The science of refractive surgery

1. THE SCIENCE OF REFRACTIVE
SURGERY
PRESENTER: DR. IDDI NDYABAWE
MODULATOR: DR. ATUKUNDA IMMACULATE
MAKERERE UNIVERSITY COLLEGE OF HEALTH SCIENCES
DEPARTMENT OF OPHTHALMOLOGY
NOVEMBER 2021

2. OUTLINE
• Introduction
• Corneal Optics
• Refractive Error: Optical Principles and Wavefront Analysis
• Corneal Biomechanics
• Corneal Imaging for Keratorefractive Surgery
• Corneal Effects of Keratorefractive Surgery
• Laser Biophysics
• Corneal Wound Healing

3. Introduction
• Goal of RF: to reduce dependence on contact lenses or spectacles for
use in ADL
• Appropriate pre-surgical evaluation: find best technique and ensure
optimal outcome for each patient.
• Categorizes: corneal or intraocular

4. .
Keratorefractive (corneal)
procedures include:
• incisional,
• laser ablation,
• lamellar implantation,
• corneal collagen shrinkage,
• Collagen crosslinking techniques.
Intraocular refractive procedures
include:
• phakic intraocular lens (PIOL)
implantation
• cataract surgery or refractive
lens exchange (RLE) with
implantation of a monofocal,
toric, multifocal, or
accommodative intraocular lens.

5. .

6. .

7. CORNEAL OPTICS
• air-tear-film interface
• anterior corneal curvature contributes greatest part to optical power
of the eye (2/3), approx. +48.00 diopters (D).
• The normal cornea flattens from the center to the periphery by up to
4.00 D.
• What do Standard keratometers and Placido-based corneal
topography instruments measure?
-the anterior corneal radius of curvature
-estimate total corneal power from these frontsurface measurements.

8. .
How do keratorefractive surgical procedures change the refractive state of
the eye?
-by altering corneal curvature.
Why is it problematic to achieve predictable results in refractive surgery?
-tolerances involved in altering corneal dimensions are relatively small
-For instance, changing the refractive status of the eye by 2.00 D may require
altering the cornea's thickness by less than 30 mm.
-minuscule changes in the shape of the cornea may produce large changes in
refraction.

9. Refractive Error: Optical Principles and Wavefront
Analysis
By what methods can we do
wavefront analysis?
• Hartmann-Shack,
• Tscherning,
• thin-beam singleray tracing,
• optical path difference.
• How important is the
information obtained from
wavefront analysis?
-calculating custom ablations to
enhance vision or correct optical
problems
-explaining patients' visual
symptoms

10. Measurement of Wavefront Aberrations
and Graphical Representations
What’s the most popular method for measuring wavefront analysis?
-Hartmann-Shack wavefront sensor
How is it done?
-low-power laser beam is focused on the retina.
-A point on the retina acts as a point source, and the reflected light is
then propagated back (anteriorly) through the optical elements of the
eye to a detector
-In an aberration-free eye, all the rays would emerge in parallel, and
the reflected wavefront would be a flat plane.
-The extent of the divergence of the lenslet images from their expected
focal points determines the wavefront error

11. .

12. .

13. What are Zernicke Polynomials?
• Mathematical formulas used to describe the surfaces
• Each aberration may be positive or negative in value and induces predictable alterations in the image quality
• The magnitude of these is expressed as a root mean square (RMS) error, which is the deviation of the
wavefront averaged over the entire wavefront.
• What are the most important Zernicke coefficients? Coma, spherical aberration and trefoil.

14. .

15. .

16. .

