Review of Cornea & Contact lenses

Critical Measurements to Improve Scleral Lens Fitting

Gathering as much data as possible can help streamline the process and provide better outcomes.

By Jason Jedlicka, OD

Release Date: September 2015
Expiration Date: September 1, 2018

Goal Statement:

This course reviews methods for capturing and interpreting essential measurements when fitting scleral lenses.

Faculty/Editorial Board:

Jason Jedlicka, OD, and Greg DeNaeyer, OD

Credit Statement:

This course is COPE approved for 1 hour of CE credit. COPE ID 46692-CL. Please check your state licensing board to see if this approval counts toward your CE requirement for relicensure.

Joint Sponsorship Statement:

This continuing education course is joint-sponsored by the Pennsylvania College of Optometry.

Disclosure Statement:

Dr. Jedlicka has no finanical interest in any products mentioned. Dr. DeNaeyer is a shareholer of Precision Ocular Metrology and received royalty payments for the Europa lens.


Fig. 1. Corneal topography display demonstrating a corneal diameter of 12.19mm, confirmed by manual measurement of 12.2mm.

Scleral lenses have always been a mainstay of a specialty contact lens practice, offering patients with irregular corneas and severe ocular surface disease alike the chance to benefit from lens wear. But the category has been enjoying a renaissance in recent years, as the evolution of scleral lens designs has given us more fitting options—namely, standard and reverse geometry designs, toric curves in the landing and transition zones, and toric and multifocal optics—that in turn allow us to offer these lenses beyond the core group of traditional scleral patients.

While this increase in scleral lens parameter options gives practitioners the ability to fit a wider range of eyes, it may make perfecting the fit itself more challenging. Technology, on the other hand, is providing information that can help us fit these lenses quicker and more accurately. 

This article reviews some of the newer approaches to the evaluation and fit.

The Limits of Diagnostic Sets

Traditionally, scleral lenses are fit using one or several diagnostic sets. Practitioners use accompanying fitting guides based on keratometry readings, ocular surface health and patient history to select an initial diagnostic lens; alternatively, they can simply choose a lens from the middle of the set to start with and adjust their selection accordingly based on the amount of lens depth needed. Some scleral lens fitters may opt to look at the profile of the eye they're fitting and use their experience to tell them which lens is most appropriate.

Fig. 2. HVID ruler for measuring horizontal corneal diameter.

These techniques, however, while sometimes accurate, are difficult to teach and unreliable overall. Keratometric readings provide little information about the ocular surface, even when combined with ocular history such as a diagnosis of keratoconus or surgical procedures. If blind-selecting a lens, practitioners face the issue of identifying the necessary depth adjustments—assuming the fitting set is appropriate for the eye shape to begin with. Thus, a more appropriate measurement system is needed.

Selecting a Lens

While scleral lenses, unlike corneal lenses, vault the cornea to rest on the sclera, it is still vital for the practitioner to understand the contours of the patient's cornea to ensure adequate but not excessive vault, which, after allowing for lens settling, should be approximately 150μm to 250μm centrally and then and taper back to eventually land on the sclera just past the limbus. Inadequate vault can result in cornea/lens touch and associated problems, as well as difficulty with lens removal due to capillary attraction; excessive vault can inhibit oxygen flow, patient comfort and ease of application. Aligning a scleral lens properly also lessens the need for high prescriptions that may reduce visual acuity, which can occur especially when fitting steep corneal lenses in keratoconus, for example. Corneal topography can be used to ascertain corneal diameter or horizontal visible iris diameter (HVID), corneal apex location and the sagittal height of the cornea at a 10mm chord. 

Anterior segment depth measurements obtained using optical coherence tomography (OCT) or Scheimpflug imaging is another way to improve the fit of a scleral lens. The initial fitting process can be streamlined by use of OCT or Scheimflug imaging, as these instruments obtain objective measurements of the depth of the cornea and sclera out to nearly 15mm, thereby providing a known starting point for diagnostic fitting.  In addition, these images allow the fitter to see the contour of the cornea and sclera, to help determine if a particular fitting set may be more ideal than others. OCT can also be used to evaluate the success of a fit at follow up by providing precise measurements of the tear reservoir and edge contour to the sclera after a period of wear, without having to remove or manipulate the lens on eye. 

Fig. 3. Corneal topography demonstrating the apex of the cornea located approximately 4.5mm inferior to the corneal center. This shape may be better fit with a scleral lens with a reverse geometry design.

