Considerations for lens parameters and corneal indentation following orthokeratology

April 14, 2019Chi Keong Kwan

Empirical contact lens fitting provides advantages in customising contact lenses to match the individual corneal profile. This case report describes how to adjust the parameters of an Orthokeratology (Ortho-K) lens in order to solve the issue of corneal indentation caused by a suboptimal lens fit.

Case report:

In 2015, a twelve-year-old caucasian female was referred to the University by an optometrist seeking myopia management advice with Ortho-K lenses.

Her subjective refraction was

R: -6.00/-0.75x180 (6/6)

L: -6.00/-0.50x12 (6/6)

No contradiction for Ortho-K lens wear was noted. After discussing the pros and cons of using Orthokeratology in myopia control, the patient’s mother elected to proceed with Ortho-K. A pair of Forge Ortho-K lenses were ordered using the EyeSpace contact lens design software, corneal topography and spectacle refraction. When the lenses arrived a delivery appointment was scheduled and the patient achieved visual acuity of 6/6 in either eye after two-weeks of overnight lens wear. The patient had been following the care and maintenance guide provided by EyeSpace.

During a routine follow-up visit in May 2017, a corneal indentation ring was noted with sodium fluorescein at the mid-peripheral cornea in either eye. Distortion of mire on the mid-peripheral cornea was observed on the corneal topography map at the same location (Fig.11 & 12). The concentric rings showed a crooked appearance and the distance between the rings demonstrated unevenness. The location of the indentation mark coincided with the location of the reverse curve (Z-Zone) and alignment curve (AC) when the lenses were placed on the eye. The patient demonstrated good unaided monocular VA (6/6) with subjective refraction of plano in either eye.

A new pair of lenses were ordered with a blended curve design, a decrease in back optic zone diameter and a decrease in the lens-corneal angle of the alignment curve.

This change aimed to reduce the mechanical pressure on the corneal epithelium at the junction between the reverse curve and alignment curve. With this modification, epithelial disruption in the corneal mid-periphery was alleviated. As a result, the indentation mark with fluorescein and distorted topographic mires on both eyes were no longer present.

Table 1. Examination details of both eyes.

Table 2. Parameters of old lenses.

Table 3. Parameters of new lenses.

                              Figure 1. Fitting of the old lens on the Right eye

                                      Figure 2. Fitting of the old lens on the Left eye

                         Figure 3. Corneal indentation ring without a lens on the Right eye

                       Figure 4. Corneal indentation ring without a lens on the Left eye

                       Figure 5. The simulated fit of the old lens on the Right eye

                      Figure 6. The simulated fit of a new lens on the Right eye

                    Figure 7. The simulated fit of the old lens on the Left eye

                         Figure 8. The simulated fit of a new lens on the Left eye

                  Figure 9. The corneal mire map of the Right eye in baseline on 22-May-2017

                 Figure 10. The corneal mire map of the Left eye in baseline on 22-May-2017

        Figure 11. The corneal mire map of the Right eye in follow up visit on 31-May-2017

         Figure 12. The corneal mire map of the Left eye in follow up visit on 31-May-2017

           Figure 13. The corneal mire map of the Right eye in follow up visit on 26-Jul-2017

                Figure 14. The corneal mire of the Left eye in follow up visit on 26-July-2017

Discussion

The use of an empirical fitting approach

The trial fitting of a contact lens can be performed using a diagnostic trial fitting set or through an empirical approach. Instead of going through the physical trial fit process using standard trial lenses from a lens bank, an empirical method uses refraction and corneal topography results for lens ordering. In an empirical approach, lens parameters can be customised to the individual eye’s profile, using corneal data such as diameter, toricity eccentricity, sagittal height and spectacle refraction. It has been reported that more than 90% of the eyes could be fitted with an RGP successfully using empirical fitting.

Another clinical use for empirical fitting occurs in patients with moderate to high astigmatism in which the cylindrical range is not covered in a standard trial lens kit. Michaud et al. achieved success when fitting RGP and soft lenses on a cohort of 20 subjects with a minimum level of 2.50D astigmatism using an empirical fitting system. The visual acuity (VA) with empirically fitted RGP and soft lenses showed no significant difference to the VA with spectacle corrections.

