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Laboratory science

Precision and accuracy of an artificial anterior chamber system in obtaining corneal lenticules for lamellar keratoplasty

Behrens, Ashley MDa,b,1; Dolorico, Arlene M.T MDa,1; Kara, David T MDd,1; Novick, Lee H MDa,1; McDonnell, Peter J MDa,b,1; Chao, Lawrence C MDa,c,1; Wellik, Sarah R MDa,1; Chuck, Roy S MD∗,a,1

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Journal of Cataract & Refractive Surgery: October 2001 - Volume 27 - Issue 10 - p 1679-1687
doi: 10.1016/S0886-3350(01)00896-3
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Abstract

In 1877, von Hippel attempted the first human corneal transplant using an anterior lamellar approach.1,2 He used a trephine to delineate the margins of the keratectomy and dissected the lamellar corneal section in a free-hand fashion. The procedure was time consuming and difficult to perform.

Several years later, Barraquer introduced the microkeratome to effectively change the refractive power of the cornea by severing a superficial cap from its surface.3 He also proposed using his invention for a modified lamellar keratoplasty by harvesting the tissue in both host and donor corneas with the microkeratome.4 This technique met with limited success.5–7 Its major drawback was the inaccuracy of the intended lenticule diameter and shape obtained with the microkeratome.6,7

These devices have been shown to be successful in achieving lamellar cuts of the cornea.8–10 Thus, they may represent an important advance for lamellar keratoplasty compared with other techniques for harvesting corneal discs in donor and recipient corneas.11–16 The use of a microkeratome significantly decreases the surgical time and produces a much smoother interface, promoting better apposition of donor and recipient bed surfaces and improved optical performance.17

One common problem with this technology is obtaining an adequate size and thickness match between the donor and the recipient stromal bed.6,7 In addition, whole ocular globes are necessary to use this instrument to cut the donor cornea since the suction ring is designed to work on the globe surface of the intact globe.

Two decades ago, Maguen and coauthors18 and Maguen et al.19 described an ingenious artificial anterior chamber that enabled corneoscleral rims to be cut using a microkeratome. However, fears about microkeratome use hindered the spread and development of this promising idea. With the development of new refractive surgery techniques such as laser in situ keratomileusis (LASIK) during the past decade,20,21 technically improved microkeratomes have regained acceptance in the ophthalmic surgical community.

Recently, a new artificial anterior chamber model with a metal base to allow the translation of a manual microkeratome was developed (ALTK System®, Moria/Microtech). This device was designed to perform lamellar keratectomies on corneoscleral rims, the usual form of preserving donor corneas in tissue eye banks. This study evaluated the precision and accuracy of this system in obtaining corneal lenticules for lamellar keratoplasty.

Materials and methods

A new artificial anterior chamber was used to perform anterior lamellar keratectomies (Figure 1). The device consists of a stainless steel structure with 3 screw-type safety rings. The lower ring sustains a metal device that covers the superficial sclera and maintains a tight fit on the metal base of the chamber to avoid leakage. A second ring in an intermediate position approximates the chamber on the former structure to tighten the sclera from above. A third ring located superiorly is adjusted to modify the height of the microkeratome plate. This plate is a gearless track to guide the microkeratome head translation at a constant height along the corneal pass. Depending on the height at which this plate is positioned, more (lower position) or less (higher position) of the cornea is exposed, resulting in a larger or smaller lenticule diameter, respectively. The chamber is connected to an infusion system with a reservoir of saline solution, placed 1.2 m above the chamber level. An expansion air chamber is located within the infusion line at 10.0 cm of the connection to the chamber.

Figure 1.
Figure 1.:
(Behrens) Artificial anterior chamber with 3 safety rings. A superior microkeratome plate is provided with guiding grooves for microkeratome head translation.

The LSK One® microkeratome (Moria/Microtech) was used to cut the corneas. It consists of a single-piece metal head connected to a nitrogen-gas-driven turbine handpiece. The blade oscillates at a rate of 15 000 oscillations/min with an orientation of 25 degrees to the cut plane, according to the manufacturer. The grooves on the base plate of the artificial anterior chamber are designed to fit into the microkeratome head, so its pass across the cornea is uniform.

After the study was approved by the University of California Irvine Institutional Review Board/Ethics Committee, corneoscleral rims (n = 47) not suitable for corneal transplantation and preserved in Optisol® were obtained from the Doheny Eye and Tissue Bank. The mean age of the 18 women and 29 men at the time of death was 57.1 years ± 14.8 (SD) (range 17 to 74 years). The procedure was performed no longer than 15 days after death. Upon availability, corneas were sequentially assigned to 9 groups of 5 corneas each. Three lenticule diameters (7.0, 8.0, and 9.0 mm) were tested using 3 microkeratome head thicknesses (180, 300, and 360 μm) for each diameter. The lenticule diameters and microkeratome head thicknesses in the 9 groups are shown in Table 1.

