SCLERAL CONTACT LENSES have entered into mainstream optometry, as many O.D.s are now prescribing them as a treatment for severe dry eye disease and as an alternative to spectacle wear for patients who have corneal irregularities, such as keratoconus. Lens fitting via slit lamp biomicroscopy makes the learning curve steep for the beginner and challenges the expert when it comes to difficult cases. Thanks to recent advances in technology, however, other devices can be incorporated into the fitting process that will improve one’s efficiency and success with scleral lenses.
Here, I discuss this technology.
1 ANTERIOR SEGMENT OCT
Determining measured central clearance values over the cornea by using a slit beam to estimate the fluorescein-stained tear reservoir as compared to the known thickness of a fit scleral lens is not always accurate, secondary to subjective interpretation. Anterior segment (AS)-OCT is a more reliable method.
Specifically, you take a horizontal AS-OCT image with the scleral lens in situ and then use the device’s software calipers to measure the clearance between the back surface of the scleral lens and the anterior surface of the cornea (see Figure 1).
A total of 50% of scleral lens settling takes place during the first 30 minutes of wear, reveals a study in March’s Contact Lens & Anterior Eye. Thus, you can accurately gauge the final central clearance of a fit scleral lens with an AS-OCT measurement 30 minutes after lens application.
AS-OCT can also be used to assess limbal lens clearance, which can be challenging via slit lamp because clearance values are often low and difficult to detect. (For low clearances, fluorescein is not observable under 20 μm.)
To accurately read the AS-OCT, make sure to locate the front and back surface of the lens and then the cornea. If the lens lands on the cornea without clearance, the lens’ back surface and the front corneal surface can be indistinguishable.
Device limitations: Haptic landing evaluation and measurement. Regarding the former, lens index changes can cause the scleral lens’ edge to look artificially depressed, making the fit appear compressed. Therefore, the haptic section should also be assessed with diffuse white light using the slit lamp. When it comes to the latter limitation, AS-OCT can only measure one meridian of the scleral lens at a time; measured distance of the lens beyond the corneal limbus is limited.
Recently, corneo-scleral topography that allows practitioners to measure the scleral surface and also fit customized scleral contact lenses has become commercially available. These instruments use either Fourier transform profilometry or a structured light approach to measure both the cornea and scleral surface.
Measurement of the anterior ocular surface, including the sclera, gives you data of an eye’s sagittal height and scleral shape, enabling you to improve scleral lens design. Without this measurement, fitting decisions are based upon observations and guestimates of diagnostic lenses. Because the amount of mismatch between the lens and the eye is not always obvious, interpretation can be challenging.
In fact, scleral shape studies using cornea-scleral topographers show that scleras are non-rotationally symmetrical and can have toricity orientated with-the-rule, against-the rule, or oblique (Figures 2, 3, 4) and have no apparent shape relationship with corneas.
So, how does the cornea-scleral topographer measure the scleral surface? First, fluorescein is required to recognize the clear tissue of the cornea and the bulbar conjunctiva. Depending on the device, measurement of the anterior ocular surface is either one single measurement or a series of three measurements with the patient looking in different gazes.
Data analysis of the measured sclera includes mean sagittal height at any measured cord diameter or measured sagittal height at any specified location. These measurements can be a key indication of how steep the fit lens will need to be to successfully vault the corneal surface.
Scleral shape is then analyzed to determine whether the haptic back surface must be spherical, toric or multi-meridian to achieve an ideal match between the scleral lens haptic and the scleral surface.
Customized scleral lens software designs a lens from the measured sclera. Specifically, you can input the amount of desired clearance of the scleral lens, and fitting software will then determine the scleral lens parameters necessary to achieve these clearance values. Also, automated algorithms can adjust the geometry of the lens to ensure an even vault across the cornea. This is especially helpful for matching a reverse geometry scleral design with an oblate corneal surface. Automated adjustments to peripheral curve widths can be implemented to obtain adequate limbal clearance. If the sclera has a relatively toric shape, then fitting algorithms design a customized toric back surface. For scleras that have significant asymmetry without a toric component, a multi-meridian back surface lens can be designed that matches the scleral shape. Further, scleral obstacles, such as pingueculas or conjunctival blebs, can be measured, enabling the customized design of scleral lens notches or edge lifts.
Device limitations: One must learn to operate the cornea-scleral topographer, but this is less difficult than lens fitting via slit lamp biomicroscopy. Also, the lids have to be retracted, and the eyes need to be still during topographic acquisition.
The AS-OCT and corneo-scleral topography can provide invaluable measurements that take the guess work out of diagnostic scleral lens fitting. (See “Case Examples,” p.24). OM
A 42-year-old female with keratoconus reported for a contact lens fitting. Patient history revealed she failed in soft and GP contact lenses.
After evaluation and discussion, I decided to fit her right eye with a scleral contact lens. Corneo-scleral topography was performed. Her 16 mm mean sagittal height was minimal at 4.09 mm; however, she did have significant against-the-rule scleral toricity. (Figure 5).
Fitting software was used to design a 16 mm scleral lens that centrally vaulted her central cornea by a pre-settled value of 300 μm. The haptic section of the lens was designed with a back-surface toricity that matched scleral toricity. A spherical-cylinder over-refraction of a scleral lens in situ showed residual astigmatism. The final lens power included front-surface toricity that was stabilized by the back-surface toric design. The patient was satisfied with the comfort and vision of the scleral lens.
A 73-year-old female patient with granular dystrophy reported for a contact lens evaluation post-penetrating keratoplasty of her right eye.
Although the corneal graft was clear, corneal topography showed severe irregularity. Corneo-scleral topography was performed, and the resulting 3D model showed a severely flat and oblate graft (Figure 6). The scleral elevation map revealed significant oblique scleral toricity.
The software algorithm was able to design a 16.5 mm scleral lens with 11.00D of reverse geometry to achieve a 300 μm pre-settled central clearance and an even vault over the patient’s oblate corneal surface. The dispensed lens also incorporated toric back-surface toricity to match the patient’s scleral surface. The dispensed lens had 305 μm of central corneal clearance, measured with AS-OCT, and even haptic alignment. The patient was happy with the vision and comfort, and only one remake was necessary for a power modification.