Once successful vitrectomy and cataract extraction have been achieved, the appropriate intraocular lens (IOL) power calculation must be performed to select a suitable IOL. To achieve a desired postoperative refraction, the selection must include consideration of the optical shape of the lens (biconvex, convexplano, planoconvex, or meniscus) and the IOL placement (bag or sulcus), and it is dependent on the preceding determination of the appropriate power. These considerations are directly dependent on the media in the vitreous compartment, that is, vitreous, balanced salt solution, silicone oil, or gas. Because these media have different indices of refraction, they will have a dramatic impact of the amount of refraction occurring at the back surface of the IOL as well as the speed that light travels through the posterior compartment (posterior IOL to retina).

SELECTING THE CORRECT OPTICAL

SHAPE OF THE IOL

BSS

The most common replacement material for the vitreous is BSS. Because it has a refractive index similar to vitreous (this is due to the high water content of the vitreous), index of refraction adjustments to power are unnecessary. Therefore, only the location of the lens will dictate IOL power. If the IOL is to be placed in the sulcus, the power should be decreased by approximately 0.5 diopter (D) for a standard-power IOL to maintain the desired refractive outcome. The sulcus is approximately 0.25 mm anterior to the in-the-bag plane, so the effective anterior chamber depth (ACD) is about 0.25 mm less. For a 20-D lens it is a 0.50-D difference, but for a 40-D lens it is a 1.0-D difference, and for a 10-D lens it is only a 0.25-D difference. If the IOL is placed in the bag or the optic is captured beneath the anterior capsule, no power change is necessary.

LENS MATERIAL AND POSITION

The IOLs manufactured today are biconvex, convexplano, planoconvex, or meniscus.1 As mentioned above there are two effects of changing the media in the posterior compartment of the eye. The first effect is a result of the change in the effective power of the posterior surface of the IOL due to a refractive medium other than the normal vitreous. The second effect is due to the light traveling through the posterior compartment at a speed different from that of the vitreous. These two effects must be considered independently because they are uniquely different for each lens shape and material.

Foldable three-piece IOLs have haptics, which, whether extruded polymethylmethacrylate (PMMA) or polypropylene, have almost no memory. They therefore remain at the same angulation in the ciliary sulcus. In some patients with crowded anterior segments, this may be the source of iris chafe leading to iris depigmentation and pigmentary glaucoma. Plate silicone IOLs are contraindicated for sulcus fixation. Polypropylene haptics, when placed in the ciliary sulcus, may erode into the sulcus, and the material has been implicated as a cause of low-grade inflammation. Therefore, the best choice for small-incision

IOL implantation would be a foldable optic material, with the optic captured within the anterior or posterior capsule to prevent iris chafe, combined with extruded PMMA haptics for sulcus compatibility. If optic capture is impossible, the incision should be enlarged and a one-piece PMMA IOL with at least 2.5-degree angulation or a 0.5-mm step vault should be implanted. This will prevent iris chafe. The haptic should have a large radius of curvature for broad sulcus contact and stability. In addition, an optic of 6.0 mm should be considered in an effort to minimize the effects of possible IOL decentration resulting in edge glare.

SILICONE OIL

The second most common replacement of the vitreous is with silicone oil. The difference in the refractive outcome with silicone in the vitreous compartment is due to the difference in the refractive index of silicone (1.405 to 1.420) for silicone oil and vitreous (1.336). Because silicone oil has a much higher index of refraction than vitreous, the silicone always reduces the back-surface power of the IOL compared to the power with vitreous. A flat (plano) back surface is the only curvature that is unaffected by the silicone oil. It is therefore recommended that convexplano lenses be used when silicone is in the posterior compartment to eliminate the effect of the posterior surface power. This shape also reduces the refractive change that occurs when the silicone is removed.

An excellent study on the ideal shape of an IOL when silicone oil is used demonstrated that a meniscus lens can actually neutralize both the shape and path length effect of silicone, such that when the silicone oil is removed there is no optical effect.2 Meniscus lenses, however, have many other optical disadvantages such as spherical aberration. More importantly, they have the highest degree of induced astigmatism when tilted.3,4 Because of these factors the lens shape of choice still remains the convexplano IOL.

