ICRS: Corneal Biomechanics Effects
INTRODUCTION AND MECHANISM OF ACTION
In 1987, intrastromal rings were introduced as synthetic, intracorneal implants for the correction of various degrees of myopia. An elastic modulus, or modulus of elasticity, is the mathematical description of an object's or substance's tendency to be deformed elastically (i.e., nonpermanently) when a force is applied to it. In keratoconus, the corneal elastic modulus is reduced due to pathology in the corneal stroma.(13) From a biomechanical perspective, the resistance to deformation is reduced in relation to the reduction of the elastic modulus that leads to increased strain and protrusion in the cornea. The consequence is increased curvature and corneal thinning, the hallmarks of keratoconus. Since stress is defined as applied force divided by cross-sectional area, stress focally increases in the zone of corneal thinning.(14)
Intrastromal corneal ring segments (ICRSs) are placed in the mid-corneal peripheral stroma at approximately two-thirds of depth outside the central optical zone to reshape the anterior corneal surface while maintaining the prolate profile (positive asphericity) of the cornea.(1) Intracorneal ring segment acts as spacer elements between the bundles of corneal lamellae producing a shortening of the central arc length (arc shortening effect) that is proportional to the thickness of the device.(2) As a consequence of this effect, the central portion of the anterior corneal surface tends to flatten and the peripheral area adjacent to ring insertion is displaced forward.(3) Corneal changes induced by the ICRS must be in relation to the structural properties of the collagen framework in the corneal stroma. The stroma accounts for 90% of corneal thickness, and evidently its mechanical properties define for the most part the mechanical properties of the whole corneal structure.(3)
According to Barraquer, when material is removed from the central area of the cornea or added to the periphery, a central flattening effect is achieved. (4) In contrast, when material is added to the center or removed from the corneal periphery, the central surface curvature is steepened. The corrective results vary in direct proportion to the thickness of the implant and in inverse proportion to its diameter.(5) The thicker and smaller the device, the higher the refractive result achieved. The diameter of the ring is proportionally inverse to the flattening intensity. Thus, the smaller the diameter, the more tissue is added (ring thickness) with higher myopic correction.(4)
ICRS create an arc-shortening effect on the corneal lamellae, flattening the central cornea. For the correction of astigmatism, the end point of each segment may produce a traction force on the surface, producing additional flattening on this reference axis. In addition, the presence of a corneal inlay may provide biomechanical support for this ocular tissue. (6) Topographic interpretation of eyes that underwent ICRS implantation shows overall flattening of the cornea, dislocation of the corneal apex towards the center, preservation of corneal asphericity, and decreased surface irregularity. (6) ICRS change the arc length of the anterior corneal curvature. The refractive effect achieved is directly related to the thickness of the segments. Placing the ring segments in the periphery of the cornea causes local separation of the corneal lamellae which results in shortening of the corneal arc length. This flattening effect on the cornea reduces myopia by lowering its optical power. Increasing the thickness of ICRS also causes greater degrees of local separation and increased corneal flattening. Thus, the degree of corneal flattening (correction) achieved by ICRS is directly related to their thickness.(7)
The initial enthusiasm regarding INTACS for the correction of myopia has faded for multiple reasons, including a limited range of correction, induced astigmatism, and slow visual recovery. Nowadays, the use of ICRS has evolved into an important therapeutic intervention for corneal ectatic diseases such as keratoconus(8), pellucid marginal degeneration(9), and post-laser surgery-induced ectasia(10). Redistribution of corneal curvature leads to a redistribution of corneal stress, interrupting the biomechanical cycle of keratoconus disease progression, in some cases.(11)
Apart from INTACS, many other types of ICRS have are available, including Ferrara rings, Kerarings, Cornealring, which are different in geometrical design and diameter.(17)
· Intacs have a hexagonal transverse shape with 8.1 mm external diameter and 6.8 mm internal diameter. There is another design of Intacs segment (Intacs SK), with inner diameter of 6 mm, an oval cross-section shape and two different thicknesses (400 μm and 450 μm). The refractive effect is modulated by the Intacs' thickness (0.25-0.45 mm, with 0.05-mm increments) and current designs have a predicted myopic range of correction from -1.00 to -4.10 D.(18)Recently, a new Intacs segment (Intacs SK) design, with inner diameter of 6 mm and oval cross-section shape, was produced (Addition technology Information, AAO, NV, USA). Two different Intacs SK thicknesses can be obtained: 400 µm (for steep K-value of 57-62 D and cylinder > 5 D) and 450 µm (for steep K-value > 62 D and cylinder > 5 D). However, there are currently no published reports on these new segments.
