In recent years, with the development of various materials, designs, and the advance in manufacturing technology, contact lenses have been widely used for the purpose of beauty treatment as well as for the existing purposes of vision correction and medical treatment. Accordingly, serious side effects (e.g., eye diseases and changes in the corneal shape) have been caused due to the negligence in the management of contact lenses and long-term wearing. Therefore, in this study, the changes in the corneal shape, which are the side effects due to the wearing of contact lenses, were investigated. The effects of the materials, refractive power, and design of contact lenses on the corneal shape (e.g., changes in corneal thickness, radius of curvature, high order aberration, and refractive power) were examined.
For the subjects of this study, 13 adult males and females (26 eyeballs) (5 males and 8 females) were selected from those who have no experience of vision correction surgery or Ortho-K lenses, have no other diseases or drug use that can affect corneas and eyeballs, and have normal visual acuity with a corrected visual acuity of more than 1.0. The subjects wore the lenses at 11:00～12:00 a.m., which was at least three hours after getting out of bed. The lenses were removed at 07:00～08:00 p.m., which was after eight hours (i.e., daily wearing time), and then the test was performed immediately.
The differences in the corneal thickness between pre-wear and after wearing the lenses were compared using 5 kinds of lenses: hydrogel disposable -3.00D (S1), hydrogel disposable +3.00D (S2), silicone hydrogel disposable -3.00D (S3), silicone hydrogel disposable toric -3.00 -1.25D (T1), and hydrogel extended-wear toric -3.00 -1.25D (T2). The changes in the corneal thickness, the curvatures of the anterior and posterior corneal surfaces, and HOA (spherical aberration and coma aberration) before and after wearing the lenses were investigated at the center part of the cornea, the zones 3 mm, 5 mm, and 7 mm away from the center of the cornea, and the positions in each different direction of the cornea (nasal side, temporal side, inferior side, and superior side). In the case of the equipment used for the measurement, ORB ScanⅡ (Bausch & Lomb Inc, ver 3.14) was used to measure the corneal topography and thickness, and Zywave (Bausch & Lomb Inc, ver 5.20) was used to analyze the high order aberration.
For measuring the changes in the corneal thickness, S1 and S2 were compared to examine the difference depending on the power; and S1 and S3 were compared to examine the difference depending on the material. Also, T1 and T2, which had the same design but consisted of different materials, were compared; and S3 and T2, which had the same water content but consisted of different materials, were compared.
In the case of the hydrogel soft contact lenses (T2 lens) with low oxygen permeability, the corneal thickness showed distinct increasing patterns. Especially, the changes in the thickness were about 6.27% at the center of the cornea, and about 5.79% at the 3 mm zone. Also, they were about 4.77% at the 5 mm zone, and about 3.8% at the 7 mm zone. In addition, the changes in the thickness depending on the direction of the cornea were 5.62% on the nasal side, 5.44% on the temporal side, 4.83% on the superior side, and 3.14% on the inferior side.
The examination of the curvature of the anterior corneal surface indicated that the changes in the vertical and horizontal curvatures were negligible. However, the curvature of the anterior surface became slightly flat, the curvature of the posterior surface became steep, and the total corneal curvature became flat. The examination of the spherical refractive power and the astigmatism indicated that the spherical refractive power increased in every lens excluding the S2 Lens, which was a hyperopic lens; while the astigmatism generally showed a decreasing pattern. There was no statistically significant change in the HOA, but the total HOA and CA increased.
If oxygen concentration decreases below a dangerous level, the decrease in aerobic metabolism induces the accumulation of lactic acid at keratocyte, the reduction in the metabolism rate of epithelium, and the increase in the lactic acid production of epithelium, which promote keratitis; and corneal edema is increased by the accumulation of lactic acid at keratocyte and the decrease in the pumping action of endothelium. Therefore, oxygen permeability (Dk) is one of the most important material properties of contact lens materials. For extended-wear soft contact lenses, 75～89 Dk/t is required to prevent hypoxia and edema; and for daily-wear soft contact lenses, 20～34 Dk/t is required to prevent edema. Among the 5 kinds of contact lenses with different water contents, materials, and designs that were used in this study, the change in the corneal shape was small for the lens with an oxygen permeability (Dk) of more than 28, and the change in the corneal shape was the largest for the lens with a very low oxygen permeability.