Changes in Foveal Fixation Axis Position Between Seated and Supine Postures and Their Potential Applications in Refractive Surgery

Summary

The aim of this study is to analyze changes in the position of the Foveal Fixation Axis (FFA) when measurements are taken with the patient in seated versus supine positions using the Ergofocus device and to discuss the potential impact of these changes on refractive surgery outcomes.

Abstract

The correct centration of the ablation in laser refractive surgery is an unresolved classic challenge. This is usually performed at the pupillary center or corneal vertex, but ideally should be done with the point through which the patient's eye truly aligns when looking. Aligning the ablation with this axis is believed to yield the most accurate and effective refractive outcomes.

This study proposes the use of a new device designed to center ophthalmic lenses, specifically progressive addition lenses, with the foveal fixation axis (FFA). This device showed an acceptable repeatability and users prescribed with progressive addition lenses with its measurements have an elevated adaptation rate and satisfaction. Because foveal fixation axis measurements have proven to be suitable for progressive addition lenses adaptation, it should be of interest to analyze whether these measurements can also be used as a proper reference in refractive surgery procedures, helping to improve centration of the laser ablation. Preoperative refractive surgery exam includes measurement of corneal and pupil characteristics that are conducted with the patient upright, but surgery is performed with the patient lying down, and a rotational movement of the eye has been described with this change of position. The purpose of this study was to analyze the change in FFA position with the change of the patient's position from seated to supine position, discussing the impact on refractive surgery procedures.

Introduction

An accurate optical centration is a key factor to achieve a satisfactory outcome in refractive surgery (laser and intraocular lenses)^1,2. However, there is currently no established consensus on the optimal technique for assessing centration in individual patients to ensure a satisfactory surgical outcome².

The pupillary center and the corneal vertex are the main references considered for this centration^2,3. The discrepancy between the visual (corneal vertex) and pupillary (pupillary center) axes is known as the angle kappa⁴. Even the combination of these two methods has been tested⁵. The pupillary center has the advantage of the light near that center appears brighter than the light entering near the edge (Stiles Crawford effect)⁶. Nevertheless, the changes in the pupil size with changes in illumination mean that the position of the pupillary center is not constant⁷. Meanwhile, corneal vertex is defined as the closest point to the center of the Placido image on corneal topography² and is closely related to the first Purkinje image⁸. On the other hand, corneal vertex is usually determined with a corneal topo- or tomography and its translation to the surgery process is not easy⁹. Moreover, the excessively theorical definition of the visual axis means that corneal vertex is not always coincident with that point¹⁰. Studies to date that have focused on the corneal-apex method have reported encouraging outcomes for correcting myopic refractive errors and coma aberrations⁹. When the distance between the corneal vertex and the pupillary center is large, evidence suggests that the ablation should be centered closer to the corneal vertex¹¹. In hyperopic patients, it is generally recommended to center the ablation on the corneal reflex or corneal apex to achieve optimal results. Additionally, when the Kappa angle exceeds five degrees, centration should be guided by the coaxially sighted corneal light reflex¹². For myopic patients with a large Kappa angle, some authors recommend adjusting the centration to align with the corneal reflex or the first Purkinje image¹³.

Foveal fixation axis (FFA) was first defined by Chang et al. who described it as the line that directly links the fixation point with the fovea¹⁰. This definition derives from the absence of a consensus regarding the definitions of various eye axes used for refractive surgery centration and the theoretical complexity of some axes which hinders their clinical measurement. Meanwhile, FFA is defined as an imaginary line connecting two points. Although it remains hypothetical at this stage, patients with a high angle kappa may be those who benefit most from FFA-based centration strategies in refractive surgery. These individuals are more likely to present significant discrepancies between pupillary-based and foveal-based centration, which could impact visual outcomes.

