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Vanderbilt University School of Engineering Department of Biomedical Engineering EYE MUSCLE PROSTHESIS Team Members: Brett Boskoff Matt Patton Jeffrey Tse Advisors: Leonard Pinchuk, Ph.D., D.Sc. Stewart Davis, M.D. Innovia LLC Date of Submission: April 24, 2007 1 ABSTRACT An ophthalmic complication encountered with increasing frequency in functional endoscopic sinus surgery (FESS) is the inadvertent amputation of the medial rectus muscle. The primary function of the medial rectus muscle is adduction - moving the eye toward the nose. Loss of medial rectus function due to inadvertent amputation of the muscle results in the affected eye turning outward and unable to adduct. Subjectively, the patient complains of incapacitating double vision. Current surgical remedies to correct this condition include transposition of remaining functional eye muscles or fixating the eye to the lining of the medial orbital wall. Transposition surgery involves moving two adjacent muscles to replace the loss function of the affected muscle. However, a potential complication of this procedure is that blood supply to the front of the eye might be compromised, leading to ischemia and vision side effects. Fixating the globe to the medial orbital wall will align the eye in the primary position, but fails to correct the lost function of the medial rectus muscle and prevents the eye from abduction - unable to turn the eye outward. While the symptom of double vision is eliminated on primary gaze, the patient remains seeing double on both adduction and abduction. The primary objective of this project is to develop an eye muscle prosthesis to partially restore the loss function of the damaged medial rectus muscle. The prosthesis consists of a coiled spring encased in a biocompatible polymer. The coiled spring prosthesis provides tension in the primary position to balance the antagonist eye muscle, while possessing linear elasticity to permit eye movement initiated by the antagonist muscle. Furthermore, the spring has inherent stiffness to restore the eye to the primary 2 position upon antagonist muscle relaxation. The insulating polymer prevents fibrous tissues from enveloping and negating the action of the coil. A proof of concept model is constructed to simulate the clinical scenario. 3 INTRODUCTION The Merck Manual defines paralytic strabismus as one or more of the eye muscles that move the eye in a different direction become paralyzed. Paralysis of the extraocular muscles most commonly arises from oculomotor (third), trochlear (fourth), and abducens (sixth) nerve palsy. The third and sixth cranial nerves innervate the extraocular muscles that move the globes horizontally and vertically, while the fourth nerve rotates the eyes. Extraocular muscle paralysis due to dysfunction of a cranial nerve in one eye results in the affected eye not working in concert with the other eye, leading to diplopia, or double vision. Cranial nerve paralysis can be congenital or acquired. Brain injuries or tumors can increase intracranial pressure, which, in turn, can compress and damage these nerves. Ischemia due to diabetes or stroke can lead to nerve palsy. A non-paralytic condition called ocular fibrosis syndrome is a progressive congenital disorder in which the extraocular muscles become severely inflamed and slowly lose their elastic and contractile properties. Patients afflicted with condition have difficulty in opening the eyelids and unable to move the eyes. Trauma to the extraocular muscles from motor vehicle accidents or surgery can also result in lost of function. In recent years, there has been an increase in the frequency of inadvertent amputation of the medial rectus muscle during functional endoscopic sinus surgery (FESS), a procedure commonly performed by otolaryngologists on patients with chronic sinus disease. This catastrophic complication results in the eye turning outward due to the unopposed action of its antagonist, the lateral rectus muscle. There are several adverse consequences, both psychological and physical, that result from strabismus. The eyes of a patient with strabismus, due to medial rectus 4 amputation, are no longer properly aligned with one another since the damaged eye loses the ability to adduct. Since the patient’s eyes are now focusing on two different points in space, symptomatic diplopia occurs. Depending on the severity of strabismus, the resulting double vision can lead to debilitating alteration in depth perception, disorientation, headaches, and emotional stress. Previous Solutions The currently available treatment solutions do not offer a complete, effective remedy to this problem. One way to “correct” the symptoms of strabismus is for the patient to wear an eye patch. While an eye patch may fix the double vision associated with this condition, it completely eliminates use of the affected eye, limiting the patient’s vision to one eye. In order to view