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An Ocular Surface Prosthesis for Keratoglobus and Terrien’s Marginal Degeneration

Mahadevan, Rajeswari*; Fathima, Asra; Rajan, Rajni; Arumugam, Amudha Oli

Author Information
Optometry and Vision Science: April 2014 - Volume 91 - Issue 4 - p S34-S39
doi: 10.1097/OPX.0000000000000200


Keratoglobus (KG) is a rare, bilateral, non-inflammatory ectatic condition of the cornea, characterized by thinning and protrusion of the entire cornea. Because of bulging, the cornea appears to be larger than its usual diameter. Even minimal trauma can lead to acute corneal edema due to Descemet membrane rupture because the stroma is uniformly thin.1,2 Sridhar et al reported the prevalence of KG in association with pellucid marginal corneal degeneration (PMCD) as 12.9% among 116 eyes of 58 patients diagnosed with PMCD in a tertiary eye care center in south India.3 Terrien’s marginal degeneration (TMD) is another uncommon presentation, with thinning of peripheral cornea. It can be unilateral or bilateral, progressing from superonasal to other quadrants. Sporadic cases of TMD have been reported in literature, and the exact prevalence is unknown. In a study by Wang et al, 25 eyes with TMD were surgically treated between 2008 and 2010 at a Chinese eye hospital.4 In early stages, these conditions can be well managed with spectacles, and as the corneal astigmatism power increases and becomes more irregular, corneal gas-permeable (GP) lenses will be the best option. Several articles5–7 have discussed the fitting aspects of GP contact lenses in ectatic corneas such as KG, TMD, and PMCD. When the condition is severe, intralimbal or corneal rigid contact lenses may not be a suitable option because of poor fitting characteristics.

Prosthetic replacement of ocular surface ecosystem (PROSE) device [also known as Boston ocular surface prosthesis (BOSP)] is very useful for patients with no other contact lens options. It is primarily designed to improve visual acuity in irregular and ectatic corneas and also to provide healing and comfort in ocular surface disorders.8–10 The vault-control spline provides a precise and predictable mechanism for adjusting the corneal clearance independently of the base curve and internal optical zone. Adequate vault provides optimal corneal apical clearance of 100 to 200 μm.11 Minimal vault was considered to be less than 100 μm of corneal apical clearance and excessive or generous as greater than 200 μm of corneal apical clearance.12

This case report describes the challenges while fitting PROSE device, during the trial, and after dispensing, in a unique case of KG with TMD, to achieve a successful outcome. To the best of our knowledge, this is the first report in literature describing the prescription of scleral devices with sagittal depth of greater than 7.00 mm.

Central corneal thickness measurements were obtained using Cirrus high-definition optical coherence tomography (HD-OCT) (Model 4000, version 6; Carl Zeiss Meditec, Dublin, CA, USA). HD-OCT allows repeatable and standard clinical measurements of corneal thickness changes.13 Measurements were taken by a single examiner. Anterior segment five-line raster of length 6 mm with spacing was used for imaging. The raster lines were positioned at the center of cornea during all acquisitions. Measurements were obtained using “caliper” tool in micrometer scale.


A 22-year-old female patient diagnosed with bilateral KG 12 years back presented to the clinic for further management. She was using spectacles for the past 12 years. The best-corrected visual acuity (BCVA) with −3.00 to 5.00 × 140 and −1.50 to 7.00 × 055 was 20/20 and 20/50 in the right and left eyes, respectively. Binocularly, the patient did not complain of diplopia even with anisometropia. Slit-lamp biomicroscopy revealed peripheral thinning of cornea in both the eyes along with central stromal scarring (Fig. 1).

Left eye of the patient showing keratoglobus and peripheral cornea thinning.

Keratometry showed irregular mires with 46.25 D @ 064; 42.00 D @ 154 in the right eye and 50.75 D @ 140; 42.00 D @ 050 in the left eye. Corneal topography was performed for both eyes (Fig. 2) and the Sim K was 46.13 D @ 077; 41.81 D @ 167 in the right eye and 55.19 D @ 132; 40.27 D @ 042 in the left eye. Ultrasonic pachymetry was measured in different quadrants, but nasal and temporal quadrants were not measurable. The central corneal thicknesses were 421 µm and 430 µm, superior corneal thicknesses were 174 µm and 171 µm, and inferior corneal thicknesses were 439 µm and 404 µm in the right and left eye, respectively.

Topography map of the right and left eyes, respectively, showing keratoglobus in both eyes, left eye greater than right eye.

The patient was cautioned to avoid ocular trauma and eye rubbing and was recommended corneal GP lenses. Trial with corneal GP lenses was performed with the patient’s consent. Suitable steeper and larger diameter lenses were tried, and all the lenses popped out immediately after blink because of excessive edge lift and lid-induced lens ejection.