Table 1
Table 1:
Lenticule diameter and microkeratome head thickness in the 9 groups

Technique

To reduce the number of air bubbles beneath the cornea, rims were placed on the chamber base after the infusion was released. Once the cornea was stabilized and centered and the absence of air bubbles was confirmed, the infusion was closed, the superior metal support was placed and locked by turning the first ring clockwise, and the second ring was turned counter-clockwise to elevate the chamber height and tighten the scleral skirt between the support and the chamber. The infusion was then released, and mechanical scraping with a #11 Beaver blade was performed to remove the epithelium in all cases. (Some of the preserved corneas had loose epithelium that might have introduced errors in thickness measurements if left in place.)

Corneal thickness measurements.

After the epithelium was removed, the infusion was closed and the corneal thickness measured using an ultrasound pachymeter (model 850, Bausch & Lomb) in the center of the cornea and in 4 quadrants on the vertical and horizontal meridians at the intersection of the aperture margin observed through the cornea (Figure 2). The infusion was then opened and the intrachamber pressure measured by tonometry (Tono-Pen XL®, Mentor).

Figure 2.
Figure 2.:
(Behrens) Corneal surface and reference lines on the stromal bed after keratectomy, showing the horizontal and vertical diameters taken for planimetry. Points represent the location of the tip of the pachymeter probe to measure the central and quadrant thicknesses (original magnification ×8.5).

Intended diameter measurements.

The provided applanation lenses were placed on the cornea to determine the plate height for the desired diameter, turning the second ring counterclockwise or clockwise depending on the guiding circle marks on the lenses. The plate, with a central opening of 11.9 mm, was positioned over the exposed cornea.

Keratectomy.

The same experienced surgeon performed all keratectomies using the right hand, in a right to left microkeratome pass fashion, to avoid bias related to surgeon experience and hand dominance. Drops of saline solution were applied to the corneal surface, and keratectomy was performed by passing the microkeratome head with its oscillating blade at a relatively constant speed along the plate. Blades were reused no more than 3 times to avoid deterioration in the microkeratome's cutting properties.22

After the lenticule was obtained and removed from the stromal bed, the infusion was closed and pachymetry measurements were taken at the same reference points as before the keratectomy. Total lenticule thickness was calculated by subtraction.

Stromal bed photography (Canon EOS® 500), ×4.8 magnification (+14D Macro Lens, Quantaray), was recorded with open infusion for planimetry before superficial irrigation to avoid morphological changes from hydration. Paper prints were scanned to images of 1200 × 839 pixel resolution and processed with a digital imaging software (Scion Image for Windows, Scion Corp.) to measure the horizontal and vertical diameters of the residual stromal bed keratectomy. Digitized images were measured at ×21 magnification in the 1024 × 768 pixel screen area. Units were calibrated to 52.0 ± 2.8 pixels/linear millimeter. Differences in the vertical and horizontal meridians were also recorded to assess circularity of the lenticule.

The accuracy of the tested system was based on the mean value obtained compared with the expected value, while the precision was based on the variability or standard deviation of the mean.

Scanning Electron Microscopy

Two selected corneas (7.0 and 9.0 mm diameters, 360 μm head) were prepared for scanning electron microscopy (SEM). Specimens were fixed in 10% buffered paraformaldehyde, immersed in osmium tetroxide, dehydrated with graded alcohols, and dried using increasing concentrations of hexamethyldisilazane. Samples were gold sputtered and examined under a scanning electron microscope (Philips XL 30).

Statistical Analysis

Calculations were made using the SYSTAT 9.0 statistical software for Windows (SPSS Inc.). Descriptive statistics (mean, standard deviation, minimum and maximum values) were performed with continuous variables. Between-group comparisons were performed using nonparametric tests (Mann-Whitney U and Kruskal-Wallis for unpaired samples, Wilcoxon for paired samples). The Fisher exact test was used for categorical variables and the Spearman rank correlation coefficient, for bivariate correlation analysis. A P value of 0.05 or less was considered statistically significant.

Results

Instrument Handling

Before keratectomy.

The tested instrument was relatively easy to operate. Air penetration in the anterior chamber was almost completely avoided by placing the cornea with irrigation open from the side of the chamber. Large scleral skirts were preferred to eliminate chamber leakage and corneal slippage from the holder and to achieve centration. Two corneoscleral rims were not watertight after the superior metal support was placed, and the procedure could not be performed. The rims measured less than 16.0 mm in diameter in 1 meridian.

