Over the past few years I have increasingly prescribed prism to many of the patients that I see in the course of having developed a niche consulting practice. The parents of some of the pre-teens and teenagers that I see ask me if their child can wear contact lenses if they need prism. My standard answer is that prism can’t be incorporated into the contact lenses, but that if prism is still indicated, it can always be used as an adjunct together with contact lenses. Since the prism is often used to aid academic performance, it can function like learning or reading lenses. During the summer or at active times, the contact lenses may suffice with less need for prism.
We say that because it’s the simpler answer, although the technically correct answer is that prism can be incorporated into contact lenses, as it is with most toric lenses. Of course the prism is typically in the base down direction to weight the lens at the bottom so it minimizes lens rotation during blinking.
So theoretically, if you wanted to incorporate a relatively small amount of yoked base down prism you could. The interesting question is now much prism is operative in the area of the optic zone of the lens. A nice article appeared in Contact Lens and Anterior Eye back in 2015 that looked at this issue.
Their main conclusions were as follows:
- All toric soft contact lens (TSCL) designs evaluated had vertical prism in the optic zone except ACUVUE OASYS® for ASTIGMATISM, which had virtually none.
- TSCL designs utilising prism-ballast and peri-ballast for stabilization have vertical prism in the central optic zone.
- In monocular astigmats fit with a TSCL, vertical prism imbalance could create or exacerbate disturbances in binocular vision function.
The mean amount of vertical prism in the optic zone (rounded off) for several representative lenses was reported as follows:
Air Optix for Astigmatism = 0.5^
Biofinity Toric and SofLens Toric = 0.8^
PureVision Toric and PureVision 2 for Astigmatism = 1.0^
I raise this issue for the same reason that I blogged awhile back about the prism base down thinning used in Progressive Addition Lenses. It’s something to be aware of when patients seem to have more trouble adapting to a new Rx than you would have expected. Although we’re talking about relatively low amounts of yoked prism, base down yoked prism can be powerful particularly for sensitive patients. If that is the case, then Acuvue Oasys for Astigmatism with essentially no prism in the optic zone may be desirable.