BostonSight PROSE (Prosthetic replacement of the ocular surface ecosystem)
Scleral contact lenses
Scleral contact lenses are lenses that rest on sclera and do not touch the cornea with a space present between scleral lens and cornea. This is termed as corneal clearance of scleral lenses and is due to vaulting of lens. The lens is filled with fluid prior to insertion in the eye. The fluid can be unpreserved saline or normal saline (0.9% sodium chloride). These lenses consist of three parts, scleral (haptic) portion that rests on sclera, vault, that is responsible for corneal and limbal clearance of the lens; and optical portion of the lens. The optical portion of scleral lenses is usually 0.2 mm larger than the horizontal visible iris diameter. However, while fitting most of the scleral lenses, more attention is given to the haptic portion, the corneal (and limbal) clearance, and the edge of the lens. All these factors affect the “on-eye” performance of scleral lenses. When fitted, the lens should not move on to the eye. The scleral lenses can be air-ventilated (fenestrated) or fluidventilated (non-fenestrated). This ventilation helps in providing oxygen to the ocular surface without compromising the physiology. With fenestrated lenses, there is a possibility for the air bubble coming into the visual axis and compromising visual acuity, but this is not so with the fluid-ventilated lenses.
Types of lenses
Scleral lenses are lenses with bearing only on the sclera with diameter of the lens being 15 mm and above.
- Miniscleral have diameter between 15 and 18 mm
- True scleral lenses have >18 mm diameter (more than 6 mm bearing on sclera).
Mini-scleral have less corneal clearance as compared to true sclerals. Various lenses available include mini-scleral design (MSD), Maxim, Jupiter, Boston mini-scleral, and Tru-scleral. The true scleral lenses are Jupiter, Pullum lenses, Tru-scleral, and PROSE (prosthetic replacement of the ocular surface ecosystem).
Prosthetic Replacement of the Ocular Surface Ecosystem (PROSE)
The Prosthetic Replacement of the Ocular Surface Ecosystem (PROSE) is a computer-assisted custom-designed lens that is made using a computerized lathe machine. The acronym PROSE is used in conjunction with the words device and treatment, where the PROSE device is the specific design that is customized to each individual eye to deliver the PROSE treatment effect. PROSE devices have been reported to be effective and safe in managing a variety of conditions including chronic graft-versus-host disease, Steven–Johnson Syndrome, debilitating ocular surface disease, and following laser in situ keratomileusis surgery.
PROSE devices are made of fluorosilicone/acrylate polymers, either oprifocon A or hexafocon B, having a Dk/t of 85 or 127x10−11 cm2 mL O2/s mL mmHg [ISO/Fatt] respectively. The diameter of the device can be made between 15 and 24 mm. In 1994 the US Food and Drug administration (FDA) approved PROSE (scleral devices) for the treatment of corneal disorders. The PROSE device consists of an optic zone, a transition zone and a scleral zone. The device is designed using spline functions, which create seamless junctions in the device between the optic zone and the scleral zone. The splines allow modification of the corneal vault (sagittal height) or distance between cornea and back surface of the optic zone independent of the base curve radius. This advanced control over device design allows for a complete customization for each patient’s eye and makes it possible to fit these lenses on any ocular surface without causing any damage, impingement, or compression on the ocular surface. Channels in the back surface of the scleral zone radiate tangentially from the center. During blinks these channels create a pumping mechanism to allow tear exchange underneath the device.
