Myopia Profile


Choosing an instrument to measure axial length

Posted on March 15th 2021 by Connie Gan

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As you get into more myopia control, choosing an instrument to measure axial length can be overwhelming. Read more for the different options.

The most common parameter to monitor myopia progression is change in refractive error. However, as clinicians around the world become more proactive with prescribing myopia control strategies, axial length data becomes an increasingly valued piece in the clinical picture. If you aren't currently equipped with the appropriate equipment, how does one go about choosing an instrument to measure axial length?

Why is axial length measurement so important?

The main 'why' behind myopia control is to reduce the rate of axial length elongation. Whilst we can normally expect a direct correlation between axial length and refractive error, this is not always the case. A reduction in refractive progression does not always correlate to reduced axial length progression. Axial length measurements are also a valuable tool in monitoring the success of orthokeratology treatment in reducing myopic progression, as refraction is intentionally altered. It can also change one's clinical management plan, such as in cases of a mismatch between myopia and axial length. An example of this is a low myope with long axial length, where a higher-than-expected risk for future pathology may lead to a more proactive management strategy.

As biometric instruments are not yet traditionally used in optometric practices,  practitioners may wonder which ones would be the most reliable to help monitor treatment success in myopia control. RM (link) and LF (link) asked the Myopia Profile Facebook community this precise question about choosing an instrument to measure axial length.

RM Ok brains trust. I am looking to incorporate axial length measurement into my myopia clinic, but I can't find a good resource to investigate instrument options. Any suggestions would be very helpful! Thanks!LF Just wanted to get some advice. We are looking at getting some equipment to measure AXL as we are keen to put more myopia management into practice. Just wondering if anyone was happy to share what equipment they use, and what would they recommend? Is it worth while also getting a machine to measure Keratometry and an auto refractor as well? Currently looking at Oculus Myopia Master (the included myopia software from BHVI looks inticing), and the Topcon MYAH Also, we already have a Oculus Pentacam, and want to start OrthoK after following Paul’s online course. Is the Pentacam easy to use for orthoK, and does anyone have any experience or helpful tips for using the Pentacam for orthok? Thanks in advance

The community's comments on their favourite instruments are summarised below.

Interferometry measurement instruments

Interferometry is a non-contact, optical biometry measure. Examples include the Zeiss IOLMaster, Haag-Streit LensStar and the OCULUS Pentacam AXL. These instruments have traditionally been used by ophthalmologists to measure corneal curvature and axial length and facilitate intraocular lens implant power calculations for cataract surgery. They have high repeatability and accuracy,1-5 and have been used in myopia control studies of orthokeratology,6-9 atropine8-10 and MiSight.11

PC The “gold standard” commonly used in research is the IOL Master but my opinion is that any optical biometer is sufficient. The key is to use the same machine as there will be variances between instruments…KG I have a second hand IOL master … If I didn’t already have the IOL master I’d be looking at one of the combo machines you’ve mentioned…

Pros: high repeatability and accuracy, reliable, non-contact measurement technique

Cons: single function instrument, typically quite expensive for primary eye care 

A-scan Ultrasonography

RM Thoughts on ultrasound?HWN variability of pressure of indentation is a factor. Especially small mm change is a large D equivalent.RM I did a repeatability study on ultrasound for axial length measurements 25 years ago when I was enrolled in a PhD on myopia - I seem to recall I had to take about 15 measurements and throw out outliers before I could trust the numbers... Non-contact would be better, no doubt...PC Not reliable enough. I had an ultrasound before optical biometry. Not even comparable in terms of repeatability and workflow efficiency. There are outliers even with IOL Master when it’s such tiny changes we are trying to measure to assess progression.

A-scan ultrasound biometers have been used in several myopia control research studies to monitor axial length - typically older studies on bifocal and progressive spectacle lenses12-14 and atropine.14-15 Ultrasound measurement requires anaesthesia and corneal applanation, so children may be intimidated by the drop instillation and/or the probe's proximity to the eye. There is a steeper learning curve in getting consistent and repeatable measurements due to applanation pressure and is dependent on the operator.

Interferometry versus ultrasound - which is more accurate?

