Microphacoemulsification
INTRODUCTION
Cataract surgery is the most common intraocular surgery procedure performed in the world. From 1967, year in which Kelman introduced the phacoemulsification technique,1 begun a new era for cataract surgery: more invasive procedures were abandoned, and we assisted in a decrease in incision size from the 10.0 mm required for the intracapsular cataract extraction to 7.0 for extracapsular cataract extraction and ultimately to the small incisions (3.2 to 2.8 mm) used for phacoemulsi-fication.1, 2 The use of smaller surgical instruments, foldable intraocular lenses (IOLs), and more advanced management software for the phaco units, further allowed to reduce incision size and tissue trauma and to promote faster functional recovery. Clinical trials have found that the length of the incision is directly proportional to the amount of induced astigmatism and inversely proportional to its stability over time.3 The bimanual microincision phacoemulsification technique is a less invasive variation of traditional coaxial phacoemulsification and allows cataract extraction through incisions of 1.5 mm or smaller.4, 5
So nowadays, cataract surgery, is considered both a therapeutic procedure to remove cataract and a refractive surgery, as patients often claim to obtain a postsurgical excellent visual rehabilitation increasingly refractive expectations. A remaining problem, however, is the accurate calculation of intraocular lens (IOL) power which is necessary for attaining the desired postoperative refraction. Nevertheless, in surveys of members of the American Society of Cataract and Refractive Surgery and the European Society of Cataract and Refractive Surgeons about the complications of foldable IOLs that require explantation or secondary intervention, incorrect IOL
power is the most common indication. For the three most commonly used IOLs (3-piece acrylic, 3-piece silicone, and 1-piece acrylic), incorrect power is the leading indication for removing or exchanging an IOL. These trends have been consistent over the past 4 years.6
Highly accurate IOL power calculations result from optimizing a collection of interconnected details. The keratometry technique, method of axial length measure-ment, IOL power calculation formula, optimized lens constant, and configuration of the capsulorhexis, all individually influence the final refractive outcome. For this reason, focusing on a single item such as the axial length measurement or the IOL power calculation formula is usually insufficient to ensure consistent accuracy over a wide anatomical range. The surgeon must consider the process as a whole while simultaneously optimizing each component.7
KERATOMETRY
Ophthalmologists and often accept without question corneal power measurements by keratometry or simulated keratometry, but not all measurements have the same level of accuracy or reproducibility. It should be remembered that keratometry errors have a 1:1 correlation with postoperative refractive errors at the spectacle plane. For example, if the keratometry reading is off by 0.50 D, the result will be a 0.50 D postoperative refractive error at the spectacle plane, even if all other aspects of the IOL power calculation and surgery are perfect. Add in other small errors such as variable corneal compression induced by applanation A-scan biometry or the use of an older 2 variable formula in axial hyperopia, and a 1.00 D deviation from the target refraction is not difficult to imagine.8
To maximize keratometry accuracy, first, make the decision to use a single instrument for all pre- and postoperative measurements in order to limit the number of variables. For manual measurements, switching to a Javal-Schiotz–style keratometer will to help improve accuracy. Autokeratometry is quick and easy, but it typically requires multiple measurements to confirm accuracy. The simulated keratometry feature of many topographers is an excellent way to objectively determine the axis of astigmatism, but it can sometimes be less accurate than careful manual keratometry for measuring the central corneal power.
Second, regularly check your keratometer against a set of standard calibration spheres and consider keeping a logbook of these evaluations . Third, if the results for any patient vary, ask a second staff member to confirm the measurements to ensure accuracy. Finally, if the keratometry mires are unreliable or distorted, obtaining a topographic axial map may help uncover something unsuspected such as a false form of keratoconus.
AXIAL LENGTH MEASUREMENTS
One of the most common reasons for an incorrect IOL power is an error in the axial length measurement. The familiar and trusted 10 MHz applanation A-scan biometry is probably no longer accurate enough to consistently satisfy contemporary patients’ expectations.
The reason is that measurements by the applanation technique produce a falsely short axial length and sometimes widely different results due to varying degrees of corneal compression and axial alignment.
Immersion A-scan biometry is unquestionably a more reliable method. This technique causes no corneal compression and measurements can be of very high quality and quite reproducible.
At present, optical coherence biometry using the IOL Master (Carl Zeiss Meditec AG, Jena, Germany) is unquestionably the most accurate way to measure axial length prior to cataract surgery. Optical coherence biometry’s use of a short-wavelength light source (instead of a longer wavelength sound beam) increases axial length measurement accuracy when compared with ultrasound.9 For challenging axial length measurements (e. g., in eyes containing silicone oil, extremely short nanophthal-mic eyes, or extremely long myopic eyes with posterior staphyloma), the accuracy of optical coherence biometry is unparalleled. The one disadvantage of the technique is that it is an optical method. Axial opacities such as a
corneal scar, dense posterior sub capsular plaque, or vitreous hemorrhage may decrease the signal-to-noise ratio to the point that reliable measurements are not possible. In the typical North American ophthalmology practice, optical coherence biometry is unable to measure between 5% and 15% of patients, and immersion ultrasound is required.