The patient returned to the clinic after 2 years with complaints of decreased vision in the right eye for a month following blunt trauma by her own hand 2 months earlier. On an emergency consultation elsewhere, she was prescribed topical medications for 3 weeks that relieved the pain. The patient had no complaints in the left eye and was not on any medication. The uniocular BCVA was 20/60 in each eye with −4.00–8.00 × 150 in the right eye and −6.50–8.25 × 050 in the left eye. Slit-lamp biomicroscopy revealed prominent corneal nerves in both the eyes and peripheral corneal thinning. In the right eye, corneal scarring and vascularization was noted inferiorly from 5 to 8 o’clock positions (Fig. 3). Central pachymetry was 417 and 419 μm in the right and left eyes, respectively. The patient was referred for a custom fit using PROSE trial.

Slit-lamp photo of the right eye showing scarring and vascularization in keratoglobus eye after trauma.

Visit 1

Trial 1 was performed after explaining the fitting goals of PROSE (Table 1). Based on our clinical experience, the commonly used sagittal depths in ectatic corneas range from 4.85 to 5.50 mm as a generous vault would be desired because of the progressive nature of such conditions. Devices with higher sagittal depths of 6.40 mm were selected to provide adequate corneal clearance to the bulged cornea. BCVA with PROSE was 20/20 and 20/30 in the right and left eyes, respectively. Both eyes showed minimal corneal clearance and the haptic did not show any impingement or edge lift with the devices. The corneal clearance was probably minimal because of protrusion of the cornea in KG. Insertion and removal of these devices was difficult. As devices with higher vault than 6.40 mm were not available in the trial set, new devices (trial 2) were ordered with higher vault and smaller diameter for easier handling (Table 1). They were designed with spherical and steeper haptics to understand the actual scleral toricity as the previous trials were with toric haptics.

Parameters of PROSE devices customized for keratoglobus and Terrien’s marginal degeneration

Visit 2

Trial 2 was performed for both eyes with the new devices. The patient complained of discomfort, redness, and pain upon insertion of the device in the right eye. Repeated attempts for trials in OD led to conjunctival congestion. Hence, trials in the right eye were deferred based on the opinion of the corneal clinician. The right eye was probably sensitive to lens wear because of corneal changes after trauma. Visual acuity in the left eye was 20/25 and fit assessment showed optimal fit after 1 hour of device wear with adequate corneal clearance which reduced to minimal after 3 hours of wear, but conjunctival impingement staining at 3 and 9 o’clock positions was noted on device removal after 1 and 3 hours of wear. A new device (trial 3) was customized for the left eye with CAD/CAM software.

Visit 3

Trial 3 was performed in the left eye with increased sagittal depth and flatter haptic to avoid conjunctival impingement (Table 1). Scleral toricity was provided at 12 and 6 o’clock conjunctival areas to avoid edge lift resulting from flatter haptic. BCVA with PROSE was 20/20. Fit assessment revealed minimal corneal clearance after 1 hour of device wear, and no conjunctival impingement staining was seen after removal. After 3 hours, the corneal clearance remained thin and uniform but without any area of corneal touch and the haptic fitted well with no edge lift or impingement. After 6 hours of device wear, the vault remained the same. Though 3 and 9 o’clock conjunctival impingement staining was observed with fluorescein dye after device removal, the patient was comfortable. Central corneal thickness of 417 μm before device insertion increased to 443 μm after 6 hours of device wear probably secondary to lens-induced corneal edema.

Visit 4

Device 4 with higher sagittal depth of 7.40 mm was tried (Table 1). The corneal clearance observed was not uniform and minimal because the device was minimally decentered inferiorly. The decentration could have been a result of the spherical haptic. The trial 3 device with sagittal depth of 7.00 mm was dispensed because it centered well and the patient was comfortable with the device. Proper education and training was given for insertion, removal, and cleaning of the device. She was asked to wear trial 3 device for less than 4 hours.

Visit 5

On 1-week follow-up, the patient was comfortable with the trial 3 device and had used it regularly 3 to 4 hours every day. Visual acuity with the PROSE device in the left eye was 20/20 and the fit was noted to be the same as before.

Visit 6

During the first month follow-up, the patient was wearing the trial 3 device for 3 to 4 hours daily. She complained of mild occasional conjunctival redness after removal of the device. The visual acuity and fit were similar to that of previous visits. The patient was asked to come for follow-up in 3 months and to report immediately any signs of recurrent redness.

Visit 7

On 3 months’ follow-up, she was comfortable wearing the trial 3 device, up to 4 hours a day, but complained of frequent conjunctival redness after removal of the device. The visual acuity and fit were similar to that of previous visits. The patient was counseled about the symptoms of corneal edema and advised to wear the device not more than 4 hours daily. A new device with higher vault and looser peripheral haptics was ordered (Table 1).

Visit 8

After 3 months, the patient returned to the clinic for trial with device 5. She had worn the old device for 3 hours before visiting the clinic. OCT images of the anterior segment were taken with the patient’s old device to measure corneal thickness which was 448 μm. Corneal clearance was almost absent centrally and around 44 to 64 μm in the periphery (Fig. 4).

HD-OCT images for the left eye showing the corneal clearance, which is almost nil centrally with patient’s device.

