Corneal oedema during reverse piggyback scleral lens wear

One clinical approach to address poor front surface wettability during scleral lens wear is the use of a “reverse piggyback” system (a soft contact lens applied to the anterior surface of a scleral lens). The aim of this study was to compare the magnitude of corneal oedema induced following short‐term reverse piggyback scleral lens wear and standard scleral lens wear.


INTRODUC TION
Traditional piggyback contact lens systems (a rigid contact lens placed on top of a soft contact lens) are used to improve the fit, comfort and tolerance of a corneal rigid contact lens 1 or minimise the potential mechanical interaction of the back surface of the lens with the anterior corneal epithelium. 2 Although scleral contact lenses vault the cornea and are more comfortable than corneal rigid lenses due to reduced corneal and/or eyelid margin interaction, 3 several cases of piggybacking corneoscleral or scleral lenses with an underlying soft lens have been reported to reduce fluid reservoir debris, 4 aid initial scleral lens application in neophyte wearers 5 and increase wearing time. 6 The use of reverse piggyback contact lens systems (a soft contact lens placed on top of a rigid contact lens) has also been described. For example, to ensure a corneal rigid contact lens does not dislodge during contact sports, 7 or to incorporate a therapeutic tint for cosmesis, light sensitivity 8 or occlusion therapy. 9 Reverse piggyback systems for scleral lenses have also been used in clinical practice to improve front surface wettability (particularly in cases of Sjogren's syndrome) when other approaches have failed (e.g., lens coatings or treatments) and to correct residual refractive error in the office during a trial fitting with diagnostic lenses.
A drawback of piggyback lens systems is reduced oxygen delivery to the cornea. For traditional piggyback systems, the soft contact lens is an additional barrier to the diffusion of atmospheric oxygen through the lens system to the cornea and also reduces tear exchange or mixing compared with a corneal rigid lens alone. 10 In a reverse piggyback lens system for a scleral lens, the addition of a soft lens acts as an additional barrier to oxygen, with no impact on tear exchange since the soft lens does not alter the interaction between the scleral landing zone and conjunctiva. Since corneal oxygen delivery is compromised during scleral lens wear due to the increased fluid reservoir thickness, 11 increased lens thickness 12 and reduced tear exchange 13 compared with other contact lens designs, piggybacking a scleral lens should be used with caution in clinical practice, typically as a last resort after exhausting other possible solutions to address poor front surface wettability.
Weissman and Ye 14 modelled the oxygen tension within the post-lens tear layer during traditional piggyback lens wear as a function of soft and corneal rigid lens oxygen transmissibility and determined that oxygen availability was greatest for lenses with highest oxygen permeability/ lens thickness (Dk/t) values. However, the oxygen within the tears began to plateau when the Dk/t of either the soft or rigid lens approached 60. As expected, these data indicate that lens materials with high oxygen permeability should be used for piggyback systems, and corneal oxygen uptake data following short-term wear (5 min) of various piggyback systems support this modelling. 15 For scleral lenses, there is also a threshold of ~100 Dk, whereby further increases in material oxygen permeability have minimal impact on corneal oedema for healthy eyes. 16 To date, no studies have quantified the potential hypoxic effect of adding a soft contact lens to a scleral lens. Therefore, the aim of this study was to examine the effect of short-term, open-eye, reverse piggyback scleral contact lens wear upon central corneal oedema in healthy eyes compared with scleral lens wear alone.

Participants
Ten young (mean ± SD age 22 ± 6 years), healthy adults (4 males, 6 females; 8 Asian, 2 Caucasian) were recruited from the staff and students of the Queensland University of Technology. Participants underwent an initial ophthalmic screening including corneal topography (E300, Medmont, medmo nt.com.au) and slit lamp biomicroscopy to assess suitability for inclusion in the study. Potential participants with irregular corneas, a history of prior ocular injury or surgery or those who were unable to achieve an acceptable fit with a diagnostic scleral lens were excluded. One participant was an intermittent soft contact lens wearer but discontinued lens wear for 24 h prior to each measurement session to minimise the potential effect of habitual soft lens wear on the cornea. None of the other participants were soft or rigid lens wearers. The study was approved by the Queensland University of Technology Human Research Ethics Committee and conducted in accordance with the tenets of the Declaration of Helsinki. All participants provided written informed consent.

