Introduction
The idea of removing the lens for refractive purposes dates back the 18th century. Abbé Desmonceaux was probably the first to propose such a surgery in France in 1776 for a patient with high myopia. In the last decades of the 19th century, the first systematically conducted operations of clear lens exchange were carried out by a Polish ophthalmologist named Vincenz Fukala in Vienna. Fukala’s indications for surgery included poor vision, inability to work, and myopia of −13 diopters (D) or higher. He operated on children with progressive myopia or young adults; the upper age limit was 40 years.
The first step of Fukala’s procedure was the dissection of the clear lens in order to observe a clear pupil at the end of surgery. Postoperatively, the eye was treated with atropine; then, several days were to elapse after washing out the swollen lens fragments. If symptoms of intraocular inflammation, pain, or photophobia arose, the remaining lens material was removed by needle. Following this procedure, most patients for the first time in their lives achieved robust visual acuity. Fukala recommended this surgery for both eyes to establish binocular vision. Despite many opponents, this operation was widespread among ophthalmologists in Europe. As a result of high rates of postoperative retinal detachment and other complication—such as ocular infection, retinal hemorrhages, and glaucoma—the procedure was gradually abandoned at the beginning of the 20th century.
Currently, as a result of advances in small-incision technique, cataract surgery has evolved from being primarily considered as a method of opaque lens removal to a procedure yielding the best postoperative refractive result. As the incidence of complications has significantly decreased, the use of lens removal as a refractive procedure has emerged. A significant advantage of lens refractive surgery compared to corneal surgery is that it covers a wider range of refractive errors. In high refractive errors both phakic intraocular lens (IOL) and refractive lens exchange (RLE) might be considered.
According to the European Registry of Quality Outcomes for Cataract and Refractive Surgery (EUREQUO), RLE is the second most frequently performed noncorneal refractive procedure. Furthermore, there has been a significant increase in number of RLE cases over time. The age of patients undergoing this procedure is older than patients having other refractive procedures; one of the reasons might be early cataract formation but also the possibility to correct presbyopia. This group of patients is socially better situated, thus able to afford this costly treatment. With increasing numbers of performed RLE procedures and overrepresentation of myopic patients, in certain western countries the population of phakic myopes reaching “cataract age” will likely be lower in future.
Pearls in Surgical Technique
The surgical technique for RLE is a modification of standard cataract surgery. The main differentiating elements are transparency and softness of the crystalline lens, absence of cataract, and presence of an abnormal ocular anatomy because of high refractive error. The best approach to RLE includes a minimally invasive surgery, either with coaxial or biaxial methods, through the smallest possible incision. Micro-incisional cataract surgery (MICS), with the final incision of 1.6 to 1.8 mm for IOL implantation, improves the visual and surgical outcomes while also reducing the risk of complications.
The procedure is typically performed under topical anesthesia. RLE can be conducted more conveniently and safely using a bimanual technique. In a biaxial procedure surgery, the steepest corneal meridian is marked, and two incisions are performed 90 degrees apart from each other. Currently, 19 G (1/1.1 mm) and 21 G (0.7 mm) instrumentation is employed for MICS. Thus a relatively wider incision should be made to enable unhampered manipulations within the anterior chamber (AC): 1.2 mm internally and 1.4 mm externally for 19 G tools and 1 mm for 21 G. One of the incisions should be located in the positive meridian of the cornea—it will be enlarged for IOL implantation. Another approach is to create a third incision for IOL implantation in the positive meridian shortly before IOL introduction into the eye. Likewise, with femtosecond laser-assisted cataract surgery (FLACS), the multiplanar clear corneal incision for IOL implantation is performed as a separate incision at the beginning of the procedure subsequent to the docking of the laser. It is believed that femtosecond (FS) corneal incisions exhibit less damage to the cornea and allow faster healing. Moreover, femtoincisions prove to be stable, more resistant to deformation and leakage, and reproducible at various intraocular pressures. Another advantage is that they do not change corneal high-order aberration significantly, featuring favorable results in triplanar configuration.
After the incision, a 1% preservative-free lidocaine solution diluted 1:1 with balanced salt solution or pure 1% lidocaine is injected into the AC. As a result of small incision size, the continuous curvilinear capsulorhexis (CCC) has to be carried out with a bent capsulotomy needle or 23-gauge vitrectomy-style micro-incisional capsulorhexis forceps. The capsulotomy construction is significant for the final position of the IOL. An inaccurate prediction of the effective lens position (ELP) has been identified as the biggest source of error in IOL power calculations. Hence, performing an FS laser capsulotomy might enhance the degree of circularity and improve IOL centration. Moreover, FS laser capsulotomies compared to manual CCC manifest less distortions over time. This is believed to provide a more stable refractive result, although no significant differences were observed for the ELP, corrected distance visual acuity, or refractive error.