17. Fourier analysis
• alternative method of evaluating the output from an aberrometer.
• sine wave-derived transformation of a complex shape
• Shapes here are more detailed, theoretically allowing for the
measurement and treatment of more highly aberrant corneas

18. Give examples of 3rd, 2nd, 1st and zero order
aberrations
3rd order aberrations:
-spherical aberration
-coma
-trefoil
2nd order aberrations:
-myopia
-hyperopia
-Regular astigmatism
1st order aberrations:
-vertical prisms
-horizontal prisms
Zero order aberrations:
-piston

19. What factors affect high-order aberrations?
• Pupil size
• Age
• Conventional surface ablation or LASIK.
What’s the most significant higher-order aberration?

20. Spherical aberration
• When does it occur?
-When peripheral light rays impacting a lens or the cornea focus in
front of more central ray
• RFs?
-after myopic LASIK and surface ablation
• CFs?
-halos around point images

21. Coma
• What happens in coma?
-With coma, a third-order aberration, rays at one edge of the pupil
come into focus before rays at the opposite edge do. The effective
image resembles a comet, having vertical and horizontal components

22. RFs for coma
• Decentered corneal grafts
• Keratoconus
• Decentred laser ablation

23. Trefoil
• can occur after refractive surgery and produces less degradation in
image quality than does coma of similar RMS magnitude

24. Which type of ablations are used to reduce pre-
existing higher order optical aberrations? Why?
• wavefront-optimized and wavefront-guided ablations
• Because they induce fewer higher-order aberration

25. Corneal biomechanics
• In which part of the cornea are fibrils more closely packed?
-anterior 2/3rds
• How do keratorefractive procedures alter corneal biomechanical
properties?
-directly (eg, radial keratotomy weakening the cornea to induce
refractive change)
-indirectly (eg, excimer laser surgery weakening the cornea by means of
tissue removal).

26. .
• What explains the variety in corneal biomechanical response to
different keratorefractive procedures?
-lack of uniformity of biomechanical load throughout the cornea
• LASIK Vs PRK, which one has greater overall effect on corneal
biomechanics? Why?
-LASIK
-Because a flap is created and ablation occurs in deeper, weaker
corneal stroma.

27. Corneal Imaging for Keratorefractive Surgery
• What parameters do we assess for in corneal imaging?
-Corneal shape,
-curvature,
-thickness profiles
• How is the degree of asphericity of the cornea quantified?
-Q value; Q=0 (spherical corneas), Q<0 (prolate corneas), Q>0 (oblate
corneas)
• How is a normal cornea? Prolate.
• How do prolate corneas minimize spherical aberrations? Due to their
relatively flat peripheral curve

28. .
• Why do we have degrading of optics of the eye after conventional
refractive surgery for myopia?
-flattening of the corneal center
-corneal asphericity increases in the oblate direction
-oblate contours induce more spherical aberrations, because peripheral
cornea here is steeper than centre

29. Corneal topography
• Detailed information about corneal curvature
• Evaluated using keratoscopic images, captured by Placido disk
patterns, converted into computerized color scales
• Referenced from the vertex normal

30. Axial power and curvature
• Axial power representation
comes from the supposition that
the cornea is a sphere and that
the angle of incidence of the
instrument is normal to the
cornea.
• Based on concept of ‘axial
distance’.

31. .

32. Corneal topography and astigmatism
• Normal: uniform colour centrally
with a natural flattening in the
periphery
• Regular astigmatism has uniform
steepening along a single
corneal meridian
• Bow-tie pattern.

33. .
• Irregular astigmatism is nonuniform corneal steepening from many
causes. Cannot be corrected by cylindrical lenses
• Evaluated by corneal topography. Finding; nonorthogonality of steep
and flat axes.
• Traditional excimer laser ablation can treat spherocylindrical errors
but does not effectively treat irregular astigmatism

34. .

