A Quantitative Measurement of Ocular Rotation in Small Incision Lenticule Extraction: Double Marking of Corneal Limbus and Sclera

Summary

Here, we present a protocol for quantitatively measuring intraoperative ocular rotation using a corneal-scleral double-marking technique. This method assists researchers in measuring the effect of ocular rotation during refractive surgery and provides specific, scientific operational approaches for the implementation of clinical research.

Abstract

The small incision lenticule extraction (SMILE) has become a mainstream surgical procedure for correcting myopia and astigmatism. Nevertheless, the lack of an active cyclotorsion-tracking system prevents it from correcting rotational errors, which may compromise astigmatic correction accuracy. Evidence regarding the influence of intraoperative cyclotorsion on SMILE outcomes remains scarce, largely because of the unavailability of viable and dependable methods of measuring the degree of rotation during an operation. To address this gap, this protocol describes a novel corneoscleral double-marking technique to overcome a fundamental methodological limitation. It involves creating positional and dye-based marks on both the corneal and scleral surfaces, enabling direct observation and quantitative measurement of ocular rotation under a surgical microscope or a slit lamp. The primary goal of this protocol is to establish a standardized, objective approach for obtaining precise, quantitative data on cyclotorsion that has previously been inaccessible. The representative results of the statistical analysis of this data indicate that ocular rotation amplitude is an independent risk factor for residual astigmatism after SMILE surgery and has a significant impact on it (β = 0.520, P < 0.001). The analysis of astigmatism vectors also confirmed this. Therefore, the availability of this quantifiable data is pivotal. It provides an essential research tool to systematically investigate the specific impact of cyclotorsion in SMILE surgery. The measurements obtained are crucial for advancing studies on postoperative astigmatic outcomes, developing accurate predictive models for surgical accuracy, and refining treatment nomograms.

Introduction

The small incision lenticule extraction (SMILE) is an advanced type of corneal refractive surgery, and has a number of benefits over conventional laser in situ keratomileusis (LASIK)^1,2,3,4. Now, SMILE has become mainstream practice in the correction of myopia and astigmatism^5,6. Nevertheless, the SMILE surgery is prone to causing under-correction and regression in the correction of patients with moderate to high myopia and astigmatism^7,8. Multiple factors contribute to the variability in astigmatic correction after SMILE, including the magnitude and type of preoperative astigmatism, surgical centration, Kappa angle, and ocular rotation^9,10,11. Among these, ocular rotation (positional cyclotorsion resulting from changes in patient posture) is identified by Chow et al. as one of the most crucial variables influencing the accuracy of astigmatic correction^12,13. This is because the femtosecond laser system does not have an integrated cyclotorsion tracking^14,15.

In order to obtain precise quantitative data on the magnitude of ocular rotation, a goal that was previously difficult to achieve. Other researchers proposed an artificial intelligence (AI)-powered system for estimating the rotation angle of the eyeball, which requires accurate segmentation of the optic disc and macula and calculates the rotation angle based on these features^16,17. The approach, however, is operationally difficult and not suitable for corneal refractive surgeries such as SMILE. Therefore, we developed a standardized and objective method - the double corneal-scleral marking. The quantifiable data generated by this method hold fundamental value and constitute a key tool for systematically studying the impact of ocular rotation in SMILE surgery. Based on this measurement, it is crucial to deepen research on postoperative astigmatism, construct a surgical accuracy prediction model based on multi-center and big data, and build an ocular rotation amplitude prediction model based on ocular features and AI.

In summary, this simple, repeatable, and easily integrated into clinical practice solution, by reliably capturing the intraoperative rotation indicators, compensates for the shortcomings of existing research methods and provides a solid data-driven research foundation for ultimately achieving better accuracy and predictability in managing SMILE surgery-induced astigmatism and optimizing the clinical treatment pathway.

Protocol

All the ocular samples used in this study were collected from the Ophthalmology Department of the Second Affiliated Hospital of Guizhou Medical University. They were approved by the Ethics Committee of the Second Affiliated Hospital of Guizhou Medical University (Approval Number: 2022-lunshen-127), which complies with the Helsinki Declaration. All the subjects signed the informed consent form before the surgery. Please proceed as follows.

