以利于排除干扰,锁定目标观察。测量结果为各直肌滑车的长度为4．6～5．8mm,始端至角巩膜缘的距离为13．8～18．0mm,其中以内直肌滑车最宽厚,内直肌滑车的厚度与相对应的肌肉厚度比值最大可达1．11。结论直肌滑车三维立体模型为临床医师了解人眼直肌滑车的结构提供了直观图像;为直肌滑车的影像学研究提供了三维解剖学依据。对直肌滑车及相关组织结构进行测量的数据将有助于建立人眼直肌滑车三维可视化数据库。（中华眼科杂志,2007,43：977-981）     

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.     

Ocular kinematics,vergence, and orbital mechanics

Important aspects of ocular kinematics relate to the geometric configuration of extraocular muscles (EOMs). The orbital layer of each rectus EOM inserts on a connective tissue ring called a pulley that deflects the EOM path. Global layer fibers of each EOM pass through the pulley to insert on the sclera. The orbital layer thus controls linear translation of the pulley, regulating the EOM's pulling direction, while the global layer rotates the eye. The active pulley hypothesis (APH) states that pulleys are actively positioned to regulate ocular kinematics. The coordinated control postulate of the APH proposes that during conjugate visually guided eye movements, rectus pulleys move the same anteroposterior distance as their insertions, but the inferior oblique pulley moves with vertical gaze by half the amount as the inferior rectus insertion. These motions, observable by magnetic resonance imaging (MRI), shift the pulling directions of these EOMs by half the ocular angle, mechanically implementing a 'linear oculomotor plant' appearing mathematically commutative to the brain and consistent with Listing's Law of ocular torsion. In the non-converged state with the head upright and stationary, rectus pulleys move little transverse to the EOM axes. During convergence and during the static torsional vestibulo-ocular reflex, MRI shows that the rectus pulley array rotates around the line of sight. Oblique EOM orbital layers may implement this shift.     

Ocular kinematics,vergence, and orbital mechanics

Important aspects of ocular kinematics relate to the geometric configuration of extraocular muscles (EOMs). The orbital layer of each rectus EOM inserts on a connective tissue ring called a pulley that deflects the EOM path. Global layer fibers of each EOM pass through the pulley to insert on the sclera. The orbital layer thus controls linear translation of the pulley, regulating the EOM’s pulling direction, while the global layer rotates the eye. The active pulley hypothesis (APH) states that pulleys are actively positioned to regulate ocular kinematics. The coordinated control postulate of the APH proposes that during conjugate visually guided eye movements, rectus pulleys move the same anteroposterior distance as their insertions, but the inferior oblique pulley moves with vertical gaze by half the amount as the inferior rectus insertion. These motions, observable by magnetic resonance imaging (MRI), shift the pulling directions of these EOMs by half the ocular angle, mechanically implementing a ‘linear oculomotor plant’ appearing mathematically commutative to the brain and consistent with Listing’s Law of ocular torsion. In the non-converged state with the head upright and stationary, rectus pulleys move little transverse to the EOM axes. During convergence and during the static torsional vestibulo-ocular reflex, MRI shows that the rectus pulley array rotates around the line of sight. Oblique EOM orbital layers may implement this shift.     

轴性高度近视眼直肌PULLEY功能位置的动态磁共振成像研究

目的用MRI研究轴性高度近视眼4条直肌pulley的功能位置,探讨轴性高度近视继发性眼球运动障碍的病因。方法轴性高度近视12例(22眼)根据眼屈光度、眼轴长度以及有无眼球运动受限分为A、B两组。A组无眼球运动受限;B组有眼球运动受限。应用动态MRI技术,获取眼球原在位及上转、下转、内转、外转位时的冠状位MRI图象。应用计算机图像处理软件测量各层面MRI图像眼球垂直方向转动时水平直肌、眼球水平方向转动时垂直直肌的横截面质心,根据其坐标值建立直线回归方程,统计求得眼球垂直转动时内、外直肌径路及眼球水平转动时上、下直肌径路直线回归曲线斜率变化最大的一点(直肌pulley的位点),将A组、B组、正常对照组进行比较。结果A组与正常对照组4条直肌pulley的位点比较无显著差异(P>0.05);B组与正常对照组内直肌、上直肌、下直肌pulley的位点比较无显著性差异(P>0.05);B组外直肌pulley的位点较正常对照组向颞下移位(P<0.01);A组与B组内直肌、上直肌、下直肌pulley的位点比较无显著性差异(P>0.05);B组较A组外直肌pulley位点向颞下移位(P<0.05)。结论外直肌pulley的位点向颞下移位可能是引起轴性高度近视眼继发性眼球运动障碍的主要病因之一。     