Because biconvex lenses may already be in place when a vitrectomy is performed, a discussion of the effect on biconvex IOLs is appropriate. The reduction in power of the biconvex IOL will be a function of the difference in the index of refraction of the IOL and silicone oil compared to the IOL and vitreous. If the IOL is made of PMMA with an index of refraction of 1.491, then the back surface power of the IOL would be proportional to 0.155 (1.491 1.336) when vitreous is in the eye and 0.086 (1.491 1.405) when silicone is in the eye. The ratio of 0.086 to 0.155 is 0.555, indicating that the backsurface power of a bi-

convex IOL will be reduced to 55.5% of its original power. For an equiconvex 20-D IOL with each surface 10 D, the backsurface becomes 5.55 D, so a 4.45- D hyperopic shift occurs due to the reduction in backsurface power of the IOL.

The reduction in power of the light rays passing through the silicone is proportional to the length of the vitreous cavity. For normal length eyes, where the vitreous cavity in pseudophakia is about 18.25 mm, the focal length is increased by about 1 mm or about 3 D. The total shift for an equiconvex PMMA IOL would therefore be approximately +7.45 D; +4.45 D due to the backsurface power and +3.00 D due to the path length in silicone.

It is now easy to see why the hyperopic shift can be minimized by using a convexplano IOL, in which the posterior surface is plano. This configuration of the IOL eliminates the reduction in power of the posterior surface and leaves only the factor due to the path length of silicone, about 3 D. It also minimizes the change in refraction when the silicone is removed. This condition is one of the only indications for specifically implanting a convexplano IOL. Convexplano lenses can be recognized without all of the specifications from the company because their A constants are usually from 116.0 to 116.5 (ACD from 4.0 to 4.5 mm), whereas biconvex lenses have A constants that usually exceed 118 (ACD of 5.0 mm). However, it is always best to check with the manufacturer to be sure.

SELECTING THE PROPER MATERIAL

OF IOL TO BE COMPATIBLE WITH

SILICONE OIL

Finally, silicone oil has been shown by Apple5 to have an affinity for currently available soft IOLs (silicone and acrylic). The silicone oil adheres to the IOL unevenly, reducing the quality of the image formed on the retina dramatically. Surface-modified and hepa- rin-treated PMMA has the lowest affinity. The recommended IOL is therefore convexplano and is a surface modified PMMA IOL. Although the incision size must be a few millimeters larger with PMMA compared to foldable IOLs, the optical benefits of PMMA in the presence of silicone oil far outweigh the disadvantages.

AXIAL LENGTH MEASUREMENTS

INVOLVING SILICONE OIL

To perform an IOL power calculation, an accurate measurement of the axial length is necessary. Accurate silicone measurements cannot be made with

silicone oil in the eye because the oil is extremely dense acoustically, attenuating most of the ultrasound energy. The only instrument that is capable of performing accurate axial length calculations in an eye filled with silicone oil is one that performs laser interferometry (Zeiss IOLMaster). If this is not available, ultrasonic axial length measurement is unreliable because the speed of sound through the silicone oil is also much slower (980 m/sec to 1080 m/sec) compared with vitreous or saline where the speed is 1532 m/sec. Although adjustment for the sound velocity can be made, the measurements are quite variable and often misleading as to the true axial length.6

It is better to measure the eye before silicone oil is injected so that the true axial length is known. More and more retina surgeons are performing A scans before retina surgery to have an accurate axial length measurement. If silicone oil is already present and the axial length is unknown, then it is better to use the axial length from the other eye for the calculation. If silicone is present in both eyes, then it is best to guess an average axial length based on the patient’s refraction before any signs of cataract. Because the normal eye is 23.5 mm in axial length, 1 mm is added for every 3 diopters of axial myopia. Although these are only approximations, they are usually much more accurate than those measured with silicone oil in the eye.

IOL POWER CALCULATIONS

INVOLVING SILICONE OIL

Once the axial length has been accurately determined, then an IOL calculation can be performed. IOL power calculations should be performed using a thirdor fourth-generation theoretical IOL formula like the Holladay 1,7 SRK/T,8 Hoffer Q,9 or the Holladay 210 (fourth-generation IOL formula). A decision must be made for the appropriate target postoperative refraction for the patient. The major consideration is when or if the silicone oil is to be removed. In most cases this is not known, so it is best to consider that the silicone oil will never be removed and choose the target refraction based on this assumption. However, in a few cases, it may be planned to remove the silicone within a few weeks so the patient may be +3 D hyperopic with a convexplano lens while the silicone is in place. Once the silicone is removed, the patient will be plano. If the patient is emmetropic with the silicone, the refraction would be 3 after the silicone is removed. Although it is always possible to perform a lens exchange or secondary piggyback IOL, it is always best to try to minimize the number of surgeries on the eye and make these decisions in advance.

