Richard L. Lindstrom, MD,
Douglas D. Koch, MD,
Robert H. Osher, MD,
Li Wang, MD, PhD,
Mitchell P. Weikert, MD
Contents
|
• |
Patient Selection And Evaluation |
|
|
• |
The Cataract Incision |
|
|
• |
Peripheral or Limbal Corneal Relaxing Incisions |
|
|
• |
Conclusions |
For over a century, it has been recognized that cataract incisions influence astigmatism.[1–3] Only in the past 15 years, however, have cataract surgeons mounted serious investigations aimed at measuring and minimizing astigmatism induced by cataract surgery. These efforts have paralleled but, until recently, have lagged behind the success of intraocular lenses (IOLs) in correcting the spherical refractive error precipitated by removal of the crystalline lens.
The term refractive cataract surgery has entered general ophthalmic usage. The term implies a coordinated and encompassing attention to both the spherical and astigmatic components of refraction. The current goal of refractive cataract surgery may or may not be to emmetropia; for some patients, the postsurgical target may be slight residual astigmatism, which contributes to depth of field. The critical difference between modern refractive cataract surgery and cataract surgery of a decade ago is the very existence of a target. Today's refractive cataract surgeon determines the starting point (the pre-existing astigmatic condition of the patient), knows the astigmatic effects of various approaches and selects a surgical plan that optimizes the refractive outcome for the individual patient. It is a degree of precision that was previously unattainable, requiring a depth of surgical planning that was previously unnecessary.
In addition to the cataract incision, whose size, location, and configuration help determine the astigmatic effects of surgery, the cataract surgeon now has two additional options in his armamentarium for correcting astigmatism: (1) corneal relaxing incisions (CRIs) and (2) toric IOLs. CRIs can be characterized as those made in the corneal mid-periphery, e.g. 6–8-mm zone, so-called “astigmatic keratotomy” (AK), and those made peripherally, so-called “peripheral or limbal corneal relaxing incisions” (PCRIs). CRIs, particularly AK, were first studied purely for their corneal refractive effects.[2,][4,][5] In terms of both its nomenclature and the actual clinical practices that it incorporates, refractive cataract surgery, therefore, represents a true marriage of refractive and cataract surgical specialties.
This chapter describes the astigmatic effects of various cataract incisions, the techniques of PCRIs, two different approaches to AK, and toric IOLs. The reader will find no hard-and-fast rules herein. Depending on the cataract extraction technique employed, the degree and meridian of pre-existing astigmatism, and a host of other variables, the same patient might find effective treatment in a variety of ways. We hope to convey a sense of the general principles involved and to sketch out treatment approaches to the range of astigmatic errors that most cataract surgeons commonly encounter.
Patient selection and evaluation
People with over 0.5–0.75diopters (D) of astigmatism usually require some kind of optical correction. Astigmatic errors of 1–2D may reduce uncorrected visual acuity to the 20/30 or 20/50 level, whereas an astigmatic error of 2–3D may correspond to visual acuity between 20/70 and 20/100.[6]
Up to 95% of eyes have some degree of naturally occurring astigmatic error. The incidence of clinically significant astigmatism reported in the literature varies between 7.5% and 75%.[6] From 3% to 15% of eyes may have astigmatic refractive errors greater than 2D.[7] The incidence of postcataract surgery astigmatism greater than 2D may be as high as 25% to 30%.[8,][9]
Obviously, in a patient with little or no pre-existing astigmatism, cataract surgery should be designed to be as astigmatically neutral as possible. For patients with significant degrees of pre-existing astigmatism, two types of approaches can be employed as a function of the type of cataract incision. The surgeon can (1) operate on the steep corneal meridian and select the type of cataract incision that will produce the desired amount of against-the-wound flattening or (2) make a small incision at a favored location (e.g. clear corneal temporal incision), factor in the small amount of astigmatic change induced by this decision, and supplement it with either CRIs or implantation of a toric IOL. Obviously, CRIs can also be used postoperatively to further modify the result.
Careful patient selection is crucial in avoiding postoperative surprises and unhappy patients. As a rule of thumb, some form of astigmatic surgery should be considered in patients in whom a standard cataract operation will result in 1D or more of postoperative astigmatism and whose fellow eye (1) has 1.5D or less of astigmatism, (2) has astigmatism at a different meridian than that of the operative eye, or (3) has a similar amount and meridian of astigmatism and is itself an imminent surgical candidate.[10]
The rationale, surgical methods, and risks are discussed with the patient preoperatively. As noted previously, the target may be a slight undercorrection of the pre-existing astigmatism because some patients are bothered by the shift in astigmatic meridian brought about by an overcorrection, and a small amount of residual astigmatism can provide pseudophakic patients with reasonably good uncorrected near and distance vision.
The sections that follow address the cataract incision, CRIs (PCRIs and AK) and toric IOLs separately, and in greater detail. However, cataract incision manipulation and these other approaches are dual partners in the treatment of a wide range of astigmatism in people with varied personalities and lifestyle requirements.
Alignment
Accurate astigmatic surgery is highly sensitive to precise meridional alignment. Vector analysis demonstrates that a misalignment of only 15° results in a 50% reduction in the astigmatic correction. A 30° misalignment will maintain the preoperative astigmatism magnitude, but produce a large shift in the astigmatic axis. Misalignment errors in excess of 30° actually result in a net increase in the magnitude of the astigmatism.[11]
Various approaches can be taken to minimize alignment errors. Whenever possible we make small drawings of the patient's eyes when they are seen in the office preoperatively. The patient's head is carefully positioned to insure that it is vertically oriented in the slit lamp. We then look for prominent conjunctival, corneal, or iris features that are likely to be visible when the patient is dilated as seen in the operating room (Figure 24-1). It is particularly helpful to indicate landmarks that provide a clear indication of the 90° and 180° meridians, because these can be easily identified relative to the position of the vertical or horizontal slit-lamp beam.
|
Figure 24-1 Identifying the steep meridian: find on the cornea, iris or conjunctiva one or more landmarks that are likely to be visible in surgery. |
An alternative approach is to mark the eye prior to entering the operating room. Topical anesthetic drops are administered, and the patient is asked to sit upright on the surgical stretcher. A marking pen is used to indicate either the 6:00 o'clock and 12:00 o'clock or 3:00 o'clock and 9:00 o'clock positions.
A third option is to mark the eyes as the patient is lying on the operating room table. For the majority of patients, this approach works extremely well. However, a small percentage of eyes rotate as the patient moves from an upright to a supine position. Swami et al.[12] demonstrated that 8% of eyes (20/240) had a deviation of greater than 10°.
A fourth option is to perform intraoperative keratoscopy to identify the major meridian and quantitate the amount, which should be consistent with the preoperative keratometry measurements and corneal topography. This can be achieved with a device like the Hyde-Osher ruler manufactured by Ocular Instruments, which is accurate for identifying 1.5D or more.