In application to myopia, the IMI Clinical Trials and Instrumentation report explains that ultrasonography is limited in resolution to about 0.30D, whereas interferometry measurements have resolution of around 0.03D, making this technique an order of magnitude better as a measure of myopia progression. Cycloplegic autorefraction, by comparison, has a repeatability of ±0.21D.16

In orthokeratology lens wear, a study comparing ultrasound and interferometry (IOLMaster) measurements of axial length found the latter to be more repeatable. The authors concluded that "measurements with the IOLMaster can replace the measurements from the two A‐scan ultrasonic biometers used, however, the reverse is not true."17

Measurement of axial length with non-contact interferometry instruments is more accurate and repeatable than by A-scan ultrasound biometry.

Combination instruments

There are also a range of instruments that combine several functions in one, which offers increased convenience for the practitioner and patient. Some are designed specifically for myopia management, with additional testing capacity and software outputs to streamline both testing and clinical communication.

Optopol REVO

SR Optopol Oct is awesome

Axial length + OCT. The Optopol REVO takes axial length measurement by using OCT imaging across the entire length of the eye. It has good axial length repeatability, and its measurements are in good agreement with those obtained on the IOLMaster.18


JL Topcon will soon introduce an affordable device incorporating topography and axial length... much less expensive than the likes of the IOL MasterJL … I understand it will be a modification of an existing topographer, adding the axial length capacity...

Axial length + corneal topography + pupillometry + dry eye assessment + progression reports. Topcon has released the Aladdin and the Myah (known as Aladdin-M in USA), both of which measure axial length and corneal topography. The Myah / Aladdin-M also includes pupillometry and dry eye assessment capabilities. Reporting to track progression include refraction (extra input required) and axial length charts. The Topcon technology uses non-contact, interferometry measurement of axial length, and has been demonstrated to be comparable to the IOLMaster.19

OCULUS Myopia Master

AT I feel that most of the biometer devices out there are designed specifically for ophthalmology especially iol calculation and thus very expensive. The manufacturer should consider a device more specific to optometry which does auto ref, topography and axial lengthJT I love the ease of using the myopia master, it gives you a really accurate read on AXL, if the signal strength is no good it will either discard it or won’t give you a reading. Takes up to 6 measurements per eye.

Axial length + keratometry + autorefraction + progression reports + risk factor analysis outputs. The OCULUS Myopia Master is a multi-function instrument, featuring autorefraction, axial length measurement and keratometry. It also has software developed in cooperation with the Brien Holden Vision Institute that provides axial length growth chart outputs and provides risk factor assessment to support clinical communication. The take-home report for parents provides the results, and also provides support on understanding myopia. The report can be printed or sent via email, together with the date for the next consultation, directly from the software. The OCULUS technology uses non-contact, interferometry measurement of axial length, which has been demonstrated to be in agreement with the IOLMaster.20 Since the Myopia Master was designed especially for the Optometrist, the combination of measurements provide utility for all patients, not just myopia management, through the Autorefractor and Keratometer functions.

Pros: repeatable and accurate, non-contact measurement, multi-functional, space- and time-saving, more affordable than single-function interferometry instruments, designed for primary eye care

Cons: none come to mind!

Take home messages:

  1. There are many considerations in choosing an instrument to measure axial length, including affordability, size and ease of use.
  2. Interferometry measurement instruments are more accurate than ultrasound measurements, but have traditionally been used by ophthalmology and been expensive for primary eye care.
  3. Multiple function instruments which employ interferometry, especially those specifically designed for myopia, provide useful clinical functionality and affordability for primary eye care practitioners, as well as tools for reporting and clinical communication.

Further reading

Meet the Authors:

About Connie Gan

Connie is a clinical optometrist from Kedah, Malaysia, who provides comprehensive vision care for children and runs the myopia management service in her clinical practice.

Read Connie's work in many of the case studies published on Connie also manages our Myopia Profile and My Kids Vision Instagram and My Kids Vision Facebook platforms.

About Kimberley Ngu

Kimberley is a clinical optometrist from Perth, Australia, with experience in patient education programs, having practiced in both Australia and Singapore.

Read Kimberley's work in many of the case studies published on Kimberley also manages our Myopia Profile and My Kids Vision Instagram and My Kids Vision Facebook platforms.

This content is brought to you thanks to unrestricted educational grant from

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