Effect of fiber post and restoration type on biomechanical behavior of endodontically treated premolars: Finite element analysis and in vitro evaluation

When a tooth suffers deep decay or a crack that reaches its nerve, dentists often perform a root canal to save it. This life-saving procedure removes the infected tissue inside, but it also leaves the tooth drier and more brittle, like a dried-out twig compared to a fresh, living branch. The tooth is also frequently weakened by the large cavity needed to reach the infection, especially in the upper premolars, which sit in a part of the mouth that takes heavy chewing forces from the side. To fix this, dentists must cover the tooth with a strong cap, but they face a difficult choice. They can grind away a significant amount of the remaining healthy tooth to fit a full crown, or they can try to preserve more of the tooth with a partial cover called an onlay or an overlay. Another common debate is whether to insert a small, flexible fiber rod deep into the root to act as a support beam. For years, the dental community has argued over which combination of cap and support offers the best protection against the tooth snapping in half, a failure that often means the tooth must be pulled out entirely.

Researchers at the West China Hospital of Stomatology set out to settle this debate by testing these different strategies on a model that mimics the human mouth. They focused on the upper premolar, a tooth known for being particularly prone to breaking after a root canal. The team created two parallel investigations: one using a sophisticated computer simulation to map out invisible forces inside the tooth, and another using real, extracted human teeth to see how much force they could actually withstand before breaking. They tested six different scenarios. Some teeth received a full crown, some a partial onlay, and some a partial overlay. Within each of these three shapes, they tested two variations: one with a fiber post inserted into the root and one without. They applied a steady, angled push of 300 Newtons to the chewing surface of each model and tooth, a force designed to simulate the kind of sideways pressure these teeth face during normal eating.

The computer simulations revealed a clear pattern in how stress traveled through the tooth. The models with the partial overlays showed the lowest levels of internal stress in both the restoration and the tooth structure. Interestingly, while the overall stress levels in the tooth and cap remained comparable between groups with and without posts, the fiber post significantly changed where that stress went. The presence of the post dramatically reduced stress in the resin core and resulted in stress dispersion on the cervical segment of the premolars. Without the post, the highest stress concentrated dangerously at the neck of the tooth, right where the root meets the crown. With the post in place, that stress was spread out more evenly, reducing the danger of a crack starting at that weak spot.

When the researchers moved to the physical test with real teeth, the results regarding strength were surprisingly uniform. No matter which type of cap was used or whether a fiber post was inserted, the amount of force required to break the tooth was statistically the same. The teeth held up equally well across all six groups. However, the story changed when the researchers looked at how the teeth broke. The type of failure mattered just as much as the force required to cause it. Some breaks were minor, occurring only in the crown or the top part of the tooth, which a dentist could fix by making a new cap. Others were catastrophic, with cracks running deep down the root, making the tooth impossible to save. The group with the overlay restoration and a fiber post had the highest rate of these fixable breaks, while the group with a full crown and no post had the lowest rate of fixable breaks and the highest rate of total loss.

The fiber post proved to be a crucial factor in saving the tooth, even though it did not change the total force required to break it. By acting as a flexible support that matched the stiffness of the natural tooth root, the post helped guide the forces away from the fragile neck of the tooth and reduced stress concentration in the cervical segment and buccal root. This prevented the cracks from traveling deep into the root, turning what would have been a lost tooth into one that could be repaired. The study suggests that while the shape of the cap—whether full or partial—does not significantly change how much force the tooth can take, the decision to use a fiber post can be the difference between a tooth that needs a simple repair and one that must be extracted. The researchers concluded that for these weakened teeth, the best strategy to ensure a tooth remains in the mouth is to use a fiber post, which helps distribute stress and encourages a break pattern that can be fixed, regardless of the specific type of ceramic cover chosen.