Viscosupplementation by intra-articular injections of hyaluronic acid (HA) is used to treat symptomatic osteoarthritis. Exogenous HA remains in the joint cavity for a short period of time (days) while claimed pain relief period lasts over months. There is a clear lack of understanding of viscosupplementation mechanism of action. Here, we hypothesize that HA penetrates the cartilage and contributes to the restoration of its mechanical quality.

Confocal microscopy and bio-indentation were used to confirm HA penetration into cartilage and modulation of cartilage quality. Bio-indentation was performed on rat distal femurs incubated overnight in HA solutions, using phosphate buffered saline (PBS) as control. For this proof-of-concept evaluation, measurements of elastic modulus (MPa) and of maximal force (µN) were recorded before and immediately after exposure with HA, as well as after an additional washout with PBS. Cartilage thickness at the site of indentation was evaluated by contrast enhanced computed tomography with an ionic contrast agent. Indentation depths were located in the upper part of hyaline cartilage. Ostenil®, commercial product containing 1 % HA, induced a decrease in indentation depth in the range of forces influencing the whole cartilage thickness, together with an increase of the elastic modulus. Then, bio-indentation and size distribution of HA chains via SEC-SLS were assessed for a range of commercially available products. The results showed higher modulation of cartilage quality in the presence of 0.25–1 MDa HA chains.

The present in vitro study suggests that HA modulates cartilage quality and might thus explain the long-term beneficial effect of viscosupplementation.