Bowman Layer Onlay Grafting for Advanced Keratoconus

For patients with advanced keratoconus, the treatment conversation has historically led to a difficult crossroads: continue struggling with increasingly poor vision, or undergo a full corneal transplant with its months-long recovery and meaningful risk of rejection. A newer procedure — Bowman layer onlay grafting — is changing that calculus, and the published results are encouraging enough that we believe our patients should know about it.

The Problem With Advanced Keratoconus

Keratoconus causes the cornea to thin and steepen into a cone shape, distorting vision progressively. For early and moderate stages, we have effective tools: corneal crosslinking (CXL) halts progression, and scleral contact lenses restore clear, comfortable vision by vaulting over the irregular cornea.

But for patients whose disease has progressed to advanced stages — with maximum keratometry (Kmax) values above 70 diopters and significant corneal thinning — crosslinking may no longer be safe to perform, and even well-fitted scleral lenses can struggle to fully correct the extreme irregularity. Traditionally, the only remaining option has been a corneal transplant: either penetrating keratoplasty (PK) or deep anterior lamellar keratoplasty (DALK).

Both of those procedures work — but they come with substantial trade-offs. They require entering the eye surgically, carry real risks of infection and rejection, demand 12 to 18 months of visual rehabilitation, and necessitate years of topical immunosuppression. For a young patient in their twenties or thirties, that is a significant burden.

What Is Bowman Layer Onlay Grafting?

The Bowman layer is the thin, tough membrane that sits just beneath the corneal epithelium (the outermost layer). In Bowman layer onlay grafting, a surgeon isolates this membrane from a donor cornea and places it on top of the patient's cornea — not inside it. The procedure works like this:

1. Epithelium removal — the surface layer of the cornea is gently removed (similar to the first step of traditional crosslinking)
2. Graft placement — a single or double donor Bowman layer membrane is stretched onto the exposed corneal surface
3. Drying and adherence — the graft is allowed to dry onto the cornea, creating a natural bond
4. Bandage lens — a soft contact lens is placed to protect the surface while the epithelium regrows over and around the graft

The entire procedure is completely extraocular — the surgeon never cuts into the corneal stroma and never enters the anterior chamber of the eye. This is the fundamental safety advantage over any form of corneal transplant.

What the Research Shows

A prospective case series published in the American Journal of Ophthalmology (2024) evaluated 21 eyes that underwent Bowman layer onlay grafting for advanced, progressive keratoconus. The results were notable:

• All 21 surgeries were uneventful — no intraoperative complications
• Corneal flattening of up to 6 diopters — mean Kmax decreased from 76 D preoperatively to 72 D postoperatively
• Follow-up of 6 to 36 months (mean 21 months) — results remained stable over time
• Scleral lens wear preserved — patients maintained their best corrected contact lens visual acuity (BCLVA)
• Diminished rejection risk — the Bowman membrane is acellular (contains no living cells), which dramatically reduces the immune response compared to full-thickness corneal grafts

Longer-term data on Bowman layer transplantation (the related "inlay" technique, where the membrane is placed inside the stroma) shows stability out to 5 years, with measured keratometry values remaining flat after the initial post-operative decrease. A separate study demonstrated that contact lens tolerance was preserved up to 8 years after the inlay procedure in eyes with progressive advanced keratoconus (Kmax > 69 D).

Onlay vs. Inlay: Two Approaches

The research literature describes two Bowman layer techniques. Understanding the difference helps clarify where the onlay approach fits:

	Bowman Layer Inlay	Bowman Layer Onlay
Graft placement	Inside a stromal pocket (mid-cornea)	On top of the cornea (surface)
Requires stromal dissection?	Yes — femtosecond laser or manual	No — completely surface-level
Cornea entered?	Partially (stromal pocket)	No — entirely extraocular
Thin cornea candidates?	Risky — thin corneas may perforate	Suitable — no perforation risk
Long-term data	Up to 8 years published	Up to 3 years published (newer)

The onlay technique was developed specifically to address the limitation of the inlay approach in the thinnest, most advanced corneas — precisely the cases where creating a stromal pocket carries the highest risk of perforation. For these patients, the onlay graft offers a safer path to corneal flattening.

How This Fits Into the Keratoconus Treatment Ladder

We think of keratoconus management as a stepwise approach. Bowman layer onlay grafting occupies a specific and valuable position on this ladder:

1. Monitoring — for subclinical or stable early disease, watchful waiting with serial topography
2. Corneal crosslinking — halts progression in early-to-moderate disease (now available as epi-on with EpiOxa)
3. Scleral contact lenses — restores clear vision at any stage, vaulting over the irregular cornea
4. Bowman layer grafting — flattens advanced corneas, preserves scleral lens wear, avoids transplant risks
5. Corneal transplant (DALK or PK) — reserved for cases where all other options have been exhausted

The goal at every rung is to delay or avoid the next one. Bowman layer grafting adds a meaningful intermediate step between scleral lenses alone and a full transplant — one that can buy years or decades of stable vision.

What This Means for Our Patients

While Bowman layer onlay grafting is not yet widely available at every center, the published evidence is growing and the surgical community is paying attention. At EyeCare Center of Orange County, Dr. Bonakdar has managed hundreds of advanced keratoconus cases with custom scleral lenses, and we work closely with corneal surgeons who perform these emerging procedures.

If you have advanced keratoconus and have been told a transplant is your only option, it is worth knowing that alternatives like Bowman layer grafting exist — and that the evidence supporting them is real and peer-reviewed, not speculative.

We can evaluate your corneal topography, stage your disease, and discuss whether you might be a candidate for this or other intermediate treatments before considering a transplant. Call our keratoconus line at (949) 693-4900 to schedule your evaluation, or learn more about our keratoconus treatment program.

References

• Birkenfeld JS, et al. "Bowman Layer Onlay Grafting as a Minimally Invasive Treatment for the Most Challenging Cases in Keratoconus." American Journal of Ophthalmology. 2024;261:195-204. PubMed.
• van Dijk K, et al. "Bowman layer transplantation: 5-year results." Cornea. 2018;37(12):1550-1554. PubMed.
• Luceri S, et al. "Long-Term Outcomes of Bowman Layer Inlay Transplantation for the Treatment of Progressive Keratoconus." Cornea. 2022;41(9):1150-1157. PubMed.
• Davidson J. "A Less Risky Procedure for Patients With Keratoconus." Modern Optometry — Significant Findings. ModernOD.