Changes in corneal structure with continuous wear of high-Dk soft contact lenses: a pilot study

Dr Klaus Ehrmann - Research Manager in Technology and Project Director for the Bifocal Contact Lens Project at the Vision Cooperative Research Centre, Sydney Australia

Klaus Ehrmann’s professional qualifications include a B.Eng awarded from the FH Technical College in Aalen, Germany; a MSc in Machine Design from the Cranfield University, UK; and a PhD in Biomedical Engineering from the University of New South Wales, Australia. He has held engineering R&D positions in the US, Israel, Germany and the UK. Prior to joining the University of New South Wales for his doctoral degree, he worked for several years for the National Physical Measurement Laboratory in Germany. For the last seven years he has been responsible for instrument development and metrology at the Vision CRC. His research interests concentrate on developing innovative methods for ocular, contact lens and vision related measurements.

Optom Vis Sci. 2003 Jun;80(6):440-6. PMID: 12808404 PubMed - indexed for MEDLINE]

Gonzalez-Meijome JM, Gonzalez-Perez J, Cervino A, Yebra-Pimentel E, Parafita MA.

Department of Physics (Optometry), School of Sciences, University of Minho, Braga, Portugal.

Review

The authors of this paper investigated the effect of extended wear of silicone hydrogel lenses on corneal thickness and topography. Under the hypothesis that mechanical interactions may cause changes in corneal morphology, they followed six subjects for 15 months while measuring thickness and topography at regular intervals and at three different locations ranging from central to 8 mm diameter. The anterior corneal surface was measured with a corneal topographer based on a Placedo Disc. Corneal thickness was measured with an ultrasonic probe.

Despite the technical difficulties in obtaining precise measurements, particularly of peripheral corneal thickness, the results from this study show an initial significant flattening of the central cornea, followed by a moderate steepening after the first three months. The effects at the peripheral cornea were less pronounced. The initial regression of corneal steepness with high Dk lens wear has been reported before, but the general conclusions were that this is due to recovery from previous low Dk lens wear and its associated corneal changes. In this study, only neophytes were enrolled, to exclude regression as a possible mechanism. Even more intriguing is the subsequent recovery of corneal curvature to almost baseline level after 12 months of lens wear followed by a slight steepening again after lens removal. This unexpected behavior asks for new models and additional experimental investigations.

Given the high stiffness of Lotrafilcon A material, mechanical interactions, similar to orthokeratology type effects on the cornea, are one possible explanation. Although not demonstrated yet in controlled clinical trials, anecdotal evidence from practitioners point in that direction. All 6 subjects in this study were fitted with the 8.6 mm base curve lenses. No fitting data was given, but with a K reading range of 42 D to 45 D, it is likely that some of the subjects would have been better fitted with the steeper 8.4 mm lens. The lens manufacturer recommends to fit the 8.4 mm lens for corneas steeper than 44 D to achieve optimum comfort and movement. A flat lens on a steep cornea will exert excessive pressure on the central cornea. Pressure related thinning and migration of epithelial cells would also explain the observed reduction in central corneal thickness. It would have been interesting to assess curvature and thickness changes over a 24 hour period and compare it with patterns typically observed in RGP orthokeratology patients.

The large variability within the data for measurement of central corneal curvature would have justified the assessment of changes in refractive error. In particular, over-refraction under high illumination with pupil sizes restricted to 2-3 mm, should provide accurate data that closely correlates with central corneal curvature. To complete this stream of investigation, the effect of different rates of flattening across the diameter of the cornea on spherical aberration would confirm the curvature measurement and also indicate if a simple change in lens power would fully restore visual acuity. No indications were given in the paper how visual performance was affected or if subjects were refitted with different lens powers, as their corneal curvature changed by more than 0.5 dioptres.

As the authors indicated in their discussion, only preliminary general conclusions can be drawn from this study, not only because of the small number of subjects, but also because their age range was rather restricted and any trends observed were rather inconsistent between subjects. Despite these limitations, the authors have highlighted a fascinating aspect of continuous wear with silicone hydrogel lenses and we await studies that collect addition clinical variables to help identify particular cause and effect mechanisms.