A matrix is something that offers support to something more fragile, and in the knee it usually refers to the cartilage matrix.
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.
The Basic Science of Articular Cartilage: Structure, Composition, and Function. Fox AJS, Bedi A and Rodeo SA. Sports Health. 2009 Nov; 1(6): 461–468.