A new device (Ergofocus) was developed to measure FFA for the centration of ophthalmic lenses, specifically progressive addition lenses. This device has demonstrated good repeatability; difference in outcomes compared with traditional pupillary distance measurements used for lens centration, suggesting non interchangeable devices because FFA usually is not coincident with pupil center¹⁴. Moreover, users of progressive addition lenses prescribed using FFA measurements were able to adapt successfully, including those who had previously been unable to adapt to these lenses¹⁵. Therefore, FFA measurement appears to be appropriate for prescribing progressive addition lenses and may be useful in other ophthalmic procedures that require precise optical centration, such as refractive surgery procedures¹⁰.

Preoperative refractive surgery examinations are typically conducted with the patient in an upright position. Procedures such as corneal topography or tomography are used to collect relevant eye data, including pupillary diameter and center, kappa angle, corneal vertex, and others, such as corneal power, astigmatism, and other relevant measurements, to guide the surgery and ensure accurate optical centration^16,17.

However, during surgery, patients lie in a supine position, and a rotational movement of the eye has been documented with this change in posture¹⁸. Therefore, a change of the FFA position when moving from a seated to a supine position is expected, but no previous studies have assessed or reported on this issue.

The purpose of this study is to analyze changes in FFA position measured with the FFA measurement device in both seated and supine positions, and to evaluate the clinical relevance of these changes for refractive surgery planning and outcomes.

Protocol

This study involved 30 healthy participants with a monocular best corrected visual acuity better than 0.2 logMAR to allow accurate target fixation at far and near distances. Participants with a previous history of strabismus, amblyopia, nystagmus, significant visual field loss, and those with systemic diseases such as multiple sclerosis, Parkinson's, Alzheimer's, and myasthenia gravis were excluded from the study. The study followed the tenets of the Declaration of Helsinki and was approved by the Aston University College of Health and Life Sciences Research Ethics Committee.

NOTE: After determining the refractive error using objective and subjective refraction, four different measurements were conducted: FFA, static cyclotorsion, Kappa distance, and motor dominant eye (DE). FFA and static cyclotorsion were measured in seated and in supine positions. The measurement protocols are described below.

1. FFA measurement

NOTE: FFA positions were measured with the FFA measurement system with participants in seated and supine positions. The order of measurements in the seated and supine positions was defined with a randomization table, ensuring that 50% of the sample started in each posture.

