Demystifying Declination Angle

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ErgoPractice News – July 2017

Measuring Product Benefits, Not Product Features

Note: Please share this article with anyone looking for loupes. This information may make the difference between buying a pair of ergonomic loupes and buying a pair only labeled ergonomic!

Frequent Stories From Clinicians Who Have Been Experiencing Neck Pain

Many clinicians are sending us posture photos in order to analyze their working postures because they have been experiencing serious neck pain. Many of them believe they purchased ergonomic loupes with large declination angles. Why can’t their loupes with large, ergonomic declination angles prevent their neck pain? This issue will answer that question.

The Loupe Ergonomics Bandwagon

As workplace ergonomics becomes more valued and discussed, many loupe companies are claiming more of their products are more ergonomic. In loupe ergonomics, this is often done by claiming large declination angles.

Loupe declination angle is intended to be a measurement of how well loupes help the user look downward.1 The greater the declination angle, the less the user has to tilt their head. Research shows that continually tilting your head more than about 25 degrees forward can cause discomfort, chronic pain, and even career-ending injury.2, 3

But since there is no governing board to standardize this measurement, different companies may choose whatever measurements maximize the appearance of ergonomics in their products.

The Error of Measuring the Declination Angle Without the User

The effective declination angle should be measured when loupes are worn by the users. The reference line should be the line connecting the corner of the eyes and the top of the ear. The effective declination angle of a pair of loupes depends on the user’s facial features, such as nose height. If frame temple arms are parallel to this reference line, temple arms may be used for reference.

So manufacturers use temple arms as the reference line. But if temple arms are not parallel to the true reference line, declination angles claimed can be significantly larger than true declination angles.

A common trick to maximize a claimed declination angle is to use a variable reference line, such as the aforementioned loupe temple arm. As you can see in Figure 1, this will often overstate the declination angle. The user in Figure 1 has a dangerous posture. However, one measurement technique shows a poor declination angle, the other extremely good.

Furthermore, if the temple arm is the reference line, the declination angle can change based on frame design. In Figure 2 we show how two different temple arm designs can create two different declination angles – despite an identical position of the eye and ocular. In this example, the position of the temple arm has no impact whatsoever on actual ergonomic performance. This type of measurement can be very misleading!

Measuring Ergonomic Benefits

Which do you want more? A large declination angle or to work without discomfort, pain, and risk of injury? I’m sure it’s the latter! So skip worrying about declination angle, no matter how it is measured, and instead measure your head tilt.

A good posture cannot be faked. As long as your neck tilt is less than about 25 degrees, you are working safely and experiencing the ergonomic benefits of a good pair of loupes.2, 3

To measure the declination angle, have an associate take a photo of you from the side, sitting upright with your neck in a comfortable position. Then another photo in a similar position, bending your neck to look at your target through your loupes. Comparing the difference between these two angles you will find your head tilt angle.4 If you have trouble with this, e-mail us your pictures and we can help you.

If your head tilt angle is greater than about 25 degrees, it doesn’t matter what anyone claims about your loupes, you are on your way to chronic pain and injury! (Hint: If you look like the person in Figure 3, you do not have ergonomic loupes.)

If you need help measuring your head tilt angle, please contact me at jchang@surgitel.com. Again, please share this article with anyone you know who will be buying loupes and trying to compare loupe ergonomics. As we’ve seen, it can be a tricky subject.


References:
  1. Valachi B, Practice dentistry pain-free, Posturedontics Press, Portland, OR, 2008, www.posturedontics.com
  2. Rucker LM, Surgical telescopes: posture maker or posture breaker? In, Murphy D, ed, Ergonomics, and the dental care worker. Am Public Health Assoc, Washington DC, 1998, 191-216
  3. Chang BJ, Ergonomic benefi ts of surgical telescopes: selection guidelines, J Cal Dental Assoc, Vol.30.No2., 2002: 161-169
  4. Chang, BJ, Verify Declination Angle Claims!, ErgoPractice News, December 2016; https://surgitel.com/verify-declination-angle-claims/