Preparation and fitting
A trial set and a slit lamp biomicroscope are used to evaluate the fit of a PROSE device on the eye. The device is carefully inserted after it has been filled with unpreserved physiological saline to provide a bubble-free reservoir that acts as a liquid bandage while masking any corneal surface irregularities. The optimal fit allows the device to rest on the sclera without causing compression of the conjunctival blood vessels while vaulting the entire cornea. The device is fit to barely clear the steepest point of the cornea and give a uniform thickness of the fluid reservoir. If further modification of the device is needed, it is done during the ordering process with the help of a computer aided design program. Excessive compression of the conjunctiva over the sclera by the scleral zone is avoided to prevent blocking of the scleral zone channels that allow fluid ventilation and, thus, increase the comfort of the device. Excessive compression causes a feeling of pressure and an impression into the conjunctiva circumlimbally, noticeable upon device removal. Scleral lenses mask the irregular astigmatism with the fluid reservoir. Most important aspect in fitting scleral lenses is aligning the haptic to the sclera. Some patients do have edge lifts indicating toric nature of sclera and this toricity can be added to scleral lens. This is possible with the availability of submicron lathe machines or computerized lathe machines. This reduces the amount of debris collection between the lens and the cornea. In addition, the comfort of the patients increases along with improved wear time. PROSE lenses are available in different Front surface eccentricity (FSE) values of 0.3, 0.6, and 0.8. A spherical lens has FSE of 0. The higher FSE values indicate rapid flattening of the lens from the center toward the periphery. PROSE with 0.6 and 0.8 FSE can improve vision in patients with advanced keratoconus.FSE may compensate for aberrations from the posterior surface, this may compensate for poor alignment of the optical axis of patient and of the lens, and this eccentricity may restore prolate surface thus improving vision.
Scleral lenses were earlier manufactured through the molding process (impression method). Impression technique is not used nowadays in modern scleral lens practice as it is cumbersome. In impression technique, a mold (cast) is made of the anterior ocular surface and then the cast is sent to the manufacturer for making the lens. With preformed (trial fitting) lenses, trial sets are available which helps practitioner fit the Scleral contact lenses. Nowadays, this is the most preferred method of scleral lens fitting. Various companies manufacture these lenses in various designs and give a fitting guide for the practitioners. With the advent of computerized lathe cut machines, it is possible to make lenses with submicron precision. PROSE uses a computer-assisted design and manufacture (CAD/ CAM) software for lens manufacturing and makes lenses with precision. Contact lens practitioners now can design or make changes in the design themselves on a computer after assessing the fit during trials. PROSE lenses are manufactured using Optoform lathe system for spherical lenses and Nanoform lathe machines for lenses with toricity at Boston Foundation for Sight. The computer design can be sent to the base unit for manufacturing the lenses with the help of software, allowing for a faster delivery to the end users.
Evaluation of corneal clearance
Until now, corneal clearance is assessed on slit lamp examination either by comparing it with the known thickness of the lens that is used as trial or by comparing it with the corneal thickness of the patient which at best gives only a rough estimation. With the availability of newer technology, such as anterior segment optical coherence tomography (ASOCT), it is possible to measure the amount of corneal clearance much more accurately. OCT showed that the mean value of height of vault after 1-hour challenge with scleral lenses was 0.80 mm and reduced to 0.54 mm after 4-hour challenge. Before finalizing the fitting, this reduction in corneal clearance needs to be taken into consideration. This is especially important in progressive disease such as keratoconus, lenses with higher vaults can be worn for longer days even if the cone progresses. If the vault is less, with the progression of cone, the lens may cause corneal touch resulting in poor vision and scarring. There is no correlation between change in vault or fluid and visual acuity in patients with dry eyes. By increasing the sagittal height and thereby vault, one can use these lenses over a prolonged period without compromising vision.
Scleral lenses in keratoconus are rarely associated with infectious keratitis or other adverse events. It’s asumed that patients with ocular surface disease may be more prone to these adverse events compared to keratoconus. This can be due to poor cleaning of the plunger used for the insertion and removal of these lenses, improper use of saline solution, or patient’s poor hygiene. Proper care regimen including mechanical cleaning is required to clean these lenses though one-step hydrogen peroxide is used as a disinfecting system. With improvement in technological advances with presbyopic lenses, today scleral lenses are also available for elderly keratoconus patients with presbyopic correction. Maxim scleral lens is a scleral lens for advanced keratoconus with near add design. However, literature on use of these lenses is sparse. Fitting of scleral lenses is possible after intracorneal ring segments implantation in patients having keratoconus.
There remain few hurdles in manufacturing these custom-designed lenses as this demands specialized equipment. The cost of the precision lathe machines is very high, which increases the cost of manufacturing these lenses significantly. High cost and the special training required in assessing the fitting and then modifying the design is quite challenging. Improvement in visual acuity with scleral lenses can bring down the rate of keratoplasty in patients of keratoconus and thereby significantly reduce the cost, effort, and other issues related to the maintenance of corneal grafts.
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