Dual Scheimpflug Imaging Yields Accuracy for Refractive Planning

Primavista is a refractive surgery-oriented practice located in Rome, where approximately 200 ophthalmologists perform procedures, including corneal and intraocular refractive surgeries. Each year, our ophthalmologists perform approximately 3,500 excimer laser and intraocular refractive procedures, such as refractive lens exchange and refractive cataract surgeries. In addition to my teaching duties as a Professor in the College of Optics and Optometry at the Università degli studi di Torino, I am the General Manager of Primavista.

One of the diagnostic instruments that we use on all refractive surgery patients at Primavista is the Galilei Dual Scheimpflug Analyzer (Ziemer; Port, Switzerland). The dual Scheimpflug device is essential in our practice to document the condition of the anterior and posterior corneal surfaces and the pachymetry of the cornea before and after refractive surgery.

During preoperative examination, analyzing the posterior surface of the cornea helps to identify and exclude patients with preclinical or forme fruste keratoconus and determine the shape of surgically altered corneas. The device's pachymetry feature allows us to locate the thinnest points of the cornea prior to LASIK or other corneal surgeries. The Galilei can also be used to measure anterior chamber depth before implantation of phakic IOLs.

Postoperatively, the Scheimpflug analysis helps us to evaluate the effect of the excimer laser on corneal tissue and to analyze regression and other complex or complicated outcomes.

Additionally, we use the Galilei to evaluate the extent and depth of corneal opacities, determining the correct parameters for excimer laser therapeutic procedures. We also use it for designing anterior lamellar keratoplasty procedures with the IntraLase FS femtosecond laser (Advanced Medical Optics, Inc., Santa Ana, California).

DUAL SCHEIMPFLUG IMAGING
We believe the Galilei is the most accurate instrument available to study the elevation, curvature, and pachymetry of the human cornea. Its principal advantage over other instruments is its combination of two rotating Scheimpflug cameras plus Placido ring-based technology. This allows the instrument to perform 3D analysis of anterior segment structures with a high degree of accuracy.

With single-camera Scheimpflug imaging, measurement of corneal thickness can vary based on decentration of the slit lamp. Decentration, which is frequently seen in real-world situations, can have a large effect on measured corneal thickness, introducing errors of as much as 30 µm.

Figure 1 shows an example of the effect of decentration in a model eye. In Figure 1A, the two cameras are centered, and the two Scheimpflug images of the cornea have approximately the same thickness. In Figure 1B, when the cameras are slightly decentered, the left Scheimpflug image of the cornea appears thicker and the right image appears thinner.

With dual Scheimpflug cameras, when the results from both cameras are compared, the error function still depends on decentration of the slit light, but the errors in the two cameras have opposite signs. By simultaneously looking at the same slit beam from both sides and averaging the corneal thickness readings, the resulting error becomes virtually zero. Double Scheimpflug imaging is thus capable of delivering highly accurate pachymetry data (Figure 2).

Another advantage of the dual Scheimpflug imaging system is that the cameras only need to rotate 180° to achieve full 3D measurement.

ADDING TOPOGRAPHY, TRACKING
The drawback with using a Scheimpflug-only system is that resolution of the height data on Scheimpflug images is insufficient to detect small changes in curvature between two surfaces. To address this problem, the Galilei also includes Placido ring technology. With data from Placido and Scheimpflug imaging merged, high accuracy can be obtained for both height and curvature data over the entire corneal surface.

Additionally, the instrument accurately tracks and compensates for patients' eye motion. The Galilei locates and tracks a patch on the eye, matching its location on every scan with a reference location, and it calculates the shift for each scan with a high degree of precision. Motion in the z-axis caused by vibration is also corrected using a complex calculation.

The Galilei is the ideal diagnostic imaging instrument for a high-volume refractive surgery practice. Its high-resolution Scheimpflug images provide accurate corneal pachymetry and topography, 3D analysis of anterior segment structures, densitometry, and pupillometry.

Dual Scheimpflug imaging on the Galilei improves the recognition of the posterior corneal surface and ensures accurate pachymetry. The combination of Placido and Scheimpflug technologies provides highly accurate elevation and curvature data over the entire corneal surface. The proprietary eye tracker provides reliable correction to compensate for ocular motion.

These capabilities are vital for documenting corneal parameters in routine refractive surgery cases, and they are all the more important in complex cases, such as identifying the depth and shape of a cornea that has undergone previous refractive surgical procedures.

Mauro Zuppardo, OD, PhD, is a Professor at the Università degli studi di Torino, Italy, Department of Mathematics, Physics and Natural Science, College of Optics and Optometry. He is also General Manager of Primavista, Rome. Dr. Zuppardo states that he has no financial interest in the products or companies mentioned. He may be reached at +39 0642013537.