Intracorneal Ring Segments

Background

More than 20 million people in the United States have myopia between 1.00 and 3.00 diopters (D) with astigmatism of 1.00 D or less. Several options for vision correction are available to each individual, including spectacle correction, contact lens fitting, refractive keratotomy (RK), photorefractive keratectomy (PRK),^[1]  laser-assisted in situ keratomileusis (LASIK),^{[2, 3]}  and intracorneal rings, as shown below.^{[4, 5, 6, 7]}  Nonsurgical approaches remain the least expensive and safest. Refractive surgery is an option for the subset of patients who desire freedom from glasses and lenses.^{[8, 9, 10]}

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Graphic representation of the intracorneal ring segments in place. Reprinted with permission of Addition Technology Inc.

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Intacs microthin prescription inserts treat mild myopia. Reprinted with permission of Addition Technology Inc.

The ideal refractive surgery procedure is effective, predictable, safe, and potentially reversible.^{[11, 12]} The goal of this article is to present a refractive surgical procedure in which implantation of a polymethyl methacrylate (PMMA) ring or ring segments in the peripheral corneal stroma is used to correct myopia and astigmatism.^[13] What initially was limited to correcting myopia and astigmatism now is primarily used to manage post-refractive laser surgery-associated corneal ectasia and keratoconus.^{[14, 15, 16, 17, 18]}

Device description

The intrastromal corneal ring (ICR) is designed to correct mild-to-moderate myopia by flattening the anterior corneal curvature without encroaching on the visual axis. The device is an open-ended PMMA transparent ring with an outer diameter (R2) of 8.1 mm and an inner diameter (R1) of 6.8 mm, and the curvature conforms to that of the cornea. It is precision lathe-cut to ±0.01 mm as a hexagonal-shaped section of a cone with positioning holes for manipulation.^[19] It is inserted through a peripheral radial incision made with a diamond knife at two-thirds corneal depth into a 360° peripheral intrastromal channel created with specially designed instruments. Anterior corneal curvature is changed by using rings of different thicknesses.^[20]

Intrastromal corneal ring segments (Intacs microthin prescription inserts, as shown below), or ICRS, are a more recent design modification of the ICR. The ring segments split the ring into two 150° arcs.^[21] The use of ring segments simplifies the implantation procedure. Furthermore, ring segments can be placed away from the radial incision, thereby minimizing the potential for incision-related complications. The degree of correction (greater-thicker) is determined by the thickness of the Intacs inserts, which are available in 5 sizes: 0.25 mm, 0.275 mm, 0.30 mm, 0.325 mm, and 0.35 mm.^{[22, 23, 24]}

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KeraVision Intacs microthin prescription inserts. Reprinted with permission of Addition Technology Inc.

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KeraVision Intacs microthin prescription inserts. Intracorneal ring segments. Reprinted with permission of Addition Technology Inc.

MediPhacos KeraRing is another intracorneal ring segment developed and marketed in Brazil. It is available in 2 models for 5-mm implantation in optical zones of 5 mm, 5.5 mm, and 6 mm. There are 40 different variations of thicknesses, arc lengths, and diameters, allowing for enhanced customization of corneal remodeling and refractive correction. Over 150,000 implants have been used worldwide.

Ferrara Ring is made of yellow PMMA, an inert and biocompatible acrylic used in intracorneal implants. It has a flat base with a variable arch length of 90 mm and 210 mm and a variable thickness of 0.15 mm to 0.30 mm. The apical diameters are 5 mm and 6 mm.

Surgical instruments

Addition Technology has surgical instruments specifically for use with Intacs insertion. These instruments include the following: corneal thickness gauges, glides, incision and placement marker, pocketing hook, pocketing lever, ring forceps, stromal spreader, vacuum-centering guide with vacuum system, and clockwise and counterclockwise dissectors.^[25]

Ferrara Ring has its surgical kit used to insert the intracorneal ring segments. These instruments include the following: Ferrara spatula, Suarez spreader, Bicalho guide, Pre-delaminator, Ferrara marker, Sinskey hook of 0.20 mm, Modified McPherson forceps, and an adjustable Ferrara diamond blade knife.^[26]

How do intracorneal ring segment inserts work?

