361 EVALUATION OF THE PERSISTENCE OF DERIVATIZED HYALURONIC ACID FOLLOWING INJECTION INTO THE GUINEA PIG FEMOROPATELLAR FAT PAD TISSUES

Abstract

Purpose: Hyaluronan (HA) based viscosupplements are used to reduce pain in patients with mild to moderate osteoarthritis. For the patient to receive the full benefit, these viscosupplements must be injected directly into the joint space. Extra-articular needle placement and injection of viscosupplements into the subsynovial tissues may be linked to a higher incidence of adverse reactions. In this study a guinea pig model was used to investigate the cellular response to two viscosupplements, non-animal stabilized hyaluronic acid (NASHA) and hylastan, following injection into the fat pad to simulate misinjection of the viscosupplement. These two crosslinked HA-based gels have distinctly different rheological and physical properties. NASHA is an epoxy crosslinked gel while hylastan is a non-animal HA gel lightly crosslinked with divinyl sulfone. Methods: Eighteen guinea pigs were randomized into two treatment groups of 6 animals each and 1 HA control group (1% w/v solution of unmodified HA) of 6 animals. Test or control material was injected into the fat pad of the assigned animal. Fat pads in the contra lateral joints were injected with lactated Ringer’s solution. Animals were sacrificed at 7 or 30 days following injection and the fat pads fixed in 4% paraformaldehyde for sectioning and staining with hematoxylin and eosin (H&E). Some slides were treated with hyaluronidase prior to staining with H&E to confirm the presence of HA-based material. The slides were examined by a board certified veterinary pathologist for the presence of residual HA-based material and cellular activity associated with this material. Rheological properties were characterized by a controlled stress rheometer and dynamic mechanical analysis. Results: On day 7, residual HA-based material was observed in tissues taken from both hylastan and NASHA-treated animals. Test material was noted as a basophilic substance that formed discrete clusters within the adipose tissue. The cellular response to these two materials was characterized by the presence of a thin layer of macrophages at the host-test material interface. Macrophages appeared to easily infiltrate and degrade the material in the hylastan-treated animals but not in the NASHA-treated animals. No residual control material was present in tissues from animals injected with the HA or lactated Ringer’s solution controls. At day 30, residual hylastan was not observed in any of the fat pad tissues. In contrast, residual NASHA was observed as discrete aggregates of basophilic material associated with a single layer of macrophages at the host material interface surrounded by layers of fibrous connective tissue that completely encapsulated the material. Rheological measurements demonstrate that hylastan is a softer, more cohesive gel than the NASHA material. Conclusions: We have evaluated the tissue distribution and cellular activity of two HA-based test articles following injection into the fat pads of guinea pigs. Microscopic analysis of the cellular activity associated with the dissolution of hylastan was quite different from that observed in tissue sections taken from the NASHA-treated animals. Hylastan injected tissue demonstrated a typical biomaterial response with macrophages that are facilitating the resorption of implanted material at early timepoints. This inflammatory response resolves at later times once the hylastan is completely resorbed. The NASHA injected tissue demonstrated a similar early time response, but at the later times showed a cellular and fibrous encapsulation of the injected material. Matrix Biochemistry