Downloaded from http://bjo.bmj.com/ on March 19, 2015 - Published by group.bmj.com
Editorial
Keratoprostheses: are we there yet? Venkata S Avadhanam,1,2 Christopher S C Liu1,2,3 Since the dawn of the 18th century, efforts to develop an ideal keratoprosthesis (KPro) have not yielded the dream KPro yet, but the journey to such an invention has given us a few notable devices and valuable experience with a number of biomaterials. The invention of a KPro started with a piece of silver-rimmed glass proposed by Pellier de Quengsy.1 We now have the latest iteration of Boston Type 1 KPro with a porous titanium back plate.2 Although the Pintucci KPro (Dacron mesh skirt and PMMA optic) and the AlphaCor (hydrogel matrix) were in clinical use for a good length of time, they failed to retain over the long term.1 We learnt that bio-integration is essential for the skirt and bio-inertness for the optic. Polymethyl methacrylate (PMMA) has largely solved the search for a stable and durable optic though the search for an ideal skirt material is still ethereal. As we speak, the Boston type-1 KPro and the osteo-odonto-keratoprosthesis (OOKP) have emerged as the most sustainable devices. The Boston KPro has a PMMA optic on a stem and a porous back plate made of PMMA (or more recently titanium), which sandwiches a donor graft button and transplanted onto the cornea. It is the most widely used KPro and is suitable for wet blinking eyes and those with multiple graft failures. OOKP involves a radical approach to KPro surgery. It is performed in 2 stages. In the first stage, the buccal mucosal membrane (BMM) is grafted on to the eye, which reconstitutes the new ocular surface. In the same stage a suitable single rooted tooth such as a canine is extracted with surrounding jawbone from which a lamina is prepared by inserting a PMMA cylinder through a central opening. The lamina is usually buried for 3-4 months in a submuscular pocket under the lower lid of the contra-lateral eye for vascular ingrowth. In the second stage, the BMM is reflected and the osteo-dental-acrylic 1
Sussex Eye Hospital, Brighton BN2 5BF; 2Brighton and Sussex Medical School, Brighton; 3Tongdean Eye Clinic, Hove BN3 6QB Correspondence to Christopher S C Liu, Sussex Eye Hospital, Eastern Road, Brighton, BN2 5BF, UK; [email protected]
lamina is implanted through the central corneal opening after removal of the iris, lens, and core vitreous. The BMM is then closed but the optic projects slightly through a hole in it. OOKP is the last resort of end stage ocular surface disease in keratinised, bone-dry eyes or those with impaired blink or absent lids. In general OOKP has better retention and success over the long-term than the Boston KPro. The results of OOKP achieved by Falcinelli are not met by any of his first generation disciples or by subsequent trainees.3 The surgical skills and the results of OOKP by Falcinelli remain a holy grail of the OOKP. During preparation of the lamina in stage 1, cement is applied to the posterior part of the anterior optical cylinder. Once the optical cylinder is in place within the lamina, the cement sits between the cylinder and dentine.4 It is to be noted that cement is only a space filler and not an adhesive. Judicious application is important and any excess cement has to be removed once it is set. In this issue of the BJO, Weisshuhn et al., under the title ‘OOKP and the testing of three different adhesives for bonding bovine teeth with optical PMMA cylinder’ describe the bonding strength test results of three different adhesives on experimental bovine teeth laminae. The authors conclude that among the three adhesives they tested (PMMA bone cement, universal resin cement and glass inomer cement), the universal resin cement was found to be the best alternative to the acrylic cement because of its superior bond strength and ease of application. However, we should point out that the acrylic cement mainly acts as a space filler. It should not be relied upon as an adhesive to hold the cylinder in the lamina. The optical cylinder is held in position inside the central hole of the lamina by sheer virtue of friction via a snug fit. To achieve a snug fit, the hole in the lamina should be of correct diameter and be perpendicular to the dentine-bone plate. Although it is useful to have additional tissue glue to supplement the adhesion it is not required in routine cases for cylinder fixation. Moreover, an adhesive cannot be helpful to hold the cylinder in
Avadhanam VS, et al. Br J Ophthalmol July 2014 Vol 98 No 7
place in the presence of laminar resorption. In cases of a loose cylinder fit due to an oversized aperture, an adhesive may be an attractive solution, but we fear that there could be some dissolution of the adhesive in vivo which could jeopardise the cylinder integrity leaving a potential space for microbial entry, aqueous leaks and serious complications such as retinal detachment and endophthalmitis. It is also unclear as to whether there will be toxicity to ocular structures. Hence, we strongly recommend a snug fit of the cylinder to the lamina and not to depend on tissue adhesives for that purpose. A tissue adhesive as such may have a role in uncommon circumstances as in combining two teeth (when the lamina from single tooth is too thin) to make one lamina. A synthetic OOKP lamina can avoid the problems of oral complications and may reduce laminar resorption and laminar exchange. We hope pre-fabricated synthetic OOKP laminae will one day become available. Competing Interest None.
