Highlights of ocular surgery





Many surgical procedures are performed to cure eye disorders, restore vision, prevent blindness, correct congenital abnormalities, or cosmetically improve the area in and around the eye. For each eye condition various surgical procedures, sometimes ingenious, have been devised.


To familiarize the ophthalmic assistant with some of the highlights of ocular surgery, the most commonly performed ocular procedures are outlined in this chapter.


Strabismus surgery


Preparation


When having strabismus surgery, children may be admitted to the hospital the afternoon before or on the day of surgery. Nowadays surgery is often performed on an outpatient basis because the child is more comfortable at home with his or her parents at night. Unless there is some adverse medical problem, such as asthma, this seems to be a safe outpatient procedure. Parents are often encouraged to remain with the child to alleviate fears and relieve the child’s feeling of abandonment. Older children are told that they may expect a bandage on one or both eyes after surgery, but that at least one bandage will be removed before they are sent home.


Surgery


Muscle surgery involves weakening or strengthening of the rectus or oblique muscles to improve alignment of the eyes.


The four rectus muscles insert close to the limbus, the medial rectus muscle being the closest (~5.5 mm, whereas the lateral rectus muscle is 7 mm from it). The rectus muscles would be easily visible if they were not covered with the conjunctiva and subconjunctival tissue. To isolate these muscles, the surgeon must cut through the conjunctiva and place a muscle hook under the muscle. The oblique muscles insert at the back of the globe, so surgery on these muscles is not performed at their insertions.


The following procedures weaken the extraocular muscles:



  • 1.

    Recession ( Fig. 32.1 ). The muscle is removed from its original insertion and repositioned farther back on the sclera. This loosens the grip the muscle has on the globe.




    Fig. 32.1


    Recession operation. (A) Preoperative position of muscles. (B) Muscle detached from globe. (C) Muscle reattachment to sclera at a point farther back from its original insertion.

    (From Stein HA, Slatt BJ, Stein RM. Ophthalmic Terminology: Speller and Vocabulary Builder . 3rd ed. St Louis: Mosby; 1992.)


  • 2.

    Transverse margin myotomy . Overlapping cuts are made on each side of the muscle to lengthen it. No change is made in the insertion.


  • 3.

    Complete tenotomy . The muscle or tendon is severed completely and allowed to retract. Rare today but needed occasionally.



The following procedures strengthen an extraocular muscle:



  • 1.

    Resection ( Fig. 32.2 ). A section of the muscle is removed from its insertion and the muscle is reattached to its original position. A resection shortens a muscle, thereby increasing its effective tension and pull.




    Fig. 32.2


    Resection operation. (A) Preoperative position of muscles. (B) Muscle detached from globe and an anterior portion of muscle cut away. (C) Muscle resutured to sclera at original insertion point.

    (From Stein HA, Slatt BJ, Stein RM. Ophthalmic Terminology: Speller and Vocabulary Builder . 3rd ed. St Louis: Mosby; 1992.)


  • 2.

    Advancement . This procedure is usually combined with a resection. After the resection has been completed, the muscle is repositioned ahead of, or anterior to, its original insertion. Advancement increases the arc of contact of the muscle with the globe, thereby enhancing its effective pull.



Although there are six extraocular muscles, occasionally, one may require surgery on the superior oblique and the inferior oblique muscle when overaction of these muscles occurs. For a full discussion of muscle function see Chapter 2 .


Postoperative routine


After strabismus surgery, children are usually allowed to get up as soon as the effects of the anesthetic have disappeared. Normally, the child is able to resume school activities almost immediately and sports within 2 weeks. The parents are informed that the operated eye may be red in the immediate postoperative period, but that this will gradually fade until the eye looks normal again. In some cases, the eyes are not straight in the immediate postoperative period because of swelling, hemorrhage, and trauma to the muscles, all of which check eye movements. The parents are told of these postoperative variations so that they do not become upset if the eyes are not straight on removal of the bandages.


