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Why surgeons need to understand how to center advanced surgical implants manually
With advancing technology in vision-correcting and vision-enhancing devices, surgeons should become more aware of the centering issues related to the implantation of these devices.
Focus on Refractive Surgery By Daniel H. Chang, MD, and George O. Waring IV, MD
From a surgeon’s perspective, centering treatments and devices in cataract and refractive surgery has previously not received much attention.
For intraocular surgery, as long as the edge of the optic remained outside of the undilated pupil, the IOL was considered centered. For corneal-based techniques, automated eye tracking and even pupil centroid shift compensation have allowed greater control of treatment centration.
But even with greater automated control, the functional center of the eye has remained a mystery to the surgeon. Developments, such as diffractive multifocal IOLs and intrastromal corneal inlays, have increased the need for surgeons to understand how to center these advanced surgical implants manually.
When discussing ocular centration, it is important to understand the ocular axes. There are four axes commonly used to describe the optical system of the eye (Figure 1). Two of them, the optical axis (Figure 1A) and the pupillary axis (Figure 1B), are defined anatomically based on ocular structures alone; two of them, the line of sight (Figure 1C) and the visual axis (Figure 1D), are defined functionally based on fixation. The angle formed by the pupillary axis and the line of sight is referred to as angle kappa (Figure 1C). It has historically been referred to as angle lambda as well.
While these four axes are good reference lines for describing the locations and relations of structures in the eye, the clinical application of these concepts should be approached with care. Since these axes are theoretical descriptions of a model eye, each axis may not be relevant to every eye.
For example, the pupillary axis and the line of sight can both change based on something as common place as dilation. Components of these axes, such as the nodal point, are nothing more than theoretic constructs for modeling purposes. Therefore, in strict terms, the visual axis is theoretical. Furthermore, the definitions of these axes fall apart (the points simply do not line up or may not even exist) when describing eyes with findings such as keratoconus, corectopia, or even aphakia.
Therefore, although useful for describing and communicating spatial positioning within the eye, the precise definition of these axes may not be as important as the method used to apply these concepts to clinical practice. The identification and alignment of structures and markers to determine position and to align an IOL or corneal inlay during surgery is more relevant than the identity of the axis that is being used.
The best location for centering an IOL is under investigation, but experience gained from corneal refractive surgery suggests that the best location is likely along the visual axis, coincident to the point corresponding to the corneal vertex, as seen by the coaxially sighted corneal light reflex. This is observed when the surgeon views the eye along the same axis as the patient’s fixation light/target. The coaxially sighted corneal light reflex is the first Purkinje reflection (PI) from that light source, which is coaxial to the surgeon’s view. Since reflections from non-coaxial light sources project peripherally (as in keratometry and corneal topography), the use of a coaxial light source is important.
It should be noted that for eyes with large angle kappa (or lambda), the surgeon should account for this offset to avoid treatment misplacement. An anatomic basis for the size of angle kappa exists, though certain biometric and anatomic generalizations may be made.
For example, short hyperopic eyes tend to have a macula more temporal and superiorly placed than an emmetropic eye with normal axial length. Therefore, the coaxially sighted corneal light reflex will be inferonasal to the pupil center. In extreme circumstances, this should be accounted for by considering splitting the difference between the corneal vertex and non-pharmacologically manipulated pupil centroid. Caution should be used with anomalous fixators and highly aberrated eyes.
Intraoperatively, the method of creating a coaxially sighted corneal light reflex depends on the operating microscope. In the case of a particular surgical microscope (OPMI Lumera, Carl Zeiss Meditec), the two smaller light sources are coaxial to the two oculars. A simple way to create a coaxially sighted corneal light reflex is to dim the large illumination beam and have the patient fixate on the coaxial light corresponding to the surgeon's dominant eye (Figure 2A).
When implanting a single-piece IOL (Tecnis Multifocal IOL, Abbott Medical Optics), the surgeon can move the IOL in a direction orthogonal to the contact point of the haptics with the capsular bag. Therefore, in order to adjust the location of an IOL, one simply rotates the IOL until the haptics are slightly clockwise of orthogonal to the direction that it needs to move.
Since rotating an IOL in the bag may slightly alter its position, this may be sufficient for good positioning. If the IOL is still not well centered, one can then shift the IOL slightly in the desired direction. Within a small degree of movement, the IOL will remain at its final placement position.
Postoperatively, evaluating IOL centration by slit lamp biomicroscopy can be difficult. When angle kappa is small, a well-centered IOL will appear centered on the pupil; but when angle kappa is large, or if oblique illumination and viewing angles are used, the IOL may not appear centered.
More accurate and reproducible visualization of IOL centration can be done under more controlled lighting conditions with the rings or pupil image captured by a corneal topographer. The center of the placido disk rings corresponds with the coaxially slighted corneal light reflex. Some devices, such as a particular topographer (Atlas 9000, Carl Zeiss Meditec), have a pupil image setting that shows only the coaxially sighted corneal light reflex. After implanting a diffractive multifocal IOL, the diffractive rings are visible, allowing for good determination of IOL centration (Figure 2B).
We are continually developing our understanding of how to center devices and implants, such as corneal inlays and diffractive multifocal IOLs. Starting with a fundamental understanding of the ocular axes, surgeons can learn how to use visual references intraoperatively to center devices and implants accurately for best outcomes.
(Figure 1) The four axes of the eye. The optical axis is the line connecting all three Purkinje images (A). The pupillary axis is the line orthogonal to the cornea that passes through the center of the pupil (B). The line of sight is the line connecting the fixation point with the center of the pupil (C). The visual axis is the line connecting the fixation point with the nodal point of the eye (D). Angle kappa is the angle between the pupillary axis and the line of sight (C).
(Figure 2) Intraoperative view of a left eye along the visual axis, rotated 90° counterclockwise (A). The patient is fixating on the stereo coaxial lights (arrowhead), and the center of the IOL is thus aligned to the coaxially sighted corneal light reflex. The third, noncoaxial light of the microscope has been turned off. The postoperative pupil image of the same eye demonstrates excellent alignment with the coaxially sighted corneal light reflex, or visual axis (B).
(Figures courtesy of Daniel H. Chang, MD)
Daniel H. Chang, MD, is in private practice in Bakersfield, CA. He did not indicate any proprietary interest in the subject matter.
George O. Waring IV, MD, is assistant professor of ophthalmology and director of refractive surgery, Medical University of South Carolina, Storm Eye Institute, and medical director, Magill Vision Center, Charleston, SC. He did not indicate any proprietary interest in the subject matter.