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The cornea breathes easily under minisclerals

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By Stephen J Vincent
PhD

David Alonso-Caneiro
PhD

Michael J Collins
PhD

Contact Lens and Visual Optics Laboratory, School of Optometry and Vision Science, QUT

 

Scleral contact lenses are primarily used for the correction of irregular corneal optics as the post-lens tear layer effectively neutralises the majority of corneal astigmatism.

Some concerns have been raised about oxygen transmissibility through scleral lens materials and the post-lens tear layer.

Based on theoretical modelling, Michaud et al1 suggested that a scleral lens with a Dk of 100, thickness of 300 µm and central corneal vault of 400 µm would induce hypoxic-related corneal swelling greater than four per cent.

We examined the change and recovery in corneal thickness associated with short-term (three hours) miniscleral contact lens wear using a lens with the same material (Dk of 100) and fitting characteristics (corneal vault of 400 µm) and observed no evidence of clinically significant corneal swelling.2

The corneal thickness of 10 healthy adults (mean age: 27 ± 5 years) with no corneal abnormalities was measured before, immediately following three hours of miniscleral lens wear (Irregular Corneal Design 16.5, Paragon Vision Sciences, USA) and three hours after lens removal using a Pentacam HR system (Oculus, Wetzlar, Germany).

Central corneal vault was assessed using a high resolution optical coherence tomographer (Figure 1).

CL OL 22 Vincent Minisclerals Figure 1 - F

Figure 1. A miniscleral contact lens imaged on eye with (A) sodium fluorescein, (B) scanning laser ophthalmoscope and (C) OCT cross-section displaying the thickness of the post-lens tear layer

Pachymetry difference maps were generated (the change in corneal thickness immediately following lens removal and three hours after lens removal) after controlling for diurnal fluctuations using customised software. These maps represent changes due to lens wear only (Figure 2).

CL OL 22 Vincent Minisclerals Figure 2 - F

Figure 2. Pachymetry difference maps (scale in mm) illustrating the change in corneal thickness over the central 6 mm immediately following (A) three hours of miniscleral lens wear and (B) a further three hours after lens removal

Over the central 6 mm of the cornea, only 5.0 ± 17.9 µm of swelling (0.85%) occurred, similar to that reported by Mountford et al3 following short-term full scleral lens wear (121 Dk, 800 µm centre lens thickness) of approximately 1 µm swelling per hour of lens wear.

This lower than predicted level of swelling may be due to some level of tear exchange occurring beneath the lens.

The greatest magnitude of corneal swelling was ~8 µm (1.4%), observed in the inferior cornea. Significant thinning relative to baseline corneal thickness was observed three hours following lens removal (13.4 ± 10.5 µm, 2.3% thinning).

This corneal thinning (or ‘de-swelling’) observed three hours after lens removal to levels below the baseline corneal thickness (referred to as corneal ‘overshoot’) had been observed previously in response to a hypoxic corneal environment following both soft and rigid lens wear or overnight eyelid closure.4

This response is poorly understood but is thought to be due to corneal hydration factors including tear film evaporation rates and endothelial fluid influx or ion pump capacity.5

Further research investigating the physiological corneal changes associated with longer-term miniscleral contact lens wear in abnormal corneas is required.

Modern high Dk miniscleral contact lenses, in conjunction with a moderate post-lens tear layer (403 ± 204 µm), do not induce clinically significant corneal oedema following three hours of lens wear as predicted by recent theoretical modelling.

  1. Michaud L, van der Worp E, Brazeau D, Warde R, Giasson CJ. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye 2012; 35: 6: 266-271.
  2. Vincent SJ, Alonso-Caneiro D, Collins MJ. Corneal changes following short-term miniscleral contact lens wear. Cont Lens Anterior Eye 2014; 37: 6: 461-468.
  3. Mountford J, Carkeet N, Carney L. Corneal thickness changes during scleral lens wear: effect of gas permeability. Int Contact Lens Clin 1994; 21: 19-22.
  4. O'Neal MR, Polse KA, Sarver MD. Corneal response to rigid and hydrogel lenses during eye closure. Invest Ophthalmol Vis Sci 1984; 25: 7: 837-842.
  5. Odenthal MT, Nieuwendaal CP, Venema HW, Oosting J, Kok JH, Kijlstra A. In vivo human corneal hydration control dynamics: a new model. Invest Ophthalmol Vis Sci 1999; 40: 2: 312-319.


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