Corneal Transplantation- Dental Lamina as Keratoprosthesis; a Tooth for an Eye

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This article is an excerpt from a manuscript published by the McMaster University Medical Journal (http://www.mumj.org). It is reprinted here as permitted by the MUMJ copyright notice which states:

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Corneal Transplantation- Dental Lamina as Keratoprosthesis; a Tooth for an Eye

Abstract

 

            Corneal diseases are among the major causes of global blindness, second only to cataracts in overall importance. The epidemiology of corneal blindness includes various infectious and inflammatory eye diseases that cause corneal scarring, which ultimately leads to functional blindness. Anatomically, the cornea is the outermost layer of the eye and is primarily responsible for light refraction which allows for central and peripheral vision. In cases where less invasive treatments for corneal disease fail, corneal transplant is indicated, and it has the highest success rate of any transplant procedure. The simplest form of corneal transplant, a Penetrating Keratoplasty (PKP), is performed by transplanting a donor cornea from a cadaver. The authors will review a more complex form of corneal transplant, the Osteo-Odonto Keratoprosthesis (OOKP), which is a two stage procedure whereby dental and buccal tissue is auto-transplanted to serve as a synthetic cornea. Our purpose is to inform readers about the relevant anatomy, two-stage procedure, surgical interprofessionalism, indications, contraindications, complications, patient outcomes and future scope of OOKP.

 

Introduction

According to the World Health Organization, in 2001 there were greater than 135 million individuals with severely impaired vision in both eyes, with hundreds of millions experiencing monocular vision loss.11 In recent years, the epidemiology of blindness has changed, shifting away from traditional infectious causes, such as trachoma and leprosy, to other causes such as cataract or corneal dysfunction. Ocular disease affecting the cornea presents in a wide variety of infectious and inflammatory conditions. In addition, the prevalence of corneal blindness varies from country to country and even from one population to another, depending on many factors, such as availability and general standards of eye care.

Corneal transplantation, referred to as Penetrating Keratoplasty (PKP), is a procedure performed by ophthalmologists to treat severe visual impairment or pain in eyes whose outermost transparent covering (the cornea) cannot be treated by non-invasive means. A PKP involves replacing the cornea with donor cadaveric corneal tissue, and according to the Canadian National Institute for the Blind1, its 85% success rate is highest of all tissue transplantations. When a PKP is unsuccessful, a Keratoprosthesis is conducted whereby a non-donor, synthetic cornea is grafted. For over 200 years, a variety of prosthetic corneas made of materials such as champagne corks and hydroxyapatite have been implanted in place of cadaveric tissue.7 The ideal keratoprosthetic would improve optical quality, have a specifiable power, be accepted by the patient’s immune system, allow for ocular examination and last the patient’s lifetime. While none of the synthetic corneas used thus far show all these traits, the use of dental lamina tissue is the most frequent form of Keratoprosthesis.7

As of September 2009, the United States of America joined Italy, Japan, Singapore and other nations in being host to a special Keratoprosthesis; the modified Osteo-Odonto Keratoprosthesis (OOKP).2 This successfully attempted procedure, performed in 2009 at Miami’s Bascom Palmer Eye Institute, is yet to be reviewed in scientific literature. However, OOKP is a highly cited topic because it is an example of interdisciplinary patient care in which eye specialists, dental surgeons, anaesthesiologists, and other medical professionals collaborate on a multi-stage procedure. Strampelli fashioned the procedure in the early 1960’s and to date nearly 600 surgeries have been attempted.7 OOKP is a last resort treatment for ocular problems rooted in corneal dysfunction and has very stringent criteria for its recipients.

Ocular Anatomy

            With adequate light, the visual system allows images to be viewed and constructed for our interpretation. Light waves reflect from objects and the cornea is the first surface through which they travel, as they head through the pupil toward the lens. Approximately 75% of the refraction (bending of light waves) is attributed to the cornea, which is an essential part of focusing images to a specific point on the retina5. When the image is correctly projected onto the retina at the posterior of the eye, neural tissue will transmit the information to the brain.

