Ординатура / Офтальмология / Английские материалы / Mastering theTechniques of Lens Based Refractive Surgery (Phakic IOLs)_Garg, Alio, Dementiev_2005

Accurate biometry also plays an important role in IOL power determination. The use of partial coherence interferometry (IOL Master, Carl Zeiss Meditec, Dublin, CA) for axial length measurement improves the predictive value of postoperative refraction,21 and it has been shown equivalent in accuracy to immersion ultrasound.22

It is interesting to note the smaller difference between simulated keratometry and the Eff RP in the LTK group as compared to the incisional keratorefractive surgery groups. One possible explanation of this difference is that the LTK corneas had undergone regression from treatment and therefore returned to a less distorted anatomy.

The IOL calculation formula plays a critical role in obtaining improved outcomes. The Holladay II formula is designed to improve determination of the final effective lens position by taking into account disparities in the relative size of the anterior and posterior segments of the eye. To accomplish this goal the formula incorporates the corneal white-to-white measurement and the phakic lens thickness, and uses the keratometry (or Eff RP) values not only to determine corneal power but also to predict effective lens position. We have found that the use of the Holladay II formula has increased the accuracy of our IOL power calculations.23

Our study has been limited to eyes which have undergone incisional and thermal keratorefractive surgery. Ongoing research will help to determine the most effective methods of calculating IOL power in eyes which have had lamellar keratorefractive surgery such as photorefractive keratectomy or laser in situ keratomileusis. It appears that further modification is necessary in these situations because of the inaccuracy of the standardized values of index of refraction.24

We continue to tell our patients as part of the informed consent process that IOL calculations following keratorefractive surgery remain a challenge, and that refractive surprises do occur. We explain that further surgery (e.g., placement of a piggyback IOL) may be necessary in the future to enhance uncorrected visual acuity. We defer any secondary procedures until a full

three months postoperatively and document refractive stability before proceeding.

REFERENCES

1.Drexler W, Findl O, Menapace R, et al. Partial coherence interferometry: a novel approach to biometry in cataract surgery. Am J Ophthalmol 1998;126:524-34.

2.Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens power calculation according to Haigis. Graefes Arch Clin Exp Ophthalmol 2000; 238:765-73.

3.Giers U, Epple C. Comparison of A-scan device accuracy. J Cataract Refract Surg 1990;16:235-42.

4.Watson A, Armstrong R. Contact or immersion technique for axial length measurement? Aust NZ J Ophthalmol 1999; 27:49-51.

5.Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification. J Cataract Refract Surg 2001; 27:188-97.

6.Sanders DR, Retzlaff JA, Kraff MC. A-scan biometry and IOL implant power calculations. Focal Points. San Francisco, CA, American Academy of Ophthalmology 1995; 13(10).

7.Fenzl RE, Gills JP, Cherchio M. Refractive and Visual Outcome of Hyperopic Cataract Cases Operated on Before and After Implementation of the Holladay II Formula. Ophthalmology 1998; 105:1759-64.

8.Hoffer KJ. Intraocular lens power calculation in radial keratotomy eyes. Phaco and Foldables 1994;7(3):6.

9.Holladay JT. Understanding Corneal Topography, The Holladay Diagnostic Summary, User’s Guide and Tutorial, EyeSys Technologies, Inc, Houston, TX, 1995.

10.Celikkol L, Pavlopoulos G, Weinstein B, Celikkol G, Feldman ST. Calculation of intraocular lens power after radial keratotmy with computerized videokeratography. Am J Ophthal 1995; 120:739-50.

11.Speicher L. Intraocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12(1): 17-29.

12.Hamilton DR, Hardten DR. Cataract surgery in patients with prior refractive surgery. Curr Opin Ophthalmol 2003; 14(1): 44-53.

13.Zeh WG, Koch DD. Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity. J Cataract Refract Surg 1999;25(7):898-903.

14.Chen L, Mannis MJ, Salz JJ, Garcia-Ferrer FJ, Ge J. Analysis of intraocular lens power calculation in post-radial keratotomy eyes. J Cataract Refract Surg 2003;29(1):65-70.

