Depth of intrastromal corneal ring segments by OCT

Depth of intrastromal corneal ring segments by OCT
Depth of intrastromal corneal ring segments by OCT

Naftali Modi1,2, Haneen Jabaly-Habib1,2
1Ophthalmology Unit of the Baruch Padeh Medical Center, Poriya - Israel
2Faculty of Medicine, Bar Ilan University, Galille - Israel

PurPose. To compare the depth of intrastromal corneal ring segments (ICRS) with the expected depth value using optical coherence tomography (OCT).
Methods. This was a retrospective comparative study in an ophthalmic unit in a government hospital, the Baruch Padeh Medical Center, Poriya, Israel. Ten eyes of 8 patients with 18 ICRS were reviewed. Eleven segments were Intacs (Addition Technology, Inc.) and 7 Kerarings (Mediphacos). Using anterior segment OCT (OPKO OTI) the shortest distance from the epithelium to the segment at 3 points was measured for each segment. The 3 points are proximal, middle, and distal to the incision.
results. The mean depth of the 18 segments was 360±68 µm. The mean maximal and minimal depths were 383±70 and 336±72 µm, respectively. The mean depths of the distal, central, and proximal point measurements of all ICRS were 358±79, 361±77, and 362±59 µm, respectively; no significant difference was found. No part of the segments tended to be more superficial than others (p=0.98). There was no significant difference between Intacs and Kerarings depths (p=0.43). There was a significant difference between the expected ICRS depth and the OCT measurements (mean 480±20) and 360±68), respectively.

ConClusions. The ICRS actual depth was less than expected. There was mild variability in segment depth, both between segments and along the same segment. No significant difference was found between the depth of Intacs and Kerarings.
 

INTRODUCTION

Intrastromal corneal ring segments (ICRS) were first used to correct low myopia. They act as passive spacing elements in the midperiphery of the cornea, shortening the arc length of the anterior corneal surface and thereby flattening the central cornea (1). Several researchers have implanted ICRS in eyes with keratoconus or ectasia with good visual acuity and keratometric outcomes (1-3). The most commonly used ICRS are Intacs and Kerarings. There are several differences between the Intacs and the Kerarings. The Intacs have larger diameter and hexagonal shape in cross-section (Fig. 1), while the Kerarings have smaller diameter and triangle shape in cross-section (Fig. 2). The manufacturers recommend implanting the segments at a depth of 66% to 75% (1) of corneal thickness at the incision point, but it is a common practice to make the incision as deep as 80% of the corneal thickness. Shallow segment depth has been associated with a high rate of complications, including ring superficialization and extrusion, stromal thinning, and epithelial breakdown (4-7). Proper evaluation of the ICRS depth during the postoperative patient examination is therefore critical. Optical coherence tomography (OCT) provides more precise measurements of ICRS depth compared to slitlamp.
ניתוח קרנית

PATIENTS AND METHODS

The study was approved by the Helsinki Board at the Padeh Medical Center, Poriya, Israel (Study 22-2011). It is based on a retrospective review of the charts of Padeh Medical Center patients whose ICRS measurements were recorded between February 20, 2011, and July 1, 2011, using OCT. Collected data included segment size, the expected depth, as calibrated on the diamond knife and registered in the operation report, and actual, postoperative ICRS depths measured by OCT (OPKO Spectral OCT/SLO™) at 3 points on each segment.

The study included 18 segments (in 8 patients). All OCT depth measurements were performed in Padeh Medical Center, and all surgeries were performed by the same surgeon (N.M.). The Intacs implantation surgery was done according to the manufacturer’s (Addition Technology, Inc.) instructions: an incision was made in the steepest meridian and a tunnel for the segments was prepared under high vacuum. Implantation of the Kerarings was done without using vacuum, as instructed by the manufacturer (Mediphacos). In both segment types, the incision depth was set for 75%±5% of corneal thickness at the incision point (0%–10% deeper than the manufacturers’ instructions, based on personal experience and known common practice) (Intacs Surgeon Training Manual mk-50222-01-6/Rev.B/02-07).
 
The OCT (OPKO Spectral OCT/SLO™) has a Signal Source of Super Luminescent Diode, with wavelength of 830 nm and optical power <750 microwatts at cornea.
The typical digital on-screen axial resolution of depth scans was <6 μm, the transverse resolution (in tissue) –20 μm, and we used a scanning rate of 32 frames per second. Measuring the anterior segment requires the use of a cornea lens add-on module, which allows a detailed, higher resolution image of the cornea. The scanning laser ophthalmoscope makes it possible to measure the segments at any desired location. It is a noncontact, noninvasive method of imaging the cornea. The zoom-in option and the features of the instrument facilitate the measurement of segment depth at various points. The depth measurement was taken between the corneal surface and the upper tip of the segment. Recording the shortest distance required using the zoom-in feature (Fig. 1). For each segment, 3 scan locations were performed according to the incision; the first one was on the proximal third of the segment, the second one was on the central third of the segment, and the third location was in the more distal part from the incision. The meridian of the incision and the location of the scan was different from patient to patient since the incision is done in the steepest meridian on corneal topography. The 3 reported variables were calculated as follows:
  • The depth of  each segment  was calculated as the average of the measurements obtained at 3 points: proximal, central, and distal
  • The maximal  and minimal depth of  each segment
  • The difference between  the maximal  and minimal depths
Modi and Jabaly-Habib


Statistical methods

Mixed models were used to allow for the hierarchical structure of the data (repeated measures in patients). The model comparing locations included the random effect of patient and the fixed effect of location. The SAS software was used for the statistical analysis.
TABLE I -  THE NUMBER AND TYPE OF SEGMENTS
IMPLANTED IN EACH PATIENT

דוקטור מודי נפתלי

 
מרכז רפואי מעיין לרפואת עיניים
the shallowest in this dataset.


