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The Effect of Chemical Properties on the Mechanical Properties of Occlusal Splint Materials

1. Introduction

Occlusal splints are removable dental appliances fabricated from polymeric materials and prescribed for the management of temporomandibular disorders (TMD), bruxism, and occlusal instability. Their clinical efficacy depends substantially on the mechanical behaviour of the constituent materials during function. Under masticatory loading, occlusal splints are subjected to cyclic compressive, tensile, and shear forces that demand materials with well-defined elastic and mechanical performance characteristics (1). It is now well-established that the chemical composition of a dental polymer - including monomer type, degree of crosslinking, polymerization mechanism, and filler incorporation - fundamentally governs the resulting mechanical properties of the fabricated appliance (2,3). Understanding the molecular-to-macroscopic relationship between chemical and mechanical properties is therefore of direct clinical relevance, particularly as new manufacturing routes such as CAD-CAM milling and additive three-dimensional (3D) printing introduce novel resin chemistries into the splint material landscape (4).

This section examines how the chemical properties of occlusal splint materials influence seven key mechanical parameters: tensile modulus, Young's modulus (modulus of elasticity), elastic properties, yield stress, the stress-strain curve, lateral strain (Poisson's ratio), and the stiffness of dental polymers.

2. Chemical Properties of Occlusal Splint Materials

The dominant polymer in conventional occlusal splint fabrication is polymethyl methacrylate (PMMA), available in heat-polymerized, cold-polymerized (autopolymerized), and injection-moulded forms (1). The chemical architecture of PMMA is defined by the methyl ester side chain attached to the methacrylate backbone, which confers its characteristic transparency, low density, and biocompatibility. Beyond PMMA, newer materials including polyetheretherketone (PEEK), bis-acryl composites, and urethane dimethacrylate (UDMA)-based photopolymers have been introduced, particularly for digitally fabricated appliances (4,5).

The relevant chemical parameters that modulate mechanical behaviour include:

Monomer and co-monomer chemistry: The ratio of monomers such as bisphenol A glycidyl methacrylate (BisGMA) to triethylene glycol dimethacrylate (TEGDMA) directly affects the degree of conversion and, consequently, the crosslink density and mechanical properties of the resultant polymer network (6).
Degree of conversion (DC): A higher DC results in greater crosslink density, which increases stiffness and fracture resistance but may reduce flexibility (6).
Crosslinking agents: Bifunctional monomers such as ethylene glycol dimethacrylate (EGDMA) create covalent bridges between polymer chains, increasing rigidity and resistance to deformation (3).
Filler content: Inorganic particulate fillers (silica, glass, zirconia) incorporated into the polymer matrix restrict chain mobility, increase elastic modulus, and reduce deformation under load (6).
Polymerization method: Heat-curing generates more complete polymerization and higher crosslink density compared with cold-curing, with direct consequences for strength and modulus values (3).

3. Tensile Modulus

The tensile modulus, also referred to as the modulus of elasticity in tension, quantifies the resistance of a material to tensile deformation within the elastic range. It is defined as the ratio of tensile stress (force per unit area) to the resultant tensile strain (proportional change in length) in the linear portion of the load-deformation curve. The chemical determinants of tensile modulus in dental polymers are primarily crosslink density and chain rigidity.

In PMMA-based splint materials, the presence of longer, more flexible methyl methacrylate chains with limited secondary crosslinking results in a moderate tensile modulus. Incorporation of crosslinking monomers such as EGDMA increases the network stiffness and raises the tensile modulus proportionally (3). In contrast, PEEK - with its rigid aromatic backbone of alternating ether and ketone linkages - demonstrates substantially higher tensile modulus values, rendering it a stiffer and dimensionally more stable alternative (5). Cold-polymerized PMMA, due to incomplete conversion and residual monomer, typically yields lower tensile modulus values compared with heat-cured equivalents (3).

4. Young's Modulus (Modulus of Elasticity)

Young's modulus (E) is the fundamental expression of material stiffness in the elastic deformation regime, expressed as:

E = σ / ε

where σ is the applied stress (Pa or MPa) and ε is the resulting strain (dimensionless). It represents the slope of the linear (elastic) region of the stress-strain curve and is a material constant determined exclusively by the interatomic and intermolecular bonding forces - and therefore by chemical composition - rather than by the geometry or size of the specimen (7).

In dental polymers, E is governed principally by the nature of the monomer repeat unit, the degree of crosslinking, and filler content. A stiffer polymer backbone (e.g., aromatic rings in PEEK or BisGMA-based composites) confers a higher Young's modulus, whereas flexible aliphatic chains in soft-liner or flexible splint polymers result in a lower E value. For PMMA denture base materials, typical E values range from approximately 2,000 to 3,500 MPa, while PEEK may exhibit values exceeding 4,000 MPa (5). Chemical modifications such as the addition of metallic nanoparticles or carbon nanotubes have been demonstrated to increase the Young's modulus of PMMA by restricting chain segment mobility at the nanoscale (3).