15.Maeda N, Klyce SD, Smolek MK, McDonald MB. Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia. Cornea 1997;16(5):517-24.

16.Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double K method. J Cataract Refract Surg 2003;29:2063-68.

17.Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery (editorial). J Cataract Refract Surg 2003;29:2039-42.

18.Gills JP, Gayton JL. Reducing pre-existing astigmatism. IN: Gills JP, Cataract surgery: the state of the art. Thorofare, NJ: SLACK, 1998;53-66.

19.Nichamin L. Refining astigmatic keratotomy during cataract surgery. Ocul Surg News April 15, 1993.

20.Fine IH, Packer M, Hoffman RS. Use of power modulations in phacoemulsification. Choo-choo chop and flip phacoemulsification. J Cataract Refract Surg 2001;27(2): 188-97.

21.Rajan MS, Keilhorn I, Bell JA. Partial coherence laser interferometry vs conventional ultrasound biometry in intraocular lens power calculations. Eye 2002;16(5):552- 6.

22.Packer M, Fine IH, Hoffman RS, Coffman PG, Brown LK. Immersion A-scan compared with partial coherence interferometry: outcomes analysis. J Cataract Refract Surg 2002;28(2):239-42.

23.Packer M, Fine IH, Hoffman RS. Refractive lens exchange with the array multifocal intraocular lens. J Cataract Refract Surg 2002; 28(3):421-4.

24.Hamed AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002;109:651-58.

Georges Baikoff (France)

INTRODUCTION

Today, the increasingly popular development of phakic implants as well as the FDA agreement obtained for the VERISYSE implant, means that a very precise preoperative evaluation of the dimensions of the anterior chamber is essential.

Axial measurements are evaluated with ultrasonography (A Scan, B Scan),1,2 with optical procedures (Slit Lamp,3 IOLMaster4). The relationship between different eye structures can be studied in reduced areas with ultrasonography (A Scan, B Scan, ultrasound biomicroscopy (UBM),5 and posterior segment optical coherence tomographer.6

The Scheimpflug technique7 gives a complete image of the anterior chamber, but the complex and inaccurate mathematical reconstructions make it difficult to evaluate the anterior segment precisely. The disparities in the obliquity of the cross sectional and projection plane of photographic images can sometimes lead to measurements being obtained through extrapolation.

It is only recently that complete axial cross sections of the anterior segment are possible with ultra high frequency ultrasound equipment (ARTEMIS)8 and with the anterior chamber optical coherence tomographer (AC OCT).9

Not only is the OCT simple to use but it is possible to obtain a very precise analysis of anterior segment modifications during accommodation and ageing of the eye. Development of this new imaging technique has enabled us not only to show that in certain cases there is a possibility of contact between a phakic implant and

the anatomical structures of the eye but also to evaluate the internal dimensions of the anterior chamber with precision.

OPTICAL COHERENCE TOMOGRAPHY TECHNOLOGY FOR THE ANTERIOR CHAMBER

Today, the AC OCT with its 820 nm wavelength, is a well-known posterior segment imaging device,6 and by 1994 IZATT et al9 had already suggested using it for anterior segment imaging. It was only in 2001, with the introduction of a high speed AC OCT using a 1310 nm wavelength that good quality, easy to interpret images, were obtained.10,11

Analysis of the eye is a noncontact procedure during which the patient fixes an optical target. The target’s focus can be adjusted with positive or negative lenses to compensate the patient’s spherical ametropia and obtain images of the eye unaccommodated. The target can be defocused by using negative lenses to induce natural accommodation of the studied eye.

There is no undue pressure on the anterior segment because there is no contact, the images are obtained in just a few seconds and only modifications to the studied eye are taken into account under physiological conditions. This examination is therefore very different from the ultrasound explorations which require stimulation of the fellow eye or the Scheimpflug technique where pilocarpine drops are used to obtain an artificial accommodation.

The image acquisition system provides a video image of the examined zone and stores the last seven images taken at a rate of 8 frames per second. At the end of the examination, the images are reviewed by the examiner and only the best shots are retained.