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The cataract incision
An incision of the cornea or sclera creates tissue gape. This gape causes corneal flattening along the meridian of the incision and steepening in the meridian 90° away (Figure 24-2), with the magnitude of this determined by several factors (see following two paragraphs). To compensate, wounds can be closed with sutures. Sutures produce local tissue compression, resulting in peripheral flattening and central steepening along the meridian of the incision and flattening 90° away (Figure 24-3).
|
Figure 24-2 Following a limbal incision, tissue gape produces flattening along the meridian of the incision and steepening 90° away. |
|
Figure 24-3 Sutures create peripheral flattening and central steepening along the meridian of the incision and steepening 90° away. |
The suture-induced net steepening persists for several months postoperatively. Over several years, however, progressive flattening occurs. The net result is an against-the-wound astigmatism.
Factors that affect the astigmatic change produced by a cataract incision include its length, meridional location, radial location (e.g. corneal, limbal or scleral), construction, and wound damage, such as thermal injury. With larger incisions, intrinsic patient factors can be important, as variations in wound healing can lead to markedly different astigmatic effects. Sutures have a temporary affect, but rarely produce changes that persist beyond 2 years, with the possible exception being those instances in which tissue is actually damaged or displaced by the sutures and heals in this new configuration.
Using scleral flap recessions of varying widths in eyebank eyes, Samuelson, Koch and Kuglen[13] have shown the direct relationship that exists between incision length and against-the-wound corneal flattening (Table 24-1). Notably, clinically significant flattening (0.5D or more) occurred only in incisions longer than 3mm.
Table 24-1 -- Induced corneal flattening along meridian of incision (scleral pocket incision) in eyebank eyes
|
Incision length (mm) |
Mean flattening (standard deviation, D) |
|
2.0 |
0.07 (0.10) |
|
2.5 |
0.10 (0.17) |
|
3.0 |
0.24 (0.17) |
|
3.5 |
0.47 (0.35) |
|
4.0 |
0.74 (0.45) |
|
4.5 |
1.00 (0.46) |
|
5.0 |
1.07 (0.41) |
|
5.5 |
1.40 (0.56) |
From Samuelson SW, Koch DD, Kuglen CC. Determination of maximal incision length for true small-incision surgery. Ophthalmic Surg 1991;22:204–207.
Suture versus sutureless
Properly constructed scleral incisions up to 7mm wide can be self-sealing in the absence of sutures. The key to watertightness is the anterior entry into the anterior chamber, which creates a valve effect as intraocular pressure compresses the mouth of the incision closed. However, we suspect that sutured scleral incisions heal more rapidly and perhaps more completely than unsutured incisions. It is, therefore, possible that sutureless incisions are more prone to late wound sliding.
For corneal incisions, the vast majority can be left sutureless. However, unsutured clear corneal incisions may permit the ingress of surface fluid following minimal patient manipulation.[14,][15] This may increase the risk of postoperative endophthalmitis, which has increased in frequency in the era of clear corneal cataract incisions.[16] We recommend suturing these incisions if they are greater than 4mm in length or if the incision is not watertight at the conclusion of surgery.
Incision location
As a general rule, for any given incision size and construction, the further the incision is from the center of the cornea, the less the surgically induced astigmatism. For small incisions, most surgeons have adopted the clear or near-clear corneal approach. Fortunately, these incisions are typically sufficiently small that they induce little astigmatism despite their anterior location. For incisions longer than 4°mm, the limbal and particularly scleral incisions offer greater astigmatic stability. Conversely, if against-the-wound drift is desired, these larger incisions can be placed more anteriorly in order to attempt to achieve the desired astigmatic change.
Incision size
Planned extracapsular Incisions
Curved scleral incisions concentric with the limbus and closed with interrupted 10-0 nylon or polyester sutures are recommended for planned extracapsular surgery.[10] Interrupted sutures are probably more prone to inducing excessive early steepness on the meridian of the incision, compared with continuous sutures. However, for these large incisions, interrupted sutures have two advantages: (1) they reduce the risk of excessive flattening along the meridian of the incision, and (2) they offer the opportunity to cut single sutures, which gives greater latitude in modifying astigmatism postoperatively. These incisions can typically drift 1–3D in the first few years after surgery, and against-the-wound flattening of up to 5D can, rarely, occur.
Enlarged phacoemulsification incisions
Incisions 6.5–7.5mm wide are used for implantation of 6–7mm polymethylmethacrylate (PMMA) optic lenses after phacoemulsification. The incisions may be curved, as with the extracapsular incision, or straight. An incision of this size can be expected to drift 1–2D against the wound. If properly constructed, these incisions can be left unsutured, or they are closed with running shoelace suture, interrupted sutures, or a continuous horizontal suture. The advantage of a continuous horizontal suture is that it can be tightened sufficiently to provide watertight closure and perhaps to minimize late wound sliding without inducing excessive astigmatism. Long-term follow-up is needed to assess the stability of incisions closed in this manner. Additional techniques for suturing enlarged phacoemulsification incisions described by Masket and Shepherd (Video Journal of Cataract and Refractive Surgery, volume V, issue 3) are illustrated in Figures 24-4 and 24-5, respectively.
|
Figure 24-4 The Masket continuous horizontal suture closure consists of a posterior radial bite, two right-to-left bites concentric with the limbus, and an anterior-posterior radial bite. |
|
Figure 24-5 Technique for single vertical mattress suture. |
“Small-incision” cataract surgery
Incisions of 3mm or less are used for insertion of foldable small-incision lenses after phacoemulsification. These incisions were originally made in the sclera or limbus, but clear or near-clear corneal incisions are now the most popular choice.
We have reviewed the ophthalmic literature regarding the astigmatic change induced by small scleral limbal and corneal incisions, and the summary of these results are shown in Tables 24-2 and 24-3.[17–30] Interestingly, the results from these clinical studies mirror the results that were found in the cadaver eye study previously performed by Samuelson and Koch.
Table 24-2 -- Surgically induced astigmatism by scleral tunnel incisions
|
Incision length (mm) |
Surgically induced astigmatism (D) |
|
3.0–3.5 |
0.20–0.40 |
|
4.0 |
0.42–0.72 |
|
5.0–5.5 |
0.35–0.89 |
Table 24-3 -- Surgically induced astigmatism by clear corneal incisions
|
Incision length (mm) |
Surgically induced astigmatism (D) |
|
3.0–3.5 |
0.20–0.68 |
|
4.0 |
0.36–0.56 |
|
5.0–5.5 |
0.46–1.24 |
If we define true “small incision” surgery on an astigmatic basis such that less than 0.5D is induced then this definition would pertain to scleral incisions measuring 4mm and corneal incisions measuring 3.0–3.5mm.
