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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
Recommended