Limbal Relaxing Incisions
Limbal Relaxing Incisions (LRI) are partial thickness incisions made at the corneal periphery for the treatment of corneal astigmatism. LRIs are considered to fall under the spectrum of incisional astigmatism treatments known as Corneal Relaxing Incisions (CRIs). Within this category, arcuate (or sometimes, "astigmatic") keratectomy (AK) can also be included. This latter term generally refers to astigmatic incisions in the cornea that are placed in closer proximity to the visual axis compared to the peripheral placement of LRIs.
Definition of Corneal Astigmatism
Abnormal curvature of the cornea with at least two axes of steeper and flatter curvature, which can result in abnormal focusing of light and impaired vision. Astigmatism typically results from curvature abnormalities of the front (anterior) surface of the cornea. Visit EyeSmart from the American Academy of Ophthalmology for a brief, patient-friendly description of astigmatism.
Classification of Astigmatism
Astigmatism may also be classified as regular and irregular.
- With-the-rule: Steep axis of the cylinder is within 15 degrees of the 90 degree vertical meridian (75 - 105 degrees)
- Against-the-rule: Steep axis of the cylinder is within 15 degrees of the horizontal meridian (165 - 015 degree)
- Oblique: Steep axis of the cylinder is not within 15 degrees of the horizontal or vertical meridians (16-74 degrees and 106-164 degrees)
- Whenever the two main axes of astigmatism are not symmetric and/or do not lie 90 degrees apart (orthogonal), the astigmatism is considered irregular. Causes of irregular astigmatism include corneal dystrophy or degeneration, ocular surface disease, corneal ectatic disease such as keratoconus, or prior corneal surgery.
The patient may present with a long history of astigmatism. Astigmatism, similar to myopia (nearsightedness) or hyperopia (farsightedness), is generally present and correctable at a young age with spectacles or contact lenses. In adults, astigmatism can also be corrected surgically with LASIK, PRK, SMILE, or limbal relaxing incisions (LRI).
Important items to note in the preoperative exam include:
- Close examination of the peripheral cornea, particularly in the areas where the incsions will be placed.
- Precise manifest refraction
- Keratometry to confirm diopters of corneal astigmatism
- Corneal topography to confirm axis of corneal astigmatism, and to classify as regular or irregular, symmetric or asymmetric
- Pachymetry at planned incision sites if possible
- Corneal Tomography (Orbscan, Pentacam, Galilei) may be considered
At least two distinct retinoscopic reflexes in different axes
The most common symptom is decreased visual acuity. The patient might notice that straight lines are not straight. Patients with even small amounts of cylinder may notice halos, glare and shadowing, especially in low light conditions and at distance.
Astigmatism is also measured during refraction. However, this measurement may include corneal and lenticular astigmatism. Therefore, this measurement cannot be used solely for determination of AK procedures. The next step is to determine how much of the measured astigmatism is a result of corneal irregularity.
Corneal astigmatism power and axis can be measured in many different ways, including the following:
- Keratometry: Manual Keratometer, Automated Keratometer, IOL Master
- Corneal Topography (Placido-disc based)
- Corneal Tomography (Scanning Slit or Scheimpflug imaging)
Defining the astigmatism
The goal of astigmatism management is to leave the patient with as little astigmatism as possible. Some patients can tolerate up to 1.0 diopter in any axis and still maintain 20/40 vision at distance and J1 at near. Typically, with-the-rule and against-the-rule astigmatism are better tolerated than oblique astigmatism.
The most crucial element of surgical planning for astigmatic keratotomy is determining the amount and location of the astigmatism to be corrected. This is also perhaps the most difficult aspect of this procedure. As mentioned above, preoperative astigmatism measurements are done in several ways. It is common for many surgeons to rely on manual keratometry as their measurement; however many use either corneal topography or Scheimpflug imaging. It is important to remember that the manual refraction may include any lenticular-induced cylinder as well, so often times the refraction will be misleading. The manual keratometry axis of astigmatism should match the topographical or Scheimpflug axis.
If the preoperative cylindrical values or axis vary with the several methods of measurements, the surgeon may opt to correct the astigmatism after the postoperative refraction has stabilized. Some surgeons opt to 'split the difference' (ie. 2 diopters @ 90 degrees on IOL master K's and 1 diopter @ 90 degrees on topography, the surgeon may treat 1.5 diopters.)
Choosing the procedure
Corneal Relaxing Incisions (CRIs) can be divided into 2 groups: limbal relaxing incisions (LRIs) and arcuate keratotomy (AK). The surgeon must evaluate the pros and cons of each in conjunction with the needs of each individual patient.
The pros of LRIs include:
- Easier to perform, less dependent on pachymetry, less likely to result in overcorrections, quicker post-op stabilization of refraction, postoperative topography is smoother/more homogenous (coupling).
- LRIs are best for low to moderate amounts of astigmatism (< 3 diopters).
The cons of LRIs include:
- A larger incision (typically one or two incisions 1-3 clock hours in arc length)
The pros of AKs include:
- Shorter incision, more powerful (correct a larger amount of astigmatism), placement in smaller optical zone (therefore low coupling ratio), 'multifocal' effect (better depth of focus)
The cons of AKs are:
- More discomfort, greater risk of corneal perforation (more dependent on accurate pachymetry), may cause more corneal irregularity and irregular astigmatism
- AKs have been noted to have a higher risk of overcorrection so they are more often performed on patients with higher amounts of astigmatism.
