Cataract surgery in patients with a history of corneal refractive surgery (i.e. RK, Lasik, PRK).
The surgical procedure in these patients is identical to all cataract procedures, but the refractive outcome, focusing ability without glasses, is much less predictable.
The calculations to determine the proper IOL power in these patients is much less accurate than in patients with no history of corneal refractive surgery.
The calculation of IOL power in part depends on the shape of the corneal surface, measured by keratometry.
Keratometry measurements combined with other parameters, i.e., length of eye, are designed to give the patient 20/20 vision at a specific working distance. Various formulas are available for this purpose (see “lens calculators”).
Corneas with no history of refractive surgery have a fairly uniform, central curvature (power) in contrast to post refractive surgery corneas where the corneal power has more variability making the effective corneal power calculation much less accurate.
The second major source of error in the accuracy of the power of the IOL is the “effective lens position” (ELP). This variability is the same for all patients, with or without corneal refractive surgery. The ELP is the final distance between the cornea and the implant after healing is complete, approximately 2 months. The behavior, contraction, fibrosis, of the lens capsule (remaining part of the original cataract) determines the ELP and may vary from patient to patient and is not totally predictable prior to surgery.
The ELP can be approximated based on the specific IOL design and from historical data about this distance. So in patients with previous corneal refractive surgery, the ELP uncertainty adds to the corneal power uncertainty as far as the final errors in refractive, focusing outcome.
In addition to traditional refractive surprises that may cause problems with premium IOL patients, a significant number of patients are also dissatisfied after newly developed multifocal IOL implants which is limiting the use of this type of premium lens by cataract surgeons. In addition to even small residual refractive errors in these patients, the reasons for dissatisfaction among these patients also include, but are not limited to, intolerance of this new optical system (halos, glare, lack of neuroadaptation). In addition, over time, the possible development of unrelated retinal or optic nerve disease make this optical system undesirable.
The solutions that may work for refractive surprises alone such as limbal incisions or laser touch-ups won’t work for most of these patients.
However, these issues can be addressed by an IOL explantation and replacement with a different type of optic, but surgeons face a dilemma in deciding on the timing of the second surgery. On the one hand, if the exchange is done too early, patients may not have had enough time to adapt to the multifocal implant. On the other hand, if the exchange is done too late, the exchange procedure becomes more difficult with an increased risk of complication.
All of these potential errors can be corrected after the primary cataract procedure if the primary surgery utilized the Precisight® lens system. This correction involves replacing the front lens element with a new front lens based on the final refractive, focusing, outcome surgical procedure. This enhancement procedure would also be applicable to patient dissatisfaction, lack of neuro-adaption, to the optical system used in the primary surgery (monovision). The enhancement procedure utilizes the same surgical incision as was used in the primary surgery, and does not change the ELP. Therefore, the refraction at the time of enhancement represents the only variable and therefore the enhancement outcome should be extremely accurate, predictable.
The Precisight® IOL system, with its exclusive docking station technology, allows partial IOL exchange without any time limits since only the front optic is removed and exchanged. Time, healing, does not affect the difficulty of this exchange procedure. IVO is currently working on a multifocal front optic, in order to help surgeons to allow their patient implanted with multifocal IOL to have several safe surgical alternatives at any postoperative time interval.