This manuscript shows each step of customizing a cryolite glass prosthetic eye including some major advantages of the use of cryolite glass for manufacturing an eye prosthesis compared to poly(methyl methacrylate). In addition, this manuscript gives ophthalmologists better insight into the ocularistic care that could improve interprofessional collaboration.
In Germany, Austria, and Switzerland, over 90% of ocularists still manufacture customized prostheses using cryolite glass from Thuringia. The present manuscript demonstrates this long-forgotten technique in detail. This manuscript shows some major advantages of manufacturing prosthetic eyes using cryolite glass in comparison to poly(methyl methacrylate) (PMMA). These advantages include a lighter weight of the prosthesis, higher levels of patient satisfaction, and only one appointment necessary for the customized manufacturing. Potential risk of breakage seems not to be a critical disadvantage for glass prosthetic eye wearers. However, in some patients, manufacturing a well-fitting prosthetic eye is not possible or reasonable due to anophthalmic socket complications such as post nucleation socket syndrome, scarred fornices, or an orbital implant exposure. This article gives ophthalmologists a better insight into ocularistic care in order to improve the essential interprofessional collaboration between ocularists and ophthalmologists.
The purpose of the present manuscript is to comprehensively demonstrate the technique of manufacturing a customized cryolite glass prosthetic that is long forgotten outside the German-speaking countries (Figure 1). This manuscript also focuses on major advantages of this technique. These include a very smooth surface of the prosthesis due to fire polishing, the light weight of the prosthesis due to the hollow design, high levels of patient satisfaction, and the need of only one appointment for manufacturing of the customized prosthesis1,2,3,4,5. This article also gives ophthalmologists better insights into ocularistic care in order to improve essential interprofessional collaboration1,2,3,4,5.
In 1832, the glassblower Ludwig Uri Müller from Thuringia, Germany, developed the cryolite glass prosthetic eye based on the class-leading models made in France4. Benefits of cryolite glass included a better look, better tolerability, easier processing, and longer durability than previous glass eyes4,6,7,8. Herman Snellen, a Dutch eye surgeon, used this cryolite glass to produce a lightweight hollow prosthetic eye in 18804,6,7,8. This lightweight prosthetic eye, the Snellen ‘reform eye’, increased the volume of prosthetic eyes, resulting in better fitting into larger eye sockets following the introduction of enucleation procedures made possible by the development of anesthesia and asepsis4,8. Twenty years later, cryolite glass had become the most commonly used material for prosthetic eyes. Germany developed into the manufacturing center of prosthetic eyes globally2,4,5,7,8. At the start of the second world war, German cryolite glass eyes became unavailable outside of the German-speaking area. Therefore, (poly)methyl methacrylate (PMMA) became a substitute material for prosthetic eyes4,7,8, and today PMMA is the most commonly used material for prosthetic eyes globally4,5,8. Notwithstanding, in German speaking countries, over 90% of ocularists still manufacture customized prostheses using the cryolite glass from Thuringia2,3,4,5,7,8,9,10,11,12,13. Each customized cryolite glass prosthetic eye is produced in two major steps: the first step is to produce a "half-done" cryolite glass eye that conforms to a white sphere with an iris and a pupil (Figure 2). The second and decisive step is to customize the "half-done" cryolite glass prosthetic eye for the respective patient. To that end, a "half-done" cryolite glass eye is selected from thousands (Figure 3) based on the best matching iris color to the patient's healthy fellow eye.
The following protocol presents customizing a selected "half-done" cryolite glass eye for a specific patient. This step lasts about 25–35 min.
All procedures performed in the following protocol involving human participants were in accordance with the ethical standards of the institutional research committee of the University of Cologne and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
1. Prosthetic eye customization
- Select one of the "half-done" cryolite glass eyes based on the best matching iris color to the healthy fellow eye of the patient (Figure 3).
