Anatomopathological Aspects and Clinical Correlation of Conjunctival Bleb Fibrosis Following Encapsulation of a Biodegradable Collagen Matrix Implant in Glaucoma Surgery

Filtering surgery remains the most widely used surgical intervention for glaucoma.¹ These procedures aim to create a fistulous tract between the anterior chamber and the subconjunctival space to facilitate the drainage of aqueous humour and reduce intraocular pressure (IOP). As a result, aqueous humour accumulates in a subconjunctival reservoir known as the filtration bleb. The primary cause of surgical failure is excessive formation of fibrotic tissue around the new drainage pathway,^{2, 3} which compromises proper functioning of the bleb.

Glaucoma surgery is intrinsically pro-fibrotic² due to a chronic pro-inflammatory conjunctival environment that often results from prolonged use of topical medications, preservative agents in eye drops, hypersensitivity to active compounds, and a high prevalence of dry eye disease. Furthermore, postoperative tissue exposure to aqueous humour, which is rich in pro-inflammatory cytokines, promotes fibroblast activation.⁴

Fibrotic scarring involves the excessive synthesis of extracellular matrix (mainly consisting of dense, organised collagen fibres³) and the persistence of myofibroblasts that should normally undergo apoptosis.² Effectively modulating this fibrotic response is crucial for improving surgical outcomes.

Advances in tissue engineering have provided alternative strategies to modulate wound healing and reduce scar formation after glaucoma surgery, such as biodegradable implants. Unlike antimetabolites, whose non-specific inhibition of cell proliferation may lead to significant ocular toxicity, these implants promote the formation of functionally effective filtering blebs while degrading into non-toxic byproducts, thereby eliminating the need for surgical removal.⁵

Ologen^® (Aeon Astron Europe B.V., Leiden, the Netherlands) is a biocompatible, biodegradable implant composed of collagen (90%) and glycosaminoglycan (10%).^{5, 6} These components are organised in a three-dimensional system, creating a 95% porous structure that acts as a biological scaffold for the healing process,^{7, 8, 9} facilitating fibroblast growth inside the implant and guiding their migration to create a loose, randomly organised deposition of connective tissue similar to physiological conditions.^{6, 7, 8, 9} This is the mechanism underlying the implant’s modulation of wound healing.^{8, 10, 11, 12} However, it is important to highlight that the implant itself does not have antifibrotic properties and could therefore hypothetically cause a secondary tissue reaction in the subconjunctival space.¹³ To avoid this reaction and modulate its immunogenicity, the collagen is purified with pepsins that eliminate telopeptides, which are the N- and C-terminal amino acid sequences responsible for the antigenicity of the collagen molecule.^{10, 11, 14, 15}

Various published surgical series comparing patients undergoing surgery with and without the implant have yielded inconclusive results regarding its potential to enhance the antifibrotic effect of other techniques (summarised in Tables 1^{3, 7, 8, 9, 16-27} and 2^{12, 13, 14, 15, 28-41}). In this case series, we present four cases of glaucoma surgery in which this biodegradable collagen-glycosaminoglycan implant failed to prevent fibrosis and was associated with an encapsulating fibrotic response surrounding the implant, which ultimately compromised surgical success.

Case 1

A 56-year-old female patient with moderate primary open-angle glaucoma (POAG) underwent non-penetrating deep sclerectomy (NPDS) with an intrascleral Esnoper V Clip implant, subconjunctival Ologen^® implantation (6 mm×2 mm), and a subconjunctival injection of 0.1 mL 0.01% mitomycin C (MMC). The patient exhibited progressive bleb flattening during the postoperative follow-up, causing a rise in IOP and requiring additional hypotensive medical therapy. One year after the initial surgery, the patient underwent surgical bleb revision. Intraoperatively, fibrotic conjunctiva and Tenon’s capsule were observed encasing the intact Ologen^® implant. Histopathological analysis revealed focal areas of dense fibrosis within the collagen matrix (Figure 1), with disorganised collagen bundles and scattered inflammatory cell infiltration, indicating a chronic foreign body reaction. Additionally, no significant degradation of the implant was observed.

Case 2

An 85-year-old female patient diagnosed with advanced POAG presented with inadequately controlled IOP despite maximally tolerated topical treatment. She underwent NPDS with a subconjunctival MMC injection and Ologen^® implantation (6 mm×2 mm). During the postoperative course, the bleb developed an avascular appearance with cystic spaces and fluorescein leakage (Figure 2A). The patient was therefore scheduled for surgical bleb revision. Intraoperatively, the Ologen^® implant was found to be intact 18 months after implantation, surrounded by a dense fibrotic capsule resembling a “ring-of-steel” reaction. This caused retraction of Tenon’s capsule, leaving an area of thin, avascular limbal conjunctiva. Histopathological analysis (Figure 2B) confirmed the absence of Ologen^® degradation, minimal fibroblast migration into the scaffold, and a thick wall of organised collagen deposition surrounding the implant.

