Simulating the Mechanics of Lens Accommodation via a Manual Lens Stretcher

The overall goal of this technique is to provide a more feasible and inexpensive method for accurately studying lens biomechanics. This method can help answer key questions in the ocular biomechanics field about how the lens changes with the loss of accommodation and the onset of presbyopia. The main advantage of this technique is that it accurately monitors the ability of the lens to accommodate at a low material cost and a high procedural robustness.

After obtaining the eye sample, firmly grasp the side of the eye and use a razor blade to make a small incision along the side three millimeters away from the cornea deep enough to reach the vitreous. Using sterilized scissors, carefully continue the incision around the circumstance of the eye using the forceps to remove the posterior eye tissue once it has been excised. Isolate the lens, zonules, and the attached vitreous using forceps as necessary to disconnect the zonules and the ciliary body from the ciliary muscles while maintaining connection with the lens.

Then remove the excess vitreous so the lens can lay flat on the manual lens stretcher. Take care to keep the lens, zonules, and a minimal amount of vitreous intact while removing the excess vitreous. To assemble the stretcher, insert 10 millimeter shoe bottoms and their corresponding shoe tops into the bottom plate of the stretcher such that a five milliliter gap remains between the back wall of the shoe indent and the shoe itself.

Using curved forceps, place the extracted lens face up in the middle of the bottom plate so that the shoes are supporting the lens over the central hole. Snap the corresponding top of the shoes into place clipping only the zonules and the vitreous. Then insert the plates into the plate case and the stopper screw into the hole located on the side of the bottom plate and insert the plate case into the base.

To measure the lens, place an imaging system directly above the apparatus at the appropriate distance for obtaining images and place a ruler in the field of view for the accurate sizing and scaling of the images in post processing. Next, firmly yet smoothly rotate the wrench in a clockwise direction to stretch the lens and obtain images of the lens. To ensure sample-to-sample consistency, it is essential to rotate the wrench smoothly while stretching the lens.

When all of the images have been acquired, rotate the wrench in the counterclockwise direction to restore the sample to its resting state and obtain the images of the final resting state of the lens. To analyze the data, upload the images to the appropriate image analysis software and use the point to select at least 40 points around the circumference of the lens. Under the analyze menu, select measure to yield the location of each selected point and fit the location points with MathLab to yield a radius and chi square of the fit.

Then use the photographed ruler to convert the pixel radius and error into metrics and perform paired two-tailed t-test analysis to compare an individual lens before and after stretching. In this representative analysis of 10 porcine eye samples, the lenses demonstrated an over 4%increase from their original radii validating the lens stretching protocol with both literature consistency and accuracy. When human lenses were tested, a dramatic decrease in the elasticity of the lens was observed in correlation with the age of the donor as expected from previous lens elasticity studies.

Once mastered, this lens stretching technique can be completed in less than 10 minutes if it is performed properly. While stretching the lens, it's important to remember to provide a continuous wrenching force until the stopper screw has been reached to minimize the variability between samples. Using this manual lens stretching technique, we can monitor the lens stiffness of patients with a variety of disease pathologies including patients with diabetes, cataracts, or presbyopia.

After its development, this technique paved the way for researchers in the field of ocular biomechanics to explore the properties of the lens during the loss of accommodation and/or onset phases of presbyopia. After watching this video, you should have a good understanding of how to use a manual lens stretcher to obtain valuable biomechanical insight about the lens. Don't forget that working with biological tissue can be extremely hazardous and that precautions such as wearing proper laboratory attire and sterilizing all of the instruments prior to use should always be taken while performing this procedure.