Are Fluid Pressures During Cataract Surgery Harmful?

This research summary is based on the work of Alex McMullen, PhD student in Biomedical Engineering at the University of Rochester, NY, USA, under the supervision of Mark Buckley, Associate Professor of Biomedical Engineering and principal investigator of the Buckley Soft Tissue Biomechanics Lab.

The study is presented in the article “Investigating the Tolerance of Corneal Endothelial Cells to Surgical Fluid Pressure Using Intact Porcine Eyes” published in Translational Vision Science & Technology (November 2024, Vol. 13, 27. doi: 10.1167/tvst.13.11.27). [1]

The primary objective of this research is to determine whether the fluid pressures typically used during cataract surgery contribute to iatrogenic corneal endothelial cell (CEC) loss.

Abstract

More than 20 million cataract surgeries are performed every year worldwide, and an estimated 1-2% of cataract patients will suffer from a surgical complication known as pseudophakic corneal edema (PCE). PCE refers to irreversible corneal swelling after cataract surgery and intraocular lens (IOL) implantation. This complication is caused by incidental surgical damage to the non-proliferative corneal endothelial cells (CECs) located on the posterior side of the cornea that are vital for maintaining corneal transparency. Patients with this condition can experience blurred vision, severe pain, and even blindness, oftentimes needing a corneal transplant to restore their vision. For readers unfamiliar with the procedure, an animated overview of standard cataract surgery provides helpful visual context.

Mechanical loads imparted on CECs during cataract surgery may play a role in acute CEC loss, since they are much different from those normally experienced by CECs in physiological conditions. For instance, cataract surgery subjects CECs to fluid pressures >60 mmHg (with peak pressures exceeding 400 mmHg) at least half of the duration of the procedure, which is drastically different from the range of 9-21 mmHg of intraocular pressure (IOP) they are normally exposed to in physiological conditions. During surgery, these elevated pressures are intentionally used to stabilize the anterior chamber, the fluid-filled space at the front of the eye, so that the surgeon can safely maneuver instruments [2]. While Buckley Soft Tissue Biomechanics Lab had previously shown that acute CEC injury occurs at contact pressures >43 mmHg (during contact-based indentation) [3], it remains unclear if surgical fluid pressure contributes to CEC damage during cataract surgery.

Aim: Using microfluidics to reproduce cataract surgery fluid-pressure conditions

In this study [1], Alex McMullen et al. developed a custom experimental platform that enabled precise application of measurable fluid pressures in the anterior chamber of intact eye explants, as well as a novel in situ viability staining and imaging method that together allowed for assessment of CEC injury triggered by fluid pressure alone. In clinical cataract surgery, intraocular pressure is regulated by controlled fluid inflow and outflow from the surgical system. Accordingly, the experimental pressure loading used here is not intended to replicate the surgical procedure, but rather to isolate and study the effects of elevated fluid pressure on corneal endothelial cells. This approach was used to determine if surgically-relevant magnitudes of fluid pressures applied for surgically-relevant durations resulted in acute CEC injury in porcine corneas.

Required Materials:

OB1 MK3+ pressure controller (Elveflow)
Microfluidic pressure sensor – MPS (Elveflow, P/N: MPS-V2-S-3)
50 mL microfluidic reservoir, 2 ports (Elveflow, P/N: KPT-M-2)
Elveflow software interface (Elveflow)
Syringe pump (custom-built in lab)
Pad 4 Pigs (eyecre.at GMbH)
IX81 inverted microscope with fluorescence imaging capabilities (Olympus)
3D printed eye holder (custom-built in lab)
PrecisionGlide needles (BD)
Hexidium iodide (Thermo Fisher)
SYTOX Green nucleic acid stain (Thermo Fisher)

Experiment Setup

Inducing fluid pressure with microfluidics

Two needles (“perfusion needle” and “vacuum needle”) were inserted into the anterior chamber of fresh enucleated porcine eyes by piercing their tips through the cornea just anterior to the limbus.

