Optical interferometry for dental hard tissue mechanics

Teeth act as a mechanical device in the mouth, and consequently experience stress and strain during chewing. Investigating the hard tissue response to these forces provides a better understanding of their effects on dental material properties and microstructural adaptations. Traditional mechanical testing tends to determine properties averaged over a large volume of regional tissue and is, therefore, unable to provide information on the spatial variation of the characteristics of dental hard tissues.¹ Furthermore, numerical modeling efforts to understand the stress distribution patterns in teeth often make grossly simplified assumptions (for example, that the elastic modulus of dentin is constant). Thus, conclusions drawn on these bases—although useful as a first step—can be misleading.

Biomechanical studies on biological systems are considered laborious in experimental mechanics. Limiting factors include the varied material characteristics of hard tissues, complexity in their geometry, their boundary conditions, difficulties in fabricating anatomically scaled models, and unavailability of a sensitive technique that can interpolate data from a clinically related experiment. One solution is optical interferometry, which uses non-contact, whole-field, and real-time techniques to enable testing of hard tissue specimens in human teeth. This approach provides stress/strain information with high sensitivity, accuracy, and adequate spatial resolution, allowing further material property correlation.

Our research aims to study dental hard tissue biomechanics using customized optical interferometry methods. Specifically, we tailored digital photoelasticity and digital moiré interferometry to determine the response of dental hard tissue to mechanical or thermal forces, and to examine the role of compositional/microstructural variation on dentin mechanics. This knowledge is key to understanding the risk factors that increase predilection to fractures in teeth, and is also useful for developing biomimetic strategies and biomaterials that restore the mechanical integrity of treated teeth.

Our digital photoelasticity setup combines an image processing system coupled with a circular polariscope: see Figure 1(A). Analyzing the resultant images^1–3—see Figure 1(B)—enabled us to acquire qualitative and quantitative information on the nature of stress distribution within dental structures. The experiments showed that tooth structure during function experiences distinct bending stress, which is distributed to a high degree along the upper part (cervical region) of the tooth root to the supporting bone. The magnitude of bending stress decreased conspicuously towards the root tip. This experiment also highlighted the role of dento-osseous structure geometry and material property gradients at the tooth-bone interface in stress distribution and bone remodeling.^{1, 2}

Figure 1. (A) Schematic of the digital photoelastic interferometry setup for determining stress distribution in a post-core restored tooth. (B) Phase-shifted photoelastic fringe pattern (wrapped image).

Figure 2. (A) Schematic of the digital moiré interferometry setup. (B, C) U and V field moiré fringe pattern (where U and V are deformations along the x and y axes, respectively).⁴

We observed a marked correlation between the pattern of principal stress distribution and the degree of mineralization and mechanical properties in dentin, showing that tooth material is biologically adapted and functionally graded. Digital photoelastic experiments on restored tooth models showed that large restorations and retentive devices in a tooth would increase regions of stress concentration and tensile stress, which in turn could increase the propensity to dislodgement and fractures.³

Our customized interferometry system combines two- or four-beam moiré interferometry with a digital image processing system. We used this system to study the role of free water and bound water on dentin mechanics.^4–7 The experiments showed that the free water in the dentin surface, porosities, and tubules are lost rapidly during partial dehydration. Loss of free water resulted in distinct patterns of residual strains in dentin. Rehydration processes reversed this effect completely.⁷

We also conducted digital moiré experiments to study the role of free water on the in-plane strain response in dentin structure. We observed hydration-induced (free water) distinct in-plane strain gradients perpendicular and parallel to the dentinal tubules in the dentin tissue. We also discovered the distinct role of free water on the stress-strain distribution within dentin, and on the mechanical integrity (fracture behavior) of teeth.⁶

In conclusion, optical interferometry-based techniques are non-contact, whole field, and real-time techniques that offer distinct advantages to study the response of dental hard tissues to functional forces.

In future, our photomechanical research aims to determine how functional forces regulate the tissue microstructural adaptation to optimize the functional requirements in human teeth. This work will have a crucial role in the development of strategies that can replace diseased dental hard tissues with material designs that function in a biomimetic manner.