The chosen image is then interpreted with specific software which readjusts the dimensions of the images by eliminating the distortions induced by corneal optical transmission differences. After reconstruction of the image, all the required anterior chamber measurements can be done: anterior chamber diameter, anterior chamber depth, corneal pachymetry, crystalline lens radius of curvature, crystalline lens thickness, irido-

corneal angle opening. The prototype’s resolution is approximately 14 μm. The infra-red light beam is stopped by the pigments, therefore a satisfactory view of the different structures situated behind the epithelium pigment layer of the iris or of the anterior uvea is not possible.

INTEREST OF THE AC OCT AND THE STUDY OF ACCOMMODATION WHEN IMPLANTING PHAKIC IOLs

Measurement of the Anterior Chamber’s Internal Diameter

One of the key points in improving anterior chamber angle-supported implant tolerance lies in correctly adapting its size with the anterior chamber’s internal diameter. Until today, we had to rely on approximate measuring methods, such as white-to-white, sometimes improved by using a graduated plastic sizer when inserting the implant. However, these measuring means are relatively inaccurate and do not give a precise evaluation of the anterior chamber diameter.

Figure 6.1 clearly demonstrates the interest of this type of anterior segment preoperative imaging (AC OCT, Scheimpflug, ultra high frequency ultrasound) to evaluate the internal diameter dimensions before surgery (Fig. 6.1).

We were surprised12 when we compared the anterior chamber’s diameter on the 0°, 45°, 90° and 135° axes. The vertical diameter appeared larger than the horizontal diameter in 74 percent of the cases. The mean difference between the vertical and horizontal axis is more significant for eyes with small diameters than eyes with large diameters. The average difference is approximately 300 μm (Fig. 6.2), which is more than the examination measuring or reproducibility error which is not more than 50 μm. In the future, this phenomenon must be taken into account in order to chose the implant. The largest diameter must be taken into account when choosing a angle-supported implant. Indeed, if one chooses an implant with an overall diameter equal to the eye’s horizontal diameter, which is generally the smaller of the two diameters, there is a risk that the implant rotates or is unstable. On the contrary, to avoid

Figure 6.1: Aspect/dimension of the anterior segment photographed with the AC OCT (Courtesy: Elsevier)

Figure 6.2: In 74 percent of normal nonoperated eyes, the vertical diameter is greater than the horizontal diameter (Courtesy: Elsevier)

implant rotation and ensure its stability, it is essential that the implant is fitted both in size and orientation to the largest diameter, i.e. generally the vertical one. This is to avoid an implant that is too big being placed on the smaller diameter which would inevitably lead to oversizing and pupil ovalisation.

Endothelium Safety Distance

Retrospective studies have shown that a 1.5 mm distance must be respected between the edge of the IOL’s optic and the corneal endothelium. This minimum safety distance avoids the risk of endothelial cell loss secondary to contact between the implant and the endothelium in particular when the patient rubs his eyes. Anterior segment imaging software should therefore include this safety distance.

Studying accommodation and crystalline lens ageing13,14 has shown that the crystalline lens increases in volume with age and during accommodation.

Figure 6.3: Endothelial safety distance (Courtesy: Elsevier)

Possibility of Contact Crystalline Lens/Implant

Having studied numerous series of phakic implants,15 we were able to show evidence of contact of different models of implants with the crystalline lens. Having dilated a hyperopic patient with an ARTISAN implant, we discovered a contact between the lower edge of the implant and the crystalline lens. Likewise, during accommodation, the posterior face of a hyperopic patient’s PRL phakic implant came into contact with the crystalline lens. In a patient implanted 10 years ago with an angle-supported IOL, we noticed that the crystalline lens came into contact with the implant’s posterior face because the crystalline lens had increased in volume with age (Fig. 6.4).

These different elements should encourage manufacturers to include in their software the profiles of the different implants available so as to be able to simulate their position in the anterior segment either accommodated or unaccommodated. Simulating anterior segment ageing would give us an indication of how long an implant will be tolerated (Fig. 6.5).

In a previous work,13 we demonstrated that because the crystalline lens thickens with age, there is a forward movement of its anterior pole which reduces the depth of the anterior chamber by 18 μm to 20 μm per year. As the irido-corneal angle recesses remain fixed, the crystalline lens distorts and pushes the iris forward modifying its relationship with an angle-supported

Figure 6.4: Contact between a ZB5M implant and the crystalline lens 10 years after implantation (Courtesy:

Elsevier)

Figure 6.5: Forward thrust of crystalline lens with ageing (Courtesy: Elsevier)

implant and or iris fixated implant. We also found16 that with ARTISAN phakic implants, there was an unusually high percentage (6%) of pigment dispersions in hyperopes. This has been confirmed by Saxena and Landesx.17

In this study, we measured the distance between the crystalline lens’ anterior pole and the line represented by the horizontal internal diameter of the anterior chamber (Fig. 6.6). In subjects having developed pigment dispersion, this rise was much higher than average. If the crystalline lens rise is above 600 μm, there is a 75 percent risk of developing pigment dispersion, (Fig. 6.7)and this complication can lead to removal of the implant and even extraction of the crystalline lens. If the crystalline lens rise is known on the day of surgery, as well as the statistical forward movement of the crystalline lens, it is possible to estimate the ARTISAN implant tolerance period knowing that the critical level is around 600 μm according to the following formula:

T = number of safe years, S = danger level in microns,

Figure 6.6: Crystalline lens rise. Distance between anterior pole of the crystalline lens and the line connecting two angle recesses at 3 o’clock and 9 o’clock (Courtesy: Elsevier)

Figure 6.7: Diagram : Crystalline lens rise vs AC depth. Eyes having developed pigment dispersion are displayed in red (Courtesy: Elsevier)

F = rise measured in microns on the day of examination, = yearly reduction of anterior chamber or yearly progression of crystalline lens’ anterior pole in microns.

(Figs 6.8 and 6.9)

This notion of the crystalline lens rise should also be applied to angle-supported anterior chamber implants. The implant’s vault measures the implant’s posterior face rise with regards to the baseline joining the tip of the implant’s footplates. It is therefore easy to understand that if the crystalline lens has a rise equal to or superior to the implant’s vault, there will be contact between the posterior face of the implant and the crystalline lens. It is necessary today to take into account the implant’s vault and the crystalline lens rise when considering surgery to know whether an angle-supported anterior chamber

Figure 6.8: Clinical aspect of pigment dispersion behind an ARTISAN implant (Courtesy: Elsevier)

Figure 6.9: AC OCT anterior segment cut of an ARTISAN implant in a case of pigment dispersion. Note: the presence of iris pigments between the crystalline lens and the implant and flattening of iris (Courtesy: Elsevier)

implant is indicated or not and for how many years it will be tolerated. We consider this notion to be just as essential as anterior chamber depth. To ensure safe anterior chamber implantation, endothelial safety distances as well as crystalline lens safety distances must be respected. With ageing, endothelial safety distances remain constant whereas the distance between implant and crystalline lens gradually decreases over the years. (Fig. 6.10)

Can Anterior Segment Imaging Indicate that one Particular Implant is Preferable Over Another?

Studying accommodation in an albino patient18 showed

Figure 6.10: Different anterior chamber safety distances measured from the angle recess to recess baseline (Courtesy: Elsevier)

that all the structures of the anterior uvea were malleable and mobile. The only stable elements of the anterior segment are the cornea and the uvea insertion at the corneo-scleral junction, that is to say the irido-corneal angle; the iris, the sulcus, the ciliary body and the crystalline lens show significant modifications during accommodation.

In our opinion, these elements define the iridocorneal angle as the most stable structure and the least affected by accommodation. This could be another fact in favor of angle-supported implants, as long as the problem of pupil ovalisations has been definitely solved as they are the result of inaccurate preoperative measurements.

Studying the ciliary body and the sulcus in an albino patient showed evidence of important diameter variations of these two structures during accommodation.18 In its present state, the Visante™ OCT technology does not allow us to routinely study the posterior chamber in a normal subject.