Patient 7 had 1 segment removed due to extrusion (the upper one). In patient 8, who had 2 segments in the same cornea, the lower segment was found to be less deep than  the upper one; however, the upper segment of the samepatient was deeper than 5 other segments, some of which were in upper and some in lower corneal position. The shallowest segment depth was found at the distal point of segment 18; the greatest depth was at the central point of segment 14. Segment 14 was implanted in the same cornea, using the same incision as segment number 13, which was also one of the deepest segments. In the same patient but in the fellow eye, the segments (numbered 11and 12) were not remarkably deep. Both the maximal and the minimal values measured in segment 18 were the shal lowest of the dataset; in segment 14, the maximal value was the deepest of the dataset (530 μm), and the minimal segment 14 value (450 μm) was surpassed only by one other measure (obtained in segment 13 as noted).A significant variability was observed in the depth mea-
surements of the various patients (p=0.06). No part of the segment tended to be more superficial than others (p=0.98; Fig. 3).Though our sample is rather small (8 patients: 4 with INTACS and 4 with Kerarings), it is of interest to compare the depth measurements between Intacs and Kerarings. Table III displays the results of comparing the average, minimal, and maximal depths, and the differences in each subgroup. There was no statistically significant difference between the 2 subgroups.
 

DISCUSSION

The OCT measurement is one of the few ways to measure precisely the depth of the ICRS in the corneal stroma. The vertical resolution of the OCT is 6 μm according to the manufacturer’s manual, supporting the validity of the measurements obtained herein. The current study demonstrates that the mean measured depth of the segments was 120 μm less than the planned depth (360±68 μm, instead of the expected 480 μm). In other words, with a mean corneal pachymetric thickness of ~600 μm at the ICRS insertion area, the mean postoperative segment depth 

ניתוחי עיניים ירודים

would be at 60% of corneal thickness instead of at the intended corneal depth of 80%. In the study of Gorgun et al (8), the anterior segment OCT–measured distance from Keraring apex to anterior corneal surface was significantly smaller than target depth calculated intraoperatively, as in our study. Similar results as well were reported by Kouassi et al (9), regardless of the ICRS insertion technique: mechanical or femtosecond laser-assisted. In their study of 76 eyes with Intacs, the mean difference between the preoperative expected depth and final segment implantation was 76.64±48.76 μm in the manual group and 85.85±33.02 μm in the femtosecond-assisted technique, with no statistically significant difference between the groups. Thus using the femtosecond laser might make the procedure easier but not deeper than the manual technique. In their study, Lai et al (10) found superiorly placed segments at 60%–64% of corneal depth and inferiorly placed segments at 40%–72% of corneal depth. Previous publications had shown higher complication rates in shallow segments, mainly due to protrusion (11). Additional shallowing of Intacs over time was shown by Kamburoglu et al (12), assisted by Pentacam camera.
There is no published study comparing Intacs to Keraring. Based on the findings of the current study and the previous mentioned studies, surgeons should be encouraged to maintain constant vigilance, in an attempt to achieve proper depth while creating the tunnel.
To our knowledge, no study to date has examined the possibility of a correlation between the segment depth and change in keratometry. It would be interesting to measure the change in keratometry in deeper segments vs shallower ones.
We found different depth measurements along the same segment. This might be noticed on slit-lamp examination with high magnification. This finding could be attributed to the cornea’s variable thickness at different points, which has been well established. The surgeon should consider the corneal thickness along the tunnel and not only at the incision site during the ICRS insertion.
We noticed a bulging into the anterior chamber in many of our patients. This bulging, which has been already published (13), varied both among patients as well as between segments within the same cornea. It is technically impossible for the OCT to measure the part of the stroma which is posterior to the segments since the posterior border of the endothelium is not sharp enough to determine the proper point for measurement; however, it was our impression that the sum of the depth of the segment and the depth of the part of the cornea posterior to the segment would be greater than the original corneal thickness. This might be a result of changes to the lamellas, caused by the segments.
Results of the current study suggest that no specific part of the segment tends to be deeper or shallower than other parts. Lai et al (10) found that the distal and inferior portions of segments tended to be placed at relatively shallow depths. The difference between their results and those of the current study might be explained by the small number of samples used in both studies: 8 segments of 4 patients in the study of Lai et al and 18 segments of 8 patients in the current study. To confirm this explanation, studies with larger samples are needed.
No significant difference was found between the mean depth measures of Kerarings and Intacs, despite the difference in segment placing techniques.
The average depth of ICRS in this study was less than planned, suggesting that surgeons need to double their efforts to achieve proper depth. While the depth of the segment differed from one point to another, there was no observed tendency for one part to be deeper than others. The measured depth of Intacs and Kerarings did not differ significantly.
 


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