To cite Avadhanam VS, Liu CSC. Br J Ophthalmol 2014;98:849. Published Online First 3 March 2014
▸ http://dx.doi.org/10.1136/bjophthalmol-2013303141 Br J Ophthalmol 2014;98:849. doi:10.1136/bjophthalmol-2014-304994
REFERENCES 1
2
3
4
Gomaa A, Comyn O, Liu C. Keratoprostheses in clinical practice - a review. Clin Experiment Ophthalmol 2010;38:211–24. doi:10.1111/ j.1442-9071.2010.02231.x Boston KPro News. 2011;Fall: No 8. http://www. masseyeandear.org/gedownload!/2011%20KPro% 20newsletter.pdf?item_id=70213024 (Accessed 1 October 2013). Falcinelli G, Falsini B, Taloni M, et al. Modified osteo-odonto keratoprosthesis for treatment of corneal blindness:long-term anatomical and functional outcomes in 181 cases. Arch Ophthalmol. 2005;123 (10):1319–29. Liu C, Paul B, Tandon R, et al. The osteo-odonto-keratoprosthesis (OOKP). Semin Ophthalmol 2005;20:113–28. doi:10.1080/ 08820530590931386.
849
Downloaded from http://bjo.bmj.com/ on March 19, 2015 - Published by group.bmj.com
Keratoprostheses: are we there yet? Venkata S Avadhanam and Christopher S C Liu Br J Ophthalmol 2014 98: 849 originally published online March 3, 2014
doi: 10.1136/bjophthalmol-2014-304994 Updated information and services can be found at: http://bjo.bmj.com/content/98/7/849
These include:
References Email alerting service
This article cites 3 articles, 0 of which you can access for free at: http://bjo.bmj.com/content/98/7/849#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
Editorial
Keratoprostheses: are we there yet? Venkata S Avadhanam,1,2 Christopher S C Liu1,2,3 Since the dawn of the 18th century, efforts to develop an ideal keratoprosthesis (KPro) have not yielded the dream KPro yet, but the journey to such an invention has given us a few notable devices and valuable experience with a number of biomaterials. The invention of a KPro started with a piece of silver-rimmed glass proposed by Pellier de Quengsy.1 We now have the latest iteration of Boston Type 1 KPro with a porous titanium back plate.2 Although the Pintucci KPro (Dacron mesh skirt and PMMA optic) and the AlphaCor (hydrogel matrix) were in clinical use for a good length of time, they failed to retain over the long term.1 We learnt that bio-integration is essential for the skirt and bio-inertness for the optic. Polymethyl methacrylate (PMMA) has largely solved the search for a stable and durable optic though the search for an ideal skirt material is still ethereal. As we speak, the Boston type-1 KPro and the osteo-odonto-keratoprosthesis (OOKP) have emerged as the most sustainable devices. The Boston KPro has a PMMA optic on a stem and a porous back plate made of PMMA (or more recently titanium), which sandwiches a donor graft button and transplanted onto the cornea. It is the most widely used KPro and is suitable for wet blinking eyes and those with multiple graft failures. OOKP involves a radical approach to KPro surgery. It is performed in 2 stages. In the first stage, the buccal mucosal membrane (BMM) is grafted on to the eye, which reconstitutes the new ocular surface. In the same stage a suitable single rooted tooth such as a canine is extracted with surrounding jawbone from which a lamina is prepared by inserting a PMMA cylinder through a central opening. The lamina is usually buried for 3-4 months in a submuscular pocket under the lower lid of the contra-lateral eye for vascular ingrowth. In the second stage, the BMM is reflected and the osteo-dental-acrylic 1
Sussex Eye Hospital, Brighton BN2 5BF; 2Brighton and Sussex Medical School, Brighton; 3Tongdean Eye Clinic, Hove BN3 6QB Correspondence to Christopher S C Liu, Sussex Eye Hospital, Eastern Road, Brighton, BN2 5BF, UK; [email protected]
