Robert J. Weinstock, MD
Perhaps in the not-so-distant future we will be performing clear lens extractions on plano presbyopic patients and replacing the natural lens with a clear, soft, jelly-like material that can accommodate fully. However, until that time, the only implants available in the United States that can somewhat mimic the natural action of a physiologic prepresbyopic eye are the Crystalens and its sister toric version called the Trulign (Bausch & Lomb) accommodating implants. Many surgeons around the globe have turned to these lenses as their premium implant of choice because of their ability to provide high-quality vision at distance while also allowing the patient to maintain some spectacle independence in the middle and near zones as well. The Crystalens and Trulign are really just an early step in the evolution of a lens implant’s ability to perform a more physiologic function and allow the eye to have a full natural range of vision without the need for glasses. The distinct advantage of the Crystalens and Trulign is that they do not use any multifocality to achieve middle and near vision and therefore do not suffer from the pitfalls of having refractive or diffractive optical rings that cause loss of contrast sensitivity that is well-documented in the use of multifocal implants. However, surgeons must be aware that the Crystalens accommodating implant also has its own drawbacks, such as the challenge of achieving precise refractive outcomes and the inability of the lens to fully accommodate so that all patients are completely spectacle independent and have a full range of accommodation. As the Crystalens design improves and other accommodating implants are brought to market in the future, surgeons will have the luxury of a variety of implants to choose from when trying to achieve spectacle independence for their patients. The Crystalens’ modified plate haptics with flexible hinge design is the novel feature allowing reduced spectacle dependence compared with conventional monofocal implants (Figure 17-1).
Currently, there are 3 versions of the Crystalens that are available for the surgeon: the Crystalens AT50SE, Crystalens HD, and Crystalens AO. These 3 lenses were brought to market in that particular order, and currently most surgeons are using the Crystalens AO due to the aspheric optic that has been incorporated into the lens design. Some surgeons still prefer to use the Crystalens AT50SE, which does not have an aspheric optic design, and other surgeons also use the Crystalens HD, which has a small area of thickening in the central 1.5 mm of the optic, causing induced negative spherical aberration and increased depth of field. Crystalens HD tends to perform better at near than the other models; however, its refractive targeting outcome must be near plano to ensure proper functioning and high visual satisfaction. The Crystalens AO is a little bit more forgiving in terms of the exact refractive outcome that is necessary to have high-quality vision and a happy patient. Its aspheric design with no induced spherical aberration allows for a slightly positive amount of spherical aberration. All models have a square-edge design to inhibit posterior capsule opacification and are made of the proprietary copolymer, Biosil. The Crytalens comes in 0.5 diopter (D) power steps from 8 to 30 and 0.25 D steps from 18 to 22. The Trulign is built on the same exact platform as the Crystalens AO, except it incorporates a toricity power in addition to the spherical power (Figure 17-2). It comes in 3 different toric powers designed to cover a range of preoperative corneal astigmatism. It comes in 1.25 D, 2.00 D, and 2.75 D correlating to 0.83 D, 1.33 D, and 1.83 D correction at the corneal plane. There are small marks in the peripheral optic adjacent to the hinge to guide axis placement.
A critical aspect to successful Crystalens and Trulign usage is proper patient selection. When a surgeon is performing his or her initial cases, it is important to find patients with very reasonable expectations who can accept that there still may be a need for light reading glasses after the procedure. A patient who has a normal-shaped eye with average axial length and K-value readings will aid the surgeon in picking the correct implant power. The absence of any other ocular pathology or history of previous ocular surgery will also stack the cards in favor of the surgeon for achieving success with his or her initial cases. Patients who have had previous vitrectomies, glaucoma surgeries or lasers, pseudoexfoliation, history of ocular trauma, advanced significant epiretinal membrane pathology, or other vascular compromises to the retina or optic nerve should not be considered candidates when first starting to use the implant. It is possible to correct mild to moderate amounts of astigmatism with laser or manual limbal relaxing incisions (LRI), but ideally for the first few Crystalens patients, it is better to choose an eye that has less than 1 D of astigmatism present on the cornea. For over 1 D of astigmatism, a Trulign can be used. In addition, if a patient has had prior corneal refractive surgery such as photorefractive keratectomy, LASIK, radial keratotomy, or conductive keratoplasty, it will be very challenging to acquire accurate keratometric values and proper intraocular lens (IOL) power selection. A more advanced biometric approach is needed for the patients, and the use of intraoperative aberrometry is useful.
