# Influence of Ferrule Height and Uniformity on Fracture Resistance of Post-core Crowns: An In Vitro Analysis

## TL;DR

This study shows that a continuous 2-mm ferrule around a tooth improves its fracture resistance and makes failures more restorable compared to non-uniform or no ferrule designs.

## Contribution

The study introduces a novel comparison of uniform versus non-uniform ferrule configurations in post-core crown restorations using an in vitro model.

## Key findings

- A continuous 2-mm ferrule preserves tooth strength and increases restorable failure likelihood.
- Non-uniform ferrules offer partial protection but are less effective than uniform ones.
- Tooth specimens with no ferrule had the lowest fracture resistance and highest catastrophic failure rates.

## Abstract

Objective

To evaluate the effect of uniform versus non-uniform ferrule configurations on fracture resistance and failure mode in endodontically treated maxillary central incisors restored with cast dowel-core and metal-ceramic crowns.

Methods

In this in-vitro, controlled study, 50 extracted maxillary central incisors were equally allocated to five groups: intact crowned (CRN), root-canal-treated crowned (RCT/CRN), uniform 2-mm ferrule (2FRL), non-uniform ferrule (0.5/2FRL; 0.5 mm proximal/2 mm labial-lingual), and no ferrule (0FRL). Specimens were statically loaded at 45° to failure; peak load (N) was the primary outcome. Failures were classified as restorable or catastrophic, and reliability was summarized as P(load >400 N, >500 N) under a normal model fitted to each group.

Results

Groups differed significantly (analysis of variance (ANOVA), p<0.001; very large effect). CRN, RCT/CRN, and 2FRL were comparable, indicating preserved strength when a continuous 2-mm ferrule engages coronal dentin. The non-uniform ferrule showed reduced resistance versus these groups (Tukey p<0.001) yet exceeded 0FRL, which performed worst. Analysis of covariance (ANCOVA) confirmed these patterns and identified greater coronal height as independently favourable. Ferrule adequacy shifted failure towards restorable patterns and increased the probability of exceeding clinical thresholds: highest for CRN/RCT/CRN/2FRL, intermediate for the non-uniform ferrule, and minimal without a ferrule.

Conclusion

Ferrule configuration is the primary determinant of mechanical behaviour. A continuous 2-mm ferrule preserves strength and favours restorable fractures; a non-uniform ferrule offers partial but meaningful protection and is preferable to none. Clinically, preserve coronal dentin and create a 2-mm circumferential collar whenever feasible.