Occasionally, a child will show an allergic response to the sutures used. This is likely to occur 2 or even 3 weeks after surgery, during a period when recovery is virtually complete. The lids suddenly swell, the conjunctiva balloons out, and the child’s eyes generally look dreadful. This is a rather innocuous event, which subsides within 3 or 4 days without causing any complications. Occasionally, however, a suture granuloma may develop.


Questions often asked about muscle surgery include the following:



  • 1.

    Can vision be lost because of muscle surgery? No. Because the muscles are attached on the surface of the globe, the eye itself is never opened.


  • 2.

    Are the eyes usually straight after one procedure? Yes, in most cases. However, undercorrections and overcorrections do occur and no ophthalmic surgeon can say with certainty which patient will require further surgery. Therefore parents are generally informed that two procedures may be necessary to straighten the eyes. With this approach, parents are not disappointed or bitter if reoperation becomes necessary and they are extremely happy if surgery results in a complete success after the first procedure.


  • 3.

    Can muscle surgery improve the vision of an adult who has a turned eye that is amblyopic? No. Strabismus surgery on an adult is strictly cosmetic. The turn can be corrected so that the position of the eyes appears normal, but the vision is not affected for better or worse.


  • 4.

    Can the eyes of an adult with strabismus be straightened? Yes. Age is no barrier to a cosmetic strabismus procedure.


  • 5.

    Can the eyes be straightened with orthoptic exercises to avoid surgery? Usually, orthoptics is an adjunct to ophthalmic surgery and not a substitute for it. Strabismus may be corrected with the use of glasses, eyedrops, or orthoptic exercises in some patients. When possible, nonsurgical methods are used first and, in small degrees of strabismus, may result in a correction.



Cataract surgery


Phacoemulsification (phaco) has evolved to be the state-of-the-art cataract surgical procedure. Patient satisfaction is extremely high with the development of a painless procedure and a rapid return of vision. The widespread use of phacoemulsification is related to improvements in surgical techniques, including incision construction, advances in machine technology, developments in intraocular lenses, and the ability to perform surgery without injections under topical anesthesia. These advancements, along with the potential complications, are discussed in this section. However, femtosecond (FS) laser may fast surpass phacoemulsification.


A cataract is opacity of the lens of the eye ( Fig. 32.3 ). The opacity may be minimal in size and faint in density so that the transmission of light is not appreciably affected, or it may be large and opaque so that light cannot gain entry to the eye’s interior. When the cataract is pronounced, the examiner cannot see the interior of the patient’s eye with any clarity and conversely the patient cannot see the examiner very clearly.




Fig. 32.3


(A) Normal eye, clear lens. (B) Eye with cataract.


A cataract is removed if it endangers the health of the eye or interferes with the patient’s ability to function. No visual level can be identified on the Snellen chart because contrast sensitivity, glare, pupillary constriction, and ambient lighting may significantly affect a person’s functioning ability, even with an early cataract. The visual demands of patients can vary depending on occupation, stage of life, or whether they are driving, and so on.


Preoperative evaluation


The cataract, when it becomes mature, obscures all details of the fundus. When this occurs, efforts should be made to evaluate the health of the interior of the eye before cataract surgery. This evaluation has important prognostic importance. It can be performed by the following methods:



  • 1.

    Two-point light discrimination test . Two lights are held a measured distance (60 cm) from the affected eye—the fellow eye being firmly covered—and are gradually separated until they can be identified as two lights. These measurements are recorded. The normal separable amount that two lights can be identified varies with the preoperative acuity. For visions reduced to hand movements, the lights should be identified about 12.5 cm apart. For visions better than 20/400, they should be identified about 5 cm apart.


  • 2.

    Light projection in all quadrants . An assessment of active sensory retina in all quadrants should be performed by asking the patient to determine the position of a small transilluminator light.


  • 3.

    Ultrasound . Ultrasound (or high-frequency waves) passes through the dense cataract and identifies any interference between the lens and the retina by rebounding off any firm obstruction. Abnormalities in the ultrasonogram can confirm the presence of a tumor mass, hemorrhage, or detached retina behind the lens. The B-scan is the main method of evaluating the area behind the lens. However, A-scan measurements (see following text) may detect defects in the central pathway.