Another important consideration in patient selection is the general disposition and personality of the patient. Since there is an out-of-pocket expense, many patients have different levels of expectations with regard to how they think vision should be postoperatively. A consistent message coming from both the surgeon and the staff about realistic expectations is paramount for a good result and a happy patient postoperatively. The patient needs to understand that although this technology is advanced and has the ability for the lens to move/accommodate inside the eye, there is no guarantee that the patient is not going to need reading glasses or rely on corrective lenses after surgery. Additionally, the patient needs to understand that certain situations can arise in the operating room that would preclude the surgeon from using the Crystalens or Trulign, and a standard implant may be used in its place. It is best to inform patients that although the implant does an excellent job of providing a continuous range of vision from distance to near, in certain situations where the print is extremely small or there is not enough ambient light, there might likely be a need to use a low-power pair of over-the-counter reading glasses. Since, as with any procedure, there are unexpected outcomes and refractive surprises that can occur as the eye heals, patients must be informed that additional vision correction procedures may be needed to fine-tune or augment the visual outcome, including LRIs, laser corrective surgery, and possibly even an implant exchange in rare cases. Patients who accept these considerations and understand that they will occasionally need a light pair of reading glasses for fine print and are mostly seeking independence from glasses in the distance and middle range are the ideal candidates for the Crystalens and Trulign. It is wise for a surgeon to listen closely to the staff and surgical counselors, because often times these members of the team spend more time with the patient and may have more insight into the patient’s expectations than the surgeon does. If the staff feels that the patient may not be a good candidate from a personality perspective and that the patient’s expectations are too high, it would be prudent for the surgeon to discuss things further with the patient or even decide not to use a premium implant in that particular case. Many surgeons like to use a preoperative questionnaire in evaluating a patient for premium implants to help guide the decision process. The Dell questionnaire is a nice tool for evaluating personality type and visual demands (see Figure 3-3).
When setting up expectations for a Crystalens patient, it is important for the surgeon to realize that, unlike multifocal IOLs, the Crystalens and Trulign very rarely cause any nighttime visual disturbances and it is very unusual to have a patient complain of nighttime glare, halos, or reduced contrast sensitivity. However, the near vision, especially for very small print, is not as clear with the Crystalens/Trulign as it would be with a multifocal IOL with a strong near power. This distinction is very important for a potential premium implant patient to fully grasp in regard to the expectations and the pros and cons of different IOL technology. If a surgeon does desire to make a person completely spectacle independent with the Crystalens or Trulign, there is an opportunity to use them in a modified or full monovision arrangement. This is especially helpful if a patient has a history of successful monovision contact lens wear in the past.
As with any cataract surgery, accurate preoperative measurements are required to achieve an excellent refractive outcome. This is especially true with Crystalens/Trulign patients because the surgeon is required to deliver a precise refractive outcome in order for the patient to have reduced spectacle dependence and a high-quality visual result. In addition to a standard A-scan that is performed prior to cataract surgery, the surgeon would be wise to obtain additional refractive measurements to help guide the procedure. Many surgeons perform multiple axial length measurements as well as multiple corneal curvature measurements using different diagnostic technologies. It may be worthwhile to employ 2 forms of A-scan technology to help choose the proper IOL for the patient. A very common protocol is to perform both an IOLMaster (Carl Zeiss Meditec) or Lenstar (Haag-Streit) as well as an immersion A-scan on each patient. It is recommended that a surgeon also perform a very thorough evaluation of the corneal curvature and regularity of the corneal architecture. This can be performed with any topography machine, and some surgeons are even using wavefront imaging to obtain a reading of optical aberrations that are present in the optical system. It is important not to underestimate the value of manual keratometry in evaluating patients preoperatively for Crystalens/Trulign surgery, and once all of the preoperative measurements are taken, it is valuable to spend time evaluating and comparing all of the values to determine which are the most accurate readings. It is very helpful when different technologies result in the same or similar implant power readings, giving the surgeon a high degree of confidence in the lens power selection. If the readings from different machines do not corroborate each other, it is prudent to repeat the measurements and try to figure out what the most predictable readings are, especially with keratometric values in patients who have previously undergone refractive corneal surgery. These preoperative measurements not only help guide the surgeon in IOL power selection but also identify any unusual corneal pathology that may preclude a desired refractive outcome. The surgeon must also determine how much corneal cylinder exists and what mode of technology, if any, should be employed to correct the corneal astigmatism at the time of the cataract surgery or afterward. Occasionally, a surgeon will find a significantly higher amount of corneal astigmatism that was not present on manifest refraction because the lenticular astigmatism was counterbalancing it. The Trulign should be used in these situations, along with corneal arcuate incision if needed. Another important measurement that should be done preoperatively is an endothelial cell count to ensure that the patient has healthy corneal endothelium to prevent a situation in which there is prolonged corneal edema postoperatively and an undesirable refractive outcome. If the patient has a borderline endothelial cell count, it would be best to use a dispersive viscoelastic to coat and protect the cornea during the surgical procedure. If the endothelial cell count and clinical examination reveal that there is a high likelihood of prolonged corneal edema or the patient may have a diagnosis of Fuchs endothelial dystrophy, using the Crystalens would be contraindicated.
