L&T: RxPertise

Jan
2011

POSITION OF WEAR

Facing Reality


Optician Jim Voss of John Boys Smith Vision Center uses a Shamir Panorameter to take Position of Wear measurements.
Photograph courtesy of Jim Voss

By Barry Santini


“No one’s ever done that before!”

Eyecare professionals who routinely measure the pantoscopic tilt, vertex distance and face form angle, pupillary distance and pupil height on each and every client are already familiar with such exclamations from their patients. Sure it’s an ego boost for practices to hear their clients recognize out loud such higher level differentiation in professional service. But for these ECPs, the real importance of taking and using these measurements lies in their burning desire to deliver the full potential of today’s most sophisticated, free form lens designs. Only through the use of accurate and precise position of wear values will your clients be able to also enjoy the fullest visual benefit of their doctor’s Rx.

Most of us have not used these measurements because until recently, no single vision or progressive lens would allow for their inclusion in the design process. With the advent of free form manufacturing, real time lens design optimization that includes every aspect of how an individual’s lenses are positioned in front of their eyes is now possible. The resulting eyewear is as tailored to your patient’s visual needs and fitting tastes as a bespoke suit is to their body. All that is required is becoming comfortable with using not just one or two, but all the fitting parameters for every pair of eyewear you dispense.

WHAT IS POSITION OF WEAR?
In the past, we routinely tilted, bent and adjusted eyewear or sunwear to make them fit as comfortably as possible. Occasionally, clients prompted us to fit their frames in a specific way in order for them to see their best. But if you’re like me, you didn’t give much thought to the reasons behind why these individual client preferences for frame angle or position would impact their visual satisfaction. We simply complied with their requests and often hoped for the best. More often than not, fitting for the best vision was kind of a hit or miss situation—more pantoscopic tilt here or less face form angle there. With higher-power prescriptions we would be keen about comparing the fitted vertex distance with the refracted value on the Rx. And with progressives, we would follow the manufacturer’s advice and increase the angle of pantoscopic tilt and try to fit the new eyewear as close to the eye as possible.

Today, we have a better way of ensuring the best vision is obtained from every Rx. Position of Wear (POW) is really a short hand term for describing how your client’s fitted eyewear has repositioned their lenses from those used to test their vision in the exam room.

SUCCESS STORIES WITH POSITION OF WEAR
Significant increases in patient satisfaction are typical for ECPs that routinely take and employ position of wear measurements. Dr. Tom Nicolai, OD, of Davis Duehr Dean Eyecare in Portage, Wis. notes that he was unhappy with most of the multifocal or traditional progressives he tried before he donned a set of POW optimized progressives himself. “Best lenses I’ve ever worn. Comfortable from the day I put them on,” Dr. Nicolai notes. Dispensers at Davis Duehr EyeCare have also become “much attentive to all their measurements since they began taking POW values for their progressives.” Dr. Nicolai has also seen a significantly lower non-adapt rate with POW optimized progressives.

Jim Voss, dispensing optician at John Boys Smith Vision Center in Ellensburg, Wash. says that taking POW measurements “sets yourself apart from your competition.” He has also noted greater client satisfaction when dispensing POW optimized lenses. Voss favors the POW optimized lenses particularly for wrap sunwear, and prefers to use companies that supply their prescriptions as a complete, finished product.

Craig Chasnov of Eyetopian Optical in Ft. Myers, Fla. has found the latest optimized lenses essential for use in wrap sunglasses, but with a personal twist: “I perform my own compensation calculations.” Chasnov finds that his long experience with how to make wrap eyewear work ensures the best quality for his clients.

TAKING POSITION OF WEAR MEASUREMENTS
Unlike a traditional order for single vision eyewear, measuring for POW SV lenses requires inclusion of pupil height for proper optimization, so it is a good idea to obtain POW values for every pair of eyewear you dispense. There are two ways to take position of wear measurements: manually and digitally. With either way, the most essential requirement is repeatability. This means that several sets of measurements taken for a single client by a single dispenser should yield found values within 0.5mm range. If not, practice until you can reliably produce values within this range. This applies to both the manual or digital methods. In all cases, the frame style selected must be fitted to the preferred position before these measurements are taken.

MANUAL METHOD
Since the dispenser is actively involved with tools and devices when taking POW values manually, we’ll call this the “hands-on” approach. (See chart above.)

PDs—Most lens designs require a visual axis measurement. But some may ask for a geometric pupil measurement. When using a ruler to obtain monocular visual axis measurements, a penlight or other light source must be employed to obtain the corneal reflex. Remember that a “digital” pupillometer is really a manual instrument with a digital readout of the value obtained.

Pupil Heights—When obtaining heights, maintaining appropriate and dispenser posture is challenging. I suggest a few measurements be taken and retaken to ensure proper fidelity.

Vertex Distance—A distometer, once used solely for cataract and other high-power fittings, is the tool of choice. Be aware that this measurement may need adjustment because of how lenses are bevel-positioned within a frame—especially for the higher-plus powers. The final thickness of plus lenses will impact your effective vertex value.