Liliana Werner et al studied the internal diameter of the anterior chamber on 20 phakic and pseudophakic cadaver eyes with the ARTEMIS (L Werner et al Poster A.S.C.R.S. San Diego 2004, Poster SFO Paris 2004, oral communication A.A.O/I.S.R.S. Subspecialty Day Meeting New Orleans 2004) and found, as we did, that in most cases, the anterior chamber’s internal vertical diameter and the sulcus’ vertical diameter were statistically larger than their horizontal diameters. More recently, in a

patient with an ICL and having developed bilateral cataract, we were able to establish that the patient had a very high crystalline lens rise (Fig. 6.11). This complication is probably due to the forward thrust of the crystalline lens, Gonvers19 demonstrated that in ICL patients, the risk of cataract dramatically increased with age.

Figure 6.11: High crystalline lens rise in a patient having developed cataract after an ICL (Courtesy: Elsevier)

SUMMARY

In the light of these studies, it appears that the AC OCT or other similar techniques (Scheimpflug, ultra high frequency ultrasound) available in everyday practice are going to become essential when scheduling a phakic implant in a patient where LASIK is contraindicated. Static and dynamic study of the anterior segment as well as new software are going to become necessary to simulate the anatomical relationship of the implant and the anterior chamber during accommodation and ageing. The safety distances required in the anterior segment will be specified and we will probably be able to predict a safety period during which the implant will be well tolerated and after which it will probably be

necessary to remove it.

REFERENCES

1.Kurtz D, Manny R, Hussein M; COMET study group. Variability of the ocular component measurements in children using A-Scan ultrasonography. Optom Vis Sci 2004; 81,1: 35-43.

2.Hamidzada WA, Osuobeni EP. Agreement between A- mode and B-mode ultrasonography in the measurement of ocular distances. Vet Radiol Ultrasound 1999;40,5:502- 7.

3.Krogsaa B, Fledelius H, Larsen J, et al. Photometric oculometry. II. Measurement of axial ocular distances with slit-lamp microscopy. Clinical evaluation and comparison with ultrasonography. Acta Ophthalmol (Copenh) 1984; 62,2:290-9.

4.Sheng H, Bottjer CA, Bullimore MA. Ocular component measurement using the Zeiss IOLMaster. Optom Vis Sci 2004;81,1:27-34.

5.Mishima HK, Shoge K Takamatsu M, et al. Ultrasound biomicroscopic study of ciliary body thickness after topical application of pharmacologic agents. Am J Ophthalmol. 1996;121,3:319-21.

6.Puliafito C, Hee MR, Schuman JS, et al. Optical Coherence Tomography of Ocular Diseases, Thorofare NJ Slack Inc, 1996.

7.Boker T, Shequem J, Rauwolf M, et al. Anterior chamber angle biometry: a comparison of Scheimpflug photography and ultrasound biomicroscopy. Ophthalmic Res 1995; 27 Suppl 1:104-9.

8.Kim DY, Reinstein DZ, Silverman RH, et al. Very high frequency analysis of a new phakic posterior chamber intraocular lens in situ. Am J Ophthalmol 1998;125, 5:725-29.

9.Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmology 1994;112,1, 584-89.

10.Radhakrishnan S, Rollins AM, Roth JE, et al. Real-time optical coherence tomography of anterior segment at 1310nm, Arch Ophthalmology 2001;119,8:1179-85.

11.Huang D, Swanson EA, Lin CP, et al. Optical Coherence Tomography. Science 1991;254:1178-81.

12.Baikoff G, Bourgeon G, Jitsuo Jodai H, et al. Evaluation of the measurement of the Anterior Chamber’s internal diameter and depth: IOLMaster vs AC OCT. J Cataract Refract Surg (submitted).

13.Baikoff G, Lutun E, Ferraz C, et al. Static and dynamic analysis of the anterior segment with opticla coherence tomography. J Cataract Refract Surg 2004;30:184350.

14.Koretz J, Strenk S, Strenk L, Semmlow J. Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study. J Opt Soc Am 2004; 21:346-54.

15.Baikoff G, Lutun E, Ferraz C. Contact between 3 phakic intraocular lens models and the crystalline lens: an anterior chamber optical coherence tomography study. J. Cataract Refract Surg 2004;30:2007-12.

