Distorted corneas: How to regularise them with minimally invasive surgery

Corneas distorted by trauma, infection, dystrophies or refractive surgery all feature anterior corneal surface irregularities that may be analysed as a sum of high order aberrations (HOAs).

Spectacles or intraocular lenses (IOLs) cannot correct HOAs, and these are often underestimated or overlooked by ophthalmologists. Even radial order HOAs (ie, spherical aberration or secondary astigmatism) reduce contrast sensitivity, while odd radial order ones (ie, coma and trefoil) reduce visual resolution. Highly aberrated corneas often feature irregular astigmatism that may be considered as a sum of HOAs and astigmatism. An irregular and highly aberrated cornea hampers precise biometry (Figure 1) and reduces postoperative visual acuity when cataract surgery is planned.

In the past, often the usual treatment for highly aberrated corneas was penetrating keratoplasty or deep anterior lamellar keratoplasty, with the long rehabilitation time and possible complications related to such invasive surgeries. Corneal reshaping attained with customised ablation may reduce or eliminate corneal HOAs.

In phototherapeutic keratectomy (PTK), excimer laser is used to treat corneal disorders such as surface irregularity, epithelial instability and superficial opacity. Sequential customised therapeutic keratectomy (SCTK), a recent evolution of PTK, is a transepithelial, customised, multistep PTK addressing corneal irregularities, in particular HOAs.

SCTK improves visual quality by eliminating superficial corneal HOAs and pursues a more physiologic, possibly prolate, corneal profile. In an eye with a highly aberrated cornea, SCTK may eliminate glare and halos caused by HOAs and optimise the precision of preoperative biometry.

One of the advantages of SCTK is that the correction of corneal HOAs requires removal of a minimal amount of tissue, thus preserving corneal biomechanics. SCTK is a multistep procedure, featuring custom transepithelial corneal topography-based PTK and intraoperative topographic monitoring. In the laser platform we adopted, it is possible to select which HOA should be treated as a priority, ignoring all the others, thus minimising ablation depth or volume. After ablation, wet PTK with sodium hyaluronate 0.25% masking fluid is performed to remove residual microirregularities and achieve a regular stromal bed, as similar as possible to the physiological Bowman layer.

After this first treatment, as a second step, the patient is brought to an examining lane and intraoperative topography, tomography and corrected distance visual acuity are performed. The third step of the procedure involves repeating intraoperative topography-guided ablation, followed by wet PTK. Thereafter, intraoperative topography/tomography and visual acuity are once more performed. If the surgeon deems that the preoperative target has been reached, they could decide to stop.

The target includes 1 or more of the following 3:

1. Improvement of preoperative CDVA of at least 2 lines, regardless of the increase or decrease of refractive error
2. All keratoscopic rings are visible without the use of masking fluid
3. Low curvature gradient (lower than 4.00-5.00 D/mm) in the 6.5 central mm is observed, evaluated with the corneal curvature gradient map

The corneal curvature gradient is defined as the difference between the curvatures of 2 points and is calculated as the 1st derivative of the tangential curvature map in the radial direction. Indications for SCTK include corneal diseases such as superficial dystrophies or degenerations, stromal irregularities caused by trauma, corneal refractive surgery, recurrent erosions, corneal opacities and neovascularisations. Because minimal tissue is ablated, patients with a history of surface or intrastromal photorefractive surgery may be safely re-treated.

Complications of refractive surgery amenable of treatment with SCTK include decentration, haze, scars, small optical zones and LASIK interface problems (Figure 2 [post PRK], Figure 3 [post LASIK],
Figure 4 [post radial keratotomy]).

Corneal surface reshaping with SCTK prioritises the elimination of HOAs over the correction of ametropia. For this reason, spherical refractive error may increase after SCTK, but when subsequent cataract surgery is planned, IOL implantation easily eliminates lower-order aberrations, attaining planned refraction (i.e, emmetropia) (Figure 5). A residual cylinder, now regular once HOAs (i.e., coma) are eliminated, may be corrected by a toric IOL.

Performing SCTK before cataract surgery thus improves refractive outcome with the latter procedure. SCTK can also increase the pool of candidates for premium IOLs by eliminating HOAs.