Otago Glaucoma Surgery Outcome Study: factors controlling capsule fibrosis around Molteno implants with histopathological correlation.

OBJECTIVE: To describe the histopathology of Molteno implant capsules in cases of primary and secondary glaucoma and to correlate them with surgical technique and clinical outcomes in quiet eyes.
DESIGN: Human tissue study with clinicopathological correlation. MATERIALS: Seventy-five autopsy eyes or surgical pathology specimens obtained between 4 days and 23 years after insertion of Molteno implants were studied. Basic histologic features common to all bleb capsules were described, and the thickness was measured in 28 specimens from quiet eyes. MAIN OUTCOME MEASURES: Histologic features of capsules, including wall thickness, distribution of inflammatory cells, and presence or absence of fibrodegeneration, were assessed by light microscopy.
RESULTS: Without aqueous flow (first stage of 2-stage insertion), the episcleral plates of Molteno implants were encapsulated by a very thin (20-60 micro m) avascular collagenous layer. The second stage of 2-stage insertion, with delayed drainage of aqueous and early temporary postoperative intraocular pressure (IOP) increase to 25 to 35 mmHg, produced thin (190-250 micro m) permeable capsules with fewer fibrovascular than fibrodegenerative components. Insertion of nonligatured implants with immediate aqueous flow produced thicker capsules (300-600 micro m) composed of an outer fibrovascular layer and an inner fibrodegenerative layer of approximately equal thickness. Three-stage insertion of modified Molteno implants with temporary externalization of aqueous flow onto the conjunctival surface and postoperative IOP not exceeding 12 mmHg produced the thickest (375-700 micro m) heavily fibrosed and impermeable capsules composed entirely of dense fibrovascular tissue without a fibrodegenerative layer.
CONCLUSIONS: Capsules around functioning Molteno implants evolved through a series of histologic stages. Without aqueous flow, the episcleral plate of the implant stimulated encapsulation by a thin avascular collagenous layer. With aqueous flow, an immediate inflammatory reaction developed in the episcleral connective tissues that included collagenous and vascular components. After a variable delay, a fibrodegenerative process developed in the deeper layers of the capsule. The fibrodegenerative process may depend on sufficient increases of IOP for aqueous to displace interstitial tissue fluid from the deeper layers of the capsule. The final thickness and permeability of the capsule probably depend on the relative intensity and timing of these opposing processes, which were influenced by surgical technique and postoperative management.