The first implants of peripheral intracorneal rings in humans were reported in 1966, targeting potential changes in refraction.1 Later, further improvements included changes in geometry (different arc lengths and heights, adjustable rings) and materials (synthetic gels or rigid materials).2 Currently, the most frequently implanted ring models are made of polymethylmethacrylate (PMMA), have a triangular or hexagonal shape, and are available in several dimensions.3
In 1987, intrastromal corneal ring segments (ICRSs) were proposed to achieve refractive adjustment by flattening the cornea. Nosé et al.4 made the first implants in human corneas in 1991, in Brazil. Almost 10 years later, Colin et al.5 reported the results of ICRS implantation to treat patients with keratoconus. Since then, ICRSs have been used to regularize the corneal shape and reduce corneal astigmatism and higher order aberrations, improve visual acuity to acceptable limits, and delay, or eventually prevent, a corneal keratoplasty.6–8 Changes in ICRS thickness and size, combination of techniques, and the addition of femtosecond lasers to dissect more foreseeable channels represent an improvement toward more predictable results.9–12
Predictive Factors and the Theory Through Which it Works
Understanding the responsive biomechanical properties of the cornea definitely helps in determining the best treatment option, predicting the results, and eventually anticipating postoperative corneal behavior.13 The changes in corneal structure induced by additive strategies can be roughly predicted by Barraquer's law of thickness: when a comparable amount of material or tissue is either added to the periphery of the cornea or removed from the central cornea, a resultant flattening effect is achieved.14 Following that rule, an inferior ICRS should be placed in an inferior cone. However, in an astigmatic laser treatment where tissue is removed from the periphery (in the flattest axis), leading to steepening, ICRS implantation as an additive procedure presents the opposite effect. Because the addition occurs in the periphery, a flattening effect occurs at the same meridian. Nevertheless, a laser treatment to correct a with-the-rule astigmatism should be at 180° and the ring should also be placed in the same position. Working at a deeper corneal depth, the ICRS presents no anterior lamellar effect and causes a steepening of the meridian of implantation and flattening the perpendicular one (the open axis), as opposed to what Barraquer's law would predict and to what is expected in excimer laser procedures. The primary explanation for this differential response is associated with the mechanical action. The ICRS acts by pushing and redistributing the corneal tissue,12 forcing the lamellae to detour around the ring and causing a small local bump. In previous studies, it has been discussed that the ICRS rotates within the cornea due to tissue wound healing. It apparently is not an actual rotation but rather a result of the plane shape of the rigid triangular ICRS inserted into the soft and curved cornea.13
The results of a ring implant correlate directly to its thickness and inversely to its distance to the visual axis or optical zone. The thicker the implant and the smaller the optical zone, the higher the corrective results. As shown in previous studies, the ICRS flattens the cornea and the effect persists for an extended period.6,8,15
Several authors have been trying to understand the best way to predict the results of ICRS implantation through the analysis of different parameters. Some studies7,12 found that the higher the preoperative astigmatism in advanced stages of keratoconus, the lower the predictability of its correction. Others showed that patients with more advanced forms of the disease presented the greatest reduction in spherical equivalent (SE) or keratometry.10,16
To date, refraction and corneal topographic pattern and the preoperative aberrometry data have been implied as significant factors to be considered in the planning of an ICRS implantation.17 To further understand the predictability, some studies12,18 investigated the visual and refractive outcomes of keratoconic eyes with coincident and not coincident topographic and comatic axis. They found satisfactory refractive results and a significant reduction of corneal coma-like aberrations assigned to the choice of the arc length and thickness of the ICRS implanted because the higher thickness of ICRS implanted, the greater effect obtained.12,18
Kling and Marcos13 developed a finite-element model to predict the corneal response to different ICRS geometrics in corneas with and without keratoconus. In addition to confirming that the anterior corneal shape is the most important parameter to determine corneal deformation and refractive achievements after ICRS implantation, they also found that posterior surface and the corneal apex shift have a significant contribution, although less representative than the anterior curvature. Nevertheless, up to now, there is no definitive predictive model derived from these studies that can be clinically useful.
A preliminary study19 revealed that when ICRSs are used to correct ectasia after refractive surgery they redistribute the corneal stress by shortening lamellae and changing corneal shape without altering the viscoelastic response of the corneal tissue.9,19
The major shifts in refraction and topographic findings after ICRS implantation usually occur in the early postoperative period.20 Late postoperative variations probably occur due to intrinsic biomechanical changes in those with unstable disease. Piñero et al.21 found that mean keratometry and the difference between corneal hysteresis and corneal resistance are factors that would allow some prediction for the postoperative visual outcome in a short term.
As seen, studies5,16,20,22 have shown that the ICRS implantation is more effective in those with moderate stages of the ectatic disease and that the unpredictability of this surgery, when performed on eyes with initial grades of keratoconus, remains a concern. Preoperative corrected distance visual acuity (CDVA) is also a useful predictor of success after ICRS because patients presenting a markedly decreased vision are those who significantly gain more lines after the implantation.16
Peña-García et al.23 recently proposed a new way to predict the results in patients with early stages of the disease, specifically grade II. This nomogram takes into account refractive astigmatism only, excluding corneal astigmatism and internal astigmatism. The internal astigmatism is the vectorial difference between the refractive cylinder and the corneal cylinder. They demonstrated that the gain in CDVA was greater when there was a relative alignment between flattest keratometry and the refractive cylinder axis (when using negative cylinder) and concluded that the higher internal astigmatism, the lower the gain in CDVA. Therefore, the most predictable outcomes would be reached when the refractive and keratometric axis are perfectly aligned, and the internal astigmatism is low.23 Although larger series and prospective studies are needed to confirm these findings, this nomogram is an alternative tool to improve the success rate in patients with early stages of keratoconus.
