Ординатура / Офтальмология / Английские материалы / Master Techniques in Cataract and Refractive Surgery_Hampton Roy, Arzabe_2004

ACKNOWLEDGEMENTS

We want to acknowledge the valuable help from Howard Goodman, MD and to Mr. Mooky Ben-David for their contributions to the photographic material used in this chapter.

REFERENCES

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For cataract patients with extreme refractive errors, implanting 2 IOLs in 1 eye, piggyback style, provides an opportunity to treat the refractive error along with the cataract. The piggyback lens implantation strategy (Figure 15-1) can be used to treat high hyperopia,1-5 extremely high myopia,6 and high astigmatism (using toric lenses). In many cases, clear lens replacement with piggyback lenses is the ideal procedure for those who are beyond the range of many other procedures.

Although the piggyback implantation technique is essentially the same for any patient, different patient populations present unique issues that require special attention. For example, high hyperopes often present with disproportional anatomy that causes challenges to power calculations, while high astigmats receiving torics require meticulous attention to the axis of implantation.

PIGGYBACKS IN HIGH HYPEROPES

Measurements and Calculations

The highly hyperopic patient presents the surgeon with 2 potential problems. The first is the possibility of surgical complications that may arise from the structural nature of the hyperopic eye. The second is implanting adequate power

while maintaining good optical quality and an accurate refraction. Calculating the correct IOL power in highly hyperopic patients presents unique challenges due to the difficulties in obtaining accurate measurements in short eyes, and the limitations of most IOL formulas.7

Accurate measurement of axial length in hyperopic eyes is especially important because any error is greatly magnified in proportion to the length of the eye. Yet it is in short eyes that accurate measurements are most difficult to obtain. Ultrasound axiometers are calibrated with average velocities for normal length eyes. These velocities are incorrect for short eyes, causing significant measurement errors.8 However, this problem can be corrected by applying the correct tissue velocity to the aqueous, lens, and vitreous.

Performing applanation biometry is frequently difficult in short-eye cases with shallow anterior chambers because it can be difficult to distinguish the initial "bang" echo from the iris and establish perpendicularity. Decreasing the ultrasound gain may be necessary when this occurs so each echo can be visualized; however, doing so can make the scan more difficult to perform. The most significant problem with applanation biometry is that the cornea is easily indented even in the hand of the most skilled ultrasound technician. Even the slightest indentation can cause significant measurement errors, which are magnified when the eye is short.8-10

Acquiring axial length measurements through non-con- tact biometry, whether immersion or the IOL Master (Zeiss

Meditec, Germany) provides superior results in these cases.11,12 First, it is impossible to indent the cornea and shorten the axial length. Thus, by their very nature, these methods are more reliable than applanation. Immersion biometry allows visualization of the corneal echoes, ensuring perpendicularity and improving accuracy. In order to obtain the most accurate measurement, the skilled ultrasound technician will watch for consistency of echo height, axial length, lens thickness, and anterior chamber depth readings.

Optimizing axial length measurements does not guarantee the desired outcome. In a study by the author performed with Dr. Jack Holladay,8 several hyperopic patients were examined and more detailed anatomical measurements were taken. Most of the short-eye cases had normal anterior segment dimensions (corneal diameter, keratometry, and anterior segment length) but shortened posterior segments. Only about 20% of eyes with axial length less than 21 mm had disproportionately small anterior segment sizes.7

Based on these observations, we can conclude that many third-generation power formulas systematically generate hyperopic errors in power calculation among extreme hyperopes because they assume the anterior segments are proportionate the shortened posterior segment.7 Thus they predict the IOL position to be too anterior, resulting in hyperopic error in approximately 80% of the cases.

Holladay has reported that prediction accuracy in short eyes is significantly improved with the Holladay II formula, which incorporates additional measurements to take into account the unusual anatomy found in most high hyperopes.7 He reported a decrease in mean absolute error among short eye cases from about 4.50 D with other formulas to a little less than 1.00 D with the Holladay II formula. He also found that about 4% of eyes with average axial lengths have anterior segment sizes that are large or small relative to the posterior segment. These patients may also benefit from the use of an IOL formula based on more measurements.

Furthermore, when the piggyback technique is used in high hyperopes, power calculations must be adjusted again. By measuring the distance from the iris to the IOL vertex, Holladay and Gills8 determined that the anterior-most lens is in the usual position while the posterior-most lens is pushed back, causing additional hyperopic error. Apparently the anterior lens pushes the posterior lens further back due to the elastic nature of the capsular bag. Thus, additional power must be factored into the equation. The Holladay II formula provides such adjustments.7 For these reasons, the Holladay II is the formula of choice for calculating piggyback IOLs. We have found improved accuracy in our piggyback cases after switching to this formula.13

Figure 15-1. Piggybacked IOLs.

