Diagnostic instruments may offer more sensitive response

Technological advances in equipment available refine clinical results.

---Testing is performed on the functional prototype Zeiss ALM.

Diagnostic and test equipment has become more sensitive, allowing surgeons to refine their IOL calculations and even increase productivity in the clinic.

Diagnostic equipment can not only improve clinical accuracy, but also boost patient volume, said Daniel S. Durrie, MD.

Refractive surgeons often overlook patient flow as they seek ways to increase patient volume, Dr. Durrie said. Better diagnostic instruments can improve refractive quality outcomes and provide results more quickly.

Dr. Durrie worked with his physician practice management company (PPMC), NovaMed, and bought two Epic 2000 (Marco Technologies, Jacksonville, Fla.) units to increase the speed and quality of refractions. One unit costs $60,000, yet he will add a third.

NovaMed provided capital and advice, and also did patient flow studies. He relied on the PPMC because it was difficult to convince his partners that he needed to buy two $60,000 refractors and two $40,000 topographers to increase profits.

“They are a good value because the patient likes them, it speeds the work-up and it requires less training for the technicians,” he said.

Dr. Durrie conducts manifest refractions on the Epic unit. New patient exams and 3-month exams involve cycloplegic exams by the physician. The combination of those exams and another diagnostic instrument, the Orbscan (Bausch & Lomb Surgical, Claremont, Calif.), provides him with all the information needed.

He needs two Orbscan units to conduct 450 exams a week.

“If someone wants to do refractive surgery and have the patients happy with their overall experience, then they have to be willing to purchase the necessary equipment required to have good flow in the clinic,” Dr. Durrie said. “These patients that are seeing well, you want them to come back and not have them tell their friends they had to wait an hour. It’s a better overall experience for the patients.”

Now he can see 120 patients per day and not rush them through the process. He was only able to do 80 per day before using the diagnostic equipment.

“We had unhappy patients, we didn’t get a lunch hour and we were getting done at 5:30,” he said. “Now we get done at 4:30 and have a lunch hour and have happy patients.”

LASIK pachymeter

Optical biometry Principle: dual beam partial coherence interferometer

Increased accuracy in measurement is one goal behind an updated laser pachymeter being offered by Paradigm Medical (Salt Lake City). The device, the P-55L, uses a new handpiece and transducer to increase sensitivity to 195 µm. The previous model, the P-55, has a sensitivity of 270 µm.

The extra sensitivity will allow surgeons to use the device on patients who need between 3 D and 15 D correction, for example, instead of the previous 6 D to 15 D range, said Robert Hall, director of marketing at Paradigm.

The device uses 99 measurement points and a transducer that runs at 20 MHz, instead of the previous 12 MHz.

Paradigm wanted to make a pachymeter to detect thinner corneas because laser in situ keratomileusis (LASIK) surgeons demanded one.

“When you go through this thinning process with LASIK or PRK [photorefractive keratectomy], everyone wants to measure it,” he said. “Surgeons also are running into more thin surfaces as they perform more corneal surgery. There’s a real need to measure thinner membranes as they run into a wider cross section of patients for the LASIK procedure.”

The new probe will reflect the cost of the better transducer, which is more difficult to make. The new device will start at $6,995. The company will still offer the previous model for $3,000 less. General ophthalmologists or optometrists could use this less sensitive unit to check for corneal swelling in extended-wear contact lens patients.

“It’s because LASIK is such a popular and growing procedure that we’re modifying some of our equipment to reflect that we are LASIK capable,” Mr. Hall said.

Post-phaco refractions

Optical and acoustical biometry Optical and acoustical axial length

While refraction after laser surgery is generally stable, refractions after cataract surgery are generally not as accurate, one study concluded. Daniel Byles, FRCOphth, conducted the study to detect refractive surprises while the patients were still on the operating table.

Immediately identifying the incorrect lens power would allow the surgeon to correct the error before the patient left the operating room, Dr. Byles said.

“Preoperative biometry is becoming increasingly accurate; however, postoperative refractive surprises can still occur,” Dr. Byles said. “If refraction at this stage was able to reliably predict refraction at final discharge, then it would be possible to detect refractive surprises before the patient has left the operating theater. Corrective action such as implant exchange can then be undertaken, preventing the patient from having to return to the theater at a later date.”

According to company literature, the Nikon Retinomax 2 hand-held autorefractor can be used in any position and takes measurements in 0.12 seconds. A “Quick Mode” function allows for measurements from 0.14 seconds to 0.07 seconds.

Dr. Byles conducted a prospective study on patients undergoing phaco with a foldable acrylic implant by a superior, corneal incision. The anterior chamber was inflated with balanced salt solution at the end of the procedure to a pressure adequate to allow the wound to close.

At the end of surgery, an autorefraction was performed with a Retinomax autorefractor. Suggested refraction was then carried out at the patient’s final outpatient screening for discharge.

Dr. Byles conducted refractions on 57 patients who underwent phaco and received a posterior chamber IOL. The majority of patients were performed under peribulbar anesthesia. About one-third were done under general anesthetic. Only two patients had a suture placed at the end of surgery and these were removed at least 1 week before final refraction.

Refraction was done immediately postoperatively and at a mean of 10 weeks later, although some patients were followed as soon as 1 week. The two readings were compared.

Sixty-one percent of Retinomax readings were within 1 D of the final refraction. Eighty-eight percent were within 2 D. The mean absolute error between the two was 1.04 D with a standard deviation of 0.92 D. Twenty-four patients had repeated readings taken with the Retinomax. The mean difference between readings was 0.8 D with a standard deviation of 0.55 D.