1. Preoperative preparation

1. Routinely irrigate the conjunctival sac of the surgical eye with 0.9% sodium chloride solution, disinfect the ocular surface with 5% povidone-iodine solution, and instill topical anesthetics as part of preoperative preparation (0.4% Oxbacillin Hydrochloride Eye Drops).
   NOTE: Use standard SMILE surgical disinfectants and anesthetics for preoperative preparation, as these are non-specific, routine agents.

2. Surgical parameter design

1. Perform the surgery using the femtosecond laser system¹⁸.
2. Design the laser parameters based on the preoperative examination data. Set the incision position at 90°, make the incision length 2 mm, and determine the center position by the corneal apex (CA).

3. Rotating mark method

NOTE: Maintain the patient in a flat, straight-ahead gaze position throughout all subsequent steps, whether in a sitting position or a lying position.

1. Design the ocular rotation measurement system according to Figure 1A, B, and critically, ensure an accurate simulation of the spatial relationship between seating (examination) and supine (surgical) ocular positions for intraoperative marking and postoperative measurement.
   NOTE: The "primary position" is defined as the state in which the patient's eyes are maintained in a level, straight-ahead gaze, with the pupils aligned horizontally. The core innovation of this protocol lies in utilizing physical markers created on the ocular surface in the supine position as fiducial references to link the two postures, enabling direct spatial calibration of the measurement system, the slit lamp, in the sitting position.
   1. First, with the patient's head securely fixed in the supine surgical position, place a precise vertical (90°) reference mark on the ocular anatomy.
   2. Subsequently, during the seated examination, once the patient reestablishes the same anatomical eye position, align the slit lamp's measurement reference, its 90° illumination beam, directly with this fiducial mark.
      NOTE: This calibration procedure ensures that the angular coordinate system of the slit lamp in the seated position is spatially registered with the coordinate system defined during supine surgery.
2. Follow the steps below for the marking operation:
   1. Upon completion of supine laser scanning, which is one of the steps in the SMILE surgery procedure¹⁸ (Figure 2A), use a sterile syringe needle and marking ruler (Figure 2B) to precisely mark the center of the incision.
   2. Following lenticule removal, which is one of the steps in the SMILE surgery procedure¹⁸, carefully color the marking points with a blue marking pen (Figure 2C, D). Then, remove the eyelid opener after removing any excess dye.

4. Rotating measurement method

NOTE: Throughout all subsequent procedures, ensure the patient maintains a stable sitting posture and constant forward gaze.

1. Immediately post-surgery, measure and record ocular rotation magnitudes with the patient seated under a slit lamp microscope.
   1. Under the wide light band (Figure 3A), locate the marking position. Adjust the slit lamp to the finest light band setting.
   2. Then, rotate the band to parallel the marked incision (Figure 3B). Finally, read the rotation amplitude indicated on the slit lamp rotation dial at this time (Figure 3C).

Results

This study measured the effective rotation amplitude during SMILE procedures using a dual-marking technique on both the sclera and cornea. What we have collected is the ocular rotation amplitude of the Chinese population. Figures 1A,B illustrate the schematic diagrams of the eyeball positions in the supine (during surgery) and sitting (immediately after surgery) positions, clearly contrasting the ocular rotation caused by the changes in body position. Fi...

Discussion

Current studies blame astigmatism correction divergences during SMILE surgery on various factors, including ocular rotation, a vital yet poorly measured clinical factor^12,13. The artificial intelligence approach to measuring rotation that is based on the division of the optic disc and fovea in fundus images is hard to apply to the SMILE surgical environment^16,17. Even though S...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Disposable sterilized syringe	Shandong Weigao Group Medical Polymer Products Co., Ltd.	CNDA Registration Number: 20163141593	Mark
Femtosecond laser treatment machine	Visumax, Carl Zeiss Meditec AG	100799115	Treatment
Marking ruler	Dongguan Tangde Medical Technology Co., Ltd.	Guangdong Medical Device Registration Number20202021011	Auxiliary marking
Medical marking pen	Dongguan Tangde Medical Technology Co., Ltd.	Guangdong Medical Device Registration Number20202021011	Mark
Slit-lamp microscope	Topcon Corporation	ZCKP020191110100041	Observation and Measurement