35. Limitations of corneal topography
• tear-film effects
• misalignment (misaligned corneal topography may give a false
impression of corneal apex decentration suggestive of keratoconus)
• instability (test-to-test variation)
• insensitivity to focus errors
• limited area of coverage (central and limbal)
• decreased accuracy of corneal power simulation measurements (SIM
K) after refractive surgical procedures
• decreased accuracy of posterior surface elevation values in the
presence of corneal opacities or, often, after refractive surgery (with
scanning-slit technology)

36. Corneal tomography
• Assesses spatial thickness profiles best
• Scanning-slit tech
• Schleimpflug-based imaging systems
• Elevation-based tomography is especially helpful in refractive surgery
for depicting the anterior and posterior surface shapes of the cornea
and lens

37. .

38. .

39. Indications for Corneal Imaging in Refractive
Surgery
• 2/3 of patients with normal corneas have a symmetric astigmatism pattern that is
round, oval, or bow-tie shaped.
• Asymmetric pattern also seen: asymmetric bow-tie patterns, inferior steepening,
superior steepening, skewed radial axes, or other nonspecific irregularities.
• Irregular astigmatism: abnormal tear film, contact lens warpage, keratoconus and
other corneal ectatic disorders, corneal surgery, trauma, scarring, and
postinflammatory or degenerative conditions.
• When underlying etiology is in question: CL wearers
• To demonstrate effects of keratorefractive surgery.

40. .
• Why aren’t the patients with keratoconus or other ectatic disorders
routinely considered for ablative keratorefractive surgery?
-because the abnormal cornea has an unpredictable response and/or
progressive ectasia.

41. .
• Why do we need to do Topographic analysis all patients being
considered for refractive surgery?
-in order to identify patients who should not undergo the procedure
Which conditions are absolute contraindications for refractive
surgery?
Corneal ectatic disorders: KC, PMD

42. .
• What’s the topographic pattern in keratoconus?
• substantial inferonasal or inferotemporal steepening

43. .
• What’s the topographic pattern in PMD?
-inferior steepening, which is most dramatic between the 4 and 8
o'clock positions, with superior flattening. "crab-claw“ appearance.

44. .
• Which patient posses the greatest difficulty in preoperative
evaluation for refractive surgery?
-the one in whom KC ultimately develops but who shows no obvious
clinical signs at the time of examination.
Which corneal thickness variant suggests an ectatic disorder?
-corneas that are not thicker peripherally

45. .
• What newer technologies can help as screening tests for keratoconus?
-high-resolution anterior segment optical coherence tomography (OCT),
-ultra-high-frequency ultrasound,
-hysteresis analysis
• What parameters do they assess?
-the relative position of the posterior and anterior apex,
-epithelial thickness,
-Corneal biomechanical properties

46. .
• What’s the role of corneal topography after PRK?
-identifying the irregularity, magnitude, and meridian of
postoperative astigmatism
What’s the solution for resulting refractive error and corneal
shape after PRK?
-Conventional, wavefront-optimized, wavefront-guided, or topography-
guided ablations may be considered

47. Corneal effects of keratorefractive surgery
• How do keratorefractive procedures induce refractive changes?
-by altering corneal curvature
• List some methods of corneal refractive procedures:
-incisional,
-tissue addition or subtraction,
-alloplastic material addition,
-collagen shrinkage,
-laser ablation

48. .
• What factors determine a patient’s satisfaction after refractive surgery?
-successful correction of refractive error
-creation of a corneal shape that maximizes visual quality.
Prolate versus oblate corneas: which ones increase spherical aberrations?
-Oblate
What complaints are had by patients with substantial spherical
aberration?
-glare
-halos
-decreased night vision

49. Incisional techniques
• What do incisions do to the curvature of the cornea?
-All incisions cause a local flattening of the cornea.
• How do radial incisions affect the corneal curvature?
-lead to flattening in both the meridian of the incision and the one
90deg away.
How do tangential incisions affect the corneal curvature?
-They cause coupling. lead to flattening in the meridian of the incision
and steepening in the meridian 90deg away that may be equal to or
less than the magnitude of the decrease in the primary meridian

50. .

