In eyes with keratoconus, the cone usually induces significant astigmatism and an asymmetric corneal power dioptric distribution and leads to significant coma-like aberrations. In fact, it has been proposed that coma-like aberrations could be a good indicator for early detection and grading of keratoconus.1
Recent studies have shown that intrastromal corneal ring segments (ICRS) implantation is an effective method to regularize the corneal shape and reduced the astigmatism and corneal high-order aberrations in patients with clear corneas and contact lens intolerance.2–11 Nomograms to implant the ICRS are based mainly on the distribution of ectatic area and on the refractive error; ICRS are usually implanted at the flattest meridian and the incision is generally performed on the steepest topographic axis. Hence, it seems that these nomograms focus mainly on the astigmatism correction. However, it is important to keep in mind that the two main optical aberrations induced in the keratoconic eyes are astigmatism and coma-like. Recently, Alfonso et al.11 reported that ICRS implantation provides good visual and refractive outcomes and is a predictable and safe procedure in paracentral keratoconus with coincident topographic and coma axis.
In the current prospective study, we present the visual and refractive outcomes of keratoconic eyes with no coincident topographic and comatic axes that were implanted with a Ferrara-type ICRS of 150° of arc inferiorly placed. The purpose of this approach of implantation was to reduce both low-order astigmatism and corneal coma-like aberrations in patients with moderate keratoconus in whom the difference from coma and flat central keratometric axis is between 30° and 75°.
Patients and Methods
This prospective study included patients with keratoconus that had ICRS implantation at the Fernández-Vega Ophthalmological Institute, Oviedo, Spain. The tenets of the Declaration of Helsinki were followed, and full ethical approval from the institute was obtained. After receiving a full explanation of the nature and possible consequences of the study and surgery, all patients provided informed consent.
Inclusion criteria were keratoconus, contact lens intolerance, a clear cornea, maximum keratometry (K) reading (simK2) up to 53 diopters (D), minimum K reading of 40 D or more, and minimum corneal thickness, at the implantation optic zone, more than 400 μm. In addition, inclusion criteria considered stages I and II according to the Amsler-Krumeich keratoconus classification and eyes must have fulfilled the following conditions:
The differences between the flat axis of the corneal cylinder measured with a Javal keratometer and an Orbscan IIz topographic system (Bausch & Lomb, Rochester, NY) had to be less than 30°.
Aberrometric surface map measured with the CA-100 Corneal Analyzer system (Topcon, Tokyo, Japan) with the coma axis between 30° and 75° of the flat topographic axis (4.5-mm pupil).
Posterior elevation map measured with the Orbscan IIz system with a maximum height point located at the inferotemporal quadrant at 1 to 2 mm of the pupil center.
Pachymetric map measured with Vistante anterior segment optical coherence tomography (Carl Zeiss Meditec, Inc., Jena, Germany) showing the lowest thickness point located at the inferotemporal quadrant at 1 to 2 mm of the pupil center.
The morphology of the eyes that fulfill these conditions is demonstrated in Figure 1. Exclusion criteria included previous corneal or intraocular surgery, history of herpetic keratitis, diagnosed autoimmune disease, systemic connective tissue disease, endothelial cell density less than 2,000 cells/mm2, cataract, history of glaucoma or retinal detachment, macular degeneration or retinopathy, neuro-ophthalmic disease, and history of ocular inflammation.
Figure 1. Preoperative corneal topography (Orbscan II; Baush & Lomb, Rochester, NY: anterior and posterior float, keratometry, and pachymetry) and coma wavefront map for a 4.5 mm pupil (CA-100 Corneal Analyzer; Topcon, Tokyo, Japan). Note the topographic (blue arrow: 22°) and coma (green arrow: 83°) axes.