Cortical cleaving hydrodissection should be performed in two distal quadrants. Although in RLE the nucleus is soft, the use of specially designed symmetric prechoppers, such as Alió-Scimitar MICS (Katena Inc.) might yield cutting the nucleus in half without placing any asymmetric pressure on the zonules. For RLE during phacoemulsification, high values of fluidics are recommended, though with low phaco power. Further, short power modulation techniques, such as hyperpulse or ultrapulse, may decrease the risk of corneal wound burn, as the off-time cycle permits cooling of the phaco-tip and cornea. The use of femtolaser-assisted lens fragmentation might be beneficial, as it decreases the energy required to emulsify the lens, ensuring that endothelial cells have less exposure to phaco-energy.
Following emulsification of the nuclear segments, the cortical material remaining in the capsular bag is removed with irrigation/aspiration. Separation of irrigation and aspiration in two independent handpieces prevents generating vortex currents at the end of the phaco-tip. Another advantage of the bimanual technique is the feasibility to remove the subincisional cortex without switching handpieces. It is worth highlighting that MICS enables outstanding AC stability, as the irrigation handpiece is constantly within the AC. As well, the impermeability of two smaller incisions is greater than with a larger incision. Thus the incidence of intraocular hypotony and the risk of AC collapse declines considerably, resulting in decreased risk of posterior vitreous detachment during surgery. This is likely quite beneficial, particularly in myopes.
The value of MICS is that it can be performed with most phacoemulsification platforms. The parameters favor fluidics with high levels of irrigation/aspiration pressure, rather than phaco power. A Venturi pump system may be recommended, as it provides fast reaction and great flexibility. Standard infusion tools could be insufficient regarding hydrodynamics; hence, particular MICS high-inflow tools should be employed. The major disadvantage of bimanual phacoemulsification lies in the current limitations of IOL technology.
IOL Power Calculation
The calculation of IOL power does not differ from those calculations made when a cataract is present. Yet, based on the absence of other ocular pathologies and long life expectancy, the patient may be more demanding. Furthermore, the loss of accommodation, particularly in pre-presbyopic patients, should be discussed meticulously. An alternative might be the use of multifocal or accommodative lenses. Optical coherence-based biometry with integrated keratometry has become a gold standard in IOL power calculations. The actual desired postoperative refraction should also be discussed since a small degree of myopia (−0.5 D) may be desirable in the case of monofocal IOL use.
The parameters taken into consideration for IOL calculations are the axial length of the eye, corneal curvature, and AC depth. The Haigis formula utilizes the formerly mentioned measurements and can be treated as the first choice for use ( Table 29.1 ). It has a rather small postoperative median absolute error and can be used with eyes of all axial lengths. Westin et al. claim that the use of Haigis’s formula resulted in better biometry prediction in the RLE cohort compared to patients undergoing cataract surgery. For eyes under 22 mm in axial length, the Hoffer Q formula should be applied for comparative assessment. The SRK-T formula manifests a lower predictive accuracy in short eyes. For that reason, it should be used for comparative purposes only in eyes over 22 mm of axial length. The Holladay 1 formula might be the second choice for eyes with an axial length of 22 to 26 mm. The Holladay 2 is a 4th-generation formula that takes into account the disparities in the anterior segment by adding the corneal white-to-white diameter and lens thickness. This might facilitate estimating the exact position of the IOL and shows benefits in eyes under 22 mm of axial length.
| Criteria | Axial Length < 22 mm | Axial Length 22 mm | Axial Length 24.5 mm | Axial Length > 26 mm |
|---|---|---|---|---|
| 1st choice formula | Hoffer Q, Haigis | SRK-T, Haigis | SRK-T, Haigis | SRK-T, Haigis |
| 2nd choice formula | Holladay 2 | Holladay | Holladay | − |
With most of the currently used devices for IOL calculation, the ability to determine true corneal power is limited. The relationship between the anterior and posterior corneal surfaces is fixed and estimated based on an empiric “keratometric index.” Such evaluation leads to overestimation of astigmatism in with-the-rule astigmatism, whereas in eyes with against-the-rule astigmatism it could be underestimated. Therefore assessing the optical power of the posterior corneal surface—specifically, its astigmatism—with a Scheimpflug analyzer could potentially increase the refractive outcome in RLE. This issue was known to be important in IOL calculations in eyes that underwent a corneal refractive surgery, as it removes corneal tissue. Subsequently, the relationship between the front and back surfaces of the cornea is altered, invalidating the use of this standardized index of refraction.
Another important issue after a corneal refractive procedure and in irregular corneas is the inaccuracy of keratometry—it performs the measurements from small regions or paracentral points of the cornea. This might be insufficient for a postsurgical cornea, that has a wide range of curvature, even in the central 3-mm region. The strategy for calculating IOL power following keratorefractive surgery depends on the preoperative data that can be obtained—in particular, preoperative keratometry and refractive error. These data should be employed for assessing the corrected postoperative keratometric values. The clinical history method is the most reliable to evaluate the corneal power after the refractive procedure. In this method, the spherical equivalent change is subtracted from the original lens power. Finally, the Holladay 2 formula is used for IOL calculation. Many strategies exist, and the recommended practice pattern is presented in Fig. 29.1 . It should also be noted that newer formulas—such as Camellin-Calossi, Shammas-PL, Haigis-L, and Barrett True-K—might provide a low prediction error. However, to date, a clear advantage of applying these formulas has not been proven.