Surgical Technique

All patients undergoing lens extraction surgery receive a thorough explanation of the type of anesthesia to be used, what to expect during surgery, and the risks involved. This is especially important for high hyperopes because compliance during surgery is critical. Topical anesthesia can be used for these patients, just as for cases with average axial lengths. However, managing complications is certainly more difficult under topical anesthesia, and high hyperopes are at greater risk for certain complications such as shallow anterior chambers; iris prolapse; pupillary block while the pupil is dilated, which can result in a hard eye; and choroidal effusion or fluid misdirection. Many surgeons may prefer regional anesthesia for these cases.

Piggybacking IOLs requires close attention to phacoemulsification technique, both because of the higher risk associated with the population that usually receives piggyback IOLs, and because of the potential long-term complications that can arise with 2 IOLs implanted in the bag. We perform meticulous cortical clean-up, and polish the posterior capsule, which reduces the risk of intralenticular cellular growth.

For myopic or hyperopic primary piggyback cases, we use 2 PMMA single-piece biconvex IOLs with an optic size of 5.5 mm. Both IOLs are placed in the capsular bag. PMMA IOLs require the use of a self-sealing scleral-tunnel incision. CCIs are avoided in these cases to reduce the risk of infection associated with wound leak. The incision is placed at the steep meridian to correct preexisting astigmatism. In some cases limbal or CRIs are also performed to correct preexisting astigmatism.7

Figure 15-2. Suturing piggybacked toric lenses together eliminates the possibility of counter-rotation.

CORRECTING HIGH

ASTIGMATISM WITH

PIGGYBACK TORIC IOLS

While astigmatism greater than 5.00 D is rare, it can be visually disabling, typically limiting the quality of vision. Patients with very high astigmatism often have associated corneal pathology, making the astigmatism more challenging to correct, and the result more unpredictable with incisional procedures alone. Before the availability of toric IOLs, the amount of surgical correction required to correct high astigmatism involved multiple limbal and CRIs, which often caused corneal distortion and visual aberrations. We have had excellent results correcting high astigmatism by implanting 2 toric IOLs, even if the patient would not have otherwise required piggyback implantation for high hyperopia. By implanting 2 toric lenses piggyback style, corneal disturbance can be significantly reduced, providing safer, less invasive surgery.

Because the maximum correction with a single toric lens is 2.40 D with the 3.50 D toric IOL, we utilize 2 toric lenses to correct astigmatism greater than 5.00 D. Implanting two 3.50-D toric IOLs back to back theoretically doubles the effective correction. Patients with less than 5.00 D of astigmatism may receive a single toric IOL, astigmatic keratotomy, strategic wound placement and size, or a combination of procedures, depending on the individual. Correcting higher levels of astigmatism typically requires more than 1 technique. However, reducing high astigmatism to a manageable level with piggyback toric IOLs makes residual astigmatism simpler to correct.

A concern with any toric implantation is rotation of the lens. A rotation up to 30 degrees will reduce the effective astigmatism correction, and a rotation greater than 30

degrees will actually worsen the astigmatism.14 Rotation in toric piggybacks is even more problematic, because the effect of off-axis rotation will be doubled. Even worse is the possibility of the lenses counter-rotating.

We have implanted piggyback toric lenses in a relatively small number of patients with few problems of postoperative rotation, and no counter-rotations. The risk of rotation with 2 toric lenses is smaller than if 1 is used, because the piggyback system fills the bag.

The potential risk of counter-rotation can be completely avoided by suturing the IOLs together through the fixation holes. Because the cylindrical component is incorporated on the anterior surface of the toric lens, we suture the lenses back to back (rather than front to back) to decrease the possibility of dimpling the optic. If the optics were compressed, there could be a reduction of the effective cylinder correction.

Suturing the lenses together necessitates a larger incision, which can actually be useful in correcting some of the incision if it is placed at the steep axis. The impact of the incision on the corneal astigmatism must then be taken into account when calculating the amount of desired correction. To suture toric lenses together, the edges of the lenses are held with a soft lens grabber. The IOLs are secured with tying forceps and 9-0 nylon, using 1 throw and 1 knot through the fixation holes at each end (Figure 15-2).