16.Baikoff G, Bourgeon G, Jitsuo Jodai H, et al. Pigment Dispersion and Artisan Implants. The crystalline lens rise as a safety criterion. J. Cataract Refract Surg (submitted).

17.Saxena R, Landesz M, Noordzij B, Luyten G. Three-year Follow-up of the Artisan Phakic Intraocular lens for Hypermetropia.: Ophthalmology 2003;110:1391-95.

18.Baikoff G, Lutun E, J. Wie, Ferraz C. Anterior chamber Optical Coherence Tomography Study of Human natural accommodation in a 19-year-old albino. J Cataract Refract Surg 2004;30:696-701.

19.Gonvers M, Bornet C, Othenin Girard P. Implantable contact lens for moderate to high myopia: relation of vaulting to cataract formation. J Cataract Refract Surg 2003; 29(5): 918-24.

Mahipal Sachdev

Sri Ganesh

Sathish Prabhu (India)

INTRODUCTION

The word “Phakic” refers to those eyes that still have their natural internal lens. IOL stands for “intraocular lens.” In the phakic IOL procedure, an intraocular lens is placed inside the eye in front of the patient’s natural lens. Phakic IOL procedures are being used on severely myopic and hyperopic patients who may not be candidates for the more common laser procedures such as PRK, LASEK, and LASIK.

HISTORY OF THE PROCEDURE

Strampelli and Barraquer (1950’s)—A biconcave angle supported lens was introduced by Strampelli and later popularized by Barraquer. These lenses were abandoned due to serious angle and endothelium related complications.

Dveli (1980’s)—Restarted phakic myopia lenses with 4 soft angle-supported loops.

Baikoff (1980’s)—Dr Baikoff from France introduced an angle supported myopia lens with Kelman-type haptics.

Jan Worst and Fechner (1986)—Introduced phakic myopia lens of iris claw design. This Artisan-Worst lens is a peripheral iris fixated anterior chamber lens. It is now marketed as Verisyse phakic IOL by AMO.

Fyodorov-Professor Fyodorov of Russia introduced the concept of a soft phakic lens in the space between the iris and the anterior surface of the crystalline lens and is marketed nowadays by Starr Surgical USA as an implantable contact lens (ICL).

PHAKIC IOLs

These are available in three styles:

1.Anterior chamber angle fixated IOL—NuVita (Bausch & Lomb), Kelman duet, I care (corneal), Vivarte (Ciba vision)

2.Iris supported phakic IOL—Verisyse/Artisan (AMO/ Ophtec)

3.Plate lens that fits between the iris and the crystalline lens—Staar implantable contact lens (ICL), PRL (Ciba)

INDICATIONS

•Patients not suitable for Lasik/Lasek due to high powers or thin corneas

REQUIREMENTS

•Age above 18 years

•Stable refraction for one year

•AC depth >3.0 mm

•Endothelial count >2000 Cells/C.mm

•No other ocular pathology.

CONTRAINDICATIONS

Contraindications include the following:

•Myopia other than axial myopia

•Corneal dystrophy/endothelial cell count <2,000 cells/cmm

•Anterior chamber depth less than 3.00 mm

•History of uveitis

•Presence of anterior/posterior synechiae

•Glaucoma or IOP higher than 20 mmHg

•Evidence of nuclear sclerosis or developing cataract

•Personal or family history of retinal detachment

•Diabetes mellitus

Some of the above contraindications are relative on

the discretion of the surgeon and the needs of the patients.

The presence of amblyopia is not a contraindication for the implantation of the phakic IOL

Anterior chamber depth is an important consideration for all three types of lenses, whether it is angle-supported, iris claw, or posterior chamber lenses. A depth of less than 3.00 mm is a contraindication. The angle-supported or the posterior chamber lens may cause crowding of the angle, and the iris claw lens will encroach on the central depth of an already shallow anterior chamber.

ANGLE SUPPORTED ANTERIOR

CHAMBER PHAKIC IOLS

These phakic IOLs are broadly categorized into 2 groups—Rigid and foldable lenses (Tables 7.1 and 7.2).

1.The vision membrane is a silicon, anterior chamber and angle-fixated phakic IOL that employs a combination of refractive and diffractive optics to