an image located peripherally to the covered, damaged eye, the patient must now turn their head. Furthermore, an eye patch is not an aesthetically pleasing option for the patient and can subsequently have negative psychological side effects on the patient. Another option available for patients with strabismus is the use of a set of special prism spectacles. These glasses consist of a series of small mirrors positioned at specific angles to cater to a patient’s specific situation. The mirrors reflect light off of each other so that the affected eye can ultimately view objects in front of them. Although the glasses have the ability to realign the field of vision of the eyes, there are drawbacks associated with this solution. The prism spectacles are bulky devices, protruding several inches in front of the face when being worn. Like the eye patch, these special glasses are not aesthetically pleasing and therefore cause similar psychological side effects. 5 A motorized version of these spectacles has been proposed, which would theoretically move the mirrors change position through mechanical control. By changing angles depending on where the patient desires to look, the motor would technically act as a substitute for the impaired extraocular muscles. The problems with this idea are numerous. The modification of the prism spectacle device would still not produce an aesthetically pleasing option for the patient. Moreover, in order for this device to work, the spectacles would need to be neurologically connected to the nerves that innervate the extraocular muscles. The technology needed to create this corrective option is far too advanced to be considered practical. Progression of treatment solutions led to a procedure to permanently suture the eye into a fixed position. This currently used procedure entails anchoring a titanium Tplate to the orbital wall and suturing the bone plate to the eye using a non-absorbable suture material. This surgical technique yields a normal eye appearance for the patient, thus mitigating the psychological side effects but at the cost of the affected eye’s motor function. The eye is permanently tethered in the primary position, rendering it incapable of rotating horizontally: the patient cannot adduct or abduct their eye and must therefore turn their head to focus on objects not in their current field of view. Furthermore, movement about the horizontal axis is also affected, as the ability to elevate and depress the eye also becomes limited as a result of this procedure. Another option is the insertion of an elastic, silicone band into the orbit of the affected eye to restore mobility. While this device may be temporarily effective, it cannot serve as a permanent solution. The elastic band is not biocompatible and slowly loses its function as fibrous tissue envelops it. 6 Extraocular muscle surgeons have attempted to reposition the remaining extraocular muscles by means of surgical transection. Surgically readjusting extraocular muscles such as the superior and/or inferior rectus would weaken the forces of these muscles and create unwanted vertical deviations upon innervation of the particular extraocular muscles involved with the affected eye. This approach implies that the patient retrain their eye muscles to work in tandem with the other eye; regardless of the training and recovery time for this procedure, this method is illogical. Blood flow to the eye will undoubtedly be affected, causing ocular ischemia, vision loss, and scarring. DESIGN METHODOLOGY Objective The main objective for this project was to create an eye muscle prosthetic device to correct complex strabismus by partially restoring the medial rectus muscle function. Accomplishing this task first entailed analyzing the design features and drawbacks of the current and preceding treatments intending to correct strabismus caused by the complete loss of or decreased function of an extraocular muscle. Current surgical remedies include passive fixation of the affected eye to the thick lining of the orbital wall or transposition of other, unaffected extraocular muscles to compensate for the lost motor function of the affected extraocular muscle. One current treatment solution involves permanently tethering the eye in the primary position. This procedure secures the eye to a titanium T-plate, which is anchored to the orbital wall, with a non-absorbable suture. However, once the globe is fixed within the orbit in this fashion, the eye’s ability to abduct, adduct, elevate, and depress is 7 compromised. Although this solution provides lasting resolution to ocular misalignment caused by strabismus in any vector, the patient will still observe diplopia, albeit less severe and less frequent than that caused by complex strabismus. When the patient rotates their normal, unaffected eye from the primary position, the opposite, fixed eye remains in the primary position, preventing the eyes from focusing on the same object