Structurally, the cornea is a 550µm thick transparent tissue sitting over the pupil and iris, making up the outer covering of the anterior chamber. It is a dome-shaped, avascular structure composed of 5 layers. The epithelial layer is most superficial, followed by Bowman’s membrane, stroma, Descemet’s membrane and endothelium.8 Changes in the shape, consistency, or composition of the cornea via tissue injury or disease produce significant affects on visual acuity.

Schematic of  Eye.

 Image copied from National Eye Institute, http://www.nei.nih.gov/

Indications and contraindications

Strampelli described the original OOKP forty years ago after using a patient’s own tooth root and alveolar bone to create an optical cylinder.9 Efforts to follow this procedure were unsuccessful until Falcinelli refined and reintroduced it to Britain in the mid 1990s3,4. Patients were recommended to undergo the modern or Falcinelli OOKP after a history of failed PKPs due to graft-versus-host immune-mediated rejection or if suffering from bilateral corneal blindness secondary to an array of corneal diseases which impair normal refraction. Some of these conditions include Stevens-Johnson syndrome, which is a hypersensitivity autoimmune reaction against certain drugs and infections leading to corneal scarring, chemical burns, trachoma (Chlamydial infection), and refractory dry eyes. Indications for an OOKP beyond the initial insults to the cornea are being able to count fingers, perceive hand motion and light perception. Contra-indications would include a lack of teeth, children under the age of 17, advanced glaucoma, or irreparable retinal detachment.7 Pre-operative assessment is completed by an ophthalmologist who assesses whether the retina and optic nerve is normal and functioning and by a maxilla-facial surgeon who completes an oral examination to decide which tooth (typically a canine) is most appropriate for implantation and to assess the health of the buccal membranes (tissue from inside of cheek).

Osteo-Odonto Keratoprosthesis: The Procedure

The procedure is typically completed in two stages separated by two to four months.7 During the first stage a canine tooth is extracted with the entire root and a portion of the jaw bone. The top part of the tooth (the crown) is removed and the remaining root, bone and tissue are shaped into a cube, referred to as dental lamina. A hole is drilled in the centre of the dental lamina and a clear, small plastic optical cylinder is fitted into the hole and sealed with dental cement. This optical cylinder will act as the media for light to travel through and focus on the retina. The dental lamina is inserted through a slit below the lower eyelid of the non-operating eye.

Schematic of  Tooth.

Image copied from Studio Dentaire, www.studiodentaire.com/en/glossary/cusp.php

The last step of stage one involves superficial keratectomy (removal of the superficial layer of the cornea) and replacing it with a piece of buccal mucous membrane, which is sutured to the sclera (outer layer of eye adjacent to cornea) thus creating a new ocular surface. Then follows a two to four month waiting period, which allows the dental lamina to heal and become vascularized.

Stage two involves moving the dental lamina and the tissue which will have grown over the optical cylinder from underneath the eyelid to the centre of the eye. The buccal mucous graft tissue is lifted and a hole is created in the center of the patient’s remaining original cornea wide enough to fit the optical cylinder. The iris is then removed (iridodialysis), the lens extracted, and an anterior vitrectomy (removal of vitreous, the gel-like fluid which supports the eye) is performed to create space for the new implanting tissues. The dental lamina, including the optical cylinder, is then inserted through the corneal hole and put into position by sutures to the host cornea and sclera. The eye is then re-inflated with filtered air. The final step involves creating a hole in the buccal mucous graft tissue wide enough to allow light to enter the optical cylinder and then suturing the buccal graft back onto the sclera. The patient should be able to see within two weeks. Immediate post-operative care entails symptom relief including prednisolone (corticosteroid) and antibiotic prophylaxis. Follow-up is life-long with an ophthalmologist.    

Schematic of completed OOKP.