Imagine the cornea as compressed arcs of fiber covering the eye. If a spacer element is inserted between those fibers, effectively pushing them apart, then the arc must flatten to accommodate the inserted element. The corneal ring complies with Barraquer and Blavatskaya postulates. According to these postulates, an addition in the cornea periphery results in its flattening, and the ring diameter determines how much the cornea will be flattened. In other words, the more tissue is added (increasing ring thickness), the smaller the diameter, the greater the myopia correction that will be obtained. Intracorneal ring segments inserts shorten the corneal cord length, produce flattening across the entire cornea, and maintain the positive asphericity of the cornea.^[27]

Advantages of intracorneal ring segment inserts

Several benefits of using intracorneal ring segment inserts exist. It is a minimally invasive outpatient procedure with low morbidity. Since the surgery is completed in the peripheral cornea, the central optical zone is not disturbed. Results are rapid and predictable. There is a reduced risk for visual adverse effects and a long-term convenient refractive correction. It is removable and exchangeable. There is central and peripheral flattening of the cornea, maintaining corneal asphericity (prolate).^[28]

Intracorneal ring segments now may be used to treat irregular steepening (irregular astigmatism) of the cornea caused by keratoconus.^{[29, 30]} There is stabilization or delay of cone progression. The intracorneal ring segment is made of acrylic, an inert and biocompatible material; therefore, there is a lack of rejection. There is a high patient satisfaction rate. It can combine with other procedures like contact lens fitting, corneal collagen cross-linking, PRK, phakic IOL implantation, and intraocular lenses. There is no evidence of any interference with a corneal transplant.

Disadvantages of intracorneal ring segment inserts

The technology is limited for use in low-to-moderate myopia with low astigmatism, corneal ectasia, and keratoconus.^[31] It remains a relatively new technology with emerging complications (eg, diurnal fluctuations in visual acuity and stromal opacifications).^{[32, 33, 34, 35]} Of further concern is the prolonged intraoperative elevated intraocular pressure.^{[36, 37]}

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History of the Procedure

The KeraVision Ring was conceived by AE Reynolds, OD, in 1978. Kera Associates was formed in 1980 to develop the ICR and other concepts. In 1995, the first preclinical study on Reynolds' product was performed.^[38]

In 1991, the first human clinical trials began in Brazil with 360° ICR. The rings were implanted in the corneas of nonfunctional human eyes. Shortly after that, a similar study was performed in the United States. Implantation in sighted eyes in the United States was implemented in 1993 as part of a US Food and Drug Administration (FDA) regulated phase II study.^[39]

Implantation in contralateral eyes began in 1994. A preliminary astigmatism study began in Brazil at this time. In 1995, the US phase II myopia trial began for 150° ICRS.^[40] In 1996, the US phase III for -1.00 to -3.50 D of myopia began.^[41] CE (Conformite Europeene) marking was granted, permitting the commercial sale of the ICR in the European Union. A Pan-European myopia study began for ICRS. In 1998, HPB approval was granted in Canada, while preliminary hyperopia studies were started in Mexico.^[42]

A premarket approval (PMA) application was filed with the FDA. In 1999, Intacs inserts was adopted for commercial sales of the ICRS, and FDA approval was granted for the commercial sale of Intacs inserts for -1.00 to -3.00 D of myopia with less than -1.00 D of astigmatism in the United States. After a promising introduction into the refractive market, KeraVision, the maker of Intacs, experienced financial difficulties and eventually sold its assets to Addition Technology in April 2001. Since its introduction, however, the use of Intacs has expanded from treating patients with low-to-moderate myopia with low astigmatism to treating patients with keratoconus.^[43]

Table 1. Evolution of Intacs Inserts

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See Table

Addition Technology (Intacs) is the popular intracorneal ring segment used in Australia and the United States. In Europe, Ferrara Ring intracorneal ring segments are popular. Ferrara Ring acquired Addition Technology. Ferrara Ring has been in development since 1986, and over 100,000 implants have been used with excellent results.

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Indications

According to the FDA, Intacs are intended for the reduction or elimination of mild myopia (-1.00 to -3.00 D spherical equivalent at the spectacle plane) in patients who are aged 21 years or older, in patients with documented stability of refraction as demonstrated by a change of less than or equal to 0.50 D for at least 12 months before the preoperative examination, and in patients where the astigmatic component is +1.00 D or less.

In 2004, Intacs was given humanitarian device approval by the FDA for use in patients with keratoconus. The inserts now may be used to reduce irregular steepening (irregular astigmatism) caused by keratoconus. Intacs inserts help restore clear vision in patients with keratoconus by flattening and repositioning the cornea. Intacs inserts are intended for patients with keratoconus who no longer can achieve adequate vision using contact lenses or glasses and for whom corneal transplantation is the only remaining option.^[14]

Table 2. Predicted Nominal Correction and Recommended Prescribing Range

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See Table

The main indication for the Ferrara Ring implant is for the management of progressive keratoconus (ie, with a gradual decrease of uncorrected visual acuity [UCVA] and best-corrected visual acuity [BCVA] and progressive corneal steepening). The Ferrara Ring implant also is indicated in patients with unsatisfactory BCVA with glasses and intolerance to contact lenses and in post-LASIK corneal ectasia when the condition worsens.

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Relevant Anatomy

The cornea is a transparent, avascular tissue continuous with the opaque sclera and semitransparent conjunctiva. The cornea is covered by tear film on its anterior surface and bathed by aqueous humor on its posterior surface.^[44]

The cornea measures 11-12 mm horizontally and 9-11 mm vertically in adults. The average corneal thickness is 0.5 mm (500 µm) centrally and 0.7 mm (700 µm) peripherally.

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Contraindications

Intracorneal ring segments are contraindicated in the following patients:

• High degrees of myopia, hyperopia, or astigmatism
• Patients younger than 21 years, with stable refractive error
• Patients with existing collagen vascular, autoimmune, or immunodeficiency disease
• Women who are pregnant or breastfeeding
• Patients with previous anterior segment trauma^[45]
• In the presence of existing ocular conditions, such as recurrent corneal erosion syndrome, corneal dystrophy, or other corneal pathology that may predispose the patient to future complications
• Patients who are using one or more of the following prescription medications that may affect corneal healing or vision: isotretinoin (Accutane), amiodarone (Cordarone), and/or sumatriptan (Imitrex)
• Patients with corneas that are too thin
• Patients with unrealistic expectations

Intracorneal ring segments should not be used in patients with keratoconus who have any of the following characteristics:

• Can achieve functional vision daily by using contact lenses
• Are younger than 21 years
• Do not have clear central corneas
• Have a corneal thickness of less than 450 µm at the proposed incision site
• Have other options to improve their functional vision
• Presence of apical opacities in very advanced keratoconus, usually with K readings above 75 D; postoperative results in these cases are generally poor, and the best treatment for them is lamellar or penetrating keratoplasty

Warnings include the following:

• Increased intraocular pressure results from the use of the vacuum-centering guide.
• The surgeon should limit the continuous application of the vacuum to 3 minutes or less and no more than 750 mBar. In some instances, reapplication of the vacuum may be necessary, and it is advised that a 5-minute "reperfusion" phase be allowed before reestablishing suction.
• It is not advisable to use intracorneal ring segments in patients with systemic diseases likely to affect wound healing (eg, insulin-dependent diabetes, severe atopic disease).
• It is not advisable to use intracorneal ring segments in patients with a history of ocular manifestations of herpes simplex virus or herpes zoster virus.
• Resterilization and/or reuse of intracorneal ring segments are not recommended.

Precautions include the following:

• The thicker diameter (0.35 mm) ring segments are associated with a higher probability of a reduced outcome than the thinner diameter (0.25-0.30 mm) ring segments. Dissatisfaction from the reduced outcomes may lead to a higher removal rate of thicker diameter (0.35 mm) ring segments.
• Patients with myopia of -1.00 D carry a higher risk for overcorrection.
• The long-term effects of Intacs on endothelial cell counts have not yet been established.^[46] A temporary decrease in corneal sensations may occur in some patients. Patients with large scotopic pupils may experience some visual symptoms.^[47] They must be properly advised. In some instances, some patients may experience a decrease in contrast sensitivity.^[48]
• No studies have been performed to establish the safety and effectiveness of other refractive procedures after the removal of Intacs.^[49]
• The safety and effectiveness of Intacs have not been established for the following:

  • Patients with progressive myopia or astigmatism, nuclear sclerosis or other crystalline lens opacity, corneal abnormality, or previous corneal surgery or trauma
  • Patients younger than 21 years
  • Corneas that are flatter than 40 D or steeper than 46 D
  • Corneas with a central thickness of less than 480 µm or peripheral thickness of less than 570 µm
  • Patients with greater than -3.50 D of myopia or with astigmatism greater than +1.00 D
  • In long-term use^[50]

   

  

   

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Imaging Studies

Corneal topography

Corneal topography development provides surgeons with effortlessly understood color-coded maps of corneal curvature and quantitative indexes of irregular astigmatism that correlate with potential visual acuity.

Modern instrumentation produces a videokeratograph, generally in the form of a color-coded contour map.

Different manufacturers use different methods (eg, Placido, 40 scanned slits, combination Placido and 40 scanned slits, phase-modified laser holography, raster stereography).

Using video keratography in the preoperative and postoperative evaluation of all patients undergoing refractive surgery is valuable.

Dual Scheimpflug analyzer

The dual Scheimpflug analyzer is a high-precision optical system for corneal topography and 3-dimensional anterior eye segment analysis based on a Revolving Dual Channel Scheimpflug Camera and a Placido Disk.

Placido imaging furnishes high-accuracy curvature data.

Scheimpflug imaging is optimal for precise elevation data.

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Other Tests

Pachymetry

The pachymeter (optical, ultrasonic) is used to measure corneal thickness.

Accurate determination of corneal thickness preoperatively allows the surgeon to set the depth of the incision to two-thirds of the measured result.

Orbscan II by Orbtek uses 40 scanned slits and Placido methods to provide anterior and posterior corneal curvature and data on corneal thickness.

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Procedures

A manual corneal trephination set to create a corneal tunnel for intrastromal corneal ring segments has been available since the introduction of ICRS.Most femtosecond laser units are able to create similar tunnels using a specific program. The femtosecond laser–created tunnels are more precise and result in less morbidity than the manual approach.

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Medical Therapy

Medical therapy is limited to broad-spectrum topical antibiotics and corticosteroids for uncomplicated cases.

See Postoperative details and/or articles on ocular pharmacology for more information.

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Surgical Therapy

See Intraoperative Details for the surgical procedure.

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Preoperative Details

The placement of the intrastromal ring tunnel and ring segments is similar, and both can be performed with topical anesthesia. The patient and the operative field are prepared and draped in the usual sterile fashion for ophthalmic surgery. A lid speculum is used for globe exposure.

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Intraoperative Details

Manual intrastromal corneal pocket creation

The corneal center is identified and marked with a Sinskey hook.

A 2-mm long epithelial impression is created at the 12-o'clock position, where the ring segments are to be placed at an 8-mm diameter optical zone.