Incision configuration and manipulation
The configuration of the incision may also influence wound stability and eventual against-the-wound drift. A straight or frown-shaped incision appears to induce less against-the-wound astigmatic change than the traditional curved incision parallel to the limbus (Figure 24-6).
|
Figure 24-6 Induced astigmatic drift against the wound tends to be greatest with traditional curved incisions and least with frown-shaped incision configurations. |
Pre-existing astigmatism can be reduced through the use of scleral flap recession on the steep corneal meridian. The approach has the advantages of (1) requiring only one incision (AK may be obviated), thereby minimizing wound-healing variables, and (2) avoiding the potential complications of corneal incisions, such as irregular astigmatism and glare.
To perform the technique, a trapezoidal scleral flap is made, centered meticulously on the steep meridian (Figure 24-7). The curvilinear base of the flap is located 2mm behind the limbus, and the lateral walls of the flap are cut to within 0.5mm of the cornea. The width of the flap at the limbus should slightly exceed the anticipated size of the incision into the anterior chamber (e.g. for a 6°mm incision, the flap measures 7°mm at the limbus and 8mm posteriorly). The flap should be approximately two-thirds of the depth. As with standard incisions, the flap is dissected into clear cornea to enhance watertightness.
|
Figure 24-7 Configuration of the trapezoidal scleral flap and running suture closure for scleral flap recession. Note that the anterior edges of the flap are slightly lateral to the intended incision into the anterior chamber. Each suture bite exits in the bed of the flap to create a barrier that prevents posterior migration of the flap. |
The flap is recessed and secured with a running 9-0 nylon suture anchored at each end and tied centrally. The suture pattern, shown in Figure 24-7, forms a barrier that prevents posterior migration of the flap and assures its stable fixation in the recessed position.[31]
With this technique up to 4–5D of astigmatism can be corrected. Each 0.25mm of recession produces about 1D of astigmatic correction; the maximum recession is about 1mm. The goal is to slightly overcorrect at surgery, as measured by qualitative or quantitative intraoperative keratometry. In the presence of significant undercorrection, the suture can be removed and the flap advanced an additional amount.
Corneal relaxing incisions
The combination of CRI with cataract surgery (Figure 24-8) is fundamental to the current definition of refractive cataract surgery. A number of surgeons in the early 1980s, among them Fenzl, Lindstrom, Martin, Neumann, Nordan, Tate, Terry, and Thornton, began investigating surgical techniques to correct naturally occurring astigmatism. In 1983, Osher began a study that addressed the correction of pre-existing astigmatism by combining transverse relaxing incisions with cataract surgery. He presented preliminary results at general meetings from 1984 to 1990.[32]
|
Figure 24-8 Transverse astigmatic keratotomy combined with cataract surgery. |
Osher's original technique consisted of placing a single straight corneal relaxing incision in the periphery perpendicular to the steep meridian at the end of surgery and then adding a second parallel incision on a 7–10.5mm-diameter optical zone. Maloney[33] described a more aggressive approach in which he placed two pairs of transverse incisions before phacoemulsification. Other surgeons attempted to quantify the effect of adding transverse corneal incisions to cataract surgery by varying incision length,[34] number of incisions,[35]optical zone size,[36] or incision depth.[37] Merlin[38] introduced arcuate incisions, and Thornton[39] and Lindstrom[40] became leading advocates while refining diamond blade technology.
Lindstrom[40] found that the coupling ratio, the amount of flattening in the incised meridian divided by the amount of steepening in the opposite meridian, was approximately 1:1 when a straight 3mm keratotomy or a 45° to 90° arcuate keratotomy was used at 5–7mm-diameter optical zones. The maximal effect of either straight or arcuate incisions occurred when incisions were placed around a 5–7mm-diameter optical zone. Although most of the effect was achieved with the first pair of incisions, a 20% to 30% additional effect could be attained with a second pair of incisions. The effect could not be increased by placing more than four relaxing incisions in the cornea.
Thornton[39] described what he believed was the geometric advantage of arcuate incisions, the use of which seems to be growing in popularity. He stated that true 1:1 coupling can occur only when the corneal circumference is unchanged, which is achieved only with short, concentric arcuate incisions. A straight transverse incision increases the overall corneal circumference, creating a flatter cornea and necessitating a compensatory addition of power to the IOL. Furthermore, a shorter arcuate incision achieves the same result as a longer straight incision.
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Peripheral or limbal corneal relaxing incisions
Hollis and Gills first investigated the use of limbal relaxing incisions (LRIs) centered along the steep corneal meridian to correct pre-existing astigmatism during cataract surgery. As the single or paired relaxing incisions are placed just inside the limbal vessels, they are actually more appropriately called “peripheral corneal relaxing incisions” (PCRIs). Because they are placed at the peripheral cornea, a potential advantage of the incisions over AK is the minimal risk of inducing irregular astigmatism.
The criterion for PCRIs in conjunction with a temporal clear-corneal incision is pre-existing with-the-rule keratometric astigmatism of ≥0.75D or pre-existing against-the-rule keratometric astigmatism of ≥1.25D. This criterion was derived from a study of the astigmatic effect of a standard 3.2–3.5mm temporal clear-corneal incision, which produces approximately 0.3D of with-the-rule change. The length and number of PCRIs are determined according to a nomogram based on age and preoperative corneal astigmatism (Table 24-4). This nomogram is designed for use in combination with 3.2–3.5mm temporal clear-corneal incision with PCRIs made near the end of the cataract surgery, and it is conservative in order to minimize the risk of overcorrections. PCRIs typically cause a mild hyperopic shift of approximately 0.2D, and this should be taken into account when selecting IOL power.
Table 24-4 -- Nomogram for peripheral corneal relaxing incisions to correct keratometric astigmatism during cataract surgery (temporal 3.2 to 3.5mm clear corneal incision)
|
Pre-operative astigmatism (D) |
Age (year) |
Number |
Length |
|
With-the-rule |
|||
|
0.75–1.00 |
<65 |
2 (or 1 x 60°) |
45° |
|
≥65 |
1 |
45° |
|
|
1.01–1.50 |
<65 |
2 |
60° |
|
≥65 |
2 (or 1 x 60°) |
50° |
|
|
>1.50 |
<65 |
2 |
80° |
|
≥65 |
2 |
60°–70° |
|
|
Against-the-rule |
|||
|
1.00–1.25* |
–– |
1 (or 2 x 30°) |
35°–40° |
|
1.26–2.00 |
–– |
1 (or 2 x 40°) |
45° |
|
≥2.00 |
–– |
2 |
45° |
From Wang L, Misra M, and Koch DD. Peripheral corneal relaxing incisions combined with cataract surgery. J Cataract Refractive Surg 2003;29:712–722.