IOL CONSTANT OPTIMIZATION
Surgeons must personalize the lens constant (Holladay 1 Surgeon Factor; SRK/T A-constant; Holladay 2 or Hoffer Q anterior chamber depth; Haigis a0, a1, and a2) for a given formula in order to make adjustments for a variety of practice-specific variables, including different styles of IOLs, keratometers, and variations in A-scan biometry calibration. Most IOL power calculation programs provide either internal software or specific recommendations for how to go about lens constant optimization.10
SURGICAL TECHNIQUE
The configuration of the capsulorhexis can affect refractive outcomes if a surgeon is implanting a single-piece acrylic or a three-piece assembled IOL. If the capsulorhexis’
diameter is larger than the lens optic, the forces of capsular bag contraction may anteriorly displace the IOL, a situation resulting in an increased effective lens power and more myopia than anticipated. A simple “rhexis rule”
is that the capsulorhexis should be round, centered, and slightly smaller than the optic. In order for the IOL power calculation formula to be most consistent and accurate, the capsular bag should completely contain the IOL.
Attention to this detail can help maximize refractive accuracy.11
In this chapter we will describe our experience of intraocular lens power calculation in that patients who underwent cataract surgery with bimanual micropha-coemulsification technique.
PATIENTS AND METHODS
A retrospective review was conducted of 690 consecutive eyes with cataracts of grade 2 to 4 according to LOCS III classification operated with bimanual microphacoemul-sification and IOL implant into capsular bag by one surgeon (GMC).
Inclusion criteria were: transparent central cornea, good preoperative pupil dilation, no history of previous eye surgery or glaucoma, no history of retinal disease, astigmatism lower than 3.0 diopters (D).
Accuracy of Intraocular Lens Power Calculation in Bimanual Microphacoemulsification
161
Preoperatively all eyes had ocular biometry with IOL Master (Carl Zeiss Inc.) (Fig. 25.1) and ultrasound biometry (Fig. 25.2) when no reading could be taken with IOL Master especially those with dense cataracts or sub capsular opacities; manual keratometry was obtained in all cases.
Eyes were stratified into groups of short, average and long axial length (< 22 mm, 22 to < 24,5, and > 24,5 respectively). Data were entered into IOL software program and we chose Hoffer Q formula for short eyes, SRK II formula for average eyes and SRK T formula for long eyes.
We compared the predicted final spherical equivalent (SE) refractive error in each eye with the actual postoperative manifest refraction SE and calculated the difference as
the error; final refraction was performed 4 weeks after surgery.
SURGICAL TECHNIQUE
Two 1.4 mm trapezoidal incisions were made in the clear cornea at 10 o’clock and 2 o’clock with a precalibrated diamond knife (E. Janach) (Fig. 25.3). A continuous curvilinear capsulorhexis (CCC) with a diameter between 5.0 mm and 6.0 mm was made with a cystotome. Hydro-dissection was performed with a 26 gauge cannula and phacoemulsification, with a 20 gauge, 30 degree-angled sleeveless probe and an irrigating chopper (E. Janach) (Fig. 25.4). Phaco fracture was by the stop-and-chop technique. Irrigation/aspiration (I/A) was performed with a 20 gauge probe with an oval section (American Medical Optics) introduced through the microincisions.12,13 Gradual suction of the cortical remnants and epinucleus was done with the aspiration probe in the dominant hand and the irrigation probe in the other, using the continuous infusion mode to avoid sudden collapse of the anterior chamber. With the irrigation probe, the lens fragments were directed toward the aspiration probe to simplify the procedure and lower turbulence in the anterior chamber.
Fig. 25.1: IOL master
Fig. 25.2: Ultrasound biometry Fig. 25.3: Microincision with precalibrated diamond knife
Fig. 25.4: Bimanual microphaco technique
Fig. 25.5: IOL for microincision (1.8 mm) implanted
Table 25.1: Surgical parameters for the Sovereign WhiteStar setup
Parameter Values
Power (%) 20 – 25
Aspiration flow (cm3/min) 24 – 28 Vacuum (mmHg)
Unoccluded 60 – 250
Occluded 80 – 300
Cortical remnant I/A (mmHg) 450 I/A = irrigation/aspiration
After the half of the capsular bag opposite the entry site was polished with the aspiration probe, the instruments were passed from one hand to the other, retracting the aspiration probe first.14 The remaining capsular bag was cleaned in the same way. Then, the IOL was inserted through a third 1.8 mm incision created at 12 o’clock (Fig. 25.5).
The phacoemulsificator used (Sovereign WhiteStar, AMO Inc, CA, USA) was the same for all surgeries and was used with the same setting parameters (Table 25.1).
