WHAT ARE MONOCULAR PD's?
It is important to understand exactly what the term monocular pd means. A common misperception is that a monocular pd is the distance from the center of the pupil (right or left) to the center of the patient's bridge when the patient's head is in the primary or "straight ahead" position, and the object of regard is at a far distance. Actually the measurement is from the point where the line-of-sight intersects the lens to the center of the bridge of the frame.
Monocular pd measurements are data for the fabrication of eyewear and must relate to the frame. Fortunately the center of the patient's bridge and the center of the frame's bridge generally coincide. Even when they do not coincide, the difference may be small and with a midrange or lower prescription power, the finished eyewear is usable. You also need to consider of this when you measure Segment Height.
When the slope of one side of the nose is steeper, if one side of the nose bulges more than the other, or if the nose is not centered right-to-left, a frame with fixed pads will be shifted to one side. The solution in such a case is to use a frame with adjustable pads and compensate the best you can for a functional and cosmetic success. In almost every case the widest part of the eyewear should be as wide as the patient's face, and it should be centered. Even if the nose is symmetrical and centered, the eyes may not be equidistant from the centerline of the nose.
It is best to use a monocular pd because we are not symmetrical and it is optimal to have the lenses in the right position to be able to see through and use the lenses correctly. Many people can have one eye positioned a few millimeters differently then the other and a binocular distance pd does not compensate for this!
Using a monocular pd would make it more likely for a patient to suceed in progressive lens wear.
EFFECT OF PANTOSCOPIC TILT
Increasing pantoscopic tilt effectively increases the width of both the reading area of a PAL and the width of the corridor. Increasing pantoscopic tilt also effectively increases the separation of the lines-of-sight at the point they pass through the lenses for near viewing. Decreasing the pantoscopic tilt narrows the width of the corridors and reading areas, and it also narrows the patient's near pd.
Obviously, adjusting the pantoscopic tilt is extremely important when fitting progressive lenses. Unlike single vision, and to some extent multifocals, the purpose of pantoscopic tilt is more than just keeping the lines of sight at right angles to the lens surfaces. With non-individualized progressives, manufacturers set a default value for the separation of the centers of the reading zones. It is up to you to find the pantoscopic angle that works best for your patient.
With individualized (free-form) lenses you specify the pantoscopic angle, vertex distance and wrap, and digital lens production incorporates your data into the lenses. The result of using accurate monocular pd's and individualized lenses helps keep the patient's lines-of-sight equally centered for better performance at near and intermediate distances.