|
* |
Especially if cataract incision is not directly centered on steep meridian. |
The location of the steep meridian is carefully determined as noted earlier in this chapter. Intraoperatively, the intended incision site is marked using one of many commercially available markers or even standard surgical calipers. The incision is made just inside the limbal vessels with a guarded diamond knife set at a depth of 600µm (Figure 24-9). In eyes receiving paired incisions along the horizontal meridian (i.e. in eyes with pre-existing against-the-wound astigmatism), the groove of the temporal clear-corneal incision is enlarged at the end of surgery to serve as the second peripheral relaxing incision, or the temporal peripheral relaxing incision can be made first by grooving at 600µm depth to planned length and then entering the anterior chamber at the 50–75% depth of this incision. A PCRI at the cataract incision site can slightly destabilize the wound, so it is important to ensure that the incision is watertight at the conclusion of surgery.
|
Figure 24-9 A view of a superior PCRI centered along 90° meridian on an eye 1 day after surgery. |
Early studies with small number of cases showed that the PCRIs were an effective method of reducing pre-existing astigmatism during cataract surgery.[41,][42] Recently, Wang, Misra, and Koch[43] reported the results in a large series of patients (93 eyes) who underwent combined clear corneal phacoemulsification and PCRIs. PCRIs significantly decreased pre-existing astigmatism, and the percentages of the eyes with keratometric astigmatism of ≤1D increased from 6% preoperatively to 51% at 4 months postoperatively. Overcorrections of 1D or more occurred in two eyes of two patients; both were over 80 years old. One of the two eyes had a corneal diameter of 10.5mm, which might contribute to the overcorrection because of both the shorter distance between the PCRI and the center of the cornea and the longer arc length relative to the corneal circumference. For this reason, we recommend measuring PCRI length by degrees instead of millimeters. There were no ocular perforations in our series, suggesting a good safety profile for using a guarded diamond knife set at a depth of 600µm when PCRIs are performed at the conclusion of cataract surgery.
We place PCRIs at the conclusion of the surgery because we had good success with this approach in our early cases and developed our first nomogram based on the results with these eyes. An advantage of performing the incisions at the conclusion of surgery is that these incisions can be omitted if there is some need to enlarge or change the site of the cataract incision. An obvious disadvantage is that there might be greater variability in corneal thickness and intraocular pressure at the conclusion of surgery, which could affect the depth of the incisions. We presume that incisions placed early in the surgery might have a greater effect and might also pose a greater risk of corneal perforation, particularly in older eyes with thinner corneas in the region of the limbus.
Astigmatic keratotomy
Lindstom and Koch's Technique
Manifest refraction, keratometry, and computerized videokeratography are performed preoperatively. For cataract patients, the surgical plan is formulated based on the intended incision and the pre-existing corneal astigmatism.
The standard nomograms shown in Tables 24-5 and 24-6 are used. The technique employs either a straight or arcuate keratotomy at the 6mm and/or 7mm zones. The nomogram, if adopted by others, needs to be adjusted to each surgeon's particular technique.
Table 24-5 -- Arcuate keratotomy 6.0mm optical zone nomogram*
|
Surgical option |
||||||
|
Age (years) |
1 x 30° |
2 x 30° or 1 x 45° |
1 x 60° |
2 x 45° or 1 x 90° |
2 x 60° |
2 x 90° |
|
20 |
0.60 |
1.20 |
1.80 |
2.40 |
3.60 |
4.80 |
|
21 |
0.62 |
1.23 |
1.85 |
2.46 |
3.69 |
4.92 |
|
22 |
0.63 |
1.26 |
1.89 |
2.52 |
3.78 |
5.04 |
|
23 |
0.65 |
1.29 |
1.94 |
2.58 |
3.87 |
5.16 |
|
24 |
0.66 |
1.32 |
1.98 |
2.64 |
3.96 |
5.28 |
|
25 |
0.68 |
1.35 |
2.03 |
2.70 |
4.05 |
5.40 |
|
26 |
0.69 |
1.38 |
2.07 |
2.76 |
4.14 |
5.52 |
|
27 |
0.71 |
1.41 |
2.12 |
2.82 |
4.23 |
5.64 |
|
28 |
0.72 |
1.44 |
2.16 |
2.88 |
4.32 |
5.76 |
|
29 |
0.74 |
1.47 |
2.21 |
2.94 |
4.41 |
5.88 |
|
30 |
0.75 |
1.50 |
2.25 |
3.00 |
4.50 |
6.00 |
|
31 |
0.77 |
1.53 |
2.30 |
3.06 |
4.59 |
6.12 |
|
32 |
0.78 |
1.56 |
2.34 |
3.12 |
4.68 |
6.24 |
|
33 |
0.80 |
1.59 |
2.39 |
3.18 |
4.77 |
6.36 |
|
34 |
0.81 |
1.62 |
2.43 |
3.24 |
4.86 |
6.48 |
|
35 |
0.83 |
1.65 |
2.48 |
3.30 |
4.95 |
6.60 |
|
36 |
0.84 |
1.68 |
2.52 |
3.36 |
5.04 |
6.72 |
|
37 |
0.86 |
1.71 |
2.57 |
3.42 |
5.13 |
6.84 |
|
38 |
0.87 |
1.74 |
2.61 |
3.48 |
5.22 |
6.96 |
|
39 |
0.89 |
1.77 |
2.66 |
3.54 |
5.31 |
7.08 |
|
40 |
0.90 |
1.80 |
2.70 |
3.60 |
5.40 |
7.20 |
|
41 |
0.92 |
1.83 |
2.75 |
3.66 |
5.49 |