resulting in double vision. In similar techniques, permanent suture material has been attached to the remnant of the transected muscle and the use of a periosteal flap based in the apex of the orbit was used to tether the eye in the primary position.1 However, these approaches met the same limitations as patients may only be orthophoric when both eyes are in the primary position and must rotate their head to remain orthophoric when looking to the sides. Therefore, it is evident that these surgical options share the same, major drawback - the eye is permanently tethered to the primary position, rendering it unable to rotate. Further advancements with respect to possible treatments have yielded potential solutions attempting to regain or replicate the lost motor function of the affect extraocular muscle, progressing from the solution of permanently tethering the eye in the primary position. One solution aims at surgically repositioning one or more of the remaining extraocular muscles to compensate for the lost motor function of the affected extraocular muscle. Myectomy, transection, or transposition of unaffected extraocular muscles, such as the superior rectus, inferior rectus, or the oblique muscles, to the horizontal plane would yield weakened muscle forces of the transposed muscle(s) and unwanted vertical deviations upon innervation of the extraocular muscles associated with the affected eye. Furthermore, the desired readjustment effects from the new extraocular muscle 8 configuration is doubtful, and the training of the control over muscle innervation for this configuration seems impractical.2 Moreover, this approach could result in compromised blood supply to the eye, leading to ocular ischemia, reduced vision, and scarring. Two studies were found that implanted an elastic, silicone band to restore the eye’s alignment and to provide an elastic band against which the antagonist could pull. In 1991, Bicas, et al. described using an elastic, silicone band to tether the eye to the orbit for paralytic horizontal strabismus.2 In 1992, Scott, et al. inserted a silicone rubber band along the course of a paralyzed extraocular muscle.3 This approach using an elastic, silicone band proved to be ineffective as the implant was rendered useless or nonfunctional due to the progressive development of a fibrous capsule around the prosthetic implant. Furthermore, with constant tension on the elastic band from the antagonist muscle, late failure is likely to be a common occurrence. Upon evaluating these currently available treatment options, we propose an eye muscle prosthetic spring implant to partially replace the function of the affected extraocular muscle. Although this prosthesis can restore extraocular muscle function of the medial or later rectus extraocular muscles, the scope of this project will focus on replacing the medial rectus. Eye Muscle Prosthesis Design The eye muscle prosthesis can be used to replace either the medial or lateral rectus muscle, but it assumes that the opposing extraocular muscle is intact and functioning. The ideal function of this prosthesis is based upon the inherent mechanical nature of a spring coil. The spring will provide the balancing, antagonistic force to readjust the eye back to the primary position, will allow the eye to turn in the direction of 9 the intact, unaffected muscle upon innervation, and will exert the necessary force to return the eye back to the primary position upon relaxation of the innervated muscle. The last two spring functions will be elaborated further: when the opposing muscle to the prosthesis is innervated, the muscle contracts, the spring prosthesis stretches, and the eye rotates in a direction away from the prosthesis; furthermore, when the eye is turned and the innervated muscle relaxes, the mechanical energy stored in the stretched spring will restore the eye back to the primary position. The spring will be encased in a biocompatible, elastomeric polymer tubing, which will prevent fibrous tissue growth from enveloping the spring coil and compromising its function. The general prosthesis design is shown in figure 1. The distal end of the prosthesis Figure 1: Magnified schematic of the prosthesis design, showing the spring coil encased in SIBS and attached to the suture platform. is attached to the insertion stump of the involved muscle on the globe; the proximal end is sutured to the anterior end of the vertical bar of the titanium T-plate. Non-absorbable sutures are used to connect the muscle stump to the muscle-suture platform, the muscle-suture platform and the distal end of the spring, and the proximal end of the spring to the vertical bar of the T-plate. The horizontal bar of the T-plate is screwed into the nasal bone, while the vertical bar of the T-plate extends along either the medial or lateral orbital wall towards the apex of the orbit where it is connected to the coiled spring. 