· Ferrara: In 1986, Ferrara started implanting modified PMMA rings in rabbit corneas and, in 1994, developed a technique of corneal stroma tunnel construction for implanting the rings. Ferrara rings are available in European countries and South America. The cross-section of a Ferrara ring is triangular in shape to reduce the photic phenomena by inducing a prismatic effect. (11)In 1996, he replaced the single ring with two segments, each having 160° of arc and obtained improved results for high myopia. Furthermore, he began to implant the segments into corneas with keratoconus and after keratoplasty.
The Ferrara ring segments, manufactured by Mediphakos, Inc., Belo Horizonte, Brazil, are made as computer-lathed PMMA and camphorquinone (CQ)-acrylic segments, available in two diameters (6.0 mm for myopia up to 7.00 D, 5.0 mm for higher degrees of myopia) and thickness varying from 150 to 350 µm. Typical Ferrara ring characteristics include :
· Made of PMMA CQ-acrylic segments;
· The internal and external diameters are 4.4 mm and 5.4 mm for the 5.0-mm optical zone and 5.4 mm and 6.4 mm for the 6.0 mm optical zone, respectively;
· The segment cross-section is triangular;
· Constant 600 µm base for every thickness and diameter;
· An arc of 160°;
· KeraRings (Mediphacos) are mainly identical in design to Ferrara rings, but different options of arc length are available. KeraRings were developed mainly for refractive correction in eyes with keratoconus. Each segment is available with an internal diameter of 4.40 mm and an external diameter of 5.60 mm. Intacs SK and KeraRings have a greater flattening effect on the central cornea due to their close position to the visual axis.(19) These are made of PMMA and are characterized by a triangular cross-section. Their optical zones are 5.0, 5.5 and 6.0 mm, and the flat basis width is 0.6 mm with variable thickness (0.15–0.30 mm thickness with 0.5 mm steps) and arc lengths (90°, 120°, 160°, 150°, 210, and 355°). The new model Keraring 355° is used only for nipple cones and is available with 0.20 and 0.30 mm thickness. (6)
· MyoRing (DIOPTEX GmBh, Linz, Austria): This ICR is a 360° continuous full-ring implant for myopia and keratoconus. Its diameter ranges from 5–8 mm and the thickness ranges from 200–320 microns. See more information at www.dioptex.com (6)
For selection of the ICRS size and position, the surgeon must consider refraction, keratometry, and corneal thickness. Implant manufacturers usually suggest a nomogram to be followed for the implantation of the ICRS.(6)
The surgical planning is based on the location of the steepest axis, the extent of ectatic area, and the refraction. A line is drawn along the steepest axis of the corneal topography. The incision is made on this axis and the segments are implanted along it. If the conical area is equally distributed along the steepest axis, symmetrical segments are implanted A single segment is also used in asymmetrical cases. Suggested sites for complete nomogram descriptions are: www.mediphacos.com, www.ferrararing.com.br, www.additiontechnology.com and www.dioptex.com. (6)
There are some differences among surgeons in choosing the implantation depth of the ICRS. Ertan and Kamburoglu used Intacs segments implanted at about 70% of the corneal depth. Segment extrusion occurred in three eyes of patients with severe keratoconus.(20)Alió et al. also used Intacs implanted at 70% of the corneal depth without intraoperative complications.(21)In another study, Wachler et al. inserted Intacs at a depth of 66%. During surgery, one eye experienced a superficial channel dissection with anterior Bowman’s layer perforation. Two eyes had a transient superficial inflammatory reaction, which resolved within one week. Segment migration and externalization were found in one eye on the first postoperative day.(22)Coskunseven et al. and Wijdh and Rij reported implantation at 75% of the corneal depth for corneal ring segments. Other surgeons have used 80%.(6)
