Prism Ballasted Front Surface Toric RGP's Rotational Effects As A Function Of The Base Curve.

Studies have suggested that it is difficult to predict the final position of the prism base when fitting a prism ballasted front surface toric RGP. This study was designed to assess the effect the base curve to cornea fitting relationship has on the rotation of prism-ballasted front surface toric RGP's. Twelve subjects wore lenses fit on K, 0.50D steeper than K and 0.50D flatter than K. Each fit was then measured for amount of rotation. The results indicate that fitting a prism~ballasted front surface toric RGP on K or 0.50D steeper than K produced less rotation than fitting this type of lens 0.50D flatter than K. Many lens designs have been created in an effort to correct residual astigmatism. One of these designs is the prism-ballasted front surface toric RGP. While this lens is used for the correction of residual astigmatism, it is often difficult to assess or predict the amount of rotation of the prism base and influence of the lower lid on nasal rise. A study conducted by Fairmain (1967) suggests that the influence of the lower lid on nasal rise accounts for 10-15 degrees of rotation. In 1964, Goldberg concluded we could predict that a prism-ballasted front surface toric RGP will
have a final prism base position 20 degrees nasally. In contrast to Goldberg's finding, Westerhout (1971) demonstrated that the prism base would position itself only 10 degrees nasally. In order to help practitioners correct residual astigmatism by the use of prism-ballasted front toric RGP's, it is necessary to predict consistently the final position of the prism base. It is the intent of this study to incorporate the idea of predicting the final position of the prism base by examining base curve to corneal fit and the effect that this has on rotation of the lens.