Unfortunately, there are only a few studies specifically assessing the predictability (percentage of eyes within ± 1.00 diopters [D] of the intended correction), efficacy, and safety of this procedure (Table 1). They revealed that there was a significant decrease in the cylinder after surgery, associated with satisfactory levels of efficacy and safety. Despite its theoretical reversibility, ICRS implantation presents a significantly lower predictability compared to excimer laser procedures.24,25
Predictability, Safety, and Efficacy Indexes of ICRS Procedures
The primary indications for ICRS implantation include keratoconus in moderate to advanced stages with clear corneas and patients with ectasia after excimer laser treatment with unsatisfactory CDVA or contact lens intolerance.6,8,10,26 ICRS implantation can also be an alternative for visual rehabilitation in patients with corneal irregularities after radial keratotomy, penetrating keratoplasty, pellucid marginal degeneration (PMD), or after trauma.6,26,27
Depending on the cone location, there are different options of rings to be implanted. Longer arc lengths are suitable for nipple or central cones, whereas shorter arc lengths are preferred in astigmatic cones.11 Thicker implants are better in patients with steep keratometry (the thicker the ring, the greater the flattening effect). In thinner corneas, thinner segments are required.11,14
Those patients with a markedly restrictive visual acuity are more prone to positive outcomes from ICRS implantation,16 as opposed to eyes with 0.1 logMAR or better that present a higher chance of unpredictable results.23
There is no consensus in the literature about a particular cut-off number for maximum keratometry and there are reports of acceptable visual results even in advanced stages.10,26,28
Channel Dissection Methods
Previous studies showed the femtosecond laser can efficiently dissect the ring channel and that there are no significant differences compared to a mechanical spreader regarding uncorrected distance visual acuity, CDVA, SE, maximum keratometry value, surface irregularity, or surface asymmetry indices. In fact, the femtosecond laser method produces fewer and less severe complications than the mechanical one.3,29
The surgery is performed under topical anesthesia after miosis achieved with 2% pilocarpine. The central cornea is marked with a felt-tipped pen. The incision site is at the steepest topographic axis of the cornea given by the topographer.5
A diamond knife is used to create a temporal radial 1-mm incision approximately 70% to 80% of the corneal thickness on the steepest axis of the topographic map. The intrastromal tunnels are initiated using a pocketing hook (formerly a stromal spreader). A guide blade is used to assess the incision's length and verify the adequacy of the pocket. Pockets should be at the same depth across the full width of the incision, within the same stromal plane, and as long as the stromal spreader. A semi-automated suction ring device could be used to help dissection of a stromal plane.15 Following channel creation, the ring segments are inserted using a modified McPherson forceps. The rings are correctly positioned with the aid of a regular Sinskey hook and an inverse Sinskey hook.5,8
The procedure is typically done using topical anesthesia. Peripheral ultrasound pachymetry or a Scheimpflug/optical coherence tomography pachymetry map is needed to ensure sufficient corneal thickness and appropriate depth of ICRS placement. The tunnel is created at approximately 70% to 80% of the corneal thickness. Then, a vertical entry cut is created by the femtosecond laser to allow access for ring placement in the tunnel. The creation of the intrastromal tunnel with the femtosecond laser is complete within 15 seconds with no manipulation of the cornea. One or two ICRSs are inserted using a modified McPherson forceps in the created tunnels. The preferred incision location is related to the position of segments, allowing a safe distance between the incision and the closest part. In the case of two segments, both should be further placed at least 1 mm away from the lip of the incision, in a fashion that avoids touching each other on the other side.3,30
The closure of the wound with a single 10-0 through and through nylon suture is optional (both techniques). A soft contact lens with an approximate correction can be placed on the eye and antibiotic, steroid, and cycloplegic drops are given to the patient postoperatively.3,26,28,30 Theoretically, solely based on surgeon's skill, the femtosecond laser-assisted procedure should generate a more accurate stromal dissection with less depth variability compared with mechanical tunnel creation, possibly translating into a more predictable refractive result.
Currently Available Rings
There are five models of rings currently available (Table 2).
Intracorneal Ring Models
Intacs (Addition Technology, Inc., Fremont, CA) has PMMA curved segments with an arc length of 150°; transverse hexagonal base; conical longitudinal section; external diameter of 8.1 mm, inner diameter of 6.77 mm; and thickness of 250 to 450 µm. There is a new design (Intacs SK) with an internal diameter of 6 mm; oval cross-section; and thickness of 400 µm (for keratometries 57.00 to 62.00 D and cylinder < 5.00 D) and 450 µm (for keratometries > 62.00 D and cylinder > 5.00 D).
Keraring (Mediphacos, Belo Horizonte, Brazil) has PMMA segments; a triangular base of 600 µm; an apical diameter of 5 mm or 6 mm; 150 to 350 µm thickness change with every 50 µm; and 90°, 120°, 150°, 160°, 210°, and 340° of arc. There are two models: SI5 (optical zone of 5 mm) and SI6 (optical zone of 5.5 mm or 6 mm). The SI5 induces central corneal flattening and astigmatism reduction and is preferred in eyes with advanced keratoconus. The SI6 is a better choice for eyes with early keratoconus, where the ectasia surface area is still small and does not reach or affect the central cornea. The desired effect of the ICRS is astigmatism reduction, not central flattening.13
The Ferrara Ring Segment (Ferrara Ophthalmics, Belo Horizonte, Brazil) has segments of PMMA Perspex CQ; a diameter of 6 mm for myopia up to −7.00 D and 5 mm for high myopia; 150 to 350 µm thickness change every 50 µm; 4.4-mm inner diameter and 5.4 mm external diameter to 5-mm optic zone and 5.4-mm inner diameter and 6.4-mm external diameter to 6-mm optic zone; triangular base with a constant 600 mm; and 90° to 210° of arc.
The Corneal Ring (Visiontech, Belo Horizonte, Brazil) has PMMA segments with spindle cross-section; an inner diameter of 4.7 mm; thickness of 150 to 350 µm change with every 50 µm; and 155° (standard) and 220° (special) of arc.
MyoRing ICCR (Dioptex GmbH, Linz, Austria) is a PMMA 360° continuous full ring to be implanted into a corneal pocket. The pocket is created using the Pocket-Maker microkeratome (Dioptex GmbH) or by femtosecond laser technology.9 It is suggested to keep the width of the corneal pocket entry less than 5.5 mm. Based on the grade of myopia and keratoconus, the diameter of the MyoRing ranges from 5 to 8 mm and the thickness ranges from 200 to 320 µm. This ring features a convex anterior and a concave posterior surface with an 8-mm radius of curvature.
Surgical Planning and Patient Selection
Single, Paired, and Continuous ICRS
Several ICRS types are available on the market, with differences in the arc, thickness, and zone of implantation. The segments can be continuous, single, or paired.
