The use of poly(styrene-block-isobutylene-block-styrene) as a microshunt to treat glaucoma

Abstract

The InnFocus MicroShunt® is a minimally invasive glaucoma drainage microtube used to shunt aqueous humor from the anterior chamber of the eye to a flap formed under the conjunctiva and Tenon's capsule. The safety and clinical performance of this device approaches that of trabeculectomy with mitomycin C, the current 'gold standard' treatment for advanced glaucoma. The invention of a new biomaterial called poly(styrene-block-isobutylene-block-styrene) or 'SIBS' is the enabling factor which led to the success of this product. SIBS is ultrastable with virtually no foreign body reaction in the body, which manifests as clinically insignificant inflammation and capsule formation in the eye. The lack of capsule formation enables unobstructed flow through the 70 µm lumen tube and the achievement of controlled low intraocular pressure, which is important for the management of glaucoma. This article summarizes the integration of SIBS into a glaucoma drainage device and confirms its functionality with clinical success over a 2-year period.

Keywords: biocompatibility; medical device; soft tissue; stent.

Figure 1.

A simplified chemical structure of SIBS with a central block that comprised polyisobutylene (incorporated initiator not shown) and end blocks of polystyrene (M is an integer >N)

Figure 2.

The InnFocus MicroShunt showing its dimensions (mm) and placement under the limbus with its proximal end in the anterior chamber and distal end under the conjunctiva and Tenon’s capsule

Figure 3.

Schematic of implantation procedure: (i) an anesthetic is injected under the conjunctiva; (ii) an incision is made below the limbus and a flap dissected above the sclera with a blunt scissor; (iii) three sponges saturated with mitomycin C are placed in the flap for ∼3 min followed by irrigation with sterile saline solution; (iv) a shallow 1 mm wide, 1–2 mm deep scleral pocket is formed 3 mm below the limbus; (v) a 25-G needle is advanced through the scleral pocket into the anterior chamber; (vi) the MicroShunt is maneuvered through the scleral pocket and needle tract with a forceps and the fins of the device wedged snugly into the scleral pocket; (vii) flow of aqueous humor from the anterior chamber to the flap is confirmed by drop observation and (viii) the distal end of the device is tucked under the conjunctival/Tenon’s flap and the flap closed with multiple interrupted 10-0 nylon sutures

Figure 4.

Change in intraocular pressure (mm Hg) with time for the MicroShunt implanted with and without cataract surgery

Figure 5.

Typical eye with the InnFocus MicroShunt at 1 and 2 years. The white arrows in the upper photographs point to the proximal end of the MicroShunt in the anterior chamber