Evidence for rectus extraocular muscle pulleys in rodents

PURPOSE: Extraocular rectus muscle (EOM) pulleys are important determinants of orbital biomechanics in humans. In this study, the authors evaluated orbital connective tissue morphology, specifically characterizing rectus muscle pulleys, in the rat, a species with laterally placed eyes, afoveate vision, and a less complex visuomotor repertoire than primates. METHODS: Adult rat orbits were paraffin processed and serially sectioned for histochemical and immunohistochemical staining. Frozen sections of enucleated globes with intact EOMs and associated connective tissue were also studied with myosin immunohistochemistry and histochemistry for the mitochondrial enzyme, nicotinamide adenine dinucleotide (NADH)-tetrazolium reductase, to delineate the orbital layer relationship with the pulley tissue. RESULTS: Focal condensations of collagenous connective tissue were found in relationship to the rectus muscles in the equatorial Tenon's fascia, similar to those described as human recti muscle pulleys. The fibroelastic pulley rings were coupled to adjacent EOM pulleys by bands containing collagen and elastin. The coupling of pulleys to the orbital walls was significantly less than that previously described in humans. As in humans, there was a dual insertion of rodent rectus muscles, with the orbital layer inserting on the muscle pulley and the global layer attaching to the sclera. CONCLUSIONS: The data support the presence of structures in the rat orbit that are the morphologic equivalent of the human rectus pulley system. Although rodent and human pulleys were similar in many respects, there were species-specific properties that may relate to established differences in orbital anatomy and/or visuomotor behavior. These data extend the rectus muscle pulley concept to rodents and may provide insight into pulley structure-function relationships.     

Evidence for a pulley of the inferior oblique muscle

PURPOSE: This study was undertaken to investigate evidence for a connective tissue pulley constraining the path of the inferior oblique (IO) muscle. METHODS: From magnetic resonance images, the cross-sectional area, path, and orbital relationships of the human IO were determined in multiple gaze positions. Rectus pulleys were directly imaged with intravenous gadodiamide contrast. Images were compared with serial histologic sections of IO muscles of humans and monkeys. RESULTS: The IO path from origin to the lateral border of the inferior rectus (IR) muscle was straight. Lateral to the IR, the IO curved to follow the globe. At the point of IR crossing, the IO moved anteriorly from infraduction to supraduction by approximately 53% of the IR insertion's travel. Gaze-related change in IO cross section was demonstrable near the IR center. The gaze-related inflection in IO path corresponded to its encirclement by a pulley consisting of a dense ring of collagen, stiffened by elastin and smooth muscle, and united with the IR pulley. Orbital layer fibers of the IO inserted on its pulley, the lateral rectus (LR) pulley, and associated connective tissues. CONCLUSIONS: Like the rectus muscles, the human and monkey IO has a connective tissue pulley serving as its functional origin. The position of the IO pulley is influenced by its coupling to the actively moving IR pulley, whereas in turn the IO orbital layer inserts on and presumably shifts the IR and LR pulleys. These intercouplings facilitate implementation by rectus extraocular muscle suspensions of a commutative ocular motor plant.     

