# A theoretical model for graft punch size calculations to prevent Descemet’s membrane folds following deep anterior lamellar keratoplasty

## TL;DR

This paper introduces a theoretical model to calculate donor graft sizes in eye surgery to prevent membrane folds in patients with steep corneas.

## Contribution

A novel theoretical model for calculating donor graft sizes to prevent Descemet’s membrane folds in deep anterior lamellar keratoplasty.

## Key findings

- Using the model, no patients developed Descemet’s membrane folds compared to three in the control group.
- The model shows that steeper corneas or larger trephine sizes require larger donor grafts to prevent folds.

## Abstract

The length of Descemet’s membrane and donor graft sizes in deep anterior
lamellar keratoplasty do not match in very steep corneas, which can lead to
Descemet’s membrane folds. The aim of this study is to establish a
theoretical model for graft size calculations for deep anterior lamellar
keratoplasty and evaluate its efficacy for preventing Descemet’s membrane
folds.

We calculated the arc diameter of the recipient bed by using the cosine
formula and developed a table to aid surgeons in donor punch size selection.
To test the usefulness of this formula, we evaluated the development of
Descemet’s membrane folds in keratoconus patients with very steep corneas (K
>60 D). In group 1, deep anterior lamellar keratoplasty surgeries were
performed using graft sizes that were determined based on our model (n=31).
In group 2, graft sizes were determined based on the empirical judgment of
the surgeon without any formal calculation (n=30).

Our theoretical calculations demonstrated that the diameter of donor punch
sizes needed to prevent Descemet’s membrane fold increases when the cornea
is steeper, or the trephine size is larger. We tested the efficacy of this
model on the clinical outcome of deep anterior lamellar keratoplasty. The
mean age (28.9 ± 10.1 years vs. 32.8 ± 8.3 years, p=0.11) and
preoperative K1 (59.2 ± 9.3 D vs. 58.1 ± 9.4 D, p=0.67), K2
(66.2 ± 6.0 D vs. 65.7 ± 7.4 D, p=0.81), and Km values (62.1
± 7.7 D vs. 61.8 ± 8.1 D, p=0.88) were similar between the two
groups. Three patients developed Descemet’s membrane folds in group 2, and
none of the patients developed Descemet’s membrane folds in group 1. These
results supported our theoretical calculations.

Adjustment of donor graft size based on the calculated arc diameter of the
recipient bed reduced the development of Descemet’s membrane folds after
deep anterior lamellar keratoplasty in steep corneas.