Singapore National Eye Centre (SNEC), 11 Third Hospital Avenue, Singapore, 168751, Singapore

Singapore Eye Research Institute (SERI), 11 Third Hospital Ave, Singapore, 168751, Singapore

Zhongshan Ophthalmic Centre, Guangzhou, China

Yong Loo Lin School of Medicine, National University of Singapore, 1E Kent Ridge Road, Singapore, Singapore

Department of Clinical Sciences, Duke-NUS Graduate Medical School, 8 College Rd, Singapore, 169857, Singapore

Abstract

Background

To assess repeatability of the Zhongshan Assessment Program (ZAP) software measurement of Anterior Segment Optical Coherence Tomography (ASOCT) images and correlate with graft trephine diameter following Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)

Methods

Retrospectively evaluated interventional case series. 121 consecutive eyes undergoing DSAEK over a 26 month period underwent ASOCT imaging 1month after their surgery. ASOCT images were processed using ZAP software which measured the graft and cornea parameters including anterior and posterior graft arc length and cord length, posterior cornea arc length (PCAL) and anterior chamber width.

Results

The graft measurements showed good repeatability on ASOCT using ZAP with high intra class coefficient and small variation in the coefficient of variation. On ASOCT, the mean recipient PCAL was 12.99+/−0.69mm and the anterior chamber width was 11.16+/−0.57mm. The mean Graft anterior arc length was 9.69+/−0.66mm and the mean Graft anterior cord length was 8.92+/−2.94mm. The mean graft posterior arc length was 9.24+/−0.75mm and the mean graft posterior cord length was 8.15+/−0.57mm. Graft posterior arc length (rho=0.788, p< 0.001) correlated best with intra-operative graft trephine diameter. The mean ratio of posterior graft arc length to PCAL was 0.712 +/− 0.056.

Conclusions

We have validated the repeatability of the ZAP software for DSAEK graft measurements from ASOCT images and shown that the graft arc length parameters calculated from the ASOCT images correlate well with the intra-operative graft trephine diameter. This software may help surgeons determine the optimal DSAEK graft size based on pre-operative ASOCT measurements of the recipient eye.

Background

Descemet stripping automated endothelial keratoplasty (DSAEK), the main form of endothelial keratoplasty in which a posterior lamellar graft is attached to the posterior corneal surface is rapidly becoming the surgical alternative to penetrating keratoplasty (PK) for patients with corneal endothelial failure

Anterior segment optical coherence tomography (ASOCT) is a non-contact imaging technique that obtains high-resolution cross-sectional images of the cornea and anterior chamber. The ability of anterior segment OCT to render tissue planes with high axial resolution is potentially useful in evaluating the cornea after corneal lamellar procedures

The aim of this study was to (i) assess the reproducibility of post DSAEK graft measurements using the modified ZAP (Zhongshan Assessment Program) software (ii) to validate the measurements obtained from the ZAP readings and correlate this with intra-operative graft trephine measurements; (iii) to retrospectively analyze the graft sizing in our first 121 DSAEK cases to assess the size of the DSAEK graft in relation to the posterior corneal arc length (PCAL).

Methods

Approval for the study was granted by the Singapore National Eye Centre and Singapore Eye Research Institute institutional review board. The study was conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from all subjects before enrollment. One hundred and twenty one consecutive eyes undergoing DSAEK at the Singapore National Eye centre over a 26 month period, performed by a single corneal surgeon (DT), had ASOCT imaging performed one month after their surgery.

DSAEK surgical technique

DSAEK was performed using a similar technique, as described by Price

Anterior segment optical coherence tomography

Anterior Segment images were scanned with the Zeiss Visante ASOCT (Carl Zeiss Meditec, Dublin, CA) within the first month post operation, with informed consent obtained from all participants, as part of our IRB approved study on DSAEK. The details of ASOCT imaging technology have been described previously

Image processing

All ASOCT images were assessed by one ophthalmologist (GT). For each image, the image file was opened using the Zhongshan Assessment Program and first, the two scleral spurs were identified, defined as the anatomical junction between the inner wall of the trabecular meshwork and the sclera (Figure

ASOCT image with markings and measurements.

