To evaluate the mechanism by which orthopedic implant wear particles induce apoptosis in immature osteoblasts in an in-vitro setting.

Stromal cells from femurs of thirty day-old Swiss Webster Mice were isolated, cultured in-vitro, and incubated with orthopedic wear particles in the micrometer size range. After incubation with wear-particles, the cells were assessed for Caspase three expression and activity in the presence or absence of specific inhibitor(s) in order to delineate potential mechanism for cellular changes previously reported.

Here we report the induction of caspase three protein expression and activity with incubation of stromal cells with titanium wear particles. Caspase three activity however was not demonstrated to be up regulated in a time dependent manner or at lower concentration of particles (2 x 107 particles/ml). However, there was a significant (P< 0.05) increase in caspase three activity with titanium particle at higher concentration (4 x 107 particles/ml) that was not reversible when the extrinsic arm of the apoptotic pathway was blocked with anti-TNFƒa antibodies.

Our previous studies have suggested that aseptic loosening of orthopedic implants may be independent of inflammatory processes, and may be associated with induction of programmed cell death. Our current results would strengthen this idea by demonstrating induction of expression and activity of caspase three involved in apoptosis in cells incubated with wear particles. In addition, titanium wear particles may induce apoptosis through direct cellular effects rather than through the extrinsic TNFƒa pathway. Delineating the mechanism by which wear particles induce apoptosis in immature osteoblasts will allow for the selection and/or development of inhibitors to the process of asceptic loosening by targeting a specific pathway.

Purpose: Peri-implant osteolysis after total joint arhtro-plasty (TJA) is a major cause of implant loosening. Cellular responses to wear particles have been reported to play a role in asceptic loosening due to their cytotoxic nature to cellular components. Purpose of this study is to evaluate the effect of orthopedic implant wear particles on immature osteoblasts in an in-vitro setting in order to further understand the mechanisms involved in asceptic loosening of implants.

Methods: Stromal cells from femurs of 30 day-old Swiss Webster Mice were isolated, cultured in-vitro, and incubated with Titanium and Ceramic (smooth and angular) particles in the micrometer size range. After 9 days of incubation the cells were assessed for Alkaline phosphatase (ALP) activity or stained for cellular changes consistent with apoptosis.

Results: Here we report both a dose-dependent decrease (P< 0.05) in ALP activity and a significant increase in programmed cell death when murine stromal cells were cultured with orthopedic implant wear particles of differing compositions. Ceramic wear particles were consistently less toxic at lower concentrations (1 x 107 to 2 x 107 particles/ml) than were wear particles composed of titanium. However, at high concentrations (4 x 107 particles/ml) all particles regardless of composition were equally toxic. These findings suggest that ceramic particles may be less cytotoxic to bone marrow stromal cells/osteoblasts than are titanium particles.

Conclusions: Previous studies have suggested that inflammatory responses to orthopedic wear particles are responsible for the asceptic loosening of orthopedic implants. In the current study however, we found that wear particles may also induce cellular apoptosis of primary bone forming cells. This suggests that the asceptic loosening of orthopedic implants may be independent of inflammatory processes, and that implant material selection should be directed, in part, by its inability to cause programmed cell death.