- Examine the fitting of the current prosthetic eye. To do so, let the patient look straight ahead. Pay special attention to the retention of the prosthesis, the viewing direction, the eye lid contour (ptosis, entropion, and ectropion), as well as to the size and volume (exophthalmos and enophthalmos) of the current prosthesis.
- Remove the current prosthetic eye with help of a contact lens suction cup for hard contact lenses.
- Examine the anophthalmic eye socket without the prosthesis and pay attention to a potential inflammation of conjunctiva, the volume filling of the orbital implant, if the orbital implant is visible through the conjunctiva, and if the fornices and sulci are deep enough for a good fitting prosthesis. If there are any major concerns regarding one of these points, an examination by an ophthalmic surgeon should be performed before manufacturing a new prosthesis.
- Take the selected "half-done" cryolite glass eye with the ocularist forceps and in the other hand take a hollow skewer that will be used later as a mouthpiece for blowing the glass prosthesis. Heat both slowly to 600 °C with a Bunsen burner while continuously rotating it, and melt the skewer at the open end of the "half-done" cryolite glass eye. Open the forceps and lay it down.
- Heat the "half-done" cryolite glass eye continuously (Figure 4). Using the healthy fellow eye as a model for the color, shape, and quantity of the conjunctival vessels, draw the vessels on the white sclera with heated glass stems in different colors (mostly red, brown, or yellow) (Figure 5).
- Heat the whole "half-done" cryolite glass eye while continuously rotating it so that the drawn vessels merge with the white cryolite glass and to produce a very smooth surface.
- Modify the shape and the volume of the of the cryolite glass prosthetic eye by suction and blowing in the mouthpiece. Keep rotating the glass eye in the flame of the Bunsen burner from time to time. Use the old prosthesis as a template for this step, but if necessary, modify the shape and the volume of the new prosthesis based on the findings of the previous examinations.
- Heat a transparent glass stem and melt it at the pupil of the cryolite glass prosthetic eye while continuously rotating the glass eye (Figure 6).
- While continuously rotating the "half-done" glass prosthetic eye, melt the glass at the rear of the prosthesis (Figure 6 and Figure 7) and reduce the volume of the rear by suction with help of the mouthpiece so that the back side shape is nearly equal to the sample prosthesis or the desired shape.
- Melt the glass stem at the front side away and heat the front side of the prosthesis again to produce a very smooth surface (Figure 8).
- Take the front side of the prosthesis with the forceps again, form the final shape of the back side with help of the skewer (Figure 9), and then melt the skewer away (Figure 10).
- Heat the whole prosthesis for fire polishing again, especially at the back side and rotate the prosthesis until the surface is very smooth all over.
- Put the prosthesis in a preheated metal container and let it slowly cool down (Figure 11).
- Insert the prosthesis and check the fitting as described in step 1.2 (Figure 12).
- If necessary, modify the shape of the prosthesis again (repeat steps 1.8–1.15).
Optimal results include a new prosthetic cryolite glass eye that fits very well, is comfortable, has a good motility, and the appearance with the prosthetic eye, including the eye lid contour, is nearly symmetrical to the healthy fellow eye (Figure 12).
Suboptimal results can result if the new prosthetic cryolite glass eye fits and is comfortable, but there are concerns regarding the cosmetic results. If a prosthesis does not fit perfectly, the appearance, including the eye lid contour, might not be symmetrical to the healthy fellow eye. In this case, the prosthesis can be redesigned eventually. Another reason for a suboptimal result despite a well-fitting prosthesis is post nucleation socket syndrome (PESS). PESS can result in an asymmetrical appearance of the prosthetic eye compared to the healthy fellow eye. In addition, due to a reduced motility of the orbital implant the motility of the prosthesis itself might not be optimal. However, fitting a new ocular prosthesis in these cases is possible and reasonable.
