Understanding Ocular Biomechanics: Unveiling the Mechanics of Vision

Introduction

Human eyes are complex structures with biomechanics involved in proper functioning. The eyes need a good balance of forces exerted by the ocular muscles. These forces change the shape of the lens allowing proper focusing of light to the retina. An imbalance in the biomechanical properties alters the vision. The study of biomechanics is important for preventing vision loss.

What Is Biomechanics and Its Importance?

Biomechanics is the study of how biological structures and systems work and react to various forces and external stimuli. In human beings, it is studying how the skeletal and muscular systems work together under different conditions. Biomechanics helps in the prediction of alterations, remodeling, and failures of tissues and organs. A proper understanding of the biomechanics of human systems helps in the management of various clinical conditions. It helps in the diagnosis, treatment planning, and prognosis of biomechanically related pathologies.

What Is Ocular Biomechanics?

The human eye is a sensory organ that reacts to light. The shape of the eye, the presence of fluids, and the maintenance of ocular pressure is controlled by biomechanical factors. The internal and external musculature, complex internal vasculature, and presence of fluids make the eye an organ that is controlled biomechanically. The study and knowledge about ocular biomechanics are utilized for clinical practice and patient benefit. The biomechanical properties of each part of the eye are discussed separately and used for the diagnosis and treatment of various ocular diseases.

What Are Corneal Biomechanics and Associated Diseases?

• The cornea is the outermost transparent tissue of the eyes, which is tough and protective in function.

• The corneal shape and the retinal image quality formed are the basis for the keratorefractive surgery.

• The cornea is made of college lamellae, resulting in high mechanical strength. The biological properties of the cornea aid in the maintenance and repair of the corneal layer.

• Cornea has optical properties and mechanical integrity to preserve the anterior corneal curvature, during a wide range of loads and hydration levels.

• Cornea has elastic and viscoelastic properties, allowing the cornea to deform reversibly under stress. The mechanical properties of corneal tissue are measured using various ex vivo and in vivo methods.

What Are the Biomechanical Pathologies Associated With Eyes?

Diseases affecting the corneal fibers change their viscoelastic properties resulting in distorted vision and surface irregularities.

Corneal Ectasia - It is a group of disorders affecting the shape of the cornea. It can occur as a result of inflammatory and non-inflammatory conditions.

• Inflammatory Conditions:

1. Keratectasia - Conical cornea, a progressive disorder with paracentral thinning.

2. Anterior Staphyloma - It is a congenital or acquired defect, characterized by progressive thinning and weakening of the sclera uveal coat.

3. Post -LASIK Surgery - Following a LASIK surgery causes mechanical deterioration of the collagen structure and the cornea starts to bulge forwards after a variable time.

• Non-inflammatory Conditions:

1. Keratoconus - It is a condition of the eye where the cornea of the eyes bulges outwards.

2. Keratoglobus - It is a non-inflammatory condition causing corneal thinning and protrusion of the cornea.

3. Pellucid Marginal Degeneration - It is a peripheral corneal thinning disorder that occurs bilaterally.

What Are Iris Biomechanics and Their Importance?

• Iris thickness is found to be different in populations and the increased thickness of the iris is related to angle closure glaucoma.

• Changes in iris dynamics increase intraocular pressure and stretch properties of the iris are also seen lower in angle closure patients due to dilation of the iris.

• These changes indicate that the biomechanical properties in response to physiological or pathological stimuli contribute to episodes of angle closure glaucoma which can be acute or chronic.

• The posterior layer and stiffer anterior layer of the stromal layer of the iris indicate complex biomechanical behavior. The sponge-like property of the iris regulates the variation in different pathological conditions.

• Imaging tools and systems provide a three-dimensional view of the iris and measure the biomechanical properties of the iris. These diagnostic tools lead to an increased understanding of the pathogenesis of angle closure.

• The studies on the biomechanical properties of the iris lead to the development of various iris replacement devices.

What Are Lens Biomechanics and Their Importance?

• The function of the crystalline lens is to focus the light from an object and to form images of the object. The accommodation of the lens is a biomechanical process that involves the action of ciliary muscles to form an image.

• For distance vision, the ciliary muscles are relaxed and increase tension to increase the focal length. Similarly, for near vision, the ciliary muscles contracts and release tension, restoring the round shape for a shorter focal length.

• The power of accommodation increases with age and the associated difficulties in focusing on objects. The functional properties of ciliary muscles remain intact, even though the power of accommodation has been compromised. The biomechanical properties of the crystalline lens are studied and benefited in the clinical management of presbyopia.

• The monovision correction or use of multifocal spectacles only improves the single distance vision and the dioptric power of the eyes is not changed.

• Understanding the biomechanical properties of the lens is beneficial for developing improved treatment strategies.

• The new treatment options developed are:

1. Drug-induced disruption of chemical bonds leads to the stiffening of the lens

2. Laser-induced stiffening of the lens.

3. Replacement of the lens tissue with biocompatible polymer material.

What Are Scleral Biomechanics and Their Importance in Glaucoma?

Glaucoma is the second largest cause of blindness. The damage of the optic nerve and death of the retinal ganglion cells with an increase in intraocular pressure, causes glaucoma. The physical deformation of the sclera with changes in its biomechanical properties is an important factor causing injury to retinal ganglion cells. Axial myopic eyes are more susceptible to open-angle glaucoma. The other mechanical changes include a large globe diameter and thin sclera.

Conclusion

The biomechanical principles of the human eye help in maintaining the eye's shape, regulating the intraocular pressure, protecting the eye from injury, and undergoing changes to give good vision. Modern treatment for eye diseases focuses on understanding the biomechanical part of eye diseases and treatment is given utilizing the biomechanical principles and brings back the vision. Ongoing studies in the biomechanics of the eye help provide a permanent cure for eye and vision problems.