  • 4.

    Blue-field entoptoscope . This device permits the patient to observe his or her own white blood cells flowing in the retinal capillaries in the macular area. This flow is visible even with a dense cataract. This entoptic phenomenon is created by an intense blue light that the patient views. With normal retinal function, the patient will describe “flying corpuscles” moving in the entire field. If the macula is not functioning, no flying corpuscles will be seen.


    This phenomenon occurs because the blue light is strongly absorbed by the hemoglobin and red blood cells, which results in the photoreceptors behind the capillaries becoming relatively dark-adapted. When a white blood cell moves through the capillary, the blue light passes through it and excites the photoreceptors behind it. Thus the passage of a leukocyte is perceived as a moving bright dot or a flying corpuscle. The intensity of the blue light can be adjusted so that sufficient light reaches the retina in cases of media opacities. This entoptoscopic effect can also be seen by looking at a clear blue sky on a bright day.


    Abnormalities in perception of the corpuscles are the result of changes in the perifoveal circulation or functional impairment of the neural elements in the retina, or both. Differentiation between the two is possible in conjunction with other tests, such as those based on fluorescein angiography and electrophysiology.


    Abnormal entoptoscopic findings include one or more of the following:




    • Total or partial absence of corpuscle perception in one or both eyes



    • Absence of pulsatile motion



    • Fewer corpuscles in one eye



    • Lower corpuscle speed in one eye



    Clinical experience with cataract patients has shown that a positive response to the blue-field entoptic test indicates a 98% probability of good postoperative macular function (visual acuity 20/40 or better). The test is especially useful when a direct view of the fundus is obscured in cases of corneal edema or scarring, hyphema, cataract and vitreous hemorrhage, membranes, or exudates.


  • 5.

    Brightness acuity tester (BAT) . This instrument, devised by Dr. Jack Holladay, can determine a significant visual loss attributed to a bright light creating a small pupil and glare (see Fig. 8.14 ). The excess light is the normal light that a person may experience when outdoors in bright sunshine. An individual with a small central cataract may be seriously affected in driving and participating in sports when the pupil contracts.


  • 6.

    Prediction of potential acuities . Interferometers and potential acuity meters (PAMs) are used in office tests to predict potential acuities in patients with cataracts and those with hazy posterior capsules after cataract extractions. This allows realistic expectations on the part of the surgeon and patient before cataract surgery or a neodymium yttrium aluminum garnet (nd:YAG) posterior capsulotomy. The interferometers pass two beams of laser light through the pupil, producing a three-dimensional interference pattern within the retina. This allows the ophthalmologist to bypass problems with most opacities of the media, as well as refractive errors. If the patient can see the interference pattern, which will appear as bands in a specific direction, this is evidence of macular function. The narrower the bands that are projected and seen, the higher is the degree of macular function.


    The PAM ( Fig. 32.4 ) is basically a pinpoint light source, a transilluminated visual acuity chart, and a lens. It projects a brightly illuminated Snellen acuity chart through an area approximately 0.15 mm in diameter. It can be used to test approximate acuities through mildly dense media because of the brightness of the stimulus and the tiny diameter of the beam used for the examination.




    Fig. 32.4


    Errors in A-scan measurements. (A) Too much pressure with the probe indents the cornea and may cause excessive shallowness of the anterior chamber and a shorter axial length. (B) Correct pressure. (C) Too little pressure may leave a thick tear layer between the probe and the cornea, resulting in a longer axial length.


    In the clinical application of these tests, the following guidelines should be used:




    • Do not shine lights into the patient’s eyes before testing because this may decrease the acuity readings.



    • Have the pupil well dilated.



    • PAM testing: drop the chinrest to enable the patient to talk without moving the head so that the acuity chart will remain visible to the patient.



    • Interferometer testing: stress that background noise will be seen (swirls of light, dots, wavy lines, half lines), but that the patient should ignore these and indicate only the direction of the lines of light seen.



    • Focus the beam in the center of the pupil at the plane of the iris, then scan the pupil until best responses are obtained.