Complicated results include the manufactured prosthesis not fitting, or if wearing it is painful. In this case, the prosthesis has to be redesigned or completely renewed. In addition, in some patients manufacturing a good fitting cryolite glass prosthetic eye is not possible or reasonable due to complications of the anophthalmic socket. Complications such as a proceeded post nucleation socket syndrome, scarred fornices, or an orbital implant exposure prevent good ocularistic care. These patients have to be examined comprehensively by an ophthalmologist, and surgical socket reconstruction has to be performed by an ophthalmic plastic surgeon.
Figure 1. Three different cryolite glass prosthetic eyes in different colors and shapes. Please click here to view a larger version of this figure.
Figure 2. A "half-done" cryolite glass eye, already merged with the hollow skewer, that is used as a mouthpiece. In the background the Bunsen burner is visible. Please click here to view a larger version of this figure.
Figure 3. Some "half-done" cryolite glass eyes in different colors. Please click here to view a larger version of this figure.
Figure 4. The "half-done" cryolite glass eye after consistent heating. Please click here to view a larger version of this figure.
Figure 5. Various preproduced glass stems in different colors. Please click here to view a larger version of this figure.
Figure 6. Melting and shaping the back side of the prosthesis. Please click here to view a larger version of this figure.
Figure 7. A second image showing melting and shaping of the back side of the prosthesis. For good stabilization, a transparent glass stem was merged with the front side of the pupil before. Please click here to view a larger version of this figure.
Figure 8. Melting away the glass stem at the front side. Please click here to view a larger version of this figure.
Figure 9. The ocularist takes the front side of the prosthesis with the forceps and forms the final shape of the back side with help of the skewer. Please click here to view a larger version of this figure.
Figure 10. Melting away the skewer at the back side. Please click here to view a larger version of this figure.
Figure 11. After completion of prosthesis, the glass eye is placed with help of the ocularists forceps in a preheated metal container in order to cool down slowly. Please click here to view a larger version of this figure.
Figure 12. Optimal result. The new prosthetic cryolite glass eye fits very well, is comfortable, has good motility, and the appearance with the prosthetic eye, including the eye lid contour, is nearly symmetrical to the healthy fellow eye. Please click here to view a larger version of this figure.
Figure 13. Front surface of the old prosthesis (template prosthesis) and the new prosthesis of the same patient. The shape of the prosthesis was copied with a very high precision by the ocularist. However, the shape of the new prosthesis was slightly modified by the ocularist due to changes of the anophthalmic socket over time for an optimal fit. Please click here to view a larger version of this figure.
Figure 14. Back surface of the old prosthesis (template prosthesis) and the new prosthesis of the same patient. The shape of the prosthesis was copied with a very high precision by the ocularist. However, the shape of the new prosthesis was slightly modified by the ocularist due to changes of the anophthalmic socket over time for an optimal fit. Please click here to view a larger version of this figure.
Following enucleation with an orbital implant, a conformer has to be inserted for two weeks (Figure 1) in order to prevent scarring of the conjunctival fornices and subsequent inserting of a prosthesis2,3,4,7,12,13. Because an early ocular prosthesis insertion improves quality of life after enucleation and ensures better rehabilitation, cryolite glass prosthetic eye wearers get their first eye prosthesis two weeks after their operation2,4,14. This cryolite glass eye prosthesis is handmade but not fully customized for the fitting due to major changes of the anophthalmic socket in the first weeks and months after operation2,4. Promptly, six weeks after enucleation, patients get a second glass prosthesis2,4. As of this date all cryolite glass prostheses are fully customized2,4.
The process of customizing the "half-done" cryolite glass eye includes some critical steps2,7. The first critical steps within the protocol include examination of the fitting of the current prosthesis (step 1.2) and examination of the anophthalmic socket without the prosthesis (step 1.4)11. The ocularist has to check the current fitting of the prosthesis as well as the anophthalmic socket in detail, because on the basis of this examination the shape of the new customized prosthesis will be possibly changed compared to the old prosthesis2,7 (Figure 13 and Figure 14). In addition, the ocularist has to check whether there are other points, such as extrusions of orbital implants requiring surgical interventions prior to adequate ocularistic care2,7. Another critical step is the slow heating and continuous rotation of the half-done cryolite glass eye. If the glass eye is not rotated continuously and evenly, it gets warped when heated. Heating or cooling the glass eye too quickly will result in breakage of the glass eye2,5,7. Furthermore, the temperature during shaping of the cryolite glass eye has to be in the correct range (nearly 600 °C) for an optimal result5. All of these steps are a matter of practice and experience resulting from over 6 years of training for ocularists.
