Artemis 2 provides 'unprecedented' diagnostic readings

By Ana Hidalgo-Simón MD, PhD

NICE - New technical progress suggests ultrasound diagnostic tools may become the slit-lamp of the 21st century, Dan Z. Reinstein MD told a clinical research symposium at the XX ESCRS Congress.

Dr Reinstein highlighted the potential of the new VHF Digital Ultrasound Arcscan Artemis 2TM across a range of anterior segment applications.
With the ability to reveal sub-surface micro-anatomy within the anterior segment and discern epithelial from stromal components within the cornea, the instrument promises to be the 'crystal ball' anterior segment surgeons have been waiting for, he said.

"No neurosurgeon would operate on a patient without a decent MRI. In the same way, our most direct path towards proper surgical management is going to come from the best and most accurate diagnostic information before surgery," Dr Reinstein said.
He described the accuracy and precision of the measurements as "unprecedented". M-scan studies within the cornea allowed investigators to determine that the interface localisation precision of the system has less than 1.0 micron of standard deviation (±0.87 µm).

This was shown experimentally to translate as the ability to make measurements within the cornea with a precision of approximately 1.0 micron.
Scanning is performed in multiple meridians, therefore providing data in 3-D. Based on this data, Dr Reinstein developed 3-D mapping techniques for individual corneal layers before and after LASIK.

A new diagnostic tool promises to provide the most accurate preoperaztive micro-anatomical information within the cornea for anterior surgeons, according to a US ophthalmologist.
The new VHF Digital Ultrasound Arcscan Artemis 2TM allows measurements to be made within the cornea with a precision of approximately 1.0 micron and 3-D scanning can be performed in multiple meridians.

It is also possible to determine the angle-to-angle or sulcus-to-sulcus dimensions of an eye, with approximately 100 micron precision and map these in 3-D for localisation of the largest axis, Dan Z. Reinstein MD said.
"We also spent considerable time developing other features that are critical to using this technology as a surgical planning device. For example, there is simultaneous optical infrared video image of the eye with each ultrasound scan.

"This is a crucial element of the system because knowledge of the position from which a scan was taken is important if scan measurements are to be used for surgical planning, both for the cornea and phakic IOL placement in the anterior segment," he added.
For LASIK surgery, the system has the advantage of being able to detect the epithelial thickness separately from the stromal component of the flap and from the residual stroma in 3-D.

All these measurements - with approximately 1.0 micron precision and in 3-D - allow the creation of maps of the thickness of each layer.
"This permits currently the most accurate planning of surgical interventions and follow-up of the evolution of each corneal component postoperatively. We can also determine very precisely the efficacy of microkeratomes in 3-D - not just at one point," Dr Reinstein explained.

In a study comparing central pachymetry by Orbscan with the Artemis 2, Dr Reinstein and colleagues found an estimated improvement of 25 microns (of standard deviation) in the accuracy of pachymetry by Orbscan compared to the Artemis 2 technology with its state-of-the-art 3-D pachymetry.

This same study showed that 36% of eyes were significantly overestimated by Orbscan, while 17% of patients were underestimated by Orbscan.
Patients overestimated could, in principle, be incorrectly deemed to be candidates for LASIK, while some would be rejected on the basis of an incorrect low thickness measurement.

"This is good for business on two fronts; good candidates are not turned away, while patients with corneas too thin are detected accurately" Dr Reinstein added.
Dr Reinstein has studied microfolds in Bowman's membrane and was able to differentiate two sub-types: Bowman's cracks, possibly caused by traumatic elevation or repositioning of the flap, and what he called 'True Microfolds' which are actually grooves in Bowman's caused by inadequate flap distension or repositioning.

He pointed out that although both lead to loss of best corrected vision, the re-lifting of patients with Bowman's cracks will not improve vision and could potentially worsen it. Therefore distinguishing microfolds form cracks becomes very important.
Such studies also led to the modification of the flap handling to a minimal-touch technique he called the 'door-opening' and 'door-closing' manoeuvres.

He also discussed his group's findings regarding the wound healing process within the cornea after LASIK and PRK, separating epithelial factors from biomechanical factors. They found that the epithelial response was unexpectedly complex. When they plotted the amount of myopia treated against the epithelial power shift produced by the surgery, they found that the epithelial response was non-linear and biphasic.

"At lower levels of myopia the epithelial power shift causes more and more myopisation with the level of myopia treated. But that is true only up to a certain point. Treating higher levels of myopia, a paradoxical epithelial hyperopic shift appears to take place," he reported.

The researchers concluded that the actual corneal power change in LASIK is degraded by an average biomechanical shift of 15%. That combines with the observed epithelial effect which accounts for about an extra 5% of variation in the final postoperative refractive error.

"This comes to a possible total of 20%. The refractive errors we are observing after LASIK can be explained by epithelial and biomechanical factors.
"And an understanding of these processes, combined with adequate modelling and predictive calculations, will enable us to further improve the accuracy of customised ablation procedures," he predicted.
Customised re-treatment
There has been a tremendous amount of interest in the use of custom ablation to repair corneas.

However, the epithelium tends to compensate for stromal surface irregularities (it is almost always thicker in the 'crevices' than over the 'bumps') and therefore the epithelium masks the true topographic and/or wavefront error being caused by an asymmetric stromal surface.

According to Dr Reinstein, systems based on topographic guidance have failed to adequately provide the surgeon with the accurate tools needed to help these patients.
This new Artemis 2 technology also promises to improve the understanding of the physiological dynamics of the anterior segment which will have implications for the insertion of IOLs.

The instrument allows the measuring of internal dimension changes from light to dark and during accommodation as the pupil and ciliary body shift position. This could be particularly important for the long-term safety of angle-supported or posterior chamber IOLs.

"In a study in which we looked at the correlation between white-to-white measurements with sulcus-to-sulcus distances, we found no significant correlation for myopic and hyperopic eyes.

"For angle-to-angle association with white-to-white measurements, we found no correlation for myopes but a weak, though significant, correlation in hyperopic patients. Unfortunately, hyperopic patients are the least likely candidates for an angle-supported IOL," Dr Reinstein said.

In presbyopic insert surgery, Dr Reinstein pointed out that the ability to accurately measure scleral thickness is going to be a very important issue because the depth of implantation and position of the implant relative to the lens equator are key success factors.

The scleral thickness and localisation of the zonular plane can be accurately measured using a special software routine developed specifically for this application.
"Using the optical image to ultrasound scan correlations, we have developed special software that enables us to calculate the exact plane on which the presbyopia inserts should be located to achieve maximum effect," he explained.
Dr Reinstein is collaborating with Jack T. Holladay MD on developing a new formula which will incorporate this improved measure of anterior chamber depth.

"We are dealing here with a technology which is enabling ophthalmologists to see beyond the surfaces they have been examining in the last few decades with microscopes. We are now entering an era of sub-surface anatomy and micro-structure to better plan surgery and treatment modalities," Dr Reinstein said.

The VHF Digital Ultrasound Arcscan technology, now embodied in the Artemis 2, was developed at Cornell University by Dr Reinstein with co-workers Ronald H. Silverman MD and D. Jackson Coleman MD.

Dr Reinstein, Dr Silverman and Dr Coleman have a financial interest in Ultralink LLC, the company producing and marketing the Artemis 2. Artemis 2 has received US FDA approval and is commercially available.

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