All eyes in the study received the Keraring SI6 ICRS (Mediphacos Inc., Belo Horizonte, Brazil). These Ferrara-type ICRS are made of polymethylmethacrylate with a triangular cross-section that induces a prismatic effect on the cornea. The apical diameter of the ICRS is 6.0 mm, and the flat basis width is 800 μm with variable thickness (150, 200, 250, and 300 μm) and arc lengths (90°, 120°, and 150°)
Before and after ICRS implantation, patients had a complete ophthalmologic examination including uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest and cycloplegic refractions, keratometry, corneal topography, corneal aberrometry, endothelial cell count, ultrasonic pachymetry, slit-lamp microscopy, Goldmann applanation tonometry, and binocular indirect ophthalmoscopy through a dilated pupil. The anterior chamber depth and pachymetric map was measured using the Visante optical coherence tomography system (Carl Zeiss Meditec, Inc.). Contact lens use was discontinued 1 month before corneal topography was performed. Diagnosis of keratoconus was established by the combination of computerized videokeratography of the anterior and posterior corneal surfaces (Orbscan IIz), K readings, and corneal pachymetry.12–14 All eyes had an inferior-superior corneal shape index more than 1.40 D (from a mean of 5 points with 30° intervals located 3.0 mm from center).
The protocol to implant the ICRS was made according to our experience. One Keraring SI6 ICRS of 150° of arc was implanted inferiorly. The thickness segment was decided based on the amount of corneal astigmatism and on the intraoperative pachymetry at the implantation zone of 6 mm (Table 1). Patients were divided into three groups as a function of the thickness segment implanted: 150, 200, and 250 μm. The implantation axis of the ICRS was coincident with the flat topographic axis.
Table 1: Thickness of the ICRS Implanted According to Amount of Corneal Astigmatism and Intraoperative Corneal Thickness
The same surgeon (JFA) performed all ICRS implantation procedures using topical anesthesia. Preoperative medications included proparacaine 0.5%, ciprofloxacin 0.3%, and oxybuprocaine ClH 0.2%. After the center of the pupil was marked and corneal thickness at the area of implantation (6.0-mm diameter) was measured by ultrasonic pachymetry, a disposable suction ring was placed and centered with respect to the pupil center. A tunnel was created at 70% corneal thickness using a 60-KHz femtosecond laser (IntraLase; AMO, Santa Ana, CA). This infrared neodymium glass femtosecond laser has a wavelength of 1,053 nm. The laser beam, which has a 3-μm diameter (spot size), is optically focused at a predetermined intrastromal depth by computer scanners, which gives a focus (dissection) range between 90 and 400 μm from the corneal anterior surface. The beam forms cavitations, microbubbles of carbon dioxide, and water vapor by photodisruption, and the interconnecting series of these bubbles forms a dissection plane. An inner diameter of 6.0 mm and outer diameter of 7.1 mm was programmed with the laser software, giving a tunnel width of 1.1 mm and an incision length of 1.7 mm on the steepest topographic axis. In all eyes, the power used to create the tunnel and the incision was 5 mJ. The procedure lasted approximately 15 seconds. Five minutes later, and after clearance of the gas bubbles, the ICRS were implanted under full aseptic conditions with dedicated forceps. The segments were placed in final position with a Sinskey hook through a dialing hole at both ends of the segment.
Postoperative treatment included combination of antibiotic (tobramycin, 3 mg/mL) and steroid eye drops (dexamethasone, 1 mg/ml) (Tobradex; Alcon Laboratories, Inc., Forth Worth, TX) three times daily for 2 weeks with tapering of the dose for 2 more weeks.
Patients were scheduled for postoperative clinical evaluation at 1 day, 1 week, and 1 and 6 months. A standard ophthalmologic examination, including manifest refraction, slit-lamp biomicroscopy, Goldmann applanation tonometry, binocular indirect ophthalmoscopy, corneal topography, corneal aberrometry, UDVA, and CDVA, was performed at all follow-up visits. All examinations were performed by the same ophthalmic technician who was unaware of the objective of the study.
