KEY CONCEPTS
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Bowman’s layer transplantation can be used for the treatment of patients with advanced keratoconus not eligible for corneal cross-linking or intrastromal corneal ring segments, effecting a reshaping and flattening of corneal curvature and improving contact lens tolerance in advanced keratoconus.
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Bowman’s layer transplantation may stabilize and halt the progression of the disease in advanced keratoconus cases for the long-term prevention of further visual loss and may prevent or delay the need for corneal transplantation.
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Stromal regeneration through cell therapy may represent a future option to restore corneal thickness, shape, and transparency in keratoconus.
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Preliminary studies of stromal regeneration techniques have demonstrated safety and moderate efficacy in the treatment of advanced keratoconus.
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Laser-assisted crescent keratectomy may be a new surgical approach to improve corneal profile and reduce higher-order aberrations in eyes with keratoconus.
Introduction
Keratoconus is regarded as a corneal ectatic disease characterized by progressive central or paracentral protrusion and thinning of the cornea, leading to myopia and irregular astigmatism with reduced visual quality and acuity. ,
The goal of the treatment is to arrest the progression of the disease and achieve visual rehabilitation. In the early stages of the disease, visual rehabilitation can be accomplished with success through optical correction of the refractive error with glasses or soft contact lenses. However, in more advanced cases, rigid contact lenses may be required to obtain a regular anterior optical surface, to compensate for high-order aberrations (HOAs) and provide better visual performance. ,
For patients with moderate keratoconus who are intolerant of rigid contact lenses or who require better vision than with glasses, the implantation of intrastromal corneal ring segments (ICRSs) is an effective option for remodeling corneal shape and reducing the amount of spherical equivalent, astigmatism, and HOA. , ,
In cases of progression of the disease or high risk of progression (children and teenagers) corneal cross-linking (CXL) can stabilize the cornea and prevent vision loss. However, in terms of efficacy and safety, the results of ICRS implantation or CXL in cases of advanced keratoconus with a thinner cornea are poor and the procedures are usually not indicated.
The treatment for these advanced cases with very poor visual acuity (VA) and/or contact-lens intolerance is usually corneal transplantation, with either penetrating keratoplasty (PK) or deep anterior lamellar keratoplasty (DALK). Although corneal transplantation can provide efficient visual rehabilitation in the short term, long-term outcomes may be compromised by adverse events.
Recently, a new surgical approach, Bowman’s layer transplantation (BLT), has been introduced to treat progressive advanced keratoconus cases that are not good candidates for ICRS implantation or CXL or as an alternative to corneal transplantation with the intention to delay PK.
In addition, new approaches that regenerate the corneal stroma with cell-based therapy through different tissue engineering techniques and techniques to reshape corneal curvature through laser-assisted crescent keratectomy are being developed.
In this chapter, we will summarize the current available literature on BLT, stromal regeneration techniques, and laser crescent keratectomy.
Bowman’s Layer Transplantation
The pathogenesis of keratoconus includes genetic, environmental, and mechanical factors. It is postulated that abnormally weak corneal collagen could lead to a progressive deformation and thinning of the cornea with mechanical factors such as eye rubbing, or overexpression of certain inflammatory factors. ,
Bowman’s layer (BL) is the second stratum of the cornea, between the basement membrane of the epithelium and the stroma. Previously considered a membrane, it is rather an acellular condensate of the anterior stroma, composed of highly compacted collagen type I and III fibrils. Its thickness is approximately 12 to 14 microns. BL does not have regenerative capacity and does not regenerate after trauma, surgical injury, or ablation with excimer laser. , It is present in primates, but not in other mammals, and its function remains unclear. It is thought that it protects the subbasal nerve plexus and the stroma against pathogens. It may play a role in modulating the epithelial-stromal wound healing process and it is considered as one of the strongest biomechanical layers of cornea.
The first report in scientific literature on the use of isolated BLT was published by Lie et al. for the treatment of a case of recalcitrant haze after photorefractive keratectomy (PRK).
