Effect of surface treatment on cell responses to grades 4 and 5 titanium for orthodontic mini-implants

Mini-implants are used to improve orthodontic anchorage, but optimal composition and surface characteristics have yet to be determined. We investigated the behavior of osteoblast-like cells on grade 4 commercially pure titanium and grade 5 titanium alloy with different surface treatments for mini-implants.

MC3T3 cells were plated on machined, acid-etched, or acid-etched grade 4 titanium enriched with calcium phosphate, or machined, anodized, or anodized and calcium phosphate-enriched grade 5 titanium disks. Surface and cell morphologies were assessed by scanning electron microscopy. Cell viability was measured by chemiluminescence, cytoskeletal organization was investigated by immunofluorescence, and real-time polymerase chain reaction for osteoblast-specific genes was performed to measure cell differentiation.

Flattened shapes and strong stress fibers were observed on the machined surfaces; cells on the rough surfaces had a spindle shape, with lower cytoskeletal polarization. Cell proliferation was highest on smooth grade 4 titanium surfaces, whereas cells quickly reached a plateau on rough grade 4 titanium; no difference was observed after 72 hours in the grade 5 titanium groups. Calcium phosphate enrichment on grade 4 titanium significantly increased the messenger RNA levels for alkaline phosphatase and osteocalcin. Osteoblastic markers were higher on the grade 5 titanium machined surfaces than on the rough surfaces, and comparable with acid-etched grade 4 titanium.

Although the grade 4 titanium enriched with calcium phosphate had the highest level of differentiation in vitro, the grade 5 titanium machined surfaces supported cell proliferation and matrix synthesis, and induced high expression of early differentiation markers. Increased mechanical resistance of grade 5 titanium makes it a potential candidate for orthodontic mini-implants.