Clinically, a material with a higher Young's modulus will deform less under a given masticatory load, which is advantageous for dimensional stability of the splint. However, an excessively rigid splint may transmit higher stress to the supporting dentition, necessitating careful material selection based on the intended clinical application (1).

5. Elastic Properties

The elastic properties of a material describe its capacity for reversible deformation - that is, the ability to return to its original dimensions upon removal of the applied load. This behaviour is governed by Hooke's Law within the proportional (elastic) limit: stress is directly proportional to strain, and no permanent deformation occurs below this threshold (7).

Chemically, elasticity in dental polymers arises from the coiled configuration of polymer chains and their capacity to straighten under stress and recoil upon load removal. High crosslink density restricts this chain movement and reduces elasticity (flexibility), while low crosslink density or the introduction of rubber-toughening agents increases elastic deformability. Flexisplint materials - flexible 3D-printed resins - are specifically formulated with UDMA-based monomers and lower crosslink density to achieve greater elastic compliance (4).

Key elastic parameters include:

Elastic limit: The maximum stress below which deformation is fully reversible.
Proportional limit: The stress at which the stress-strain relationship first deviates from linearity.
Resilience: The energy stored per unit volume at the elastic limit, relevant to shock absorption by the splint during parafunctional activity.

6. Yield Stress (Yield Strength)

The yield stress is defined as the stress at which a material undergoes its first detectable permanent (plastic) deformation. Beyond the yield point, the material no longer returns to its original dimensions upon load removal. In clinical terms, a splint material that is loaded beyond its yield stress will deform permanently, altering the occlusal relationships it was designed to maintain.

The yield stress is a function of the strength of intermolecular and interchain forces, which in turn depends on chemical composition. A higher degree of crosslinking raises the yield stress by requiring greater force to overcome the covalent network and initiate plastic flow. PMMA with crosslinking co-monomers exhibits higher yield strength than uncrosslinked PMMA (3). Residual monomer content - higher in cold-cured materials - acts as an internal plasticizer, reducing the yield stress (3). In 3D-printed splint materials, incomplete photopolymerization may result in lower yield stress values compared with conventionally cured counterparts, as confirmed by flexural strength data in the literature (4).

7. Stress-Strain Curve

The stress-strain curve is the graphical representation of a material's mechanical response from initial elastic deformation, through yielding, to plastic deformation and eventual fracture. It integrates all of the preceding properties into a single diagnostic profile of material behaviour. For dental polymers used in occlusal splints, the stress-strain curve typically exhibits:

A linear elastic region (slope = Young's modulus)
A yield point, marking the onset of permanent deformation
A plastic deformation region, where the material flows under continued stress
An ultimate tensile strength, the maximum stress before fracture
A fracture point

The chemical composition of the polymer governs the shape and extent of each region. Highly crosslinked materials (e.g., heat-cured PMMA, PEEK) show a steep elastic slope, a high yield point, and often a relatively short plastic region - indicating brittleness. Less crosslinked or rubber-toughened materials exhibit a shallower slope, a lower yield point, and a more extended plastic region - indicating ductility. The area under the entire stress-strain curve represents the toughness of the material, or its total energy absorption capacity before fracture (7).

Chemical modification strategies such as copolymerization with methacrylic acid or incorporation of elastomeric domains have been employed to shift the stress-strain profile of PMMA-based splints toward greater toughness without sacrificing stiffness (3). The stress-strain curves of contemporary 3D-printed materials demonstrate considerable variability, reflecting the diversity in resin chemistry and photopolymerization protocols employed by different manufacturers (4).

8. Lateral Strain (Poisson's Ratio)

When a material is subjected to axial tensile stress, it simultaneously undergoes lateral (transverse) compressive strain perpendicular to the loading axis, a phenomenon known as lateral strain. The ratio of lateral strain to axial strain within the elastic range is termed Poisson's ratio (ν):

ν = - ε_lateral / ε_axial

For most dental polymers, Poisson's ratio ranges from 0.30 to 0.45, with a value of 0.5 representing an ideal incompressible material (7). The chemical determinants of ν include chain flexibility, crosslink density, and the presence of filler particles. Highly crosslinked polymers and filled composites tend to exhibit lower Poisson's ratios, reflecting their greater resistance to lateral expansion under axial load. Conversely, viscoelastic or lightly crosslinked polymers may approach ν = 0.5.

In finite element analysis (FEA) modelling of dental appliances, accurate Poisson's ratio values are essential for realistic stress distribution predictions. Errors in ν assignment - often arising from insufficient chemical characterization of the material - can significantly alter stress distribution outputs and compromise the validity of computational models (7). For occlusal splints, the lateral strain behaviour determines how the material redistributes load across its cross-section under biting forces, with implications for edge chipping and crack propagation along the labial or palatal periphery.