7.32 |
|
42 |
0.93 |
1.86 |
2.79 |
3.72 |
5.58 |
7.44 |
|
43 |
0.95 |
1.89 |
2.84 |
3.78 |
5.67 |
7.56 |
|
44 |
0.96 |
1.92 |
2.88 |
3.84 |
5.76 |
7.68 |
|
45 |
0.98 |
1.95 |
2.93 |
3.90 |
5.85 |
7.80 |
|
46 |
0.99 |
1.98 |
2.97 |
3.96 |
5.94 |
7.92 |
|
47 |
1.01 |
2.01 |
3.02 |
4.02 |
6.03 |
8.04 |
|
48 |
1.02 |
2.04 |
3.06 |
4.08 |
6.12 |
8.16 |
|
49 |
1.04 |
2.07 |
3.11 |
4.14 |
6.21 |
8.28 |
|
50 |
1.05 |
2.10 |
3.15 |
4.20 |
6.30 |
8.40 |
|
51 |
1.07 |
2.13 |
3.20 |
4.26 |
6.39 |
8.52 |
|
52 |
1.08 |
2.16 |
3.24 |
4.32 |
6.48 |
8.64 |
|
53 |
1.10 |
2.19 |
3.29 |
4.38 |
6.57 |
8.76 |
|
54 |
1.11 |
2.22 |
3.33 |
4.44 |
6.66 |
8.88 |
|
55 |
1.13 |
2.25 |
3.38 |
4.50 |
6.75 |
9.00 |
|
56 |
1.14 |
2.28 |
3.42 |
4.56 |
6.84 |
9.12 |
|
57 |
1.16 |
2.31 |
3.47 |
4.62 |
6.93 |
9.24 |
|
58 |
1.17 |
2.34 |
3.51 |
4.68 |
7.02 |
9.36 |
|
59 |
1.19 |
2.37 |
3.56 |
4.74 |
7.11 |
9.48 |
|
60 |
1.20 |
2.40 |
3.60 |
4.80 |
7.20 |
9.60 |
|
61 |
1.22 |
2.43 |
3.65 |
4.86 |
7.29 |
9.72 |
|
62 |
1.23 |
2.46 |
3.69 |
4.92 |
7.38 |
9.84 |
|
63 |
1.25 |
2.49 |
3.74 |
4.98 |
7.47 |
9.96 |
|
64 |
1.26 |
2.52 |
3.78 |
5.04 |
7.56 |
10.08 |
|
65 |
1.28 |
2.55 |
3.83 |
5.10 |
7.65 |
10.20 |
|
66 |
1.29 |
2.58 |
3.87 |
5.16 |
7.74 |
10.32 |
|
67 |
1.31 |
2.61 |
3.92 |
5.22 |
7.83 |
10.44 |
|
68 |
1.32 |
2.64 |
3.96 |
5.28 |
7.92 |
10.56 |
|
69 |
1.34 |
2.67 |
4.01 |
5.34 |
8.01 |
10.68 |
|
70 |
1.35 |
2.70 |
4.05 |
5.40 |
8.10 |
10.80 |
|
71 |
1.37 |
2.73 |
4.10 |
5.46 |
8.19 |
10.92 |
|
72 |
1.38 |
2.76 |
4.14 |
5.52 |
8.28 |
11.04 |
|
73 |
1.40 |
2.79 |
4.19 |
5.58 |
8.37 |
11.16 |
|
74 |
1.41 |
2.82 |
4.23 |
5.64 |
8.46 |
11.28 |
|
75 |
1.43 |
2.85 |
4.28 |
5.70 |
8.55 |
11.40 |
From Richard L. Lindstrom, MD, Phillips Eye Institute, Minneapolis, Minnesota 55404, and Chiron IntraOptics, Irvine, CA.
|
* |
Find patient age, then move right to find result closest to refractive cylinder without going over. |
Table 24-6 -- Arcuate keratotomy nomogram for males with 7.0mm optical zone
|
Surgical option |
|||||||
|
Age (years) |
1 x 45° |
2 x 30° |
1 x 60° |
1 x 90° |
2 x 45° |
2 x 60° |
2 x 90° |
|
20 |
0.32 |
1.62 |
0.92 |
2.02 |
2.22 |
2.72 |
3.82 |
|
21 |
0.36 |
1.66 |
0.96 |
2.06 |
2.26 |
2.76 |
3.86 |
|
22 |
0.39 |
1.69 |
0.99 |
2.09 |
2.29 |
2.79 |
3.89 |
|
23 |
0.40 |
1.73 |
1.03 |
2.13 |
2.33 |
2.83 |
3.93 |
|
24 |
0.46 |
1.76 |
1.06 |
2.16 |
2.36 |
2.86 |
3.96 |
|
25 |
0.50 |
1.80 |
1.10 |
2.20 |
2.40 |
2.90 |
4.00 |
|
26 |
0.54 |
1.84 |
1.14 |
2.24 |
2.44 |
2.94 |
4.04 |
|
27 |
0.57 |
1.87 |
1.17 |
2.27 |
2.47 |
2.97 |
4.07 |
|
28 |
0.61 |
1.91 |
1.21 |
2.31 |
2.51 |
3.01 |
4.11 |
|
29 |
0.64 |
1.94 |
1.24 |
2.34 |
2.54 |
3.04 |
4.14 |
|
30 |
0.68 |
1.98 |
1.28 |
2.38 |
2.58 |
3.08 |
4.18 |
|
31 |
0.72 |
2.02 |
1.32 |
2.42 |
2.62 |
3.12 |
4.22 |
|
32 |
0.75 |
2.05 |
1.35 |
2.45 |
2.65 |
3.15 |
4.25 |
|
33 |
0.79 |
2.09 |
1.39 |
2.49 |
2.69 |
3.19 |
4.29 |
|
34 |
0.82 |
2.12 |
1.42 |
2.52 |
2.72 |
3.22 |
4.32 |
|
35 |
0.86 |
2.16 |
1.46 |
2.56 |
2.76 |
3.26 |
4.36 |
|
36 |
0.90 |
2.20 |
1.50 |
2.60 |
2.80 |
3.30 |
4.40 |
|
37 |
0.93 |
2.23 |
1.53 |
2.63 |
2.83 |
3.33 |
4.43 |
|
38 |
0.97 |
2.27 |
1.57 |
2.67 |
2.87 |
3.37 |
4.47 |
|
39 |
1.00 |
2.30 |
1.60 |
2.70 |
2.90 |
3.40 |
4.50 |
|
40 |
1.04 |
2.34 |
1.64 |
2.74 |
2.94 |
3.44 |
4.54 |
|
41 |
1.08 |
2.38 |
1.68 |
2.78 |
2.98 |
3.48 |
4.58 |
|
42 |
1.11 |
2.41 |
1.71 |
2.81 |
3.01 |
3.51 |
4.61 |
|
43 |
1.15 |
2.45 |
1.75 |
2.85 |
3.05 |
3.55 |
4.65 |
|
44 |
1.18 |
2.48 |
1.78 |
2.88 |
3.08 |
3.58 |
4.68 |
|
45 |
1.22 |
2.52 |
1.82 |
2.92 |
3.12 |
3.62 |
4.72 |
|
46 |
1.26 |
2.56 |
1.86 |
2.96 |
3.16 |
3.66 |
4.76 |
|
47 |
1.29 |
2.59 |
1.89 |
2.99 |
3.19 |
3.69 |
4.79 |
|
48 |
1.33 |
2.63 |
1.93 |
3.03 |
3.23 |
3.73 |
4.83 |
|
49 |
1.36 |
2.66 |
1.96 |
3.06 |
3.26 |
3.76 |
4.86 |
|
50 |
1.40 |
2.70 |
2.00 |
3.10 |
3.30 |
3.80 |
4.90 |
|
51 |
1.44 |
2.74 |
2.04 |
3.14 |
3.34 |
3.84 |
4.94 |
|
52 |
1.47 |
2.77 |
2.07 |
3.17 |
3.37 |
3.87 |
4.97 |
|
53 |
1.51 |
2.81 |
2.11 |
3.21 |
3.41 |
3.91 |
5.01 |
|
54 |
1.54 |
2.84 |
2.14 |
3.24 |
3.44 |
3.94 |
5.04 |
|
55 |
1.58 |
2.88 |
2.18 |
3.28 |
3.48 |
3.98 |
5.08 |
|
56 |
1.62 |
2.92 |
2.22 |
3.32 |
3.52 |
4.02 |
5.12 |
|
57 |
1.65 |
2.95 |
2.25 |
3.35 |
3.55 |
4.05 |
5.15 |
|
58 |
1.69 |
2.99 |
2.29 |
3.39 |
3.59 |
4.09 |
5.19 |
|
59 |
1.72 |
3.02 |
2.32 |
3.42 |
3.62 |
4.12 |
5.22 |
|
60 |
1.76 |
3.06 |
2.36 |
3.46 |
3.66 |
4.16 |
5.26 |
|
61 |
1.80 |
3.10 |
2.40 |
3.50 |
3.70 |
4.20 |
5.30 |
|
62 |
1.83 |
3.13 |
2.43 |
3.53 |
3.73 |
4.23 |
5.33 |
|
63 |
1.87 |
3.17 |
2.47 |
3.57 |
3.77 |
4.27 |
5.37 |
|
64 |
1.90 |
3.20 |
2.50 |
3.60 |
3.80 |
4.30 |
5.40 |
|
65 |
1.94 |
3.24 |
2.54 |
3.64 |
3.84 |
4.34 |
5.44 |
|
66 |
1.98 |
3.28 |
2.58 |
3.68 |
3.88 |
4.38 |
5.48 |
|
67 |
2.01 |
3.31 |
2.61 |
3.71 |
3.91 |
4.41 |
5.51 |
|
68 |
2.05 |
3.35 |
2.65 |
3.75 |
3.95 |
4.45 |
5.55 |
|
69 |
2.08 |
3.38 |
2.68 |