10 Although this device, when replacing the medial rectus, will provide the balancing, antagonistic force to correct strabismus by readjusting the eye to the primary position and will permit ocular abduction, the spring prosthesis cannot be innervated to contract and cause the eye to rotate in the direction of the prosthesis from the primary position. Therefore, this prosthetic implant only partially restores the function of the damaged medial rectus muscle since ocular adduction cannot be achieved. Orbital and Surgical Space Dimensions The ocular orbits are paired quadrilateral pyramidal cavities, symmetrically located on each side of the skull’s sagittal plane. The orbital bone structure is comprised of the frontal, zygomatic, maxillary, sphenoid, ethmoid, palatine, and lacrimal bones. The cavity is pyramid-shaped, with its widest portion within the orbital rim, then diverging posteriorly to the apex. The orbital rim is semi rectangular, discontinuing at the inferior nasal margin, supporting and protecting the orbit and globe. It is formed by the frontal, zygomatic, and maxillary bones.4 The medial orbital walls are parallel with respect to one another; the lateral orbital walls are divergent nearly perpendicular to one another. The lamina papyracea of the ethmoid sinus primarily composes the medial wall. The frontal process of the maxillary and lacrimal bones contributes anteriorly; the sphenoid bone contributes to the posterior portion of the medial wall. The lateral orbital wall is triangular in shape; the zygomatic bone contributes anteriorly and the greater wing of the sphenoid bone contributes posteriorly.4 The orbital floor does not extend to the apex but instead discontinues at the posterior wall approximately at the level of the maxillary sinus. The orbital floor is 11 composed of the maxillary, zygomatic, and palatine bones. The largest portion of the orbital floor is attributed to the orbital plate of the maxillary bone. The superior orbital wall is also triangular in shape and extends to the apex. The orbital roof is primarily composed of the orbital plane of the frontal bone, while the lesser wing of the sphenoid bone contributes to the posterior section.4 The approximate dimensions of both the human eye and the orbit vary among individuals; therefore, their averages were utilized for the construction of the prosthesis and the proof of concept model. The average human eye measures approximately 25 mm horizontally, 23 mm vertically, and 21 to 26 mm from anterior to posterior. The human eye is contained in a pear-shaped orbit. The dimensions of the orbit are approximately 45 mm horizontally, 35 mm vertically, and 40 to 45 mm from anterior to posterior.5 Based on these structural dimensions, the average surgical space was estimated to be 10 mm horizontally and 8 to 10 mm vertically at both the distal and lateral ends of the eye. Variations of the surgical space occur due to patient uniqueness of the orbital and nasal structures and to the exact incision location of surgery. Extraocular Muscle Forces Determination of the forces associated with extraocular muscles has been widely investigated over many years; however, due to the high amount of difficulty and invasiveness with respect to measuring the forces involved in human subjects, the data produced by the research often yields contradictory and thus inconclusive information. In 1965 (Robinson) and 1967 (Childress and Jones), the stiffness constants of the horizontal extraocular muscles were reported to be 1.2 g/deg (gram of force per degree of rotation of the eye from the median) and 1.25 g/deg, respectively.6,7 In 1981, Collins, et 12 al. published measurements of the medial rectus load constants ranging from 0.8 to 1.7 g/deg.8 In 1992, Scott, et al. stated the constant to be 0.3 to 0.5 g/deg.3 Based on the literature, it is evident that there is a large variation regarding the knowledge of extraocular muscle force constants. Material Selection Titanium T-plate The function of the bone plate is to provide an origin attachment location for the proximal end of the spring prosthesis anteriorly along the medial orbital wall. This component will be T-shaped with the horizontal bar anchored to the nasal bone and the vertical bar extending along the medial orbital wall towards the apex. The T-plate and screws will be composed of titanium because of its biologically inert, corrosion resistant, and nonmagnetic properties. The nonmagnetic property of the T-plate will not limit the patient to medical imaging modalities that do not use magnetic fields. Dacron / Polyethylene Terephthalate Muscle-Suture Platform The muscle-suture platform, which connects the medial rectus muscle stump to the prosthesis, acts as the prosthesis insertion. The suture pad will be made of Dacron, or polyethylene terephthalate (PET). Dacron is biologically inert and has an extensive history in medical applications, ranging from vascular or tissue grafts to non-absorbable sutures. Serving as the connecting component between the medial rectus insertion muscle stump and the spring prosthesis, this platform will be double woven to ensure suture stability and to prevent potential failures associated with constant tension on the device. 13 SIBS Polystyrene-polyisobutylene-polystyrene, or SIBS, is a thermoplastic, elastomeric, biocompatible copolymer, which features changeable triblock morphology, allowing modification of the material’s elastic and/or porous properties by altering the ratio of styrene to isobutylene during synthesis. This polymer does not oxidize or hydrolyze in the body; furthermore, this material does not attract polymorphonuclear leukocytes, preventing inflammation and fibrous tissue growth. SIBS’ ability to ultimately counteract the body’s innate immune response to foreign objects attests to its invaluable application in implants. Nitinol Spring Nitinol was chosen as the spring material because of its shape memory characteristics. When wound in a coil and heated above 500ºC, the cooled Nitinol spring will exhibit excellent retention of “trained shape” and spring properties despite severe compression, stretching, bending, or distortion. This alloy can be strained at least eight to ten times more than ordinary spring steel without permanent deformation. Nitinol is also corrosion resistant, biocompatible, and nonmagnetic. The nonmagnetic property of the spring will not limit the patient to medical imaging modalities that do not use magnetic fields. RESULTS Spring Testing Upon consultation with an ophthalmologist and other professionals, the appropriate load constant of the medial rectus was identified to be on the greater end of 14 the range reported by Collins, approximately 1.7 g/deg. Therefore, the objective of the subsequent spring tests was to produce a spring with a constant close to 1.7 g/deg. Based on the preliminary spring testing done at Innovia, it can be inferred that Nitinol requires a smaller mandril diameter to attain the desired spring constants. Coiling 0.006’’ diameter Nitinol wire around a 0.02’’ diameter mandril yielded a spring with a load constant of 1.59 g/deg. This value is very close to the desired spring constant and was used in the proof of concept model. Nitinol Spring Constant vs Dimensions Spring Constant (g/deg) 6 4.77 5 4 3.52 3.14 3 2.14 2 1.59 1.04 1 0 0.02 0.01 0.02 0.01 0.025 0.02 Inner Diameter (in) Graph 1: Spring constants for Nitinol springs of various wire widths (in) and mandril diameters (in). As the diameter of the Nitinol wire increases, the spring constant of the coil also increases. The prosthesis’ spring constant may not match the actual load constant of the associated muscle. However, the possibility that the brain can retrain the antagonist muscle to compensate for this deviation has been discussed. Once the prosthesis is implanted, the spring’s tension can be adjusted according to the surgeon’s specifications by changing the length of extension when the eye is in the primary position. 15 Furthermore, a main functional feature of the spring is to provide the antagonist muscle with an opposing, elastic structure to pull against. Proof of Concept Model For the proof of concept model, shown in figure 2, a partial skull (Sawbones) made of solid foam was used. A 25 mm diameter wooden sphere, imitating the eye, was mounted in the orbit. The titanium T-plate (420.71, Synthes, Inc.) and titanium screws (400.603, Synthes) were attached to the skull model using the Synthes Craniomaxillofacial tool kit. 5-0 nonabsorbable sutures were used to attach the distal and proximal ends of the spring component to the muscle-suture platform and T-plate, respectively. Approximately 4’’ of Nitinol wire Figure 2: Proof of concept model showing the orientation of the prosthesis, replacing the medial rectus muscle. (Small Parts, Inc.) with a 0.006’’ diameter was wound around a 0.02’’ diameter mandril to make a 0.3’’ long, unstretched spring. The dimensions of the Nitinol spring yields a spring constant of 1.6 g/deg. Approximately 0.5’’ of 16% styrene SIBS tubing, extruded at Innovia, was fitted onto the spring. A 5 mm x 10 mm Dacron platform, provided by Innovia, was superglued (unable to suture) to the sphere 5.5 mm medial to the limbus, the border of the cornea and the sclera. The prosthesis prototype was attached to the skull, as if replacing the medial rectus muscle; in order to demonstrate the prosthesis’ function, another similar musclesuture platform was attached 7 mm temporal to the limbus, simulating the lateral rectus. 16 Pulling this lateral suture simulates the innervation and contraction of the lateral rectus muscle causing the spring prosthesis to stretch and the sphere to abduct. Furthermore, releasing tension on the lateral suture signifies lateral rectus relaxation resulting in spring Figure 3: Proof of concept model demonstrating the function of the prosthesis, allowing sphere abduction. contraction and sphere adduction to its original, resting position (primary position). DISCUSSION Extraocular Muscle Forces With regards to the literature researched, it must be noted that the extraocular muscle force measurements were performed on normal subjects, having unoperated, unaltered eye statuses. This is relevant because in the event of accidental amputation due to FESS or unsuccessful extraocular muscle transection subsequent scarring and inflammation of the surrounding extraocular anatomy will occur. As a result, the opposing antagonist muscle must compensate by increasing the necessary muscle force to rotate the eye. In this case, special consideration must be addressed with respect to the prosthesis’ spring constant. Due to the increased muscle force of the antagonist, the replacement prosthesis will require a greater spring constant in order to correctly restore the eye back to the primary position. 17 The variation of the reported data, concerning extraocular muscle forces, could be attributed to the variety of or the inaccuracy of the methods used by which the values were obtained. These discrepancies will not be resolved until further testing is done; this entails implantation of the muscle prosthesis into animals and subsequently humans. Furthermore, it must be noted and addressed that all the literature researched related to extraocular muscle forces define the muscle force constants and muscle activation forces in terms of grams per degree and grams, respectively, even though gram is a unit of mass, not force. This could be attributed to the difficulty of measuring and defining the involved forces in terms of newtons or dynes. Converting the measurements to proper units of force entails measuring the eye’s acceleration then applying Newton’s second law of motion: force= mass · acceleration. However, this law only applies to translational motion, as the force and acceleration terms in the equation are both linear vectors. With respect to the eye, motion is governed by rotational dynamics, the only linear term being the muscle force tangential to the eye. Therefore, obtaining extraocular muscle force values in terms of newtons or dynes using the methods described in the literature is infeasible. Cost Analysis By completing a material cost analysis for each individual component, an estimated cost for the prosthesis was achieved. The T-plate structure, which is composed strictly of titanium, can be produced for approximately $1. Nitinol spring currently costs $5 a foot, which supplies the necessary spring component for three prostheses. This translates to approximately $1.67 for each individual spring. The SIBS copolymer is relatively inexpensive to synthesize and comes out to $0.50 per prosthesis. By adding the 18 individual costs for components, the total prosthesis cost is approximately $3. All of the individual components as well as the final construction of the prosthesis can be manufactured or assembled at Innovia, greatly reducing production costs. Risk Analysis In order to ensure that the eye muscle prosthesis as safe as possible for the user, a list of related factors and possible solutions to minimize the severity and probability of associated risks was devised. The first factor considered was the implant environment; the prosthesis will be subjected to the internal environment of the human body, located in the orbital region of the skull. To minimize the inherent complications due to contact, the T-plate structure will be comprised of titanium, which is corrosion resistant, biologically inert, and nonmagnetic. Similarly, the spring will be made of Nitinol, which also has corrosion resistant and biocompatible properties. The spring component will be extruded with a biocompatible copolymer, SIBS, which does not oxidize or hydrolyze in the body. Furthermore, this elastomer does not attract polymorphonuclear leukocytes, thus preventing the spring from coming into contact with the internal environment and subsequently eliciting an inflammatory response that would compromise the prosthesis’ function. Another significant risk factor deals with the duration of the prosthesis. The prosthesis serves as a permanent implant solution for associated patients; therefore, it must maintain its integrity and function for the full implant duration in the user. Using Nitinol, which is not only a corrosion resistant and biocompatible material but also an excellent shape memory alloy, as the spring material, reduces the risk of the spring losing any of its mechanicals properties through repeated loading and unloading during its 19 application lifetime. The SIBS polymer, which does not oxidize or hydrolyze in the body, reduces the risk of corrosion over time due to contact with the body’s internal environment. Moreover, this material’s ability to negate the body’s innate response to foreign objects, leukocyte aggregation, will extend the implant’s lifetime in the patient. Another risk factor evaluated was failure due to repetition. Repetitive eye rotations will subsequently