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Abstract
In orthokeratology (OK), flat centre, steeper mid-periphery rigid contact lenses are used to reshape the cornea to temporarily eliminate the myopia of the wearer, and vice versa for the hyperope. There is evidence also that OK retards the progression of myopia by reducing relative peripheral hyperopia. However, currently OK is regarded as a ‘niche’ procedure mainly due to discomfort and the perceived complicated fitting procedure associated with rigid OK lenses.
This thesis explores the potential for soft contact lenses (SCLs) to be used for OK. An understanding of the lens design parameters influencing on-eye pressure distribution of SCLs was developed by employing a finite element (FE) model.
When worn inverted, some SCLs have been reported to produce orthokeratology-like changes in corneal shape. The FE model developed was used to examine the effects of inversion on the internal stresses and shape of SCLs. Inverted SCLs were found to have drastically altered on-eye pressure profiles compared to lenses worn correctly. This was found to be due primarily to the changes in lens shape rather than internal stress, thus SCL inversion was not found to be instrumental in achieving OK effects.
Novel SCLs not requiring inversion to exhibit pressure differentials when worn were designed. These lenses produced OK-type corneal shape changes. However, they also produced a small persistent, localised steep central zone in both the lens and the cornea. Analysis of stress within the lenses demonstrated that buckling of the lens was the likely reason for this lack of central conformity.
Increasing central lens thickness mitigated buckling and the associated central corneal steepening, but only the most extreme lens design worn in the closed eye flattened the cornea centrally.
The FE model developed offers valuable insights into the principal factors producing pressure differentials beneath SCLs. If SCL OK is to become a viable alternative for the correction of myopia, central corneal steepening associated with buckling of the central portion of the lens needs to be overcome and some suggestions were made as to how this could be achieved.