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Refractive Surgery | Apr 2011

The Advantages of Intraoperative Aberrometry

The aberrometer helps to optimize cataract surgery and thus avoid the need for an enhancement.

Wavefront aberrometry is a widely used method of determining refractive error in the eyes of corneal refractive surgery patients. First employed as a diagnostic tool, wavefront aberrometry has emerged as the dominant means of refraction in LASIK, with the wavefront data used to program the excimer laser. The benefits associated with this technology’s ability to fine-tune corneal refractive outcomes are so profound that intraoperative aberrometry has now spread to cataract surgery.

The ORange Intraoperative Wavefront Aberrometer (WaveTec Vision Systems, Inc., Aliso Viejo, California) could change the way we treat patients. Unlike a traditional aberrometer, the ORange was designed specifically for use in the operating room during cataract surgery. The system allows real-time analysis of refraction, astigmatism, and aphakic lens power during the cataract procedure so that we may make adjustments while the patient is on the operating table.

Enhancements have become a common part of premium IOL surgery. If cataract surgery does not result in the targeted refraction, treatment with an excimer laser can generally achieve the desired visual outcome. By allowing us to obtain a refraction in the operating room, the ORange nearly negates the need for enhancements and thus boosts patient satisfaction. This technology also has the potential to increase the predictability of limbal relaxing incisions (LRIs), augment the accuracy of toric IOLs, and better predict the IOL power needed. The aberrometer calculates a precise refraction by using two interconnected pieces of hardware. The unit is composed of an optical head that attaches directly to the surgical microscope and is connected to the ORange’s processor and touchscreen monitor (Figure 1). At any point during surgery, we can capture a wavefront measurement. The processor then analyzes the captured data to calculate the refractive value of the eye.

IN THE OPERATING ROOM

Prior to surgery, patient data are entered into and stored on the computer. At the time of surgery, I retrieve these data and choose the appropriate function from the ORange’s menu (refraction, LRI, toric IOL, and IOL power calculation). To obtain a refraction, I instruct the patient to fixate on an internal red light, which allows me to go through both coarse and fine alignment to ensure that the ORange is focused on the corneal apex and visual axis. The coarse alignment involves positioning the crosshairs on the screen within the parameter of the four light-emitting diode dots. In the fine alignment, the goal is to center the crosshairs in the center of the reticule. Only then will the system capture the aberrated wavefront images. The ORange will acquire and analyze 40 images in approximately 20 seconds.

TIPS

The best wavefronts are obtained under normal topical anesthesia, rather than a retrobulbar block or conscious sedation. For intraoperative readings, I recommend pressurizing the eye well, either with balanced saline solution or an ophthalmic viscosurgical device but not a combination of the two. Wound construction is crucial, because a leaky incision will not maintain pressure during wavefront measurements. Some clinicians may have concern regarding the effect of corneal hydration on readings. I suggest not performing stromal hydration of the corneal wounds prior to acquiring the wavefront measurement. Intraoperative corneal hydration is usually minimal and has little effect on the quality and accuracy of the readings. Corneal edema generally begins in the hours following cataract surgery.

Initially, it seemed as if the ORange would be a useful method for checking the postsurgical refraction. Although the unit can perform this function, I have found it difficult to achieve the exact effective lens position of the IOL when inflating the anterior chamber for readings. This variability in postimplantation spherical readings prompted the development of the second-generation aberrometer head, which allows aphakic readings. Because astigmatism is not greatly affected by effective lens position, the ORange system has always been accurate for LRIs and toric IOLs.

LRIs

The use of LRIs has been fraught with variability. The procedure’s low cost and simplicity allowed it to survive the advent of laser vision correction. By allowing us to acquire preoperative measurements in the operating room, the ORange eliminates the need to mark the patient’s eye preoperatively. When LRIs are to be performed in conjunction with cataract surgery, the unit automatically and accurately accounts for any surgically induced astigmatism. In the past, we relied on LRI nomograms to help predict outcomes. Now, we can make the incisions and acquire a post-LRI refraction. If this reading shows too much residual cylinder, we can deepen or extend the LRIs to help avoid the need for a postoperative enhancement. When combining cataract surgery and LRIs, I prefer to take an initial refractive reading with the ORange before penetrating the eye. I check whether this information is similar to the preoperative refraction and aligns with the corneal topography. Next, I create an LRI that is smaller than I would normally use, which allows suitable initial depth resulting from a firm eye. I can easily lengthen the incision later if necessary. After implanting the IOL, I fill the anterior chamber with balanced saline solution until the eye is firm. I then perform intraoperative wavefront aberrometry to see if the LRI(s) must be extended. I can repeat this process until I achieve the desired result (Figure 2).

TORIC IOLS

Corneal markers help us to align a toric IOL, but all of them depend on preoperative measurements. Even the different toric calculators assume a set amount of induced astigmatism from the corneal incision, but the steepness and thickness of the cornea varies. In eyes with irregular astigmatism from keratoconus, pellucid marginal degeneration, or previous radial keratotomy, these toric calculators can be grossly off on the optimal axis of treatment.

An initial set of readings with the ORange while the eye is aphakic tells me the appropriate IOL power, the ideal axis of astigmatism, and the true amount of cylinder needed. Whereas current toric IOL calculators are conservative on cylindrical power, using the ORange has shown me that a large percentage of patients would benefit from an IOL of the next largest cylindrical power than the chosen preoperatively. I perform aberrometry again after implanting a toric lens to provide me with the refraction and to indicate the direction of rotation needed to optimize lens placement (Figure 3). I repeat the process after rotating the IOL accordingly and have found that doing so yields impressive results.

SUMMARY

Obtaining real-time refractive data allows me to act immediately to improve visual outcomes and reduces the number of postoperative enhancements I perform. The minimal extra time in the operating room has resulted in cost savings by decreasing the time I spend in the examination lane and on enhancements. The real price of suboptimal outcomes, however, is a loss of referrals.

Farrell “Toby” Tyson, MD, is the Medical Director and CEO of Cape Coral Eye Center in Cape Coral, Florida. He states that he has no financial interest in the products or companies mentioned. Dr. Tyson may be reached at tel: +1 239 542 2020; e-mail: tysonfc@hotmail.com.

TAKE-HOME MESSAGE
• The ORange can capture a wavefront measurement at any point during surgery.
• Obtaining real-time refractive data allows immediate intraoperative corrections that can improve outcomes and avoid enhancements.

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