ICRS FOR KERATOCONUS
The aim of ICRS implantation is not to treat or eliminate the existing disease, or should at least not be considered as a traditional refractive surgical procedure. However, ICRS is a surgical alternative to decrease astigmatisms and corneal abnormalities and, thus, increase the visual acuity to acceptable limits aiming to at least delay the need of corneal grafting.(15)
Intacs have been used to treat patients with keratoconus. The results are encouraging, especially in decreasing astigmatism, increasing topographical abnormalities, and minimizing the risk of further progression of corneal ectasia.(19)Khan et al. reported that Intacs SK are effective for the correction of refractive error in eyes with advanced keratoconus. They showed that the mean spherical equivalent was −6.57 D preoperatively and −2.84 D at 12 months. The mean UCVA was significantly improved 12 months postoperatively (logMAR, 0.88) than preoperatively. (23)
Colin et al.reported clinical application of Intacs for the optical correction of keratoconus patients who were contact lens-intolerant. After implantation, UCVA and corneal regularity improved, and the amount of astigmatism and spherical error were reduced. This procedure seems to be promising in terms of avoiding penetrating keratoplasty and increasing the patient’s quality of vision. In more severe cases, caution should be exercised; this surgery may not be effective in patients with severe keratoconus who have thin corneas.(8)
The placement of ICRS generates both an immediate response that interrupts the biomechanical disease progression in keratoconus and a time-dependent biomechanical response that allows subsequent improvement of vision over 6 months.(24)
ICRS ADJUVANT THERAPY AFTER LASIK
Similarly, ICRSs have been used as an adjunct to LASIK surgery. LASIK and Intacs differ in several respects. LASIK is a more versatile technique that corrects low to moderately high levels of myopia (<10 D) and myopic astigmatism (up to 5 D).(25)In contrast, ICRSs are designed to treat only low levels of myopia (up to 3 D) without clinically significant astigmatism. The dissimilarity between the underlying mechanism responsible for the induced central corneal flattening suggests that the procedures could be additive.
The use of LASIK and Intacs as sequential surgeries has been reported. We have implanted Intacs in patients as a means of treating residual myopia 2 years after LASIK. Further excimer laser surgery might have been unsafe in these patients and may have increased the risk of keratoectasia.(26)
1. Nosé W, Neves RA, Burris TE, Schanzlin DJ, Belfort Júnior R. Intrastromal corneal ring: 12-month sighted myopic eyes. J Refract Surg. 1996;12(1):20-8.
2. Burris TE, Holmes-Higgin DK, Silvestrini TA, Scholl JA, Proudfoot RA, Baker PC. Corneal asphericity in eye bank eyes implanted with the intrastromal corneal ring. J Refract Surg. 1997;13(6):556-67.
3. Piñero DP, Alio JL. Intracorneal ring segments in ectatic corneal disease - a review. Clin Exp Ophthalmol. 2010;38(2):154-67.
4. Barraquer JI. Modification of refraction by means of intracorneal inclusions. Int Ophthalmol Clin. 1966;6(1):53-78.
5. Fleming JF, Wan WL, Schanzlin DJ. The theory of corneal curvature change with the Intrastromal Corneal Ring. CLAO J. 1989;15(2):146-50.