lamina is implanted through the central corneal opening after removal of the iris, lens, and core vitreous. The BMM is then closed but the optic projects slightly through a hole in it. OOKP is the last resort of end stage ocular surface disease in keratinised, bone-dry eyes or those with impaired blink or absent lids. In general OOKP has better retention and success over the long-term than the Boston KPro. The results of OOKP achieved by Falcinelli are not met by any of his first generation disciples or by subsequent trainees.3 The surgical skills and the results of OOKP by Falcinelli remain a holy grail of the OOKP. During preparation of the lamina in stage 1, cement is applied to the posterior part of the anterior optical cylinder. Once the optical cylinder is in place within the lamina, the cement sits between the cylinder and dentine.4 It is to be noted that cement is only a space filler and not an adhesive. Judicious application is important and any excess cement has to be removed once it is set. In this issue of the BJO, Weisshuhn et al., under the title ‘OOKP and the testing of three different adhesives for bonding bovine teeth with optical PMMA cylinder’ describe the bonding strength test results of three different adhesives on experimental bovine teeth laminae. The authors conclude that among the three adhesives they tested (PMMA bone cement, universal resin cement and glass inomer cement), the universal resin cement was found to be the best alternative to the acrylic cement because of its superior bond strength and ease of application. However, we should point out that the acrylic cement mainly acts as a space filler. It should not be relied upon as an adhesive to hold the cylinder in the lamina. The optical cylinder is held in position inside the central hole of the lamina by sheer virtue of friction via a snug fit. To achieve a snug fit, the hole in the lamina should be of correct diameter and be perpendicular to the dentine-bone plate. Although it is useful to have additional tissue glue to supplement the adhesion it is not required in routine cases for cylinder fixation. Moreover, an adhesive cannot be helpful to hold the cylinder in
Avadhanam VS, et al. Br J Ophthalmol July 2014 Vol 98 No 7
place in the presence of laminar resorption. In cases of a loose cylinder fit due to an oversized aperture, an adhesive may be an attractive solution, but we fear that there could be some dissolution of the adhesive in vivo which could jeopardise the cylinder integrity leaving a potential space for microbial entry, aqueous leaks and serious complications such as retinal detachment and endophthalmitis. It is also unclear as to whether there will be toxicity to ocular structures. Hence, we strongly recommend a snug fit of the cylinder to the lamina and not to depend on tissue adhesives for that purpose. A tissue adhesive as such may have a role in uncommon circumstances as in combining two teeth (when the lamina from single tooth is too thin) to make one lamina. A synthetic OOKP lamina can avoid the problems of oral complications and may reduce laminar resorption and laminar exchange. We hope pre-fabricated synthetic OOKP laminae will one day become available. Competing Interest None.
To cite Avadhanam VS, Liu CSC. Br J Ophthalmol 2014;98:849. Published Online First 3 March 2014
▸ http://dx.doi.org/10.1136/bjophthalmol-2013303141 Br J Ophthalmol 2014;98:849. doi:10.1136/bjophthalmol-2014-304994
REFERENCES 1
2
3
4
Gomaa A, Comyn O, Liu C. Keratoprostheses in clinical practice - a review. Clin Experiment Ophthalmol 2010;38:211–24. doi:10.1111/ j.1442-9071.2010.02231.x Boston KPro News. 2011;Fall: No 8. http://www. masseyeandear.org/gedownload!/2011%20KPro% 20newsletter.pdf?item_id=70213024 (Accessed 1 October 2013). Falcinelli G, Falsini B, Taloni M, et al. Modified osteo-odonto keratoprosthesis for treatment of corneal blindness:long-term anatomical and functional outcomes in 181 cases. Arch Ophthalmol. 2005;123 (10):1319–29. Liu C, Paul B, Tandon R, et al. The osteo-odonto-keratoprosthesis (OOKP). Semin Ophthalmol 2005;20:113–28. doi:10.1080/ 08820530590931386.
849
Downloaded from http://bjo.bmj.com/ on March 19, 2015 - Published by group.bmj.com
Keratoprostheses: are we there yet? Venkata S Avadhanam and Christopher S C Liu Br J Ophthalmol 2014 98: 849 originally published online March 3, 2014
doi: 10.1136/bjophthalmol-2014-304994 Updated information and services can be found at: http://bjo.bmj.com/content/98/7/849
These include:
References Email alerting service
This article cites 3 articles, 0 of which you can access for free at: http://bjo.bmj.com/content/98/7/849#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
