Viscoelastics (ophthalmic viscosurgical devices - OVDs) are used in ophthalmology intraoperatively for the protection, coating and stabilization of tissues and eye segments. Currently, the main areas of application include cataract surgery, glaucoma surgery, keratoplasty and surgeries of the posterior eye segment

In my view intraoperative OVDs are of importance by creating space (e.g. sections of the anterior chamber, sections of the capsular bag) and by protecting the corneal endothelium. They offer additional protection, as they also save cells from damage incurred by friction caused by surgical instruments. Furthermore, intraoperative trauma due to irrigation/aspiration flow or swirling fragments of the lens nucleus in the anterior chamber is minimized. In modern cataract surgery they are used as a coating agent and for protection of the intraocular lens (IOL) during injection and implantation. There are currently numerous OVDs commercially available.

So how do you choose your OVD?

There are certain selection criteria and factors to be considered when choosing an OVD. For example, the agent should not limit the view onto the surgical field (optical clarity), it must be completely transparent, but at the same time, a small colour tint or tinge may be helpful during the removing process. An optimal OVD must be readily injectable, but cannot be allergenic and must be non-toxic. Even in prolonged surgical procedures it must not have a tendency to form lumps and at the end of surgery, it must be easily removable by suction, as residual OVDs may lead to increased intraocular pressure (IOP) as potential side effect/complication. (IOP peaks 6-24 hours postop).

The property of a specific viscoelastic is defined by either its ability to hold together and to move as one cohesive mass or, on the contrary, by its ability, despite possible turbulences, to remain dispersive in the anterior chamber.

The ophthalmic surgeon can choose between numerous viscoelastics: OVDs based on hyaluronic acid (e.g. Healon, Amvisc Plus, Provisc), methyl cellulose (Adatocel) and chondroitin sulfate-hyaluronic acid combinations (Viscoat).

Compared to hyaluronates, methyl cellulose-containing OVDs adhere generally better to the endothelium, but are more difficult to remove, for example, by suction. Compared to hyaluronates, methyl cellulose-containing OVDs adhere generally better to the endothelium, but are more difficult to remove, for example, by suction.

Figure 1: Preoperative corneal endothelial cell number.

A viscoelastic substance must fulfill three key requirements:

• Biological requirements: sterile, non-flammable, non-allergenic
• Ophthalmic requirements: perfect transparency, bubble-free after injection, easy removal from the eye, for example, by suction
• Rheological requirements: here are the main differences between individual viscoelastics, numerous factors such as the molecular weight, the concentration, elasticity, viscosity and pseudo plasticity and the manufacturing process are important.