Bowman Membrane Transplant (BMT)
Definition & Overview
Bowman membrane transplant (BMT) is a surgical procedure to treat advanced, progressive keratoconus where the donor Bowman’s layer (BL) is transplanted into a mid-stromal corneal pocket. This procedure aims to flatten the cornea and offer stabilization against further ectasia (distension) and enable the patient to continue wearing contact lenses for normal visual functioning.
BMT is a sub-category of anterior lamellar keratoplasty (ALK), which involves the selective removal of a corneal layer (stroma) and replacing it with healthy donor tissue. It uses an acellular graft, potentially avoiding complications of graft failure commonly found in penetrating keratoplasty (PK) and deep anterior lamellar keratoplasty (DALK). BMT also does not use sutures, further avoiding suture-related complications. It is a new procedure in the ophthalmic field with less intensive medical therapy and requires less stringent surveillance.
BMT is suitable for patients with progressive advanced keratoconus in whom intracorneal ring segments and ultraviolet corneal crosslinking cannot work. They should also have an acceptable contact lens-corrected vision. The Bowman’s layer gives the anterior cornea its biomechanical strength and shape. In advanced keratoconus, this layer is thin and disrupted, further weakening the cornea and leading to more ectasia. Transplanting the Bowman’s layer strengthens the anterior cornea, restores its shape, and arrests ectasia. The procedure enhances visual rehabilitation with contact lenses because it improves fitting and lens’ retention, avoiding or delaying the need for PK or DALK.
Also Known As
- Bowman Layer Transplantation
Before the Procedure
Before the surgery, the graft is prepared following laid-down ophthalmic standards. The medical history and blood screening of the donor are made. The BL is traditionally prepared from a donor globe obtained from a deceased person less than 24 hours after demise. The graft can also be obtained from an anterior corneal button of a cornea that has been stripped of the endothelium and Descemet membrane in Descemet membrane endothelial keratoplasty (DMEK) patients. Recent studies indicate that a femtosecond laser can also be used to prepare BL tissue with better outcomes than extracting the tissue manually.
The ophthalmologist takes the patient through what to expect, including the benefits and risks of the procedure. The patient's medical history is taken, as well as a clinical and ophthalmic evaluation.
The patient is put under local anesthesia and given an eye massage. A Honan’s balloon is applied for ten minutes. The patient is then placed in an anti-Trendelenburg position. During the procedure, the surgeon:
- Performs a superior conjunctival peritomy
- Uses a crescent knife to make and dissect a partial scleral tunnel outside the limbus. S/he makes the dissection into the clear cornea
- Fills the anterior chamber with air after creating a paracentesis
- Uses a DALK technique to create a manually dissected stromal pocket up to the limbus
- Removes the air and incises the cornea-scleral into the dissected pocket
- Immerses the graft in ethanol (70%) for about 30 seconds to eliminate the remaining cellular material. S/he thoroughly rinses the graft with a balanced salt solution
- Stains the graft with Trypan blue, places it on top of the glide, and uses a cannula to push it into the eye
- Removes the glide once the graft is settled inside the stromal pocket. S/he folds the graft and positions it
- Pressurizes the eye using a balanced salt solution and repositions the conjunctiva to the superior limbus
- Patches the eye
- No suturing is required
Risks & Complications
BMT is a minimally invasive approach with minimal risks and complications because it is limited to the stromal pocket, and no surface incisions and sutures are used. Therefore, there are no suture-related or postoperative surface complications. Besides, the risk of allograft rejection is non-existent due to the use of an acellular BL tissue. The chances of cataract formation and the development of glaucoma are minimized because topical steroids are rapidly discontinued.
Intraoperatively, there is the risk of a Descemet membrane perforation when the surgeon dissects the mid-stromal pocket. The surgeon aborts the procedure so that the perforation heals, and BMT surgery can be performed later. The other option is for the surgeon to perform a PK depending on the perforation's position and size.
Corneal hydrops may occur postoperatively even years after the surgery. It is possible eye rubbing and allergies may lead to the hydrops; therefore, these patients require close monitoring.
Aftercare & Recovery
After the procedure, the patient will take antibiotics for one week. Corticosteroids are also prescribed and should be taken for one month. The continuation of these steroids will depend on patient recovery.
BMT improves corneal thickness, with the patient better able to tolerate contact lenses. There is a decrease in keratometry values, and visual acuity significantly improves. In one study, a reduction of keratometry values remained stable for five years when the patient was followed up, and best-corrected visual acuity improved and stabilized over a 5-year follow up period.
In another study, the operated eye demonstrated significant corneal flattening of 8-9 D in keratometry (maximum) values a month after the procedure, and ectasia stabilized. In five years of follow up, the progression of keratoconus and severe complications were avoided in 84% of the eyes.
In yet another study, after a 4-year follow up period, 90% of the patients had experienced a cessation of ectasia progression. The corneas flattened by 8 D and best-corrected visual acuity improved in most of the sample patients.