Impression based technology is advantageous as it allows close alignment of the scleral lens to the anatomy of the anterior ocular surface. The original technique for fitting scleral lenses was impression molding. Josef Dallos, an ophthalmologist in Budapest in 1930 used Negocoll (derived from seaweed) for ophthalmic impressions.1 In subsequent years, others followed with a variety of impression techniques.
Scleral lens technology has advanced tremendously in the last decade. However, there are still situations where fitting is difficult, even with state of the art scleral lenses. For these situations, scleral lenses can be custom made from an impression, or mold, of the eye.
If any type of high irregularity is present, impression based scleral lenses may be beneficial. Impressions are helpful for significant ocular surface asymmetry or localized irregularities where the peripheral curves or diameter of a scleral lens cannot be modified. Even with lens modifications, compression or edge lift may be present. Ocular surface irregularities include pingueculae, pterygium, blebs and glaucoma drainage devices. Even in cases of moderate elevation of the ocular surface, such as a bleb, impression based technology can vault and protect the asymmetrical area.
In cases of highly irregular corneas, there may be a large difference between the flattest and steepest areas of the cornea. Examples are proud plateau grafts or advanced nipple cones. The post-lens-tear reservoir may be vastly irregular with minimal clearance in one area and excessive clearance in another area. There may be difficulty inserting a scleral lens without bubbles, or unwanted bubbles may enter the lens. Additionally, lens decentration and/or reduced visual acuity may occur.
Impression based technology is used for protection in ocular surface disease, neurotrophic keratitis, graft versus host disease, Steven Johnsons Syndrome, ocular cicatricial pemphigoid, and limbal stem cell deficiency, amongst others. Athletes, including professional athletes, and those who work in windy or dusty conditions can also benefit from this technology.
Polyvinyl siloxane impression material was introduced in 1995.2 Two cartridges mixed in an application gun are combined in the mixing tip where the polymerization process begins. This is applied to the eye in an impression shell. Polyvinyl siloxane stays in the working state longer, causes less post-impression corneal staining and the molds do not degrade as quickly. Topical anesthetic is not needed when obtaining an impression.
EyePrintPROTM is an impression-based transparent prosthetic scleral device introduced in 2013.2 An impression of the eye captures the curvatures of the entire ocular surface with 1-2 microns of accuracy. With technological advances, 3D scanning of the mold creates a customized scleral lens design. EyePrintPROTM lens options include toric, decentered optics, multifocal, optical prism in any direction and higher-order aberration correction.
Technological advances have made the manufacturing process of scleral lens production more predictable and reproducible. Impression scleral lens technology is a valuable tool for scleral lens specialists.
EyePrintPROTM lens. Image courtesy of Christine Snidt.
1. Dallos J. Personal communication to Sabell 1977.
2. Barnett M, Johns LK. Contemporary Scleral Lenses: Theory and Application. Bentham Science 2017. Volume 4 ISBN: 978-1-68108-567-8. Ch 1: 30-37.
Dr. Melissa Barnett is a Principal Optometrist at the UC Davis Eye Center in Sacramento, specializing in anterior segment disease and specialty contact lenses. Dr. Barnett is the Past President of The Scleral Lens Education Society. She lectures and publishes extensively on topics including dry eye, anterior segment disease, contact lenses and creating a healthy balance between work and home life for women in optometry. Dr. Barnett along with Dr. Lynette Johns co-edited the book Contemporary Scleral Lenses: Theory and Application with the unique perspectives and contributions of international experts.