The technique described in the protocol is used for manufacturing a hollow, double walled cryolite glass eye, a so-called 'reform eye'2,7. These reform eyes are used especially for patients after enucleation or evisceration, when they need more volume replenishment through the prosthesis2,7. When volume replenishment is not necessary, for example in phthisic eyes or in patients with microphthalmos, the technique can be modified and the cryolite glass prosthetic eye can be made very thin and single-walled. Therefore, the back wall is melted off during the customizing process2,7.
In case the customized prosthesis glass eye does not fit initially, it can be heated once more, and the shape can then be modified by the ocularist. However, reheating of the glass prosthesis eye is only possible in the first days after production2,5,7. After a wearing time of several days, the first hydrolytic material changes, and material stress occurs, and a reheating results mostly in a breakage of the prosthesis2,5,7. Of course, in some rare cases, the prosthetic eye has to be reproduced from the beginning due to a fitting error that cannot be corrected anymore (for example, when the prosthesis is too small, or the material is too thin).
Essentially, using the technique described here allows the production of cryolite glass prosthetic eyes of nearly all shapes and sizes2,7. The limitations of our technique depend on the anophthalmic socket2,3,11. In patients with a pronounced postenucleation socket syndrome (PESS) or scarring of the fornices, it is not possible or useful to customize a prosthetic eye2,7. These patients benefit more from an initial surgical reconstruction of the anophthalmic socket with following ocularistic care2,7.
After six months, patients receive a new customized glass prosthesis based on the changes of the eye socket over time2,4. Afterwards, according to the current recommendations, most adult patients in Germany get a new customized prosthesis at least once a year, primary due to hydrolytic surface changes resulting in an inflammation of the anophthalmic socket2,4,11. It is not possible to polish cryolite glass prostheses2,4,5. Of course the fitting and the design of the yearly, newly made prosthesis is adjusted and improved from time to time by the ocularist because of the patients' experiences and wishes2,4.
In contrast, PMMA prosthetic eye wearers only get one prosthesis about 6 weeks following eye loss and wearing a conformer2,4. This prosthesis is usually worn for the next 5 or 6 years2,4. PMMA prosthetic eyes have to be repolished every year due to surface changes but there is in some cases no improvement in the fitting or the design over the years2,4,5. These different approaches in the manufacturing of the prosthesis immediately following eye loss seem to result in higher levels of patient satisfaction in the cryolite glass prosthetic eye wearers compared to PMMA prosthetic eye wearers2,4,15. However, the exact reasons for these divergences between both groups remain unclear2,4,15,16.
One very important point for prosthetic eye wearers is mucoid discharge2,4,11,17,18,19,20,21,22. The reason for this discharge is a conjunctival inflammation, among other things caused by the irritation of the anophthalmic socket by the ocular prosthesis2,4,11,17,18,19,20,21,22. Clinical studies show that the fire-polished surface of cryolite glass prosthetic eyes is smoother than those of PMMA prosthetic eyes2,4,11,23. This could be a potential advantage of the cryolite glass compared to PMMA, but some studies show that there was no difference in mucoid discharge between cryolite glass and PMMA prosthetic eye wearers2,4,11. Therefore, further studies are needed to investigate this in detail2,4,11.