6. Mannis MJe, Holland EJe. Cornea. Fourth edition. ed.
7. Assil KK, Barrett AM, Fouraker BD, Schanzlin DJ. One-year results of the intrastromal corneal ring in nonfunctional human eyes. Intrastromal Corneal Ring Study Group. Arch Ophthalmol. 1995;113(2):159-67.
8. Colin J, Cochener B, Savary G, Malet F. Correcting keratoconus with intracorneal rings. J Cataract Refract Surg. 2000;26(8):1117-22.
9. Akaishi L, Tzelikis PF, Raber IM. Ferrara intracorneal ring implantation and cataract surgery for the correction of pellucid marginal corneal degeneration. J Cataract Refract Surg. 2004;30(11):2427-30.
10. Alió J, Salem T, Artola A, Osman A. Intracorneal rings to correct corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2002;28(9):1568-74.
11. El-Husseiny M, Tsintarakis T, Eppig T, Langenbucher A, Seitz B. [Intacsintracorneal ring segments in keratoconus]. Ophthalmologe. 2013;110(9):823-6, 8-9.
12. Daxer A. Biomechanics of Corneal Ring Implants. Cornea. 2015;34(11):1493-8.
13. Andreassen TT, Simonsen AH, Oxlund H. Biomechanical properties of keratoconus and normal corneas. Exp Eye Res. 1980;31(4):435-41.
14. Zare MA, Mehrjardi HZ, Afarideh M, Bahrmandy H, Mohammadi SF. Visual, Keratometric and Corneal Biomechanical Changes after Intacs SK Implantation for Moderate to Severe Keratoconus. J Ophthalmic Vis Res. 2016;11(1):17-25.
15. Brightbill FS. Corneal surgery : theory, technique and tissue. 4th ed. ed. [St. Louis, Mo.] ; London: Mosby; 2009.
16. Pérez-Merino P, Ortiz S, Alejandre N, de Castro A, Jiménez-Alfaro I, Marcos S. Ocular and optical coherence tomography-based corneal aberrometry in keratoconic eyes treated by intracorneal ring segments. Am J Ophthalmol. 2014;157(1):116-27.e1.
17. Schanzlin DJ, Asbell PA, Burris TE, Durrie DS. The intrastromal corneal ring segments. Phase II results for the correction of myopia. Ophthalmology. 1997;104(7):1067-78.
18. Alió JL, Shabayek MH, Artola A. Intracorneal ring segments for keratoconus correction: long-term follow-up. J Cataract Refract Surg. 2006;32(6):978-85.
19. Yanoff M, Duker JS. Ophthalmology. Fourth edition. ed. Philadelphia, Pennsylvania: Elsevier Saunders; 2014. xxiii, 1404 pages p.
20. Ertan A, Kamburoğlu G. Intacs implantation using a femtosecond laser for management of keratoconus: Comparison of 306 cases in different stages. J Cataract Refract Surg. 2008;34(9):1521-6.
21. Alió JL, Artola A, Hassanein A, Haroun H, Galal A. One or 2 Intacs segments for the correction of keratoconus. J Cataract Refract Surg. 2005;31(5):943-53.
22. Boxer Wachler BS, Christie JP, Chandra NS, Chou B, Korn T, Nepomuceno R. Intacs for keratoconus. Ophthalmology. 2003;110(5):1031-40.
23. Khan MI, Injarie A, Muhtaseb M. Intrastromal corneal ring segments for advanced keratoconus and cases with high keratometric asymmetry. J Cataract Refract Surg. 2012;38(1):129-36.
24. Ertan A, Colin J. Intracorneal rings for keratoconus and keratectasia. J Cataract Refract Surg. 2007;33(7):1303-14.
25. Peyman GA. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol. 1996;122(2):284-5.
26. Primack JD, Azar DT. Laser in situ keratomileusis and intrastromal corneal ring segments for high myopia. Three-step procedure. J Cataract Refract Surg. 2003;29(5):869-74.