Effects of phosphoric acid treatment of titanium surfaces on surface properties, osteoblast response and removal of torque forces

This study investigated the surface characteristics and biocompatibility of phosphate ion (P)-incorporated titanium (Ti) surfaces hydrothermally treated with various concentrations of phosphoric acid (H3PO4). The surface characteristics were evaluated by scanning electron microscopy, thin-film X-ray diffractometry, X-ray photoelectron spectroscopy, optical profilometry, contact angle and surface energy measurement and inductively coupled plasma mass spectroscopy (ICP-MS). MC3T3-E1 cell attachment, spreading, proliferation and osteoblastic gene expression on different surfaces were evaluated. The degree of bony integration was biomechanically evaluated by removal torque testing after 4 weeks of healing in rabbit tibiae. The H3PO4 treatment produced micro-rough Ti surfaces with crystalline P-incorporated Ti oxide layers. High concentration H3PO4 treatment (1% and 2%) produced significantly higher hydrophilic surfaces compared with low H3PO4 treatment (0.5%) and untreated surfaces (P < 0.01). ICP-MS analysis showed P ions were released from P-incorporated surfaces. Significant increased cell attachment (P < 0.05) and notably higher mRNA expressions of Runx2, alkaline phosphatase, osteopontin and osteocalcin were observed in cells grown on P-incorporated surfaces compared with cells on untreated machined surfaces. P-incorporated surfaces showed significantly higher removal torque forces compared with untreated machined implants (P < 0.05). Ti surfaces treated with 2% H3PO4 showed increasing tendencies in osteoblastic gene expression and removal torque forces compared with those treated with lower H3PO4 concentrations or untreated surfaces. These results demonstrate that H3PO4 treatment may improve the biocompatibility of Ti implants by enhancing osteoblast attachment, differentiation and biomechanical anchorage.