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Joseph L. Demer - One of the best experts on this subject based on the ideXlab platform.
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rectus extraocular muscle size and Pulley location in concomitant and pattern exotropia
Ophthalmology, 2016Co-Authors: Alan Le, Joseph L. DemerAbstract: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 P 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.
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location and gaze dependent shift of inferior oblique muscle position anatomic contributors to vertical strabismus following lower lid blepharoplasty
Investigative Ophthalmology & Visual Science, 2015Co-Authors: Sun-young Shin, Joseph L. DemerAbstract:Lower lid blepharoplasty is widely performed aesthetic surgery that corrects involutional changes.1 This surgery involves the region of the lower eyelid retractors, connective tissue bands extending from the region of the conjoined inferior rectus muscle (IR), and inferior oblique muscle (IO) Pulleys. This region also includes Lockwood's ligament, a connective tissue structure that supports the globe. Complications of blepharoplasty have been reported, including diplopia due to strabismus.2–8 While vertical strabismus is relatively infrequent, it is one of the most bothersome complications. The mechanism of this diplopia is not well understood. The lower eyelid has an intimate anatomic relationship with the IO and orbital bones.9,10 The IO Pulley is partly coupled to the mobile IR Pulley by elastic tissues. The lower eyelid normally moves in coordination with vertical eye position by roughly the same amount, as does the globe surface. However, while the IO Pulley is shifted by the IR's orbital layer, the IO Pulley moves only half as far at the IO Pulley and lower lid.11 This means that in infraduction, the IO Pulley more closely approximates the lower lid skin surface than in other gaze positions. Elasticity of lower lid tissues contributes to the coordinated shifts of the lower lid, IO Pulley, and eye. Fibrous adhesions of the IO-IR Pulley to the orbital floor may produce restrictive hypertropia by hindering normal posterior Pulley shift during infraduction.12 It is therefore plausible that milder changes in elastic mechanical forces in the eyelids following blepharoplasty might be transmitted to IR and IO so as to contribute to strabismus following blepharoplasty. Such a putative effect would probably be related to individual variations in lid and bony orbital anatomy. We hypothesized that the lower lid blepharoplasty could affect IO position, since the lower lid is intimately coupled to the IO-IR Pulley system. If such an effect were to occur, it should be most pronounced in patients who have shallow orbits or other anatomic features bringing the IO-IR Pulley assembly into proximity with the skin. This could cause vertical strabismus following lower lid blepharoplasty. Therefore, this study sought to investigate, using high-resolution magnetic resonance imaging (MRI), the position of IO relative to the adnexa in subjects with vertical strabismus following lower lid blepharoplasty, comparing these with controls.
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connective tissues reflect different mechanisms of strabismus over the life span
Journal of Aapos, 2014Co-Authors: Joseph L. DemerAbstract:Background Connective tissue Pulleys determine extraocular muscle force directions and Pulley heterotopy can induce strabismus. The etiology and type of Pulley abnormalities vary with patient age, resulting in different but predictable types presentations of strabismus. Methods Magnetic resonance imaging (MRI) was obtained in 95 patients with Pulley heterotopy, of whom 56 had childhood-onset pattern strabismus, and was compared with published data on 28 patients aged 69 ± 12 years who had sagging eye syndrome. Control data were from age-matched normal controls with no strabismus. Results Patients with childhood-onset strabismus had intact lateral rectus–superior rectus band ligaments and straight extraocular muscle paths but exhibited Pulley array A pattern–associated incyclorotation or V pattern–associated excyclorotation. Rectus transposition surgery collapsed patterns. Patients with sagging eye syndrome exhibited blepharoptosis, superior sulcus defect, and inferolateral displacement of rectus Pulleys with elongation of extraocular muscles that followed curved paths. Symmetrical lateral rectus Pulley sag was associated with divergence paralysis esotropia; asymmetrical sag > 1 mm, with cyclovertical strabismus. Both lateral rectus resection and medial rectus recession treated divergence paralysis esotropia. Partial vertical rectus tenotomy treated cyclovertical strabismus. Conclusions Childhood onset Pulley abnormalities are associated with A or V pattern strabismus and external anatomical features suggest that these Pulley defects are probably congenital. Adult onset Pulley defects commonly result from age-related tissue involution and external features such as adnexal laxity are also helpful in recognizing involution as a possible etiology of strabismus.
