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Examples include variations of polylactic acid (PLA), polyglycolic acid (PGA), polydioxanone, and polycaprolactone. PLA has been a desirable choice because its degradation product is lactic acid, a natural constituent of the Krebs cycle. b. These polymers are resorbed at different rates. i. PLA resorbs faster than PGA. ii. Composite products may have intermediate properties. c. Resorption allows the host tissue to assume its normal role as the load-sharing capabilities of the polymer decrease. This must be balanced with the need for maintaining mechanical properties.

Figure 5 The differentiation schema of bone marrow precursor cells into osteoclasts. Cortical cytokine regulators are indicated. M-CSF = macrophage colony–stimulating ligand; RANKL = receptor activator for nuclear factor κB ligand; OPG = osteoprotegerin. 1: Basic Science Figure 4 3. Other intracellular signaling proteins—In addi- tion to secondary messengers, GTP-binding proteins such as Ras and protein kinases can accomplish the signal transduction without secondary messengers, through kinase cascades.

D. iii. Alumina, in the form of aluminum oxide, has shown lower wear rates than conventional metal-on-polyethylene bearings. Although early clinical experience showed fracture of alumina femoral heads to be a significant complication, this problem appears to be design related, and newergeneration designs have shown significantly lower fracture rates. iv. Zirconia, in the form of zirconia oxide, has shown less clinical success than alumina when used as a bearing surface against polyethylene. Zirconia oxide has lower toughness, which makes the material more susceptible to roughening and increased wear.