Using the power vector method of Thibos and Horner,15 the refractions obtained before and 6 months after ICRS implantation were assessed. Using this notation, any spherocylindrical refractive error can be expressed by three dioptric powers: M, J0, and J45, where M is a spherical lens equal to the spherical equivalent of the given refractive error and J0 and J45 are two Jackson cross-cylinders equivalent to the conventional cylinder. These numbers are the coordinates of a point in a three-dimensional dioptric space, being the power of the vector from the origin of the three-dimensional dioptric space (M, J0, and J45). Thus, the length of this vector is a measure of the overall blurring strength (B) of a spherocylindrical refractive error. Manifest refractions in conventional script notation (S [sphere], C [cylinder] ×a [axis]) were converted to power vector coordinates and overall blurring strength (B) using the following formulas: M = S + C/2; J0 = (−C/2) cos (2 ϕ); J45 = (−C/2) sin (2 ϕ); and B = (M2 + J02 + J452)½.
Data analysis was performed using SPSS for Windows software (version 14.0; SPSS, Inc., Chicago, IL). Normality was checked using the Kolmogorov–Smirnov test, and outcomes were compared using t tests. Differences were considered statistically significant when the P value was less than .05.
This study comprised 41 eyes of 39 patients (20 men and 19 women). Table 2 shows patient demographics before the surgery. ICRS were successfully implanted in all eyes and no intraoperative and postoperative complications or segment extrusion occurred in any of the eyes included in this patient series.
Table 2: Patient Demographics
Table 3 shows the UDVA and CDVA before and 6 months after ICRS implantation for the whole sample and the 150, 200, and 250 μm groups. The efficacy indexes for the whole sample and for the 150, 200, and 250 μm groups at 6 months of ICRS implantation were 0.57, 0.58, 0.52, and 0.61, respectively.
Table 3: UDVA Before and 6 Months After ICRS Implantation
For the whole sample, 2 of the eyes lost two lines or more of CDVA and 3 eyes lost one line of CDVA 6 months after ICRS implantation. The safety index 6 months after ICRS implantation was 1.10 (Figure 2A). The safety indexes 6 months after the surgery were 1.04, 1.08, and 1.19, for the 150, 200, and 250 μm groups, respectively.
Figure 2. Change in corrected distance visual acuity (CDVA) 6 months after intrastromal corneal ring segments implantation (safety) for (A) the whole sample, and the (B) 150, (C) 200, and (D) 250 μm groups.
Table 4 shows a summary of distribution of manifest refractive error before and 6 months after ICRS implantation, for the whole sample and for each group separately, following the power vector method. For the whole sample and for each group there was a large reduction in B value after surgery (P < .001). Figure 3 shows the astigmatism component of the power vector represented by a two-dimensional vector (J0 and J45), for the whole sample and the 150, 200, and 250 μm groups. The origin of the graph (0, 0) represents an eye free of astigmatism. The spread of the postoperative ICRS implantation data from the origin is more concentrated than the spread of the preoperative data in all cases. The keratometric astigmatism changed from 2.55 ± 1.13 D before ICRS implantation to 0.93 ± 0.90 D after surgery (P < .0001) for the whole sample. Analyzing each group separately, the change in keratometric astigmatism was from 1.79 ± 0.83 to 0.52 ± 0.60 D (P = .0001) for the 150 μm group, from 2.67 ± 1.21 to 1.23 ± 0.87 (P = .0006) for the 200 μm group, and from 3.03 ± 1.21 to 1.00 ± 1.06 (P = .0001) for the 250 μm group.
Table 4: Manifest Refractive Errors Before and 6 Months After ICRS Implantation Following the Power Vector Method
Figure 3. Representation of the astigmatic vector (J0 and J45) before and 6 months after intrastromal corneal ring segments implantation for (A) the whole sample and the (B) 150, (C) 200, and (D) 250 μm groups.
Table 5 shows the root mean square (RMS) values of corneal coma-like aberrations for a 4.5-mm pupil size before and 6 months after ICRS implantation for the whole sample and for the 150, 200, and 250 μm groups.