Some studies have demonstrated that implanting a single segment may provide better results than two segments in cases of peripheral ectasia (greater reduction in astigmatism) and the option of two segments, or a longer continuous one, is better in cases of central keratoconus (more significant decrease in the spherical equivalent).31–34
A complete, or an almost complete (340°), intrastromal ring is a new technique that has also been demonstrated to be safe and efficient, especially in cases with high myopia.9,35–37 In contrast to partial ICRS surgery, in which segment selection requires complex grading of the keratoconus and distinguishing between different types of cones, continuous ring implantation uses a simple nomogram in which keratoconus is graded using the keratometry readings only.9 Complete ring implantation may have a greater flattening effect on the anterior corneal surface.
Table A (available in the online version of this article) shows the recent data about the comparative studies between single and double segments or continuous ring implantation. Most of those comparisons involve different groups of patients with keratoconus (usually two rings are used in central cones and one ring in peripheral cones), making it harder to draw any conclusion.
Studies Comparing Single, Paired, and Complete ICRS Implantation
ICRS Position and Depth
Each manufacturer provides a specific nomogram for the ICRS position and depth, which takes into account the keratoconus pattern (central, paracentral, or peripheral) and the manifest refraction or topographical astigmatism. The rings can be placed with the axis of the open part of the segment aligned with the steepest axis of manifest refraction/topographical cylinder (or the center of the segment aligned with the flattest axis). Besides the manufacturers' guidelines, a detailed description including nomograms of currently available ICRSs was published by Alió et al.38 Recent studies have also tried to provide better algorithms to obtain an adequate position for a particular segment in a given cornea.22,23 Because reducing higher order aberrations (irregular corneal shape) is a critical part of ICRS effectiveness, it seems reasonable that more advanced nomograms should include coma and relevant aberrations. Nomograms based on a neural network of clinical cases are also under development.38
Influence of Corneal Asphericity on Ring Selection
The ring selection can also be based on preoperative corneal asphericity (Q). In keratoconus, the steepening of the central cornea leads to an increase in Q value. The thicker the implanted segment, the greater the reduction in corneal asphericity, considering the normal Q value (−0.23 ± 0.08).39
The ICRS models in the market are designed to be used in a specific optical zone. Because the greater the optical zone, the lower the effect in the central cornea, there is a tendency to use a smaller optical zone for more advanced keratoconus.13,28 Larger optical zone models present a greater effect on astigmatism, whereas smaller optical zone models have a more pronounced effect on SE.40
Symmetric segments with shorter arc lengths would primarily reduce corneal astigmatism with less effect on spherical error, whereas longer arc lengths would essentially change spherical error by flattening the cornea, with limited effect on astigmatism.11 The main advantage of shorter arc length models is the considerable reduction in astigmatism, which is primarily the reason they have been used on astigmatic cones and PMD, clinical situations in which it is preferable to prioritize the astigmatism correction.41
Abad et al.42 investigated the effect of asymmetric shorter arc length segments to correct astigmatism in asymmetric keratoconus with more than 2.00 D of astigmatism and found that implantation of asymmetric shorter arc length 90°/120° Intacs SK (severe keratoconus) segments achieved 33% higher correction of astigmatism compared with 150° single-segment Intacs SK implantation.
Complications associated with the mechanical technique include epithelial defects at the keratotomy site, anterior or posterior perforations during channel creation, the extension of the incision toward the central visual axis or the limbus, superficial placement or asymmetric placement of the segments, or infectious keratitis. Persisting incisional gaping, decentration, stromal thinning, and corneal stromal edema around the incision and channel from excessive surgical manipulation are also possible complications.22,26,30 The epithelial defect rate seems to be significantly higher when the channel is created mechanically.30 Halos, glare, small white deposits, and neovascularization have been reported.15 Halos occur in approximately 10% of patients and may be related to the pupil size and smaller optical zone.
The femtosecond laser theoretically reduces the occurrence of extrusion with its higher predictability in depth.43 Migration and extrusion are commonly related to ICRSs placed too close to the incision, which impairs its actual closure.
Complications with femtosecond lasers have also been reported and include intraoperative incomplete channel creation (2.7%), postoperative segment migration (1.3%), endothelial perforation (0.6%), vacuum loss (0.1%), corneal melting (0.2%), and infection following implantation (0.1%).43
The dissection of a false or second channel can cause the leading edge of the segment to enter a plane other than the femtosecond laser-created channel. This could be solved by removing the segment, turning around, and inserting it in the opposite direction through the femtosecond channel (turnaround technique).
Extrusion, refractive failure, microbial keratitis, and corneal melting were reported to be the main leading reasons for ICRS explantation44 and, in some cases, a new implantation can be safely performed with significant visual and refractive improvements.25
Since Colin et al.5 introduced ICRS implantation for the treatment of keratoconus, several studies have shown the long-term efficacy and safety of ICRS treatment for keratoconus with variable predictability,8,20 maintaining the early satisfactory outcomes regarding visual acuity, keratometry, and corneal thickness.8
When analyzing the main studies published, the mean change in SE ranged from 0.96 to 3.85 D, the average change in mean keratometry ranged from 1.92 to 5.03 D, and the gain of lines of vision ranged from 45% to 86.6% of patients. Table 3 summarizes previously published studies and their results in patients with keratoconus.6–8,10,12,15,31,45–50 The association with CXL will be further discussed in the manuscript.
Visual and Refractive Outcomes in Different Studies After ICRS Implantation in Keratoconic Eyes
In PMD, the aim with ICRS implantation is to reshape the cornea and reduce the asymmetrical astigmatism by mostly using shorter arc length segments. It is an alternative to enhance visual acuity, especially in patients who are intolerant to contact lenses. Studies showed that ICRS placement is an effective, safe, reversible, and relatively conservative choice compared to surgical management in early and moderate PMD.51 To avoid corneal perforation in PMD cases, a thinner segment (0.25 mm) should be used.52 There is a tendency toward using a single ICRS in corneas with PMD due to its peripheral location.51
The main target of implanting ICRS for post-LASIK ectasia is to regularize the corneal shape, improve visual acuity, and avoid the need for penetrating keratoplasty. Several studies have demonstrated better visual outcomes, reduction in spherocylinder error, and keratometry.33,53,54 Patients with satisfactory visual acuity after developing ectasia or those with early stages of post-LASIK ectasia should not be considered candidates for ICRS implantation because of the risk of vision loss after implantation.
Most studies showed significant changes in SE, which implies an improvement in visual acuity, especially uncorrected distance visual acuity53 presenting an apparently reduced effect in those who had no significant vision loss caused by post-LASIK ectasia.55 Sharma and Boxer Wachler33 compared single- versus double-segment implantation for post-LASIK ectasia and observed greater improvement in the single-segment group.
When planning ICRS implantation after LASIK, optical coherence tomography may help to determine the flap thickness and to avoid the flap dissection and ring implantation within the flap interface, which could dislocate the flap and get a minimal effect from the procedure. The key is to place the ring deep enough to be right below the plane of the flap, as well as sufficiently far from the posterior face.56
High astigmatism and ametropia are some of the main limiting factors for visual rehabilitation after corneal transplant. ICRS implantation regularizes the corneal surface, improving visual acuity, reducing the manifest refraction, and avoiding another corneal transplant.55
Arriola-Villalobos et al.55 evaluated 9 eyes that had mechanical ICRS implantation after penetrating keratoplasty and observed reduced keratometry values and significantly improved corneal topography and CDVA with no cases of graft rejection. The authors recommend waiting for at least 2 years after the corneal transplant and 1 year after suture removal to avoid damaging the interface by the traction during mechanical dissection.
Lisa et al.57 reported 32 eyes that had penetrating keratoplasty that underwent ICRS implantation for astigmatism correction using the femtosecond laser technology. They showed that the UDVA and the CDVA improved significantly after ICRS implantation in approximately 90% of eyes. They proposed the following nomogram: when astigmatism is higher than 6.00 D, the ICRS must have an apical diameter of 6 mm in a 5-mm optical zone and 300 µm of thickness. This shift increases the pushing effect of the ICRS in the corneal tissue and enhances the astigmatism correction.
Most patients with corneal ectasia due to a progressive disease experience poor visual acuity due to irregular astigmatism. Only CXL has been shown to halt the progression of the disease without predictably achieving visual rehabilitation. The next step in such patients is to improve visual acuity. Although rigid gas permeable contact lenses or scleral lenses are less invasive and attractive options, the results and tolerance rates are limited in highly irregular corneas. Thus, several studies have demonstrated other options to address the visual acuity in eyes with keratoconus.58–62
CXL may theoretically provide a pivotal complementary effect in combination with the ICRS, halting the keratoconus progression.58,60,63 There is no consensus about the order of surgeries or if they should be combined into one surgical procedure.59
We are of the opinion that, due to the intense corneal remodeling following CXL,64,65 it seems reasonable that the chances of a most predictable refractive effect increase after a year. Therefore, and considering there are no further difficulties in implanting ICRSs in cross-linked corneas, waiting for the remodeling effect may be the best option.
If planned at the same procedure, riboflavin could be delivered by previous epithelial debridement (traditional technique) or inside the intrastromal pocket. No statistically significant differences were observed between both methods.63 In cases of progressive PMD, CXL should also be associated to ICRS. In 2010, Kymionis et al.66 reported a significant improvement in visual acuity in a patient with PMD who demonstrated progression after ICRS placement and then had simultaneous photorefractive keratectomy and CXL without segment explantation.
Phakic Intraocular Lens Implantation
The main benefit of the phakic intraocular lens approach is that greater degrees of spherocylindrical error can be corrected with no tissue removal and no risk of corneal haze.67 In patients with keratoconus, ICRS combined with the implantation of the phakic intraocular lens proved to be a safe and predictable procedure. Although Moshirfar et al.68 observed similar results when comparing simultaneous and sequential implantation, most studies recommended at least 6 months between procedures, when the manifest refraction and corneal topography were stable. In patients with progressive disease, CXL can also be associated.69
Photorefractive Keratectomy + CXL
Previous studies that investigated surface ablation associated with ICRS also included CXL in their protocol. A study70 with same-day topography-guided photorefractive keratectomy and CXL after previous ICRS implantation for keratoconus found a significant increase in visual acuity and reduction in keratometry, sphere, and astigmatism. In these trials, the patients were observed after ICRS implantation for at least 3 months, expecting stabilization of refraction. A maximum ablation depth of 50 µm62 was recommended. In some cases, mitomycin C 0.02% was used for 30 seconds after ablation with no consensus been established regarding this topic.62
Elbaz et al.71 showed similar results with accelerated (10 minutes of ultraviolet-A irradiation at 9 mW/cm2) or standard CXL combined with same-day transepithelial phototherapeutic keratectomy and single ICRS implantation for keratoconus, with 12 months of follow-up.
Despite the positive results, longer follow-up studies are needed to dismiss concerns of long-term complications and to corroborate these outcomes. Table 4 summarizes the latest data about this triple procedure.
ICRS + PRK + CXL
There seems to be enough evidence to consider ICRS implantation an effective and safe procedure in most cases. Although ICRS implantation did not halt the progression of keratoconus, it significantly improved the visual, refractive, and topographic parameters in the short term. However, more studies specifically demonstrating its predictability are needed. Prospective studies with longer follow-up remain necessary to develop and validate innovative nomograms, addressing not only the spherocylindrical error, but also higher order aberrations and corneal asymmetries. The initial results of combined procedures are encouraging, despite the lack of adjusted nomograms and definition of a better sequence of approach. Protocols' uniformity and collaboration between different research groups can help to enhance results with this promising procedure.
- Blavatskaia ED, Viazovskii IA, Barsegian LG. [Change in corneal curvature in intralamellar homotransplantation of discs of various diameter and thickness]. Oftalmol Zh. 1967;22:123–128.
- Fleming JF, Wan WL, Schanzlin DJ. The theory of corneal curvature change with the Intrastromal Corneal Ring. CLAO J. 1989;15:146–150.
- Kubaloglu A, Cinar Y, Sari ES, Koytak A, Ozdemir B, Ozertürk Y. Comparison of 2 intrastromal corneal ring segment models in the management of keratoconus. J Cataract Refract Surg. 2010;36:978–985. doi:10.1016/j.jcrs.2009.12.031 [CrossRef]
- Nosé W, Neves RA, Schanzlin DJ, Belfort Júnior R. Intrastromal corneal ring—one-year results of first implants in humans: a preliminary nonfunctional eye study. Refract Corneal Surg. 1993;9:452–458.
