Selective Root Canal Treatment Versus Conventional Root Canal Treatment: Effect of Pulpotomy Extent on Root Dentin Stress Distribution — A Three- Dimensional Finite Element Study

Teeth are not merely static blocks of stone; they are living structures that must withstand the relentless, shifting forces of chewing. When a tooth's inner nerve tissue, known as the pulp, becomes inflamed or infected, the standard solution has long been to remove it entirely, clean out the hollow space, and fill it with a synthetic material. This procedure, called a root canal, saves the tooth from infection but leaves it more fragile. The removal of the soft, living core changes how the tooth handles stress, often making it prone to cracking, particularly in the narrow "collar" of dentin just below the gum line. This area is critical because it is where the tooth's shape changes most abruptly, creating a natural weak point. In recent years, dentists have begun to experiment with a gentler approach: removing only the diseased portion of the pulp while leaving the healthy, deeper tissue alive. This is known as selective root canal treatment. The hope is that keeping some of the tooth's natural, living interior might preserve its strength better than removing it all. However, a lingering question has remained: does it matter how much of the pulp is left behind? Is the benefit simply about saving any amount of living tissue, or does the tooth need a specific, deeper layer of that tissue to remain strong?

To answer this, researchers at Misr International University and Ain Shams University turned to a powerful tool called finite element analysis. Instead of drilling into real teeth, which would destroy them and make it impossible to measure internal forces, they built a highly detailed, three-dimensional digital model of a human upper premolar. This virtual tooth was constructed with the same layers found in nature: a hard outer shell of enamel, a thick middle layer of dentin, and an inner chamber containing the pulp. The researchers set up a scenario where the tooth was subjected to a single, sharp push of 100 Newtons, applied at an angle to mimic the force of biting down on something hard. They then ran a series of simulations to see how the stress traveled through the tooth's structure under different treatment conditions. In every scenario, the shape of the tooth and the size of the opening drilled into the top were kept exactly the same, ensuring that any differences in stress were caused solely by how much pulp was removed and how the resulting space was filled.

The team tested five different situations. In one, they removed all the pulp from both roots and filled the space with standard gutta-percha, the material used in traditional root canals. In the other four, they treated only one of the two roots, leaving the other root fully treated as a constant reference point. For the treated root, they simulated three different depths of pulp removal: a shallow cut just at the top, a medium cut partway down the root, and a deep cut extending further down. Crucially, they did not just fill these holes with a single block of material. They modeled a realistic, two-layer repair: a thin, 2-millimeter cap of a biocompatible cement called MTA placed directly over the remaining pulp, followed by a bulk filling of composite resin to seal the rest of the tooth. This detail was vital, as it prevented the simulation from being skewed by the properties of a single filling material.

The results of these simulations revealed a clear and somewhat surprising pattern. The most significant factor in reducing stress within the critical collar of the tooth was not how deep the pulp was removed, but simply whether any vital pulp was left behind at all. When the researchers compared the teeth where some living tissue was preserved against the teeth where all pulp was removed, they found that the stress in the dentin was roughly 11.6 to 12.4 percent lower in the teeth with preserved pulp. Specifically, the stress levels in the teeth with any vital tissue ranged from 4.19 to 4.22 megapascals, whereas the tooth with no living tissue at all experienced a stress of 4.71 megapascals.

What was perhaps more striking was what the simulations did not show. The depth of the pulp removal made almost no difference to the stress levels. Whether the pulp was removed just a few millimeters or several millimeters deeper, the stress in the tooth's collar remained virtually identical, varying by less than one percent. This suggests that the biomechanical advantage comes from the presence of the living tissue itself, rather than the specific amount of it. Furthermore, the researchers looked at other parts of the tooth, such as the area where the roots split and the very tip of the root, and found no consistent pattern of stress changes based on how deep the treatment went. In every single scenario, the highest stress recorded was still far below the point where the tooth's dentin would actually break. The peak stress observed was only about 11 percent of the lowest reported strength limit for dentin, meaning that under this single, static bite, none of the treatments came close to causing a fracture.

The study concludes that for the purpose of preserving the mechanical strength of the tooth's collar, the decision to save any vital pulp is far more important than the precise depth of the removal. While the simulations showed that keeping the pulp reduced stress, the researchers noted that these were computer models of a single, instant bite. They cautioned that real teeth face years of repeated chewing and fatigue, which might tell a different story. Additionally, the study held the shape of the tooth and the access hole constant, meaning it could not account for how a different drilling technique might change the outcome. Nevertheless, the findings offer a clear direction for understanding the mechanics of these treatments: the simple act of preserving the living core of the tooth appears to be the key to reducing stress, regardless of how much of that core is left behind.