We implanted two different foldable, acrylic, micro-incision IOLs: Akreos MI60 (Fig. 25.6) and Acri.Smart 48S (Fig. 25.7). The first IOL has hydrophilic properties and has a great uveal biocompatibility, while the second one is hydrophobic, and has a better capsular biocompatibility (Table 25.2).15-17
RESULTS
The results are underlined in Table 25.3. We haven’t note differences than data reported in literature.7-10
Fig. 25.6: Akreos M160
Fig. 25.7: Acri.Smart 48 S
Table 25.2: Technical features of implanted IOLs Characteristics Akreos MI60 Acri.Smart 48S
Shape single piece single piece
Optic biconvex, aspheric standard, anterior and posterior biconvex, symmetric
Optic diameter 6.0 5.5 mm
Overall length 10.7 mm 11.0 mm
Haptic angulation 10° 0°
Material foldable acrylate foldable acrylate
Surface hydrophilic hydrophobic
A-costant (optic) 118.9 118.3
Accuracy of Intraocular Lens Power Calculation in Bimanual Microphacoemulsification
163
Table 25.3: Mean absolute error by formula
Mean Absolute difference, predicted vs actual postoperative SE refraction (D) ± SD
Axial Length, mm (range) Eyes Hoffer Q SRK II SRK T
< 22 (20.71-21.95) 31 0.62 ± 0.53 -
The smaller incision used for cataract extraction today make the surgery less invasive and safer, result in less invasive and safer resulting in less postoperative intraocular inflammation, fewer incision-related complications, lower surgical induced astigmatism and shorter total surgical time. These factors provide faster postoperative visual recovery and increased patient satisfaction. Increasingly, patients expect good refractive outcomes after cataract surgery in adition to the therapeutic benefits from treating the pathology.
Bimanual microincision phacoemulsification is an effective and safe technique to manage all types of cataract.
Also the accuracy of IOL calculation for new microincisional intraocular lenses is at least comparable to results reported in literature.
REFERENCES
1. Kelman CD. Phaco-emulsification and aspiration; a new technique of cataract removal; a preliminary report. Am J Ophthalmol 1967;64:23–35.
2. Paton D, Ryan S. Present trends in incision and closure of the cataract wound. Highlights Ophthalmol 1973;14:3–10.
3. Aliò JL. What does MICS require? The transition to microincisional surgery. In: Aliò JL, Rodriguez-Prats JL, Galal A, eds, MICS Micro-Incision Cataract Surgery. El Dorado, Republic of Panama, Highlights of Ophthalmology International, 2004;1–4.
4. Cavallini GM, Masini C. Microfacoemulsificazione bimanuale; origine e definizione. In: Cavallini GM, eds, Microfacoemulsificazione Bimanuale nella Chirurgia della Cataratta. Modena, Athena Ed, 2006;15–16.
5. Cavallini GM, Lugli N, Campi L, et al. Surgically induced astigmatism after manual extracapsular cataract extraction or after phacoemulsification procedure. Eur J Ophthalmol 1996;6:771-78.
6. Mamalis N. Complications of foldable IOLs requiring explantation or secondary intervention—2001 survey update.
J Cataract Refract Surg 2002;28:2193-201.
7. Kiss B, Findl O, Menapace R, Wirtitsch M, Drexler W, Hitzenberger CK and FercherAF: Biometry of cataractous eyes using partial coherence interferometry: clinical feasibility study of a commercial prototype I. J Cataract Refract Surg 2002;28:224–29.
8. Rajan MS, Keilhorn I, Bell JA. Partial coherence laser interferometry vs conventional ultrasound biometry in intraocular lens power calculations. Eye 2002;16:552–56.
9. Findl O, Drexler W, Menapace R, Heinzl H, Hitzenberger CK
& FercherAF: Improved prediction of intraocular lens power using partial coherence interferometry. J Cataract Refract Surg 2001;27:861–67.
10. Narvaez J, Zimmerman G, Doyle Stulting R, Chang D.
Accuracy of intraocular lens power prediction using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas. J Cataract Refract Surg 2006;32:2050-53.
11. Holladay J. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg 1997;23(9):1356-70.
12. Cavallini GM, Campi L, Masini C, Pelloni S, Pupino A.
Bimanual microphacoemulsification versus coaxial mini-phacoemulsification: Prospective study. J Cataract Refract Surg 2007;33:387-92.
13. Gimbel HV, Neuhann T. Development, advantages and methods of the continuous circular capsulorhexis technique.
J Cataract Refract Surg 1990;16:31.
14. Fine IH. Corneal tunnel incision with a temporal approach.
In: Fine IH, Fichman RA, Grabow HB. Clear corneal cataract surgery and topical anesthesia. Slack, Thorofare: NJ, 1993:
5-26.
15. Cavallini GM. Microfacoemulsificazione bimanuale; tecnica bimanuale. In: Cavallini GM, Eds. Microfacoemulsificazione Bimanuale nella Chirurgia della Cataratta. Modena, Athena Ed, 2006.
16. Tsuneoka H. Minimally Invasive bimanual phaco surgery and foldable IOL implantation through the smalllest incision.
In: Garg A, Fine IH, Chang DF, Tsuneoka H, Eds. Step by Step Minimally Invasive Cataract Surgery. Jaypee Brothers Medical Publishers (P) LTD; New Delhi, 2005;186-209.
17. Cavallini GM, Pupino A, Masini C, Campi L, Pelloni S.
Bimanual microphacoemulsification and Acri.Smart intraocular lens implantation combined with vitreoretinal surgery. J Cataract Refract Surg 2007;33(7):1253-58.