BL appears to prevent excessive bulging of the cornea. In fact, fragmentation and sectoral disappearance of BL are a pathognomonic feature in the histologic sections from keratoplasty of corneal specimens of patients with keratoconus. , , That finding led these authors to study a variant of this technique to treat progressive advanced keratoconus. They theorized that a midstromal inlay of BL could partially restore corneal curvature with a flattening effect and at the same time stabilize the disease.
DONOR TISSUE PREPARATION
Like other lamellar keratoplasty techniques, the graft is obtained prior to surgery. Graft tissue can be prepared from a corneoscleral button or a whole globe obtained less than 24 hours, with equivalent success. Corneas not valid for PK or endothelial keratoplasty because of low endothelial cell count can be also used for harvesting the BL graft. Furthermore, corneoscleral buttons used to harvest Descemet membrane endothelial keratoplasty (DMEK) grafts can be used to obtain BL tissue for another patient, allowing expanded use of the donor tissue. , ,
The donor corneoscleral button is mounted on an artificial chamber and pressurized with balanced salt solution (BSS). Epithelium is removed completely with surgical sponges. A circular 360-degree superficial incision in the BL within the limbal corneal periphery is made using a 30-gauge needle, to access a peripheral plane of dissection. With McPherson forceps, the peripherical edge of BL is lifted, then BL is carefully peeled from anterior stroma, moving from the periphery to the center over the full 360 degrees ( Fig. 34.1 ). Because of inherent elasticity of the tissue, BL graft spontaneously curls into a double or single roll with the epithelial border at the outside. BL is usually trephined with a 9- to 11-mm corneal punch. The graft is submerged in 70% alcohol to eliminate any remaining epithelial cells. The BL inlay is then rinsed with BSS and stored in organ-culture medium at 31°C until implantation.
Standardizing the harvesting of a thin BL graft is the most challenging step of the procedure. It is laborious and technically demanding, with a 30% failure rate. Tearing of the BL during preparation or an unusually thick BL graft with additional stroma attached are the main reasons for discarding the tissue. To help overcome this technical barrier, Parker et al. investigated the potential use of the femtosecond laser to prepare BL graft. Donor globes were mounted in a globe holder and, after de-epithelialization, the laser was programmed to make a superficial cut at a depth of 20 microns. However, grafts prepared using this method were statistically thicker than with the manual technique, owing to the presence of a variable amount of stromal residual tissue. , If the BL is dissected with additional stroma, visual quality and the clarity of the cornea can be affected postoperatively because of interface haze.
SURGICAL TECHNIQUE
BLT consists of implanting a circular inlay of BL in the middle of the stroma of the recipient keratoconic cornea through a stromal pocket. This midstromal pocket can be made using the manual technique for lamellar dissection in DALK described by Melles et al. , or using a femtosecond laser. The surgery can be done under topical, local, or general anesthesia.
In the manual technique, the anterior chamber is filled with air through a peripheral paracentesis. Then, the superior conjunctiva is opened and dissected and a superficial scleral incision of 5 mm in length, 1- to 2-mm outside the limbus, is made. With a set of dissection spatulas of specific design (Melles spatula set; DORC International BV, Zuidland, the Netherlands), the scleral incision is tunneled up to the superior cornea. A paracentesis is made, through which the anterior chamber is filled completely with air. The air-endothelium interface acts as a convex mirror that reflects images from the anterior and posterior corneal surfaces as a specular light reflex. The non-incised corneal tissue between the tip of the spatula and the light reflex from the air-endothelium interface is seen as a dark band directly surrounding the tip. In this way, one can control the depth of dissection, monitoring the width of this dark band to create the pocket at a depth of about 50% from limbus-to-limbus 360 degrees. When the midstromal pocket over a circumference of 8 to 9 mm in diameter is completed, the air in the anterior chamber is replaced by BSS. The BL graft is immersed again in 70% ethanol for 30 seconds, rinsed with BSS, and stained with trypan blue 0.06% for better visualization in the stroma. The BL roll is unfolded, inserted, and centered in the pocket through the scleral incision, helped by a glide (BD Visitec [Fichman]; Beaver-Visitec International, Waltham, MA) and a blunt spatula or a cannula with BSS ( Fig. 34.2 ). Postoperative treatment includes topical antibiotics and steroid on a tapering dosage. ,
Intraoperatively, a Descemet perforation during stromal dissection has been reported in 10% of cases (2/22 eyes). Management of this complication may include aborting the surgery and reattempting the dissection later when wound healing has occurred or proceeding with a consecutive PK. Intraoperative anterior segment optical coherence tomography (iAS-OCT) can be used for better visualization of the dissection plane during the surgical procedure, even in the presence of corneal scarring, edema, or blood.
To reduce the complication rate and make the stromal dissection in the host cornea easier, García de Oteyza et al. have introduced a variant of the surgical technique in which the stromal pocket is created by a femtosecond laser at a depth of 50%, instead of manually ( Fig. 34.3 ). They reported two cases of grade IV keratoconus intolerant to contact lenses treated with BLT using this technique without complications. Both patients became contact lens tolerant with an improvement in best spectacle-corrected visual acuity (BSCVA) and best contact lens–corrected visual acuity (BCLVA), keratometry, and pachymetry values after surgery.
CLINICAL OUTCOMES
van Dijk et al. were the first to report the results of BLT in 20 eyes with advanced progressive keratoconus with a follow-up to 5 to 7 years. , , , The outcomes of BLT are summarized in Table 34.1 . A significant flattening effect of −4.1 diopter (D) in mean simulated keratometry (K mean ) and −6.9 D in maximum keratometry (K max ) was observed after BLT at 1 month, without significant changes thereafter up to 5 years. This flattening effect seems more prominent in more advanced cases and with central cones. BSCVA improved from 0.05 preoperatively to 0.13 postoperatively at 12 months, and was unchanged thereafter. Of the eyes, 85% gained Snellen lines of BSCVA or remained stable. BCLVA remained stable during follow-up; however, all patients tolerated rigid contact lenses better postoperatively, whereas most were intolerant before the surgery. Thinnest corneal thickness (TCT) showed an increase after surgery, whereas central corneal thickness (CCT) remained stable. However, in 10% of cases (2/20 eyes), there was progressive corneal steepening. Intraoperatively, two eyes were complicated by perforation of the Descemet membrane during manual dissection and were excluded from the analysis. , Three cases suffered an acute hydrops 4.5, 6, and 6.5 years after BLT without previous signs of progression but with a history of atopy and continuous eye rubbing. No other complications such as adverse endothelial events or allograft rejection episodes were observed. Endothelial cell density remained stable after BTL for up to 5 years. ,
| Authors | Study Type | Indication | Sample Size (Eyes/Patients) | Mean Follow-Up (Months) | Stromal Pocket Dissection Technique | Main Outcome | Secondary Outcome | Complications | ||
|---|---|---|---|---|---|---|---|---|---|---|
| van Dijk et al. (2014) | Prospective | Progressive a advanced KC grades III and IV | 10/9 | 16 | Manual | Decrease in K mean and K max | Improvement in contact lens wearing. | None | ||
| van Dijk et al. (2015) | Prospective | Progressive a advanced KC grades III and IV | 22/19 | 20 | Manual | Decrease in K mean and K max | Improvement in contact lens wearing. Little increase in BSCVA. | Intraoperative perforation of Descemet membrane in two cases. Progressive steepening in two cases (10%). | ||
| Luceri et al. (2016) | Retrospective | Progressive a advanced KC grades III and IV | 15/14 | 12 | Manual | Decrease in anterior and posterior HOA RMS | Increase medium and posterior corneal backscattering | |||
| van Dijk et al. (2018) | Prospective | Progressive a advanced KC grades III and IV | 20/17 | 60 | Manual | Decrease in K mean and K max | Improvement in contact lens wearing. Little increase in BSCVA. | Progressive steepening in two cases (10%). Acute hydrops after 4.5 years in one case. | ||
| García de Oteiza et al. (2019) | Prospective | KC grade IV with intolerance to contact lenses | 2/2 | 3 | Femto second laser | Decreased simK1 and simK2. Contact lens tolerance. | Increased CCT and TCT | None |
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