How does Biomechanics affect you?
The biomechanical properties of ocular tissue (i.e., its material composition) determine how the eye will respond and deform when placed under stress, while growing or healing, and when undergoing surgery.
Surgeons currently only measure shape parameters prior to surgery – other primary parameters like stiffness or elasticity are not accounted for in clinical practice.
This inability to measure tissue biomechanics can lead to surgical surprises, lack of surgical precision, added diagnostic uncertainties, and missed diagnosis.
An understanding of corneal biomechanics is fundamental to the diagnosis and treatment of corneal and lens disorders.
Understanding the interactions between biomechanics and functionality of your eye will allow the physician to better screen and apply a more effective treatment for your corneal and lens related refractive disorders.
Anatomy of the Eye
The primary function of the eye is to create an image of the object we look at. In the process, the eye refracts light through the Cornea, Iris, and Lens on to the Retina.
- First, light passes the Cornea, a rigid multilayered structure, which is covered by a tear film.
- Second, light passes the Iris (Pupil), which regulates the amount of light that continues to pass to the back of the eye.
- Third, light passes the Lens, a crystalline soft body, that changes its shape and position to contribute to refraction.
- Finally, light hits the Retina - our light sensory system.
Biomechanics related to the Cornea
The cornea is the transparent front part of the eye that covers the iris. Its main function is to focus light on to the retina. While the cornea contributes most of the eye's focusing power, its focus is fixed. Very small changes to its shape or thickness will disturb its focusing capabilities - an eye will manifest this in refractive errors.
Biomechanics imaging related to the cornea focuses primarily on:
- Keratoconus and Ectasia
- Corneal Crosslinking
- Refractive Surgery
Keratoconus is a progressive ectatic disorder of the cornea characterized by bilateral, asymmetric, non-inflammatory degeneration which results in central and paracentral thinning and protrusion. Progression of the disease is thought to begin with localized degeneration of material properties, followed by a cycle of thinning, increased strain, and redistribution of stress. Clearly, biomechanics play a very critical role in the early onset of keratoconus and ectasia.
Corneal collagen cross-linking is a technique which uses UV light and a photosensitizer (Riboflavin Vitamin B2) to strengthen chemical bonds in the cornea. The goal of the treatment is to halt progressive and irregular changes in corneal shape known as ectasia. These ectatic changes are typically marked by corneal thinning and an increase in the anterior and/or posterior curvatures of the cornea, and often lead to high levels of myopia and astigmatism.
Refractive Surgery - LASIK
LASIK (laser in situ keratomileusis) is the most frequently performed of many surgical procedures designed to correct refractive errors. LASIK involves creating a corneal flap using a microkeratome or a Femto-Laser, reshaping the cornea using an Excimer laser to remove tissue from the underlying stromal bed and then replacing the flap.
Corneal ectasia is one of the most devastating complications after LASIK. Post-LASIK ectasia is considered in patients who developed increasing myopia, with or without increasing astigmatism, loss of uncorrected visual acuity, often loss of best-corrected visual acuity, with keratometric steepening, with or without central and paracentral corneal thinning, and topographic evidence of asymmetric inferior corneal steepening after LASIK procedure.
Understanding and imaging the biomechanical properties of your cornea prior to LASIK might be a safe approach to predicting if your eye will develop post-LASIK ectasia.
Biomechanics related to the Lens
The lens is centrally located in eye, behind the cornea and the iris. In addition to focusing the image and aiding visual aquity, the lens also has important functions in the protection of the sensory system of the eye, the retina, from UV radiation.
Biomechanics imaging of the lens is of particular interest for development and progression of
- Presbyopia, and
Presbyopia is the progressive inability of the eye to focus on near objects as a person ages. The cause of it has been widely studied over the years and is still subject to debate. But many ophthalmologists agree that presbyopia is caused by a loss of elasticity of the natural crystalline lens of the eye, making it difficult for this lens to change shape or “accommodate” in order to focus on close objects. Changes in the curvature of the lens as it grows over time and loss of power in the muscles attached to the lens have also been postulated as causes.
A cataract is defined as any opacification of the eye's crystalline lens, and any of these changes that then lead to a degradation in the optical quality of the lens can cause visual symptoms. As there are a wide variety of cataract types, there is a large spectrum of visual symptoms associated with cataractous changes. Age related is by far the most common type of cataract and it is divided into 3 types based on the anatomy of the human lens. There are central, peripheral and posterior cataracts. Patients commonly develop opacity in more than one area of their lens which can cause overlap in the classification of cataracts.
Most common is the yellowing and hardening of the central portion of the lens and it occurs slowly over years. As the core of the lens hardens, it often causes the lens to increase the refractive power and causes nearsightedness.