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sagging eye syndrome connective tissue involution as a cause of horizontal and vertical strabismus in older patients
JAMA Ophthalmology, 2013Co-Authors: Zia Chaudhuri, Joseph L. DemerAbstract:Importance Recognition of sagging eye syndrome (SES) as the cause of chronic or acute acquired diplopia may avert neurologic evaluation and imaging in most cases. Objectives To determine whether SES results from inferior shift of lateral rectus (LR) extraocular muscle (EOM) Pulleys and to investigate anatomic correlates of strabismus in SES. Design and Setting We used magnetic resonance imaging to evaluate rectus EOMs, Pulleys, and the LR–superior rectus (SR) band ligament at an eye institute. Participants Patients with acquired diplopia suspected of having SES. We studied 56 orbits of 11 men and 17 women (mean [SD] age of 69.4 [11.9] years) clinically diagnosed with SES. Data were obtained from 25 orbits of 14 control participants age-matched to SES and from 52 orbits of 28 younger controls (23 [4.6] years). Main Outcome Measures Rectus Pulley locations compared with age-matched norms and lengths of the LR-SR band ligament and rectus EOMs. Data were correlated with facial features, binocular alignment, and fundus torsion. Results Patients with SES commonly exhibited blepharoptosis and superior sulcus defect. Significant inferolateral LR Pulley displacement was confirmed in SES, but the spectrum of abnormalities was extended to peripheral displacement of all other rectus Pulleys and lateral displacement of the inferior rectus Pulley, with elongation of rectus EOMs (P Conclusions and Relevance Widespread rectus Pulley displacement and EOM elongation, associated with LR-SR band rupture, causes acquired vertical and horizontal strabismus. Small-angle esotropia or hypertropia may result from common involutional changes in EOMs and orbital connective tissues that may be suspected from features evident on external examination.
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magnetic resonance imaging of the effects of horizontal rectus extraocular muscle surgery on Pulley and globe positions and stability
Investigative Ophthalmology & Visual Science, 2006Co-Authors: Robert A Clark, Joseph L. DemerAbstract:The relative stability of posterior extraocular muscle (EOM) paths despite large, surgically induced changes in the location of the EOM’s insertion provided one of the pivotal insights that led to the modern concept of rectus EOM Pulleys.1 The transition in EOM path from stable posterior EOM belly to mobile tendon insertion defined the functional position of the EOM Pulley after surgical transpositions.2,3 Further enhancements in magnetic resonance imaging (MRI) have allowed the functional position of the EOM Pulleys to be detected, even during routine gaze changes in normal subjects.4,5 Histologic studies of the orbit in the region of EOM path inflections have revealed a complex, connective tissue matrix of smooth muscle, collagen, and elastin,6–10 with insertion by some fibers of the EOM’s orbital layer into the encircling connective tissue that has been said to act as each EOM’s functional origin,11,12—that is, its Pulley. Abnormalities of the EOM Pulleys, either in location13–17 or in stability,18,19 have been associated with several forms of incomitant strabismus. Despite the wealth of published information on Pulley location and stability in both normal subjects and subjects undergoing complex EOM surgery for incomitant strabismus, little is known about the effects on the EOM Pulley caused by standard EOM surgery for horizontal strabismus. Most EOM surgery involves resecting (shortening the tendon while maintaining the same insertion) or recessing (moving the insertion posteriorly) the EOM without changing the direction of its path in central gaze. Although moving the EOM insertion anteriorly and posteriorly along its original path should not tangentially displace the posterior EOM belly, extensive surgical dissection of the orbital connective tissue may destabilize the EOM Pulley during gaze shifts.5 Notwithstanding that the degree of mechanical coupling between the orbital and global layers of the rectus EOMs is quantitatively unknown, there is probably at least some transfer of force between them. The orbital and global layers of each rectus EOM form parallel laminae that, at least in midorbit, contain a similar maximum number of fibers. The orbital layer terminates well posterior to the sclera, with at least some orbital layer fibers terminating in connective tissue ensheathing the EOM at a location probably corresponding to the functional Pulley.20 There is also some attachment of global layer fibers to the Pulley tissues, although the global layer is generally thought to become contiguous with the tendon that ultimately inserts on the sclera.11 Because at least some orbital layer force would thus be directly coupled to the Pulley and because Pulleys are suspended from the anterior orbit by elastic bands of connective tissue,10 the posterior shift of the EOM insertion might be expected to shift its Pulley posteriorly. Similarly, because the orbital and global layers of a rectus EOM are in close apposition throughout the entire extent of the former, mechanical coupling between the layers would shift attached structures such as Pulleys in the same direction as the surgically shifted tendon insertion. If such interlaminar coupling were strong, resecting the EOM tendon anterior to its Pulley would be expected to shift the Pulley anteriorly. This study was designed to detect what effect, if any, standard recess or resect horizontal EOM surgery has on Pulley position and stability.