Case 3

A 56-year-old female patient with a history of diabetic retinopathy was referred for primary angle-closure glaucoma. The patient developed toxicity associated with hypotensive treatment, and glaucoma progression was noted in the left eye. Therefore, NPDS with MMC injection and Ologen^® implantation (6 mm×2 mm) was performed. Postoperatively, the bleb progressively became fibrotic, necessitating an escalation in hypotensive treatment. Bleb revision was performed three years after the initial procedure. Intraoperative findings revealed a highly retracted Tenon’s capsule that had contracted the bleb around the Ologen^® matrix, which had remained intact for three years (Figure 3A). Histopathological assessment

(Figure 3B) revealed dense fibrotic tissue surrounding exogenous mesh-like material (Ologen^®).

Case 4

A 53-year-old female patient diagnosed with pigmentary glaucoma presented with a long history of topical antiglaucoma therapy. Visual field progression warranted a combined phacoemulsification and NPDS procedure enhanced with MMC and Ologen^® implantation (6 mm×2 mm). IOP control remained inadequate a year after the procedure, with a fibrotic bleb that required needling on two separate occasions to reach target IOP. During surgical bleb revision, an intense scarring reaction was observed around the scleral flap and Ologen^® implant. The implant was encapsulated in a manner similar to a foreign body reaction. After debridement of the fibrous tissue and removal of the residual Ologen^® matrix, a single-layer pericardial graft was placed to reinforce the scleral flap. Histological examination revealed the Ologen^® material embedded within a hyalinised collagenous matrix with minimal fibroblast presence (Figure 4).

Following bleb revision, all patients demonstrated improved bleb function and better IOP control. No additional surgical procedures were required during follow-up, and IOP remained stable with or without adjunctive medical therapy depending on the case.

Numerous studies have examined the effectiveness of Ologen^® across several types of glaucoma surgery (summarised in Tables 1^{3, 7, 8, 9, 16-27} and 2^{12, 15, 28-41}). However, interpretation is challenging due to variability in study design, limited sample sizes, and differences in surgical technique, including MMC timing and dosage. The heterogeneity of methodologies, from randomised trials to retrospective studies, further limits generalisability, resulting in inconsistent and sometimes contradictory findings.

Most literature focuses on comparisons with MMC in trabeculectomy. Meta-analyses report no significant differences in IOP control or success rates at 12-24 months.^{6, 13} This implant is generally considered a safe and viable alternative to MMC; the absence of avascular blebs in the treatment groups, alongside a low incidence of postoperative complications, has been consistently reported.^{18, 25} Additionally, this matrix promotes physiological bleb vascularisation²³ and serves as a mechanical buffer that mitigates overfiltration during the early postoperative period, potentially reducing the risk of hypotony.³⁵ In contrast, MMC is often associated with diffuse bleb formation, which may contribute to sustained long-term IOP reduction,²⁵ albeit at the cost of an increased risk of early hypotony and avascular bleb development.^{18, 25}

Beyond trabeculectomy, studies on Ologen^® use with glaucoma drainage devices (GDD) indicate that the implant may improve success rates when used as a patch graft over the tube.^{28, 29} While no significant differences in IOP reduction were observed,^{29, 30, 31, 32, 33} it appears to lower the risk of tube exposure.²⁸

The biodegradable collagen-glycosaminoglycan implant modulates the deposition of connective tissue in a manner that more closely replicates physiological wound healing. It prevents direct contact between the conjunctiva and sclera, thereby acting as a biological spacer.^{12, 42} Regarding the physical characteristics of filtering blebs, studies have reported mixed findings on the height and volume occupied by the collagen matrix. Some investigations indicate that the matrix can fill the entire subconjunctival space of the bleb, whereas others suggest that it occupies only a limited volume and gradually resorbs. Early postoperative observations consistently show that blebs in eyes receiving this implant appear more prominent than those in eyes treated with MMC, likely reflecting the immediate space-occupying effect of the subconjunctival implant.