    • With the interferometers, first use horizontal and vertical bands until the best acuity has been reached and then use oblique lines to verify this.



    • With the PAM, start with large letters and ask the patient to read only the first two or three letters in each line. If at any time two or more letters are identified in a given line, that line, if it is the smallest read, is the endpoint even though the patient may not be able to detect that line again.



    • Never tell patients they should see letters or lines because this tends to upset them. Simply ask, “What do you see?” If they see letters, they will say so. With the interferometer, ask, “What do you see?” If they begin to see something, ask if there are any lines as you make them larger and larger until they see them.



  • 7.

    Endothelial cell function . Although specular microscopic examination is the standard method of evaluating the morphology and cell count of the endothelial cells, the use of slit-lamp biomicroscopy can aid the clinical observation ( Fig. 32.5 ). With the use of an objective lens in the slit lamp and careful positioning of the slit light, the endothelial mosaic may be viewed. By use of the Endo lens designed by Tomey Corporation, the image may be enhanced. The image may be compared with a grid that can be placed in the objective lens assembly. Pachymetry can be performed to measure the central thickness of the cornea. Thicker corneas, especially greater than 600 μm, increase the likelihood of a compromised endothelium. The findings of one or more of the following signs suggest an increased risk of corneal edema or decompensation following any cataract surgery: thick cornea, reduced endothelial cells, or corneal guttata.




    Fig. 32.5


    The slit lamp may be used for evaluation of cell density.


  • 8.

    Partial coherence interferometry or A-scan . Another important aspect of evaluation is to determine the required dioptric power of the intraocular lens to be chosen. Coherence interferometry is a noncontact method that allows determination of the axial length of the eye, as well as keratometric values of the cornea. With these two values, as well as specific constants for the different implants, the ideal power of the intraocular lens can be determined. The older method of calculation of the axial length of the eye is by A-scan ultrasound, which uses a probe that makes contact with the cornea (see Fig. 32.4 ). Special concerns that require an adjustment of intraocular lens formulas include patients with previous refractive surgery (e.g., laser-assisted in situ keratomileusis [LASIK], photorefractive keratotomy [PRK], SMILE, radial keratectomy [RK]), high myopia, or high hyperopia.



Preparation


Patients are often seen by the family physician or internist at least 1 week before cataract surgery to ensure that medical conditions, such as diabetes and hypertension are under control. Also the ophthalmic surgeon should be provided with the names and dosages of the medications the patient may be taking.


Patients should be instructed to wash their hair before entering the hospital because hair washing is avoided during the first few postoperative days to prevent contamination of the wound by dirty rinse water. Smoking, of course, should be discouraged because a heavy cough can easily disrupt a fresh wound or initiate bleeding. With clear corneal microincisions performed under topical anesthesia, it is not necessary to discontinue aspirin or warfarin (Coumadin). If a peribulbar or retrobulbar block is given or a scleral incision is made, it is usually best to discontinue medication as this could result in bleeding.


Surgery


The object of cataract surgery is to remove the crystalline lens of the eye that has become cloudy. This is performed under an operating microscope that permits magnification. The technique of phacoemulsification is the most common method of removing cataracts today. An ultrasonic probe that vibrates rapidly can liquefy a lens through a microincision. This small-incision surgery has resulted in an incision size that has been reduced from 10 mm for extracapsular surgery to 1.8 to 2.2 mm for phacoemulsification. Special incision construction generally eliminates the need for sutures. This leads to minimal induced astigmatism and a rapid recovery. Most surgeons today perform cataract surgery in freestanding surgical centers on an outpatient basis (see Ch. 34 ).


Phacoemulsification


In 1963 Dr. Charles Kelman commenced research to ascertain the possibility of removing a cataract through a small incision. After attempting many preliminary techniques, including crushing, cutting, and drilling the lens, he finally perfected an apparatus and tip that he used to apply an oscillating and ultrasonic frequency to emulsify the cataract. He was attempting to improve on the system of cataract surgery that, in that era, consisted of freezing with a cryoprobe or by using a capsule forceps. With the phacoemulsifier, a microincision of less than 3 mm is required. This means less tissue destruction, less wound reaction, a quicker operation, less chance of wound disruption and its attendant complications, less astigmatism, and earlier ambulation and visual recovery ( Box 32.1 ). In most cases, the patient is able to resume normal activities immediately after the operation.



