Polyacrylamide Hydrogel: An Alternative Osteoarthritis Treatment Option

Osteoarthritis (OA) exerts its painful effects by decreasing joint range of motion as inflammation sets in and cartilage breaks down. “Any of you that work in this field or have arthritis yourself know restriction of movement in the joint capsule is a key component to the disease process,” said Leigh de Clifford, BVSc, Cert. AVP, PGDipVPS, MVSc, while presenting his research findings at the 2019 American Association of Equine Practitioners (AAEP) Convention, held Dec. 7-11 in Denver, Colorado.

In horses this commonly leads to chronic and performance-limiting lameness. Veterinarians have a litany of OA treatment options at their disposal, all offering varying results. Those treatments traditionally include corticosteroid and hyaluronic acid (HA) joint injections. Another option—polyacrylamide hydrogel (PAAG)—is showing promise, according to a growing body of cross-species research.

However, no researchers had previously investigated PAAG’s effect in naturally occurring OA in a large group of racehorses. So de Clifford, who works at Matamata Veterinary Services, in New Zealand, and a team of researchers set out do just that, while also comparing PAAG results to those of corticosteroid and HA treatments.

What Is PAAG?

Parents and teachers might recognize PAAG as a messy, water-absorbing toy in their kids’ science kits (aka “water jelly crystals”). It’s also widely used as a cosmetic filler (think breast or facial augmentation), for making contact lenses, as a food packaging product (flexible cold packs), and as those water-absorbing beads that keep your houseplants’ roots moist.

PAAG’s water-absorbing nature also suggests it could, when injected into the joint, provide lubricating and cushioning effects.

For de Clifford’s study, his team used Arthramid Vet, which is 2.5% PAAG and 97.5% sterile water. Researchers on prior studies report that this specific PAAG product (which de Clifford noted uses a patented technology) is not harmful to living tissues in humans and other animals.

“Not all hydrogels are the same,” he said, citing previous research that evaluated the 11 currently available PAAG products. The investigators found Arthamid Vet to be the most biocompatible with these tissues.”

In previous studies researchers have found it integrates into soft tissue through blood vessel ingrowth, collagen deposition, and water exchange with the surrounding tissue, he said, adding that integration stabilizes the joint, improving load transfer.

The Study: Double-Blinded and Controlled

De Clifford’s study spanned 11 months and included flat-racing Thoroughbreds from a single trainer in Melbourne, Australia, who has a racing stable of more than 500 horses. “The goal was to test the product in a controlled clinical setting,” he said. “It all took place at a single training site: same track, same riders, same feed, same farrier, same management systems.”

An independent blinded veterinarian identified 33 Thoroughbreds, all in full training, that exhibited lameness grades of 1-3 on the AAEP’s five-point lameness scale (0 being sound and 5 non-weight-bearing). De Clifford and his team randomly divided the Thoroughbreds into three groups, and a second blinded veterinarian treated with:

  • Group 1: 2 mL of 2.5% PAAG;
  • Group 2: 12 mg of triamcinolone acetonide (TA, a corticosteroid); and
  • Group 3: 20 mg of sodium hyaluronan (a form of hyaluronic acid, or HA), followed by two intravenous treatments of 40 mg of HA at weekly intervals.

In total, the study included 89 joints examining veterinarians identified for treatment using flexion tests and radiographs. All horses rested for 48 hours after the injections before returning to their regular training routines. “At this point all horses were at a full-gallop stage,” said de Clifford.

The initial veterinarian then reexamined all the horses at Weeks 2, 4, and 6. He then reexamined the PAAG-treated horses at Week 12.

The researchers defined “success” as full resolution of the lameness grade at Day 0. “I’ve never had a client thank me for turning their Grade 3 (lame) horse to grade 1, so we decided they had to be sound or lame-free to be a success,” de Clifford said.

At Week 6, the examining veterinarian deemed 83% of the PAAG-treated horses lameness-free, compared to 27% of the TA-treated horses and 30% of the HA-treated horses. All successfully treated horses in the PAAG group remained sound at 12 weeks.

“The take-home is that the polyacrylamide hydrogel was found to be superior and longer-lasting than the two conventional treatments we put it up against,” de Clifford said.