Ti-6Al-4V ion solution inhibition of osteogenic cell phenotype as a function of differentiation timecourse in vitro

Metal ions released from the implant surface are suspected of playing some contributing role in loosening of hip and knee prostheses. Previous work in this laboratory demonstrated that sublethal doses of the ionic constituents of Ti-6Al-4V alloy suppressed expression of the osteoblastic phenotype and deposition of a mineralized matrix. The purpose of this work was to further explore this suppression as a function of the normal time-course of phenotype expression. Bone marrow stromal cells were harvested from juvenile rats and exposed to time-staggered doses of a solution of ions representing Ti-6Al-4V alloy. Cells were cultured for four weeks and assayed for total protein, alkaline phosphatase, intra- and extracellular osteocalcin, and calcium. Ti-6Al-4V solutions were found to produce little difference from control solutions for total protein or alkaline phosphatase levels, but strongly inhibited osteocalcin synthesis. Calcium levels were reduced when ions were added before a critical point of osteoblastic differentiation (between 2 and 3 weeks after seeding). These results indicate that ions associated with Ti-6Al-4V alloy inhibited the normal differentiation of bone marrow stromal cells to mature osteoblasts in vitro, suggesting that ions released from implants in vivo may contribute to implant failure by impairing normal bone deposition.