During keratectomy.

After the keratectomy was completed, a retrograde advance of the lenticule toward the blade and the plate was detected in 8 cases using 8.0 mm or more of the intended diameter and a head of 300 μm or more. This movement occurred immediately after the lenticule was cut loose, at the end of the cut, during the pass of the keratome head along the rest of the plate. This was avoided by interrupting the turbine activity after the keratectomy was completed. The rest of the keratome head pass was accomplished with the turbine off.

Baseline Measurements

The mean central corneal thickness was 651.4 ± 101.0 μm (range 445 to 849 μm). The mean intrachamber pressure was 53.5 ± 2.7 mm Hg (range 48 to 60 mm Hg). The intrachamber pressure did not differ significantly among the 9 groups (P = .310); some differences in central corneal thickness were observed (P = .025) (Table 2).

Table 2
Table 2:
Baseline measurements of intrachamber pressure and central corneal thickness

Variability of the Corneal Lenticule Thickness

No differences were observed in central lenticule thickness when the diameter was varied (7.0, 8.0, and 9.0 mm) with the same keratome head: 180 μm (P = .264), 300 μm (P = .564), and 360 μm (P = .949). Despite a tendency to obtain a greater central lenticule thickness when the head thickness was increased with the same diameter, a highly significant difference was observed in the 8.0 mm (P = .010) and the 9.0 mm (P = .019) groups only. No significant difference was seen in the 7.0 mm group (P = .166) (Table 3). Similarly, highly significant differences were detected in the peripheral lenticule thickness (mean of 4 peripheral quadrants) in the 8.0 mm (P = .012) and 9.0 mm (P = .008) groups using different head thicknesses, but no difference was observed in the 7.0 mm group (P = .060) (Table 4).

Table 3
Table 3:
Central lenticule thickness.
Table 4
Table 4:
Peripheral lenticule thickness.

Lenticules were thinner centrally than peripherally (P = .002). Precision and accuracy were higher in peripheral thickness measurements than in central measurements in the 180 and 300 μm groups. In the 180 μm group, the central lenticule thickness was within ±50.0 μm of the expected thickness in 4 of 15 corneas (26.7%) and the peripheral lenticule thickness, in 9 of 15 corneas (60.0%) (P = .034). In the 300 μm group, the results were 4 of 15 cases (26.7%) and 6 of 15 (40.0%), respectively, (P = .042). In the 360 μm group, the deviation from expected was comparable for central and peripheral measurements—4 of 12 (33.3%) cases were within ±50.0 μm of the expected thickness. As head thickness increased, accuracy tended to decrease (P = .045).

Regional thickness variations were observed in various quadrants. Among all groups, a mean difference of 101.1 ± 45.1 μm (range 20.0 to 201.0 μm) was obtained by subtracting the thicker and thinner lenticule values from all points measured. There were no significant differences between the mean differences in each group separately (P = .169) (Figures 3 and 4).

Figure 3.
Figure 3.:
(Behrens) Local lenticule thickness variations in the horizontal meridian in each group.
Figure 4.
Figure 4.:
(Behrens) Local lenticule thickness variations in the vertical meridian in each group.

Variability of the Corneal Lenticule Diameter

The mean obtained horizontal and vertical diameters are shown in Table 5. Variability in horizontal and vertical diameters was not significant among the 3 head thicknesses within the same intended diameter group (P ≥ .424).

Table 5
Table 5:
Horizontal/vertical lenticule diameter.

In 16 of 40 corneas (40.0%), the obtained diameter was within ±0.2 mm of the expected diameter; in 32 corneas (80.0%), the obtained diameter was within ±0.5 mm. Accuracy was higher in the 8.0 mm group: 8 of 17 corneas were within ±0.2 mm of expected (47.1%), regardless of the thickness considered (P = .041). In the 7.0 and 9.0 mm groups, the same accuracy was found in 4 of 16 corneas (25.0%).

The lenticule circularity was calculated by subtracting the horizontal and vertical diameters. Categorizing these variables, 17 of 40 lenticules (42.5%) had a difference of 0.1 mm or less; 26 of 40 (65.0%) had a difference of less than 0.15 mm and 29 of 40 (72.5%), of less than 0.2 mm. One obtained lenticule was markedly oval, with a difference of 0.7 mm, and 2 others had differences of 0.58 mm and 0.51 mm. Within groups using the same diameter, no correlation was observed with obtained central or peripheral thickness and horizontal or vertical diameter (rs ≤ 0.28).

Scanning Electron Microscopy

Surface analysis showed a smooth stromal bed, with no chatter lines in the central sector of the keratectomy and almost no chatter at the keratectomy margins. The transition of the keratectomy edge to the central bed was smooth, and no significant irregularities were observed. Tissue remnants were barely observed on the cut surface (Figure 5).