INTRAOCULAR LENS SELECTION AND TARGETING
Some surgeons prefer to do surgery on the dominant eye first with a plano target, while other surgeons prefer to do the nondominant eye first with a slightly myopic target. Regardless of this decision, it is important to check the patient postoperatively after the first eye has been completed to ensure that the eye is healing well and that the refractive target is where the surgeon intended. If it is not, then a slight adjustment can be made on the second eye to ensure a good refractive result. When selecting the IOL power in patients who have had previous corneal refractive surgeries, it is especially important to do an accurate assessment of the keratometric values and also to employ software, such as that available on the American Society of Cataract and Refractive Surgery website, to help determine the correct IOL power selection. An acceptable targeting strategy for an average patient would be to target somewhere between plano and -0.25 for the dominant eye and somewhere between -0.25 and -0.50 to the nondominant eye. This small amount of anisometropia is extremely well tolerated by the patient, and the slight difference allows the patient to have a good range of vision and often results in minimal need for reading glasses after the procedure is completed. To ensure good outcomes, some surgeons are now using intraoperative aberrometry. These machines can do aphakic autorefractions and recommend a lens power while the patient is on the operating table. They can also provide a refraction once the implant is placed in the eye to help guide the correct axis placement for the Trulign and determine whether the implant is sitting in the proper position (Figure 17-3). For average eyes, the SRK-T formula works well. For eyes with axial lengths under 22 mm, Ks flatter than 42, steeper than 47, and postrefractive surgery cornea, the Holladay 2 formula is better suited.
As with any cataract surgery, an efficient atraumatic surgical procedure is paramount for ensuring a good refractive outcome. Some important features that are specific to a good refractive outcome with a Crystalens/Trulign include creating a well-constructed corneoscleral wound and a 5- to 6-mm centralized capsulorrhexis. If too small of a capsulorrhexis is made, it can lead to capsular contraction and fibrosis of the anterior capsule. The fibrosis can then cause a shift in the lens position within the eye. Conversely, if too large of a capsulorrhexis is created, it can lead to an unstable platform for the Crystalens implant and prevent optimal performance of the lens (Figure 17-4).
When removing the nucleus and cortical material, it is extremely important for the surgeon to be as gentle as possible with the capsular bag. If there is excessive manipulation of the capsule, it can cause zonular dialysis or tearing of the capsule. If this occurs, the implant will not sit properly inside the eye and will not perform properly postoperatively. If there is unexpected damage to the capsule during surgery, it may be best to abandon the Crystalens/Trulign and use a standard monofocal implant. Another important strategy in the cataract surgery is to ensure a complete and thorough cortical clean-up. Many surgeons gently polish the capsule to ensure all endothelial cells are removed from the capsular bag; this can be a double-edged sword, because overzealous polishing of the capsule can lead to zonular tearing and damage. If cortical material or endothelial cells are left behind in any significant amount, it can affect the patient’s vision postoperatively and also cause excessive scarring, fibrosis, and irregular contraction of the capsule, leading to a displacement in the IOL position. Some surgeons even like to polish the under-surface of the anterior capsular rim.