Effects of Preparation Height, Luting Cement Type, and Thermal Aging on the Retention Resistance of 3D-Printed Crowns in Primary Anterior Teeth: An In Vitro Study
This in vitro study demonstrates that while resin cement initially offers superior retention for 3D-printed crowns on primary anterior teeth compared to resin-modified glass ionomer cement regardless of preparation height, thermal aging significantly reduces this advantage, suggesting that cement selection is more critical than preparation height for initial retention.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When a young child loses a front tooth to severe decay, the consequences extend far beyond a missing smile. The tooth is essential for clear speech, proper chewing, and the self-esteem that comes from looking like other children. Restoring these tiny, fragile teeth is one of the most delicate challenges in dentistry. The teeth are small, their inner pulp chambers are large, and there is very little hard surface left to hold a new cap in place. For decades, dentists have relied on prefabricated crowns—caps made in a factory that come in standard sizes. While these work well, they are rigid and cannot be shaped to fit the unique curves of a specific child's tooth, often requiring the dentist to grind away more healthy tooth structure than necessary to make the cap sit correctly. A newer approach uses three-dimensional printing to create a custom crown that matches the child's tooth exactly, potentially preserving more of the natural structure. However, for any crown to stay on, it must be glued firmly with a special dental cement, and it must survive the harsh environment of the mouth, where hot coffee and cold ice cream constantly stress the bond.
Researchers set out to test how well these custom 3D-printed crowns hold up under different conditions. They wanted to know if the height of the remaining tooth stump mattered, which type of glue worked best, and how the crowns would fare after being subjected to the temperature swings of a real mouth. To find the answers, they gathered 180 healthy, extracted baby front teeth. They carefully prepared each tooth by grinding it down to three specific heights: two millimeters, three millimeters, or four millimeters. They then used a digital scanner to map each tooth and a 3D printer to create a custom resin crown for every single one. Half of the crowns were glued with a resin-modified glass ionomer cement, a material known for being forgiving with moisture, while the other half were glued with a resin cement, which is often stronger initially. To simulate the wear and tear of a year in the mouth, half of the groups were subjected to ten thousand cycles of hot and cold water, while the other half remained fresh. Finally, the team pulled on each crown with a machine to measure exactly how much force was needed to pop it off.
The results revealed that the height of the prepared tooth was not the deciding factor for how well the crown stayed on. Whether the remaining tooth was two, three, or four millimeters tall, the crowns held just as well, provided the glue was chosen correctly. This finding suggests that dentists do not need to grind down a tooth to a specific height to ensure a secure fit, as long as they stay within a reasonable range. The type of cement, however, made a significant difference. Before any aging, the resin cement held the crowns much tighter than the glass ionomer cement. The resin cement required an average force of about 91 newtons to remove, while the glass ionomer needed only about 65 newtons. But the story changed when the crowns were exposed to the simulated hot and cold cycles. The strength of the resin cement dropped significantly after this aging process, falling to an average of 74 newtons. In contrast, the glass ionomer cement remained steady, showing no significant loss of strength. After the aging process, the initial advantage of the resin cement disappeared, and both types of glue performed similarly.
Perhaps the most surprising discovery concerned how the crowns actually failed. In many dental studies, crowns simply pop off cleanly, leaving the glue behind. Here, that happened in only about ten percent of the cases. In the vast majority of instances, the force required to pull the crown off was so great that it broke something else first. Often, the 3D-printed crown itself cracked, or the natural baby tooth fractured, or both broke at the same time. This indicates that the bond between the crown, the glue, and the tooth was so strong that the materials themselves were the weak link, not the glue line. The researchers noted that the way they tested the crowns—pulling straight up through a hole designed into the crown—might have concentrated stress in a way that caused these fractures. While this suggests the custom crowns are incredibly durable, it also means that the test conditions were extreme. The study concludes that custom 3D-printed crowns are a promising option for saving damaged baby teeth, offering a conservative way to restore function and appearance. However, because the crowns in the lab broke before they came loose, and because the glue types behaved differently under heat and cold, more real-world studies are needed to confirm how long these restorations will last in a child's mouth.
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