Pantoscopic Tilt
—Do not confuse this with pantoscopic angle (which is the angle the temple makes with respect to the plane of frame front). There are several manual devices, such as the plumb scale from Carl Zeiss, which can be used for this measurement. The most important aspect of proper pantoscopic measurement is to be sure that the client’s facial plane (imagine a line from the brow to the chin) is as vertical as possible before the measurement is made. Although modifying a value because of client posture is appropriate when determining the fabricated progressive height, no allowance for client facial posture is to be made for pantoscopic tilt values.

Frame Wrap Angle—Also known as face form angle and panoramic angle, one can use a simple protractor or a dedicated frame wrap measuring device, such as the Shamir Panorameter. You can also obtain individual frame wrap values from the tables provided by some companies, such as Oakley and Luxottica. An important aspect of wrap fitting is eyelash clearance. When fitting a frame or sunglass with a thin plano lens, remember that thicker Rx lenses may require increasing the vertex distance to obtain lash clearance.

DIGITAL METHOD
The dispenser is involved to ensure proper patient’s positioning when obtaining digital POW measurements. Most of these automatic devices employ an attached “frame guide,” which provides the cardinal points the digital camera requires to obtain its values. These frame guides are also important as they provide the needed information to help compensate when the client’s facial plane is not parallel to the camera plane. A full set of up to five POW values can be obtained with these newer systems, often with a precision of 0.1mm, a level rarely achievable with the manual method.

HOW DOES POSITION OF WEAR OPTIMIZE AN RX?
Position of wear is really a shorthand term for describing how your client’s fitted eyewear has repositioned their lenses from those used to test their vision in the exam room. The main benefit of position of wear Rx compensation is to deliver the best representation of the prescription as determined in the exam room. It’s easy to understand why when you take a quick look at the difference between the fitting values of the lenses used in an exam room versus how eyewear is routinely fitted:

Exam Room
  1. Vertex Distance—typically averages 13 to 14mm.
  2. Pantoscopic Tilt—Lenses of a phoroptor or trial frame are straight up and down, i.e., zero pantoscopic tilt.
  3. Face Form Angle—Lenses are held in an absolutely flat plane, i.e., zero face form angle.
  4. Pupillary Alignment—Particularly with phoroptors, they accommodate IPD in a binocular manner, and therefore cannot make allowances for monocular eye separation differences.
  5. Pupillary Height—The patient’s eye is typically aligned with the geometric (optical) center of the phoroptor’s test lenses.

Fitted Eyewear
  1. Vertex Distance—Fitted values can range from as close as 8mm to as far as 20mm.
  2. Pantoscopic Tilt—Fitted values range from 0 to 15mm; typically 6 to 8mm.
  3. Face Form Angle—Also known as frame wrap angle, fitted values range from 0 to 10 degrees. Wrap around sunglasses can range from 12 to as much as 25 degrees.
  4. Pupillary Alignment—If measured ideally, individual (monocular) values are taken. Differences between the eyes may range up to 4mm.
  5. Pupillary Height—Fitted values often find the pupil placed in most lens shapes between 3 and 7mm above the geometric (and in some cases, the optical) center of the lenses.
Position of wear compensation takes these differences into account. In determining the tilt and centration errors induced by these fitting differences, the lens optimization program will deliver a recalculated prescription that compensates for this.  The prescription values created are referred to as the compensated Rx, and a work slip is created to facilitate verification by both the fabricating laboratory and the eyecare professional. In some cases, it may be desirable to tender the record of compensated values to the patient or the patient’s record, in order to prevent potential confusion in the future. An additional benefit for the lens optimization program in knowing all the actual fitting values is that the lens’ surface can be optimized in a global manner, and not just at one or two reference points on the lens surface. The result is a much more sophisticated representation of the original prescription, and elevates lens performance to levels not achievable through traditional surfacing or casting methods.

OPTIMIZING SINGLE VISION LENSES AND PROGRESSIVE LENSES
Do single vision lenses benefit from position of wear compensation? You bet! Stronger prescriptions, those with moderate to greater cylinder power and fitting situations where the client’s pupil is significantly above the mechanical center all greatly benefit from POW optimization. Where progressive lenses optimization differs from SV is in the reading zone. The eye’s depressed reading position represents another situation where the exam room findings may depart from the in situ fitting of the eyewear. At the typical reading level of a progressive, the values for both vertex distance and lens tilt become different compared to how the near Rx is determined during refraction. This is one reason why traditional progressive designs have recommended close vertex fitting and increased pantoscopic angle.

What can be done to optimize the performance of a traditional progressive? Not much. All we can do is gain an understanding and comply with the manufacturer’s fitting recommendations. The problem with this approach is that it leaves client preferences for fit and appearance as secondary to a lens’ designer’s fitting assumptions. Not the best place for a paying customer to be, in my humble opinion.

A BLUEPRINT FOR SUCCESS

Helping to optimize your patient’s lenses, differentiating yourself from the competition and becoming more aware of the importance of taking good measurements are all tangible benefits of getting to know about position of wear. Practices that embrace the value of POW measurements clearly desire to keep their offices at the very top of their game. Today, with the growth of online eyewear flooding the optical landscape, it’s always good to know where the high ground lies.


 

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