Another aspect is the durability of both materials (PMMA vs. cryolite glass) 2,4,5. At first glance, the potential breakage of cryolite glass prosthetic eyes seems to be a problem for anophthalmic patients2,4,5. However, the mean rate of breakage is very low and amounts to only one prosthesis per 26.63 years of wear2,4,5. Damage (94%) occurs during removal or cleaning of the glass eye2,4,5. Therefore, removing and cleaning of the prosthetic eye should be done over a filled sink2,4,5. In case of damage or loss, almost all cryolite glass prosthetic eye wearers have at least one suitable replacement prosthesis2,4,5. To avoid the very rare case of damage to the prosthetic eye inside the socket and of course also to protect their healthy fellow eye, patients should wear protective glasses all the time2,4,5. In summary, a potential higher breakage rate of cryolite glass seems to be of subordinate importance and not to be a disadvantage in everyday life for the patients2,4,5.
Another advantage of the use of cryolite glass is the weight of the prosthesis2,4,5,7,24. While the Snellen 'reform eye' made of cryolite glass is hollow and therefore lightweight, most of the PMMA prosthetic eyes are solid and heavier than cryolite glass prostheses with the same volume2,4,5,24. According to the experience of the authors, these heavier prostheses might cause sagging of the lower eye lid in the course of the postenucleation socket syndrome. Of course, there are some innovative approaches to manufacturing hollow PMMA prostheses. However, these techniques are still not used in standard care2,4,5,24.
In addition, only one appointment is needed to manufacture cryolite glass prosthetic eyes2,4,5. After one hour, patients can leave the ocularist with their new prosthesis, while PMMA prosthetic eye wearers need 3–4 appointments at their ocularists and get their new PMMA prosthesis a few weeks after the first appointment2,4,5,24.
So far the recommended care regime for glass prosthetic eyes includes removal and cleaning on a daily basis while for PMMA prosthetic eyes removal and cleaning between 1–6 months is recommended11,17,18. However, the results of the newest studies suggest that cryolite glass prosthetic eyes should not be removed daily for cleaning and that further research is needed to develop a cleaning protocol11. This protocol would recommend a minimum period of wear between cleaning sessions and should consider individual conditions, such as climatic, psychological, and environmental factors11.
In recent decades, some improvements of this technique have focused on the working conditions of the patients, such as air conditioner use, ergonomic workstation designs and, in a few cases, newer devices such as novel Bunsen burners2,4,5,7. However, in the last few years some potential further developments have been described in the literature25,26. New surface coatings with antibacterial or with more hydrophilic properties are one of the main subjects of current research regarding prosthetic eyes25,26. However, these new coatings will probably be used especially for PMMA prosthetic eyes because the coating procedures mostly require very high temperatures or high pressure, very likely resulting in breakage of the glass prostheses. Another innovative approach to improve the prosthetic eyes is to fabricate a hollow ocular prosthesis with a functional lubricant reservoir to increase the artificial eye comfort27. This technique is not established, and further studies are needed to define the clinical outcome. A big issue of this suggested improvement could be the growth of bacteria within the hollow prosthesis. Thus, further studies are a high priority.
In summary, this manuscript shows each step of customizing a cryolite glass prosthetic eye, including some major advantages of the use of cryolite glass for manufacturing an eye prosthesis compared to PMMA, and gives ophthalmologists a better insight into ocularistic care. These insights help to improve the essential interprofessional collaboration between ocularists and ophthalmologists. In addition, some divergences between the PMMA and cryolite glass prosthetic eyes have not yet been identified in detail, especially the impact of the material itself, remain unanswered, and should be addressed in further studies.
Alexander C. Rokohl, Joel M. Mor, Niklas Loreck, Konrad R. Koch, and Ludwig M. Heindl have no financial or proprietary interest in any material or method mentioned in the article. The participant in this study was recruited from the Trester-Institute for Ocular Prosthetics and Artificial Eyes in Cologne that is owned and operated by Marc Trester.
No funding was received for this manuscript.
|Bunsen burner with gas and air flow over a fire-resistant worktop made from anodised stainless steel|
|Preheated metal container to 500 degree celsius|
|Pre-produced "half-done" cryolite glass eye|
|Transparent glass stem|
|Various preproduced glass stems in different colors|
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