When treating horses with osteoarthritis, veterinarians can choose from many therapy options. Viscoelastic therapies, sometimes referred to as viscosupplementation, generally focus on lubricating the joint and reducing friction between the two opposing cartilage surfaces within it. While the traditionally used viscoelastic therapy is hyaluronic acid (HA), practitioners are now using several new viscoelastic therapies in horses, including 4% and 2.5% synthetic cross-linked polyacrylamide hydrogels (PAAGs) and a naturally derived osteocushion product.

Because of their synthetic and inert properties, PAAGs are classified as medical devices that do not need to meet FDA approval, that have a shelf life of three years, and have a very low incidence of associated adverse events, said Lauren Schnabel, DVM, PhD, Dipl. ACVS, ACVSMR, professor of equine orthopedic surgery in the Department of Clinical Sciences at North Carolina State University’s College of Veterinary Medicine, in Raleigh. While both PAAGs are mostly made up of water and are highly water absorbent, they contain different percentages of polyacrylamide and are reported to have different target tissues and mechanisms of action.

The target tissue of 4% PAAG is cartilage. Schnabel said this product has been shown to bind to damaged cartilage, reduce friction within the joint to the same extent as the best HA products, and remain in the joint much longer than HA.

The target tissue of 2.5% PAAG is synovium. This product has been shown to promote cellular integration and increase joint capsule elasticity, she said. Rather than being eliminated from the horse’s system, 2.5% PAAG is absorbed fully into the synovium four to six weeks after treatment.

Treatment indications for PAAGs include a positive response to intra-articular anesthesia (diagnostic joint blocking in lameness exams) and a known diagnosis of osteoarthritis. “PAAGs are best when used in the early stages of osteoarthritis but can be used during all stages of disease,” said Schnabel. Veterinarians should consider 4% PAAG for osteoarthritis cases with known or suspected cartilage damage and 2.5% for osteoarthritis with synovitis (when the connective tissue inside the joint becomes inflamed), she added.

Naturally derived osteocushion is a medical device made from bovine collagen and elastin as well as a porcine carbohydrate source. More research is needed to help veterinarians better understand the tissue this product targets, as well as its indications, said Schnabel.

“These new viscoelastic therapies are inert and do not have inherent anti-inflammatory properties, nor do they contain growth factors, so veterinarians need to determine where they fit into a horse’s treatment plan,” said Schnabel.

She urged veterinarians to choose their treatments based on the individual horse’s needs and to strongly consider combination therapy when indicated. “In cases in which the joint is effusive (full of fluid) and inflamed, I highly recommend treating the joint first with either a steroid or orthobiologic and then following up with the viscoelastic therapy,” Schnabel said. “Additionally, if the joint has a soft tissue injury within it contributing to the osteoarthritis, like a meniscal tear or a collateral ligament tear, that soft tissue injury should be treated with orthobiologics and not a viscoelastic alone.”