Box 32.1

Phacoemulsification


Advantages





  • Small incision



  • Fewer wound problems



  • Less astigmatism



  • More rapid physical rehabilitation



  • Less risk of expulsive hemorrhage



  • Faster surgery



  • Quicker visual recovery



Disadvantages





  • Machine dependent



  • Longer learning period



  • Complications while learning



  • Expensive equipment



  • Difficult with hard nucleus



  • Need good pupil dilation



  • Difficult with small pupils




Skin and eye preparation


Before surgery, the skin around the eyelids is prepared with an antiseptic, most commonly povidone-iodine (Betadine) preparation. The eye is irrigated with a dilute solution of Betadine and balanced saline.


Anesthesia


Advances in anesthetic techniques have resulted in a dramatic change for both patients and surgeons. Retrobulbar injections into the orbit work well at providing anesthesia and akinesia. Unfortunately, the injections may be associated with complications that can include retrobulbar hemorrhage, intraocular penetration, and optic nerve penetration. The development of peribulbar injections decreases the chance of intraocular or optic nerve problems, but still can result in an orbital hemorrhage and discomfort. The use of topical anesthesia combined with intraocular lidocaine has revolutionized the way that surgery can be successfully performed.


After the superficial ocular structure is anesthetized with a topical anesthetic (e.g., tetracaine), a paracentesis is performed into the anterior chamber and 0.25 to 0.50 mL of 1% preservative-free lidocaine is injected into the anterior chamber. This results in dramatic anesthesia and usually eliminates all discomfort for the patient. The advantages of topical anesthesia are that it avoids all complications from orbital injections, provides increased safety for patients on anticoagulants, and results in an immediate recovery of vision because the optic nerve is not affected by this form of anesthesia.


Incision construction


Incision size has reduced with changes in techniques. The incision size for intracapsular cataract extraction was approximately 12 mm, with extracapsular cataract extraction 10 mm, and with phacoemulsification of around 2.5 mm. The advantages of a smaller incision are primarily less trauma to the eye, less astigmatic effect, and a quicker return to the former lifestyle. A self-sealing incision can be created in which there is an internal corneal lip of tissue that is closed off by the normal intraocular pressure. Sutures are not usually required. The induced astigmatism is minimal.


Continuous curvilinear capsulorrhexis


The technique involves making a small opening in the limbus or in the clear cornea and introducing a cystotome to cut an opening in the anterior capsule of the lens ( Fig. 32.6 ). Previously, the opening into the anterior capsule was made with a capsulotomy needle by a series of jagged punctures that converted the central capsule into a series of postage-stamp cuttings. Currently, a continuous tear opening, often called continuous curvilinear capsulorrhexis (CCC) , is made by tearing the capsule so that the edges remain sharp, well demarcated, and very strong. This prevents extension into the periphery of tears of the capsule and permits the capsule to hold the lens implant securely ( Fig. 32.7 ). This can be performed with better centration and circularity with the FS laser. The capsulotomy with a laser can be done on the line of sight, which allows the implant to be centered in the capsular bag.




Fig. 32.6


Phacoemulsification. (A) Cataract. (B) Continuous curve capsulotomy. (C) Removal of nucleus. (D) Cortex aspiration and enlargement of incision. (E) Insertion of intraocular lens. (F) Wound closure.



Fig. 32.7


(A) Can-opener capsulotomy. (B) Continuous curve capsulorrhexis.


Hydrodissection and hydrodelineation


Balanced saline can be injected into the lens to separate either the cortical material from the capsule (hydrodissection) or the nucleus from the epinucleus (hydrodelineation) . This allows the nucleus to be rotated freely within the capsular bag during the phaco technique. Separation of the nucleus from the epinucleus allows removal of the nucleus with phacoemulsification, leaving an underlying cushion of epinuclear tissue to protect against inadvertent rupture of the posterior capsule.