Jin Bo Tang - One of the best experts on this subject based on the ideXlab platform.
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recent evolutions in flexor tendon repairs and rehabilitation
Journal of Hand Surgery (European Volume), 2018Co-Authors: Jin Bo TangAbstract:This article reviews some recent advancements in repair and rehabilitation of the flexor tendons. These include placing sparse or no peripheral suture when the core suture is strong and sufficiently tensioned, allowing the repair site to be slightly bulky, aggressively releasing the Pulleys (including the entire A2 Pulley or both the A3 and A4 Pulleys when necessary), placing a shorter splint with less restricted wrist positioning, and allowing out-of-splint active motion. The reported outcomes have been favourable with few or no repair ruptures and no function-disturbing tendon bowstringing. These changes favour easier surgeries. The recent reports have cause to re-evaluate long-held guidelines of a non-bulky repair site and the necessity of a standard peripheral suture. Emerging understanding posits that minor clinically noticeable tendon bowstringing does not affect hand function, and that free wrist positioning and out-of-splint motion are safe when strong surgical repairs are used and the Pulleys are properly released.
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release of the a4 Pulley to facilitate zone ii flexor tendon repair
Journal of Hand Surgery (European Volume), 2014Co-Authors: Jin Bo TangAbstract:During primary or delayed primary repair of the flexor digitorum profundus tendon, surgeons often face difficulty in passing the retracted tendon or repaired tendon under the dense, fibrous A4 Pulley. The A4 Pulley is the narrowest part of the flexor sheath, proximal to the terminal tendon. Disrupted tendon ends (or surgically repaired tendons) are usually swelling, making passage of the tendons under this Pulley difficult or even impossible. During tendon repair in the A4 Pulley area, when the trauma is in the middle part of the middle phalanx and the A3 Pulley is intact, the A4 Pulley can be vented entirely to accommodate surgical repair and facilitate gliding of the repaired tendon after surgery. Venting the Pulley does not disturb tendon function when the other major Pulleys are intact and when the venting of the A4 Pulley and adjacent sheath is limited to the middle half of the middle phalanx. Such venting is easily achieved through a palmar midline or lateral incision of the A4 Pulley and its adjacent distal or/and proximal sheath, which helps ensure a more predictable recovery of digital flexion and extension.
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effect of a3 Pulley and adjacent sheath integrity on tendon excursion and bowstringing
Journal of Hand Surgery (European Volume), 2001Co-Authors: Jin Bo Tang, Ren Gou XieAbstract:Abstract The effect of the A3 Pulley and adjacent sheath integrity on tendon function at the proximal interphalangeal (PIP) joint was investigated in 21 fingers in 7 fresh-frozen cadaver hands. Excursions of the flexor digitorum profundus (FDP) tendons were measured when the tendons were pulled to produce PIP joint flexion of 110° from a resting position of 0°. Excursions of the FDP tendons in 10 fingers were tested within the intact sheath and after incision of the A3 Pulley, of the A3 Pulley with its proximal sheath up to the distal border of the A2 Pulley, and of the sheath between the A2 and A4 Pulleys. Eleven fingers were tested after incision of the A3 Pulley, of the A3 Pulley and its distal sheath up to the A4 Pulley, and of the sheath from the A3 to A4 Pulleys. Excursions of the FDP tendons increased to 103% ± 3% after incision of the A3 Pulley, 110% ± 4% after incision of the A3 Pulley and its proximal sheath, and 107% ± 6% after incision of the A3 Pulley and its distal sheath. Excursions increased to 116% ± 6% after incision of the sheath from the A3 to A4 Pulleys and to 119% ± 3% after incision of the sheath between the A2 and A4 Pulleys. Tendon bowstringing was 0.3 mm after incision of the A3 Pulley, 0.6 mm after incision of the A3 Pulley with its distal sheath, 0.8 mm after incision of the Pulley with its proximal sheath, 1.4 mm after incision of the sheath from the A3 to A4 Pulleys, and 1.6 mm after incision of the sheath between the A2 and A4 Pulleys. The results suggest that the sheath adjacent to the A3 Pulley plays an important role in restraining tendon bowstringing at the PIP joint, whereas the A3 Pulley alone is of little importance. This study elucidates the role of individual parts of the sheath around the PIP joint in maintaining tendon function and may guide decisions regarding the area and length of the sheath feasible for surgical release or requiring repair in the treatment of tendon lacerations. (J Hand Surg 2001;26A:855–861. Copyright © 2001 by the American Society for Surgery of the Hand.)
Vadims Poukens - One of the best experts on this subject based on the ideXlab platform.
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quantitative analysis of the structure of the human extraocular muscle Pulley system
Investigative Ophthalmology & Visual Science, 2002Co-Authors: Reika Kono, Vadims Poukens, Joseph L. DemerAbstract: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 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. (Invest Ophthalmol Vis Sci. 2002;43: 2923‐2932)
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Evidence for active control of rectus extraocular muscle Pulleys.
Investigative ophthalmology & visual science, 2000Co-Authors: Joseph L. Demer, Vadims PoukensAbstract: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. (Invest Ophthalmol Vis Sci. 2000;41: 1280 ‐1290)
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innervation of extraocular Pulley smooth muscle in monkeys and humans
Investigative Ophthalmology & Visual Science, 1997Co-Authors: Joseph L. Demer, Vadims Poukens, J M Miller, Paul E MicevychAbstract:PURPOSE Soft Pulleys stabilize paths and determine pulling directions of the extraocular muscles (EOMs). This study was conducted to characterize innervation of smooth muscles (SMs) supporting these Pulleys. METHODS Cadaveric human and monkey orbits were step and serially sectioned for histochemical and immunohistochemical staining. Before perfusion, the superior cervical ganglia of one monkey had been injected with the anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L). Immunoperoxidase staining to human SM alpha-actin confirmed Pulley SM. Monoclonal and polyclonal antibodies were used to demonstrate PHA-L, tyrosine hydroxylase, dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, neuronal nitric oxide synthase (NOS), and synaptophysin. The NADPH-diaphorase reaction was also used as a marker for NOS and the acetylcholinesterase (AChE) reaction for acetylcholine. RESULTS Pulleys, consisting of collagen and elastin sleeves supported by connective tissue containing SM, were observed around rectus muscles of humans and monkeys. The human and monkey SM was richly innervated. Axons terminating in motor end plates within SM bundles were immunoreactive to PHA-L, tyrosine hydroxylase, and dopamine beta-hydroxylase, but not phenylethanolamine-N-methyltransferase, indicating innervation of Pulley SM from the superior cervical ganglion by projections using norepinephrine. Smaller axons and motor end plates were also demonstrated in SM, using NADPH-diaphorase and NOS immunoreactivity, indicating nitroxidergic innervation, and using AchE, indicating cholinergic parasympathetic innervation. The pterygopalatine and, to a lesser extent, the ciliary ganglia, but not the Edinger-Westphal nucleus, contained cells immunoreactive to NOS, suggesting that nitroxidergic innervation to Pulley SM is mainly from the pterygopalatine ganglion. CONCLUSIONS The SM suspensions of human and monkey EOM Pulleys are similar and receive rich innervation involving multiple neurotransmitters. These complex projections suggest excitatory and inhibitory control of EOM Pulley SM, and support their dynamic role in ocular motility.
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structure function correlations in the human medial rectus extraocular muscle Pulleys
Investigative Ophthalmology & Visual Science, 1996Co-Authors: J D Porter, Vadims Poukens, Robert S Baker, Joseph L. DemerAbstract:Purpose. Fibroelastic Pulleys function like the trochlea to fix the position and pulling direction of the recti extraocular muscles within the orbit. This study characterized the fine structure of the human medial rectus muscle Pulley. Methods. Human medial rectus muscle Pulley tissue was dissected at autopsy, immersed in aldehyde fixative solution, and processed for and examined with light and electron microscopy. Results. Pulley structures were located within posterior Tenon's fascia, closely surrounding the medial rectus muscle. Pulleys were comprised of a dense collagen matrix with alternating bands of collagen fibers precisely arranged at right angles to one another. This three-dimensional organization most likely confers high tensile strength to the Pulley. Elastin fibrils were interspersed in the collagen matrix. Fibroblasts and mast cells were scattered throughout the relatively acellular and avascular collagen latticework. Connective tissue and smooth muscle bundles suspended the Pulley from the periorbita. Smooth muscle was distributed in small, discrete bundles attached deeply into the dense Pulley tissue. Conclusions. Fine structural observations confirm the existence and substantial structure of a Pulley system in association with the medial rectus extraocular muscle. The presence of Pulleys must be considered in models of the oculomotor plant. The cytoarchitecture and placement of Pulleys suggest that they are internally rigid structures and are consistent with the idea that they determine functional origins for the extraocular muscles. However, the nature of the connective tissue-smooth muscle struts suspending the Pulley system to the orbit supports the notion that the Pulley position, and thus the vector force of the eye muscles, may be adjustable.