Other scleral imaging instruments provide information regarding scleral shape and toricity to determine whether a lens should be ordered with toricity in the landing zone. Below are some methods for obtaining these measurements. 

HVID. Because the scleral lens needs to vault the entire cornea and limbus, it must be large enough in diameter to land outside the corneal-limbal zone. Corneal diameter measurements can be obtained using a corneal topographer—simply capture the topographical map and use the corneal diameter measure on the display (Figure 1). Some topographers may also offer the ability to measure HVID with a point and click line display. A handheld ruler (Figure 2), slit lamp reticule or slit lamp camera with measuring capabilities, as well as anterior segment OCT, can also be used to measure HVID. 

Corneal Shape. Evaluating corneal shape is a good next step in the scleral lens fitting process. Knowing where the corneal apex is will allow you to choose a lens with the most appropriate shape to match the cornea. If the corneal apex is within the central 4mm of the cornea, a standard geometry lens should work well; if the apex of the cornea is located outside the central 4mm, however, or if there are significant elevations (e.g., Salzmann's nodular degeneration) near the peripheral cornea, a reverse geometry lens design may be more successful (Figure 3)

Sagittal Height. Another useful measurement to aid in the scleral lens fitting process is corneal sagittal height. Found on many corneal topographers, it is the measurement between the geometric center of the cornea and the intersection of a specified chord length—in this case, 10mm (Figure 4). Average sagittal height from 10mm to 15mm for all eye types is approximately 2,000μm.1 Thus, by using the 10mm chord depth and adding 2,000μm for the sag of the 10mm to 15mm chord plus the desired vault, you can come very close to the proper sag of the lens needed. 

Fig. 4. Three-dimensional display of corneal topography shows corneal sagittal height measurement at 10.0mm of 1,906 microns.
Fig. 5. OCT image demonstrating a sagittal height of 3,760 microns at a 15mm chord.
Fig. 6. Pentacam image demonstrating a sagittal height of 4,230 microns at a chord of 14.64mm.

Take the following example: a 10mm chord demonstrates a sagittal height of 1,906μm. Based on a desired initial vault of 350μm centrally, a diagnostic lens in a 15mm diameter should be 4,256µm (i.e., 1906µm + 2,000µm + 350µm for vault = 4,256µm starting point for a 15mm lens). If the lens to be fit is larger than 15mm, the sagittal height will need to be increased incrementally, as the larger area of eye surface to be covered will mean greater overall depth. In my experience, having reviewed several scleral fits in retrospect, I find that an adjustment of approximately 300µm per millimeter of lens diameter is fairly accurate (i.e., 4,256µm + 600µm, for a 2mm diameter increase = 4,856µm sagittal height for a 17mm diameter lens).

Obtaining a cross-sectional image of the anterior segment using diagnostic imaging technology is another useful way to obtain a starting point for an initial diagnostic lens. The software included in these instruments can provide measurements for a variety of possible heights and widths (Figures 5 & 6), simplifying the initial lens selection process. 

Scleral Contour Measurements. Some instruments are capable of evaluating the shape of the sclera, which may help practitioners achieve a proper lens fit. Figure 7 shows an image of a highly toric sclera obtained from the Eye Surface Profile (Eaglet Eye), one of two such instruments (the other being the sMap3D by Precision Ocular Metrology; see "Virtually Fitting Custom Scleral Lenses"). However, while these instruments provide more information about the ocular surface than corneal topography, at this time they are so new that the data they provide cannot yet be applied universally to all scleral lenses with simple formulas or rules.

Virtually Fitting Custom Scleral Lenses

By Gregory W. DeNaeyer, OD

Fig. 1. Data from three eye positions is stitched together to form a 3D model.

The sMap3D topographer (Precision Ocular Metrology) uses a structured light approach for three-dimensional mapping to obtain measurements of the cornea and sclera with a 22mm maximum field of view. The sMap3D takes multiple triangulated measurements using a single DLP projector and two cameras positioned laterally on each side.

Fluorescein is added to the patient's eye, which is necessary for imaging the corneal and bulbar conjunctival surface. The patient is then instructed to gaze at a fixated light straight ahead while the eyelids are opened as widely as possible with assistance from the practitioner or a staff member. The practitioner focuses the eye and captures the image. Two additional measurements with the patient fixating up and down are taken in succession.