**ASOCT image with markings and measurements.**

The Zhongshan Assessment Program (ZAP, Guangzhou, China) is a proprietary non-commercially available anterior segment image processing software developed at the Zhongshan Ophthalmic centre, Guangzhou, China. This software may be requested free from the authors. The software automatically extracted the 300 x 600 8-bit greyscale (intensities from 0 to 255) image portion of the output file and performed noise and contrast conditioning

Statistical methods

Parametric and non-parametric tests were used to compare continuous variables according to data distribution. Spearman’s Rho correlation, linear regression and logistic regression analyses were used to assess factors relating to the posterior corneal arc length. P< 0.05 was considered statistically significant. Bland-Altman analysis was performed to analyze intra-observer agreement. Analysis of repeat measurements was done looking at limits of agreement, coefficient of variation and intraclass coefficient. Statistical analysis was performed by SPSS and microsoft® Excel software.

Results

Patient characteristics

A total of 121 eyes underwent DSAEK and had ASOCT done. 12 (9.9%) eyes were excluded because the ASOCT image was of insufficient quality to identify the sclera spur or to process the image with ZAP software. The remaining 109 eyes (56 right and 53 left) of 104 patients were included in this study. The mean age was 66.17+/−11.58 years and 53.2% were male (58 male patients & 46 female patients). The Sheets Glide insertion technique was used in 93 patients (85.3%) with acceptable ASOCT images. Mean pre-operative donor endothelial cell count was 2881.54 +/− 240.78 and the mean graft thickness was 194.50 +/− 43.10 um (Table

**N=109 (Mean +/− SD)**

Age (Years)

66.17 +/−11.58

Sex (Male)

58 (53.2%)

Side (OD)

56 (51.4%)

Glide Insertion technique

93 (85.3%)

Graft size (mm)

8.63 +/− 0.53

Endothelial Cell Count

2881.54 +/− 240.78

Graft thickness (um)

194.50 +/− 43.10

Posterior Cornea Arc length (mm)

12.99 +/− 0.69

Cornea Posterior Curvature (mm)

5.80 +/− 0.85

Posterior graft arc length (mm)

9.24 +/− 0.72

Posterior graft cord length (mm)

8.15 +/− 0.57

Anterior graft arc length (mm)

9.69 +/− 0.66

Anterior graft cord length (mm)

8.65 +/− 0.48

Ratio of posterior graft arc length to posterior Cornea arc length

0.712 +/− 0.516

Ratio of Anterior graft arc length to posterior Cornea arc length

0.745 +/−0.051

Ratio of posterior graft arc length to Posterior graft cord length

1.134 +/− 0.036

Ratio of anterior graft arc length to anterior graft cord length

1.122+/−0.048

Ratio of anterior graft arc length to posterior graft arc length

1.048 +/−0.048

Ratio of anterior graft Cord length to posterior graft cord length

1.096+/−0.339

Distribution of Graft Trephine size.

**Distribution of Graft Trephine size.**

ASOCT analysis

Intra-observer repeatability of graft parameters

We have previously shown good repeatability for the measurement of recipient posterior cornea arc length

**Parameter**

**Mean (SD)**

**Mean (SD)**

**P***

**Mean difference (95%CI)**

**Limits of Agreement**

**Coefficient of variation (95% CI)**

**Intraclass Coefficient (95% CI)+**

**Observer 1**

**Observer 2**

*(2-sided).

+ all p< 0.001.