A diamond blade, set at 65% of the peripheral corneal depth, is used to perform a radial incision along this mark. 

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The 1.2-mm radial incision is made with a diamond knife at the edge of a 7-mm optical zone. The diamond knife is set for approximately two thirds of c....

A vacuum-centering guide is positioned relative to the central corneal indentation.

The vacuum is increased, and the guide is circumferentially adhered onto the perilimbal conjunctiva, stabilizing the globe. This device provides a guide for the dissector. The dissector is inserted into the radial incision, and blunt dissection of the cornea at two-thirds depth is performed in the clockwise and counterclockwise directions to create two stromal channels.

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Rotation of the dissector creates the tunnel in the peripheral cornea into which an Intacs insert will be placed. Reprinted with permission of Additio....

The vacuum is released, and the vacuum-centering guide is removed.

Ring segments are inserted through the radial incisions using special forceps and positioned using a Sinskey hook nasally and temporally so the superior ends are approximately 3 mm apart. 

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The first Intacs insert is placed in the tunnel. Reprinted with permission of Addition Technology Inc.

The incision is closed with 1 or 2 interrupted 11-0 nylon sutures.

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An Intacs insert in place at approximately two thirds of corneal depth. Reprinted with permission of Addition Technology Inc.

Femtosecond Laser-Assisted intrastromal corneal pocket creation

The femtosecond laser technology used in laser vision correction, such as all-laser bladeless LASIK (Femto-LASIK) and lenticule extraction (SMILE - Small Incision Lenticule Extraction), is also ideally suited for creating incisions in preparation for intracorneal ring (ICR) implantations. When defining tunnel parameters, it even performs inclined cutting geometries and ring tunnel segments smaller than 360° with a high degree of flexibility.

Due to the precision of the tunnel creation with a femtosecond laser, complications including extrusions, are less when compared with manual dissection techniques.^[51]

The surgical procedures are performed under topical anesthesia and complete aseptic measures. The procedure is initiated by marking a reference point for centration (Purkinje reflex). Corneal thickness around the 5 mm circle was intraoperatively measured using ultrasonic pachymetry. This is correlated with the corneal thickness as detailed with the Pentacam examinations. A disposable glass lens with a suction device is applied to applanate the cornea. The femtosecond laser is used. The continuous circular stromal tunnel is created as recommended by the ring manufacturer and previous studies, at 80% depth of the thinnest location with an inner diameter of 5.00 mm, outer diameter of 5.90 mm, entry cut length of 1.40 mm, and entry cut thickness 1 mm with ring energy 1.9 mJ. The incision is created at the steepest axis. The choice of the ring's number, arc, and thickness is made following the nomogram provided by the manufacturer. The ring segment is introduced by the Keraring forceps after dilating the tunnel with a blunt dilator. Finally, a soft bandage contact lens is applied.

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Postoperative Details

Postoperatively, antibiotic-corticosteroid combination drops and/or ointment are used, and the speculum is removed. The eye may be covered overnight with a shield. Antibiotic-corticosteroid combination drops are used four to six times daily for one to two weeks. The sutures are removed 2 weeks postoperatively or longer if the incision is not healed adequately.

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Follow-up

Patients are seen postoperatively on Day 1, Week 1, and Months 1, 3, 6, and 12. The surgeon should watch for postoperative complications. An observations timeline has been outlined for potential complications. See Complications.

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Complications

Complications include localized incision-related epithelial defects, epithelial plug formation, wound dehiscence, superficial neovascularization, surgically induced astigmatism, infiltrates in the channel, transient decreased corneal sensation, and delayed infectious keratitis.^{[52, 53]} Other observations included haze and deposits around the intrastromal channel.