cause the spring to stretch and contract as well as place tension on the suture platform. The risk of spring failure due to repetition is low due to Nitinol’s shape memory properties, discussed previously. Furthermore, this elastic alloy can be strained at least eight to ten times more than an ordinary spring steel wire without permanent deformation. The Dacron muscle-suture platform has extensive experience in implants and surgical applications, attesting to its long term biocompatibility. Its established history and double woven structure suggest a low risk of failure from repetitive, oscillating tension from the spring component. It can be inferred from the Design Safe chart, in the appendix, that there is an overall low level of risk of failure associated with the eye muscle prosthesis. Future Direction As further extraocular muscle force studies and replacement prosthesis proposals are pursued, more accurate quantification of the involved forces is required. A force gauge capable of precisely measuring muscle forces of both the medial and lateral rectus muscles is needed. Since the spring is replacing the damaged extraocular muscle, a thorough understanding and accurate quantification of the extraocular muscle forces is essential. 20 Another consideration that must be addressed is the optimal prosthesis design that mitigates the consequences associated with possible failure. A design that has been proposed and assessed involves a replacing the vertical bar of the T-plate with a tube. The proximal end of the prosthesis is sutured to a stationary titanium ball, which will rest on the opening of the vertical tube of the T-plate. The suture passes through the tube and is anchored to the horizontal bar of the T-plate. In the event that the spring prosthesis requires replacement, the suture can be untied, and the spring component can be removed, while the T-plate remains screwed into the nasal bone. In the current design, if a problem arises, the whole prosthesis must be detached; the proposed design permits the spring to be independently removed in the case that spring adjustments or replacements are needed. While this extraocular muscle prosthetic device has the potential to be an effective solution to correct strabismus and the problems associated with it, further corrective measures can still be taken, as this solution does not completely restore the lost motor function of an absent extraocular muscle. The prosthesis provides partial restoration of lost medial rectus function, allowing ocular adduction from the primary position. Even with optimal device function, the prosthesis cannot achieve complete recovery of the lost functions of the medial rectus muscle; the eye is still limited in its ability to adduct from the primary position. The next step in the evolution of the eye muscle prosthesis will involve innervation of the prosthesis, which would allow the device to contract and permit ocular rotation in the direction of the prosthesis from the primary position, thus restoring full movement of the affected eye muscle. One form of technology that could potentially be used for future measures involves use of bionic neurons, or BIONs. 21 BIONs have recently been used to reanimate paralyzed muscles through means of electrical stimulation; this device can effectively take over the function of damaged neurons. Innovation This application of using a spring as a prosthetic implant device to restore extraocular muscle function is unique. The inventor of the extraocular muscle prosthesis, Dr. David Tse, Bascom Palmer Eye Institute (Miami, FL), currently has a provisional patent application with the United States Patent and Trademark Office. The eye muscle prosthesis novelly features a spring coil acting as the biasing component for the antagonist muscle. Another characteristic of this device that will ultimately distinguish itself from its predecessors is the polymeric housing wherein the spring will be contained. Innovation of this biocompatible, elastomeric copolymer, SIBS, is attributed to Dr. Leonard Pinchuk, Innovia LLC. 22 CONCLUSIONS This project’s ultimate objective was the development of an eye muscle prosthetic device to correct complex strabismus brought about by amputation, dysfunction, and/or paralysis of an extraocular muscle by partially restoring the functioning of the affected muscle. Although this prosthesis can replace the function of the medial rectus and lateral rectus, the scope of this project focuses on applying the device to the medial rectus. The main components of the device are the titanium T-plate that is anchored to the nasal bone, the Nitinol spring coil, and a biocompatible elastic polymer, SIBS, which encases and protects the spring. The spring is sutured to the eye muscle insertion and effectively acts as a replacement for the affected muscle. After prototype fabrication, the prosthesis was attached to an artificial skull model. This model successfully demonstrates the conceptual function of the prosthesis. 