## 4 Discussion
This was clearly evident from the histologic findings of the present study, wherein, new bone matrix deposition was evident not only at the edges of the defect, but also within the defect, especially surrounding the BCP particles. Hyaluronic acid (HyA) is a component of tissue extracellular matrix (ECM) which plays a key role in cell adhesion, chemotaxis, differentiation, and proliferation, signaled through several macromolecules and especially during wound healing and tissue regeneration [28; 29]. [PERSON] et al. [19] reported that HyA stimulated bone healing by accelerating the proliferation and migration of mesenchymal cells. Thus, it was suggested that HyA has a molecular mode of action which can improve the osteogenicability of bone graft materials [19]. Furthermore, HyA of a specific molecular weight when used in vitro, was reported to significantly increase alkaline phosphatase activity and stimulate osteoblastic cell proliferation and differentiation [28]. It has also been suggested that HyA combined with synthetic bone grafts could prevent connective tissue ingrowth towards the core of the grafted biomaterials [30]. The mechanisms by which HyA stimulates bone formation are mainly attributed to its physicochemical properties, whereby it forms a suitable substrate for the migration and differentiation of osteoprogenitor cells [31] and for binding proteins crucial to bone healing, such as fibrinogen, fibrin, fibronectin, and collagen [32] Moreover, HyA could protect the surgical site from bacterial contamination and reduce post-surgical inflammation associated with bone healing [20]. Contrary to the reported studies, histomorphometrical evaluation in the present in vivo study did not reveal any superior performance of adding HyA to BCP granules, grafted in the rat femoral condylar defects, at four weeks and at 10 weeks. This could possibly be attributed to the low molecular weight (\(<\)10\({}^{3}\) kDa) of the HyA used in the present study. Interestingly, in previous studies, it has been documented that HyA of higher molecular weight (\(>\)10\({}^{3}\) kDa) promoted mesenchymal stem cells (MSCs) proliferation and differentiation. It showed that the use of a higher molecular weight of HyA enhanced the formation of calcium nodules, as well as the gene expressions of ALP, RUNX-2, and OCN. For instance, [PERSON] et al. [28] reported that the capability of HyA to enhance the osteogenic and osteoinductive properties of bone graft materials was dependent on its dose and molecular weight. On the basis of their in vitro experiment, they reported that high molecular weight HyA (900 kDa and 2300 kDa), when used at a dose of 1 mg/mL, resulted in significantly pronounced osteogenic activity and mineralization on rat calvarium derived cell cultures as compared with low molecular weight of HyA and other doses [28]. The HyA biomaterial used in the present study, namely Hyadent(r), was a hydrogel consisting of long molecular chain HyA at a concentration of 14 mg/mL and has been reportedly used in the successful management of periodontal tissue loss [21]. Although the present study hypothesis of enhanced bone regeneration within rat femoral condylar bone defects using a combination of HyA and BCP was based on previous research findings, the results of the present study did not support the same. Nevertheless, it would be alluring to evaluate through future studies, whether HyA used at a specific dose and molecular weight could still enhance bone regeneration when used along with osteoconductive bone graft materials.