Once the target has been chosen, then the IOL power calculation is performed in the normal manner using the latest generation theoretical IOL formulas. Once the normal IOL is determined, the back surface and path length adjustments are made as described above and the final IOL power determined. For example, in the case above, if the “normal” IOL power is 21 D for a convexplano IOL, then the required power with silicone would be 24 D (21 + 3). If an equiconvex lens is present, then the power would need to be 31.46 D (24 + 3 + 4.46). If the patient is emmetropic with the silicone and the biconvex lens, the refraction would be almost +7.5 D of hyperopia upon removal of the silicone.

IOL POWER CALCULATIONS

INVOLVING GASES

Although gases such as air and perfluorocarbons are used, these are usually temporary, and there are problems with vision itself when gases are present in the posterior compartment.11 However, it is still worth evaluating the refraction changes that occur when gas is used to replace the vitreous. The indices of refraction of any gases used in the eye are virtually the same as air—1.000. The lower index of refraction has the reverse effect of the silicone—the posterior surface power of the IOL increases and the speed of light through the gas is much faster. Because the differences are almost three times greater than the change with silicone, the effects are almost three times as large. The result is that patients who are emmetropic with vitreous become extremely myopic (10 to 30 D) with convexplano and biconvex lenses, respectively. Because the gas ultimately absorbs and is replaced with aqueous, most surgeons simply choose the normal implant power, expecting the myopia during the absorption period.

CONCLUSION

The optimal IOL for implantation after vitrectomy is dependent on the vitreous substitute.

For BSS, with sulcus placement and optic capture within the anterior or posterior capsule, a biconvex foldable 6-mm optic, 11.5-mm extruded PMMA haptic (e.g., AMO SI 40NB or Alcon MA60) is indicated. For BSS, with sulcus placement without optic capture, a biconvex one-piece angulated PMMA IOL with a 6-mm optic and a long radius curved 11.5-mm haptic (e.g., AMO PS43NB) is indicated.

For silicone, a surface modified PMMA convexplano lens is indicated. The effective power of the IOL will be approximately 3 D less when the silicone is in place. Most surgeons try to leave the patient

emmetropic to slightly myopic with the silicone and deal with the refractive myopia of 3 D if and when the silicone is removed. The postsilicone removal myopia may be eliminated by refractive surgery (LASIK), lens exchange, or secondary piggyback IOL. Biconvex lenses have a much greater change and are therefore not recommended in these cases.

For gas, due to its temporary usage, a standard IOL may be utilized.

REFERENCES

1.Holladay JT. International intraocular lens registry. J Cataract Refract Surg 1999;25:128–136.

2.McCartney DL, Miller KM, Stark WJ, Guyton DL, Michels RG. Intraocular lens style and refraction in eyes treated with silicone oil. Arch Ophthalmol 1987; 105:1385–1387.

3.Holladay JT. Evaluating the intraocular lens optic. Surv Ophthalmol 1986;30:385–390.

4.Holladay JT, Bishop JE, Prager TC, Blaker JW. The ideal intraocular lens. CLAO J 1983;9:15–19.

5.Apple DJ, Isaacs RT, Kent DG, et al. Silicone oil adhesion to intraocular lenses: an experimental study comparing various biomaterials. J Cataract Refract Surg 1997;23:536–544.

6.Holladay JT. Standardizing constants for ultrasonic biometry, keratometry and intraocular lens power calculations. J Cataract Refract Surg 1997;23:1356– 1370.

7.Holladay JT, Prager TC, Chandler TY, Musgrove KH, Lewis JW, Ruiz RS. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg 1988;14:17–24.

8.Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990;16:333–340.

9.Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993;19:700–712.

10.Holladay JT, Gills JP, Leidlein J, Cherchio M. Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses. Ophthalmology 1996;103:1118–1123.

11.Mansour AM, Holladay JT. Visual acuity and intraocular gas. Arch Ophthalmol 1988;106:1345–1346.