- Colin J, Cochener B, Savary G, Malet F. Correcting keratoconus with intracorneal rings. J Cataract Refract Surg. 2000;26:1117–1122. doi:10.1016/S0886-3350(00)00451-X [CrossRef]
- Alió JL, Shabayek MH, Artola A. Intracorneal ring segments for keratoconus correction: long-term follow-up. J Cataract Refract Surg. 2006;32:978–985. doi:10.1016/j.jcrs.2006.02.044 [CrossRef]
- Alfonso JF, Lisa C, Fernández-Vega L, Madrid-Costa D, Montés-Micó R. Intrastromal corneal ring segment implantation in 219 keratoconic eyes at different stages. Graefes Arch Clin Exp Ophthalmol. 2011;249:1705–1712. doi:10.1007/s00417-011-1759-9 [CrossRef]
- Torquetti L, Ferrara G, Almeida F, et al. Intrastromal corneal ring segments implantation in patients with keratoconus: 10-year follow-up. J Refract Surg. 2014;30:22–26. doi:10.3928/1081597X-20131217-02 [CrossRef]
- Alió JL, Piñero DP, Daxer A. Clinical outcomes after complete ring implantation in corneal ectasia using the femtosecond technology: a pilot study. Ophthalmology. 2011;118:1282–1290.
- Ertan A, Kamburoglu G. Intacs implantation using a femtosecond laser for management of keratoconus: comparison of 306 cases in different stages. J Cataract Refract Surg. 2008;34:1521–1526. doi:10.1016/j.jcrs.2008.05.028 [CrossRef]
- Ferrara P, Torquetti L. Clinical outcomes after implantation of a new intrastromal corneal ring with a 210-degree arc length. J Cataract Refract Surg. 2009;35:1604–1608. doi:10.1016/j.jcrs.2009.04.035 [CrossRef]
- Alfonso JF, Lisa C, Merayo-Lloves J, Fernández-Vega Cueto L, Montés-Micó R. Intrastromal corneal ring segment implantation in paracentral keratoconus with coincident topographic and coma axis. J Cataract Refract Surg. 2012;38:1576–1582. doi:10.1016/j.jcrs.2012.05.031 [CrossRef]
- Kling S, Marcos S. Finite-element modeling of intrastromal ring segment implantation into a hyperelastic cornea. Invest Ophthalmol Vis Sci. 2013;54:881–889. doi:10.1167/iovs.12-10852 [CrossRef]
- Barraquer JI. Modification of refraction by means of intracorneal inclusions. Int Ophthalmol Clin. 1966;6:53–78. doi:10.1097/00004397-196606010-00004 [CrossRef]
- Torquetti L, Berbel RF, Ferrara P. Long-term follow-up of intrastromal corneal ring segments in keratoconus. J Cataract Refract Surg. 2009;35:1768–1773. doi:10.1016/j.jcrs.2009.05.036 [CrossRef]
- Vega-Estrada A, Alió JL, Brenner LF, et al. Outcome analysis of intracorneal ring segments for the treatment of keratoconus based on visual, refractive, and aberrometric impairment. Am J Ophthalmol. 2013;155:575–584. doi:10.1016/j.ajo.2012.08.020 [CrossRef]
- Piñero DP, Alió JL, Teus MA, Barraquer RI, Uceda-Montañés A. Modeling the intracorneal ring segment effect in keratoconus using refractive, keratometric, and corneal aberrometric data. Invest Ophthalmol Vis Sci. 2010;51:5583–5591. doi:10.1167/iovs.09-5017 [CrossRef]
- Alfonso JF, Fernández-Vega Cueto L, Baamonde B, Merayo-Lloves J, Madrid-Costa D, Montés-Micó R. Inferior intrastromal corneal ring segments in paracentral keratoconus with no coincident topographic and coma axis. J Refract Surg. 2013;29:266–272. doi:10.3928/1081597X-20130318-06 [CrossRef]
- Dauwe C, Touboul D, Roberts CJ, et al. Biomechanical and morphological corneal response to placement of intrastromal corneal ring segments for keratoconus. J Cataract Refract Surg. 2009;35:1761–1767. doi:10.1016/j.jcrs.2009.05.033 [CrossRef]
- Kymionis GD, Siganos CS, Tsiklis NS, et al. Long-term follow-up of Intacs in keratoconus. Am J Ophthalmol. 2007;143:236–244. doi:10.1016/j.ajo.2006.10.041 [CrossRef]
- Piñero DP, Alió JL, Barraquer RI, Michael R. Corneal biomechanical changes after intracorneal ring segment implantation in keratoconus. Cornea. 2012;31:491–499. doi:10.1097/ICO.0b013e31821ee9f4 [CrossRef]
- Peña-García P, Vega-Estrada A, Barraquer RI, Burguera-Giménez N, Alió JL. Intracorneal ring segment in keratoconus: a model to predict visual changes induced by the surgery. Invest Ophthalmol Vis Sci. 2012;53:8447–8457. doi:10.1167/iovs.12-10639 [CrossRef]
- Peña-García P, Alió JL, Vega-Estrada A, Barraquer RI. Internal, corneal, and refractive astigmatism as prognostic factors for intrastromal corneal ring segment implantation in mild to moderate keratoconus. J Cataract Refract Surg. 2014;40:1633–1644. doi:10.1016/j.jcrs.2014.01.047 [CrossRef]
- Schwartz AR, Tinio BO, Esmail F, Babayan A, Naikoo HN, Asbell PA. Ten-year follow-up of 360 degrees intrastromal corneal rings for myopia. J Refract Surg. 2006;22:878–883.
- Torquetti L, Ferrara G, Almeida F, Cunha L, Ferrara P, Merayo-Lloves J. Clinical outcomes after intrastromal corneal ring segments reoperation in keratoconus patients. Int J Ophthalmol. 2013;6:796–800.
