Effect of non-thermal plasma treatment on the bonding property of Bio-HPP post and core materials

When a tooth is damaged deep inside, leaving too little natural structure to hold a crown, dentists often insert a post into the root to act as an anchor. For decades, metal posts were the standard, but they are stiff and can sometimes crack the tooth because they do not bend like natural bone. A newer alternative, made from a high-performance polymer reinforced with tiny ceramic particles, offers a more flexible solution that mimics the tooth's natural movement. However, this material is chemically inert and smooth, making it notoriously difficult to glue securely to the tooth. If the bond fails, the entire restoration can loosen or fall out. The challenge, then, is finding a way to make this slippery, modern material stick firmly without damaging it or using harsh chemicals that might hurt the patient.

Researchers set out to solve this by testing a technique called non-thermal plasma treatment. Imagine a beam of invisible energy that can clean and chemically alter a surface without generating heat. By blasting the surface of the polymer posts with different types of gas in this energy state, the team hoped to create a surface that resin cement could grip onto tightly. They tested three specific gases: ordinary air, pure oxygen, and nitrogen. The goal was to see if this process could transform the smooth, non-stick surface of the post into something the glue could hold, and to determine which gas worked best.

To find the answer, the team took forty human teeth that had been extracted for other reasons and prepared them to receive these posts. They used a computer to design and mill forty custom posts from blocks of the polymer material, ensuring each one fit perfectly into the prepared space. These posts were then divided into four groups. One group received no treatment at all, serving as a baseline. The other three groups were treated with one of the three gases using the plasma device. After treating the surfaces, the researchers cemented the posts into the teeth using a standard dental glue and waited a week for everything to set. They then sliced the teeth into thin sections to test how hard it was to push the posts out, measuring the force required to break the bond at different depths within the root.

The results were clear and decisive. The untreated posts held on with an average force of about 8.00 megapascals, a measure of pressure. Every single group that received plasma treatment held significantly better. The posts treated with air or oxygen required roughly 13.35 and 13.42 megapascals respectively to dislodge. However, the posts treated with nitrogen gas performed the best of all, requiring an average force of 18.19 megapascals to break the bond. This means the nitrogen treatment made the posts nearly twice as strong as the untreated ones. The researchers also found that the location of the post mattered; the bond was strongest near the top of the tooth and became weaker as the post went deeper toward the tip of the root, regardless of which treatment was used.

Looking closely at the surface of the materials under powerful microscopes revealed why the treatment worked so well. The untreated posts had a relatively smooth surface with some scratches from polishing. The plasma-treated posts, however, showed tiny pits and a cleaner surface, indicating that the energy had etched away microscopic layers. Chemical analysis showed that the treatment had changed the surface composition, adding oxygen and nitrogen atoms while removing some carbon. This chemical shift created new, polar groups on the surface that could interact more strongly with the glue. While all the gases improved the surface, the nitrogen treatment created a specific type of chemical group that seemed to form a particularly robust connection with the resin cement, explaining why it outperformed the others.

The study also examined how the materials failed when pushed apart. In the untreated group, the glue simply slipped off the post, indicating a weak bond between the two. In the treated groups, the failure usually happened between the glue and the tooth itself, rather than between the glue and the post. This shift is a crucial sign of success; it means the bond between the post and the glue became so strong that the weakest point in the entire system moved to the tooth structure, not the material interface. This suggests that the plasma treatment successfully solved the adhesion problem for the polymer, making it a much more viable option for dental restoration.

While the study was conducted in a controlled laboratory setting and did not test how these bonds would hold up over years of chewing or temperature changes, the findings offer a promising path forward. The research demonstrates that non-thermal plasma, particularly with nitrogen gas, can effectively prepare these advanced polymer posts for bonding. By turning a difficult-to-stick material into one that holds fast, this technique could help dentists use these flexible, tooth-friendly posts more confidently, potentially reducing the risk of root fractures and improving the longevity of dental repairs.