1. Introduction
1. Introduction
Cataracts have blinded more people throughout the ages than any other
affliction. Wertenbaker (1982) explains a change in protein molecules within the
lens causes it to become cloudy, beginning as spots and eventually pervading
the whole lens turning it a milky yellow white. This scatters oncoming light and
blocks vision.
Triggers to cataracts included excessive glucose (diabetes),
radiation, injury, abuse of drugs or alcohol and more commonly, old age, which is
called a senile cataract.
Discussion with ophthalmic surgeons has established a critical device in
ophthalmology, the Intraocular Lens, more commonly called an IOL.
This
analysis will concentrate on these devices. To aid the initial understanding of this
project an interview with an ophthalmic surgeon was carried out at a local
hospital. The information obtained can be seen in Appendix A.
An Intraocular Lens is surgically implanted to replace the eye’s natural lens,
which is called a cataract after it becomes cloudy as shown in Figure 1.1.
Lenses are implanted to improve vision and quality of life. This procedure is very
common, especially amongst the elderly. The World Health Report published in
1998 estimated that there were 19.34 million people who are bilaterally blind from
age-related cataract. Senile cataracts represent 43% of all blindness (Foster,
2000).
Harold Ridley implanted the first IOL on November 29, 1949 in St.
Thomas Hospital in London (Physics daily, 2005). As one of the most successful
long-term implants of the 20th century, cataract surgery owes its accomplishment
to the continuous improvement of surgical techniques, instrumentation and lens
design.
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1. Introduction
Figure 1.1: Difference between a healthy and cataract lens (Aravind, 2005).
There are two basic types of IOLs: foldable and hard. Foldable lenses are made
of a soft material, usually silicone or acrylic. These soft lenses are implanted
using the phacoemulsification surgical technique.
A metal probe is inserted
within the lens capsule, known as the capsular bag, through a small incision and
vibrates, breaking the lens into small pieces which are gently sucked out of the
eye as seen in Figure 1.2. The empty lens capsule is then polished to remove
any remaining cells.
Figure 1.2: Phacoemulsification Surgical Technique, Instrument breaking up and
removing pieces of clouded lens (Cataract surgery, 2005a).
The artificial lens implant can then be rolled up and inserted through the small
incision, around 3mm in length, using a tweezers or a specially designed
injection system. Once inside the eye, the IOL gently unfolds as seen in Figure
1.3.
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1. Introduction
Figure 1.3: The Implanting and unfolding of a soft foldable IOL in the capsular bag
(Cataract surgery, 2005b).
Hard plastic lenses, usually polymethlymethacrylate (PMMA) material, are
appropriate in certain circumstances usually determined by the surgeon. Since
they cannot be folded the hard lens is placed through a slightly larger incision,
this
surgery
is
called
Extracapsular
Cataract
Extraction
(ECE).
Phacoemulsification surgery has many advantages over ECE including shorter
recovery period, faster surgery and no sutures. This surgery is also used to
implant hard lenses in the anterior chamber of the eye, just behind the iris and
pupil but not in the capsular bag. Lenses with larger, stronger haptics are
designed for the anterior chamber and are required in cases where there is
capsular rupture or a tear due to complicated surgery. Haptics are the arms of
the Lens to hold it in place within the capsular bag. The haptics and optic of a
lens are labeled in Figure 1.4. Multifocal lenses are also available, designed to
replicate the focusing power of the natural lens, but they have not been hugely
successful.
Figure 1.5: A one piece foldable IOL showing the lens body and haptics (cataract
surgery, 2005b).
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1. Introduction
Intraocular lenses are class III devices, meaning they are high risk and require
Premarket Approval Applications (PMAs) to provide reasonable assurances of
safety and efficiency. Generally they require extensive testing and human trials
before going to final deliberation by the FDA (U.S. Food and Drug
Administration). All medical devices require FDA approval before being
marketed.
Some devices are explanted, or removed, after a period of time as a result of
implant failure.
This is when the prosthesis becomes ineffective, usually
associated with pain or decreased vision. Complications are caused primarily by
mechanical trauma, inflammation or infectious complications (Carlson et al.,
1998).
The discovery of the cause of implant failure is important as it helps us
understand how to make replacement surgery more successful. Detailed studies
and examination of explanted devices is crucial to better understanding their
mechanisms of failure and work towards the development of more durable
devices.
The undertaking of this study is in tandem with the University of Bari and the
University of Siena, both in Italy. The University of Siena collect explanted and
control samples and assign them with reference numbers. It also collects some
manufacturing data and patients’ clinical data. Samples are then placed in a
sterile medium, Balanced Salt Solution (BSS) in this case, and sent here to the
University of Limerick where the physical analysis of the IOLs take place. Some
of these analyses include metrology, optical microscopy and Atomic Force
Microscopy. After these non-destructive examination techniques the samples
are then sent on to The University of Bari for surface chemical analysis. The
University of Bari finally return the samples to the University of Limerick for
destructive mechanical and thermal analysis including microhardness testing,
Scanning Electron Microscopy, and Differential Scanning Calorimetry.
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1. Introduction
Throughout this study 5 explanted samples were received. All the information
received concerning these explanted lenses can be seen in Appendix B.
The objectives of this project are:

To provide an understanding of test methods and apparatus used that will
extend the amount of information coming from explanted IOLs regarding
their mode of failure.

To establish if test methods used picked up differences between the
sample devices and if the results correlated with clinical data such as
literature.

To elucidate any parameters that contributed to the failure of the sample
devices.

To contribute towards the development of a working protocol for retrieved
ophthalmic devices.
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