3.78 |
3.98 |
4.48 |
5.58 |
|
70 |
2.12 |
3.42 |
2.72 |
3.82 |
4.02 |
4.52 |
5.62 |
|
71 |
2.16 |
3.46 |
2.76 |
3.86 |
4.06 |
4.56 |
5.66 |
|
72 |
2.19 |
3.49 |
2.79 |
3.89 |
4.09 |
4.59 |
5.69 |
|
73 |
2.23 |
3.53 |
2.83 |
3.93 |
4.13 |
4.63 |
5.73 |
|
74 |
2.26 |
3.56 |
2.86 |
3.96 |
4.16 |
4.66 |
5.76 |
|
75 |
2.30 |
3.60 |
2.90 |
4.00 |
4.20 |
4.70 |
5.80 |
|
Subtract 0.37 from each predicted value for females. |
AK is performed at the end of the cataract procedure with the eye inflated. A smaller (5mm or less) self-sealing incision is preferred when AK is combined with cataract surgery. When they are planning the AK, surgeons must factor in the expected against-the-would drift of the particular incision used.
Equipment includes an operating microscope, a Sinskey hook, 0.12 Colibri corneal fixation forceps, and various zone and incision markers. The Lindstrom arcuate marker (Katena Products, Denville, NJ) is preferred for arcuate incisions. Round 3mm, 5mm, and 7mm radial keratotomy optical zone markers and 8-, 12-, and 16-cut radial keratotomy incision markers can be used to localize the incision location and length. A skin-marking pencil or stencil ink pad is used to clarify the marks. An ultrasonic pachymeter is used to measure corneal thickness intraoperatively. A surgical keratometer is useful but not essential for intraoperative monitoring.
A vertical-blade (push) diamond micrometer knife allows the surgeon good visibility while pushing through the length of the keratotomy. The knife is calibrated with the Mastel Retiscope (Mastel, Rapid City, SD) or a similar device. Extreme care should be taken in knife selection, calibration, and maintenance to assure reproducible cuts. Balanced salt solution and an irrigation cannula are used to keep the cornea moist and to irrigate incisions.
Topical anesthesia is particularly helpful in these patients, as it permits them to fixate the filament of the surgical microscope. This permits centration as demonstrated in Figure 24-10. For cataract patients who have been anesthetized with peribulbar or retrobulbar injection, the surgeon can accurately estimate the center of the pupil with the patient's eye adjusted to be perpendicular to the microscope. As with the method shown in Figure 24-10, the pupillary center is marked with a Sinskey hook or similar device.
|
Figure 24-10 The center of the optical zone is determined by asking the patient to fixate on the microscope light, on a mark placed directly between the two oculars, or on the Mastel Aximeter (Mastel, Rapid City, SD). While the patient is properly fixating, the center of the entrance pupil is marked with a Sinskey hook. |
The keratotomy optical zone is marked with a 7mm marker (Figure 24-11). The steep meridian is marked with a skin-marking pen using intraoperative keratometry or preoperative landmarks and an axis marker (Figure 24-12). To mark the length of a 3mm transverse keratotomy, a 3mm circular zone marker is placed over the 7mm zone mark (and also over the 5mm zone mark if four cuts are planned) in the steep meridian (Figure 24-13). If arcuate keratotomy is preferred, the Lindstrom arcuate marker guides the performance of 45°, 60°, and 90° arcuate cuts (Figures 24-14 and 24-15). The use of a 16-ray, 12-ray, or 8-ray RK marker, respectively, can provide similar guidance (Figure 24-16). Arcuate incisions of more than 90° are not recommended.
|
Figure 24-11 Marking the optical zone with a 7-mm zone marker. |
|
Figure 24-12 Marking the steep meridian. |
|
Figure 24-13 Use of a 3-mm zone marker to delineate the incision length. |
|
Figure 24-14 The Lindstrom arcuate marker (Katena Products) is placed on the cornea aligned with the steep meridian. |
|
Figure 24-15 Cornea marked before astigmatic keratotomy. Perpendicular lines mark 45°, 60°, and 90° for arcuate cuts. |
|
Figure 24-16 Left, Sixteen-ray RK marker is useful to delineate 45° arcuate keratotomy. Certer, Twelve-ray RK marker is useful to delineate 60° arcuate keratotomy. Right, Eight-ray RK marker is useful to delineate 90° arcuate keratotomy. |
Intraoperative pachymetry is used at the appropriate optical zone in the steep meridian on one (for a single incision) or both sides of the cornea (Figure 24-17). The blade depth of the calibrated diamond knife is set at 100% of the thinnest paracentral pachymetry. If pachymetry is not available, setting the knife at 0.6mm for a 7mm optical zone incision appears to be safe and effective.
|
Figure 24-17 Corneal pachymetry is measured directly over the incision site; the blade is set at 90%–100% of the thinnest pachymetry reading. |
With the corneal fixation forceps held in the nondominant hand and used to grasp tissue at the limbus, the knife in the dominant hand is set into the cornea, pausing for 1 second. The knife is then guided slowly through the incision (Figure 24-18).