Image copied from http://stanford.wellsphere.com/dental-health-article/using-a-tooth-to-help-the-blind/788589- special thanks to Dr. Dean Brandon

Results

Falcinelli reported superb results as 75% of his OOKP patients were able to see 20/40 or better and 85% had anatomical stability (n=181).3 In a study by Hille et al., they found that of eight patients who underwent OOKP, four had a best visual acuity of 0.6 to 0.9 (20/33 to 20/22) and all eight patients had stable prosthesis.6 Other studies have shown similar results, whereby outcomes of OOKP are better than most methods for managing end stage corneal diseases.4,10   

 

Discussion

In reviewing the efficacy of OOKP procedures, the complications are a significant consideration. Complications of OOKP are on a wide spectrum and can involve oral, ocular or systemic complications if the patient is immunocompromised. Vitreous hemorrhaging (bleeding within the eye), retinal detachment are serious complications occurring post stage two. Movement of the optical cylinder can occur which would require additional surgery to fix it in place. The ocular or buccal membranes are always at risk for infection and prompt attention to these areas should be considered if they show any signs of pain or inflammation. Glaucoma is a major associated problem and its incidence has been cited up to 75% of OOKP patients7. One of the reasons for the high incidence of Glaucoma is that neither intraocular pressures nor visual field testing can be accurately measured in the OOKP eye. Consequently, diagnosis of glaucoma is left up to simple methods, like the patient’s subjective visual field defects.        

Although the complications of OOKP can vary in severity, majority of patients have stable implants. Because OOKPs have been shown to improve and maintain visual acuity, this procedure is an innovative alternative for managing end stage ocular surface diseases after failed PKPs. The procedure still remains technically difficult, requires special training and healthy dentine and buccal tissues. Current research is being done for creating synthetic analogues to substitute the dental lamina and as well as accurately measuring intraocular pressures (to diagnose Glaucoma) in post-transplant patients.

The future of OOKP is promising, as the literature and scientific interest is increasing. Modern OOKP surgery still has many barriers to becoming universally accepted because of the required technical and surgical expertise, but it does provide hope for restoring vision in refractory corneal blindness. The creativity of using a tooth as an eye implant should hope to inspire future interprofessional approaches to ophthalmic practice to provide the best care for patients.     

References:

1)      Canadian National Institute for the Blind. Gift of Sight: Corneal Transplants. Web Article. Accessed on Sept. 28, 2009.

<http://www.cnib.ca/en/support/gift-sight/Default.aspx.>

2)      CBN News. Tooth-to-Eye Transplant Saves Vision: Web Article. Accessed on Sept. 27, 2009. < http://www.cbn.com/cbnnews/healthscience/2009/September/Tooth-to-Eye-Transplant-Saves-Vision/>

3)      Falcinelli G, Barogi G, Taloni M. Osteoodontokeratoprosthesis: present experience and future prospects. Refract Corneal Surg. 1993; 9:193-194. 

4)      Falcinelli G, Barogi G, Caselli M et al. Personal Changes and Innovations in Strampelli’s Osteo-odonto-keratoprosthesis. An Inst Barraquet. 1999; 29(S)47-48. 

5)      Goldstein, E. Bruce. Sensation & Perception. 7th Edition. Canada: Thompson Wadsworth, 2007

6)      Hille K, Landau H & Ruprecht KW. Osteo-odonto-keratoprosthesis: A summary of 6 years surgical experience. Ophthalmologe. 2002; 99(2):90-95.

7)      Liu C, Paul B, Tandon R et al. The Osteo-Odonto-Keratoprosthesis (OOKP). Seminars in Ophthalmology. 2005; 20(2):113-128.

8)      Snell, Richard S., Lemp, Michael A. Clinical Anatomy of the Eye. England: Blackwell Services, Inc. 1998.

9)      Strampelli, B. Keratoprosthesis with osteodontal tissue. AM J Ophthalmol. 1963; 89: 1029-1039.

10)  Tay AB, Tan DT, Lye KW et al. Osteo-odonto-keratoprosthesis surgery: a combined ocular-oral procedure for ocular blindness. Int J Oral Maxillofacial Surg. 2007; 36(9) 807-813.

11)  Whitcher JP, Srinivasan M, Upadhyay MP. Corneal blindness: a global perspective. Bulletin of the World Health Organization. 2001;79(3):214-21.

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