prescribing for astigmatism
More Torics for the Toolbox
BY TIMOTHY B.
EDRINGTON, OD, MS, FAAO
Since their introduction, toric soft contact lenses have been manufactured with several different designs to maximize rotational stability. Prism ballast, toric back surfaces, double slab-off, truncation and combinations of these designs have been utilized with varying degrees of success. The most prevalent stabilization method has been the prism ballast design, in which prism is incorporated to create peripheral thickness variations in order to minimize lens rotation. The "watermelon seed" effect positions the thinnest portion of the lens periphery beneath the upper eyelid and the thickest portion positions inferiorly. Gravity plays a minor role, if at all, in the rotational position of this design.
The prism ballast design (Figure 1) is incorporated in the majority of currently available toric soft lenses. The prism effect may be attained by eccentric lenticulation or by incorporating a prism in the lenticular or carrier portion of the lens peripheral to the optic zone. Also, several manufacturers incorporate a comfort chamfer around the entire circumference of the anterior surface lens edge. This comfort chamfer is accomplished by thinning the edge profile, thereby reducing inferior lid interaction which could decrease rotational stability.
Recently, Vistakon launched its new disposable soft toric. The Acuvue Toric features a dual thin zone stabilization design. Thin zone, also referred to as double slab-off, designs have been utilized on soft torics such as the CIBA Vision Torisoft and the Wesley Jessen Optifit 3 to enhance rotational stability. Double slab-off designs generally do not incorporate prism.
Double slab-off refers to the lens edge profile being dramatically thinner on the top and the bottom of the lens and thicker through the side portions of the lens periphery. One of the two thin zones (optically it does not matter which one) will tend to position beneath the upper eyelid in order to achieve rotational stability. The thicker portion of the lens periphery positions centrally between the eyelids.
Overall lens diameter also tends to be important in lens stabilization. Larger diameter toric soft lenses allow the manufacturer more lens surface peripheral to the optic zone in order to accentuate thickness profile differences.
The CooperVision prism ballasted toric design (Figure 1) has a central toric posterior surface. Rigid lenses with toric back surfaces limit lens rotation on toric corneas. However, it is debatable as to whether a back surface toric aids that dramatically in rotational stability for soft toric designs. I guess it couldn't hurt, assuming that the corneal toricity and the lens toricity share similar axes.
Figure 1. CooperVision's prism ballasted toric design
Many practitioners prescribe front surface torics when the patient's astigmatism is primarily internal or lenticular, and they prescribe back surface torics when the astigmatism is primarily corneal. In theory, a back surface toric lens would assist stabilization more for against-the-rule toric corneas than for with-the-rule or oblique toric corneas.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. E-mail him at email@example.com.