|
Figure 24-18 Marking the incision. |
The completed incision is irrigated with balanced salt solution (Figure 24-19), and several drops of topical antibiotic are placed on the eye. Patching or cycloplegia is not routinely used. If a significant perforation occurs, subconjunctival antibiotic, topical cycloplegia, and a pressure patch are used; obviously, if chamber depth cannot be maintained, then the incision with the perforation is sutured. Perforations are extremely rare with the technique described.
|
Figure 24-19 Irrigation with balanced salt solution. |
Osher's technique
Since beginning astigmatic keratometry combined with cataract surgery for the reduction of pre-existing astigmatism in 1983, Osher's technique has gone through several revisions. The initial examination had always included careful keratometry, and each new generation of corneal topography has been added. The amount of phakic or pseudophakic astigmatism in the fellow eye must be considered in determining candidacy. Patients with significant anterior membrane dystrophy or severe Fuch's endothelial dystrophy are excluded. An explanation of the surgical plan to reduce the astigmatism is given, and the patient is informed that this procedure is elective and inexact and may result in more ocular irritation than normal for several days following surgery (although this is usually not the case). Permission to perform the astigmatic keratometry is part of the routine informed consent form for cataract surgery.
Following the initial evaluation, an operative plan is formulated. The optical zone is selected, primarily based on the Osher nomogram (Table 24-7) while keeping the length, depth, and shape of the incisions constant.[44] Principles gained through experience, such as the greater response in eyes having against-the-rule cylinder, a large corneal diameter, increasing patient age, and the perceived effect of intraocular pressure are taken into consideration. The nomogram will need to be adjusted according to each surgeon's particular technique. Since astigmatic keratotomy does not change average preoperative keratometric power, no change is needed in IOL power. After arriving at the optimal approach for the patient, a drawing is made on the chart, which is hung from the microscope next to the topography for easy reference. The drawing shows the size of the optical zone and the location of the incisions to assure proper orientation.
Table 24-7 -- Osher nomogram for 3mm T–cuts
|
Cylinder (D) |
Optical zone (mm) |
|
1.5 |
8.5 |
|
2.0 |
8.0 |
|
2.5 |
7.0–7.5 |
|
3.0 |
6.0–6.5 |
|
3.5 |
2 pair: 6 and 8.5 |
From Osher RH. Transverse astigmatic keratotomy combined with cataract surgery. Ophthalmology Clin North Am 1992;5:717–725.
In the early years of performing this procedure, the major meridians of the eye were marked in the holding room prior to surgery with a drop of topical anesthetic and a cautery while the eye was in the primary position of gaze for distance fixation. This method has been replaced by quantitative intraoperative keratoscopy. A Hyde-Osher ruler made by Ocular Instruments has a series of spherical and astigmatic circles cut out of a metallic bar. This is held between the eye and the microscope and easily identifies the steep meridian of curvature, which is marked at the limbus with two spots 180° from each other using the coaptation cautery. With the eye coaxial with the microscope, the amount of cylinder is quantitated by neutralizing the progressive astigmatic openings in the bar until a circular reflex is observed. The measurements of the axis and amount of cylinder are usually consistent with the preoperative data. If the axis is off by several degrees, the intraoperative observations are favored. If a disparity greater than 10° or 15° exists, AK is not performed – a decision that is rarely necessary.
Although initially blade depth was determined by intraoperative pachymetry, for many years Osher has simply set the blade at 690 microns for an optical zone of 6mm or greater. Formerly, AK was performed at the conclusion of the procedure to maximize visualization during the cataract surgery, but currently the incisions are made at the beginning of the procedure. The advantages include a firmer globe with a better epithelium, yielding more accurate intraoperative keratoscopy and incision depth. In addition, the healthier epithelium results in many fewer corneal abrasions, so the eye does not require patching.
The incision length is 3mm. The globe is stabilized with a multiple dull-toothed forceps held in the fellow hand. Increasing experience has resulted in consistent incision depth between 80% and 95%, which is important in achieving effective results. A second pair of incisions is reserved for cylinder greater than 3.5D. After the incisions are made, a 30 gauge cannula is used to confirm that the depth is adequate; it is then used to gently irrigate a stream of balanced salt solution into the incision to remove any trapped air bubbles or cellular debris. Complications include corneal abrasion in about 5% and microperforation in less than 1%. If a superficial abrasion occurs, a double pressure patch is applied at the conclusion of the procedure.
Astigmatic keratometry is performed in those 20% of patients, approximately, with pre-existing cylinder of ≥1.5D. In a study of this conservative approach to AK using the same nomogram in which only the optical zone was varied, all eyes except one with amblyopia enjoyed a best-corrected visual acuity comparable to that of a controlled population not receiving AK.[32] However, the uncorrected vision was outstanding with acuity of 20/40 or better achieved in 76%. Certainly only a fraction of these patients would have achieved this visual result had their cylinder not been reduced by AK. Comparison of preoperative and postoperative keratometry measurement showed that the IOL selected would have been unchanged in 87% of eyes. Although 13% showed a power change of between 0.5 and 1D, it was reassuring to find that the surgical change in the cornea influenced the IOL power less than 1D in all cases.
Femtosecond laser-assisted arcuate keratotomy
With the advent of the femtosecond laser and its ability to produce both horizontal and vertical cleavage planes within the corneal stroma, several investigators have reported their use of the laser to create corneal incisions to reduce astigmatism. Harissi-Dagher and Azar [45] used a femtosecond laser to perform astigmatic keratotomy in two patients with high levels of corneal astigmatism following penetrating keratoplasty. The postoperative refractive cylinder measured 4.9 and 4.3D, down from the preoperative levels of 8.5 and 7D, respectively. The best-corrected visual acuities also improved from 20/100 and 20/200 before surgery to 20/30 and 20/60 after. No complications were seen in either case. Kymionis et al.[46]described their use of the femtosecond laser to correct irregular corneal astigmatism following penetrating keratoplasty. They used the keratoplasty software on the Intralase femtosecond laser (Abbot Medical Optics, Inc., location) to create a single arcuate side cut that was 6.5mm in diameter. Six months following the procedure, the corneal astigmatism had decreased from 4 to 0.5D and the best spectacle-corrected visual acuity had improved from 20/50 to 20/32.
Hoffart et al.[47] compared the effectiveness of a femtosecond laser to a mechanical method using the Hanna keratome (Moria, Inc., Anthony, France) in performing AK in 20 postkeratoplasty eyes. The mean uncorrected and best-corrected acuities did not change significantly for either group. However, the mean refractive cylinder decreased from preoperative levels of 8.6 and 6.7 D to postoperative values of 3.9 and 4.7D for the laser and mechanical methods, respectively. The Hanna group had a microperforation in one case and worse alignment, in general. Both treatments were found to be effective in reducing postkeratoplasty astigmatism, with the femtosecond laser showing some advantages over the mechanized method.
