Corneal Tissue Addition Keratoplasty

The treatment of keratoconus has expanded during the past 2 decades. CXL, including established epithelium-off techniques and the epithelium-on approach recently approved by the US FDA, can stabilize progressive disease.^1,2 Advances in specialty contact lenses, such as wavefront-guided scleral lenses, have improved visual rehabilitation.³ Nevertheless, a treatment gap remains between these approaches and corneal transplantation.

Some patients with stable keratoconus have markedly irregular corneas, leaving them with poor UCVA or BSCVA or an unsustainable dependence on specialty contact lenses. These individuals may not be ready to undergo deep anterior lamellar keratoplasty or penetrating keratoplasty, and specialty contact lens wear may not entirely address the functional burden of their disease.

Corneal tissue addition keratoplasty (CTAK) is a minimally invasive anterior lamellar keratoplasty procedure in which customized irradiated donor tissue (VisionGraft, CorneaGen) is implanted in an intrastromal channel created using a femtosecond laser. Rather than replace the central cornea, CTAK adds tissue strategically to reshape the ectatic cornea while preserving the central visual axis.

CTAK may be used as part of a comprehensive approach to keratoconus management. CXL is performed to stabilize progressive disease, followed by CTAK to improve corneal shape and then the prescription of spectacles or contact lenses to further rehabilitate the patient's vision.

Why Add Tissue?

In keratoconus, corneal thinning and protrusion cause irregular astigmatism and reduce optical quality. By adding stromal tissue in a carefully selected location, CTAK can alter corneal geometry, thereby flattening and regularizing the ectatic region.

CTAK uses preserved, gamma-irradiated donor corneal stroma. The tissue is custom-shaped with a femtosecond laser to individual patient specifications. The goal is not always to create maximum flattening but to place the appropriate amount of tissue at the right location to improve the overall corneal shape and the patient's visual function.

Clinical Experience

Our initial prospective clinical trial evaluated CTAK in 21 eyes of 18 patients, predominantly with advanced keratoconus.⁴ Specialty contact lenses had failed in many of them, and their remaining option was corneal transplantation.

At 6 months, the patients' mean uncorrected distance visual acuity had improved from approximately 20/327 to 20/82 (> 5 lines), and their mean corrected distance visual acuity had improved from approximately 20/82 to 20/43 (> 3 lines). Ninety-five percent of eyes had gained at least 2 lines of uncorrected vision, and 57% had gained at least 2 lines of corrected vision.

The topographic effect was also substantial. Mean keratometry decreased by 8.44 D, and the mean point of maximum flattening on axial difference maps was 16.03 D.

Personalizing CTAK

The goal of CTAK is not the same for every patient. The procedure's potential role depends on the patient's disease severity, baseline visual function, and dependence on specialty contact lenses (Figure 1). CTAK may be able to improve both the UCVA and the BSCVA of individuals with mild keratoconus. For those with moderate disease, improving their corneal regularity and BSCVA may substantially reduce or, occasionally, eliminate their dependence on specialty contact lenses.

Patients with more advanced keratoconus will likely require specialty lenses to achieve their best vision after CTAK. More functional vision in glasses, however, may allow them to remove their contact lenses for portions of the day (Figure 1C). This is an important and sometimes underappreciated goal. Many of these patients essentially depend on specialty contact lenses during all waking hours, and maintaining this level of wear over decades can become increasingly difficult. If their BSCVA after CTAK allows them to remove these lenses in the evening or for portions of a weekend, that reduction in daily wear time may support sustainable long-term specialty contact lens use and reduce the risk of future intolerance.

There is also a practical and psychological benefit to having functional backup vision. Patients who are highly dependent on specialty contact lenses often experience chronic anxiety regarding the potential loss or breakage of these lenses and an inability to wear them because of ocular irritation or another problem. Clinicians who care for patients with keratoconus are likely familiar with the following scenario. A patient presents with a red, painful eye while continuing to wear a specialty contact lens. When asked why they did not remove the lens, they often respond, "I cannot function without my lenses." Even a temporary interruption in contact lens wear can result in a profound loss of functional vision, making it extremely difficult to convince these individuals to discontinue lens wear when clinically necessary. In contrast, a patient who can function reasonably well in glasses has an alternative.

Success after CTAK should therefore not be defined solely by contact lens independence. For some patients, it may be better UCVA, improved BSCVA, conversion to an alternative form of optical correction, fewer hours of specialty contact lens wear, or simply functional vision when a contact lens is not being worn.