Blake Hannaford - One of the best experts on this subject based on the ideXlab platform.
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modeling cable driven robot with hysteresis and cable Pulley network friction
IEEE-ASME Transactions on Mechatronics, 2020Co-Authors: Muneaki Miyasaka, Mohammad Haghighipanah, Yangming Li, Joseph Matheson, Andrew Lewis, Blake HannafordAbstract:In this article, a method of modeling robotic systems with closed-circuit cable–Pulley transmissions using the independently developed hysteretic cable stretch model and cable–Pulley network friction model is presented. The hysteretic cable stretch model captures the responses of the cable including elasticity, internal friction, material damping, and hysteresis. The cable–Pulley network friction is a function of cable tension, average individual wrap angle of cable around Pulley, and number of Pulleys. For the verification of the method, these two models are integrated into the dynamic model of the first three joints of the RAVEN II robotic surgery platform. The parameters of the developed model are specifically tuned for the RAVEN II and the performance of the model is compared against the kinematic model and a previous, simplified dynamic model (a model with exponential cable stretch and linear damping model). The result showed that even though significant improvements were not observed for the first two joints, the average error and the maximum error of the third joint, which uses smaller diameter and longer cable and more guide Pulleys, could be reduced by 10–20% and 5–10%, respectively, over the previous, simplified dynamic model. Also, analysis with various input trajectories indicated that cable-driven systems with very low speed, frequent change in the loading condition, and high cable tension benefit from the proposed method.
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Measurement of the cable-Pulley Coulomb and viscous friction for a cable-driven surgical robotic system
IEEE International Conference on Intelligent Robots and Systems, 2015Co-Authors: Muneaki Miyasaka, Joseph Matheson, Andrew Lewis, Blake HannafordAbstract:In this paper we present experimentally obtained cable-Pulley Coulomb and viscous friction for cable-driven surgical robotic systems including the RAVEN II surgical robotic research platform. In the study of controlling cable-driven systems a simple mathematical model which does not capture physical behavior well is often employed. Even though control of such systems is achievable without an accurate model, fully understanding the behavior of the system will potentially realize more robust control. A surgical robot is one of the systems that often relies on cables as an actuation method as well as Pulleys to guide them. Systems with such structure encounter frictional force related to conditions of cable and Pulley such as cable velocity, tension, type and number of Pulley, and angle of cable wrapping around Pulley. Using a couple of test platforms that incorporate cable, Pulleys, and other experimental conditions corresponding to the RAVEN II system, it is shown that cable-Pulley friction is function of tension, wrap angle, and number of Pulleys and not of magnitude of cable velocity.
Robert A Clark - One of the best experts on this subject based on the ideXlab platform.
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the role of extraocular muscle Pulleys in incomitant non paralytic strabismus
Middle East African Journal of Ophthalmology, 2015Co-Authors: Robert A ClarkAbstract: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.
