2Evolution of the Nomogram FERRARA RING NOMOGRAMFirst generation ( ): keratoconus gradeSecond generation ( ): spherical equivalentThird generation ( ): topographic astigmatismFourth generation (May 2009 to now): asphericity (Q)Evolution of the Nomogram(criteria for ring selection)
3cornea hypercorrection in axial myopia Second generation ( ): spherical equivalentFERRARA RING NOMOGRAMEye 1Eye 2Refraction: DRefraction: DAre these the same?NO !cornea hypercorrection in axial myopia
4Are these the same ? NO ! FERRARA RING NOMOGRAM Third generation ( ): topographic astigmatismFERRARA RING NOMOGRAMEye 1Eye 2Keratometry: x DKeratometry: x DAre these the same ?NO !same topographic astigmatism - different changes after ring implantation
7HOW THE FERRARA RING WORKS Nowadays:Asphericity lawAddition in the cornea periphery = asphericity change – frequently withoutconcomitant significant corneal flattening;Ring diameter = how much the cornea will be flattened, the astigmatism andasphericity will be changedRing arc length = how much the asphericity, keratometry and astigmatism will bechanged. The shorter the segment, the greater the astigmatic correction, the lesser theasphericity change.
8160 arc = 90% of keratoconus (“standard” ring) - Q negative (prolate to hyperprolate cornea)- Average to high K values- Average to high astigmatism140 arc = PMD- Q 0 or positive- Low K values- High astigmatism210 arc = Nipple- Q negative (hyperprolate cornea)- High K values- Low astigmatism
9NOMOGRAM General Guidelines IT MUST BE DETERMINED: 1. The type of keratoconus:Central NippleOval (paracentral)PMD2. The corneal asphericity (Q) or K (average)3. The pachymetry at the corneal steepest axis and probable ring track (5 mm optical zone)
10NOMOGRAM1. Type of keratoconusCentral(Nipple)Oval
12Q (asphericity) variation according to ring thickness
13Q (asphericity) and K (keratometry) variation according to ring thicknessSingle Segments (µm)∆Q∆Kp value150- 0.070.78 D0.042200- 0.311.82 D< 0.001250- 0.342.74 DPaired Segments (µm)150 – 150- 0.573.40 D150 – 200- 0.734.35 D150 – 250- 0.803.86 D0.001200 – 200- 0.865.65 D200 – 250- 1.026.27 D250 – 250- 0.995.30 D
14If this happens so it is advisable to choose a pair of segments that NOTE: (THIS IS VALID FOR ALL MAPS)The segment or pair of segments SHOULD not turn the expected postoperative cornea asphericity (Q) significantly belowIf this happens so it is advisableto choose a pair of segments thatfits this condition even if theachieved KERATOMETRICcorrection is smallerthan the desired one.AVOID POSTOPERATIVE OBLATE CORNEAS !Q (asphericity) = Quality of vision
21NOMOGRAM IT MUST BE DETERMINED: The pachymetry at the corneal steepest axis and probable ring track(5 mm optical zone)Incision (80% depth):1 segment: at 900 axis – the tip of the ring at the steepest axis2 segments: at the steepest axis
22NOMOGRAM WHY THE INCISION SHOULD BE AT 80% DEPTH? Superficial stroma Deep stroma
23NOTE: (THIS IS VALID FOR ALL MAPS) The thickest segment of a pair of segments cannot exceed half thickness of the cornea in its bed.If this happens so one has to choose a pair of segments thatfits this condition even if the achieved correction is smallerthan the desired one.
27NOMOGRAM SUMMARY GENERAL RULES Q preoperative - Q change induced by the ring = (TARGE FINAL Q VALUE)
28NOMOGRAM ASPHERICITY RING SELECTION Oblate corneas (Q < -0.23): single 140 or 160 (arc) segmentsModerately prolate corneas ( < Q < -1.00): single or paired 160 (arc) segmentsHyperprolate corneas (Q > ): paired 160 (arc) segments or 210 segment if nipple cone
29NOMOGRAMThe asphericity is the best parameter, at the present moment, for ring selection;There is a tendency to implant “less” tissue to achieve the same (or better) correction than was achieved in the past;Q value = Quality of vision