Posterior Cornea Arc length (mm)

13.08 (±0.61)

13.06 (±0.59)

0.634

0.02 (−0.06 to 0.10)

−0.32; 0.35

2.56% (1.50, 3.63)

0.959 (0.899 to 0.983)

Anterior chamber Width (mm)

11.24 (±0.50)

11.24 (±0.45)

0.942

0.00 (−0.10 to 0.10)

−0.42; 0.41

3.71% (0.25, 5.25)

0.900 (0.765 to 0.959)

Ant Graft Arc length (mm)

9.77 (±0.51)

9.783 (±0.53)

0.317

−0.02 (−0.06 to 0.02)

−0.18; 0.14

1.61% (0.94, 2.28)

0.988 (0.970 to 0.995)

Ant Graft Cord length (mm)

8.70 (±0.33)

8.73 (±0.34)

0.075

−0.02 (−0.05 to 0.00)

−0.14; 0.09

1.28% (0.75, 1.81)

0.985 (0.963 to 0.994)

Post Graft Arc length (mm)

9.43 (±0.52)

9.45 (±0.51)

0.240

−0.02 (−0.06 to 0.02)

−0.17; 0.13

1.55% (0.91, 2.19)

0.989 (0.973 to 0.996)

Post Graft Cord length (mm)

8.24 (±0.32)

8.26 (±0.34)

0.088

−0.02 (−0.05 to 0.00)

−0.12; 0.08

1.24% (0.73, 1.76)

0.988 (0.969 to 0.995)

Band Altman Plots for Intraobserver agreement of Graft Parameters (Anterior graft Arc Length, Anterior Graft cord length, Posterior Graft Arc length, Posterior Graft Cord Length).

**Band Altman Plots for Intraobserver agreement of Graft Parameters (Anterior graft Arc Length, Anterior Graft cord length, Posterior Graft Arc length, Posterior Graft Cord Length)**

Endothelial keratoplasty analysis

On ASOCT, the mean recipient posterior cornea arc length was 12.99 +/− 0.69mm and the mean anterior chamber width was 11.16 +/− 0.57mm. The mean Graft anterior arc length was 9.69 +/− 0.66mm and the mean Graft anterior cord length was 8.92 +/− 2.94mm. The mean graft posterior arc length was 9.24 +/− 0.75mm and the mean graft posterior cord length was 8.15 +/− 0.57mm. The mean ratio of posterior graft arc length to posterior cornea arc length was 0.712 +/− 0.056 (range 0.514 to 0.849) and the mean ratio of anterior graft arc length to posterior cornea arc length was 0.745 +/−0.051 (range 0.618 to 0.859).

Bivariate correlation showed that intraoperative graft trephine diameter correlated with graft anterior arc length, graft anterior cord length, graft posterior arc length and graft posterior cord length (all p< 0.001) (Table ^{2} (0.697) for determining posterior graft arc length.

**ASOCT Parameter**

**Spearman’s Rho**

**p**

Posterior Cornea Arc Length

0.298

<0.001

Cornea posterior curvature

0.011

0.909

Anterior Chamber Width

0.162

0.094

Graft Anterior Arc length

0.681

<0.001

Graft Anterior Cord Length

0.643

<0.001

Graft Posterior Arc length

0.788

<0.001

Graft Posterior Cord Length

0.636

<0.001

Graft Thickness

−0.167

0.088

Correlation between trephine diameter and graft posterior arc length.

**Correlation between trephine diameter and graft posterior arc length.**

**Independent variables in regression model**

**B coefficient* (95%CI)**

**P value**

**Adjusted R**^{2}

*B coefficient for trephine diameter.

**Final model using backward selection.

All variables in the models were tested for colinearity with acceptable tolerence.

Recipent age, sex, posterior cornea arc length

1.112 (0.935, 1.289)

<0.001

0.677

Recipent age, sex, posterior cornea arc length, AC width

1.134 (0.958, 1.310)

<0.001

0.686

Recipent age, sex, posterior cornea arc length, AC width, thickness

1.134 (0.955, 1.312)

<0.001

0.683

Recipent age, sex, Cornea posterior curvature

1.162 (0.999, 1.324)

<0.001

0.682

Cornea Posterior curvature

1.154 (0.998, 1.310)

<0.001

0.685

AC width

1.117 (0.959, 1.276)

<0.001

0.691

Recipent age, sex, Cornea posterior curvature, posterior cornea arc length, AC width

1.122 (0.951, 1.292)

<0.001

0.694

Cornea Posterior curvature, AC width**

1.112 (0.954, 1.269)

<0.001

0.697

Examining the trend of graft size, we found that the posterior graft arc length increased with posterior cornea arc length. However, we noted that in our series, eyes with larger posterior cornea arc length, the ratio of graft posterior arc length to posterior cornea arc length was less than eyes with smaller posterior cornea arc length. (Figure

Graph of Ratio of posterior graft arc length to posterior cornea arc length vs cornea arc length.

