Why does intermittent hydrostatic pressure enhance the mineralization process in fetal cartilage?

The purpose of this study was to determine which factor is the most likely one to have stimulated the mineralization process in the in vitro experimen ts of Klein-Nulend et al. (Arth. Rheum., 29, 1002-1009, 1986), in which fet al cartilaginous metatarsals were externally loaded with an intermittent hy drostatic pressure, by compressing the gas phase above the culture medium. Analytical calculations excluded the possibility that the tissue was stimul ated by changes in dissolved gas concentration, pH or temperature of the cu lture medium through compression of the gas phase. The organ culture experi ments were also mechanically analyzed using a poroelastic finite element (F E) model of a partly mineralized metatarsal with compressible solid and flu id constituents. The results showed that distortional strains occurred in t he region where mineralization proceeded. The value of this strain was, how ever, very sensitive to the value of the intrinsic compressibility modulus of the solid matrix (K-s). For realistic values of K-s the distortional str ain was probably too small (about 2 mu strain) to have stimulated the miner alization. If the distortional strain was not the factor to have enhanced t he mineralization process, then the only candidate variable left is the hyd rostatic pressure itself. We hypothesize that the pressure may have created the physical environment enhancing the mineralization process. When hydros tatic pressure is applied, the balance of the chemical potential of water a cross cell membranes may be disturbed, and restored again by diffusion of i ons until equilibrium is reached again. The diffusion of ions may have cont ributed to the mineralization process. (C) 1999 Elsevier Science Ltd. All r ights reserved.