As illustrated in Figure 2, the perfusion needle was connected to an Elveflow OB1 MK3+ pressure controller that allowed to controllably pressurize the anterior chamber with balanced salt solution and/or viability stain using the Elveflow software. Meanwhile, the vacuum needle was connected to a syringe pump that pulled fluid out of the anterior chamber and into an Elveflow microfluidic pressure sensor, giving us the ability to ensure that the anterior chamber pressure (ACP) was precisely maintained in the eye (within ±3 mmHg), downstream from the pressure controller, throughout the experiment.

Observing fluid pressure impact on corneal endothelial cells

The anterior chamber was then drained of all fluid and either:

re-perfused with a novel staining solution (hexidium iodide labels all cell nuclei, and SYTOX green only labels injured/dead cell nuclei); or
re-perfused with deionized water (which damages CECs), drained again, and perfused with the same staining solution.

As described in the setup scheme, the stain is delivered into the anterior chamber through the same needle previously used to pressurize the eye. This approach eliminates the need to remove and reinsert the needle, make additional incisions, or introduce any other instruments into the chamber.

The eyes were then brought back to a physiological pressure before removing the needles and sealing the incisions to prepare the eyes for the imaging workflow summarized in Figure 3.

The total and injured/dead CEC counts (TC and IC, respectively) obtained from image analysis were used to calculate CEC loss for each sample as a “Percent CEC Injury” (PCI (%) = (IC/TC) x 100).

Figure 3 - Imaging workflow: fluorescent z-stack corneal endothelial images were (A) acquired in intact porcine eyes using an epifluorescence microscope at 4x magnification. The acquired images were then processed by (B-1) marking locations of in-focus CECs for each plane, (B-2) fitting a 3-D surface to those points, and (B-3) using the 3-D surface fit to create height-focused circular ROIs with a diameter of 6.5 mm. Zooming into (C) a representative endothelial image demonstrates the staining assay’s ability to clearly label individual CEC nuclei. The processed ROI images were then (D) analyzed to obtain injured/dead and total CEC nuclei counts (images demonstrating how yellow markers are plotted on centroids of CEC nuclei that were identified by a custom MATLAB image analysis code).

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Key Findings: What is the impact of fluid-pressure on corneal endothelial cells?

As shown in Figure 4, no significant differences were observed in mean PCI values between the surgical fluid pressure groups and the sham control groups at either time point. Interestingly, the mean PCI values across all surgical pressure groups, including eyes perfused up to 400 mmHg (more than 25 times higher than physiological intraocular pressure), remained below the lowest levels of corneal endothelial cell (CEC) loss typically reported within one year after cataract surgery (2%–42%). In contrast, the mean PCI of the positive control group differed significantly from all other groups, confirming that the method was capable of detecting CEC injury and death. Collectively, these results indicate that fluid pressure alone does not induce clinically significant acute iatrogenic CEC damage.

Figure 4 – Quantification of injured CECs for each experimental group. There was a significant difference (p<0.0001) in PCI between the positive control group and all other groups, but no significant differences in PCI between any of the other groups. The yellow and purple dotted lines indicate the low and high ends of clinically-observed CEC loss within 1 year following cataract surgery (2% and 42%).

Conclusions

Corneal endothelial cell injury in cataract surgery is a multifactorial phenomenon

To the investigators’ knowledge, this study was the first to isolate the effects of surgical fluid pressures, at magnitudes and durations relevant to cataract surgery, on acute corneal endothelial cell (CEC) loss [1]. This was achieved through the development of novel experimental methods that relied extensively on Elveflow microfluidic instruments. The findings not only addressed a longstanding clinical question by demonstrating that surgical fluid pressures alone do not cause clinically significant acute CEC loss, but also introduced methods that will enable future investigations into the key factors underlying iatrogenic CEC injury in cataract and other intraocular surgeries.

