I don't give the flax seed oil for joint problems, as they are still young. I give it for the other benefits like shiny soft coat and less shedding. This is from a vet site.
Hexane is a chemical solvent used by many manufacturers to extract the oil from the seed, and can be extremely allergic for dogs and cats. The health benefits of supplementation with flax seed oil for pets are numerous. Flax seed oil is rich in Omega 3 and 6 Essential Fatty Acids (EFA's) which are protective against degenerative disease and illness, including bone and joint problems, cancer, and cardiovascular, kidney and auto immune disease. Flax seed oil is excellent for improving skin conditions such as dandruff or allergic irritation. It also improves the silkiness and sheen of the coat and helps to heal dry, cracked paws. The essential fatty acids found in flax oil are components of nerve cells and cell membranes. Flax oil is also one of the best known sources of lignans, which are compounds that are thought to have powerful anti cancer, antifungal and antiviral properties.