Introduction: Articular cartilage displays very little self-healing capabilities, generating a major clinical need. Here, we introduce a thermoresponsive hyaluronan hydrogel (THH) for cartilage repair. The gel is fluid at room temperature and becomes gel at body temperature. THH was obtained by covalently grafting poly(N-isopropylacrylamide) (pNIPAM) to hyaluronan. In this pilot study THH safety and repair response were evaluated in an osteochondral defect model in rabbit. Materials and Methods: THH was prepared and characterized as previously reported!1!. The in vivo study protocol was approved by the local authorities. Skeletally mature (＞ 24 weeks) female New Zealand White rabbits (Charles River), weighing 3.3 ± 0.3 kg were enrolled in the study. A defect of 2.7 mm diameter and 4 mm depth was drilled in the central area of the medial trachlear ridge. Experimental groups were: empty 1 week (n=4); THH 1 week (n=4); empty 12 weeks (n=6) and THH 12 weeks (n=6). At 12 weeks the defect sites were evaluated macroscopically according to the ICRS Cartilage Repair Assessment System and histologically with the O'Driscoll score system in hematoxylin/eosin and safranin 0 fast green stained sections. At week 1 and 12, samples from joint capsule, proximal lymph nodes, spleen, liver and kidneys were analysed for histopathological evaluation. Results: THH could be easily injected and remained within the defect throughout the study. The ICRS score showed a statistically superior outcome for THH in comparison to the empty control. O'Driscoll score gave opposite trend without reaching statistical significance (Table 1). Aslight tissue reaction was observed around THH, however vascularization and subchondral bone formation were not impeded. An upper proteoglycans rich fibro-cartilaginous tissue with relatively good continuity and lateral integration into the existing articular cartilage was observed in all cases (fig 2). For the empty group bone formation above the tidemark was observed (' in fig 2). No signs of local or systemic acute or subacute toxicity were observed in any of the harvested tissues. Table 1 - Macroscopic ICRS Score and O'Driscoll Score average values, maximum and minimum values of empty and HpN group at 12 weeks Fig. 1: Overviews of the osteochondral defect in histological sections stained with Hematoxylin and Eosin (a, b) Safranin 0 Fast Green (c, d) at 12 weeks. The borders of the defects are marked with top down arrows. Empty group: panels a, c. (*) indicates new bone formation above the level of the original tidemark (dashed line). Scale bar - 1 mm for all images. Discussion: THH was both easily injectable and not dislocated from the defect in a moving joint. The empty control achieved relatively high tissue repair. Therefore a wider or less deep defect would have been more appropriate. pNIPAM toxicity was a potential concern, however histopathology showed no signs of local or systemic acute or subacute toxicity. Conclusion: THH is easily injectable and remains into an osteochondral defect within a moving synovial joint. THH is biocompatible and does not interfere with the intrinsic healing response of osteochondral defects in a rabbit model and could therefore be a suitable cell or drug carrier for osteochondral repair.
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