Table 5: Root Mean Square Values of Corneal Coma-Like Aberrations for 4.5 mm Pupil Size Before and 6 Months After ICRS Implantation
Our results show that UDVA and CDVA improved after ICRS implantation; in addition, most of the eyes (approximately 90%) maintained or improved CDVA (Figure 2A). Based on the safety index (1.10, 6 months after ICRS implantation), the visual outcomes were satisfactory. To the best of our knowledge, this is the first study that evaluates ICRS implantation in keratoconic eyes with no coincident topographic and comatic axis, so we could not make direct comparisons with previous studies. However, it seems valuable to compare our results with those obtained in other studies that analyzed the visual outcomes after Ferrara-type ICRS insertion.
In a sample of 50 keratoconic eyes implanted with Keraring ICRS, Coskunseven et al.4 found 86% of eyes maintained or improved CDVA after surgery. In a study by Shabayek and Alio5 21 keratoconic eyes were implanted with Keraring ICRS and approximately 95% maintained or improved CDVA after implantation. Piñero et al.6 showed a significant improvement in logMAR CDVA (mean change of one line) after Keraring ICRS implantation in a sample of 35 keratoconic eyes. Kubaloglu et al.7 reported a mean gain in UDVA of 2.5 Snellen lines (approximately 95% of eyes maintained or improved their CDVA after Keraring ICRS implantation) in 100 keratoconic eyes. In a previous study of our research group10 approximately 90% of 219 keratoconic eyes maintained or improved CDVA. The results obtained for keratoconus with no coincident topographic and coma axes in the current study are also comparable to those recently reported in keratoconus with coincident topographic and coma axes.11 However, it is important to note the differences between both studies in the nomogram of ICRS implantation.
Regarding refractive outcomes, previous studies have shown that ICRS implantation is an effective procedure to reduce the refractive error in patients with keratoconus.2–11 Our results are in agreement with these results. After ICRS implantation, the overall blurring strength (B) greatly decreased. Regarding astigmatism components, Figure 3A shows that the data are more concentrated around the origin of the graph (0,0) after the surgery than before ICRS implantation.
In addition to visual and refractive outcomes, we also analyzed the changes in RMS value of corneal coma-like aberrations. We found that there was a significant reduction of corneal coma-like aberrations for the whole sample after ICRS implantation. These findings (ie, the decrease both corneal coma-like aberration and low-order astigmatism after ICRS implantation) are due to the choice of the arc length and thickness of the ICRS implanted in the current study. An ICRS of 150° of arc inferiorly implanted could have two effects: (1) the effect of ICRS with a short arc length on flattening of central K readings and on the astigmatism correction2–11 or (2) the ring inferiorly placed pushes the protrusion area of the cornea from the inferotemporal quadrant toward a more central corneal area (Figures 4A and 4B. This change in the location of the protrusion area provokes the central part of the wavefront map to become more uniform after ICRS implantation. All of these changes may explain why there was both a low-order astigmatism and coma-like aberration decrease after ICRS implantation.
Figure 4. (A) Preoperative and (B) postoperative tangential topographic map of a case analyzed in the current study.
To assess the effect of the ICRS thickness, we can analyze the results obtained in each of the groups. Our results showed good outcomes for the 200 and 250 μm groups; UDVA and CDVA significantly improved and most of the eyes maintained or improved their CDVA after ICRS implantation. In relation to refractive outcomes, the B values significantly decreased for both groups (Table 4). The decrease in the B value explains why UDVA improved greatly (a mean of more than two lines for groups) after ICRS implantation. Analyzing Figures 3C and 3D, one could observe how the data after surgery are more concentrated around the origin of the graph (0,0), showing an important decrease of astigmatism. There was a significant reduction of corneal coma-like aberrations for both groups, which could justify why the 200 and 250 μm groups had a significant improvement of CDVA after ICRS implantation. For the 150 μm group, UDVA showed a significant improvement; however, there was no significant change of CDVA after ICRS implantation. Analyzing the refractive and corneal coma-like aberrations outcomes for this group, it is possible to observe that the B values presented a significant decrease; hence, this finding explains the improvement of UDVA after ICRS implantation. In addition, the ICRS implanted in this group (150° of arc and 150 μm of thickness) had no effect on corneal coma-like aberrations. This could explain why the 150 μm group had no significant improvement of CDVA after ICRS implantation. Therefore, this type of ICRS should be implanted only in patients with low values of astigmatism and comatic aberration.
