Trial Frame Refraction
Mark E. Wilkinson, OD
February 28, 2017
This is Mark Wilkinson from the University of Iowa Department of Ophthalmology & Visual Sciences. In this presentation I will discuss the why and how of doing a trail frame refraction.
The goal of the subjective refraction, no matter how it is performed, is to achieve clear and comfortable binocular vision, if possible.
As with any refraction technique, it is important to remember that the ability of the clinician to maintain control during the refraction is directly related to their ability to communicate clearly and directly with the patient.
The starting point for a subjective refraction begins with the patient’s entrance visual acuities with their current glasses on, if they have glasses. Or, their uncorrected acuity, if the patient has no glasses.
You will want to observe how the patient sees both in the distance and at near with their current RX, if they have one.
Next you may want to perform keratometry to estimate the amount of refractive cylinder present. Keratometry is particular helpful for patients with previously unknown refractive errors.
Finally, you will perform retinoscopy or do an auto-refraction to establish an objective starting point for your subjective refraction.
You may wonder, can I use an auto refractor instead of a retinoscope to determine an objective starting point for my refraction. The answer is yes, in most, but no all cases.
Auto refraction can provide important objective documentation of the patient’s sphere and cylinder powers.
However, auto refraction may be difficult to impossible to perform for patients who need to use eccentric viewing, for those with nystagmus and for patients with media opacities.
You might next ask, why would I use a trial frame, if I have a phoropter?
To answer that question, we have to consider some of the disadvantages of a phoropter. Those include that the light reflex for retinoscopy may be poorer than with loose lenses.
Additionally, there is decreased light transmission with the phoropter when multiple lenses are used with higher refractive errors.
Eccentric viewing will be difficult to impossible for the patient to use when viewing through a phoropter.
Finally, for those patients with nystagmus, it will be difficult for them to use their null point, when viewing through a phoropter.
The advantage of using a trial frame when doing a refraction is that a trial frame and loose lenses are easier and more natural than a phoropter for patients that are difficult to refract, have high refractive errors, as well as for those who are visually impaired.
Standard refraction techniques are employed when performing a trial frame refraction on an individual with normal sight.
Just Noticeable Difference refraction techniques are used for individuals that are visually impaired.
Just Noticeable Difference (JND) is the amount of lens power needed to elicit an appreciable change in clarity or blur. The poorer the visual acuity, the larger the Just Noticeable Difference.
Just Noticeable Difference equals the denominator of the 20 foot Snellen acuity divided by 100.
For example, if your patient has 20/150, divide 150 by 100 and you get 1.50D. This give you a starting Just Noticeable Difference power of +/-0.75D.
If the patient cannot appreciate improved or worsened vision with the +/-0.75D lenses, then it is likely that there is no prescription change that will enhance their acuity.
Just Noticeable Difference refraction techniques will allow the clinician to perform an accurate refraction at any acuity level.
Just Noticeable Difference refracting techniques apply to both the sphere and the cylinder correction.
Most important, Just Noticeable Difference refraction techniques elicits reliable answers.
To find the best cylinder power and axis uses the Just Noticeable Difference technique with a Jackson Cross Cylinder (JCC),
For 20/50 or better visual acuity, you will use a +/-0.25D JCC.
For 20/63-20/100, you will use a +/-0.50D JCC.
For 20/125-20/160, you will use a +/-0.75D JCC.
For 20/200 or less, use a +/-1.00D JCC.
Cylinder axis and cylinder power are tested in the same way with the JCC as is done when using a phoropter.
Here is an example of how you would use the Just Noticeable Difference refraction technique.
Your patient presents with uncorrected acuity of 20/400 in their only seeing right eye.
They have no old spectacles and you are unable to do retinoscopy due to band keratopathy.
400 divided by 100 equals 4.00D, so you will start by having the patient compare +2.00D/-2.00D. The patient states +2.00D is clearer. Put a +4.00D lens in the trial frame.
With +4.00D in the trial frame, again ask the patient to compare +2.00D/-2.00D. If the patient still prefers +2.00D, replace the +4.00D lens in the trial frame with a +8.00D lens.
With +8.00D in the trial frame, again ask the patient to compare +2.00D/-2.00D. The patient now prefers -2.00D; replace the +8.00D lens in the trial frame with a +6.00D lens.
Check acuity. Now the patient’s acuity is 20/200.
Adjust your Just Noticeable Difference lens power and refine with +1.00D/-1.00D bracketing lenses.
If the visual acuity continues to improve to the 20/100 level, you would then do your fine tuning with +0.50D/-0.50D.
