Novel imaging device assesses tear film layer to increase likelihood of cataract surgery success
Over the past two decades, we have been able to improve the outcomes of cataract procedures considerably, but the accuracy of the refractive prediction is still far from perfect. In an endeavor to further reduce postoperative refractive errors, information concerning the true state of the ocular surface, which is the first refractive layer of the cornea after all, has become more essential. Since the cornea accounts for about two-thirds of the total dioptric power of the eye, small variations in the measured corneal shape, that can be caused by ocular surface irregularity, may have a large effect on the recommended power of an IOL.1 (See “The Tear Film: An Overview,” below.) Using reflection-based technology, the Cassini Ambient topographer allows for accurate cylinder management for astigmatic keratotomies and toric IOLs by measuring the shape, state, and quality of the tear film.
Here I discuss, how, specifically, the device accomplishes this.
THE TEAR FILM: AN OVERVIEW
The ocular surface is covered by a thin, liquid film called the tear film. The composition of the tear film is complex and plays an essential role in nourishing and protecting the cornea.
The tear film has 3 distinct layers: the top hydrophobic lipid layer, made of a thin sheet of lipids that reduces surface tension, helps to spread the tears after each blink, and protects the underlying aqueous layer from evaporation; the aqueous layer, which is the thickest layer of the tear film, and plays an important role in the oxygenation of the cornea; and the mucous layer, which compensates for corneal unevenness and reduces friction during blinking.2
In addition to nourishing and protecting the cornea, the tear film forms a smooth refractive surface over the uneven corneal surface. At each blink, the tear film is refreshed and goes through a dynamic tear build-up phase to form a tear film layer.
Local rupture in the hydrophobic lipid layer exposes the aqueous layer directly to air, leading to high tear-evaporation rates that potentially produce rupture of the tear film.3 The time between the formation of the tear film build-up and break-up of the tear film depends on the quality of the tear film (mix of lipids and water), the environmental conditions, and the pathology of the cornea.
Stability of the Tear Film
Ocular Surface Qualifier (OSQ) (Figure 1), Ocular Surface Dynamics (OSD), and Ocular Surface Visualization (OSV) recording data are presented in a complete overview in the Cassini software interface, providing immediate insight into the status of the patient’s tear film layer.
FIGURE 1: No tear film irregularity, as captured by the Ocular Surface Qualifier.
IMAGE COURTESY DEE STEPHENSON, MD, FACS, ABES, FSEE
The Ocular Surface Visualization (OSV) recording is a significant educational tool for our patients to better understand their ocular surface health and diagnosis prior to refractive cataract surgery.
Healthy tear film layers appear very even and reflect light like a convex mirror. Tear film layers that tend to break up, or evaporate quickly, appear very uneven, leading to a distortion of the reflective points (Figure 2).
FIGURE 2: Irregular tear film, as shown via Ocular Surface Qualifier.
IMAGE COURTESY DEE STEPHENSON, MD, FACS, ABES, FSEE
If irregularity is diagnosed, surgeons can then carefully assess the ocular surface dynamics and actual behavior of the tear film before continuing with power IOL calculations.
Patented Ocular Surface Technology
The Cassini Ambient employs a dual modality system for imaging of the human eye in both the visible and infrared spectrum.
Specifically, a multitude of colored and infrared LEDs illuminate the entire shape of the anterior surface, centrally and including the peripheral zone, enabling the surgeon to view a reconstruction of the patient’s cornea.
The device projects a signature pattern of color LEDs off the convex mirror constituted by the tear film toward the device’s RGB camera to ensure a direct relationship between each image point and a corresponding source point.
When the surface of the cornea is smooth, each projected color LED appears regular in the image, forming a pristine shape (Figure 3). However, during the inner blink period, the tear film changes dynamically; producing localized “dry” regions, which leads to distorted shapes of the projected color LEDs. To capture the transition from a pristine shape to distorted reflection, the Cassini Ambient processes every frame and monitors the uniformity of every reflected color LED. The distorted LED reflections are then marked to indicate potential dry regions by displaying an active orange overlay. (Figure 4)
FIGURE 3: Smooth tear film, as captured by Ocular Surface Dynamics.
IMAGE COURTESY DEE STEPHENSON, MD, FACS, ABES, FSEE
FIGURE 4: Note the distorted LED reflections.
IMAGE COURTESY DEE STEPHENSON, MD, FACS, ABES, FSEE
FIGURE 5. Smooth tear film layer
FIGURE 6. Ocular Surface Dynamics (OSD) 2 second recording
FIGURE 7. Ocular Surface Visualization (OSV) 10 second recording
Something else to keep in mind: From a keratometry reading point of view, unstable tear films affect the measured radius of curvature significantly.4 In this context, the device can be used to assess the dynamics (stability) of the tear film by recording the corneal reflection of its projected LED pattern over time.
The shape of the surface is revealed as a linear combination of Zernike polynomials. Using ray-tracing methodologies and the law of reflection, the polynomial coefficients are iteratively updated until the differences between the angles of incidence and the angles of reflection are minimized in a least squares sense — and the surface shape is determined.5 Skew ray errors are, thus, avoided and, in combination with the sampling density, this allows for highly detailed and accurate surface measurements — especially considering the axis of astigmatism and higher-order aberrations.4
Effective and Efficient
In conclusion, I find this technology to be an incredible tool for identifying real-time ocular surface disease. The Cassini Ambient provides us, as surgeons, with an accurate and detailed description of the cornea, which helps us to improve our surgical outcomes, reduce the number of postoperative surprises we get, and lessen the number of patients requiring follow-up corrective treatments. Using a unique point-to-point, re-flection-based technology, with color and infrared LED illumination, this device addresses important sources of corneal errors in refractive cataract surgery. CP
References
- Larkin H. Bright path ahead: examining six requirements for reducing errors and eliminating ‘refractive surprise.’ ESCRS Eurotimes website. May 1, 2017. Accessed May 5, 2022. https://www.eurotimes.org/refractions .
- Ang M, Baskaran M, Werkmeister RM, et al. Anterior segment optical coherence tomography. Prog Retin Eye Res. 2018;66:132-156.
- King-Smith PE, Fink BA, Nichols JJ, Nichols KK, Hill RM. Interferometric imaging of the full thickness of the precorneal tear film. J Opt Soc Am A Opt Image Sci Vis. 2006;23(9):2097-2104.
- Klein SA. Corneal Topography Reconstruction Algorithm that Avoids the Skew Ray Ambiguity and the Skew Ray Error. Optom and Vis. Sci. 1997;74(11):945-962.
- Snellenburg JJ, Braaf B, Hermans EA, van der Heijde RGL, Sicam VAPD. Forward ray tracing for image projection prediction and surface reconstruction in the evaluation of corneal topography systems. Opt Express. 2010;18(18):19324-19338.
