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.