Device 5 was inserted after 1 hour of device 3 removal. Visual acuity with the new device was 20/30; the fit assessment after insertion displayed adequate corneal clearance, and the haptic did not show any impingement or edge lift. OCT measurements of the anterior segment after device insertion showed corneal thickness of 420 μm and corneal clearance of 164 μm (Fig. 5). After 6 hours of device wear, the fit assessment was similar as seen after insertion. OCT measurements of the anterior segment revealed corneal thickness of 436 μm and corneal clearance of 144 μm (Fig. 5). No conjunctival impingement staining was observed after device removal. As the vault was better with device 5, the patient was asked to wear device 5 with restricted wearing hours.

OCT images showing the corneal clearance with the trial device 5 in the left eye. Corneal clearance of 164 μm seen after lens insertion (upper image) reduced to 144 μm after 6 hours of device wear (lower image).

Visit 9

After 3 months, she had no complaints with the device and was comfortably wearing the device for 8 to 10 hours every day. She was asked to watch for symptoms and signs like haloes or redness. She was advised again to wear the device for restricted hours with intermittent breaks.

Visit 10

After 1 month, the patient visited the PROSE clinic complaining of mild haloes after 12 hours of wear. She reported that the haloes disappeared after a few minutes of device removal. The fit and vision were as recorded in visit 8 with device 5. The symptoms of corneal edema were reinstructed and the necessity to wear the device for restricted hours with intermittent breaks was reinforced.


Fitting PROSE in a KG eye is quite challenging. The fact that optimally fitting scleral devices serve as a protective measure against trauma in such thin and protruding corneas cannot be denied. Devices with increased sagittal height and higher vault are needed in such corneas.12 In this case report, it was observed that the higher vault in the trial 2 device showed adequate corneal clearance initially, but after 1 hour it was minimal. It was reported that the PROSE device with higher vault tends to sink inferiorly.12 Steeper base curves may increase the vault, but selecting steeper base curves requires additional minus power and also reduces the fluid reservoir over the peripheral corneal surface, causing peripheral corneal staining. Limbal stem-cell deficiency in compromised corneas fitted with scleral lenses may lead to corneal re-epithelialization by the conjunctiva.14 This may result in pain, poor vision, and nonreversible opacification of corneal tissue.14 In this case report, base curve was kept constant.

Initial selection of the haptic curvature and lens diameter plays an important role while fitting the PROSE device, as it can alter the corneal clearance. In this case report, good corneal clearance was maintained by gradually increasing the sagittal depth and flattening the haptic. Increased sagittal depth, which is required in ectatic corneas like KG, makes the PROSE device sink into the conjunctiva more, causing impingement. Hence, a flatter haptic can make the PROSE device fit better, by reducing the focal pressure on the conjunctiva and landing more evenly.

Rathi et al15 showed that the vault reduces by 0.26 mm after 4 hours of BOSP wear. Hence, PROSE fitting in corneas with steeper curvature such as KG has to be carefully monitored while dispensing the PROSE device because reduced vault over a period may cause trauma to the cornea leading to corneal scarring and even corneal transplantation.

Corneal edema with device wear has to be monitored appropriately. Smith et al16 showed that 2.4% to 3.5% of corneal swelling is induced during the day with rigid GP scleral lens after 3 hours of wear. Michaud et al based on theoretical calculations of oxygen transmissibility through lens and fluid reservoir systems recommended devices with highest Dk, with a maximum central thickness of 250 μm and with clearance not exceeding 200 μm to minimize hypoxia-induced corneal swelling,17 In this report, the increase in corneal edema could also be an adaptive symptom of an initial lens wearer.18

OCT is a useful tool for imaging the sclera and cornea. It aids in fitting and also to get a better understanding of the vault and scleral compression.12,15,19,20 In this case report, OCT served as a valuable adjunct in fitting custom-made scleral lenses (trial 5). It is also observed from the case report that changing only the vault parameter in the device is not sufficient in achieving an optimal fit; other parameters like haptic and total diameter also have to be modified.


In case of KG, the PROSE device with a higher vault has to be prescribed as the vault is expected to reduce with increased wear time. Adequate vault or corneal clearance can be achieved not only by modifying the vault but also by the selection of haptic. Corneal thickness measurements during the trial can give a better idea of the corneal edema expected after dispensing the PROSE device. OCT is an efficient tool for measuring the diurnal changes in the vault over a period in such challenging cases, as PROSE fitting could require modification with time. Regular follow-ups are mandatory to ensure optimal fitting.

PROSE fitting, though complicated by the combination of KG and TMD, can be successful with customized fitting using spline technology. Scleral devices also serve as a protective measure against trauma in such thin and protruding corneas in addition to providing improved vision.


We thank Boston Foundation for Sight (Needham, MA, USA) for their continuous support in designing and manufacturing PROSE scleral devices.

Received: September 4, 2013; accepted December 17, 2013.


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Boston ocular surface prosthesis (BOSP); prosthetic replacement of ocular surface ecosystem (PROSE); keratoglobus; corneal ectasia; scleral contact lenses; optical coherence tomography (OCT); Terrien’s marginal degeneration

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