The research study investigates how normal horse joints react to injections of a 2.5% synthetic polyacrylamide hydrogel, primarily noting a response from the body’s immune system without any adverse effects or signs of joint disease.

Research Methodology

• The research was an in vivo study conducted on 10 healthy horses not showing signs of osteoarthritis (OA) or joint disease.
• These horses were administered with either 50 or 100 mg of a solution containing 2.5% injectable polyacrylamide hydrogel (2.5 iPAAG) into a total of 13 metacarpophalangeal (joints in the lower limbs of the horses) or middle carpal joints.
• The injected joints were then inspected at various intervals: immediately after injection, 14 days, 42 days, and/or 90 days post injection.
• The inspection of joints included a clinical examination, synoviocentesis (a joint fluid test), gross pathology, histology (study of microscopic tissue changes), and scanning electron microscopy.

Research Findings

• All the horses in the study remained healthy showing no adverse effects during the research period.
• Postmortem inspection of the joints did not indicate any significant abnormalities.
• The condition of synovial fluid withdrawn from joints, investigated through arthrocentesis cytology, remained normal throughout the experiment.
• However, synovial histology showed a notably higher level of macrophages (a type of white blood cell), villus hyperplasia (increase in the number of cells), and vascularization (creation of a blood supply) in the joints injected with 2.5 iPAAG in comparison to control joints.
• The electron microscope confirmed an extensive tissue integration of the 2.5 iPAAG, showing a third-dimensional (3-D) scaffolding structure with intact cross-linked strands.

Conclusions

• The results demonstrate that injecting a healthy joint with 2.5 iPAAG triggers an immune response that is predominantly led by macrophages. However, these responses were classified as a typical response to a foreign body and did not cause disease or harm.
• This study also shows no evidence of fibrosis (the thickening and scarring of connective tissues) or mineralization (the abnormal deposit of minerals).
• Integration of the gel into the tissue could be seen as early as 14 days post injection, with no unabsorbed gel found in the joint cavity at that time.