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Views: 0 Author: Site Editor Publish Time: 2025-07-17 Origin: Site
Poisson's ratio (ν) quantifies the transverse strain response of dental PMMA when subjected to longitudinal loading, such as biting forces. For PMMA, typical ν values range between 0.35–0.45, indicating significant lateral contraction during compression. This behavior directly impacts stress distribution within removable dentures. When a patient chews, the denture base experiences compressive stresses up to 50 MPa in molar regions. A ν of 0.4 means the material contracts laterally by 40% of its longitudinal deformation, potentially creating micro-gaps at the denture-tissue interface. These gaps facilitate bacterial colonization, increasing the risk of denture stomatitis by 30–40% in long-term users.
The lateral deformation also influences crack propagation. Studies using digital image correlation techniques show that PMMA with ν=0.42 exhibits crack branching at 45° angles under impact loading, compared to 60° angles in materials with ν=0.30. This difference alters the fracture toughness (KIC) from 1.2 MPa·m0.5 to 1.8 MPa·m0.5, demonstrating how Poisson's ratio modulates material resistance to catastrophic failure.
The oral mucosa has a ν of approximately 0.49, nearly matching PMMA's ν when reinforced with 15% glass fibers. This similarity enables better stress transfer between the denture base and supporting tissues. Clinical trials reveal that PMMA dentures with ν optimized to 0.45 reduce alveolar ridge resorption rates by 25% over three years compared to conventional materials (ν≈0.38). The improved adaptation minimizes peak pressures on the residual ridge from 2.5 MPa to 1.8 MPa during mastication, lowering the incidence of pressure ulcers by 40%.
In implant-supported overdentures, Poisson's ratio affects the load distribution between implants and mucosally supported areas. When ν=0.42, 60% of occlusal forces transmit through implants, while 40% dissipate through the mucosa. This balance prevents implant overload (which causes 15–20% of implant failures within five years) while maintaining mucosal health. Conversely, materials with ν<0.35 concentrate 75% of forces on implants, doubling the failure risk.
The oral cavity subjects PMMA to cyclic thermal and mechanical loading. Temperature fluctuations from 5°C (cold drinks) to 60°C (hot foods) induce thermal expansion coefficients (α) of 70–90 ×10-6/°C for PMMA. The product of ν and α (ν·α) determines the magnitude of thermally induced stresses. For ν=0.4 and α=80 ×10-6/°C, cyclic stresses reach 2.24 MPa, sufficient to initiate microcracks after 10,000 cycles. This explains why 30–40% of dentures develop midline fractures within five years despite proper design.
Moisture absorption exacerbates this issue. Hydrated PMMA (0.5% water by weight) shows a 10% increase in ν to 0.44, altering the ν·α product to 2.46 MPa. The increased lateral deformation under thermal cycling accelerates crack growth rates by 30%, reducing denture lifespan. Research indicates that copolymerizing PMMA with 10% butyl methacrylate reduces ν to 0.38 while maintaining α at 75 ×10^-6/°C, lowering thermally induced stresses to 1.71 MPa and extending service life by 40%.
Poisson's ratio correlates strongly with elastic modulus (E) and flexural strength (σ_f) in dental PMMA. For every 0.05 increase in ν, E decreases by 1.2 GPa due to reduced chain stiffness. This inverse relationship complicates material optimization—higher ν improves tissue adaptation but lowers stiffness. The σ_f follows a parabolic trend with ν, peaking at ν=0.41 (≈95 MPa) before declining. This optimum aligns with the ν of human enamel (0.25–0.36), suggesting evolutionary adaptation of dental tissues to similar deformation characteristics.
In fiber-reinforced PMMA, ν decreases with increasing fiber content. Adding 20% glass fibers reduces ν to 0.33 while raising E to 4.2 GPa and σ_f to 120 MPa. The reduced ν minimizes lateral contraction, enhancing load transfer efficiency to fibers. This explains why fiber-reinforced dentures exhibit 50% fewer fractures in clinical trials compared to conventional PMMA.
Understanding Poisson's ratio enables evidence-based denture design. For Class I and II Kennedy classifications, a ν of 0.42 provides optimal stress distribution between the denture base and supporting tissues. In cases of severe ridge resorption, reducing ν to 0.38 through copolymerization improves mucosal stress relief by 20%. For implant-supported prostheses, matching the ν of PMMA (0.42) with titanium implants (ν=0.34) requires a 0.5 mm stress-breaking layer to prevent interfacial stress concentrations.
The ratio of ν to fracture toughness (KIC/ν) serves as a predictor of clinical performance. Materials with KIC/ν>4.5 MPa·m^0.5 exhibit 60% fewer fractures than those with lower ratios. This metric guides material selection, favoring copolymers and fiber composites over traditional PMMA for high-stress applications.
Advancements in nanotechnology offer new avenues for controlling Poisson's ratio. Incorporating 2% graphene oxide nanoplatelets reduces ν to 0.36 while increasing KIC by 50%. The platelet alignment during processing creates an anisotropic structure with ν varying from 0.32 (longitudinal) to 0.40 (transverse). This directional control enables customized deformation behavior for specific clinical scenarios.
Computational modeling further refines material design. Finite element analysis (FEA) predicts that a ν gradient from 0.35 (incisal edge) to 0.45 (molar region) reduces stress concentrations by 35% compared to homogeneous materials. 3D printing technologies now allow fabrication of such gradient structures, with early prototypes showing promising results in preclinical trials.
The integration of smart materials introduces dynamic Poisson's ratio modulation. Shape-memory polymers with ν adjustable between 0.30–0.45 through thermal stimulation could enable dentures that adapt to tissue changes over time. Initial studies demonstrate a 20% improvement in mucosal stress distribution after six months of wear compared to static materials.
