New Options in the Treatment of Keratoconus Mechanism of Ac.on of Intracorneal Ring Segments Rafael I. Barraquer, MD, PhD Medical Director Barraquer Ophthalmology Center Barcelona, SPAIN
Management of Keratoconus • Traditional options – (Spectacles), RGP-CL – Special CL (piggyback, hybrid)
– Keratoplasty (penetrating)
• Prevention ?
• Current options
– (Spectacles), RGP-CL Special CLs
– IntraCorneal Ring Segments (ICRS) – Collagen Crosslinking (CXL) – MW-Thermokeratoplasty (Avedro) – Combined CXL +ICRS, PRK, pIOL, etc. – Keratoplasty: • Penetrating ? • Deep lamellar (DALK)
• Prevention – Avoid eye rubbing !! – Genetic counseling?
Historical Background
“Refrac've Keratoplasty” (José I. Barraquer, 1949)
Peripheral Annular Resec.on
For: • Keratoconus • Anisometropia
• “Corrects ~ -‐10 D”
Thickness-Curvature 18 types of procedures Relation (1964) Â Â
“Thickness’ Law” of J.I.B. (1965) Thickness CENTRAL
PERIPHERAL
INCREASE DECREASE
Steepens (KPh-H)
Flattens (KPh-M)
Flattens (KM-M)
Steepens (KM-H)
Annular Implants (J.I.B, 1964)
Homoplastic rings by E.D. Blavatskaya Inst. Filatov, Odessa, 1966
• • • • •
Experimental (rabbits) Homoplastic implants Lamellar corneal pocket Relat. superficial (“medium” thickness) “Ideal” rings 7.0 x 4.0 mm
Homoplastic Annular Implants by E.D. Blavatskaya
• ↑↑Thickness ↑↑Flattening – 100-120 mm è 6 - 8 D – 150-170 mm è 9 -12 D – 290-310 mm è 18 - 21 D
• ↑↑Diameter ↓↓Flattening (w constant thickness)
• Irregular astigmatism Zhivotovsky & Vishnevetsky (1969-71) n 2 human cases: plastic 7-5 mm intracorneal rings è corr. high myopia (“good results” @ 2 a.)
Silicone Rubber Rings (G.Sim贸n & R.I.Barraquer, 1985-88)
Rectangular section rings Flat configuration - adaptable Various: thickness & diameter
Technique: corneal pocket Experimental (rabbits, dogs)
Silicone rings
(Simón & Barraquer)
Simple procedure Can induce marked flattening Good tolerance Increasing effect with ► Ring thickness ► Smaller diameter
Keratophakia for Myopia Ring
Posi.ve (corr. H) Nega.ve (corr. M)
BUT: Thickness law works… Only if implant (or ablation) is SUPERFICIAL !
Superficial implants
è Push forward the anterior face (thickness effect)
Deep implants è Deform the posterior face (& not directly the anterior face)
è Thickness Law does not apply
Current ICRS are implanted at a deep stromal level
Ø
Indentation is basically backwards
Ø
No push-forward effect (would-be “thickness” effect)
Ø
What then? something “Biomechanic”
Mechanism of Action of Intracorneal Segments
(“Rings”)
ICR: PMMA Ring (closed) by Fleming & Schanzlin, 1987
Experimental (rabbits) Expandable (metal clamp closure) Prismatic – Hexagonal Profile Conic conf. (parallel to lamellae)
Theory/Mechanism: • Expansion è flat, M • Compression è steep, H
Ring Segments (INTACS)
Clinical application: • Myopia (W. Nosé et al., 1991-93-96) èEffective: 1,25 – 3,25 D • Keratoconus (J. Colin, 1997)
Ferrara Ring -Keraring® Arcuate Implants (P. Ferrara, 1991…)
Triangular section è Posterior base
Flat configuration è Not parallel to lamellae
Gel Injection Ring (Polyethylene) (GIAK, Sim贸n, Parel et al., 1994)
ICRS: Mechanism of action? n Nobody really knows how they work
• Pseudo-Thickness Law – The effect does increase with ring thickness & small Æ, BUT: – The impl. meridian is steepened – The “open” meridian is flattened (just the contrary of JIB’s Law !!
• NOT related to Thickness Law of José I. Barraquer – Not a negative keratophakia – They work at deep position è NO anterior lamellar effect
Biomechanical effects • Space Occupation !! – Force lamellae to “detour” around impl. – No forward thickness effect (deep location), or only a local “bump” – Torsional effect?
Space Occupa.on = Compression Similar to: • Compressive sutures • Wedge resection (sutured) • Thermokeratoplasty (tissue retraction)
Sector vs. circular effects • Assuming a compressive effect (on the detoured collagen lamellae)
• Increased tension
• When acting over a sector only… è that axis will be steepened è the opposite axis @90º will be flattened (coupling effect) • This corrects Astigmatism • This sector action is maximal with the less wide segments (90º) • The effects of two opposite implants are additive • This action is the opposite of what Barraquer “Thickness Law” would predict
However, when compression is circular… If compression is 360º: è Gen. flattening !! Just like a tightly sutured PK Corrects mostly myopia
Now, there MUST be a point where steepening (sector compression) effect gives way to flattening (circular compression) effect This should be observed as we go from narrow ( 60º, 90º) to progressively encircling (2 x160º, etc.) segments
Remember… q Early ICRS (1990s) were conceived to correct myopia q 2 equal, encircling implants
q Current uses: Keratoconus, Ectasia 1 single or 2 unequal implants
ICRS in KC & Ectasia
¿What are we trying to correct?
• As.gma.sm • Myopia • Decentered op.cs – Typical of KC & other Ectasia – Not corrected by glasses – Can be measured as Coma
¿How can we correct Coma?
• Astigmatism: • •
•
a Quadrant aberration
Maximal correction action over 90º 1 or 2 ICRS centered over the flat axis. There is coupling
Coma: a Hemispheric aberration Maximal correction action over 180º • 1 ICRS only centered over Coma/decentration axis . No coupling •
Summary Ø Ø Ø
Ø
Corneal “ring” procedures: can be traced to pioneering refractive surgery (1949) Intracorneal deep annular implants do not follow thickness law biomechanical ection Compressive “lamellar detour” theory explains observed effects of ICRS: Ø asymmSector implants steepen axis, flatten opposite axis(correct astigmatism) Ø Circular implants correct mostly sphere (myopia) Ø Wide etric implants correct decentration/coma (properly placed) New versatile implants of different width /diameter/ thickness promise selective correction of the differing features in KC & other ectasia.
Thank You