Corneal refrac+ve surgery: Are we trea+ng the wrong loca+on with the wrong correc+on?

Transcription

1 RAA

2 Corneal refrac+ve surgery: Are we trea+ng the wrong loca+on with the wrong correc+on? Raymond A. Applegate, OD, PhD College of Optometry University of Houston

3 Corneal refrac+ve surgery is arguably good enough Yet most refrac+ve surgeons would like to increase accuracy and precision to minimize and preferably eliminate surprises.

4 WFG-LASIK Induced HO RMS as a Function of Pre-op Levels 0.60 Change in HOA RMS (microns) mm pupil WFG-LASIK y = -0.70x R 2 = PreOp HOA RMS (microns) Slide courtesy of Steve Schallhorn

5 In corneal refrac+ve surgery it is argued that one gets beher visual outcomes if the surgery is centered between center of the pupil and Purkinje image I.

6 If the observa+on holds that centering the surgery between Purkinje I and the pupil center on average provides a beher outcome for a given plajorm, then the wrong correc+on on average was designed for the intended loca+on and it is unlikely the best correc+on on average for the loca+on being treated.

7 There is no shortage of papers that have looked at the issue of where to center the abla+on profile.

8 Here I wish to argue that our industry would be in the drivers seat if we could measure the op+cal errors of the eye, use these measurements to design the desired correc+on, execute the correc+on design with accuracy and precision and obtain the intended desired correc+on.

9 Today I wish to s+mulate open discussion as to the next steps in improving accuracy and precision.

10 For the sake of discussion here let s define the goal of refrac+ve surgery to reduce the op+cal errors of the eye for visually relevant light such that acuity and foveal visual re+nal image quality is equivalent or beher to what one had with single vision glasses or contact lenses.

11 U H What is visually relevant light? It is light that enters the limi+ng aperture.

12 What is the re+nal loca+on of cri+cal importance to detailed vision? The locus of fixa+on which is typically within the foveola

13 Do we have the necessary instrumenta+on to Correct the refrac+ve errors of the individual eye with sufficient accuracy and precision to insure that re+nal image is equal to or beher than the individual is able to obtain with glasses or contact lenses.

14 First, let s examine the two of the key measurement tools (corneal topographers and wavefront sensors) and how measurements are taken.

15 Both instruments require the eye to To fixate a target of interest is a fixate a target for a proper property measurement of the eye and visual system. Said differently, one asks a pa+ent to fixate the target of interest when making a measurement as the instrument is aligned to the eye.

16 Temporal Nasal

17 Location of pupil center with respect to VK center Superior Mean 0.15 mm T; 0.04 mm S SD 0.14 mm; SD 0.12 mm Inferior Temporal Nasal

18 This 2 dimensional image shows the lateral displacement of the pupil center as viewed with respect to the VK axis. Because the instrument is not required to be centered over the pupil when the eye is fixa+ng a target on the op+cal axis of the instrument. If trea+ng along the LoS the pupil border traced here does not iden+fy the corneal area that should be treated. Why?

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20 Subject-fixated coaxially sighted corneal light reflex VK Line of sight Instrument Axis Center of Pupil Fovea

21 When The the wavefront WS is translated sensor to is align not aligned the instrument properly axis of the WS to the center to the pupil, the eye rotates. WS Line of sight Instrument Axis Center of Pupil Fovea ANSI Standard

22 Alignment of Line of Sight (LoS) to VK instrument axis A camera co-axial with the VK axis captures images of entrance pupil center coincident with fiducials. When instrument axis is aligned to the line-of-sight, the region of cornea overlying the pupil is most relevant to foveal vision. Mires F LoS E LoS Camera focus plane F In general, the LoS is NOT perpendicular to the corneal surface. Courtesy Larry Thibos WFC 2015

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24 With these factors in mind, let s now consider three key factors in the design of a WFG correc+on. Design of the correc+on for the refrac+ve errors of the eye Design compensa+on for the angle of incidence of the abla+ng beam Design compensa+on for the bio-mechanical response of the cornea

25 If the monochroma+c op+cal errors of the eye are fully corrected over the largest entrance pupil of the fixa+ng eye, the eye will be diffrac+on limited in the foveal area for all smaller pupil diameters for that wavelength despite small movements of the pupil center with pupil constric+on or dila+on. Coordinate system for the coding of the correc+on.