1. FFA measurement in a seated position
   NOTE: The FFA measurement system is a device that comprises two movable slits (one horizontal and one vertical) in front of each eye. To conduct the FFA measurement in a seated position, follow the next steps:
   1. Place the participant in a comfortable position, seated in the examination chair.
   2. If FFA measurement in the seated position is performed first, place the FFA measurement device on the participant's head. The FFA measurement device has rubber bands to get adjusted to the participant's head and two nose pads to rest on the participant's nose with the slits in the 'zero' position. Adjust them to make the participant the most comfortable as possible and fix the device in place for a correct measurement. The device must be centred and adjusted to avoid unwanted movements that can affect the measurement process.
   3. Connect the FFA measurement device via Bluetooth to an Android tablet app that receives the measurements and generates an Excel file for collecting data. Connect the device with the app and write an ID for the measurement.
   4. Tell the participant to hold the fixation target. Select the same fixation object for seated and supine position measurement. Preferably, use a cross-fixation test to adapt it to the form of the two slits that comprise the device (in this study, an 8 cm length and 0.5 cm width cross was used, shown with a projector). Fixation test is placed at near distance (35 to 50 cm) to simulate the position of the laser in refractive surgery procedure. The device has a laser sensor (top and middle located) to measure the fixation distance with an accuracy of 1 mm. If FFA measurement in the supine position is performed secondly, check that the measurement distance is the same as in the seated position measurement.
   5. Measure the right eye (RE), occlude the left eye (LE).
   6. Move the vertical slit of the RE manually until the participant can see the vertical beam of the fixation cross centered in the visual field generated by the slit. To ensure that the position of the fixation cross remains constant throughout the entire measurement process, ensure that the laser point at the top and middle of the device is always pointing at the center of the fixation cross. The app calculates the distances from the zero-position based on the position of the slit as the different measurements are taken. If FFA measurement in the seated position is performed secondly, initiate the measurement from the final position of the previous measurement.
   7. Move the horizontal slit of the RE manually until the participant can see the horizontal beam of the fixation cross centered in the visual field generated by the slit. Ensure that the laser spot is pointing at the center of the fixation point.
   8. Measure the LE, occlude the RE.
   9. Repeat the process as previously performed in the RE. First, move the vertical slit and then the horizontal slit until the participant can see the light beams of the fixation cross in the center of the visual field generated by the slits, ensuring that the laser spot is pointing at the center of the cross.
   10. Press the measure button in the app to save the measurement data.
   11. Check that the measurement distance is inside the proposed range of distances (35 to 50 cm). If not, correct the position of the fixation cross and repeat the measurement procedure.
   12. If FFA measurement in the seated position is performed first, keep the slits in the final position.
2. FFA measurement in a supine position
   1. If FFA measurement in the supine position is performed first, place the FFA measurement device on the participant's head. Adjust the rubber bands to the participant's head and ensure that the nose pads rest properly on the participant's nose with the slits in the 'zero' position. Ensure that the device is correctly adjusted and centered.
   2. Place the participant in a comfortable position, lying down on an examination table.
   3. Connect the FFA measurement device with the app via Bluetooth and write an ID for this measurement.
   4. Tell the participant to hold the fixation cross over his head. If FFA measurement in the seated position is performed secondly, check that the measurement distance is the same as in the supine position measurement.
   5. Measure the RE, occlude the LE.
   6. Move the vertical slit of the RE manually until the participant can see the vertical light beam of the fixation cross centered in the visual field generated by the slit. To guarantee that the position of the fixation cross is constant across the whole measurement process, ensure that the laser spot, located at the top and middle of the device, is always pointing at the center of the fixation cross. The app calculates the distances from the zero-position based on the position of the slit as the different measurements are taken. If FFA measurement in the supine position is performed secondly, initiate the measurement from the final position of the previous measurement.
   7. Move the horizontal slit of the RE manually until the participant can see the horizontal light beam of the fixation cross centered in the visual field generated by the slit. Ensure that the laser spot is pointing at the center of the fixation point.
   8. Measure the LE, occlude the RE.
   9. Repeat the process as previously done in the RE. First, move the vertical slit and then the horizontal slit until the participant can see the light beams of the fixation cross in the center of the visual field generated by the slits, ensuring that the laser point is pointing at the center of the cross.
   10. Press the measure button in the app to save the measurement data.
   11. Verify that the measurement distance falls within the proposed range of distances (35 to 50 cm). If not, correct the position of the fixation cross and repeat the measurement procedure.
   12. If FFA measurement in supine position is performed first, keep the slits in the final position.

2. Static cyclotorsion measurement

NOTE: Static cyclotorsion was measured using the single Maddox rod method¹⁹. Cyclotorsion was determined by the difference between the ocular cyclotorsion in the seated and the supine position. A white Maddox rod was used, and the order between seated and supine position measurement was randomized.

1. Place the participant in a seated or supine comfortable position according to the randomization table. Participants must wear a trial frame that is correctly adjusted.
2. Measure RE cyclotorsion, occlude LE.
3. Ask the participant to fixate on a white light placed at a near distance (40 cm).
4. Place the white Maddox rod in front of the RE of the participant oriented at 180°. The participant is supposed to see a completely vertical line.
5. Ensure that the head of the participant is in a straight position to avoid unwanted turns that can affect the measurement.
6. Spin the Maddox rod until the participant notices that they see a completely horizontal line.
7. Measure the angular position of the Maddox rod lines at their final location to obtain the cyclotorsion value of the RE. If the lines are completely horizontal, cyclotorsion value is 0°. Use a different sign for incyclotorsion and excyclotorsion. In this study, a negative value indicates incyclotorsion and a positive value excyclotorsion²⁰.
8. Change the Maddox rod to the LE and occlude the RE to measure the LE cyclotorsion.
9. Spin the Maddox rod until the participant sees a completely horizontal line. Ensure that the head of the participant is in a straight position.
10. Measure the angular position of the Maddox rod lines at their final location to obtain the cyclotorsion value of the LE.
11. The difference in cyclotorsion between the seated and supine positions is the value of static cyclotorsion.

