Most offices and clinics perform what is called contact A-scan biometry. This involves either hand-holding the probe or using a slit-lamp to contact the cornea with the probe in order to obtain the axial length measurements used in an IOL power calculation.

The problem with the technique is an error source known as corneal compression. The probe touches the cornea and compresses it a certain amount dependent upon the skill of the biometrist.

A .4 mm compression of the cornea will result in a 1 D error in the calculated IOL power for a normal length (24mm) eye. The error would be on the plus side in the IOL power (meaning a minus error in the refractive result) because the axial length is being artificially shortened.

This can be a problem because more and more patients are demanding precision in their final outcome. It has been demonstrated that ophthalmologists can consistently arrive at post-op refractive errors within .5 D of the targeted outcome.

Corneal compression can be controlled to a degree by monitoring the anterior chamber depth (ACD) readout on the A-scan monitor, or on the printout. A decreased numerical reading, compared to the ACD of other measurements, indicates excess corneal compression. The particular scan can be eliminated from the calculation process.

Attention to this source of error requires multiple A-scan readings so that ACD comparisons can be made. This requires more time to perform the scans and more time to evaluate the scans.

If one technician in the office performs all the A-scans, it would be possible to make an adjustment in the A-constant through retrospective analysis, assuming that the technician is consistent with the degree of his/her corneal compression.  However, a study at Emory University by Rhonda Waldron, COMT demonstrated that the lower the intraocular pressure is, the more likely it is that significant corneal compression will occur,  increasing the chances that the "one biometrist" policy may still produce post-op refractive surprises.

The corneal compression problem can be solved by using an immersion A-scan technique.  A "shell" is placed on the eye to provide a "water bath" over the cornea. The water bath provides a medium through which the probe can measure the eye without touching the cornea.

For those unfamiliar with the technique, some questions come immediately to mind:

Isn’t this uncomfortable for the patient?   Practitioners of the technique say that it is well tolerated. The usual topical anesthetic is used and the patient does not have to struggle with keeping the eyelids open.

A shell is needed to contain the "water bath",  and you will need an A-scan unit that allows the corneal and retinal gates to be adjusted. Two types of shells are currently available.

The Hansen Shells

These are also know as Ossoinig shells. They are plastic cylinders with a flange at the end that touches the eye. They come in five sizes from 16 to 24mm in diameter to fit different size eyes. Hansen shells can be purchased in a set of three ($150), or a set of five ($200), from Hanson Ophth. Dev. Corp. in Iowa City (319-338-1285).

The patient must be fully reclined when using the Hansen shell. The shell is filled with saline, methylcellulose, or a 50/50 mixture of saline and methylcellulose. Practitioners say that it takes a while to learn to use these shells because there is no support for the probe and a steady hand is required.

There is a 90 minute video that demonstrates immersion A-scan using the Hansen shells.  It was produced by Myra Cherchio COMT of St Lukes Cataract and Laser Surgery Center of Tarpon Springs, Florida ($107, 800-282-9905).

The Prager Shell

2 Recline the patient.  It is helpful to have a fixation target, such as an "X", marked on the ceiling.  The patient will not be able to see the internal fixation light of the probe.

4 Retract the lids and place the shell on the eye.  An edge of the shell can be placed in the lower fornix first, with the patient looking up. The upper lid must then be retracted enough for the shell to fit between the lids, with the patient looking down.

The size (diameter) of the Hansen shell is selected according to what will fit between the patient’s lids. The shell is positioned so that the cornea is centered.

Place a paper towel or some tissues at the outer canthus to catch any drips. Tell the patient to keep both eyes open.

5 Fill the shell with an appropriate liquid.  With a Hansen shell use saline, methylcellulose (the thick solution used for gonioscopy, such as Goniosol), or a 50/50 mixture of the two. Some practitioners prefer to put in methylcellulose first, because it forms a better seal around the bottom of the shell to prevent leaks. The less expensive saline solution is then used to fill the shell the rest of the way.

The Prager shell uses saline.

Avoid air bubbles. You will want to fill the shell close to the top. If you use a partial fill, the probe tip is too close to the cornea and the probe tip spike may be confused with the cornea spike on the scan.

6 Obtain a proper scan.  Place the tip of the probe in the liquid, above the cornea. Aim the probe along the axial axis of the eye. Confirm proper alignment by observing the scan.

It will take some practice to maintain a cornea spike because the spike will appear only when the probe is aimed perpendicularly to the cornea. It is easier to find the cornea spike using the Prager shell because the probe is held in place by the shell. The cornea spike may appear as a closely separated double spike, representing the anterior and posterior corneal surfaces.

7 Evaluate the scan.  There will be five spikes instead of four, because the probe is not touching the cornea. The A-scan display must be adjusted so that all five echoes (probe tip "A", cornea "B", anterior lens "C", posterior lens "D", retina "E") are visible. Orbital fat echoes (F) should also be visible.

There must be adequate separation between the probe tip echo and the cornea echo (G) so that the measuring gate or light does not measure from the probe tip echo. Good separation is achieved by holding the probe tip away from the cornea.

Care must be taken to make sure that the measuring gates or lights of the particular instrument are in proper position to measure the cornea, lens, and retina echoes. Some instruments do this automatically, some will have to be manually adjusted, and some instruments cannot be used for immersion A-scans.

Each scan must be evaluated according to the criteria for a good scan of the optical axis. Once an acceptable scan has been obtained, repeat as necessary for confirmation.

8 Remove the shell.  Place tissues at the patient’s temple, retract the lids and gently remove the shell. Have the patient slowly close the eye to push out the liquid. Rinse with an irrigating solution.

1  The immersion technique does not guarantee consistency when comparing scans on the same eye, although results will be more consistent.

2  Automatic A-scan instruments do not always correctly identify the spikes. You may have to manually adjust the gates or lights on your scans.

3  It is difficult to tell if the probe is aligned properly by looking at the probe and the eye in the shell. Patient fixation with the fellow eye is an important aide to proper alignment. If the patient has difficulty with fixation, the contact method may have to be used.

4  One immersion technique user noted that the post-op results were off by a consistent amount when using the manufacturers suggested A-constant. She had to adjust by adding .5D to the calculated IOL power until there were enough cases to customize the A-constant.

Ophthalmic Echography, by Cynthia Kendall, published by Slack, Inc. 800-257-8290. slackinc.com