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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
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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
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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.
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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
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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.
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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.
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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
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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
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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
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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.
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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
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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
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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.
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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
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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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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