Elveflow instruments to reproduce and monitor fluid-pressure

The use of Elveflow instruments was critical in reproducing and monitoring accurate fluid pressures throughout the experiments. The OB1 MK3+ pressure controller ensured highly stable, pulseless flow with fast response times, allowing precise regulation of anterior chamber pressures (ACPs) over a wide dynamic range. In combination, the Elveflow microfluidic pressure sensor (MPS-V2-S-3) provided real-time pressure readouts with high resolution (±4 mbar / ±3 mmHg), enabling continuous monitoring and fine-tuned adjustments of intraocular pressures. These instruments were operated and synchronized using the Elveflow Smart Interface (ESI) software, which offered an intuitive platform to set input pressures, log measurements, and automate adjustments with reproducible accuracy. This integration of hardware precision and software control was essential for isolating the role of fluid pressure in corneal endothelial cell response, while also offering flexibility for protocol adjustments without compromising stability. If you want more information about this research, do not hesitate to refer to the original article.

Images were obtained from the manuscript with the approval of the authors.

Authors Information

Alex McMullen is a biomedical engineering PhD student at the University of Rochester. After earning a bachelor’s degree in mechanical engineering from Ohio Northern University and working in the medical device industry for 4 years, Alex decided to return to academia to earn a master’s degree from the University of Rochester’s Center for Medical Technology & Innovation. Alex then continued his studies at the University of Rochester in the PhD program, joining Mark Buckley’s Soft Tissue Biomechanics Lab in the Summer of 2021. Under the mentorship of Dr. Buckley, Alex is using biomechanics approaches to investigate iatrogenic injury of corneal endothelial cells. Thus far, his research has been published in Translational Vision Science & Technology, as well as presented at national and local conferences including the Association for Research in Vision and Ophthalmology’s (ARVO) annual meeting, the American Society of Mechanical Engineers’ (ASME) Summer Bioengineering Conference (SB3C), and an American Society of Biomechanics (ASB) regional meeting.

Mark Buckley is an Associate Professor of Biomedical Engineering and the principal investigator of the Buckley Soft Tissue Biomechanics Lab at the University of Rochester. Dr. Buckley received his Ph.D. in Physics from Cornell University under the mentorship of Drs. Itai Cohen and Lawrence Bonassar in 2010 and worked under Dr. Louis Soslowsky as a post-doctoral fellow at the University of Pennsylvania from 2010-2012. He joined the faculty of the Department of Biomedical Engineering at the University of Rochester in January of 2013. He has co-authored publications on diverse topics ranging from three-dimensional tracking of swimming bacteria to the use of ultrasound elastography to quantify transverse compressive strain in the Achilles tendon. Dr. Buckley is particularly interested in the mechanics of soft biological tissues including cartilage, tendon and the cornea. His research emphasizes finding ways to control and exploit their complex mechanical properties to prevent graft damage during transplantation surgeries, diagnose injury and disease, guide rehabilitation protocols, and evaluate treatment and repair strategies.

Written and reviewed by Alex McMullen, and Louise Fournier, PhD in Chemistry and Biology Interface. For more content about microfluidics, you can have a look here.

References

McMullen AJ, Dantsoho ZA, Chamness S, et al. Investigating the Tolerance of Corneal Endothelial Cells to Surgical Fluid Pressure Using Intact Porcine Eyes. Transl Vis Sci Technol. 2024;13(11):27. doi:10.1167/tvst.13.11.27
Wenzel DA, Schultheiss C, Druchkiv V, et al. Effect of elevated irrigation bottle height during cataract surgery on corneal endothelial cells in porcine eyes. BMC Ophthalmol. 2023;23(1):211.
Ramirez-Garcia MA, Khalifa YM, Buckley MR. Vulnerability of corneal endothelial cells to mechanical trauma from indentation forces assessed using contact mechanics and fluorescence microscopy. Exp Eye Res. 2018;175:73-82. doi:10.1016/j.exer.2018.06.005
Jesintha Navaratnam 1,*,Tor P. Utheim 2,3,Vinagolu K. Rajasekhar 4 andAboulghassem Shahdadfar, Substrates for Expansion of Corneal Endothelial Cells towards Bioengineering of Human Corneal Endothelium, J. Funct. Biomater. 2015, 6(3), 917-945; https://doi.org/10.3390/jfb6030917