Dear Ms. Specs in the City,
I had the following situation occur with a patient recently: They had to raise their chin to see through their single vision lenses. The Rx is -10.00D OU. I followed all of the standard troubleshooting steps and verified that the power and PD are correct, and the OC height is the same in both lenses. Need help, what am I missing?
Baffled in Buffalo!
Dear Baffled in Buffalo,
Here’s something to consider that can affect the single vision patient’s visual experience—the placement of the lens optical center height (OC) relative to the amount of pantoscopic tilt. Pantoscopic tilt is defined as a lens tilt around the horizontal axis or 180, relative to the patient’s primary gaze. Tilt changes the effective power of a lens, so you can see where this could present a problem.
Typically, the lab determines the placement of the lens optical center height in a single vision lens. As is the case with stock lenses, the lab generally places the OC or major reference point (MRP) height of a surfaced single vision lens on the datum line, the imaginary line that runs through the middle of the lens, horizontally. If the patient wears single vision lenses, and their eyes are positioned way above the datum line, you might consider providing the lab with the patient’s pupillary height in the lens. They will use this measurement along with the pantoscopic tilt measurement to calculate the correct placement of the OC height relative to the pantoscopic tilt angle that is formed by the lens tilt in front of the eyes. This measurement will ensure proper placement of the OC/MRP of the lens, meaning that it adheres to Martin’s Rule of Tilt. Ms. Specs wants you to remember this important rule: In single vision lenses, the OC heights must be dropped 1 mm below the pupillary center for every 2 degrees of pantoscopic tilt.
What are the effects if we do not adhere to the rule of tilt? The sphere power will change from the prescribed power, and unwanted cylinder will be induced along the 180 for pantoscopic tilt. The patient will experience more prism than if had we done nothing at all. Even though the prism will be in the same direction for both eyes, Base Up or Base Down, and in theory “cancel,” there are still adverse visual effects. The wrap angle around the vertical axis or the 90 can also induce power errors at the 90, which can be addressed by adding the correct amount of face form or wrap angle. Have you ever heard a patient exclaim: “I have the feeling I am walking downhill”? This is due to excessive BU Prism. Conversely, if they state: “I feel like I am walking uphill,” this is the opposite—excessive BD prism.
You can provide the lab with the patient’s pupillary center height, and they will calculate Martin’s Rule of Tilt to determine proper placement for the OC/MRP heights. Or you can mark the pupil center on the lens with the lens plane perpendicular to the floor. This height compensates for the tilt.
The details of Martin’s Rule of Tilt can be found in online links and ophthalmic optics books (Ms. Specs highly recommends you add System for Ophthalmic Dispensing 3rd edition to your optical library. This text will not let you down).
Dear Baffled, to be a dispensing star, make the extra effort to provide precision measurements so that the patient has the absolute best optical experience.
See Well and Be Well,
Ms. Specs in the City
Laurie Pierce, ABOM