- Ertan A, Colin J. Intracorneal rings for keratoconus and keratectasia. J Cataract Refract Surg. 2007;33:1303–1314. doi:10.1016/j.jcrs.2007.02.048 [CrossRef]
- Torquetti L, Ferrara P. Intrastromal corneal ring segment implantation for ectasia after refractive surgery. J Cataract Refract Surg. 2010;36:986–990. doi:10.1016/j.jcrs.2009.12.034 [CrossRef]
- Rabinowitz YS. INTACS for keratoconus and ectasia after LASIK. Int Ophthalmol Clin. 2013;53:27–39. doi:10.1097/IIO.0b013e3182774453 [CrossRef]
- Shabayek MH, Alió JL. Intrastromal corneal ring segment implantation by femtosecond laser for keratoconus correction. Ophthalmology. 2007;114:1643–1652. doi:10.1016/j.ophtha.2006.11.033 [CrossRef]
- Kymionis GD, Kankariya VP, Plaka AD, Reinstein DZ. Femtosecond laser technology in corneal refractive surgery: a review. J Refract Surg. 2012;28:912–920. Erratum in: J Refract Surg. 2013;29:72. doi:10.3928/1081597X-20121116-01 [CrossRef]
- 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;31:943–953. doi:10.1016/j.jcrs.2004.09.050 [CrossRef]
- Yeung SN, Ku JY, Lichtinger A, Low SA, Kim P, Rootman DS. Efficacy of single or paired intrastromal corneal ring segment implantation combined with collagen crosslinking in keratoconus. J Cataract Refract Surg. 2013;39:1146–1151. doi:10.1016/j.jcrs.2013.03.022 [CrossRef]
- Sharma M, Boxer Wachler BS. Comparison of single-segment and double-segment Intacs for keratoconus and post-LASIK ectasia. Am J Ophthalmol. 2006;141:891–895. doi:10.1016/j.ajo.2005.12.027 [CrossRef]
- Fahd DC, Alameddine RM, Nasser M, Awwad ST. Refractive and topographic effects of single-segment intrastromal corneal ring segments in eyes with moderate to severe keratoconus and inferior cones. J Cataract Refract Surg. 2015;41:1434–1440. doi:10.1016/j.jcrs.2014.10.037 [CrossRef]
- Jabbarvand M, Hashemi H, Mohammadpour M, Khojasteh H, Khodaparast M, Hashemian H. Implantation of a complete intrastromal corneal ring at 2 different stromal depths in keratoconus. Cornea. 2014;33:141–144. doi:10.1097/ICO.0000000000000026 [CrossRef]
- Studeny P, Krizova D, Stranak Z. Clinical outcomes after complete intracorneal ring implantation and corneal collagen cross-linking in an intrastromal pocket in one session for keratoconus. J Ophthalmol. 2014;2014:568128. doi:10.1155/2014/568128 [CrossRef]
- Jabbarvand M, Salamatrad A, Hashemian H, Mazloumi M, Khodaparast M. Continuous intracorneal ring implantation for keratoconus using a femtosecond laser. J Cataract Refract Surg. 2013;39:1081–1087. doi:10.1016/j.jcrs.2013.02.054 [CrossRef]
- Alió J, Vega-Estrada A, Sanz-Díez P, Peña-García P, Duran-García M, Maldonado M. Keratoconus management guidelines. International Journal of Keratoconus and Ecstatic Corneal Diseases. 2015;1:1–39. doi:10.5005/jp-journals-10025-1095 [CrossRef]
- Torquetti L, Ferrara P. Corneal asphericity changes after implantation of intrastromal corneal ring segments in keratoconus. J Emmetropia. 2010;1:178–181.
- Haddad W, Fadlallah A, Dirani A, et al. Comparison of 2 types of intrastromal corneal ring segments for keratoconus. J Cataract Refract Surg. 2012;38:1214–1221. doi:10.1016/j.jcrs.2012.02.039 [CrossRef]
- Rodriguez-Prats J, Galal A, Garcia-Lledo M, De La Hoz F, Alió JL. Intracorneal rings for the correction of pellucid marginal degeneration. J Cataract Refract Surg. 2003;29:1421–1424. doi:10.1016/S0886-3350(02)02038-2 [CrossRef]
- Abad JC, Arango J, Tobon C. Comparison of astigmatism correction using shorter arc length 90°/120° asymmetric Intacs severe keratoconus versus 150° single-segment Intacs severe keratoconus in asymmetric keratoconus. Cornea. 2011;30:1201–1206. doi:10.1097/ICO.0b013e3182182bc6 [CrossRef]
- Coskunseven E, Kymionis GD, Tsiklis NS, et al. Complications of intrastromal corneal ring segment implantation using a femtosecond laser for channel creation: a survey of 850 eyes with keratoconus. Acta Ophthalmol. 2011;89:54–57. doi:10.1111/j.1755-3768.2009.01605.x [CrossRef]
- Ferrer C, Alió JL, Montañés AU, et al. Causes of intrastromal corneal ring segment explantation: clinicopathologic correlation analysis. J Cataract Refract Surg. 2010;36:970–977. doi:10.1016/j.jcrs.2009.12.042 [CrossRef]
- Kanellopoulos AJ, Pe LH, Perry HD, Donnenfeld ED. Modified intracorneal ring segment implantations (INTACS) for the management of moderate to advanced keratoconus: efficacy and complications. Cornea. 2006;25:29–33. doi:10.1097/01.ico.0000167883.63266.60 [CrossRef]
- Rabinowitz YS, Li X, Ignacio TS, Maguen E. INTACS inserts using the femtosecond laser compared to the mechanical spreader in the treatment of keratoconus. J Refract Surg. 2006;22:764–771.
- Ertan A, Kamburoglu G, Bahadir M. Intacs insertion with the femtosecond laser for the management of keratoconus: one-year results. J Cataract Refract Surg. 2006;32:2039–2042. doi:10.1016/j.jcrs.2006.08.032 [CrossRef]
- Coskunseven E, Kymionis GD, Tsiklis NS, et al. One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol. 2008;145:775–779. doi:10.1016/j.ajo.2007.12.022 [CrossRef]
- Colin J, Cochener B, Savary G, Malet F, Holmes-Higgin D. INTACS inserts for treating keratoconus: one-year results. Ophthalmology. 2001;108:1409–1414. doi:10.1016/S0161-6420(01)00646-7 [CrossRef]
- Colin J. European clinical evaluation: use of Intacs for the treatment of keratoconus. J Cataract Refract Surg. 2006;32:747–755. doi:10.1016/j.jcrs.2006.01.064 [CrossRef]
- Ertan A, Bahadir M. Management of superior pellucid marginal degeneration with a single intracorneal ring segment using femtosecond laser. J Refract Surg. 2007;23:205–208.