**Graph of Ratio of posterior graft arc length to posterior cornea arc length vs cornea arc length.** Vertical line represents the 75^{th} percentile of posterior cornea arc length (13.52mm). Horizontal line represents the mean ratio of graft posterior arc length to posterior cornea arc length (0.71).

Discussion

We have previously shown the utility of the ZAP software in providing reproducible measurements of posterior cornea arc lengths from ASOCT images

Although useful in providing anatomical detail of anterior segment structures, ASOCT does have its limitations. Images need to be dewarped with software algorithms to correct for index transitions

Most of the grafts in this series were performed in eyes with pseudophakic bullous keratopathy where larger grafts with greater replacement of functional cornea endothelial cells were desirable. In our series, the mean ratio of posterior graft arc length to recipient posterior cornea arc length was 0.712 +/− 0.056 (Range 0.504 to 0.852). There was a small negative trend between the ratio of posterior graft arc length to posterior cornea arc length and the posterior cornea arc length, which suggested that we could have used larger grafts in eyes with larger posterior corneal arc lengths (Figure ^{th} percentile (13.52mm), the ratio of graft posterior arc length to posterior cornea arc length was less than the mean (0.712) in 70.4% of these eyes. This suggests that 70.4% of grafts in patients with PCAL> 13.5mm were undersized.

We feel that sizing of the DSAEK graft is not a “one size fits all” procedure. It is known that cornea dimensions can vary with ethnicity and adult stature

In future, we aim to perform further studies to prospectively analyze the effect of graft size and the ratio of graft diameter to posterior cornea arc length, on postoperative outcomes including endothelial cell count and refractive outcome. Once we establish an optimal graft size ratio, we can estimate the appropriate graft trephine diameter based on the cornea posterior arc length from the pre-operative ASOCT image. Our multiple regression analysis showed that, a model using posterior cornea arc length and AC width measured on ASOCT would allow us to reasonably estimate the graft arc length based on the trephine diameter chosen (Table

The use of ASOCT imaging and ZAP software in our study does have some limitations. It has previously been shown that there can be difficulty in detecting the sclera spurs on some ASOCT scans

Conclusion

In summary we have successfully validated the repeatability of the ZAP software measurements of ASOCT images from patients who have undergone DSAEK surgery. We have also shown that the graft arc length parameters calculated from the software correlate well with the intra operative graft trephine diameter. There will be much value in analyzing the correlation between graft parameters measured on ASOCT and DSAEK outcomes, which may enable us to optimize graft trephine diameter chosen for DSAEK surgery. The use of high resolution ASOCT scans in the future will also provide greater detail with more quantitative measurements that may be useful in studying outcomes and optimizing surgical decisions. The ability to accurately select the ideal donor diameter, to maximize endothelial cell transfer while reducing the risks of anterior chamber crowding and peripheral anterior synechiae, and predict the final refractive outcome, could help to improve graft survival, and visual outcomes, as well as reduce complications in DSAEK surgery.

Competing interests

The authors declare they have no competing interests.

Authors’ contributions

GT participated in the design of the study, performed the measurements on the ASOCT images, performed the statistical analysis and drafted the manuscript. MH advised on the design of the study, developed the software used for image measurements and advised on drafting the manuscript. DT performed the surgery on all patients in the study, advised on the design of the study and advised on the drafting of the manuscript. JM conceived of the study and participated in the design and coordination of the study as well as the drafting of the manuscript. All authors read and approved the final version of the manuscript.

Financial disclosure

National Research Foundation-Funded Translational & Clinical Research (TCR) Program Grant [NMRC/TCR/002 - SERI/2008 - TCR 621/41/2008].

Pre-publication history

The pre-publication history for this paper can be accessed here: