Mokbel T., Samra W.A.

Mansoura University, Mansoura, Egypt

 SUMMARY

Aim. To еvаluаtе the safety, efficacy and possible additive role of flat fitting RGP lens following riboflavin-UVA corneal collagen cross-linking (CXL) in management of progressive keratoconus.

Subject and Method. Prospective comparative study of 30 eyes without RGP lens and 34 eyes fitted with RGP lens with apical bearing (flat fitting) from 2 to 10 weeks postCXL. None of the patients had been wearing contact lens (CL) before CXL. The effect of the CXL without and with RGP lens was measured by the change from the baseline of best spectacle corrected visual acuity (BSCVA), refractive sphere and cylinder, maximal keratometry value (K_max), simulated keratometry, astigmatism and I-S value. The results of these measurements are reported pre-CXL and at 6-month intervals up to 24 months post-CXL.

Result. In the eyes without CL, none of the study parameters showed a significant change from baseline. Only in the group of eyes fitted with CL post-CXL, several parameters showed a significant improvement: refractive cylinder, I-S value, K_max, simulated keratometry and astigmatism.

Conclusion. RGP lens with flat fitting can be a safe additive factor in improvement of topographical parameters following CXL.

Keywords: cornea, keratoconus, keratometry, contact lens, collagen cross-linking, dinamic.

INTRODUCTION

Corneal collagen cross-linking (CXL) is a technique in which extra cross-links are created in the corneal stroma to increase the rigidity of the corneal tissue [1]. Clinical reports indicate that CXL is effective to stop the progression of keratoconus and conclude a statistically significant improvement of visual acuity and topography in a majority of patients [2–6]. However reports on the efficacy of CXL should ideally only include patients who are not wearing CL due to topographic changes occurred by rigid gas permeable (RGP) lens fitting in patients with keratoconus. However, such patients present with the need of both stabilization through CXL and optical correction with RGP lenses.

Different strategies can be adopted when fitting RGP contact lenses to keratoconic patients, these include «apical bearing» (flatter fitting) and «apical clearance» (steeper fitting) fittings as explained by Korb et al. [7] and «three-point touch» fit which is often favored over a flatter or a steeper fit as it is believed to relieve some of the pressure on the cone by distributing the weight of the RGP lens over more of the cornea [8]. Nevertheless, the literature suggests that many keratoconic patients are still fitted with RGP contact lenses using a flat-fitting technique, which can offer comparatively better visual acuity than steeper-fitting lenses [9]. At present, no published study has addressed the effect of flat lens fittings in keratoconic eyes treated with CXL.

The purposes of this study are to еvаluаtе the results of UVA/riboavin CXL for stabilization of progressive keratoconus in patients who have never worn CL before and to address the possible influence of flat fitted RGP lens on the visual and topographical results.

 
MATERIALS AND METHODS

This prospective longitudinal study was conducted on 64 eyes of 35 patients with progressive keratoconus that underwent riboflavin/UVA CXL treatment. Females were more frequent than Males (20 vs 15). The average age of the patients was 27.6 ± 8.2 years (range, from 18 to 35 years). None of the patients had a history of CL wear. 30 eyes did not need CL correction, while 34 eyes were fitted with Rose K2 lenses 2 to 10 weeks post-CXL treatment.

This study was conducted in accordance with the principles of the Declaration of Helsinki. All patients read and signed the informed consent form; for patients under 18 years, both parents were required to sign.

 
Ophthalmological examination and follow-up

Clinically and instrumentally documented keratoconus progression was considered in the study. Progressive keratoconus was defined as 1 or more of the following changes over 12 months: an increase of 1.00 diopter (D) or more in the steepest K, an increase of 1.00 D or more in manifest cylinder, or an increase of 0.50 D or more in the manifest refraction spherical equivalent. All stages of keratoconus were treated except advanced cones with scarring (stage 4 of the Krumeich classification). Exclusion criteria included a history of corneal surgery, corneal pachymetry less than 400 mm, a history of chemical injury or delayed epithelial healing, and any other ocular or systemic disease.

Patients were examined at baseline and at 6, 12, 18 and 24 months post-CXL. Subjective parameters were registered: best spectacle corrected visual acuity (BSCVA), refractive sphere and cylinder. Objective parameters were established on a corneal topographer (Shinnippon, Tokyo, Japan): K_max, IS value, simulated keratometry measurements (K1, K2) and simulated astigmatism.

