Geometric and Optical Characteristics of Soft Contact Lenses ON and OFF the Human Eye

Table of Contents
Introduction........................................................................................................... 2
Literature review................................................................................................... 3
Hydrogel............................................................................................................. 4
Silicon hydro-gel................................................................................................. 4
The difference in design..................................................................................... 5
Discomfort from wearing contact lenses............................................................6
Summary of the work.......................................................................................... 13

Introduction
Soft contact lenses are medical devices largely used to correct the
eye refractive error for more than 125 million people worldwide. Unlike
spectacle lenses which are used in conventional glasses, soft contact
lenses should be designed to fit the user’s eye besides being able to
correct his vision. During the fitting process, the surface of the contact
lens deflects to fit the eye shape; therefore, the optical power of the soft
lens on-eye differs from the one originally manufactured and tested off-
eye. The aim of this report is to investigate the geometric and optical
characteristics of soft contact lenses ON and OFF the human eye.
Soft contact lenses have had a huge effect on the world contact lens
market ever since they turned out to be broadly accessible in the primary
1970s. Since their institution, the sum of soft contact lenses being
recommended globally has consistently risen and it is majorly the sale of
soft contact lenses that is accountable for a business that is approximated
to be worth about $8 billion yearly. A study by (Welford ,1986) has shown
that soft lenses presently make up approximately 86% of all contact lens
refits globally. The fullness of the contact lens market with soft lenses has
happened mostly for two reasons. First, soft lenses offers the wearer what
they perceive as the most essential needs for effective contact lens wear;
good comfort and good vision. The main hindrance with inflexible lenses is
commonly accepted as being their deprivation of comfort, and specifically,
their primary uneasiness. Second, improvements in manufacturing know-

how have led the business to soft lenses and, yet further, to the idea of
disposability (Weissman and Gardner, 1984).
Problems with eyesight are the main reason why almost 50% of the
population in industrialised countries wears optical lenses. Myopia is one
of the major eye disorders leading to blindness. Understanding its causes
and treatment can be a way of reducing its progressiveness. The disease
is said to be an interplay of genetics and environmental factors. It is
higher and prevalent to the individuals whose parents are myopic which
indicate that it is genetically associated. Also, the study indicates that it
also associated with education and the amount of time spent doing work
which tends to increase optical blur. The condition has resorted to
pathological problem increase, therefore, the effort to reduce myopia
problem has been devised.
Literature review
According to Austin (1999), indicate that the problem of short-
sightedness and problems associated with myopia can be corrected by
use of contact lenses, however, the writer considers the disadvantages
associated with contact lens wearing and effects of the thickness of the
contact lens have on the eye. Cooperand Tkatchenko (2018) suggest that
the use of spectacles and contact lenses is one of effective measure to
reduce short-sightedness are pathological problems associated with
myopia. Contact lenses are made to correct the visual aspect of the
patient also to fit the eye of the patient. There are major comparisons to
contact lenses on the eye and off the eye which affect the design of the

contact lens. These are concerns to the design and validation to their
perspective. There are different types of contact lenses, here we shall look
and examine two types of soft contact lenses. These are silicone hydro-gel
and Hydro-gel.
Young (2018) proposes that contact lens materials are nice
examples of bio-materials. A bio-material might be designated as an
artificial or natural material that is appropriate for institution into living
matter, particularly as part of a medical implement. The term covers a
wide range of expertise, comprising tissue manufacturing, synthetic
organs, bio-ceramics, clinical implements, and implantable medicine
delivery structures. Contact lenses are categorised as a clinical implement
in a majority of nations. Very few individuals are possibly to get through
life minus having a particular type of bio-material instituted in their bodies
(Yap, Archer, Gobbe, and Reinstein, 2016). The most widespread
examples used today consist of contact lenses, dental fillings, heart
valves, intraocular lenses, and stents. This list in its own stresses just how
varied bio-materials should be in order to fulfil their very particular end
use; for instance; a contact lens material has very unique characteristics
to a material used for dental fillings.
Hydrogel
The hydrogel is made from gel-like water containing plastic called
hydro-gel. The lenses are very thin, elastic and take the shape of the front
part of the eye. The lenses were introduced in the 1970s which made
contact wear of the soft lenses more popular since they were comfortable.

The soft lenses were alternative of lenses made from Plastic material
polymethyl methacrylate (PMMA) which took weeks to adapt and many
people could not successfully wear them due to their discomfort.
Silicon hydro-gel
Silicon hydro-gel is the more populous, porous and advanced type of
the hydro-gel lenses. They were introduced in 2002. They allow more
oxygen to reach the cornea. For bio-material to be effective in its usage, it
follows that it should similarly be bio-compatible. Bio-compatibility is the
ability of a material to perform with a suitable host reaction in a particular
usage. A “suitable host reaction” would comprise not having poisonous or
hurtful impacts on biological structures (Patel, 1998). Bio-materials
produced for use as contact lenses should not just fulfil all these
necessities for safe application in the eye, but in addition they should
likewise have very particular attributes like being transparent and stay
like that on-eye, be at ease and be comparatively inexpensive to produce.
The difference in design
To examine major comparison characteristics of the contact lens on
the eye and off the eye, we are going to look at research done to find the
differences in design. A research was conducted to find the optical
performance of the two mentioned lenses to examine their on and off the
eye performance. A monochromatic aberration of contact lenses was
measured on the eye using Hartmann ocular aberrometer lambda= 850
and another one was measured off eye using single pass contact lenses
lambda 540. The soft contact lens was put in a wet cell to compare the

wavelength aberration of the two and knowledge on refraction (Sterner,
Aeschlimann, Zurcher, Osborn Loren, & Kakkassery, 2016).
The measure of both sphere aberration was similar to those of on-
eye and off the eye. However, when the high powered lens was used there
were noticeable differences in power sphere. The on-eye exhibited lower
power than the measured power from off the eye. Longitudinal spherical
lens varies in aberration when powered in hydro-gel as expected from
geometric optic theory. Aberration from measurement on the longitudinal
sphere on off and on-eye deviated significantly from the expected of a
thin lens with spherical surfaces due to surface asphericities. Walline,
Long, and Zadnik (2004) suggests that the difference of lens aberration
can be modified using tear lens or as relative lens thickness modification
caused by lens flexure. This finding of the study indicates that the
difference between the on-eye and off-eye does not reflect on the
manufacturer error but because of lens interaction as a result of lens
flexure or the tears forming behind the lens.
Discomfort from wearing contact lenses
Whittle (2007) denotes that there are almost 140 million of contact
lens wearers around the globe. Despite the introduction of soft lenses, the
majority of these people report the discomfort of wearing contact lenses.
It is found through research that the discomfort is as a result of dryness
caused by the tear exchange beneath the contact lens which leads to
replenishment of fluid between the ocular surface and the lens. Before the
wear, the exchange is normally lower but after the wear, the tear