9. Stiffness of Dental Polymers

Stiffness, while often used synonymously with Young's modulus, is more precisely defined as the resistance of a body to deformation under an applied force and is influenced by both the material's intrinsic elastic modulus and the geometric dimensions of the specimen (cross-sectional area and length). In the context of material science, stiffness is governed by E and is therefore fundamentally chemical in origin.

For occlusal splint materials, stiffness is determined by:

Polymer chain rigidity: Aromatic backbones (PEEK, BisGMA-based resins) confer greater stiffness than aliphatic chains (PMMA, soft liners).
Crosslink density: Increasing the molar ratio of crosslinking monomers raises stiffness.
Filler volume fraction: Higher inorganic filler loading increases the composite stiffness in accordance with the rule of mixtures (6).
Degree of polymerization: Incomplete conversion leaves residual monomer that acts as a plasticizer, reducing stiffness.

A systematic review and meta-analysis by Valenti et al. (2024) demonstrated that milled (CAD-CAM) occlusal splint materials did not consistently outperform heat-cured conventional PMMA in terms of mechanical properties, including flexural strength, suggesting that polymerization chemistry rather than fabrication route is the primary determinant of mechanical performance (4). Furthermore, Prpic et al. (2023) reported that most 3D-printed occlusal splint materials exhibited lower flexural strength and surface hardness compared with milled or cold-polymerized counterparts, attributed to incomplete photopolymerization in layer-by-layer additive fabrication, with flexural strength values ranging from 46.1 to 106 MPa across materials tested (8). These findings highlight the critical role of the chemical curing process in determining final stiffness and mechanical performance.

10. Summary

The chemical composition of occlusal splint materials is the primary determinant of their mechanical performance. Table 1 summarises the relationships described above.

Mechanical Property	Chemical Determinant	Effect
Tensile modulus	Crosslink density, chain rigidity	Higher crosslinking -> higher tensile modulus
Young's modulus (E)	Monomer type, filler content, DC	Aromatic backbone and fillers -> higher E
Elastic properties	Chain flexibility, crosslink density	Lower crosslinking -> greater elasticity
Yield stress	Crosslink density, residual monomer	Higher crosslinking -> higher yield stress
Stress-strain curve	Overall polymer architecture	Governs shape and toughness
Lateral strain (Poisson's ratio)	Chain flexibility, filler content	Stiffer materials -> lower ν
Stiffness	E, chain rigidity, polymerization extent	Higher DC and fillers -> greater stiffness

A thorough understanding of these structure-property relationships is essential for the informed selection of splint materials in clinical practice and for the development of next-generation polymer systems with optimized mechanical profiles for specific occlusal loading environments.

References

Zafar MS. Prosthodontic Applications of Polymethyl Methacrylate (PMMA): An Update. Polymers (Basel). 2020;12(10):2299. doi:10.3390/polym12102299 [PMID: 33049984]
Raszewski Z, Nowakowska-Toporowska A, Nowakowska D. Update on Acrylic Resins Used in Dentistry. Mini Rev Med Chem. 2021;21(1):20-29. [PMID: 33634758]
Khan AA, Fareed MA, Alshehri AH, Aldegheishem A, Alharthi R, Saadaldin SA. Mechanical Properties of the Modified Denture Base Materials and Polymerization Methods: A Systematic Review. Int J Mol Sci. 2022;23(10):5737. doi:10.3390/ijms23105737 [PMID: 35628546]
Valenti C, Federici MI, Coniglio M, Betti P, Pancrazi GP, Tulli O. Mechanical and biological properties of polymer materials for oral appliances produced with additive 3D printing and subtractive CAD-CAM techniques compared to conventional methods: a systematic review and meta-analysis. Clin Oral Investig. 2024. doi:10.1007/s00784-024-05772-6 [PMID: 38916682]
Zhang Y, Zhang W, Yang M. Comprehensive review of polyetheretherketone use in dentistry. J Prosthodont Res. 2025;69(2):189-203. [PMID: 39756889]
Branco DS, Schneider LFJ, Consani S, et al. Chemical and Mechanical Properties of Experimental Dental Composites as a Function of Formulation and Postcuring Thermal Treatment. Int J Dent. 2018;2018:1386924. doi:10.1155/2018/1386924
Craig RG, Powers JM. Restorative Dental Materials. 11th ed. St. Louis: Mosby; 2002.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical Properties of 3D-Printed Occlusal Splint Materials. Dent J (Basel). 2023;11(8):199. doi:10.3390/dj11080199 [PMID: 37623295]

Note on citation style: This write-up uses Vancouver style (numbered superscript citations in order of appearance) as required for a PhD thesis in dental/biomedical sciences. Reference numbers are listed sequentially as they appear in the text. All PMIDs are provided for verification.

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