51. .
• What’s the recommended depth of a corneal incision? Why? What’s
the rationale?
-85-90% deep
-to retain an intact posterior lamella and maximum anterior bowing of
the other lamellae
• If you have a given corneal incision, in which age group is greater
effect faced?
-The same incision has greater effect in older patients than it does in
younger patients

52. List the important variables for radial and
astigmatic surgery
• Age
• Number of incisions
• Depth of incisions
• Length of incisions

53. Tissue addition or subtraction techniques
• Which exceptional lamellar procedure is still used to date?
-laser ablation technique
What is keratomileusis? Who discovered it?
-carving of the anterior corneal surface to correct myopia. Discovered
by Barraquer.
What is epikeratophakia? What is its other name?
-adding carved donor tissue to the surface to induce hyperopic or
myopic change. A.k.a: Epikeratoplasty

54. Alloplastic material addition techniques
• How can we alter corneal shape using alloplastic material addition
techniques?
-by adding alloplastic material such as hydrogel on the surface or into
the corneal stroma to modify the anterior shape or refractive index of
the cornea.

55. Collagen shrinkage techniques
• How can collagen shrinkage be used to alter corneal shape?
-by altering corneal biomechanics, through heating collagen to induce
changes in corneal curvature
• At what temperature does heated collagen shrink?
58-76 degC
• Why are thermokeratoplasty and conductive keratoplasty avoided in the
central cornea? Where should they be used?
-scarring
-can be used in midperiphery

56. .

57. Laser biophysics
• Laser-tissue interactions: 3 types:
-photoablation; most important
-photodisruption,
-photothermal
How does photoablation work?
-breaks chemical bonds using excimer (from "excited dimer") lasers or other
lasers of the appropriate wavelength.
-Laser energy of 4 eV per photon or greater is sufficient to break carbon-
nitrogen or carbon-carbon tissue bonds

58. .
• What are ArF lasers?
-excimer lasers that use electrical energy to stimulate argon to form dimers
with the caustic fluorine gas.
-They generate a wavelength of 193 nm with 6.4 eV per photon.
Why is light of wavelength 193nm suitable for operating on the surface of
tissues?
-very low tissue penetrance
-capable of great precision, with little thermal spread in the tissue
-lack of letharlity to cells;… nonmutagenic… hence safe
At what wavelength do we start to see mutagenicity?
250nm

59. .
• What is the use of femtosecond laser in refractive surgery? On what
principle does it work?
• -creating corneal flaps for LASIK
• -to create channels for intrastromal ring segments
• -for lamellar keratoplasty and PKP.
• Works by photodisruption. Matter turned into plasma. 1053nm
infrared beam.

60. .
Photothermal effects:
-achieved by focusing a holmium:YAG laser with a wavelength of 2.13
mm into the anterior stroma.
-The beam's energy is absorbed by water in the cornea
-resulting heat causes local collagen shrinkage and subsequent surface
flattening.

61. Fundamentals of excimer laser photoablation

62. .
• In surface ablation, laser applied to Bowman’s layer and anterior
stroma.
• In LASIK, combination of an initial lamellar incision with ablation of
the cornea, stromal bed.
• Thickness and diameter of LASIK flap depends on:
-Instrumentation
-Corneal diameter
-Corneal thickness

63. .
• Ultrathin flap: 80-100micrometers
• Standard flap: 130-180micrometers
• Why do we embrace multizone treatment algorithm?

64. .
• Why should we retain enough stromal tissue after creating a LASIK
flap and ablation?
-to maintain adequate corneal structure
• Historical standard for minimum thickness of stromal bed tissue:
250micrometers.
• List the types of photoablating lasers:
-Broad-beam lasers
-Scanning-slit lasers
-Flying spot lasers

65. Wavefront-optimized and wavefront-guided
laser ablations
• Why are conventional laser treatment profiles likely to induce some
degree of higher-order aberration?
• Because conventional laser treatment profiles have small blend zones and
create a more oblate corneal shape postoperatively,
• How does wavefront-optimized laser ablation improve postoperatice
corneal shape?
-by taking the curvature of the cornea into account and increasing the
number of peripheral pulses; this approach minimizes the induction of
higher-order aberrations
-quicker than wavefront-guided technology

66. .
• How is wavefront guided laser ablation different from conventional
excimer laser and wavefront optimized laser ablation?
-WFGLA, treats both low order and high order aberrations by applying
complex ablation patterns to the cornea to correct the wavefront
deviations; unlike the later two which rely on subjective refraction
alone.

