Ceramic Overlays Lowered Stress, but Fiber Posts Did Not Raise Fracture Resistance
Endodontically treated maxillary premolars remain difficult teeth to restore because the loss of tooth structure changes how biting forces travel through the crown and root. In this study, investigators paired finite element analysis with an in vitro fracture test to examine whether a fiber post and the choice of ceramic restoration could improve biomechanical behavior after root canal treatment. The answer was more restrained than many clinicians might expect. Ceramic overlays produced the lowest maximum von Mises stress in the modeled premolars, yet the addition of a fiber post did not significantly improve fracture resistance across the tested groups.
The work matters because premolars are exposed to angled functional loads and often retain less natural structure after endodontic therapy. Once the pulp is removed and an MOD cavity is prepared, the tooth no longer behaves like an intact crown-root unit. Stress concentrates in cervical and internal regions, especially when the restoration must bridge missing cusps and weakened dentin. A restorative strategy that looks mechanically sound on paper can still fail to translate into stronger fracture performance in the laboratory.
Six Restoration Scenarios Under a 300 N Oblique Load
A three-dimensional model of an endodontically treated maxillary premolar with an MOD cavity was built and tested across six scenarios: crown with no fiber post, crown with fiber post, onlay with no fiber post, onlay with fiber post, overlay with no fiber post, and overlay with fiber post. The model received a 300 N force at a 45° angle to the occlusal surface, a loading direction that better resembles chewing forces than a straight vertical push. von Mises stress served as the main biomechanical output in the computer analysis.
In parallel, 72 extracted human maxillary premolars were divided into six groups of 12 and subjected to fracture resistance testing. Fracture modes were recorded and then examined by scanning electron microscopy. Statistical analysis used two-way ANOVA and Tukey’s HSD testing with α = 0.05. The design allowed the same clinical question to be viewed from two angles: how the tooth-restoration complex deforms under force, and how much force it can actually take before failure.
Ceramic overlays came out ahead in the finite element model. They generated the lowest maximum stress values, regardless of whether a fiber post was present. In practical terms, the overlay geometry appears to spread load across a wider area of the remaining tooth, reducing focal concentration in vulnerable segments. This is a mechanical advantage, not a guarantee of higher fracture survival.
The fiber post also changed the stress picture. Its insertion dispersed stress along the cervical segment of the premolar, again independent of restoration type. Cervical redistribution can be useful if the goal is to blunt peak stress in one region, but it does not necessarily restore the tooth to near-original strength. Posts can reinforce a root only if the whole restorative system works together; otherwise, they may simply shift where the forces land.
No Significant Fracture Advantage Across the Six Groups
The in vitro results were blunt. No significant differences in fracture resistance emerged among the six treatment groups. In other words, the restoration type and the presence or absence of a fiber post did not translate into a measurable mechanical advantage under the study conditions. The data did not support the idea that either crowns, onlays, or overlays clearly outperformed the others in raw fracture resistance, even though they behaved differently in the finite element model.
One sentence captures the central tension in this study: lower modeled stress did not automatically become higher fracture resistance.
The fracture-mode data added a more clinically relevant layer. The proportion of restorable fractures was highest in the OvP group and lowest in the CNP group. Restorable fractures are the failures clinicians prefer because the tooth can sometimes be repaired rather than extracted. Even so, the authors did not report a statistically significant fracture-resistance boost from fiber post placement, so the better restorable-fracture proportion should be viewed as a directional signal rather than proof of superior strength.
What the Stress Patterns Suggest About Tooth Survival
Biomechanically, the findings fit a familiar pattern in restorative dentistry. Ceramic overlays can preserve more of the tooth’s structure than full crowns, and preserving structure usually helps distribute force more naturally. Full crowns may offer coverage, but they also require more preparation and may not necessarily improve the tooth’s ability to resist oblique loading once the endodontic weakening has already occurred. Onlays and overlays attempt to strike a balance between coverage and conservation, and the overlay configuration appears to have reduced stress most effectively in the finite element model used here.
The cervical stress dispersion seen with fiber posts is also consistent with the idea that posts modify internal load transfer rather than acting as a simple brace. A post can help retain core material and alter strain patterns inside the root, yet the surrounding dentin remains the critical substrate. When that dentin is already compromised by cavity design and endodontic access, the mechanical ceiling may stay low no matter how the restoration is configured.
Why the Laboratory Signal Still Needs Careful Clinical Reading
The study brings together computational and bench testing, but the two systems answer slightly different questions. FEA can identify where stress accumulates, while fracture testing asks how much force the tooth can tolerate before it fails. A low-stress restoration may still fracture at a force similar to that of a higher-stress design if the crack pathway, adhesive interface, or root anatomy shifts the failure mode. The SEM examination of fracture modes helped characterize failures, but the abstract does not provide detailed subgroup breakdowns beyond the restorable-fracture proportions.
For clinicians restoring endodontically treated maxillary premolars, the practical message is cautious rather than dramatic. Fiber posts altered stress distribution and were associated with a numerically higher proportion of restorable fractures in some groups, yet they did not significantly increase fracture resistance. Ceramic overlays produced the most favorable stress profile in the finite element analysis, but the laboratory fracture test did not show clear superiority across all groups. Restoration choice still has to account for remaining tooth structure, occlusal loading, adhesive control, and the specific geometry of the preparation.
Funding came from the National Key Research and Development Program of China and the Research and Development Program of West China Hospital of Stomatology, Sichuan University. The authors reported no conflicts of interest. Even with those details in place, the central limitation remains straightforward: the abstract does not show a clinically decisive fracture-resistance winner among crown, onlay, and overlay designs, with or without a fiber post, under the test conditions used here.