In vitro Simulation of Periodontal Ligament in Fatigue Testing of Dental Crowns

Should the load-to-fracture test for CAD/CAM monolithic molar crowns be standardized and how? A systematic review and finite element analysis

The load-to-fracture test is widely used to evaluate crowns made of new CAD/CAM materials, even though its validity in predicting clinical performances is often questioned. Despite its limitations, the test is useful in assessing the load-bearing capacity of crowns subjected to accidental overloads and setting up step-stress regimes for fatigue testing. This study combined a systematic review (SR) and finite element analysis (FEA) to assess whether the test should be standardized and how. The SR evaluated load-to-fracture studies of monolithic CAD/CAM molar crowns published in Q1 and Q2 journals. Findings from 85 studies highlighted the lack of standardization in test methods, particularly regarding loading head type and die material. This variability led to a wide dispersion of fracture load results, limiting the utility of the load-to-fracture test. The FEA evaluated the influence of loading head type and die material on tensile stress distribution in lithium disilicate (LD) and polymer-infiltrated ceramic network (PICN) crowns. Eight in vitro conditions were simulated, varying the loading head (4 mm and 10 mm spheres, inverse V-shaped device, opposing teeth) and die material (stiff, E = 207 GPa; non-stiff, E = 13 GPa). The FEA confirmed that the stress distribution and peak tensile stress in LD and PICN crowns depend significantly on these factors as well as the crown material properties, with the peak stress variation from LD to PICN ranging from -4 % to 237 %. Using larger-diameter spheres with a die material approximating dentin in stiffness resulted in stress distributions more representative of clinical conditions.

Influence of remaining coronal tooth morphology with resin abutment and fiber post on static and dynamic fracture resistances

This study aimed to clarify the fracture resistance of resin abutments built on endodontically treated roots with the remaining coronal teeth via static and cyclic loading tests. Endodontically treated bovine roots, which had a remaining coronal tooth covered with an occupied area for a quarter and half of the circumference at the tensile side or covered the circumference at both the tensile and compressive sides, were fabricated to build up to the resin abutment. Fracture resistance was evaluated via static and cyclic loading tests by applying a load of 30° to the tooth axis. Half of the circumference of the remaining coronal tooth showed a significantly higher static fracture load and survival rate. The remaining coronal tooth on the compressive side improved the dynamic fracture resistance associated with severe fractures. The occupied area and location of the remaining coronal tooth affected the static and dynamic fracture resistances.

A novel digital workflow for fabricating artificial periodontal ligament using three-dimensional printing flexible resin: A dental technique

The fabrication of periodontal ligament (PDL) models for in vitro dental studies has seen a wide range of techniques and materials being utilized. This paper introduces a novel dental technique that employs a digital workflow for the fabrication of artificial PDL using three-dimensional printing of flexible resin. This innovative approach offers several advantages, including enhanced accuracy and realism in simulating PDL. The digital workflow facilitates a streamlined fabrication process, ensuring efficiency and precision. By presenting this novel technique, this digital approach contributes to the advancement of in vitro dental research, providing researchers with a reliable and realistic model for studying various dental phenomena.

An in vivo evaluation of clear aligners for optimal orthodontic force and movement to determine high-efficacy and periodontal-friendly aligner staging

Objectives

To investigate the effect of aligner displacement on tooth movement and periodontal health to improve the efficiency of aligner treatment and explore the mechanism in vivo.

Methods

A two-tooth site was established by a finite element (FE) model to virtually evaluate aligner staging. A randomized controlled experiment was conducted when the tooth sites in beagles were treated with fixed or aligner appliances with different movement and force, and tooth movement and internal structure were recorded during the alignment. After sacrificing five dogs, bone-periodontal ligament (PDL)-tooth specimens were removed and processed to conduct uniaxial compression and tensile tests as well as micro-CT imaging and histological analysis.

Results

Three displacements of 0.25, 0.35 and 0.45 mm were obtained from FE analysis and applied in beagles. In general, aligners had poorer performance on movement compared to fixed systems in vivo, but the aligner with a staging of 0.35 mm had the highest accuracy (67.46%) (P < 0.01). Loaded with severe force, fixed sites exhibited tissue damage due to excess force and rapid movement, while aligners showed better safety. The PDL under a 0.35-mm aligner treatment had the highest elastic modulus in the biomechanical test (551.4275 and 1298.305 kPa) (P < 0.05).

Conclusions

Compared to fixed appliances, aligners achieve slightly slower movement but better periodontal condition. Aligners with an interval of 0.35 mm have the highest accuracy and best PDL biomechanical and biological capacities, achieving the most effective and safest movement. Even with complexity of oral cavity and lack of evaluation of other factors, these results provide insight into faster displacement as a method to improve the efficacy of aligners.

Effect of bone defect width and a ferrule on the fracture characteristics of vertically fractured teeth reattached with adhesive resin cement: An in vitro study

This study aimed to clarify the effects of vertical bone defect width and a ferrule on fracture of the fragments of fractured tooth reattached with adhesive resin cement (reattached tooth). The reattached tooth was built up by a fiber post and composite resin core for abutment and formed to the abutment with or without a ferrule. The vertical bone defect was fabricated with a V-shaped defect in different widths. The fracture load was evaluated using a universal testing machine. The vertical bone defect did not affect the fracture load, but a ferrule increased the root fracture load. For the specimens without a ferrule, debonding between the composite resin core and the root at the coronal loading side and fractures at the apical side of the root were found. In conclusion, the ferrule at abutment could affect fracture load and modes, and the bone defect width did not.

The effects of a PDL analogue on occlusal contact forces

BACKGROUND

Previous bench-top studies examined the details of the mechanical environment of rigidly-fixed occluding teeth. It was demonstrated that during each chomp, contacting molar teeth experience in-occlusal-plane forces (F_lateral ) that are highly transient in magnitude and direction.

OBJECTIVES

The objectives of this study are to identify F_lateral behaviors that are attributable to the presence of a visco-elastic periodontal ligament (PDL) analogue, and to asses the necessity of incorporating it into future studies.

METHODS

A weighted maxillary molar denture tooth was lowered onto, and raised from, a matching mandibular molar 10 times. The latter was supported by a load cell that continuously measured F_lateral . For statistical purposes, the test was repeated with 21 (n = 21) different occlusal relationships obtained with 0.05 mm incremental shifts of the lower assembly.

RESULTS

Overall, the results are similar to those of rigid attachment but the details of the F_lateral profiles are very different.

CONCLUSION

The PDL plays a major role in the mechanical environment of occlusion, suggesting that, in general (not necessarily always), it should be integrated into studies of occlusion.