Candidacy

Patient selection begins with identifying a meaningful functional problem that CTAK has the potential to improve. Contact lens history is particularly important. The risk-benefit calculation is very different for someone who achieves comfortable 20/20 vision in a scleral lens and wears it successfully throughout the day compared to someone who cannot insert a scleral lens, tolerates a corneal gas permeable lens for only a few hours, or has nonfunctional vision whenever the lens is removed.

Anatomically, we generally prefer it if the area of greatest steepening and the thinnest region lie within the central 6 to 8 mm of the cornea. Patients with peripheral ectasia and true pellucid marginal degeneration may have less favorable outcomes after CTAK placement. Dense central scarring is a potential contraindication because CTAK does not remove or improve an opacity. Extra care is also required in cases of post-LASIK ectasia to determine whether the remaining stroma beneath the LASIK flap is adequate for the CTAK procedure.

A Customized Procedure

Tomography drives CTAK planning. The position and dimensions of the cone are evaluated, and these data are used to generate a customized tissue design and corresponding map for femtosecond laser channel placement (Figure 2). The tissue is cut to the approved specifications and delivered for implantation.

Surgery is performed under topical anesthesia. A femtosecond laser is used to create the stromal channel and entry incision. The channel is opened and fully dissected, and the donor tissue is inserted and advanced to its planned position. Accurate placement is critical because the topographic effect depends on the tissue's final location. In uncomplicated cases, the procedure can be performed without corneal sutures.

CTAK Is Not CXL

CTAK is not a substitute for CXL. The latter procedure is performed to increase corneal biomechanical stability and reduce the risk of disease progression, whereas the goal of CTAK is to alter corneal shape. The procedures are synergistic. Progressive keratoconus should be addressed independently.

In our initial study,⁴ 11 eyes had previously undergone CXL and were confirmed to be stable before CTAK. Surgically, prior CXL did not increase the difficulty of CTAK or the risk of complications during corneal channel dissection.

We have performed CTAK followed by CXL on the same day in several cases with excellent results. The optimal timing and sequencing of CXL and CTAK continue to be investigated.

CTAK Before Cataract Surgery

CTAK may have an important role in patients with keratoconus who are approaching cataract surgery. In this population, a long-term consideration is the potential loss of the dexterity needed to insert, remove, and care for specialty contact lenses. Maximizing their functional vision without a specialty lens therefore assumes increasing importance.

The irregular cornea in these eyes can make both IOL power calculation and postoperative refractive correction challenging. In select patients, we found that performing CTAK before cataract surgery provided a staged approach to visual rehabilitation. CTAK flattened and regularized the cornea, which was then allowed to stabilize before cataract extraction.

The implantation of a Light Adjustable Lens (RxSight) can be particularly useful in this setting because residual spherical and cylindrical refractive errors can be addressed postoperatively. Although further study is required to identify appropriate patients, timing, and refractive predictability, we have found this combination to be a promising strategy for visual rehabilitation in select individuals with keratoconus and cataract.

Expanding the Keratoconus Treatment Spectrum

Modern keratoconus management is becoming increasingly individualized. CXL can stabilize the disease. Topography-guided surface ablation provides a subtractive method for reshaping select corneas.⁵ Intrastromal corneal ring segments can alter corneal geometry, but this intervention is limited by the segments' fixed dimensions and may be complicated by extrusion or inflammatory reactions to the PMMA material.^6,7 Deep anterior lamellar keratoplasty and penetrating keratoplasty remain essential options for advanced disease, particularly when significant central scarring is present.

CTAK is additive rather than subtractive and uses biologic rather than synthetic material. The procedure preserves the central cornea and can be customized to the individual ectatic pattern. For the appropriate patient, CTAK expands the treatment options between optical rehabilitation and corneal transplantation. It can provide an opportunity to substantially reshape an irregular keratoconic cornea while preserving the host cornea and maintaining future surgical options. Importantly, the goal of treatment can be tailored to the individual.

As clinicians' ability to stabilize, reshape, and optically rehabilitate keratoconic corneas improves, treatment is becoming less about choosing a single procedure and more about identifying which component of the disease—and which aspect of the patient's visual function—needs to be addressed. Within that framework, CTAK provides a unique and effective treatment designed to improve corneal shape across a wide spectrum of keratoconus patients.