The observations timeline consists of the following:

• Immediate (1-7 d)
• Early (1-4 wk)
• Intermediate (after 4 wk)
• Immediate postoperative observations may include epithelial defects, lamellar channel haze, under-correction and over-correction, and incision-healing responses.
• Early postoperative observations may include sterile infiltrates, epithelial cysts or plugs, and infectious keratitis.
• Late postoperative observations may include positioning hole deposits, lamellar channel deposits, and infectious keratitis.^{[54, 55]}

Exchange or explantation

An exchange procedure can be done in selected patients with under-correction or over-correction. In selected patients who are dissatisfied, an explantation procedure can be done. The procedures can be done with the patient using only topical anesthesia. The surgical steps are listed below:

• Early in the postoperative period, a Sinskey hook can be used to reopen the original incision. If the original incision is healed, a radial incision is made at the original site at the same depth as the initial incision.
• Fibrous tissue is gently loosened with blunt dissection.
• A Suarez spreader is used to dissect the channel opening.
• A Sinskey hook then is used to engage the positioning hole and to rotate the segment out of its channel.
• In exchange procedures, a new ICR segment can be placed into the previously made channel.
• The incision is closed with a suture.
• The same postoperative procedure is followed as with the original insertion.

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Outcome and Prognosis

A summary of the safety and efficacy variables at 12 months after surgery is as follows:

• Uncorrected visual activity (UCVA) 20/20 or better (74%); UCVA 20/40 or better (97%)
• Mean refractive spherical equivalent (MRSE) ± 0.50 D (69%); MRSE ± 1.00 D (92%)
• Greater than or equal to 2 lines loss best spectacle-corrected visual acuity (0%); increased cylinder greater than or equal to 2.00 D (0%)
• Corneal topography demonstrated that, although general flattening of the central cornea occurs, the normal positive asphericity of the cornea is maintained after placement of the ring and ring segments.^[56]
• Transient dry eye may follow the placement of Intacs inserts, but the tear film quality returns within one week after surgery.^{[57, 15, 58, 59, 60]}

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Future and Controversies

Applications of intrastromal ring segments include the following:

• Myopia with astigmatism - Going beyond the range that the FDA has approved; currently, the upper limit is at -3.00 D spherical equivalent at the spectacle plane.
• Astigmatism - Applications in patients with pure astigmatism; currently, the upper limit is at +1.00 D of astigmatism.
• Presbyopia - In presbyopes and patients previously treated with LASIK or PRK
• Therapeutic indications - The use of Intacs in patients with mild corneal ectasia not deemed to be candidates for LASIK or PRK^{[16, 61]}
• Therapeutic indications - By flattening the central corneal protrusion, intracorneal ring implantation is a promising new therapy for patients with early-to-moderate pellucid marginal degeneration (PMD) intolerant of contact lenses.^{[62, 63, 64, 65]}
• Therapeutic indications - Intracorneal ring segments now are used to manage ectatic corneal diseases.^[66]
• Combination refractive procedures – The LASIK-Intacs method and the Intacs-LASIK method significantly improved visual acuity and refraction based on limited experience. LASIK followed by Intacs may be the preferred procedure for safety, convenience, and lower induced cylinder.
• Enhancement - The correction of residual myopia following maximum corneal sculpting with LASIK or PRK (posterior stromal bed too thin for further ablation); improving the conditions of patients with decreased nighttime visual function (including halos, induced myopia, and decreased contrast sensitivity) following LASIK and PRK, associated with constricted optical zones.^[67]
• Enhancement - Implantation of Intacs in eyes with myopic regression after LASIK and PRK resulted in a good refractive outcome and improved uncorrected visual acuity.^[68]

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Medication Summary

Topical broad-spectrum antibiotic-steroid ophthalmic drops given one drop four to six times daily for 1-2 weeks are sufficient to prevent infection and control inflammation. Pure topical corticosteroid drops may be extended for an extra week depending on the residual corneal inflammation.

Preservative-free ophthalmic lubricating drops may be given as needed for dry eye symptoms.

There is no need for oral antibiotics or anti-inflammatory medications.

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