26 The real world is polychroma+c. Would it not be beher to align to the achroma+c axis to minimize the impact of transverse chroma+c aberra+on

27 Thibos/Bradley achroma+c alignicator

28 From: Location of Achromatizing Pupil Position and First Purkinje Reflection in a Normal Population Invest. Ophthalmol. Vis. Sci ;56(2): doi: /iovs Figure Legend: AcP positions relative to those of PI. Those eyes where distances between AcP and PI were found to be statistically significant (P < 0.05) are shown with a red outline.

29 Location of pupil center with respect to VK center Superior Mean 0.15 mm T; 0.04 mm S SD 0.14 mm; SD 0.12 mm Inferior Temporal Nasal

30 Transverse chroma+c aberra+on is important factor limi+ng visual performance for small pupils (e.g., pupil inlay (1.6mm), it is not an important factor for larger pupils.

31 Design compensa+on for the angle of incidence of the abla+ng beam Less effec+ve abla+on More effec+ve abla+on

32 Design compensa+on for the angle of incidence of the abla+ng beam Center of pupil Center of cornea VK axis

33 Each different axis will have a Given corneal eleva+ons are measured with corneal slightly topography different it is likely eye the rota+on compensa+on for the angle of incidence was designed using data gathered with respect to the VK axis. Shihing the compensa+on on the cornea with respect to LoS will shih the resul+ng spherical aberra+on correc+on with respect to the eye s inherent spherical aberra+on.

34 Design compensa+on for the biomechanical response of the cornea The cornea is not a piece of Plas+c J Refract. Surg Jul-Aug;16(4):

35 Biomechanical Response to Refractive Surgery Ruberti JW, Sinha Roy A, Roberts CJ. Corneal Biomechanics and Biomaterials. Annual Review of Biomedical Engineering, 2011 Aug 15; Vol. 13, pp Biomedical Engineering Cynthia J. Roberts, Ph.D.

36 Biomechanical Central Flattening and Peripheral Steepening Enhances a Myopic Procedure Reduces the effect of a Hyperopic Procedure Flattening (hyperopic shift) in a non-refractive PTK Including the PTK profile in one axis of an astigmatic procedure Induces unintended para-central and peripheral shape changes that result in!! Biomedical Engineering Cynthia J. Roberts, Ph.D.

37 Corneal response as a func+on of magnitude of the correc+on Tangen+al corneal topography maps N =25 N =321 N =635 N =622 < 2 D 2 to 4 D 4 to 6 D Magnitude of the correc+on 6 to 8 D TangenQal dioptric maps courtesy of Cynthia Roberts

38 Compensa+on for the biomechanical response is another sph. aberra+on correc+on and has to be appropriately registered with the LoS. Center of pupil Center of cornea VK axis

39 Each different axis will have a slightly different eye rota+on

40 Laser plajorm alignment to the eye Most all laser plajorms for corneal refrac+ve surgery have eye tracking to place the surgery in the intended loca+on. How is the eye rotated when the landmarks for registra+on are selected?

41 Temporal Nasal

42 Displacing SA correc+ons with respect to an underlying SA induces coma The most common induced aberraqon arer refracqve surgery is coma

43 Combine: A misalignment of the ahempted correc+on; with a small misaligned compensa+on for the angle of incidence correc+on; with a small misalignment for biomechanical response correc+on; and accuracy and precision goes down.

44 All measurements of the refrac+ve errors of the eye, design of the correc+on and implementa+on of the correc+on need to be made using a common reference system.

45 Next Steps to improving outcomes Common reference system Know the design of each individual eye and their proper+es Use op+mizing algorithms with objec+ve metrics Two step procedure

46 Next Steps to improving outcomes Common reference system Know the design of each individual eye and their proper+es Use op+mizing algorithms with objec+ve metrics Two step procedure

47 Next Steps to improving outcomes Common reference system Know the design of each individual eye and their proper+es Use op+mizing algorithms with objec+ve metrics Two step procedure

48 Next Steps to improving outcomes Common reference system Know the design of each individual eye and their proper+es Use op+mizing algorithms with objec+ve metrics Two step procedure

49 T H A N Y O U K