Promising results with refined technique

Bowman layer transplantation can be simpler and easier as an onlay procedure.

Dermot McGrath

Posted: Thursday, April 1, 2021

Jack Parker MD, PhD

Isolated Bowman’s layer transplantation (BLT) appears to offer a promising new treatment modality for a growing range of indications including corneal scarring from herpetic disease, corneal haze after refractive surgery or advanced corneal ectasia from keratoconus, according to Jack Parker MD, PhD.
“For the past decade, Bowman’s layer transplantation has been an important part of how we treat patients with advanced corneal scarring from refractive laser or advanced corneal ectasia from keratoconus. As we refine the technique further as an onlay procedure, it is poised to spread more rapidly as the technique becomes simpler and easier, particularly if the graft can be prepared in a more facile manner,” Dr Parker said at the online 11th EuCornea Congress.
Explaining the background to BLT, Dr Parker, in private practice at Parker Cornea, Birmingham, Alabama, and at NIIOS-USA, San Diego, California, said that while little is known about the function of Bowman’s layer in the eye, there is greater understanding of some of the layer’s descriptive features.
“These descriptive features include the fact that it provides an anatomical separation between the epithelium and the underlying anterior stroma and that this separating barrier tends to be mechanically strong,” he said.
Over a decade ago, Dr Parker and co-workers first noticed that dysfunctional healing after refractive laser occurred in the absence of the ablated Bowman layer in these patients.
“We therefore reasoned that we may be able to treat these scars by replacing Bowman’s layer. We performed an experiment in which an isolated donor Bowman’s layer was harvested from a donor cornea. The patient’s epithelium was debrided and the Bowman’s layer graft was placed into the cornea and allowed to heal. We observed that restoring this barrier between the anterior stroma and the overlying epithelium prevented recurrence of the scar,” he said.
Once the relevance of the separation function of Bowman’s layer had been established, the next step was to try to take advantage of the mechanical strength of Bowman’s layer grafts by implanting them in patients with advanced keratoconus.
Once the graft had been harvested from the donor cornea, the surgery was performed by dissecting a pocket in the mid-stroma of the recipient cornea and then sliding the isolated Bowman’s layer graft inside on top of a surgical glide and unfolding it,” he said.
The results of the first surgeries were better than anyone expected, said Dr Parker.
“What we observed is that these eyes would often experience something like eight or more dioptres of corneal flattening, which is already a huge benefit. But the real virtue of the procedure seemed to be that the corneas would be stiffened, with around 90% of the eyes seeing the progression of their keratoconus arrested with Bowman’s layer implant and dramatic improvements in the corneal curvature,” he said.
The outcomes also appeared to be durable, said Dr Parker. “We followed the original cohorts of Bowman layer recipients for up to 10 years and what we saw is that the graft appears to be well integrated in the mid portion of the recipient stroma, and the shape of the cornea was flatter and stable,” he said.
A recent refinement of the transplantation has been to use a Bowman’s layer onlay graft, explained Dr Parker.
“An inlay graft is placed into the middle of the recipient cornea, whereas an onlay graft is placed on to the surface of the recipient cornea. With the onlay approach, we have observed that we candebride the patient’s epithelium and place the Bowman’s layer graft on top of the cornea and allow it to dry and fixate without sutures or glue or dissecting a pocket,” he said.
The technique has shown promising results in patients with ectasia and fluctuating vision after radial keratotomy.
“What we observe is a normalisation of the shape of the cornea. The ectatic areas get flatter and the flat, sunken, depressed areas tend to get steeper as the cornea remodels with the presence of the Bowman layer graft,” he said.
Dr Parker said that Bowman layer onlay grafting may also have applications in the treatment of corneal haze as a result of herpetic scarring.
“We noted that debriding the epithelium and then placing a Bowman layer onlay graft over the surface of the cornea allowed a normal healing response to finally occur so that the cornea is clear without the recurrence of corneal scarring,” he said.
Going forward, Dr Parker said further research is needed into Bowman layer onlay procedures which are technically easier to perform than inlay grafts.
“We also need new studies on preparation of the graft tissue itself. The Bowman’s layer graft is typically prepared manually in the eye bank, but it is a tedious and time-consuming process. There have been experiments using a femtosecond laser for dissection, but so far these grafts are much thicker than those prepared by hand, and there is some concern that the extra thickness may optically obscure the effect of the transplant,” he concluded.
Jack Parker: