Vertex distance is a frequently overlooked reason patients complain about new glasses. Take the high-plus presbyope, for example, who complains that he has trouble reading with his new prescription. Though your staff has checked and rechecked the glasses-pupillary measurements, seg height, lens power, the adjustment-he still complains that he has to hold reading materials very close in order to see them clearly. Since this new prescription is only slightly different than the old one and the base curve is exactly the same, the problem seems even more perplexing.

The likely culprit? The often ignored calculation of vertex distance (VD).

The problem is that vertex distance-that is, the distance, measured in millimeters, between the patient's cornea and the back of the ophthalmic lens-has an effect on the perceived power of the lens. It's even more important in higher power lenses:+/-5.00D or more. In such lenses, there can be significant differences in power if the VD of those new lenses is different from the testing VD used during the refraction.

Here's the problem. The perceived power of a lens varies when the lens is moved further toward or away from the eye. When the VD is higher than during the exam, a plus lens becomes stronger. With increased vertex distance, a minus lens, however, gets weaker.

The ideal solution is, of course, for the two vertex distances-in the glasses and in the exam-to be the same. If the in-exam VD can't be replicated, there is another solution-recalculate the lens power using the rotary chart that accompanies the distometer.

The benefits of a distometer

It can help staff select appropriate frames prior to ordering lenses. Say you have just completed an exam of a patient who requires a high powered lens. To avoid problems and to help your staff, the next step could be to write the examining vertex distance figure on the patient's chart. During frame selection, staff can then use the examining VD figure on the chart to calculate the vertex distance of various prospective frames. This would help eliminate frames that were inappropriate. When making such calculations, it's important to remember that the average vertex distance of spectacle lenses that are fitted properly is 8-12 mm.

Making a final frame selection that allows the lenses to be close to the examining VD will help avoid both remakes and non-revenue-producing return office visits to handle complaints.

Another way to avoid trouble is to calculate the vertex distances of the glasses upon delivery of the finished eyewear. This will help keep patients from having to come back for readjustments. And to help even more, consider guiding these patients to frames with adjustable nose pads. This bridge design allows you to adjust the vertex distance by altering the adjustment of the pads.

How to use a distometer

It's important to use this device correctly. Here are tips-some of which are taken from Distometer-Instructions For Use, Haag-Streit Service, Waldwick, N.J.-to help make it work for your office.

Frames. First, tell the patient to put the frames on as they will usually be worn. That's important, because you might place the frame on the face differently than it will actually be worn.

The device. While stressing the importance to the patient of keeping his or her eyes closed, put the stationary arm of the device on the closed eye lid. Then slowly depress the plunger until the distometer's moving arm touches the backside of the lens.

The scale. Next, read the vertex measurement, which appears in millimeters on the scale. When taking note of the measurement, don't forget to consider lid thickness. Though some distometers do the lid calculation automatically, you can do it yourself by simply adding 1mm to your total.

Rotary chart. The rotary chart that comes with the distometer makes refiguring lens power easy. The red scale on the chart applies to minus lenses, the black to plus lenses. To use the chart, first calculate the vertex distance of the lenses on the distometer as described above. Then, using the vertex distance as determined during the initial exam process, align that in-exam vertex distance on the chart with the prescribed spherical power of the lens. Without moving the dial, read the vertex distance of the new spectacles. The recalculated lens power for the new spectacles is adjacent to that VD.

Staff training. Encourage staff members to practice on one another using this device. It will help them understand, among other things, the importance of making sure patients keep their eyes closed during the measuring process.

One benefit of correctly using devices like a distometer and understanding the possible effects of deviating from an examining vertex distance is, of course, creation of better products-and better vision-for your patients. They also save your business both time and money in terms of unscheduled and no-charge return office visits. And, they'll help cut down on material costs incurred by your operation. All of these represent sound reasons to take the time to make sure your staff understands how to avoid vertex distance problems. EB