 
CXL Treatment

After topical anesthetic eye drops administration, 10 % alcohol was applied on the corneal epithelium for 30 seconds. The epithelium was mechanically removed within the central 8.5 mm diameter area using a Beaver blade. Next, riboflavin (0.1 % solution, 10 mg riboflavin-5-phosphate in 10 ml dextran-500 20 % solution) (Ricrolin, Sooft Italia, Montegiorgio, Italy) was applied every 3 minutes for 30 minutes until the stroma was completely saturated and aqueous stained yellow.

Ultraviolet-A irradiation was accomplished using a commercially available UVA system (UVX, Peschke Meditrade, Switzerland). Before treatment, the intended 3 mW/cm² surface irradiance (5.4 J/cm² surface dosage after 30 minutes) was calibrated using a UVA meter (LaserMate-Q, LASER 2000, Wessling, Germany). During treatment, riboflavin solution was applied every 5 minutes to ensure saturation and balanced salt solution (BSS) was applied every 3 minutes to moisten the cornea. Topical anesthetic drops were instilled every 10 minutes throughout the procedure.

A drop of Moxifloxacin 0.5 % (Vigamox, Alcon) and a bandage contact lens (AcuVue, Johnson and Johnson Vision Care Inc.) were applied at the end of the surgery. Postoperative treatment included Vigamox eye drops 5 times daily for 1 week and Diclofenac 0.1 % eye drops (Voltaren, Novartis Ophthalmics) 3 times daily for 1 week. Fluorometholone eye drops (FML, Allergan Inc.) were used twice daily for 1 month.

 
Contact lenses fitting

All eyes were fitted with a reverse-geometry lens design namely Rose K IC after measuring precisely lens parameters according to a special trial set of this design consisting of 19 lenses with base curves (BC) ranging from 6.5 to 8.4 mm. All lens overall diameters in the fitting set are 11.2 mm with changing power to approximate final lens power. All lenses in the fitting set have a standard edge lift design. However, we can order the lens with a BC ranging from 5.7 mm to 9.3 mm, with diameter ranging from 9.4 mm to 12 mm, with any power and with standard, flat or steep edge lift.

The goal of the central lens-cornea relationship was «apical bearing» which could be obtained by ordering the lens 0.2 flatter than optimal central fit «three point touch». When an optimal central lens-cornea relationship was obtained, the peripheral edge lift was еvаluаtеd. If the trial lens gave a desirable edge lift, then standard edge lift was ordered for that eye. If minimal or excessive edge lift was observed, increased or decreased edge lift was ordered respectively. After finding the optimal trial lens for each eye, an over-refraction was performed while the subject was wearing the trial lenses to determine the contact lens power. All Rose K lenses were ordered from David Thomas Company (UK) with high Dk materials eg Boston XO₂.

 
Statistical method

For statistical еvаluаtion of the study parameters, the 64 keratoconus eyes were divided into two groups, depending on the necessity of CL fitting for optical correction:

– group 1: keratoconus eyes without CL correction (30 eyes);

– group 2: keratoconus eyes fitted with CL post-CXL (34 eyes).

To detect any influence of CL wear on the results, the above-mentioned parameters were еvаluated for each group separately. All statistical analyses were done using student t test and the significance was adjusted at 0.05.

 
RESULTS

Pre-CXL data were available for all eyes of this study group. Patents characteristics are summarized in table 1. No significant difference was demonstrated in any studied parameters between both groups. For group 1, none of the study parameters showed a significant cant change (all p values > 0.05) compared to baseline at different time points (table 2).

Table 1. Pre CXL patients characteristics

Table 2
The pre and post operative measured parameters of the studied patients without CL (mean ± SD)

  Note. *p is significant at < 0.05.

For group 2 in whom reverse geometry CL was flatly fitted for optical correction after CXL, several parameters on topography showed a clearly significant improvement from baseline: K1, K_max, simulated astigmatism and I-S value at 12, 18 and 24 months of follow up. K2 showed a significant improvement only at 18 and 24 months. Also refractive cylinder was significantly improved at 6, 12 and 18 months follow up (table 3).

Table 3
The pre and post operative measured parameters of the studied patients with CL (mean ± SD)

 Note. *p is significant at < 0.05.

 In order to establish the failure rate in this study, the evolution of K_max in individual eyes needs to be investigated. 2 eyes of a 17 years old boy in the first group (non-CL wearers) had obvious treatment failure. They developed 3 diopters at 24 months,accompanied by a subjective decrease in BSCVA.

