A Scheimpflug system images the anterior segment with a camera perpendicular to a slit beam, creating an optical section of the cornea and lens. The images taken during the examination are digitized in the main unit, and all image data is transferred to a computer interface bus. When the examination is finished, the computer calculates a 3-dimensional virtual model of the anterior segment, from which all additional information is derived. Approximately 25,000 data points are used to calculate topographic corneal thickness, corneal curvature, anterior chamber angle, volume, and height. Pachymetry and topography of the entire anterior and posterior surface of the cornea from limbus to limbus are calculated and displayed. Opacity in cornea and lens may be quantified. Rotating and dual-camera Scheimpflug imaging systems are in clinical use today for anterior segment imaging.
In the Pentacam (Oculus, Lynwood, WA), a rotating Scheimpflug camera takes a maximum of 2 seconds to generate a complete image of the anterior segment. Any eye movement is detected by a second camera and corrected for in the process. A 3-dimensional model of the anterior segment may be calculated from as many as 25,000 true elevation points.
The Galilei (Ziemer USA, Wood River, IL) dual Scheimpflug camera incorporates Placido disc technology to improve curvature information on the central cornea. A 3-dimensional model of the anterior segment may be calculated from many true elevation points, as described with the Pentacam.
Scheimpflug analysis generates images of the anterior segment in 3 dimensions using a noncontact method. It provides anterior and posterior surface topography of the cornea that is derived from true elevation measurements, in contrast to the Orbscan IIz (Bausch & Lomb, Rochester, NY), which derives its posterior elevation map mathematically. The decision as to whether to perform enhancement surgery may rest upon accurate depiction of the posterior curvature. This is important to the refractive surgeon, for it is believed that the Orbscan IIz overestimates elevation of the posterior curvature, especially after LASIK procedures.
The Pentacam overview display provides the ophthalmologist with a vast amount of information. This includes Scheimpflug image with densitometry measurement of cornea and lenticular opacity; camera/slit lamp position; keratometry; mean eccentricity; pupil diameter; a virtual eye image that depicts the anterior and posterior surface of the cornea as well as the planes associated with the iris and lens; anterior chamber analysis; and a color map of the physician’s choice including details such as corneal thickness, elevation, curvature, anterior chamber depth, or refractive power (Figure 7). Four-color maps may be loaded into a single display, which enables the physician to create an individual printout with the important maps needed for daily work. Keratoconus detection maps may be generated to assist in the classification and diagnosis of keratoconus prior to contact lens fitting or refractive surgery. The Holladay report is useful in determining the equivalent keratometric power in relation to pupil size in patients with previous refractive surgery and cataract. Zernike wavefront representation of the anterior and posterior surface of the cornea may be calculated based on the measured height data. A unique feature is that image acquisition time is short and does not require an expert examiner.
The Galilei image representation offers a versatile standard display that includes an anterior instantaneous curvature, pachymetry, anterior elevation, posterior elevation, pupil indices, and keratoconus detection. The final map representation may be altered to fit the physician’s specific need regarding refractive, cataract, glaucoma, or keratoconus applications. As with the Pentacam system, image acquisition time is short and does not require an expert examiner.
With the Pentacam, the interface between the anterior LASIK flap and the residual posterior bed may clearly be seen, even months to years after surgery. This provides valuable information for the refractive surgeon prior to enhancement surgery so as not to violate the residual 250 to 300 µm of posterior stroma (Figure 8). In addition, cornea and lens opacity may be graded in an objective manner with the densitometry feature, using a scale from zero to 100. The degree of cataract formation can be documented and followed over time. If there is clinical evidence of disease in both cornea and lens, it may be possible to determine which structure is most likely the cause of reduced vision.
With respect to corneal applications, Scheimpflug analysis is an excellent way to quantify the severity of Fuchs endothelial dystrophy and to assess clinical improvement after endothelial keratoplasty. Specifically, using the densitometry function of the Pentacam, a normal anterior and posterior densitometry reading of the cornea is between 25 to 30 densitometry units. An increase in the anterior and/or posterior corneal spike usually denotes clinically significant corneal edema. This becomes significant in the range of 45 to 50 densitometry units. Anatomic attachment and deturgesence of the endothelial disc may be followed postoperatively after Descemet’s stripping automated endothelial keratoplasty (DSAEK).
Prior to phakic refractive implant surgery, precise estimation of anterior chamber dimensions can be done (ie, angle width, anterior chamber depth measurement). Postoperatively, after phakic refractive implant surgery, accurate measurement of the IOL distance from the endothelium and crystalline lens may be determined, as well as the effect on the anterior chamber angle opening. Complicated formulas are no longer necessary to estimate the IOL power in patients with cataract and previous refractive surgery. With the Holladay report, the equivalent keratometric power may be determined in relation to pupil size.
Compared to Placido disc-based topography, which provides an eye image and rings to display distortion of the anterior surface, the visual information that the Pentacam provides may be more difficult to interpret initially. This issue has been overcome with the advent of the Galilei, which has incorporated Placido disc technology into its format.
With respect to image resolution, compared to anterior segment optical coherence tomography (OCT) and very-high-frequency (VHF) ultrasonography (ultrasound biomicroscopy, or UBM), iris detail and angle visibility are poor in glaucoma and refractive implant applications when using the Scheimpflug analysis. An opaque cornea will limit the accuracy of the Scheimpflug imaging, a problem that is not apparent with UBM. Calculation of the residual stromal bed (RSB) is considerably easier with anterior segment OCT and UBM, due to higher resolution with the latter. The clinical accuracy and usefulness of the Zernike representation of the wavefront aberrations produced by the anterior and posterior cornea has not been determined or compared to other types of wavefront aberrometry.