Machine design


Phaco equipment consists of a phaco tip that is inserted into the eye, a phaco handpiece that allows rapid vibration of the tip to liquefy the nucleus, and the machine that allows adjustments of a variety of parameters. The parameters that can be varied during each case include the amount of fluid infused into the eye, a vacuum level that allows suction of lens material, and the phaco energy that controls vibration frequency of the tip.


Phaco technique


There are a variety of techniques to emulsify and remove the nucleus, which may vary depending on the density of the cataract. A chopping technique uses a “chopper” to divide a nucleus into small segments before being emulsified. The phaco tip with high vacuum impales the nucleus and the chopper is used to divide the lens into fragments. A “divide and conquer” approach may be used in which a deep trench is created in the nucleus, and instruments are used to crack the nucleus into multiple pieces that can be safely removed from the eye. A “flip” technique involves lifting or floating the nucleus above the capsule before emulsification is performed.


Sutures


The sutureless closure has resulted in more rapid rehabilitation after cataract surgery. The basic principle of sutureless incision is the creation of a valve-like self-sealing wound that is relatively small. The valve permits the incision to withstand unusual stress or intermittent raised intraocular pressure, which may follow in the postoperative period. The most common incision is made into clear cornea. If a scleral incision is made, various shapes of design have been advocated that lessen the degree of induced astigmatism. These include variations of straight incisions or a curvilinear incision, which is sometimes labeled the “frown and smile” incision.


Femtosecond laser


Advancements in technology have resulted in the development of laser cataract surgery ( Fig. 32.8 ). Combining an FS laser (see following text) with a sophisticated imaging technique of optical coherence tomography (OCT) allows the laser to perform corneal wound construction (main incision and side-port incision); corneal relaxing incisions to reduce astigmatism; capsulorrhexis of an exact size, shape, and centration ( Fig. 32.9 ); and fragmentation of the nucleus. The laser procedure is typically performed outside of the surgical operating room. After this procedure, the patient is taken to the surgical suite, where the eye is prepped and draped in the usual manner. A dull instrument is used to open the corneal wound incisions, and a viscoelastic substance is placed in the anterior chamber. A forceps is then used to grab the central capsule and pull this out from the eye. Hydrodissection and occasionally, hydrodelineation are performed. A phaco tip is then placed into the eye, and the nucleus, which was previously dismantled by the laser, is removed from the eye. The amount of phaco energy to remove the nucleus is significantly lower than with standard phacoemulsification. In fact, a high percentage of cases can be performed without any phaco energy. The remaining cortical material is aspirated from the capsular bag. The implant is then inserted through the small phaco incision, typically around 2.2 mm.




Fig. 32.8


Laser cataract unit for wound construction, limbal relaxing incisions, capsulorrhexis, and fragmentation of the nucleus.

(Courtesy Optimedica, Santa Clara, CA.)



Fig. 32.9


Comparison of capsulorrhexis done manually (A), versus with a laser (B).

(Courtesy Optimedica, Santa Clara, CA.)


The main advantages of laser cataract surgery include less dexterity required on the part of the surgeon, an easier learning curve for beginner surgeons, more accurate and consistent corneal incisions, corneal relaxing incisions, and a perfectly round capsulorrhexis. In addition, because there is less energy used to liquefy the cataract, there is less intraocular turbulence, which can result in clearer postoperative corneas and a quicker visual recovery.


Intraocular lenses


Historically, one of the major problems after cataract surgery was the use of aphakic spectacles. Older adult patients had to bear the attendant magnifications and distortions by spectacles following cataract surgery. Contact lenses were developed, especially those that can be worn overnight or for extended wear, to avoid the handling difficulties of insertion and removal that are a constant hazard to the insecure older adult aphakic patient. The solution today has been in the direction of intraocular lens implants, which Ridley introduced in 1949. Through the pioneering efforts of Cornelius Binkhorst of Holland, Peter Choyce of England, Edward Epstein of South Africa, and Fyderov of Russia, the intraocular lens has become the major form of visual rehabilitation after cataract surgery. With the use of sodium hyaluronate (Healon) and other viscoelastic substances, endothelial damage is minimized during implant surgery. Magnification induced by spectacles and contact lenses has been reduced to zero with intraocular lenses through the positioning of the implant within the eye ( Fig. 32.10 ).




