Abstract: The invention relates to a kit of parts, consisting of compositions (C1) and (C2) in separate containers, for use in the treatment or prevention of osteoarthritis in a vertebrate, wherein: (a) composition (C1) is a liquid hydrogel formulation comprising a polymer gellant with chemically cross-linkable groups; (b) composition (C2) is an aqueous auxiliary formulation comprising either one or more dissolved cross-linkers Z having two or more reactive groups that can chemically cross-link the polymer gellant by forming covalent bonds; or comprising one or more dissolved compounds Y, selected from the group consisting of oxidant, oxidase, peroxidase and combinations thereof, that initiate or mediate chemical cross-linking of the polymer gellant; wherein said treatment comprises combining compositions (C1) and (C2) to form a liquid hydrogel composition (C3) and administering said liquid hydrogel composition (C3) to the synovial fluid or to the cavity of a synovial joint of the vertebrate, to form a cross-linked hyd