Stability of gold bead tissue markers

Significant soft tissue features in the orbit and elsewhere are not resolved by MRI or any other imaging method. We describe a new method that uses tiny ( approximately 0.1 mm diameter) gold beads as markers to visualize movements of such tissues with high spatial resolution ( approximately 100 microm) and moderate temporal resolution ( approximately 100 ms). We describe bead fabrication, implantation, imaging, and image processing to extract three-dimensional bead coordinates. We then present results of an experiment to determine the stability of gold bead tissue markers (GBTMs) over time in normally moving orbital tissues. Most beads (76%) implanted in sclera, muscle, tendon, and connective tissue were highly stable over the 6-month measurement period. Beads that were judged unstable drifted only a few 100 microm. Bead flows with gaze suggested that posterior Tenon's capsule moves with the globe, that the lateral rectus belly may sideslip, producing "bridle forces," and that the posterior medial rectus pulley sling moves freely anteriorly and posteriorly, but hardly vertically, as required by the "coordinated active pulley" hypothesis. The GBTM method seems applicable to study such short time course phenomena as extraocular muscle (EOM) and connective tissue movement as a function of gaze and such long time course phenomena as myopic eye growth.     

Functional anatomy of the anophthalmic socket: insights from magnetic resonance imaging

PURPOSE. Changes in anophthalmic socket anatomy can significantly compromise esthetics and motility after enucleation. This study evaluated such changes by using magnetic resonance imaging (MRI) of enucleated and fellow orbits. METHODS. High-resolution, surface coil MRI was performed in five patients after enucleation for uveal melanoma. Images were analyzed quantitatively to determine extraocular muscle (EOM) volume, maximum diameter, and length; orbital, optic nerve (ON), and orbital fat tissue (OFT) volume; and implant position. The fellow orbit was used as the control. Patients evaluated their satisfaction with the surgical results and were clinically examined. RESULTS. Rectus EOM volume was slightly but not significantly reduced on the surgical compared with the control side. Oblique EOM and OFT volumes were unchanged. Rectus EOM length was significantly reduced in the surgical side, but maximum EOM diameter in central gaze position was slightly but not significantly greater on the enucleated side. Rectus EOM paths were not qualitatively changed by enucleation and continued to exhibit the influence of the connective tissue pulleys, which retained motility, as appropriate to EOM contraction. Implants were located significantly posterior to the normal globe location. CONCLUSIONS. Enucleation does not significantly change EOM volume, but shortens EOM paths, a change that would be expected to alter their mechanical properties. EOM pulleys appear to retain their functional role in enucleated orbits.     

Extraocular muscle sideslip and orbital geometry in monkeys

The belly of each extraocular muscle is elastically coupled to both the globe and orbit. The dependence of muscle planes on gaze angle must be determined experimentally. In monkeys, radio-opaque markers were implanted along the upper and lower margins of a lateral rectus. A scleral search coil was implanted in the other eye. With the eye in various gaze positions, X-ray images were made to show the LR in the lateral view. We found that as the eye rotates vertically over 50 deg (+/- 25 deg), the point of tangency of the LR with the globe slips an average of 5.1 mm vertically with respect to the globe, allowing this point--and so the muscle plane--to remain approximately fixed relative to the orbit. The results of quantitative orbital dissections are presented in support of the sideslip calculations.     

Isometric force development in human horizontal eye muscles and pulleys during saccadic eye movements

Purpose: The connective tissue elements forming the check ligaments and portals of the human eye muscles have recently been ascribed with a pulley function. Active positioning of the pulleys over orbital layer contraction during eye movements has been suggested. Other studies have instead demonstrated fibrous tissue connections between all parts of the muscle and the pulleys. We aimed to compare the isometric force developed at the muscle tendon and at the pulleys of the horizontal eye muscles, and to investigate which eye muscle structures might exert force on the pulleys. Methods: Isometric force development was recorded from the lateral and medial rectus muscles in six patients operated for strabismus under topical anaesthesia. Two strain gauge probes were used, each attached with 5–0 silk sutures either to the muscle tendon or to the pulley. The eye muscles were activated by horizontal saccadic eye movements in steps from 30 degrees in the off‐direction to 30 degrees in the on‐direction of the muscles. Results: The forces developed at the tendon and pulley were almost identical with respect to amplitude and other parameters. No differences were found in forces developed at the pulleys of the medial and lateral rectus muscles. Conclusions: The results support the presence of fibrous tissue connections between all eye muscle fibres and pulley structures, rather than orbital fibre control of the pulley.