Fig. 2. Scleral elevation map and toricity.
The sMapPro software is able to stitch together the images taken in straight, up, and down positions to produce a three-dimensional model of the patient's eye (Figure 1). Stitching is a necessary step to obtain maximum area of the sclera that is occluded by the lids despite the eyelids being held open. A stitched model is required for measurement of the vertical meridians to determine accurate toricity measurements and over all sagittal depth value, which are used for custom fitting.

The sMapPro software gives sagittal depth data at any specified chord. Corneo-scleral topography and elevation maps can be evaluated. Scleral toricity can also be calculated from any specified radius from center (Figure 2). The virtual fit screen allows the practitioner to send the data directly to Visionary Optics for analysis and design of a custom Europa Scleral lens. A diagnostic lens does need to be applied for over-refraction to determine final lens power.

Fig. 3. Virtual fit scleral lens on a keratoconus patient.
Alternatively, the practitioner can custom fit the lens using the software's virtual fitting plots. sMapPro software allows for complete specification of any lens parameter to virtually adjust for corneal and limbal clearance, as well as custom back surface toricity. The sMapPro recommends a starting base curve based upon any desired initial amount of central corneal clearance. Peripheral curve toricity is calculated based upon the patient's maximum scleral toricity. The fitting software adjusts peripheral curve widths to ensure limbal clearance. Figure 3 shows a virtual fit of a Europa scleral lens for a patient with keratoconus.

Dr. DeNaeyer is clinical director of Arena Eye Surgeons in Columbus, Ohio, and a consultant to Alcon, Visionary Optics, Bausch + Lomb and Aciont. He is also the designer of the Europa scleral lens (Visionary Optics) and a shareholder for Precision Ocular Metrology (sMap3D).

Assessing Lens Fit

Fig. 7. Corneal and scleral topography obtained with an eye surface profiler indicate a highly toric scleral contour. Fig. 8. OCT images of an adequately vaulted scleral lens (top) and inadequately vaulted scleral lens (bottom).

Once the scleral lens is on the eye, OCT can be used to assess lens fit by providing information on central vault, limbal clearance and landing zone in relation to the sclera. The amount of desired central vault and limbal clearance varies somewhat from one lens design to another, as well as from one fitter to the next. Generally speaking, a settled scleral lens should have between 150μm and 250μm of central vault, which tapers down to a fraction of that (20μm to 40μm) over the limbus to eventually land on the sclera. A scleral lens with excessive or inadequate vault, or one that lands on or inside the limbus, needs to be reordered with the appropriate adjustments made to fix these deficiencies. Keep in mind these parameters may change somewhat from initial application to a time several hours later as the scleral lens settles into the tissue. 

With respect to limbal clearance, there is no "magic number"; rather, as long as some amount of clearance exists, the limbus should be able to tolerate the lens. Note, however, excessive limbal clearance may allow for conjunctival prolapse and possibly sectoral hypoxia due to a thick tear reservoir. Figure 9 demonstrates several OCT images of scleral lenses with varying degrees of clearance over the limbal area.


Fig. 9. OCT images of scleral lenses over the limbus, demonstrating ideal, inadequate and possible excessive clearance. Fig. 10. OCT images of scleral edge profiles. The top image appears to have a proper alignment to the sclera, the middle image is loose and the bottom image is tight.

OCT imaging can also be used to evaluate edge profiles. This is helpful in ensuring that the landing zone of the lens is acceptable in all quadrants. A scleral lens that is too flat will demonstrate edge lift on OCT. This flat edge will encourage debris to accumulate under the lens and fogging of the vision over the course of the day. A scleral lens that is too tight will demonstrate an appearance of lens "digging in" to the conjunctival-scleral complex. This tightness will create discomfort and redness over time, and may have more significant long-term effects on the ocular surface. OCT can be particularly helpful in comparing edge fit along different meridians in helping to determine if toric landing curves might be helpful to improve a scleral lens fit. Figure 10 shows several edge profiles. 

Fitting scleral lenses in the past has been more of an art than a science in many respects. The future of scleral lens fitting figures to be more scientific, driven by precise ocular surface measurements and software that can customize a lens to the individual eye. In the immediate term, using the technology that is available will streamline the fitting process while we wait for a technological revolution in scleral lens fitting to occur. 

Dr. Jedlicka is a clinical assistant professor at the Indiana University School of Optometry and president of the Scleral Lens Education Society.

References

  1. Kojima, R. Eye shape and scleral lenses. Contact Lens Spectrum. April 1, 2013.