A Comparison Of The Optics Of Back Surface Toric (Spherical Front) Versus Bitoric Rigid Contact Lenses.

Three critical concerns in fitting contact lenses have been defined physical, physiological, and optical. The physical concern is for the best lens-cornea bearing relationship, which permits optimal lens positioning while maintaining lens mobility. The physiological concern is for adequate oxygen availability, and the optical concern is for just that - optimal optics. Most authors seem to feel that the costly and more complicated design of a bitoric lens, compared to a back toric (spherical front) lens, is necessary for the astigmaismt. According to some, a bitoric should be the lens of first choice for a toric cornea. Because the tear film is of equal and opposite curvature from the toric base curve of the contact lens, but has a lower index of refraction, the toric base curve is only partially neutralized. This phenomenon is induced astigmatism. Therefore, except for the case when the refractive astigmatism is greater than the corneal astigmatism, a bitoric lens is necessary. The back toric design aligns well with the cornea, but does not provide good acuity in most cases because the induced cylinder compounds physiological astigmatism. The main optical advantage of a bitoric lens is correction of all forms of astigmatism - corneal, physiological, and induced.
The purpose of this study was to compare the optics of two different lens designs for toric corneas, while monitoring the physical and physiological concerhs discussed above. The hypothesis was that the optics of a custom designed bitoric lens would be subjectively better than that of a laboratory designed back toric (spherical front) lens. Subjects were provided with one pair of each lens design and, after one week of wear with each pair, they completed a questionnaire concerning optics and comfort. Clinical assessment included visual acuity, overrefraction, biomicroscopy, fluorescein pattern evaluation, and keratometry.