CHOOSING THE BEST Pi
Mirror, mirror on the wall, which lens is the best of all
The most important part of the Pi in Eye (Prelex) surgery is the artificial biocompatible implant, which allows one to see far and near in each eye. These are known as Presbyopic implants or Pi. They are made by artificial means and not taken from cadavers. These lenses do not propel any allergic response when situated in the eye. Even in certain cases the lenses can be interchanged with similar lenses. Currently, the FDA has approved several different types of Pi lenses in the US.
A mono-focal lens is an accommodating lens. With this lens all light rays are brought to focus at one point. Front and back movements as well as the rounded shape of the implant inside the eye intensify the depth of field. Mono-focal lenses may not be able to permit clear vision on all three regions. Setting the focus of the two eyes a little differently solves this problem. Doing this creates the term mini mono-vision. For this lens the leading eye is fixed for distance, middle, and some near while then on-leading eye is set for near, middle, and some distance. The brain then fuses the pictures to produce all three regions of vision with depth perception. The multifocal Pi lens splits inbound light. While for the older refractive lenses there were interchanging regions for far and close up vision. The mature lenses have therefore decreased into disfavor as patients were not pleased with the class of vision. The current top-notch choice is the diffractive “Pi” lens. For this lens there are apodaizations or rings infused on to the lens during the engineering development. These rings separate the incoming light into two pictures allowing the brain to pic the desired image. For instance, if you stood looking at a distant tree in front of a window and held your finger in front of your eyes you would then notice that the sign got blurry but if you focused at your finger then your finger would seem blurry. Our brains concentrate on what or attention is focused on. Individuality, employment, interests, and athletics all influence the decision for the final lens to be chosen for the patient. Occasionally different lens implants can be used chosen for the two eyes. A doctor should always consider the patients needs before selecting the best lens. The choice process for a lens demands thorough evaluation and thoughtfulness of all obtainable choices.
FIGURE 7.1 Presbyopic Implants (FDA Approved)
Crystalens is an accommodating back chamber intraocular lens made of silicone.
It is also a modified foldable plate haptic lens with hinges across the plates and includes two flexible colored polyamide loops attached extremity of the plates.
The opic zone is small at 4.5mm. This lens should not be placed in the ciliary sulcus. Also it should not be implanted if there is a zonular rupture or if the bag is not intact.
Additionally, YAG capsulotomies should be delayed for a minimum of twelve weeks after the lens implant surgery. Moreover the capsulotomy should be smaller than the optic size or the lens can get dislocated. The cryalens does not drastically absorb UV light from the sun so protective UV sunglasses should be worn after surgery. This lens moves mechanically to yield 1.0 diopter of monocular space. Lens safety for this lens is good as hinge movement of one billion cycles at ten cycles per second has been documented. Roughly 95% of visible light is transmitted. This lens has undergone several refinements and currently the 5th generation Crystalens and Crystalens AO changes are available. Crystalens HD was lately stopped as AO has proven to be more operational.
Figure 7.2 A Crystalens
Crystalens transmits 100% of light rays regardless of the eye pupil size. Its aberration-free aspheric lens permits great contrast sensitivity. In addition, individuals who have an astigmatism can accept this lens better. It also permits some torque without upsetting vision. Patients display better vision seven years post surgery than at the time of surgery.
Restor is an acrylic IOL that partakes the fundamental construction of a central optic and two arms or haptics for stabilization. This lens can be a one or three-piece foldable lens that is implanted into a bag or ciliary sulcus. It consists of nine steps of decreasing heights giving energy for certain lighting conditions and activities. This lens does not require any ciliary muscle or movement to function. The +3.0 diopter on the lens yields a power on the cornea equal to wearing +2.5 readers, which allows for near vision. A refractive region that leads light to distal focal point encompasses the diffractive region. The asphencity of the optic counteracts for the corneal circular irregularities. The chromophore filters blue light without impacting color vision or the quality of vision. Certain optical effects may be expected including halos around light during low light or in nighttime settings.
