Feature, Optical Dispensing

Vertex distance, every millimetre counts

High-powered lenses still require the back vertex distance to be measured otherwise the wearer may not get the best out of their lenses. Steven Daras dissects this underused dispensing measurement.

Vertex distance is a vital but oftenunderutilised measurement in dispensing spectacles. It is the distance from the back of the correcting lens surface to the apex of the cornea and is often called the back vertex distance (BVD) for this reason.

Steven Daras

In the past BVD measurements were routinely used when dispensing high powered post-cataract aphakic corrections. When correcting aphakia, prescribers included the test BVD on the prescription due to the high plus powers being prescribed. This allowed whoever dispensed the prescription to compensate for any changes required to the lens power due to the spectacle frame sitting at a different plane or BVD.

A stronger lens has a different effect on light when it sits at different distances from the eye. This can cause greater or lesser bending of light at the corneal plane affecting the eye’s ability to refract the light. Thus, compensating the lens power allows the wearer to see the same with the new lenses sitting in the new plane (frame BVD) as they originally saw in the original plane (test BVD). This allows the wearer to get the same visual effect with their new spectacles as they did during their eye test.

As aphakic prescriptions declined, so did recording BVD measurements at test and during dispensing. However, all highpowered lenses including high minus still require the BVD to be measured and be considered in both instances. Otherwise, the wearer may not get the best visual performance from their lenses. Recently I assisted a colleague with a client who had purchased glasses elsewhere but was still having trouble despite having her glasses remade.

Her original prescription read R -8.75/- 1.50 90 L -7.75/-1.50 90. The onsite optometrist confirmed the refraction and measured the test BVD, then her fitted frame BVD was measured and her prescription was compensated. The lenses were reordered as R -9.25/-1.50 90 L -8.00/-1.50 90. She was extremely happy with the result. The 6mm change in vertex distance had been compensated and she now has the same vision through her spectacles as when tested, so every millimetre counts.

When ordering ‘as worn’ progressive powered lenses (PPL) lens laboratories require the frame BVD as well as other specific measurements. These allow optimisation of their lenses to perform in the actual plane of wear. Knowing how far away from the eye the lens sits is important. If this measurement isn’t given, the lab will apply a set of pre-set values (averages) that allows them to complete the order but may not allow the lens to work as the lens designers had planned.

Thus, BVD’s should be measured for: all ‘position of wear’ PPL designs; on all highpowered lenses especially greater than ± 8.00 D (British standards recommend ≥ ± 5.00 D); when matching previous spectacles; and when the test vertex distance is written on the prescription as this is an instruction from the prescriber to create the same effect in the new frame plane as their test BVD during refraction.

Test and frame BVD’s will vary as there is no average BVD due to head and facial features differing from person to person. The trial frames or phoropters sit differently on different faces so test BVDs vary. American Optical (AO) stated that an average for their phoropter’s test BVD is 13.75 mm and suggested 14 mm be used.

In a study of 189 of their patients, a group of ophthalmologists specialising in refractive surgery and refractions (Weiss et al, 2002) found quite a difference to this advice. Using the AO phoropter, they found that the range of actual test BVDs measured on their patients varied between 10mm and 34mm with the test BVD ‘average’ being 20.4mm (see image above).

Some prescribers use their own ‘standard’ test BVD figures of 16mm, 14mm, while some use 12mm, so who is correct? Whilst low powered spectacle prescriptions may explain why we don’t see test BVDs written on these, there is a need in stronger Rxs.

Unfortunately, in practise there are very few test BVDs written on higher-powered spectacle prescriptions. This means that unless the fitted spectacle frame sits at the same BVD as the test lens, then there is little or no chance of the wearer seeing as well as when tested. Research would tend to suggest that where applicable every test and frame BVD should be measured, recorded and compensated to achieve best practice.

ABOUT THE AUTHOR: Steven Daras is Course Coordinator of Optical Dispensing TAFE Digital, co-author of the Practical Optical Dispensing and Practical Optical Workshop textbooks, a popular conference speaker and a director and secretary of ADOA.

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