|MadSci Network: Medicine|
Dear Rafe, First, let me apologize for not answering your question sooner. The academic summer included meetings and a little vacation time for me. The basic answer to your question is not a question of rejection, but rather of anatomy. You are partially correct in inferring load bearing demands as a limiting factor. The load there is very high, and to my knowledge, "cloned" cartilage is as yet not as tough and durable as the natural version. But primarily, it has to do with the location. There's nothing mysterious in the local environment, but rather, there is simply no room for a surgeon to work there without causing more damage to the joint than would be repaired by the implant. I checked some websites that offer anatomical illustrations, but couldn't find a really good one that illustrates the specifics, so you may want to consult an anatomy text to explore this further. Basically, the medial collateral ligament, the one that connects the femur to the tibia on the "inside" of the knee, and the cartilage meniscus that cushions the top of the tibia just inside that ligament (the "medial chondyle", the inner of the two "knobs" at the top of the tibia) and bears the load transferred from the femur, are "welded" to the medial surface of the tibal chondyle. There's no way to access the medial side of the upper surface of the tibia to add cartilage without stripping these away and causing all sorts of potential additional problems. On the outer side, there is a space between the lateral collateral ligament and the tibia, giving a surgeon more room to work. Also, the lateral chondyle (i.e., the outer "knob") of the tibia is less completely covered by cartilage meniscus, again giving more access surgically without disturbing as much of the normal knee structures. I hope this answers your question, and again, my apologies for making you wait. Paul Odgren, Ph.D. Dept. of Cell Biology University of Massachusetts Medical School Worcester
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