Different concepts of 'matrix'
Cartilage cells are unusual cells, not bound together in tissues like most cells, but living in little separate pockets, called lacunae, and supported within a firm 'jelly' which is the matrix.
Another 'matrix' that one hears about in knee surgery is the matrix that is used in a laboratory upon which cultured cartilage cells are 'seeded' to produce a cell-rich membrane that can be cut to shape to fit a hole in the cartilage of the knee to encourage cartilage regrowth there.