Scleral lens fitting
Kerectasia Alignment Tangent Torus (Capricornia Contact Lenses, capcl.com.au) diagnostic scleral lenses were used for this study. These are non-fenestrated lenses with a spherical landing zone made from hexafocon A material (Dk 100 × 10 −11 (cm 2 /s)(ml O 2 /ml × mmHg)), with a total lens diameter of 16.5 mm. The sagittal height, back vertex power and central lens thickness varies for each diagnostic lens and these thickness profiles have been quantified previously. 17 The initial scleral lens was selected based on the average corneal sagittal height measured over a 10 mm chord using a videokeratoscope (E300; Medmont, medmo nt.com.au). In accordance with the manufacturer's fitting guide, an additional 2000 μm was added to extrapolate this corneal sagittal height value to a 15 mm chord (the primary functional diameter of the lens), and an additional 400 μm was then added to provide an initial central fluid reservoir thickness between 200 and 400 μm. The selected diagnostic lens was inserted into the participant's left eye with saline (Lens Plus Ocupure, Abbott Medical Optics, abbott. com) and fluorescein, without an air bubble, and assessed with slit-lamp biomicroscopy to ensure no corneal bearing or conjunctival impingement. The initial central fluid reservoir thickness was measured at the normal to the tangent

Key points
• Consistent with theoretical modelling, piggybacking a highly oxygen-permeable soft contact lens onto a scleral lens resulted in minimal additional central corneal oedema. • The major barrier to atmospheric oxygen delivery in scleral or reverse soft-scleral piggyback lens wear is the fluid reservoir. • A reverse soft-scleral piggyback contact lens system is safe for short-term wear in healthy eyes and may be used to address residual refractive error or a non-wetting surface during fitting.
of the corneal apex using the in-built callipers of an anterior segment optical coherence tomographer (Spectralis, Heidelberg Engineering, heide lberg engin eering.com). If the initial central fluid reservoir thickness was outside of the 200-400 μm range, the lens was removed and an alternative diagnostic lens with a different sagittal height was selected, and the fitting and assessment process repeated.

Reverse piggyback system
A delefilcon A daily disposable soft contact lens (Dk 140 × 10 −11 (cm 2 /s)(ml O 2 /ml × mmHg)), Dailies Total 1®; Alcon, alcon.com 18 was used for the reverse piggyback system, with a base curve of 8.5 mm, diameter of 14.1 mm and back vertex power of −0.50 D. The mean ± standard deviation central thickness for five of these lenses measured using optical coherence tomography was 147 ± 4 μm. This lens was chosen because it is highly oxygen permeable, highly wettable and exhibits a long non-invasive tear breakup time, 19 and so is a likely candidate for reverse piggyback systems in clinical practice.

Measurement sessions
The diagnostic scleral lens that provided an acceptable fit with an initial central fluid reservoir thickness between 200 and 400 μm was used for the two measurement sessions conducted on separate days separated by at least 24 h (scleral lens condition and reverse piggyback condition). To minimise the influence of any potential carry-over effects, the order of lens wear was balanced across the participants (i.e., five participants wore the scleral lens for the first measurement session and then the reverse piggyback system for the second measurement session, and the other five participants wore the lenses in the reverse order). Measurement sessions were performed during office hours (i.e., 09:00-17:00 h, at least 2 h after waking) and at approximately the same time of day (median start times 10:11 h for the scleral lens condition and 10:29 h for the reverse piggyback condition). For the scleral lens condition, the appropriate scleral lens was applied to the left eye with preservative-free saline, and the central corneal thickness and fluid reservoir thickness were measured at the tangent to the normal of the corneal apex using optical coherence tomography (Spectralis, Heidelberg Engineering, heide lberg engin eering.com) by a single examiner. This instrument has an axial resolution of 3.9 μm 20 and a high level of intra and interobserver repeatability for central corneal thickness measurements (coefficient of variation less than 5%). 21 A volumetric scanning protocol was used which included eleven 8.3 mm horizontal line scans separated vertically by 278 μm centred on the pupil, with nine B-scans averaged per line scan. The values used in the analysis were the average of the three line scans closest to the pupil centre. Participants remained in the laboratory during 90 min of lens wear, and the central corneal thickness and fluid reservoir thickness were then remeasured by the same examiner using the same scanning protocol prior to scleral lens removal. This period of lens wear was chosen as corneal oedema reaches a peak after about 90 min of lens wear in young healthy eyes. 22 For the reverse piggyback condition, the same procedure was followed, except that the soft contact lens was applied to the front surface of the scleral lens after the baseline measurements of central corneal thickness and fluid reservoir thickness were captured. After 90 min, the soft lens was removed, and these same parameters were remeasured with the scleral lens still in place. This ensured that the OCT measurements for both the scleral lens and reverse piggyback conditions were all obtained through the same scleral lens for each participant and therefore eliminates potential variations in thickness measurements that might arise due to magnification effects from the additional soft contact lens. 23