Fig. 32.10


Magnification induced by aphakic spectacles, contact lenses, and intraocular lenses.


The present-day success of intraocular lenses is a result of more skillful microsurgery, as well as better design, finish, and fixation of the lenses. In addition, a better understanding of positioning of the lenses within the capsular bag, the use of the YAG laser for capsular opacification, and the better management and minimization of complications have led to significant success with intraocular lenses. Their use is indicated in virtually all patients undergoing cataract surgery.


Lens materials and design


Intraocular lenses are composed of an optical portion, called the “optics” of the lens, and the “haptics” ( Fig. 32.11 ). The optics portion has a dioptric power that permits focusing light from afar onto the retina. The “size” of the optics varies from 5 to 7 mm in diameter. The term haptics is from the Greek word haptesthai meaning “to lay hold of.” The haptics refer to the method of holding the optical portion in place in the human eye, which consists of loops that are made of either polymethyl methacrylate or Prolene. Polymethyl methacrylate is noteworthy as a hard, firm, inert material that has been singled out for the manufacture of quality intraocular lens optics and is inert in the human body. Loops made of this material are commonly used instead of Prolene. Prolene is a suture material that is also relatively inert in the human body and provides a softness and pliability that permit its support of the optical portion of the lens. Acrylic and silicone lenses have been developed, which are also inert inside the eye and can be folded so as to be inserted through a microincision. Titanium and metal loops have disappeared in the manufacture of intraocular lenses because of the adverse reaction they produce on the human retina. Loop designs are more flexible so as to permit greater adjustments within the structure of the eye itself to variations of the ocular changes that occur with each blink and contraction of the rectus muscles. This in itself has been a major step forward in the design of intraocular lenses.




Fig. 32.11


Intraocular lenses.

(From Stein HA, Slatt BJ, Stein RM. A Primer in Ophthalmology: A Textbook for Students . St Louis: Mosby; 1992.)


The shape of the optical portion may be planoconvex , in which case the anterior portion of the lens is convex , whereas the back surface is flat. It may have reverse optics, in which the back surface of the lens is convex and the front surface is flat. It may alternatively be biconvex , in which both sides of the optical portion are convex. Some lenses are made aspheric , in which there is an alteration in power from the center of the lens to the periphery. Because of microincision surgery, foldable lenses are preferred.


Designs have incorporated an ultraviolet filter into the optical portion of the lens. This eliminates wavelengths in the ultraviolet spectrum less than 400 nm. The health of the cornea can be determined by specular microscopy on corneal cell density (see Ch. 43 ) or by a guestimate using a × 1.6 objective lens with a slit lamp (see Fig. 32.5 ).


The power of the intraocular lenses varies from eye to eye. The use of optical coherence allows for the most accurate measurement of the axial length of the eye to determine the required power of the implant. The more common powers are about + 18.00 to + 22.00 diopters, but lenses are available for any power, including minus power and very high plus power.


Intraocular lenses also may be classified according to their position and their method of fixation. Anterior chamber lenses ( Fig. 32.12 ) include lenses that lie in the anterior chamber of the eye. These may be angle-supported, in which case they are supported in the angle of the anterior chamber, or they may be iris-supported, in which case they may be attached with or without sutures to the iris. These lenses have almost become obsolete and are used only for special purposes. Lenses are usually positioned in the posterior chamber and they may be supported by capsular support, in which case they may be called in-the-bag lenses (see Fig. 32.14C ) because they are fitted directly into the capsular bag that contained the former crystalline lens, or they may be sulcus-supported ( Fig. 32.13 ), in which case they lie in front of the remainder of the anterior capsule and are supported in the sulcus of the eye ( Fig. 32.14B ).


Jun 26, 2022 | Posted by in OPHTHALMOLOGY | Comments Off on Highlights of ocular surgery
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