RINGS CHOSEN BASED ON CORNEAL ASPHERICITY

PAULO FERRARA EYE CLINICBELO HORIZONTE – MG

PAULO FERRARA

RING ARC Q K Cyl

140

160

210

ASPHERICITY x KERATOMETRY x ASTIGMATISM

• Corneal asphericity (Q) – corneal shape;

• Oblate …………………….Prolate;

• “Normal”: mild prolate;

• Keratoconus: hyper prolate cornea;

• Normal Q value: - 0.231

ASPHERICITY

1. Yebra-Pimentel E, González-Méijome JM, Cervino A, et al. Asfericidad corneal en una

poblácion de adultos jóvenes. Implicaciones clínicas. Arch Soc Esp Oftalmol 2004: 79:385-392

Q (asphericity) variation according to ring thickness

Q (asphericity) and K (keratometry)

variation according to ring thickness

The range (in ∆Q and ∆K) refers to mean preoperative values to mean postoperative values.

Single Segments (µm) ∆Q ∆K p value

150 - 0.07 0.78 D 0.042

200 - 0.31 1.82 D < 0.001

250 - 0.34 2.74 D < 0.001

Paired Segments (µm)

150 – 150 - 0.57 3.40 D < 0.001

150 – 200 - 0.73 4.35 D < 0.001

150 – 250 - 0.80 3.86 D 0.001

200 – 200 - 0.86 5.65 D < 0.001

200 – 250 - 1.02 6.27 D < 0.001

250 – 250 - 0.99 5.30 D 0.001

NOTE: (THIS IS VALID FOR ALL MAPS)

The segment or pair of segments SHOULD not turn the expected postoperative cornea asphericity (Q) significantly below -0.23.

If this happens so it is advisable

to choose a pair of segments that

fits this condition even if the

achieved KERATOMETRIC

correction is smaller

than the desired one. AVOID POSTOPERATIVE OBLATE CORNEAS !

Q (asphericity) = Quality of vision

ASPHERICITY: RING SELECTION

Oblate corneas (Q < -0.23) : single 140 or 160 (arc) segments

Moderately prolate corneas (- 0.23) < Q < -1.00) : single or paired160 (arc) segments

Hyperprolate corneas (Q > - 1.00) : paired 160 (arc) segments or210 segment if nipple cone

CLINICAL CASES

Q = 0,13

Cyl = -5.2 D

K1 = 39.8 D

K2 = 44.9 D

PREOP

SURGICAL

STRATEGY??

CLINICAL CASES

AF 150/140

Q = 0,08

Cyl = -2.2 D

K1 = 40.7 D

K2 = 42.9 D

POSTOP

IF THE TOPOGRAPHIC

ASTIGMATISM WAS USED

FOR RING SELECTION

(160/250):

OBLATE CORNEA

HIPERCORRETION

CLINICAL CASES

Cataract + PMD

VA: 20/400

+3.00-6.00x85

Q = 0,34

Cyl = +10.2 D

K1 = 48.1 D

K2 = 37.9 D

PREOP

SURGICAL

STRATEGY??

CLINICAL CASES

AF 150/140

Q = 0,65

Cyl = +6.6 D

K1 = 46.3 D

K2 = 39.7 D

POSTOP

VA: 20/30

+0.50-3.00x85

3 mo after ICRS:

Phaco + IOL

VA: 20/20

+2.00-3.00x85

CLINICAL CASES

High Astigmatism

Amblyopia

BCVA: 20/80

-2.50-4.50x20

Q = -0.31

Cyl = -3.5 D

K1 = 41.2 D

K2 = 44.7 D

PREOP

SURGICAL

STRATEGY??

CLINICAL CASES

AF 150/140

BCVA: 20/30

-3.50

Q = -0.26

Cyl = -1.5 D

K1 = 41.4 D

K2 = 43.0 D

TO BE

FOLLOWED

BY PRK

POSTOP

RESULTS

Q variation (∆Q) from preoperative to postoperative, according to

the ICRS type (160 / 210) and thickness implanted.

Q variation (∆Q) from preoperative to postoperative, for the 150-140 = 0.023

- The asphericity should be the main factor to be considered in surgical planning;

- As the technique has an orthopedic effect, the postoperative goal should always be surface re-shaping (turn the postoperative Q close to -0.23) rather than corneal flattening.

CONCLUSION