Use of 2.5% polyacrylamide hydrogel mitigates joint capsule stiffness and cartilage degradation in experimental equine osteoarthritis

Objective: To evaluate the symptom- and disease-modifying effects of injectable 2.5% polyacrylamide gel (iPAAG) in an experimental model of equine osteoarthritis (OA). Methods: 16 horses underwent osteochondral fragment creation (day 0) in 1 randomly-selected middle carpal joint (MCJ). On day 14, the MCJ was injected with 2 mL of phosphate-buffered saline (n = 8) or 2.5% iPAAG (8); high-speed treadmill exercise commenced on day 16 to induce OA. Clinical parameters (lameness, joint effusion, range of motion, flexion response) and synovial fluid were collected weekly and biweekly, respectively. Radiographs were obtained before surgery, at day 14, and at day 70. Postmortem evaluation (day 70) included gross, arthroscopic, and histopathologic evaluation of the MCJs and measurement of joint capsule stiffness. Results: There were no significant differences in clinical parameters or endpoint arthroscopic, macroscopic, or synovial membrane histopathology scores between groups. In 2.5% iPAAG-treated limbs, the third carpal bone had significantly lower (better) total articular cartilage histology scores (6.38 ± 4.10 vs 11.25 ± 4.20) and proteoglycan staining scores of the intermediate articular cartilage layer (0.38 ± 0.74 vs 1.88 ± 1.25) compared to saline-treated OA joints. Saline-treated OA joints had significantly increased joint capsule stiffness versus contralateral sham-operated MCJs (1,824 ± 1,105 vs 1,002 ± 495 Pa), whereas 2.5% iPAAG-treated joints did not (1,099 ± 647 vs 1,184 ± 504 Pa). Conclusions: Treatment with 2.5% iPAAG showed some ability to mitigate the negative effects of OA on articular cartilage. Treatment also resulted in retention of normal joint capsule elasticity during the progression of OA. Clinical Relevance: Treatment with 2.5% iPAAG may result in retention of normal joint capsule elasticity in horses with naturally occurring OA.