23 ACKNOWLEDGEMENTS We would like to express our utmost gratitude and appreciation to our advisors, Dr. Len Pinchuk and Dr. Stewart Davis, and to the other employees at Innovia for their support and insight. Innovia’s mission in the research, development, and manufacturing of biomedical products to improve the quality of life epitomizes the engineering principles we seek to embrace. Furthermore, we owe special thanks to Dr. David Tse for allowing us to develop his invention; his patience and compassion undoubtedly inspired our success. REFERENCES 1. Goldberg R.A., et al. “Use of Apically Based Periosteal Flaps as Globe Tethers in Severe Paretic Strabismus.” Archives of Ophthalmology. 118 (2000): 431-437. 2. Bicas, Harley. "A Surgically Implanted Elastic Band to Restore Paralyzed Ocular Rotations." Journal of American Association for Pediatric Ophthalmology and Strabismus. 28 (1991): 10-13. 3. Scott, Alan, et al. "Eye Muscle Prosthesis." Journal of American Association for Pediatric Ophthalmology and Strabismus. 29 (1992): 216-218. 4. Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger. (1918): 5c. The Exterior of the Skull. 5. Toris C.B., Yablonski M.E., Wang Y.L., Camras C.B. "Aqueous Humor Dynamics in the Aging Human Eye." American Journal of Ophthalmology. 124 (1999): 407-12. 6. Robinson, D. A. “The Mechanics of Human Smooth Pursuit Eye Movment.” Journal of Physiology. 180 (1965): 569. 7. Childress, D.S. and Jones, R. W. “Mechanics of Horizontal Movement of the Human Eye.” Journal of Physiology. 188 (1967): 273. 8. Collins, Carter, et al. "Extraocular Muscle Forces in Normal Human Subjects." Association for Research and Vision and Ophthalmology. 20 (1981): 652-664. 24 APPENDIX SPRING TESTING GRAPHS Spring Wire Spring Constant vs Dimensions Spring Constant (g/deg) 3 2.5 2 1.5 1 0.5 0 0.02 0.01 0.02 0.01 Inner Diameter (in) Elgiloy Spring Constant vs Dimensions Spring Constant (g/deg) 3 2.5 2 1.5 1 0.5 0 0.02 0.01 0.02 0.01 0.025 0.02 Inner Diameter (in) INNOVATION WORKBENCH 25 0.025 0.02 - Directions for Innovation 1. Find a way to eliminate, reduce, or prevent [the] (Strabismus) in order to avoid [the] (Diplopia), under the conditions of [the] (Extraocular Muscle Dysfunction or Amputation). 2. Find a way to eliminate, reduce, or prevent [the] (Extraocular Muscle Dysfunction or Amputation) in order to avoid [the] (Strabismus), under the conditions of [the] (Ophthalmoparesis or Adverse Effect of Medical Procedure). 3. Find a way to eliminate, reduce, or prevent [the] (Ophthalmoparesis or Adverse Effect of Medical Procedure) in order to avoid [the] (Extraocular Muscle Dysfunction or Amputation). 4. Find an alternative way to obtain [the] (Surgical Implant) that offers the following: provides or enhances [the] (Correction of Eye to Primary Position), eliminates, reduces, or prevents [the] (Diplopia). 5. Find an alternative way to obtain [the] (Correction of Eye to Primary Position) that offers the following: provides or enhances [the] (Regaining of Antagonist Muscle Force), does not require [the] (Surgical Implant). 6. Find an alternative way to obtain [the] (Regaining of Antagonist Muscle Force) that offers the following: provides or enhances [the] (Regaining of Complete Muscle Motor Function), does not require [the] (Correction of Eye to Primary Position). 26 7. Find an alternative way to obtain [the] (Regaining of Complete Muscle Motor Function) that does not require [the] (Regaining of Antagonist Muscle Force). 8. Find an alternative way to obtain [the] (Covering of Affected Eye) that offers the following: provides or enhances [the] (Negation of Diplopia) eliminates, reduces, or prevents [the] (Diplopia) does not cause [the] (Loss of Vision in Affect Eye). 9. Resolve the contradiction: The useful [the] (Covering of Affected Eye) should provide [the] (Negation of Diplopia) counteracts [the] (Diplopia) and avoids [the] (Loss of Vision in Affected Eye). 10. Find an alternative way to obtain [the] (Negation of Diplopia) that does not require [the] (Covering of Affected Eye). 11. Find a way to eliminate, reduce, or prevent [the] (Loss of Vision in Affected Eye) under the conditions of [the] (Covering of Affected Eye). Prioritize Directions - Directions selected for further consideration High Priority: 1, 4, 5, 10 Long Term: 6, 7 Other / Irrelevant: 2, 3, 8, 9, 11 - List and categorize all preliminary ideas Current surgical remedies for paralyzed or absent extraocular muscle involve fixing the globe in the primary position (Correction of Eye to Primary Position), leaving the eye unable to adduct or abduct. We propose a spring coil system to replace the paralyzed or absent extraocular muscle (Regaining of Antagonist Muscle Force). Future considerations would be to utilize a bionic neuron to innervate the amputated or dysfunctional muscle in conjunction with the spring coil system (Regaining of Complete Muscle Motor Function). CONCEPT MAP 27 DESIGN SAFE 28 29