After establishing the spherical power as describe above, VA is 20/200. 200 divided by 100 = 2.00D. This indicates that you will start with a +/-1.00D JCC.
With the JCC oriented for power at 90/180 degrees, ask the patient which is clearer.
Patient states that +1.00D axis 180 is clearer. Put a +2.00D axis 180-cylinder lens in the trial frame.
Remember to adjust the sphere power by adding -1.00D to the current sphere power in the trial frame to maintain the Circle of Least Confusion on the retina.
Now, with a +2.00D axis 180-cylinder lens in the trial frame, again ask the patient to compare +1.00D to -1.00D axis 180.
If the patient still prefers +1.00D axis 180, replace the +2.00D axis 180-cylinder lens in the trial frame with a +4.00D axis 180-cylinder lens.
With a +4.00D axis 180-cylinder lens in the trial frame, add an additional -1.00D sphere to the current sphere power in the trial frame.
Again asked the patient to compare +1.00D to -1.00D axis 180. If the patient now prefers -1.00D axis 180, replace the +4.00D axis 180-cylinder lens in the trial frame with a +3.00D axis 180-cylinder lens and adjust the sphere power by adding +0.50D sphere power to the existing sphere power in the trial frame.
If the acuity has improved to the 20/100 level, now refine with a +0.50D/-0.50D JCC.
Once the cylinder power is determined, repeat the same process to determine the cylinder axis using the standard technique of 15-degree interval change in axis until reversal, then 5 – 7-degree interval changes in axis if the patient’ visual acuity allows for this level of discrimination.
Remember to recheck the cylinder power if there is a significant change in the cylinder axis.
Let’s talk about trial frame logistics.
As you prepare to start refract with a trail frame, it is important to become familiar with how to use a trial frame and trial lenses.
Here are pictures of a couple of commonly used trial frames.
To start using a trial frame, it is important to familiarize yourself with how to adjust the temples, nose pad, pantoscopic tilt, cylinder axis and Pupillary Distance.
To adjust the temples, slide the temples all the way out before putting the trial frame on your patient. Once on, slide the temples forward to achieve a snug, but comfortable fit.
The nose pad can be moved forward or back and up and down for optimum positioning using the adjustment points on the trial frames noted by the arrows.
Pantoscopic tilt is adjusted using these adjustment points on the trial frame temples.
Cylinder axis is adjusted using these adjustment points on the trial frame.
Pupillary distance is adjusted for each eye using these adjustment points on the trial frame.
When using trial lenses, take the trial lenses for the right eye from the left side of the trial lens case lenses for plus and minus spheres and cylinder lenses.
You will use the lenses from the right side of the trial lens case lenses for the left eye.
This will allow you to easily look at the trial lens case to know what lenses are in the trail frame at any time, for each eye.
Put spherical lenses in the back wells of the trial frame.
Put cylindrical lenses in the front wells of the trail frame.
It is important to adjust the front lens holders on the trial frame to allow you to slide the lenses in from the temporal sides of the trial frame.
This is because it will be both difficult and uncomfortable for the patient if you try to slide the lenses in from the nasal side.
Finally, put reading correction lenses in the front wells of the trial frame, in front of the cylinder lenses if they are being used.
To avoid patient discomfort, recognize that you will need to support the trial frame with your other hand when putting trial lenses into the trial frame.
When using a trial frame, you get an extra option when asking which is better, one or two with spherical lenses.
With the trial frame, you have option 3, which is without either of the JND lens choices.
This slide shows how you use the JCC for cylinder axis determination.
The red and white dots straddling the axis.
This slid shows how you use the JCC for cylinder power determination.
The red and white dots on axis.
Practice using the trial frame as well as the Just Noticeable Difference technique
when acuity is 20/50 or worse, or if regular refraction techniques are not working.
Remember to decrease power interval as acuity improves.
When in doubt, use a trial frame.
It is important to remember that for patients with reduced vision, the only intervention may be to prescribe appropriate glasses!
In summary, you will want to prescribing a conventional prescription for patients with reduced vision, when the patient sees a qualitative improvement in their vision.
Additional reasons to prescribe a conventional prescription is to improve vision for intermediate tasks such as writing, sewing, using a video magnifier or a computer.
It is important to remember that a conventional prescription can be prescribed to facilitate the use of optical devices, especially if the patient has a significant refractive error.
Finally, remember that you can incorporate the spectacle Prescription into some optical devices, if the prescription is significant and/or improves acuity.
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