From this quarter's "Sports Health"-
"The Basic Science of Articular Cartilage: Structure, Composition, and Function"- Fox, Bedi, Rodeo; p. 461.
"Articular cartilage is devoid of blood vessels, lymphatics, and nerves & is subject to a harsh biomechanical environment. Most important, it has a limited capacity for intrinsic healing & repair...injury to articular cartilage is recognized as a cause of significant musculoskeletal morbidity...it's unique & complex structure makes treatment, repair, or restoration challenging for all."
Always keep articular cartilage on your mind when rehabing weight bearing joints. It's my opininion that this stuff lags behind other connective tissue in the healing process. I know I always preach getting the athlete weight bearing as safely possible, and I stick to that. However, a good eval will clue you in on what joint surfaces may have gotten beat up and need to be protected. Prudent use of chain reaction biomechanics by way of a slant board, U/LE drivers can help you stear away from grinding that articular cartilage early on. Spin bikes & aquatic therapy come in handy.