 
COMPLICATIONS

After cross-linking, one eye showed an epithelial defect with asymptomatic iritis like reaction. Tfhis complication resolved after a month of topical steroid therapy, so the treatment was continued. Apart from the usual CL problems like irritation, allergy and dry eye, no relevant adverse events were observed from flat fitting of the CL in the second group at 6, 12, 18 and 24 months.

 
DISCUSSION

In this investigation, we systematically applied special RGP contact lenses with a reverse-geometry design (Rose K IC) in keratoconic eyes after CXL. All eyes in CL group were fitted with apical bearing type. The aim was to еvаluаte how both clinical and topographical parameters change with respect to this type of fitting.

 The results of CXL in this study agreed with the findings of previous authors who noted that progressive keratoconus could be stabilized by UVA/riboflavin CXL [1, 6, 10–12]. However, most of them reported a statistically significant improvement of a majority or even all of the studied parameters, while, our results show a more differentiated picture. Taking into account the influence of CL wear: In the eyes without CL, none of the study parameters showed a significant change from baseline (all p values > 0.05). Only in the group of eyes fitted with CL post-CXL, several parameters showed a significant improvement including the refractive cylinder, I-S value, K_max, simulated keratometry and astigmatism.

 The findings of this study agree with the previously published literature that demonstrated improvement in most of topographical values only in the group of eyes fitted with CL post-CXL [13]. Other studies reported better LogMAR visual acuities and reduced aberrations with flatter-fitting RGP [14, 8, 9, 15, 16]. This report supports the original hypothesis by Zadnik and Mutti that the back surface of an apical bearing RGP lens will «flatten» and «mould» the irregular corneal distortions typically found in keratoconic eyes [16]. This corneal moulding will effectively give the anterior corneal surface a more «normal» curvature profile by exerting pressure on the apex of the cornea, perhaps regularizing its shape similar to what made by orthokeratology.At present, the long-term effects of wearing flat-fitting contact lenses on ocular surface are unknown. The literature shows that flat-fitting lenses may cause damage at the cone apex leading to anterior stromal scarring [7, 9, 17]; such changes in the cornea could induce additional aberrations and perhaps even limit the improvement visual performance given by the contact lens. Although contact lens wearing does not necessarily cause this scarring, it is likely to exacerbate it [8]. The hypothesis that acceleration of scarring can be caused by flat-fitting lenses was addressed by the study by Korb [7] but the sample size was very small, having only seven keratoconus subjects (14 eyes). Despite of that debated risk of corneal scaring imposed by flat fitted CL, most CLEK Study patients wear flatfitting lenses. Overall, rigid lenses were fitted an average of 2.86 D (SD +/− 3.31 D) flatter than the first definite apical clearance lens.

 At present, the long-term effects of wearing flat-fitting contact lenses on ocular surface are unknown. The literature shows that flat-fitting lenses may cause damage at the cone apex leading to anterior stromal scarring [7, 9, 17]; such changes in the cornea could induce additional aberrations and perhaps even limit the improvement visual performance given by the contact lens. Although contact lens wearing does not necessarily cause this scarring, it is likely to exacerbate it [8]. The hypothesis that acceleration of scarring can be caused by flat-fitting lenses was addressed by the study by Korb [7] but the sample size was very small, having only seven keratoconus subjects (14 eyes). Despite of that debated risk of corneal scaring imposed by flat fitted CL, most CLEK Study patients wear flatfitting lenses. Overall, rigid lenses were fitted an average of 2.86 D (SD +/− 3.31 D) flatter than the first definite apical clearance lens.

 However, in our study we didn’t face any scar from such fitting during follow up period. This is can be explained by the stiffness of the central part of the cornea induced by CXl prior to CL wearing and also may be due to high Dk material of the manufactured lens and the adjusted edge lift fitting that allows good tear exchange underneath the lens.

 Also, we do not know, however, if the improved topographic parameters are permanent after CXL treatments combined with CL wear or if the reported effects are transient warpage phenomena [18]. In order to elucidate this question, patients would have to stop CL wear, which is ethically not feasible because it would involve a long period of incapacity to pursue their everyday activities.

 In conclusion, we have found that CXL has led to stabilization of keratoconus in all studied eyes, but improvement of topographic values could only be established in the group of patients wearing flat fitted CL after the CXL treatment So, RGP lens with flat fitting can reshape the keratoconic cornea without significant adverse reactions adding a safe toll in improvement of clinical and topographical parameters following CXL.

 However, further investigation into long-term changes in such parameters with different RGP lens designs and fittings in eyes with keratoconus will be needed. This may address the efficacy of apical bearing fitting with CXL in management of keratoconus and help to improve our current understanding of how contact lens fit affects the optical performance.

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