Although AK was performed in postkeratoplasty eyes in these cases, they do demonstrate the feasibility of using the laser to create corneal incisions that reduce astigmatism. The laser can be used in a similar fashion to create astigmatism-reducing corneal incisions following cataract surgery. Care must be taken to allow the corneal wounds to heal adequately before applying the suction ring required for use with the femtosecond laser. While the laser provides a very controlled and potentially repeatable method for performing AK, several disadvantages are associated with its use for AK: (1) there is an added expense for the disposable materials and treatment, (2) the treatment can not be performed at the time of cataract surgery, and (3) well-tested nomograms are needed.
Toric intraocular lens
The toric IOL was devised by Shimizu, Misawa, and Suzuki[48] and has been used clinically since 1992. The first toric IOL introduced was a nonfoldable 3-piece posterior chamber lens; foldable toric single-piece IOLs are available currently. Advantages of astigmatism correction with toric IOLs over CRIs are reversibility and excellent optical quality with no induction of irregular astigmatism. The surgical technique of toric IOL implantation involves careful preoperative marking of the correct meridion for IOL alignment, as well as intraoperative rotation of the toric IOL to orient the its axis markings along the steep corneal meridian.
Staar toric IOL models (STAAR Surgical Company, Monrovia, CA) have a toric anterior surface, spherical posterior surface, two positioning marks along the long axis on the anterior surface, and two 1.15mm fenestrations at the opposite ends (Figure 24-20). The lens is available in powers ranging from 9.5 to 28D with cylindrical adds of 2 and 3.5D, which theoretically correct 1.4 and 2.3D of astigmatism at the corneal plane, respectively. Thus, patients with 1.5–3.5D of regular pre-existing astigmatism are candidates for implantation of these lenses.
|
Figure 24-20 The Staar toric intraocular lens. |
Postoperative rotation of this plate haptic toric IOL is a significant problem. Leyland et al.[49] reported that 18% (4 of 22) of IOLs rotated more than 30°. Sun et al.[50] reported that 18% rotated between 20° and 40°, and 7% rotated more than 40°. In the study by Till et al.[51] 6% of IOLs rotated more than 31°. This, presumably, results in a greater than 10% incidence of surgical reintervention to reposition the implants. Modifications in this plate haptic toric IOL design are needed to address this problem. An additional drawback in the use of this toric IOL is that the only cylindrical adds currently available are 2 and 3.5D.
A one-piece acrylic toric IOL (Alcon Surgical, Inc.) was recently approved for use within the US. The lens design employs the widely used Acrysof single-piece platform with the toric correction on the posterior surface (Figure 24-21). The IOL is available in powers ranging from 5 to 30D with three levels of cylindrical correction (1.5, 2.25, and 3D), which correspond to 1.03, 1.55, and 2.06D of astigmatism correction at the corneal plane, respectively. In the US FDA clinical trial, 494 eyes were randomized to implantation of the toric IOL or a standard spherical single-piece acrylic IOL. At 6 months following surgery, 420 eyes were available for analysis (211 with the toric IOL and 209 controls). Ninety-four percent of eyes receiving a toric IOL achieved monocular UCVA of 20/40 or better, compared to 79% of controls. Postoperative refractive cylinder was ≤0.50D in 61% of toric patients and 19% of controls. The IOL showed excellent rotational stability with a mean postoperative rotation of 4°. Ninety-seven percent of the IOLs rotated less than 10°. In addition, 94% of patients receiving bilateral toric IOL implantation were spectacle-free for distance activities.
|
Figure 24-21 The Alcon Acrysof toric intraocular lens. |
Several post-market studies have evaluated the performance of this toric IOL. Bauer et al.[52] looked at the outcomes of Acrysof toric IOL implantation in a prospective study of 53 eyes in 43 patients. Patients were somewhat evenly split between the T3 (16 eyes), T4 (14 eyes), and T5 (23 eyes). Thirteen eyes in the T5 group had the potential to be fully corrected, while 10 eyes had astigmatism levels that could only be partially corrected. Greater than 90% of eyes achieved uncorrected visual acuities of 20/40 or better, while almost 80% had uncorrected visual acuities of 20/25 or better. Residual refractive astigmatism was less than 0.75D in 74% of eyes and less than 1D in 91%. The mean IOL misalignment was 3.5 ± 3°. Weinand et al.[53] found the rotational stability of the IOL to be excellent. They evaluated the IOL position using digital photographs taken immediately after implantation and 6 months later. By referencing features on the conjunctiva and IOL, they found a median postoperative IOL rotation of only 0.7° in a group of 17 eyes, with a maximum rotation of 1.8°. Chang [54] compared the rotational stability of the Acrysof toric IOL (100 eyes) to the Staar toric IOL (90 eyes). He found that 90%, 99%, and 100% of the Acrysof toric IOLs were aligned within 5, 10, and 15°, respectively. The Staar toric IOL had 70%, 90%, and 97% aligned within 5, 10, and 15°, respectively. The mean rotation of the Acrysof IOL was 3.35 ± 3.41°, while the mean rotation of the Staar toric IOL was 5.56 ± 8.49°. He concluded that both IOLs showed a small degree of postoperative rotation, with the Acrysof toric IOL demonstrating greater stability that was statistically significant.
Astigmatism correction with toric IOLs carries several advantages. Typically there is a need for only one surgical procedure. Also, the induction of irregular corneal astigmatism is avoided and the single-piece toric IOL demonstrates long-term stability. The most ideal way to manage residual astigmatism would be to modify the IOL after it has been implanted using a non-invasive approach. The Calhoun Vision silicone IOL, currently in development, may offer this possibility. The refractive power of this IOL can be modified with laser irradiation after implantation. Differential irradiation of the optic can alter its refractive power to correct astigmatism, as well as other higher-order aberrations. This IOL may have the potential to correct a wide variety of postoperative refractive errors.
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Conclusions
Refractive cataract surgery requires meticulous planning and surgical technique in order to optimize the spherical and astigmatic refractive outcomes. Patient expectations are increasing; for many, excellent uncorrected visual acuity is a primary goal of surgery.[55] The development of multifocal and accommodating IOLs heightens the clinical mandate for precision in reaching targeted refractive goals. The practitioner who has mastered the current tools and philosophies in dealing with astigmatism in cataract surgery may be in the best position to incorporate new technologies as they emerge.
Although no single best approach to astigmatism correction in cataract surgery has yet been established, it likely that evolving toric and adjustable IOL designs will ultimately provide the most consistent refractive results and superior optics. However, by incorporating the general principles outlined in this chapter, surgeons will be able to transition into the new subspecialty of refractive cataract surgery and greatly enhance their surgical outcomes using current technology. By paying meticulous attention to results, each surgeon will inevitably create his or her own nomograms, tailored not only to the technical aspects of a preferred surgical approach, but also to the visual demands of the individual patient.
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