The cytotoxic effect of different concentrations of titanium particles on osteoblasts was studied in vitro. It was found that the viability of the osteoblasts was inversely proportional to the particle concentration. Phagocytosis of particles by the osteoblasts was evident and was demonstrated to be responsible for cell necrosis. Moreover, during and after phagocytosis, the osteoblasts released products that were cytotoxic for other osteoblasts, as established with a conditioned medium assay. The titanium particles thus had both a direct and an indirect effect on osteoblast viability. It also was observed that the titanium particles induced a process of programmed cell death (apoptosis) when co-cultured with osteoblasts. The results of this study suggest that not only is the amount of wear debris generated important, but the local accumulation of the debris also may have a significant impact on bone cell function.

Tissue engineering offers the possibility of replacing damaged human ligaments with engineered ligament tissues. Hence, we attempted to culture in vitro ligament tissues by seeding human anterior cruciate ligament (ACL) and medial collateral ligament (MCL) cells onto synthetic biodegradable polymer fiber scaffolds. The ACL and MCL cells readily attached to the scaffold fibers. These cells and their secreted matrix soon surrounded the scaffold fibers and bridged the gaps in between. Beginning at 2 weeks, portions of the scaffolds were completely filled with tissue matrix. By 5 weeks, the scaffolds became single bundles of tissue. Thus the cell/fiber system appears to be a viable system for culturing ligament tissues. Additionally, cell proliferation under mechanical and biochemical stimuli was studied for up to 4 days. Whereas mechanical stimulus and transforming growth factor enhanced proliferation, inflammatory agents (lipopolysaccharide and complement C5a) had a negative effect. This work can thus contribute to a sound strategy for culturing replacement ligament tissues in vitro.

Inflammation and accumulation of matrix metalloproteinases (MMPs) in synovial fluids may be involved in the poor healing ability of the anterior cruciate ligament (ACL) after injury. With a rat ACL rotating injury model, we found that levels of IL-1beta, IL-6, and TNF-alpha were significantly higher in synovial fluids after ACL injury. MMP-2 activity and global MMP activity in synovial fluids also increased significantly in a time-dependent manner. Ex vivo studies showed that all tissues contributed to the elevation of MMP-2 in synovial fluids, especially synovium and the injured ACL. We concluded that although the regular wound-healing mechanism also occurs after ACL injury, accumulation of MMP activity in the synovial fluids, due to all of the intraarticular tissues, may be at least one of the important reasons why an injured ACL cannot be repaired.

Numerous studies have demonstrated the capacity of mechanical strains to modulate cell behavior through several different signaling pathways. Understanding the response of ligament fibroblasts to mechanically induced strains may provide useful knowledge for treating ligament injury and improving rehabilitation regimens. Biomechanical studies that quantify strains in the anterior cruciate and medial collateral ligaments have shown that these ligaments are subjected to 4-5% strains during normal activities and can be strained to 7.7% during external application of loads to the knee joint. The objective of this study was to characterize the expression of types I and I11 collagen in fibroblast monolayers of anterior cruciate and medial collateral ligaments subjected to equibiaxial strains on flexible growth surfaces (0.05 and 0.075 strains) by quantifying levels of mRNA encoding these two proteins. Both cyclic strain magnitudes were studied under a frequency of 1 Hz. The results indicated marked differences in responses to strain regimens not only between types I and I11 collagen mRNA expression within each cell type but also in patterns of expression between anterior cruciate and medial collateral ligament cells. Whereas anterior cruciate ligament fibroblasts responded to cyclic strains by expression of higher levels of type-I collagen message with almost no significant increases in type-I11 collagen, medial collateral ligament fibroblasts exhibited statistically significant increases in type-111 collagen mRNA at all time points after initiation of strain with almost no significant increases in type-I collagen. Furthermore, differences in responses by fibroblasts from the two ligaments were detected between the two strain magnitudes. In particular, 0.075 strains induced a time-dependent increase in type-I11 collagen mRNA levels in medial collateral ligament fibroblasts whereas 0.05 strains did not. The strain-induced changes in gene expression of these two collageiis may have implications for the healing processes in ligament tissue. The differences may explain, in part, the healing differential between the anterior cruciate and medial collateral ligaments in vivo.