Nevertheless, several studies report a higher incidence of encapsulated blebs and an increased need for postoperative needling.^{8, 9, 27} In a study by Min et al.⁴³, encapsulated blebs developed more frequently and more rapidly in eyes receiving this matrix compared to those undergoing conventional trabeculectomy. Most encapsulated blebs were observed between the second week and the first month postoperatively. The authors proposed that non-contractile, collagen-producing fibroblasts play a key role in this process. Because this implant is biodegradable, it may facilitate migration of fibroblasts into the bleb, contributing to early encapsulation. To mitigate this, the authors suggested strategies such as soaking it in relatively high concentrations of antimetabolites, applying antimetabolite eye drops in the early postoperative period, or injecting antimetabolites directly into the bleb during the first one to two weeks after surgery.

In a case reported by Voykov and Rohrbach¹¹, the implant remained in place one month after surgery, but no filtering bleb was observed. Histopathology showed no foreign body reaction or inflammatory infiltrate, with early fibrosis and vascularisation confined to the implant margins. These findings align with those of Rosentreter et al.²³, who reported fibroblast infiltration of the Ologen implant, encasement by a collagenous pseudocapsule, accumulation of α-smooth muscle actin-positive myofibroblasts, collagen III deposition at the implant borders and within the pseudocapsule, strong fibronectin expression, and positive prolyl-4-hydroxylase staining in both the implant and overlying epithelium.

Studies on NPDS have demonstrated substantial reductions in IOP and the need for glaucoma medications.^{34, 35, 36} These observations are consistent with our own clinical findings. Similarly, subconjunctival devices such as XEN, PRESERFLO, and Ex-PRESS demonstrate comparable IOP reductions and medication usage between eyes treated with antimetabolites and those receiving this biodegradable collagen-glycosaminoglycan implant.^{15, 37, 38, 40, 41} Although late bleb fibrosis requiring subsequent needling or 5-fluorouracil was more frequently reported in the XEN group without Ologen,³⁸ the incidence of fibrosis was higher in the PRESERFLO group when Ologen was used.⁴⁰ In fact, individual genetic predisposition to a fibrotic response may also contribute to surgical outcomes. A hyper-fibrotic gene expression profile has recently been identified in patients experiencing surgical failure (with conjunctival overexpression of genes such as VEGFA or CXCL8).⁴⁴ We hypothesise that patients with this hyper-fibrotic profile may be inherently more prone to exaggerated scarring responses in general, and specifically to Ologen^®, undermining its intended modulatory function.

This collagen matrix is designed for complete degradation within 90 to 180 days.^{19, 35} However, factors such as the degree of cross-linking, type of collagen preparation, and enzymatic processing can influence this timeline.³⁰ In our clinical experience, degradation timelines deviated markedly from expectations: in Case 2, Ologen^® remained intact for 18 months after implantation, and in Case 3, it persisted for 3 years. This phenomenon may be attributable to an encapsulating immunological reaction surrounding the implant, which prevents enzymatic degradation by tissue proteases. Similar histopathological findings have been reported by Selvan et al.⁴⁵, who described a “ring-of-steel” reaction encasing the intact implant in both eyes of the same patient. This phenomenon was observed following the use of MMC-soaked Ologen^® and was thought to result from the prolonged localised effect of MMC and the resultant excessive fibroblast proliferation in the surrounding tissue. The observations with GDD and NPDS support the hypothesis of Stephens and Sarkisian¹² that this implant could become encapsulated when not directly exposed to aqueous humour, potentially hindering its biodegradation. In addition, other possible complications associated with Ologen^® include implant migration and the development of symptomatic blebs, which may reflect an altered subconjunctival wound-healing response. These potential adverse events should be considered when interpreting the overall clinical performance of the implant.⁴⁶

In summary, Ologen^® and MMC demonstrate comparable efficacy across various glaucoma surgical modalities. Although MMC may achieve marginally lower IOP in selected contexts, it carries an increased risk profile. Our findings do not challenge the overall safety of this biodegradable collagen-glycosaminoglycan implant but rather highlight that its biological behaviour may be unpredictable in selected patients. The matrix appears to reduce severe complications such as hypotony and tube erosion. However, as observed in our series, it may also be associated with encapsulation, delayed degradation, and bleb fibrosis, even when used in conjunction with MMC. These findings highlight the considerable variability in tissue response to biodegradable implants and suggest a potential role of patient-specific fibrotic predisposition. Emerging evidence indicating distinct conjunctival gene expression profiles associated with early surgical failure supports the concept that a hyper-fibrotic molecular phenotype may influence wound-healing responses to collagen-based implants. Consequently, careful patient selection, meticulous ocular surface preparation, and the future integration of genetic or molecular profiling of hyper-fibrotic phenotypes may help to optimise surgical outcomes and refine the role of collagen-based implants in glaucoma surgery.