Figure 5.
Figure 5.:
(Behrens) Scanning electron microscopy of the stromal bed after keratectomy. Top: View of a 7.0 mm−360 μm sample with a small residual flap at the end of the microkeratome pass (12 o'clock) (original magnification ×15). Bottom: Keratectomy edge showing a smooth surface with no chatter lines (original magnification ×50).

Discussion

The development of different models of artificial anterior chambers has enabled corneal surgeons to mimic operating on whole donor globes using corneoscleral disks obtained from eye banks.23,24 Krumeich and Swinger25 designed one to obtain lamellar samples for epikeratophakia, as well as to obtain corneal buttons using a trephine in penetrating keratoplasty (PKP).26 Naumann et al.27 and Seitz et al.28 describe another modified anterior chamber to obtain corneal buttons for PKP using the nonmechanical excimer laser trephination. The model tested here is intended for both lamellar keratectomy and full-depth corneal trephination from the epithelial side. We focused on the lamellar approach. One important factor in a system such as this is cut reproducibility. As an elementary principle, the obtained donor lenticule for an anterior lamellar keratoplasty should ideally match the recipient stromal bed in thickness and diameter to promote better adhesion, avoid interface problems, and improve optical results. If a microkeratome is used for both recipient and donor corneas, the shape of the lenticule and the bed tend to be similar, apposition is easier to accomplish, and fewer sutures are required for shorter periods of time, which reduces suture-related complications.29,30

One concern in previous studies using similar equipment is the difficulty of obtaining a whole circular lenticule with an accurate target diameter.6,7 Our results showed that in more than three fourths of the donor corneas, the diameter was within ±0.5 mm of the target, which demonstrates a higher accuracy than the variable results (up to ±1.0 mm) in other series.7 Almost 73% were reasonably circular. Complete circularity is difficult to obtain because donor corneas are not spherical. However, the degree of circularity and cut decentration were easily predicted by looking at the image on the applanation lens, which closely matched the residual stromal bed obtained (Figure 6).

Figure 6.
Figure 6.:
(Behrens) Keratectomy decentration. Top: Applanation lens in place before keratectomy; meniscus (arrows) shows a decentered applanation shape compared to reference lines of the applanation lens (bars). Bottom: Same specimen, showing the decentered cut. Arrows show the keratectomy margins and bars, the difference in the measured distance from the edge to cut margin in 2 quadrants. (original magnification ×4.7).

There was tendency toward variable thickness with the sector of the lenticule studied. Overall, the central lenticule area was thinner than the periphery. Unfortunately, not many studies in the literature have examined the regional thickness variations in lenticules obtained using microkeratomes; this hinders appropriate comparisons. However, studies in animal models have shown that local differences exist, at least in the horizontal meridian.10,22,31 One possible explanation for the obtained central thinning may be related to the fluctuation in the intrachamber pressure during the microkeratome pass, which may decrease disproportionately when the cornea is flattened when the keratome blade reaches the center. Variability in microkeratome translation speed may also play a role.

Local thickness changes may be important if these variations are not the same in the recipient eye. With a mean difference of 100 μm observed between 2 sectors of the same lenticule, significant irregular refractive changes may be introduced to recipient corneas. Less accuracy of the lenticule thickness in thicker cuts might relate to differences in the hydration over the stroma. Postmortem and organ-cultured corneas show more posterior than anterior swelling.32 The variability in thicker cuts may be higher, especially when physiologic corneal hydration occurs after transplantation.

New and more accurate methods of corneal thickness measurement such as optical coherence tomography and high-frequency ultrasound have been proposed, especially for LASIK surgery.33,34 Use of these methods to assess 360-degree dimensional changes in lenticules of both donor and host corneas is of major interest to understand and predict refractive changes after lamellar keratoplasty.

The morphological surface analysis by SEM revealed the high quality and smoothness of the cut obtained with this system. No other manual method of dissection (air,12 fluid,35 or viscoelastic substances15) is able to match the cut smoothness obtained using the current-generation microkeratomes. This may reduce the interface problems frequently observed in lamellar keratoplasty.

In summary, the instrument tested is another useful tool for lamellar corneal surgery. Randomized controlled clinical studies are ongoing to assess the significance of the observed variations in the donor lenticule dimensions on the optical result. These data will be necessary to determine the ideal dimensions of the donor and recipient geometries needed to obtain optimal vision. Although ideally the donor diameter should match the recipient diameter, factors such as variability in postmortem hydration and regional lenticule thickness will have to be considered.

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© 2001 by Lippincott Williams & Wilkins, Inc.