- Kymionis GD, Aslanides IM, Siganos CS, Pallikaris IG. Intacs for early pellucid marginal degeneration. J Cataract Refract Surg. 2004;30:230–233. doi:10.1016/S0886-3350(03)00656-4 [CrossRef]
- Kymionis GD, Tsiklis NS, Pallikaris AI, et al. Long-term follow-up of Intacs for post-LASIK corneal ectasia. Ophthalmology. 2006;113:1909–1917. doi:10.1016/j.ophtha.2006.05.043 [CrossRef]
- Alió J, Salem T, Artola A, Osman A. Intracorneal rings to correct corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2002;28:1568–1574. doi:10.1016/S0886-3350(01)01275-5 [CrossRef]
- Arriola-Villalobos P, Díaz-Valle D, Güell JL, et al. Intrastromal corneal ring segment implantation for high astigmatism after penetrating keratoplasty. J Cataract Refract Surg. 2009;35:1878–1884. doi:10.1016/j.jcrs.2009.05.060 [CrossRef]
- Abad JC. Management of slipped laser in situ keratomileusis flap following intrastromal corneal ring implantation in post-LASIK ectasia. J Cataract Refract Surg. 2008;34:2177–2181. doi:10.1016/j.jcrs.2008.06.049 [CrossRef]
- Lisa C, García-Fernández M, Madrid-Costa D, Torquetti L, Merayo-Lloves J, Alfonso JF. Femtosecond laser-assisted intrastromal corneal ring segment implantation for high astigmatism correction after penetrating keratoplasty. J Cataract Refract Surg. 2013;39:1660–1667. doi:10.1016/j.jcrs.2013.04.038 [CrossRef]
- Kymionis GD. Corneal collagen cross linking - PLUS. Open Ophthalmol J. 2011;5:10. doi:10.2174/1874364101105010010 [CrossRef]
- Coskunseven E, Jankov MR, Hafezi F, Atun S, Arslan E, Kymionis GD. Effect of treatment sequence in combined intrastromal corneal rings and corneal collagen crosslinking for keratoconus. J Cataract Refract Surg. 2009;35:2084–2091. doi:10.1016/j.jcrs.2009.07.008 [CrossRef]
- Yeung SN, Lichtinger A, Ku JY, Kim P, Low SA, Rootman DS. Intracorneal ring segment explantation after intracorneal ring segment implantation combined with same-day corneal collagen crosslinking in keratoconus. Cornea. 2013;32:1617–1620. doi:10.1097/ICO.0b013e3182a738ba [CrossRef]
- Colin J, Velou S. Implantation of Intacs and a refractive intraocular lens to correct keratoconus. J Cataract Refract Surg. 2003;29:832–834. doi:10.1016/S0886-3350(02)01618-8 [CrossRef]
- Coskunseven E, Jankov MR, Grentzelos MA, Plaka AD, Limnopoulou AN, Kymionis GD. Topography-guided transepithelial PRK after intracorneal ring segments implantation and corneal collagen CXL in a three-step procedure for keratoconus. J Refract Surg. 2013;29:54–58. doi:10.3928/1081597X-20121217-01 [CrossRef]
- Alió JL, Toffaha BT, Piñero DP, Klonowski P, Javaloy J. Cross-linking in progressive keratoconus using an epithelial debridement or intrastromal pocket technique after previous corneal ring segment implantation. J Refract Surg. 2011;27:737–743. doi:10.3928/1081597X-20110705-01 [CrossRef]
- Hafezi F, Koller T, Vinciguerra P, Seiler T. Marked remodelling of the anterior corneal surface following collagen cross-linking with riboflavin and UVA. Br J Ophthalmol. 2011;95:1171–1172. doi:10.1136/bjo.2010.184978 [CrossRef]
- Kymionis GD, Tsoulnaras KI, Liakopoulos DA, Paraskevopoulos TA, Kouroupaki AI, Tsilimbaris MK. Excessive corneal flattening and thinning after corneal cross-linking: single-case report with 5-year follow-up. Cornea. 2015;34:704–706. doi:10.1097/ICO.0000000000000424 [CrossRef]
- Kymionis GD, Grentzelos MA, Portaliou DM, et al. Photorefractive keratectomy followed by same-day corneal collagen cross-linking after intrastromal corneal ring segment implantation for pellucid marginal degeneration. J Cataract Refract Surg. 2010;36:1783–1785. doi:10.1016/j.jcrs.2010.06.044 [CrossRef]
- Poulsen DM, Kang JJ. Recent advances in the treatment of corneal ectasia with intrastromal corneal ring segments. Curr Opin Ophthalmol. 2015;26:273–277. doi:10.1097/ICU.0000000000000163 [CrossRef]
- Moshirfar M, Fenzl CR, Meyer JJ, Neuffer MC, Espandar L, Mifflin MD. Simultaneous and sequential implantation of intacs and verisyse phakic intraocular lens for refractive improvement in keratectasia. Cornea. 2011;30:158–163. doi:10.1097/ICO.0b013e3181eeb0dd [CrossRef]
- Coskunseven E, Sharma DP, Jankov MR, Kymionis GD, Richoz O, Hafezi F. Collagen copolymer toric phakic intraocular lens for residual myopic astigmatism after intrastromal corneal ring segment implantation and corneal collagen crosslinking in a 3-stage procedure for keratoconus. J Cataract Refract Surg. 2013;39:722–729. doi:10.1016/j.jcrs.2012.11.027 [CrossRef]
- Kremer I, Aizenman I, Lichter H, Shayer S, Levinger S. Simultaneous wavefront-guided photorefractive keratectomy and corneal collagen crosslinking after intrastromal corneal ring segment implantation for keratoconus. J Cataract Refract Surg. 2012;38:1802–1807. doi:10.1016/j.jcrs.2012.05.033 [CrossRef]
- Elbaz U, Shen C, Lichtinger A, et al. Accelerated versus standard corneal collagen crosslinking combined with same day phototherapeutic keratectomy and single intrastromal ring segment implantation for keratoconus. Br J Ophthalmol. 2015;99:155–159. doi:10.1136/bjophthalmol-2014-304943 [CrossRef]
Predictability, Safety, and Efficacy Indexes of ICRS Procedures
|Author||Design||Eyes (n)||Surgical Procedure||Follow-up (mo)||Predictability||Efficacy Index||Safety Index|
|Alió et al.31||Prospective, comparative||11/15||1 segment/2 segments (Intacs) + femto||12||Group I: reduced cylinder from 5.36 ± 2.77 to −2.89 ± 1.39 D. Group II: reduced cylinder from −4.65 ± 2.29 to −2.26 ± 1.33 D (P < .05)||0.92/0.9||1.53/1.63|
