Titanium Oxide: A Bioactive Factor in Osteoblast Differentiation

Abstract

Titanium and titanium alloys are currently accepted as the gold standard in dental applications. Their excellent biocompatibility has been attributed to the inert titanium surface through the formation of a thin native oxide which has been correlated to the excellent corrosion resistance of this material in body fluids. Whether this titanium oxide layer is essential to the outstanding biocompatibility of titanium surfaces in orthopedic biomaterial applications is still a moot point. To study this critical aspect further, human fetal osteoblasts were cultured on thermally oxidized and microarc oxidized (MAO) surfaces and cell differentiation, a key indicator in bone tissue growth, was quantified by measuring the expression of alkaline phosphatase (ALP) using a commercial assay kit. Cell attachment was similar on all the oxidized surfaces although ALP expression was highest on the oxidized titanium alloy surfaces. Untreated titanium alloy surfaces showed a distinctly lower degree of ALP activity. This indicates that titanium oxide clearly upregulates ALP expression in human fetal osteoblasts and may be a key bioactive factor that causes the excellent biocompatibility of titanium alloys. This result may make it imperative to incorporate titanium oxide in all hard tissue applications involving titanium and other alloys.

Figure 1

SEM images of Ti-6Al-4V and γ-TiAl alloys. (a), (b): untreated, (c), (d): oxidized at 500°C, (e), (f): oxidized at 800°C, and (g), (h): MAO at 225 mA, 4 mins.

Figure 2

SEM micrographs of hFOB 1.19 cells on a glass coverslip (positive control), GTi and TiV (untreated alloys), thermally oxidized TiV5 and GTi5 (500°C), TiV8 and GTi8 (800°C), micro arc oxidized (MAO) TiV (200 mA and 225 mA at 3 min and 4 min), and MAOGTi (200 mA and 225 mA at 3 min and 4 min) disks. Magnification 1000x.

Figure 3

SEM micrographs of hFOB 1.19 cells seeded on a glass coverslip (positive control), GTi and TiV (untreated alloys), TiV5 and GTi5 (500°C), TiV8 and GTi8 (800°C), MAOTiV (200 mA and 225 mA at 3 min and 4 min), and MAOGTi (200 mA and 225 mA at 3 min and 4 min) disks and incubated for 10 days (3 days at 33.5°C and subsequently 7 days at 39.5°C). Magnification 3500x.

Figure 4

Alkaline phosphatase activity on thermally oxidized and MAO treated Ti-6Al-4V and γ-TiAl alloys. ALP activity on positive control (glass coverslips) and untreated Ti alloys is also shown.