Determining the Axis

When evaluating the preoperative axis of astigmatism, manual keratometry and corneal topography frequently differ, due to the different ways the instruments acquire the readings. We identify the steep axis preoperatively with corneal topography and manual keratometry. In the event of a discrepancy, the surgical keratometer is the final arbiter. Using the surgical keratometer before, during, and after insertion of the IOL ensures the most accurate placement of the toric lens, astigmatic keratotomy and cataract incision. We have found the surgical keratometer to be a vital tool in the operating room and believe it to be even more accurate than corneal topography. Precise identification of the steep axis is especially critical for patients receiving piggyback toric lenses, since the effect caused by a slight off-axis placement is doubled.

Correct orientation of the lens is determined during surgery using the surgical keratometer. The axis marked on the lens is aligned with the steep axis identified with the surgical keratometer.

Correcting Rotation of the Piggyback Toric

System

If the toric lenses are placed off axis, or if they rotate postoperatively, they can be rotated back into position. IOLs usually do not rotate on their own once capsular fusion takes

Figure 15-3. Interlenticular opacification (ILO), or cellular ingrowth, into the piggyback lens interface.

place,15 and are difficult to rotate surgically after 2 weeks postoperatively, as breaking the adhesions risks zonular disruption.

When we surgically rotate toric lenses, we routinely use a 30-gauge needle on a syringe with Xylocaine (AstraZeneca, Waltham, Mass). We first anesthetize the cornea with topical Alcaine (Alcon, Fort Worth, Tex) and Xylocaine jelly. Using the 30-gauge needle, we enter the anterior chamber at the desired axis of rotation. We then remove a small amount of aqueous and reinject with Xylocaine to prevent the patient from jerking during the lens rotation. Before the lenses are rotated into the correct position, they are freed 360 degrees with a gentle rocking motion, loosening any adhesions. The tip of the needle is simply placed along the edge of the lenses, and both lenses are easily rotated into position.

COMPLICATIONS AND RISKS

Intralenticular Opacification

Intralenticular opacification (ILO), a long-term complication of piggyback lenses, has been reported.16-19 ILO is cellular growth between piggybacked lenses that is often characterized as Elschnig pearl formation, and may even result in a fibrous membrane formation between the lenses. ILO has been reported primarily in acrylic lenses with longterm follow up, although it has also been seen to a lesser degree in PMMA and silicone piggybacks.16-19

Gayton19 has reported an incidence of ILO of 43% among his acrylic piggybacks and 22% among his PMMA piggybacks. He has reported a number of cases with thick, opaque membranes that have severely impacted vision and

Figure 15-4. Polishing the capsule removes more lens epithelial cells and may reduce the incidence of ILO.

required surgical removal. Moreover, both Gayton18 and Shugar16 have reported a shift in refraction among cases with significant ILO.

We conducted a study of all our piggyback cases with at least 2-year possible follow-up and examined them at slit lamp to determine the extent of the problem in our practice. We examined 50 eyes of 34 patients, 22 eyes with piggybacked PMMA lenses, 19 with silicone, and 9 with 1 PMMA and 1 silicone lens.

We found 3 eyes, or 6%, showed signs of ILO, 2 with piggybacked PMMA (Figure 15-3), and 1 with mixed PMMA and silicone. In these 3 cases, we saw only mild interface growth, with no impact on visual function, no shift in refraction, and no symptoms of glare or shadows. We found no cases of ILO in double-silicone piggybacks, even with both in the bag.

We found a much lower incidence and severity of ILO in our practice than reported by Gayton18 or Shugar,16 which may be due either to a difference in IOL material, because we do not use acrylic, or to a difference in surgical technique. Dr. Apple has implicated incomplete removal of epithelial cells as a possible cause of ILO.16,19 Because we routinely polish the capsule in all our cataract cases (Figure 15-4), we effect a more complete removal of lens epithelial cells at surgery, which may have significantly lowered our incidence of ILO.

While we believe that polishing the capsule is an important step for all lens extraction cases, meticulous attention to removal of all epithelial cells is especially crucial in piggyback cases. While the causes of this complication are not yet well understood, and methods of treatment still under study, careful polishing of the capsule and removal of all lens epithelial cells may significantly lower the incidence and severity of the problem.

CONCLUSION

The piggyback lens implantation technique provides the opportunity to correct or reduce extreme refractive errors in cataract and refractive surgery patients. When using the piggyback technique in high hyperopes, the often unusual anatomy of short-eye patients presents unique challenges in power calculation and surgical technique. In high astigmats, the piggyback technique used with toric lenses can theoretically correct up to 5.00 D of cylinder, while suturing the lenses together avoids any possibility of counter-rotation. The risk of the long-term complication of interlenticular opacity, or cellular growth between the lenses, is reduced with the use of PMMA and silicone lenses, and meticulous cleaning of the capsule.

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