Histologic and cytologic changes in normal equine joints after injection with 2.5% injectable polyacrylamide hydrogel reveal low-level macrophage-driven foreign body response

Authors: Lowe J, de Clifford L, Julian A, Koene M
Publication: Proceedings of the Equine Veterinarians Branch of the NZVA, Volume 2024 Proceedings, pp 31-44, Jun 2024
Publisher: New Zealand Veterinary Association

Animal type: Horse
Article class: Conference paper

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Abstract:

Objective: The data presented in this paper are derived from an in vivo study performed to characterise the nature of the synovial integration process of a 2.5% synthetic cross-linked injectable polyacrylamide hydrogel (2.5 iPAAG) injected IA in horses.

Animals: Ten healthy horses not suffering from OA or signs of joint disease were administered 50 or 100 mg 2.5 iPAAG in a total of 13 metacarpophalangeal or middle carpal joints.

Methods: Injected joints were examined at 0, 14, 42, and/or 90 days postinjection. Parameters investigated included clinical examination, synoviocentesis, gross pathology, histology, and scanning electron microscopy.

Results: All horses remained clinically normal, with no adverse events recorded throughout the study period. Gross postmortem did not reveal any significant findings. Arthrocentesis cytology parameters remained within clinically normal levels throughout the study. Synovial histology demonstrated that cellular infiltration of macrophages, villus hyperplasia, and vascularisation were significantly higher in 2.5 iPAAG-injected joints compared to controls. Scanning electron microscopy confirmed that the 2.5 iPAAG demonstrated an extensive tissue integration as a 3-D scaffolding structure with intact cross-linked strands.

Clinical relevance: Results confirm that an IA injection of 2.5 iPAAG induces a typical foreign body response that is predominately macrophage driven with no evidence of fibrosis or mineralisation. Integration of the gel is evident by 14 days, with no free gel remaining in the joint cavity at this time.

Keywords: equine, 2.5% iPAAG, histology, synovitis, macrophage

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