DIGITEYES 3: Printing "Patient-Specific" (but not quite custom) Eyes
Last time we looked at a study where the authors digitized the first step of the prosthetic eye making process: a 2016 study where they replaced the physical impression with a touchless scan. As I noted, this seems to me an advance in principle, since the impression is currently the most uncomfortable part of the process for the patient. While they then went on to 3d print the prototype, the final prosthetic eye was handmade using traditional methods.
Today we’re going to look at a series of papers that approach the problem from the opposite end: they keep the analog impression, but they digitize (parts of) the manufacturing of the final prosthesis. These changes primarily affect what goes on behind the scenes and do not directly improve the patient experience. Nonetheless, there is reason for patients to take interest in these developments too: they are starting to chip away at the challenges that we must meet in order to transform the analog process into a digital one, ultimately giving us access to new tools that can create new possibilities for what we can achieve in making your ocular prosthesis.
I am treating these papers together because they seems to me to be steps that build on each other. The first study in this series, published in 2017, takes a CT scan of a wax model of the socket, and converts the data into a three-dimensional format which was then printed. Though the final painting and clear capping was done by hand, the patients went on to wear the 3d printed prosthesis. (Compare this to the previous study, where the 3d printed component was simply used as a prototype for the final prosthesis.)
What is interesting is that the software they used to convert the scan into a 3d model also allowed them to print the prosthesis hollow. This is an advantage to some patients who are wearing large heavy prostheses that put too much weight on the lower lids, resulting a droopy look. Making a prosthesis hollow by conventional methods is so time consuming that it is almost never done, whereas it seems like it is pretty straightforward to do this with the digital software.
The next papers (2019 and 2021) appear to build on its foundation: They begin in a similar fashion by taking an impression by conventional means, and then scanning it. However, here they take a further, bold step: rather than having the ocularist paint the iris and veins on the final eye they printed a digital image of an iris and veins onto the final eye. In the first study, they took a photograph of an iris and sclera and printed a modified digital image of it onto the final prosthesis. Note that the photo had to be “modified”: you cannot simply take a picture of an eye and print it, since the camera captures all kinds of interference from the ambient lighting that they need to get rid of digitally in order to have a viable image. This can be a complex and finicky process, and in the second study the authors adopt a more efficient (but less patient specific) “diagnostic tool” for choosing a stock image from a digital library that roughly matches their natural colouring.
In addition to the work they did on digitizing the painting process they also performed cytotoxicity tests for the printer resins to evaluate the safety of the new materials used. In the 2019 study this was done through cell assays, as they did not yet have anyone wearing their printed prostheses. But 2021, in addition to further in vitro testing of cytotoxicity and strength, they also performed a small clinical case study of 3 patients wearing these prosthesis over a month and monitored conjunctival inflammation. No instances of averse reaction were observed.
What is the value in these developments?
In this series I have outlined some benchmarks for measuring the value of digital tools in this application.
1) improves result (better prosthesis)
2) more efficient (faster/easier)
3) increases access (more people can get it)
Let's look at how these developments measure up.
The authors claim that the patients wearing these printed eyes were happy with the results. However, objectively it is hard to say definitively that these printed eyes are better than handmade ones. The only concrete improvement I can see in the result is the ability to print the prosthesis hollow.
The authors of the 2017 study also claim to have increased efficiency. They claim to have sped up the fabrication process, stating that the “computer aided” eye took 2.5 hours to fabricate, whereas a hand-made eye takes 10 hours. However I cannot for the life of me make sense of this, since they still are still using conventional methods to take the impression, fit the wax model, and paint the iris and sclera. There is no way that all of that, plus 3D scanning, modelling, and printing, can be done in 2.5 hours. Perhaps the authors mean that the printing itself took 2.5 hours, but to compare that to “10 hours” as the length of time it takes to process the plastic in the conventional method is not accurate, as this step only takes 45 minutes.
Instead of claiming to have reduced the time it takes to make an eye the authors of the 2019 and 2021 studies claim to have increased the efficiency in terms of the skilled labour time specifically. Their method requires less time from the trained ocularist because they have replaced the hand-painting with an automated transfer of a digital image. The authors celebrate the fact that these methods do not require the skills and labour of a traditional ocularist to achieve a "patient specific" result.
In certain contexts, this is in fact a valuable advancement, as it can help achieve our third benchmark of "increasing access" to ocular prostheses. In some markets the demand for ocular prosthetics far outstrips the supply of skilled practitioners available to make them, which is the case in India and Korea where these studies were performed. In those markets most patients are currently fitted with stock eyes that are mass produced and only approximate the shape and colour of their eye, rather than having an eye made to their exact individual specifications. In such circumstances these 3d “patient specific” eyes would be an improvement from what is currently available to them.
However, here in Canada we have enough ocularists to keep up with the need for ocular prosthetics. What limits patient access to these services is that some patients have to travel great distances to see an ocularist, since there isn’t sufficient demand to warrant having an ocularist in every town. But once they get to us, we have enough skilled practitioners in each province to do the work for our patients in a timely fashion. Analog methods remain preferable in the Canadian context as they allow us more latitude to customize the shape and colour to the exact specifications of our patients. Given the choice, I expect most patients would take an eye custom-made by the hand of a skilled ocularist over a digital "patient-specific" eye that merely scans the impression and chooses the colouring from a library of stock images.
Even though this iteration of digital eyes may be better suited to a different market, I am excited by the achievements of these studies. First, they have begun testing the biological and physiochemical safety of some printer resins for use in the socket, and the results look promising. Secondly, they have taken a stab at the challenges of printing the iris and scleral colouring. Ultimately they found it impractical to convert a photograph of an individual’s iris and sclera onto the finished piece, and have reverted to a “colour diagnostic tool” that helps choose from a library of prepared images. As we move forward in this series we will be looking more at what goes into painting an eye by hand, and why digitizing this process is not quite as simple as snapping a photo.
 Ruiters S, Sun Y, de Jong S et al (2016): Computer-aided design and three-dimensional printing in the manufacturing of an ocular prosthesis. Br J Ophthalmol 100:879-881. Note that in 2021 the same authors published a follow up study that expands the scope of the digital modeling to include mirroring of the front curve of the cornea (Ruiters et al. (2021): Three-dimensional design of a geometric model for an ocular prosthesis in ex vivo anophthalmic socket models. Acta Ophthalmol 99:221-226).  Alam M, Sugavaneswaran M, et al (2017): An innovative method of ocular prosthesis fabrication by bio-CAD and rapid 3-D printing technology: A pilot study. Orbit 36-4: 223-227.  2017:223  Ko J, Kim S, et al (2019): Semi-automated fabrication of customized ocular prosthesis with three-dimensional printing and sublimation transfer printing technology. Sci Rep 9:2968.  Ko J, Kim S, et al (2021): A Pilot Study of Ocular Prosthesis Fabrication by Three-dimensional Printing and Sublimation Technique. Korean J Ophthalmol 35:37-43.  They use a light scanner (Cara Scan 3.2) rather than the CT scanner used in the 2017 paper.