Data analysis
To provide an approximate correction for any small differences in the initial central fluid reservoir thickness between the scleral lens and reverse piggyback conditions, which has the potential to impact corneal oedema, 11 a correction factor was calculated for each participant using published data which quantified the variation in corneal oedema in young healthy eyes as a function of central fluid reservoir thickness. 11 This approach has been used previously to provide an estimated corrected value for corneal oedema. 24,25 Both the raw data and corrected data are presented in the results. Data were analysed using SPSS statistical software version 27.0 (IBM, ibm.com). The normality of the data was confirmed using the Shapiro-Wilk test and a two-tailed paired t-test was used to compare corneal oedema between the two lens conditions. Data are presented as the mean and standard deviation with a p < 0.05 considered statistically significant.

R ESULTS
The average centre thickness of the scleral lenses used for both experimental conditions was 414 ± 57 μm. The initial and final central fluid reservoir thickness and the raw and corrected central corneal oedema for the scleral lens and reverse piggyback lens condition are displayed in Table 1.
No statistically or clinically significant differences in central corneal oedema (raw or corrected) were observed between the two conditions (p > 0.05). After 90 min of lens wear, the corrected central corneal oedema for the scleral lens condition was 2.02 ± 0.76%, and 2.32 ± 1.15% for the reverse piggyback condition.

DISCUSSION
There was no statistical or clinically significant increase in central corneal oedema following short-term reverse piggyback scleral lens wear relative to the scleral lens wear control condition after correcting for variations in fluid reservoir thickness (mean difference + 0.30 ± 1.20%). This finding is consistent with previous experiments, which revealed that substantial thickness increases for high Dk scleral lenses did not result in significant changes in central corneal oedema in the short term. For example, Fisher et al. 12 reported a 1% increase in central corneal oedema under open eye conditions when increasing the centre thickness of a scleral lens (Dk 141) from 150 to 1200 μm. Together, these findings indicate that during scleral lens wear (with or without an additional reverse piggyback soft lens), the major barrier to atmospheric oxygen reaching the cornea is the thickness and oxygen permeability of the fluid reservoir beneath the scleral lens (Dk of tears or saline ~80), and the lack of tear exchange. Therefore, the fluid reservoir thickness of reverse piggyback scleral lens systems should be minimised as much as clinically practical.
Using Fatt's resistance in series approach to estimate the total Dk/t of a piggyback system, 26 the average Dk/t of the scleral lens system used in this experiment was 13.6 (scleral lens centre thickness 414 μm and material Dk 100, central fluid reservoir thickness 259 μm and fluid Dk 80). The addition of the soft contact lens (soft lens centre thickness 147 μm and material Dk 140) as part of the reverse piggyback system would theoretically reduce the overall Dk/t of the system to 11.9. Based on the model of Weissman and Ye (derived from their figure 1), 14 this would reduce the oxygen tension within the fluid reservoir from approximately 41.4 mmHg to 32.2 mmHg (a 21% reduction). Using the same modelling, a 21% reduction in oxygen availability would also occur for the average scleral lens parameters in this experiment if the scleral lens thickness was increased by 200 μm or if the fluid reservoir thickness was increased by 150 μm.
The limitation of the current study is the use of participants with healthy corneas and the short duration of lens wear. Patients requiring a reverse piggyback scleral lens system are likely to be those with severe dry eye that contributes to poor front surface lens wettability such as graft versus host disease or Sjogren's syndrome, and this would require longer periods of lens wear. Future research examining the effects of reverse piggyback scleral lens systems (potentially using a range of different soft lenses) on corneal oedema, visual quality and lens wettability in patients with chronic ocular surface disease would provide useful information about the potential for reverse piggyback scleral lens systems for long-term clinical use.
In conclusion, the use of a reverse piggyback system is likely safe for short-term use (e.g., a soft lens placed over a scleral lens to correct residual refractive error during a diagnostic lens fitting in the office) in healthy eyes. However, further research is required regarding long-term use in eyes with corneal disease.