Tecnis lens is designed of a UV blocking hydrophobic acrylic. It is like a magnifying glass in that it is biconvex shaped with diffractive rings on the back surface. The addition of the lens is that it is a plus 4 diopter. It is mad in a patented diamond cryolathing process which means the material and the shape are formed in the one step. The Tecnis lens is a clear transmiting more light although the increased number of rings can sometimes cause glare.
The Restor lens offers vision at all distances retaining the natural three-dimensional visions. The most popular lens is this lens and fits the necessities of most patients. Seldom though some patients are bothered by glare, haloes, or lack of crispness of vision.
Also individuals with the Restor lens need more light in dark settings. The Tecnis lens on the other hand does not require extra light in dark settings. Therefore it is popular in individuals employed in low light conditions. The downside of the Technis lens is that it may cause glare and haloes while driving and vision for computers is not flawless. The Crystalens has the least glare and is the best choice for computer use. Yes, it can be confusing but not one lens is best. If one lens were best then only one lens would exist. It is the job of the doctor to fit each individual with the best lens for his or her lifestyles. Here are some overall Advantages and Disadvantages of each lens:
Advantages: Clear vision for 2.5/3 zone
Disadvantages: May need glasses occasionally
Advantages: Full range of vision (3/3)
Disadvantages: Extra light for reading
Advantages: Nearly full range
Disadvantages: Glare, computers & intermediate vision may be a problem
Let’s look at some examples to clarify how the ideal lens should be chosen:
53 yr. old baseball player and businessperson:
Baseball involves depth perception and force to hit the ball.
Sometimes people play under artificial lights. If the person is
adaptable, then Restor would be appropriate. Crystalens would
be better if he/she wanted sharpest vision and was willing to
wear readers at the office.
63 yr. old sewer and avid reader:
Sewers require depth perception to thread the needle and they
need to be able to work and read in lower light conditions. This
may be a person best suited to Tecnis.
57 yr. old skier and CEO:
Here again depth perception is important. Crystalens is usually
preferred as reflection of light from snow and ice can cause glare.
49 yr. old actor and backpacker:
An actor needs to concentrate on emoting not straining to
read cue cards. As there is the heat and light from the overhead
light fixtures, Mini-monovision with Crystalens will work both for
her acting and hiking needs.
75 yr. old retired man:
HE would rather enjoy the company of his grandchildren than
search for glasses. Tecnis or Restor would give that
59 yr. old surgeon:
Suregeons are a challenging group to most appropriately
fit lens type. A surgeon using microscopes may want to avoid
any glare and prefer Crystalens. They can adjust the eyepiece
on the microscope to allow for the mini monovision. General
practitioners, internal medicine doctors may want Restor.
Figure 7.6 Lifestyle questionnaire. This helps in matching the Pi based on work, sports and leisure activities
Choosing a Pi for your needs
There is seldom the condition when a patient has already has one lens implant in an eye and with a surgeon who does not do Pi. The question asked is, should the second eye have the modern advanced implant? And, what, if at all, should be done with the first eye? It is essential to have an broad conversation about monofocal lenses, accommodative lenses and the various types of multifocal lenses. The simplest tactic is to advance with a Pi in the second eye and leave the earlier operated eye alone. This would still free them from the continuous dependence of glasses. It does, conversely, take time to alter to the different visions in the two eyes.
Almost all individuals in this scenario are content with their choice. If the patient is happy with combined monofocal and Pi lens and able to adjust, then nothing more need be done. If the patient wants a similar lens in the other eye, it needs to be looked at by a surgeon on a case-by-case basis. If the previous eye surgery has recently been performed and the eye has yet to undergo a YAG procedure then an exchange of implants may be possible. In this case the old monofocal implant is cut with micro scissors and pulled out though a small opening. A Pi lens will then be inserted through the opening. Although, if the procedure had been performed many years ago and the patient is content with vision in that eye, then it is safer to leaven the first eye alone.