Comparing the results of the three groups, we can observe that the corrective results in terms of astigmatism, B, and RMS values of coma-like aberration vary in direct proportion to the thickness of the implant. These findings are in agreement with previous studies showing that the higher thickness of ICRS implanted, the higher effect obtained.16
In the current study, no complications occurred during surgeries or over the entire follow-up period. It has been reported that the use of the femtosecond technique to create the corneal tunnel makes the ICRS implantation safer and provides a significant reduction of complications (such as extrusion ring) due to the precise depth of the implantation with the femtosecond laser.2–4 Although there were no complications in any case, it is important to note that approximately 10% of the eyes lost lines of CDVA, and further long-term prospective studies should be performed to analyze the potential risk factors for losing lines of CDVA.
Our results suggest that one Ferrara-type ICRS of 150° of arc with a thickness of 200 or 250 μm implanted inferiorly may reduce both low-order astigmatism and coma-like aberrations in keratoconic eyes with no coincident topographic and comatic axes, providing an improvement of UDVA and CDVA values.
- Alió JL, Shabayek MH. Corneal higher order aberrations: a method to grade keratoconus. J Refract Surg. 2006;22:539–445.
- Ertan A, KamburogluBahadir 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] .
- 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] .
- 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] .
- 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] .
- Piñero DP, Alió JL, Teus MA, Barraquer RI, Michael R, Jiménez R. Modification and refinement of astigmatism in keratoconic eyes with intrastromal corneal ring segments. J Cataract Refract Surg. 2010;36:1562–1572 doi:10.1016/j.jcrs.2010.04.029 [CrossRef] .
- 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] .
- 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] .
- 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] .
- 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] .
- 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] .
- Maeda N, Klyce SD, Smolek MK. Comparison of methods for detecting keratoconus using videokeratography. Arch Ophthalmol. 1995;113:870–874 doi:10.1001/archopht.1995.01100070044023 [CrossRef] .
- Rabinowitz YS, Rasheed K, Yang H, Elashoff J. Accuracy of ultrasonic pachymetry and videokeratography in detecting keratoconus. J Cataract Refract Surg. 1998;24:196–201 doi:10.1016/S0886-3350(98)80200-9 [CrossRef] .
- Fam HB, Lim KL. Corneal elevation indices in normal and keratoconic eyes. J Cataract Refract Surg. 2006;32:1281–1287 doi:10.1016/j.jcrs.2006.02.060 [CrossRef] .
- Thibos LN, Horner D. Power vector analysis of the optical outcome of refractive surgery. J Cataract Refract Surg. 2001;27:80–85 doi:10.1016/S0886-3350(00)00797-5 [CrossRef] .
- Barraquer JI. Modification of refraction by means of intracorneal inclusion. Int Ophthalmol Clin. 1966;6:53–78.
