2008-09-13
Piggyback IOL Dr. Warren Hill specializes in Piggyback IOL Intraocular Lens Power Calculations. World Renowned (Renown) cataract surgeon. Piggyback IOL: The patient in this case underwent cataract extraction and IOL implantation in 1998 with the goal of postoperative monovision. Unhappy with monovision, she returned to the operating room in 2001, where her surgeon implanted a piggyback IOL to balance her refractive error. She now has two IOLs, or is polypseudophakic. The patient presented for a routine examination with dilation. With 20/20 vision in the operated eye and no complications noted, she was instructed to return for routine follow-up. The patient made an emergency visit for lid dermatitis of the contralateral eye, and the iris-captured piggyback IOL was noted. The patient was unaware of the abnormal pupil shape, but did note degradation in her vision (20/25-3). It is unclear whether the dilation 3 months prior allowed the IOL to be captured, but this is most likely the case. With piggyback IOLs, one IOL is implanted in the capsular bag, the other in the ciliary sulcus. In primary cataract surgery, the two lenses are used to create additional IOL power, or in secondary surgery they are used to change an unacceptable post-operative refraction. The patient’s surgeon repositioned the sulcus IOL without sequelae. It has not been decided whether a mild miotic is necessary to prevent the sulcus IOL from repositioning anteriorly again. Hence, in patients with known piggyback IOLs, caution must be exercised when considering pharmacological dilation.
When both patient and surgeon are unhappy with the postoperative refraction following cataract surgery, removing the original intraocular lens may not always be the best approach. When the first IOL is within the capsular bag, an ideal solution may be to place another IOL in the ciliary sulcus. This second lens has come to be known as a "piggyback" IOL.
According to Holladay, there are several reasons why this approach may be better than a lens exchange. First, removing the original lens may rupture the capsule and/or loosen zonules. Second, inserting a piggyback lens is technically much easier than attempting a lens exchange. And third, the true cause of the refractive error is usually unknown. If the original lens was mislabeled, then a lens exchange may further compound the refractive problem.
The refractive vergence formula
In 1997, Holladay elegantly described a method for pseudophakic and aphakic intraocular lens power calculations, independent of axial length.
Holladay JT: Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg 1997; 23: 1356 - 1370
When significant refractive deviations are seen, the refractive vergence formula is very helpful in understanding how much optical power must be added to, or subtracted from, an eye at the level of the anterior chamber, ciliary sulcus, or capsular bag. This formula also works well for the aphakic eye.
The power of the IOL to be implanted is determined by the following:
ELPo = effective lens position.
Ko = net corneal power.
IOLe = IOL power.
V = vertex distance.
PreRx = pre-op refraction.
DPostRx = desired post-op refraction.
The Effective Lens Position (ELPo) is the distance from the secondary principal plane of the cornea to the principal plane of the thin-IOL equivalent. The keratometric power of the cornea (Kk) is converted to the net optical power of the cornea (Ko) as follows: Ko = Kk * 0.98765431
If the keratometric power (Kk) is 44.50 D, then...
Ko = 44.50 D * 0.98765431 = 43.95 D
The net optical power of the cornea (Ko) would then be 43.95 D
For a simple Excel spreadsheet with the refractive vergence formula, go to our Download Area and download the R-verg-Hill.xls (Microsoft Excel file).
Let's say, for example, that during cataract surgery, the operating room
the staff mistakenly handed you a +22.00 D posterior chamber lens when the calculations
called for a +18.00 D lens to be placed in the capsular bag (ELPo = 5.55 mm).
Not a surprise, the patient is now -3.25 D more myopic than planned. In spite
of aniseikonic bases curves, the smallest eye size possible and a very close
vertex distance, the image disparity cannot be tolerated and she is requesting
that you find some other solution. With postoperative keratometry of 44.25/44.75
x 090, the refractive vergence formula would tell you that a -4.00 D posterior
chamber lens, placed in the ciliary sulcus (ELPo = 4.80), will achieve a postoperative
refraction close to -0.25 D.
Another example. If an aphakic patient had a refraction of +12.50 D sphere (vertex distance of 10 mm) and keratometry of 45.00/45.00 x 090, it would take a +19.50 D anterior chamber IOL (ELPo = 3.50 mm) to achieve a postoperative refraction of approximately -0.25 D.
In our practice, we typically use the refractive vergence formula for this type
of IOL calculation. The Holladay IOL
Consultant has a much more sophisticated form of the refractive vergence
formula (known as Holladay R), which is highly recommended.
Formula