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magnetic resonance imaging of the effects of horizontal rectus extraocular muscle surgery on Pulley and globe positions and stability
Investigative Ophthalmology & Visual Science, 2006Co-Authors: Robert A Clark, Joseph L. DemerAbstract:The relative stability of posterior extraocular muscle (EOM) paths despite large, surgically induced changes in the location of the EOM’s insertion provided one of the pivotal insights that led to the modern concept of rectus EOM Pulleys.1 The transition in EOM path from stable posterior EOM belly to mobile tendon insertion defined the functional position of the EOM Pulley after surgical transpositions.2,3 Further enhancements in magnetic resonance imaging (MRI) have allowed the functional position of the EOM Pulleys to be detected, even during routine gaze changes in normal subjects.4,5 Histologic studies of the orbit in the region of EOM path inflections have revealed a complex, connective tissue matrix of smooth muscle, collagen, and elastin,6–10 with insertion by some fibers of the EOM’s orbital layer into the encircling connective tissue that has been said to act as each EOM’s functional origin,11,12—that is, its Pulley. Abnormalities of the EOM Pulleys, either in location13–17 or in stability,18,19 have been associated with several forms of incomitant strabismus. Despite the wealth of published information on Pulley location and stability in both normal subjects and subjects undergoing complex EOM surgery for incomitant strabismus, little is known about the effects on the EOM Pulley caused by standard EOM surgery for horizontal strabismus. Most EOM surgery involves resecting (shortening the tendon while maintaining the same insertion) or recessing (moving the insertion posteriorly) the EOM without changing the direction of its path in central gaze. Although moving the EOM insertion anteriorly and posteriorly along its original path should not tangentially displace the posterior EOM belly, extensive surgical dissection of the orbital connective tissue may destabilize the EOM Pulley during gaze shifts.5 Notwithstanding that the degree of mechanical coupling between the orbital and global layers of the rectus EOMs is quantitatively unknown, there is probably at least some transfer of force between them. The orbital and global layers of each rectus EOM form parallel laminae that, at least in midorbit, contain a similar maximum number of fibers. The orbital layer terminates well posterior to the sclera, with at least some orbital layer fibers terminating in connective tissue ensheathing the EOM at a location probably corresponding to the functional Pulley.20 There is also some attachment of global layer fibers to the Pulley tissues, although the global layer is generally thought to become contiguous with the tendon that ultimately inserts on the sclera.11 Because at least some orbital layer force would thus be directly coupled to the Pulley and because Pulleys are suspended from the anterior orbit by elastic bands of connective tissue,10 the posterior shift of the EOM insertion might be expected to shift its Pulley posteriorly. Similarly, because the orbital and global layers of a rectus EOM are in close apposition throughout the entire extent of the former, mechanical coupling between the layers would shift attached structures such as Pulleys in the same direction as the surgically shifted tendon insertion. If such interlaminar coupling were strong, resecting the EOM tendon anterior to its Pulley would be expected to shift the Pulley anteriorly. This study was designed to detect what effect, if any, standard recess or resect horizontal EOM surgery has on Pulley position and stability.
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magnetic resonance imaging after surgical transposition defines the anteroposterior location of the rectus muscle Pulleys
Journal of Aapos, 1999Co-Authors: Robert A Clark, Arthur L Rosenbaum, Joseph L. DemerAbstract:Abstract Introduction: Connective tissue Pulleys serve as the functional origins of the rectus extraocular muscles (EOMs) and constrain the sideslip of the posterior EOM bellies after transposition surgery. Anterior to the Pulleys, EOM paths appreciably displace to reach their transposed insertions. The inflection points in the EOM paths from minimal posterior displacement to maximal anterior displacement should define the anteroposterior location of the EOM Pulleys after transposition. Methods: Contiguous cross-sectional magnetic resonance images were obtained in planes perpendicular to the long axis of the orbit over its entire anteroposterior extent before and after operation in 6 patients who underwent rectus muscle transposition surgery. Four patients underwent full tendon width transposition of the vertical rectus muscles laterally for lateral rectus palsy. Two of these patients had augmentation of the transposition with sutures that fixated the temporal margins of the transposed muscles posteriorly to the sclera adjacent to the borders of the lateral rectus muscle. One patient underwent full tendon width transposition of the horizontal rectus muscles superiorly for superior rectus palsy. One patient underwent full tendon width transposition of both lateral rectus muscles inferiorly for "A" pattern esotropia. Paths of EOMs were defined relative to the area centroid of the orbit. Pulley locations were inferred from EOM paths. The postoperative change in EOM Pulley location was obtained by subtracting the preoperative Pulley location from the postoperative Pulley location for each image plane. Results: For all patients, the postoperative change in EOM belly location was relatively small posterior to the globe-optic nerve junction. The 2 patients with abducens palsy who underwent placement of posterior augmentation sutures, however, demonstrated a significantly larger displacement of the posterior vertical rectus paths compared with similar patients who did not receive augmentation sutures. For all horizontally transposed vertical rectus muscles and inferiorly transposed lateral rectus muscles, the inflection of the EOM path began 3 mm anterior to the globe-optic nerve junction. For the superiorly transposed medial rectus muscle and lateral rectus muscle, the inflection began 6 mm anterior to the globe-optic nerve junction. Conclusions: The anteroposterior locations of the EOM Pulleys can be defined by analysis of EOM displacement after transposition surgery. Augmentation of transpositions by posterior suturing displaces the EOM Pulleys substantially more than nonaugmented transpositions. (J AAPOS 1999;3:9-14)