3. Kappa distance

NOTE: Kappa distance and its X and Y components were determined with a corneal topography. The device automatically calculates and provides X and Y components based on the participant's fixation and corneal alignment.

1. Place the participant in a comfortable seated position in the examination chair. Tell the patient to position their chin and forehead on the support of the topographer.
2. Instruct the participant to fixate the internet target light of the device to ensure a correct alignment.
3. Follow the instructions of the device to capture the corneal topography and the kappa distance values.

4. Motor dominant eye determination

NOTE: Motor DE is determined with the "hole in hands" method, a slight variation of the "hole in card" method²¹.

1. Place the participant in a comfortable seated position in the examination chair.
2. Project a small circle or point of light as a fixation point at distance vision.
3. Instruct the participant to form a small opening by overlapping their outstretched hands.
4. Tell the participant to look through that small opening to the distant fixation point.
5. Tell the participant to close the RE and LE alternatively and to determine which eye is focusing on the fixation point. The eye fixating on the fixation point is the motor DE.
   NOTE: Statistical analysis was performed using the SPSS 27.0 statistical package for Mac. A descriptive statistical analysis was performed of FFA, cyclotorsion, and Kappa angle parameters. Asymmetry has been defined as the difference between the FFA of both eyes, either RE and LE or DE and non-dominant eye (NDE). The sample was tested for normality with the Kolmogorov-Smirnov test, and comparisons were made with the Wilcoxon signed-rank test. P < 0.05 was considered statistically significant. To examine the associations between FFA, cyclotorsion, and Kappa angle, Spearman's rho correlation was used. Using G*Power, a sample size of 30 participants was calculated to be sufficient to detect a minimum difference of 0.75 mm in FFA measurements between positions, with 90% power and a standard deviation of 2.50 mm."

Results

Thirty subjects (18 women and 12 men) with a mean age of 25.93 ± 6.82 years (range: 18 to 47 years) and a spherical equivalent of -1.55 ± 2.28 D (range: -9.50 to +1.00 D) were included in the study. Participants showed a mean LogMAR visual acuity of -0.06±0.05 (range: -0.2 to 0.1). 19 participants presented the RE as the DE compared to 11 who presented LE.

Analysis of FFA distance in seated and supine positions showed a statistically significant difference in the RE (P = 0.02), with supine FFA...

Discussion

The correct optical centration of the ablation in refractive surgery is an unresolved classic challenge². This study addresses the influence of cyclotorsion, kappa angle, and the position of the FFA to clarify its interrelation. The cyclotorsion produced when the patient lies down on the surgical table is a fundamental parameter for the correct performance of refractive surgery^22,23. There are mechanisms to prevent the influence of the cyc...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ergofocus System	Lentitech, Spain	https://lentitech.com/ergofocus-system/	Device designed to measure Foveal Fixation Axis position. It was originally designed to improve the adaptation process to progressive addition lenses.
Oculus Keratograph	Oculus, Germany	https://www.oculus.de/es/productos/keratograph-5m/	Device to make corneal topographies. In this study this study was used to measure the kappa distance in the X and Y components.
White Maddox rod	Promocion optométrica, Spain	9756-E	Device for measuring phoria, in this case cyclophoria.
WAM-5500	Rexxam, Japan	https://www.rexxam.co.jp/eye-care/products/wam5500.html	Refraction-Keratometer that allows the patient to keep natural binocular vision duringmeasurement.