- AKs may also risk a loss of best-corrected spectacle acuity
Relative Contraindications for CRIs
- Autoimmune disease
- Peripheral corneal disease
- Terrien's or furrow degeneration
- Prior corneal surgery, particularly incisional procedures
Either LRIs or AK may be used in conjunction with toric intraocular lenses (IOLs) and strategic cataract incision placement to treat high astigmatism.
Astigmatism may also be managed with Toric IOLs alone.
The surgeon should consider the age of the patient as the effect of the relaxing incision have been found to increases with age.
A simple breakdown of treatment options
Astigmatism less than 1.0 diopter can typically be adequately treated with CRIs.
Astigmatism ranging from 1.0 diopter to 3.0 diopters can be corrected with a toric IOL or CRIs, although toric IOLs are regarded to give more reliable results.
Astigmatism greater than 3.0 diopters may corrected with a combination of toric IOL or CRIs and/or strategic cataract incision placement.
The surgeon may opt to utilize an online calculator (See Additional Resources section below for link) for guidance. Online LRI calculator results are based on the preoperative keratometry measurements and anticpated surgeon induced astigmatism.
Many surgeons have developed nomograms. Table 1 is a representative nomogram. (Courtesy of Louis D. "Skip" Nichamin, M.D.)
Corneal relaxing incisions may be done either in conjunction with cataract surgery or after.
There are many nomograms widely available. Ideally, each surgeon develops their own by monitoring their outcomes.
While CRIs may be done after surgery, it is easiest for the beginning surgeon to do them in the OR in conjunction to routine cataract surgery. CRIs are often done at the beginning of the case when the eye is still firm.
Prior to draping, with the patient in a sitting position, the cornea is marked. There are many strategies for marking the cornea such as 3:00 and 9:00, or 12:00 and 6:00. Most CRI corneal marking sets have an instrument designed for making preoperative orientation marks.
A diamond knife is most commonly utilized for the CRI incision. Disposable and reusable, metal CRI knives are also available. Both often have preset depths for surgeon convenience. The usual depth is 600 microns but may range from 450 to 650 microns. It is helpful to have handy a print out of the patient's topography and/or CRI calculator print out to ensure appropriate orientation of the CRI. An arc-shaped incision is made in the clear cornea close to the limbus (approximately 0.5 mm). The episclera is grasped with 0.12 forceps. The arc incision is drawn towards the surgeon for best control. It is important to press the CRI blade firmly against the cornea to ensure consistent depth of the incision and to create the incision with one pass to avoid irregularities.
Depending the amount of astigmatism to be corrected, paired incisions may be employed. For asymmetrical regular astigmatism, the paired incisions may be different lengths, greater in the axis with the larger amount of cylinder.
Alternatively, some surgeons prefer to do the CRI upon completion of the cataract surgery.
Alcon ORA system Intraoperative Aberrometer utilizes Talbot-Moire's interferometry, a form of wavefront analysis which has a dynamic range of -20 to +20 D. The device is attached to the surgical microscope. At any point in the surgery, the ophthalmologist may take a measurement. The monitor will display the refractive error of the eye. Surgeons who have this technology available in their operating room are able to complete the cataract extraction portion of the procedure. Intraoperative measurements are taken and the appropriate lens and ancillary techniques (LRIs or CRIs/AKs) can be employed. Alternatively, this diagnostic tool may be used to confirm preoperative calculations.
Femtosecond Lasers may also be used during cataract surgery. They can create the capsulotomy, side-port and main incisions in addition to nuclear defragmentation. The surgeon can elect to have laser-created astigmatic keratotomies at the time of the procedure as well. However, they may be less predictable than other methods of correcting astigmatism, and could decrease best corrected visual acuity.
Surgical follow up
Postoperative follow up should be per routine for cataract surgery when done together. Manifest refractions should be performed. Postoperative corneal topography is also helpful to identify treatment effect.
Possible complications of CRIs include infection, overcorrection, undercorrection, perforation of the cornea, induced astigmatism, discomfort and decreased corneal sensation.
Overcorrection: Wait for the refraction to stabilize. The incision may be cleaned (with Sinskey hook or similar instrument). Then the wound is sutured with a 10-0 nylon. Placing CRIs perpendicular to the original incisions may induce irregular astigmatism so this method of correction is discouraged.
Undercorrections may be corrected by enlarging the original incision.
Incisions should be examined carefully at time of placement for corneal perforation. If a perforation is noted, it should be sutured with a 10-0 nylon if it is not self-sealing. This will limit the effect of the procedure, but the suture may be removed several weeks later and the effect of the CRI should be apparent after suture removal.
The vast majority of patients do exceedingly well. Even routine cataract surgeries are transitioning into 'refractive' procedures. As patient expectations for improved refractive outcomes increase, cataract surgeons will find CRIs are a useful and easy to master tool to achieve optimal postoperative results.
- AMO online LRI calculator:
- Boyd K, Mendoza O, Turbert D. What Is Astigmatism? Symptoms, Causes, Diagnosis, Treatment. American Academy of Ophthalmology. EyeSmart® Eye health. https://www.aao.org/eye-health/diseases/astigmatism-4. Accessed March 03, 2023.
- ↑ Chang J. Femtosecond laser-assisted astigmatic keratotomy: a review. Eye Vis (Lond) 2018;5:6. doi: 10.1186/s40662-018-0099-9.