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EYE MUSCLE PROSTHESIS
Brett Boskoff1, Matthew Patton1, Jeffrey Tse1
Advisors: Stewart Davis, M.D.2, Leonard Pinchuk, Ph.D., D.Sc.2
1Department
of Biomedical Engineering, Vanderbilt University, Nashville, TN
2Innovia LLC, Miami, FL
INTRODUCTION
DESIGN METHODOLOGY
An ophthalmic complication encountered with increasing frequency in
Functional Endoscopic Sinus Surgery (FESS) is the inadvertent amputation
of the medial rectus muscle. This extraocular muscle contributes to ocular
adduction, rotation of the eye to inward towards the nose. Medial rectus
amputation causes the affected eye to abduct and remain in the outward
position, unable to adduct, resulting in strabismus and subsequent
incapacitating double vision.
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 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.
Current surgical remedies to correct this condition include transposition of
remaining functional eye muscles or fixation of the eye to the lining of the
medial orbital wall. Transposition involves operating on the three
remaining rectus muscles, compromising blood flow to the eye and possibly
leading to further complications. While fixating the globe to the medial
orbital wall keeps the eye in the primary position, it fails to correct the lost
function of the medial rectus muscle and renders the functional lateral
rectus muscle nonfunctional.
This project ultimately aims at developing an eye muscle prosthesis to
partially restore the function of the damaged medial rectus extraocular
muscle, although it can be similarly applied to replace the function of the
lateral rectus. 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 position upon antagonist muscle relaxation. A completed
prosthesis prototype was attached to an artificial skull, effectively serving
as a proof-of-concept model.
PROOF OF CONCEPT
The distal end of the prosthesis 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.
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.
MATERIAL SELECTION
CONCLUSIONS
Materials
• Titanium T-plate (Synthes)
• Titanium screws (Synthes)
• 4’’ Nitinol wire, 0.006’’ diameter (Small
Parts)
• 0.5’’ of 16% styrene SIBS tubing
(Innovia)
• 5 mm x 10 mm Dacron platform
(Innovia)
• Non-absorbable suture
PROJECT 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. Furthermore, the project aims to develop a working
prototype and incorporate it into a proof of concept model.
Nitinol Spring Constant vs Dimensions
Spring Constant (g/deg)
Eye patch
Prism spectacles
Permanently tethering the eye into a fixed primary position
Transection of remaining extraocular muscles
Elastic silicone band
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.
ACKNOWLEDGEMENTS
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PREVIOUS SOLUTIONS
The prosthesis prototype was attached to the skull, as if replacing the medial
rectus muscle; in order to demonstrate the prosthesis’ function, another similar
muscle-suture platform was attached 7 mm temporal to the limbus, simulating the
lateral rectus. 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 contraction and sphere adduction to its
original, resting position (primary position).
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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.
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Inner Diameter (in)
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