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1 Hull and East Yorkshire Eye Hospital, Fountain Street, Anlaby Road, Hull HU3 2JZ, UK
2 Department of Ophthalmology, Leeds General Infirmary, Clarendon Wing, Belmont Grove, Leeds, UK
Keywords: contact lenses; corneal lenses
In the 1950s, chronic corneal hypoxia and its attendant complications were associated with scleral contact lenses (ScCLs) compared to corneal contact lenses (CCLs).1 Changes in mechanical design to improve transfer of oxygenated tears between the corneal-tear film interface were partially successful, but accompanied by increased complexity of lens fitting and design.2 These included fenestrations, slots, truncations, and channels. Apart from being time consuming and technically difficult to manufacture, these modifications were almost invariably associated with trapping of air bubbles behind the ScCL, resulting in reduced vision and localised corneal desiccation. Without a sealed tear film, the ScCL rested progressively more on the corneal apex and limbus—that is, settling back, which caused corneal erosions, scarring, and hypoxia.
The advent of fluorocarbon silicone/acrylate co-polymer ScCLs resulted in greater utility because of high gas permeability.3 One major criticism of ScCLs has been the suboptimal visual acuity achieved when compared to CCLs.3 In this study we compared the best corrected visual acuity (BCVA) in patients with RGP ScCLs who failed a trial of CCLs.
Method and results
The case notes of 15 patients prescribed ScCLs were reviewed over an 18 month period. The reasons for discontinuing CCL use included discomfort, excessive mobility, poor fit, short wearing times, and subjective lens intolerance. There were 18 eyes in 15 patients whose average age was 37 years (18–80). There were eight males and seven females.
The BCVA varied according to the pre-existing pathology. These were post-penetrating keratoplasty (seven); keratoconus (six), and herpetic scarring (two). Mean astigmatism was 9.7D (3.5–18D). CCL average BCVA was 6/18, but with ScCLs was 6/9, of which eight (44%) achieved 6/5, p = 0.1; 2 test.
The greatest improvement occurred in the keratoconus group (BCVA 6/18 with CCLs; to 6/9–6/5 with ScCLs); followed by the keratoplasty group (with ScCL 6/9 in five cases and 6/18 in two cases from pre-existing corneal scarring). In all cases the scleral lenses were well tolerated. No complications were noted.
Comment
The relatively close apposition of the cornea to a CCL provides a stable refractive interface. In a normal cornea, the centre is assumed to be spherical and regular so that a singly curved CCL can be made based on keratometry readings. In corneas with highly abnormal topography such as high astigmatism, severe flattening, apical protrusion, thinning, and scarring, the nature of the refractive interface between the cornea, precorneal tear film, and contact lens is altered because the above assumptions no longer hold true.
ScCLs vault the cornea, which eliminates the need for close alignment to the cornea. This compensates for very abnormal corneas giving good BCVA that can be difficult to achieve with CCLs.4 As the power of CCLs increases, positional stability and accuracy of fit decreases. High power CCLs tend to be bulkier, thicker, and with a larger diameter that alters the centre of gravity. These CCLs tend to sag or droop with axis mislocation so vision is through the peripheral lens and not the optic zone. Induced prismatic effects cause reduced vision, lens intolerance, and discomfort. This is exacerbated by edge sensation from high edge lift. Lens instability with excessive frictional mobility on the cornea also increases the potential for erosions, scarring, and intolerance. ScCLs retain positional stability and tend not to be associated with the aforementioned problems.
Based on the findings of this study, we think the use of ScCLs should not be prejudiced because of the perception that they are optically inferior to CCLs. The optical and therapeutic benefit of ScCLs should not be underestimated. They can have an important role in management of patients where surgery is undesirable or high risk.
References
Tan DT, Pullum KW, Buckley RJ. Medical applications of scleral contact lenses: 1. A retrospective analysis of 343 cases. Cornea 1995;14:121–9.
Tan DT, Pullum KW, Buckley RJ. Medical applications of scleral contact lenses: 2. Gas-permeable scleral contact lenses. Cornea 1995;14:130–7.
Pullum KW, Buckley RJ. A study of 530 patients referred for rigid gas permeable scleral contact lens assessment. Cornea 1997;16:612–22.
Schein OD, Rosenthal P, Ducharme C. A gas-permeable scleral contact lens for visual rehabilitation. Am J Ophthalmol 1990 15;109:318–22.