Thickness of the ICRS Implanted According to Amount of Corneal Astigmatism and Intraoperative Corneal Thickness
|Intraoperative Corneal Thickness (μm)||Corneal Astigmatism (D)|
|1.00 to 2.00||2.25 to 3.00||3.25 to 4.00||4.25 to 5.00|
|400 to 450||150||150||150||150|
|450 to 500||150||200||250||250|
|Characteristic||Whole Sample||150 μm||200 μm||250 μm|
|Age (y)||31.72 ± 9.47 (20 to 50)||29.98 ± 9.32 (20 to 46)||31.43 ± 8.97 (21 to 49)||31.99 ± 9.64 (23 to 50)|
|Mean SE (D)||−3.65 ± 3.34||−2.32 ± 2.44||−3.35 ± 3.24||−4.99 ± 3.73|
|Mean refractive sphere (D)||−2.25 ± 3.24||−1.27 ± 2.49||−2.10 ± 3.18||−3.16 ± 3.74|
|Mean refractive cylinder (D)||−2.81 ± 1.40||−2.09 ± 0.94||−2.50 ± 0.82||−3.66 ± 1.72|
|Mean keratometric cylinder (D)||2.55 ± 1.14||1.79 ± 0.83||2.67 ± 1.21||3.04 ± 1.01|
|Mean K minimum (D)||44.62 ± 2.52||44.25 ± 2.47||44.31 ± 1.82||45.33 ± 3.25|
|Range K minimum (D)||42 to 50||42 to 50||42 to 48||42.75 to 50|
|Mean K maximum (D)||47.12 ± 2.83||46.04 ± 2.34||46.98 ± 1.88||48.36 ± 3.79|
|Range K maximum (D)||43.50 to 52||43.75 to 51||44 to 51||43.50 to 52|
UDVA Before and 6 Months After ICRS Implantation
|Group||UDVA Before ICRS||UDVA After ICRS||P||CDVA Before ICRS||CDVA After ICRS||P|
|Whole sample||0.76 ± 0.41a||0.53 ± 0.46||.0006||0.13 ± 0.14b||0.07 ± 0.09||.0007|
|150 μm||0.60 ± 0.44a||0.50 ± 0.44||.03||0.12 ± 0.18||0.08 ± 0.10||.15|
|200 μm||0.86 ± 0.32a||0.51 ± 0.49||.02||0.08 ± 0.07b||0.02 ± 0.03||.03|
|250 μm||0.80 ± 0.43a||0.58 ± 0.48||.01||0.19 ± 0.14b||0.11 ± 0.10||.005|
Manifest Refractive Errors Before and 6 Months After ICRS Implantation Following the Power Vector Methoda
|Group||Before ICRS||After ICRS||P|
| M||−3.655 ± 3.340||−2.168 ± 2.843||< .0001|
| J0||0.001 ± 0.100||−0.259 ± 0.621||.07|
| J45||−0.101 ± 1.227||0.168 ± 0.586||.06|
| B||4.160 ± 3.100||2.903 ± 2.700||< .0001|
| M||−2.318 ± 2.444||−1.750 ± 1.951||.004|
| J0||−0.247 ± 0.771||−0.257 ± 0.667||.46|
| J45||−0.046 ± 0.870||0.216 ± 0.460||.09|
| B||2.927 ± 1.971||2.114 ± 1.747||< .0001|
| M||−3.346 ± 3.240||−2.721 ± 3.292||.004|
| J0||−0.409 ± 0.936||−0.406 ± 0. 258||.41|
| J45||0.119 ± 0.887||0.076 ± 0.336||.44|
| B||3.747 ± 3.045||2.943 ± 3.146||.001|
| M||−4.991 ± 3.734||−3.205 ± 3.008||.001|
| J0||0.576 ± 0.999||−0.053 ± 0.777||.04|
| J45||−0.350 ± 1.690||0.215 ± 0.835||.03|
| B||5.513 ± 3.519||3.487 ± 2.894||.0005|
Root Mean Square Values of Corneal Coma-Like Aberrations for 4.5 mm Pupil Size Before and 6 Months After ICRS Implantation
|Group||Before ICRS (μm)||6 Months Afer ICRS (μm)||P|
|Whole sample||0.80 ± 0.53||0.61 ± 0.59||.02|
|150 μm||0.72 ± 0.60||0.73 ± 0.71||.41|
|200 μm||0.66 ± 0.42||0.44 ± 0.31||.03|
|250 μm||0.99 ± 0.56||0.65 ± 0.43||.01|