immunogenic – they are protected from the immune system by the matrix.
– failure in osteochondral allografts is often related to surface incongruity and failure for this interface to heal.
– cortical allografts DO undergo stress fracture
– regarding XRT – steroids may paradoxically DECREASE the rate of AVN.
– the most common sarcomas post XRT are osteosarcoma, fibrosarcoma, and MFH
– biodegradable implants often show huge amounts of surrounding osteolysis by 12 weeks. (in up to 50% of patients)
– hyaluronic acid is important for weeping lubrication (the primary type of lubrication in joints) – it gets squeezed out of the cartilage matrix.
– the modulus of elasticity of UHMWP – may decrease with time, as it is a viscoelastic substance.
– the modulus of UHMWP is less than all metals
– in terms of acetabular wear: wear tends to decrease with age, and is calculated to be about .1 mm per year, although much of this might be creep, not wear.
– acetabular wear is almost always ABRASIVE. 3rd body wear may also occur, but this is just a specific type of abrasive wear.
– chondrocytes tolerate a lower PO2 than synovial cells – in fact, chondrocytes probably tolerate a lower PO2 than a lot of cells!
– in the zones of cartilage:
– superficial – cells are flat, spindle shaped; very little proteoglycan; collagen tangential to surface
– transitional – cells are more round, hypertrophied
– deep – cells are plump, spherical, with lots of proteoglycan; collagen perpendicular to surface
– zone of calcified cartilage
– subchondral bone
– the tide mark is between the deep zone and zone of calcified cartilage. It is somewhere between 100 nanometers to 5 micrometers (microns) in thickness, depending on who you read.
– glycosaminoglycans that make up proteoglycans are predominantly chondroitan sulfate, dermatin sulfate, and keratin sulfate. They join with a core protein to make proteoglycan (90% are called aggrecan). They are negatively charged and thus hydrophilic.
– the ratio of glycosaminoglycans in proteoglycans changes with age – chondroitan-4 sulfate decreases with age, while chondroitan 6 sulfate and keratin sulfate increase with age. Certainly, the aging process leads to less proteoglycans in general and thus a loss of some of the water carrying capacity of the cartilage.
– hyaline cartilage – chondrocytes make up less than 10% of tissue volume and less than 5% of wet weight, while the collagen makes up over 50% of the dry weight. The water content of cartilage is about 65-80% the total wet weight.
– remember: chondroitan 4 sulfate DECREASES with age
chondroitan 6 sulfate and keratin sulfate INCREASE with age.
– in osteoarthritis, there is an increased degradation of aggrecan and type II collagen fibers, and the cells try to keep up with this by increasing the synthesis of aggrecan and collagen (eventually doesn’t keep up though).
– in general, chondrocytes have a high metabolic rate as they maintain the extracellular matrix. Over time, they become larger and no longer divide.
– with aging, cartilage becomes stiffer; the protein content increases, the water content decreases, and cartilage proteoglycans decrease in mass and size
– in piezoelectric materials (such as bone), the electric potentials are produced by strain in the ORGANIC COMPONENTS – ie. the collagen and proteoglycans. This does not depend on tissue viability.
– when bending is applied to the bone, the negative charges are on the concavity (compression side), the positive charges on the convexity (tensile side). The negative charges are associated with bone formation, the positive with bone resorption.
– for compartment syndrome – two definitions of critical pressure: 30 mm Hg, or a difference between diastolic and compartment pressure of less than 20 (ie. the compartment pressure creeps up to within 20 of diastolic pressure).
– probably the biggest influence on nerve regeneration is the Schwann cell. The regeneration comes from the nerve substance 1 cm