67. .
• Which type of ablations do higher order aberrations require?
-non-radially symmetric patterns of ablation (which are often much smaller
in magnitude than ablations needed to correct defocus and astigmatism).
How do we generate a 3D map of the planned ablation?
-Using the difference between the desired and the actual wavefront
What’s the role of accurate registration?
-to ensure that the ablation treatment actually delivered to the cornea
matches the intended pattern
How is such accurate registration achieved?
-using marks at the limbus before obtaining the wavefront patterns or by iris
registration, which matches reference points in the natural iris pattern to
compensate for cyclotorsion and pupil centroid shift.

68. .
• How satisfactory are the results obtained from wavefront-optimized
and wavefront guided ablations for myopia, hyperopia and
astigmatism?
-excellent, with well over 90% of eyes achieving 20/40 or better
uncorrected distance visual acuity (UDVA)
Which patients are exception in terms of outcomes of customized
treatments?
-patients with substantial preoperative higher-order aberrations

69. Topography-guided laser ablations
-link the treatment to the corneal topography rather than to the
wavefront data.
-may offer significant benefit in the treatment of highly aberrated eyes,
such as eyes with previous RK or PKP.

70. Corneal wound healing
• All forms of keratorefractive surgery are exquisitely dependent on
corneal wound healing to achieve the desired results.
• By either modifying or reducing wound healing or exploiting normal wound
healing
• How does PRK use above principle?
-PRK requires the epithelium to heal quickly, and with minimal stimulation of
the underlying keratocytes, to avoid corneal scarring and haze.
How does lamellar keratoplasty use this principle?
Lamellar keratoplasty requires intact epithelium and healthy endothelium
early in the postoperative period to seal the flap.

71. .
• Why do we need regeneration of corneal nerves?
-regeneration of the corneal nerves is crucial to a normal ocular surface
and good visual function.
-Delay or difficulty in re-innervation can lead to problems with corneal
sensation and tear-film stability and to dry eye symptoms.
On what factor does the cascade of corneal wound healing depend?
-on the nature of the injury

72. .
• Regarding surface ablation, where do we find corneal haze?
-In subepithelial anterior stroma.
• What circumstances lead to greater haze formation?
-deeper ablation
-increased surface irregularity
-prolonged absence of the epithelium
How is the corneal epithelium secured to the stroma?
-By hemidesmosomes and anchoring fibrils

73. .
• Why do some surgeons use MMC on stromal bed?
-To decrease haze formation
• What’s the role of Vitamin C in excimer laser?
-Protecting the cornea from UV light damage by the excimer laser.
Why is it that haze formation is not seen to occur in the central flap
interface after LASIK?
-either due to lack of significant epithelial injury and consequent
subcellular signaling or
-to maintenance of some intact surface neurons

74. .
• What kind of healing pattern do we observe in LASIK?
-Very little long-term evidence of healing between the disrupted lamellae
and only typical stromal healing at the peripheral wound
What holds the lamellae in position after LASIK? Evidence for this
phenomenon?
The lamellae are initially held in position by negative stromal pressure
generated by the endothelial cells aided by an intact epithelial surface
Evidence: Even years after treatment, the lamellar interface can be broken
and the flap lifted, indicating that only a minimal amount of healing occurs.
LASIK flaps can also be dislodged secondary to trauma many years
postoperatively.

75. References
• AAO Book 11