A prediction of final refraction shows no systematic bias and that overall the mean spherical equivalent at the end of surgery was the same as the mean final spherical equivalent. However, the mean error of each reading was ± 1 D with a range from –3 D to +5.6 D, and a standard deviation of 0.92 D.

Readings within the operating theater were within 1 D of final refraction in only 58% of the cases and 88% were within 2 D. When compared with the accuracy of routine preoperative ultrasound biometry and the SRK-T formula, autorefraction showed a higher mean error than standard deviation and was predictive within 1 D or 2 D of the final outcome.

“It should be noted that only three patients had an outcome within 2 D away from that predictive biometry,” he said. “There was more than 3 D in seven patients. The highest error was 5.6 D more hypermetropic in the final outcome. Clearly, this means that autorefraction under these conditions is not able to identify refractive surprises reliably.”

Variable conditions

Variable measurement conditions in the operating theater appear to be the main reason for the inaccurate prediction on final outcome, Dr. Byles said.

Problems included the difficulty in accurately aligning the autorefractor and globe in the presence of peribulbar and general anesthesia. The second main factor is variation in anterior chamber inflation at the end of surgery. Excessive inflation to achieve a watertight wound seal induced hypermetropia, plus the shallower anterior chamber in the softer eye induced an apparent myopia, he said.

Manipulation at the corneal rim also could influence readings by inducing irregular astigmatism in the immediate postoperative period.

“Under the conditions of the present study, the Retinomax does not provide a reliable means of avoiding postoperative refractive surprises,” Dr. Byles said. Accuracy would increase with more standardized anterior chamber inflation, with topical anesthesia and by fixation and accurate alignment.

Takeshi Kamiya, product manager of ophthalmic instruments for Nikon USA, said that the Retinomax gave good data as soon as 1 minute after LASIK, but not immediately after phaco.

“Right after surgery, the eye changes a lot,” Mr. Kamiya said. “It’s not stable. That’s why a physician can’t get good data.”

Still, many physicians ask him how to sterilize the Retinomax so they can bring it into the operating room. The electronics make that difficult, so many buy a cover for the device and then bring it in, he said.

A new device

Another biometry device uses light, not ultrasound, to develop a more accurate way of measuring the eye.

Wolfgang Haigis, PhD, MS, a physicist with the University Eye Clinic Wuerzburg in Germany, helped develop the IOLMaster biometry device (Carl Zeiss Jena GmbH, Jena, Germany), a non-contact instrument due for release in Germany this year.

Dr. Haigis is head of the Zeiss biometry laboratory and for the past 20 years has studied ultrasound, ultrasound biometry and optical lens calculation.

The instrument, a two-beam partial coherence interferometer, measures axial length, anterior chamber depth and corneal radius in 0.4 seconds and could aid surgeons’ efforts to select the right IOL power.

The basic principle of the device originated from the laboratory of Prof. Adolf F. Fercher in Vienna.

The device produces two beams by means of an optical delay path. These two beams of short coherence light are reflected onto the surfaces of the eye of the cornea and then the retina. If the delay path is equal to the optical pathway of the eye, interference results and the signal can be evaluated.

“We have to make clear that there is a difference between what we measure acoustically and what we measure optically,” Dr. Haigis said. “Acoustically we measure the distance of the inner limiting membrane, whereas optically the dominant signals stem from the retinal pigment epithelium. So optically our results will end up in a longer axial eye length.”

Researchers compared biometric and keratometric measurements of the IOLMaster to the Grieshaber Biometric System to calibrate the machine. Research on the IOLMaster correlated optical and acoustical measurements in 500 eyes. This assessed the machine’s operations and allowed researchers to calibrate the instrument. They then selected another 155 cases and made comparative measurements.

Dr. Haigis said the procedure was reproducible, with a mean deviation of 10 µm between acoustically and optically derived measures of axial length.

Immersion ultrasound reproducibility, measured for five consecutive axial length measurements, was 22 µm ±24 µm. The IOLMaster had a mean reproducibility of 23 µm ±15 µm.

“The new biometry device provides highly accurate measurements of axial length, corneal radii and anterior chamber depth,” Dr. Haigis said.

For Your Information:

• Daniel S. Durrie, MD, practices at the Eye Center of Kansas City, 55200 College Blvd., Overland Park, KS 66211; (913) 491-3737; fax: (913) 491-9650. Dr. Durrie is the director of refractive surgery for NovaMed. Dr. Durrie has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Wolfgang Haigis, PhD, MS, is a physicist with the University Eye Clinic Wuerzburg, Universitats-Augenklinik, Josef-Schneider-Str.11, D-97080 Wurzburg, Germany; (49) 931-201-5640; fax: (49) 931-201-2454. Dr. Haigis has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Robert Hall can be reached at Paradigm Medical, 1127 W. 2320 South, Ste. A, Salt Lake City, UT 84119; (800) 742-0671; fax: (801) 977-8973. Mr. Hall is director of marketing at Paradigm Medical. Mr. Hall did not participate in the preparation of this article.
• Daniel B. Byles, FRCOphth, can be reached at Bristol Eye Hospital, Lower Maudlin St., Bristol, BS1 2LX, England; (44) 1179230060; fax: (44) 1179284856. Dr. Byles has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Takeshi Kamiya can be reached at Nikon USA Instruments Division, 1300 Walt Whitman Road, Melville, NY 11747-3064; (800) 438-8782; fax: (516) 547-5251. Mr. Kamiya is product manager of ophthalmic instruments at Nikon USA. Mr. Kamiya did not participate in the preparation of this article.