|Alfonso et al.7||Prospective||115 (stage I), 84 (stage II), 20 (stage III)||1 or 2 segments (Keraring) + femto||6||Lower predictability for astigmatism correction in stage III||0.7/0.73/0.36||1.13/1.18/1.28|
|Ertan & Kamburoglu10||Retrospective noncomparative||155 (stage II), 83 (stage III), 68 (stage IV)||2 segments (Intacs)/mechanical||10.39 ± 5.04||No data||No data||No data|
|Alfonso et al.12||Cohort study||56||1 or 2 segments (Keraring)/mechanical||6||78.6%||0.6||1.14|
|Jabbarvand et al.37||Prospective, nonrandomized, consecutive||98||MyoRing + femto||12||74%||No data, efficacya: 22.5%||2.2|
|Studeny et al.36||Retrospective, consecutive, nonrandomized interventional||22||MyoRing (femto) + CXL (intrastromal pocket)||12||Statistically significant reductions of sphere cylinder and spherical equivalents were found 1 month after surgery, but no significant changes in manifest refraction were detected during the remaining follow-up||No data||1.7|
Intracorneal Ring Modelsa
|Arc length||150°||90°, 120°, 150°, 160°, 210°, 355°||90°, 120°, 160°, 210°||155° or 220°||360°|
|Thickness (mm)||0.25 to 0.45||0.15 to 0.35||0.15 to 0.30||0.15 to 0.35||0.2 to 0.32|
| Inner||6.77||No data||4.4||No data||5|
| Outer||8.10||No data||5 to 6||No data||8|
Visual and Refractive Outcomes in Different Studies After ICRS Implantation in Keratoconic Eyes
|Author||Design||Eyes||Procedure||ICRS Model||Follow-up (mo)||Mean Change in SE (D)||Mean Change Keratometry (D)||Gains of Lines of CDVAa|
|Colin et al.49||Prospective, nonrandomized||10||Mechanical||Intacs||12||2.12||4.60 (max K)||No data|
|Alió et al.31||Prospective, nonrandomized||26||Mechanical||Intacs||12||1-segment group: 3.27; 2-segment group: 1.92||1-segment group: 4.69; 2-segment group: 1.92||1-segment group: 81.81%; 2-segment group: 86.67%|
|Colin50||Prospective, nonrandomized||57||Mechanical||Intacs||12||1.53 (6 mo)||3.70 (6 mo)||62%|
|Alió et al.6||Retrospective||13||Mechanical||Intacs||48||1.45||2.56||No data|
|Ertan et al.47||Retrospective||118||Femtosecond||Intacs||12||3.85||3.90||73.7%|
|Rabinowitz et al.46||Retrospective||Mechanical: 10; Femtosecond: 20||Mechanical/femtosecond||Intacs||Mechanical: 12; Femtosecond: 6||Mechanical: 2.96; Femtosecond: 2.98||Mechanical: 2.52; Femtosecond: 2.91||No data|
|Kanellopoulos et al.45||Prospective, nonrandomized||20||Mechanical||Intacs||12||3.46||2.95||No data|
|Shabayek & Alió29||Prospective, non-randomized||21||Femtosecond||Keraring||6||0.96||2.24||70%|
|Coskunseven et al.48||Retrospective||50||Femtosecond||Keraring||12||3.12||3.07||68%|
|Ertan & Kamburoglu10||Retrospective||306||Femtosecond||Intacs||4||3.09||2.79||No data|
|Torquetti et al.15||Retrospective||35||Mechanical||Ferrara||5 years||No data||5.03||74.3%|
|Alfonso et al.7||Prospective||219||Femtosecond||Keraring||6||Stage I: 1.22; Stage II: 2.16; Stage III: 2.11||No data||Stage I: 52.16%; Stage II: 65.48%; Stage III: 60%|
|Alfonso et al.12||Cohort||56||Femtosecond||Keraring||6||2.33||No data||44.9%|
|Torquetti et al.8||Retrospective||36||Mechanical||Ferrara||10 years||No data||3.01||No data|
ICRS + PRK + CXL
|Author||Type of Study||Eyes||Order of Procedure||Follow-up (mo)||Ablation Depth (µm)||Mitomycin After PRK||Outcomes||Complications|
|Kymionis et al.66||Case report||1||PRK + CXL 12 months after ICRS||9||No data||No data||CDVA improved to 20/25 and significant improvement in topographic findings||No|
|Conskunseven et al.62||Prospective case-series||16||ICRS followed by (6 months) CXL followed by (6 months) PRK (non-topography-guided)||6||50||No data||Significant changes in UDVA, CDVA, SE, and K-values||No|
|Kremer et al.70||Interventional case-series||45||PRK+ CXL 6 months after ICRS (femto)||12||40||No data||Significant changes in UDVA, CDVA, K-values, and mean subjective cylinder||11.1% of treated eyes developed mild haze|
|Elbaz et al.71||Retrospective||16||t-PTK+ CXL+ ICRS (femto): same day; 1 group: accelerated CXL; 2 group: standard CXL||12||50||No data||Mean UDVA and K-values improved significantly in both groups, whereas mean CDVA and SE showed statistically significant improvement only in the accelerated group. There was no statistically significant difference between groups||Mild haze in one eye|
Studies Comparing Single, Paired, and Complete ICRS Implantation
|Author||Design of Study||Year||Eyes, Single/Paired||Type of Ectasia||Surgical Procedure||Follow-up (mo)||Patient Selection||Outcomes|
|Alió et al.31||Prospective, comparative, consecutive||2005||11/15||Keratoconus||Mechanical||12||Single: inferior cone (0.45 mm). Paired: when steepening extending at least 1 mm above and beyond the 180° meridian of the cornea–central cone (0.45 mm inferior and 0.25 mm superior).||Similar effect was obtained in the reduction of the refractive cylinder and the keratometric readings.|
|Sharma & Boxer Wachler33||Retrospective, comparative||2006||17/20||Keratoconus post-LASIK||Mechanical||3.1||Matched groups. The author did not mention if the single or paired were chosen by ectasia location.||Single-segment induces more physiologic corneal shape changes and improved postoperative results in keratoconus than double-segment.|
|Yeung et al.32||Retrospective, comparative||2013||38/47||Keratoconus||Femto + CXL (same day)||12||Single: inferiorly decentered cone associated with a flat or normal superior cornea. Paired: if the cone was within the central 5-mm zone.||Reduced corneal astigmatism, improved UDVA, and maintained CDVA. Safe and effective treatment for progressive keratoconus when the preoperative CDVA was at least 20/60.|