For Ortho-K lens fitting, Chan et al. achieved a 90% first fit success rate using computerised aided empirical method among 51 myopic children. In another study using corneal topography results and refraction data customised Ortho-K lenses were designed to induce hyperopic central corneal steepening of +2.00D for monovision correction in a group of emmetropic presbyopes

In our case study, empirical fitting was adopted. The Forge Ortho-K lenses used in our case were virtually designed on the EyeSpace contact lens design software using subjective refraction, HVID and corneal topography map data. The optimisation of the lens to cornea relationship is achieved using the company’s lens fitting software, including modification of the lens profile in different zones. According to the company, 83% of cases were fitted successfully without alteration to the software’s selected parameters and the remainder were successfully fit following the second lens remake.

Corneal indentation and troubleshooting

During the visit in May 2017, indentation rings were noted in the mid-peripheral cornea of either eye with corresponding mire distortion shown on the corneal topography map. When matching the diameter of the indentation ring (6.5mm) to the lens, it corresponded to the reverse curve. Therefore, the ring was believed to be caused by the mechanical pressure due to a “too steep” lens-corneal angle at the alignment curve position (Figure 5 and 7). A re-design of the lens profile was made attempting to improve the landing of the alignment curve on the mid-peripheral cornea of both eyes (Figure 6 and 8). Following the lens modification, the epithelial disruption was alleviated. The corneal indentation was no longer present in either eye and less distortion was noted in the corneal mire maps.

Corneal epithelial indentation ring is suggestive of a steeply fit rigid contact lens or strong lens adherence. Swarbrick and Holden suggested that in patients with overnight RGP wear, a bound lens edge was believed to cause indentation ring on the cornea with corneal distortion. Other underlying causes of this have been postulated. These include lenses that are flat centrally which in turn decentres a lens into the flatter peripheral cornea, eyelid pressure, post-lens tear thickness and viscosity, larger lenses and insufficient axial edge lift.

Among Ortho-K wearers, Chan et al reported that corneal indentation was found in 4% of the children fitted with Ortho-K lenses using an empirical method after the first night of lens wear, but indentation marks disappeared in the subsequent follow-up visits4. Lui and Edwards reported that patients who demonstrated the location of the corneal indentation ring in overnight Ortho-K wear correspond to the bearing zone of the Ortho-K lens.

The authors commented that the indentation mark was related to lens adherence and excessive pressure on the cornea. The indentation was believed to be caused by a steeply fit lens and these corneal indentation cases were solved by flattening the back optic zone radius, increasing the use of lubricant and encouraging conscious blinking to the patient. Considering the results from the published studies, corneal indentation is associated with lens binding and/or localised mechanical pressure while the underlying causes could be multifactorial.

In our case, the corneal indentation ring was at the reserve curve and alignment curve zone. The excessive localised pressure was relieved by adjusting the reverse curve and alignment curve profile, with reduction of the slope of the lens-corneal angle at the alignment curve zone. Although epithelial indentation is not true staining, it indicates mechanical disruption of the corneal epithelium. One study has demonstrated continued use of an extended wear RGP in an asymptomatic wearer with corneal indentation ring and corresponding staining subsequently developed corneal ulcers in both eyes. As persistent corneal indentation suggests chronic compression of the cornea, intervention is required. As in our case, we recommended that the patient cease lens wear until complete corneal recovery prior to refitting the lens.

Conclusion

In this case report, empirical fitting with simulated fluorescein pattern was adopted for Ortho-K lens fitting. The corneal indentation was solved following the adjustment of the reverse curve and alignment curve profile which has alleviated the localised pressure on the mid-peripheral cornea during overnight Ortho-K lens wear. It demonstrates the advantages of applying an empirical approach to troubleshooting lens fitting issues in Orthokeratology. This approach demonstrated a reduction in patient chair time than with other methods of trial lens fitting and troubleshooting.

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