CLSA Eye Witness Spring 2011 - (Page 26)

A Good Starting Point Jennifer chancellor, coa, ncle-ac ith everything you do, you should have a good starting point. I love to hike. It is one of my favorite pastimes. I especially enjoy hiking with my husband. While my ability to endure a long hike is, in his words “admirable,” having a hiking partner that is always prepared is extremely beneficial. My husband will map the terrain, gauge the distance, check the weather, and pack the bags. A good starting point is crucial to a long hike in a remote area. If you do not start correctly, the hike will be more difficult and the task of getting to the end will take longer than necessary. We can use this analogy when fitting patients with contact lenses. At the beginning of a contact lens fit, vertexing the refraction back to the corneal plane can help provide a better starting point and help us determine the contact lens Rx much more efficiently. But what exactly does it mean by “vertexing a lens” and why is it necessary? The term vertex is often used in geometry, to define one type of point. It is used to explain the corners of geometrical shapes (Figure 1). A Figure 1 W Figure 2 + B C A better description of vertex in relation to the optical field is to define vertex as the point where the axis of the lens intersects with the surface (either front or back) of the lens. The actual vertex distance is an indication of how far the ocular (again, either the front or back) side of the lens is from the front of the cornea. Moving a lens closer to or further away from the eye has the effect of changing the total power of the optical system without changing the power of the lens. When fitting a patient with contact lenses, we typically start with a refractive correction that is determined for spectacle vertex distance, and then select a diagnostic lens based on the vertexed contact lens prescription. To expand on the explanation of vertexing, it is important to understand how moving a lens affects the power of the optical system. If the distance between the back of the lens and the eye changes, the effective power of the lens will also change. A better way of understanding the effective power is to refer to it as the actual power the wearer is seeing (or perceiving). The effective power at the corneal 26 w w w. c l s a . i n f o | plane will always increase in plus power relative to the spectacle plane. The perceived power of the lens varies when the lens is moved toward or away from the eye. For instance, if a plus lens is moved away from the eye and the light rays are no longer focused on the retina, the perceived power is stronger than needed and the actual power of the lens will need to be decreased. If the same plus lens is moved toward the eye, the perceived power will seem weaker, necessitating a lens that is stronger in plus to get the same effective power. The exact opposite will occur with a minus lens (Figure 2). With spectacles, it is often understood the glasses will have the same amount of distance in between the cornea and lens as the refraction. Since contact lenses rest on a tear film on the cornea, the distance in between the cornea and the lens will not be the same as it was in the phoropter during the refraction. To achieve the same power at the contact lens plane as the spectacle plane, plus power has to be incorporated into the contact lens Rx. Assuming a standard 12mm vertex for the contact lens prescription, both minus and plus contact lenses will always require less minus (more plus) than the spectacle plane. While assuming a 12mm vertex distance will suffice in many cases, you may want to know the actual vertex distance when stronger powers are involved. For instance, if the refraction was performed at a 15mm distance, a greater compensation may be needed for higher powers (over – 15.00 or +11.00 | EyEWitnEss spring 2011 c o n ta c t l e n s s o c i e t y o f a m e r i c a http://www.clsa.info http://WWW.CLSA.INFO

Table of Contents for the Digital Edition of CLSA Eye Witness Spring 2011

CLSA Eye Witness Spring 2011

CLSA Eye Witness Spring 2011 - (Page Cover1)
CLSA Eye Witness Spring 2011 - (Page Cover2)
CLSA Eye Witness Spring 2011 - (Page 1)
CLSA Eye Witness Spring 2011 - (Page 2)
CLSA Eye Witness Spring 2011 - (Page 3)
CLSA Eye Witness Spring 2011 - (Page 4)
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CLSA Eye Witness Spring 2011 - (Page Cover3)
CLSA Eye Witness Spring 2011 - (Page Cover4)
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