A preliminary study of shaker-based optical coherence elastography for assessment of gingival elasticity

Highlights

• A shaker-based OCE system is developed.

• For the first time, elasticity imaging of in vivo gingiva is realized.

• Shear modulus and Young’s modulus of in vivo gingiva are quantified.

• The system provides noninvasive, real-time, high resolution and sensitivity imaging.

• The proposed method shows great potential for clinical stomatology.

Abstract

Studies on the changes in biomechanical properties of the gingiva are significant, since they are closely related to the pathological process of gingivitis and periodontitis. Although some research has been conducted, the currently used techniques are not feasible for in vivo and clinical measurements due to their disadvantages, such as the invasive nature and the requirement of thin sectioning. In this study, we developed a shaker-based optical coherence elastography system which provides noninvasive measurement, high axial resolution, high displacement sensitivity, and real-time imaging, and explored its potential for the detection of gingival elasticity. The system was tested by an agar phantom model and an in vivo rabbit model, where the shear wave propagations were visualized and the corresponding shear modulus and Yong’s modulus were quantified, suggesting the feasibility and reliability of our system. As far as we know, this is the first in vivo elasticity imaging of the gingiva, thus providing a powerful tool for rebuilding gingival elasticity, which may widely benefit clinical medicine in the future.

Introduction

Periodontitis, a chronic inflammatory disease occurring in the periodontal tissue, is considered as a major problem in the global burden of oral diseases due to its high frequency and non-ignorable impact on quality of life [1]. However, the pathogenesis of periodontitis is so complex that it has not been fully elucidated [2]. It has been acknowledged that during the occurrence and development of periodontitis, the primary pathological change is inflammation of gingival tissues (also termed as gingivitis) [3]. Therefore, early detection of gingivitis may provide accurate diagnosis of initial periodontal pathology. The gingiva is a collar of masticatory mucosa around the cervical portion of the teeth which serves as a major protective barrier between the environment of the mouth and the rest of the periodontium [4]. It is composed of two different stratified epithelia, and a densely collagenous lamina propria [5]. In the beginning of gingivitis, biomechanical properties of the gingiva may change with the hyperplasia or disappearance of collagen fibers before structural changes occur [5]. Therefore, knowledge of gingival elasticity is important for the early prevention and diagnosis of gingivitis and periodontitis. Additionally, the elastic information may be useful for a deeper understanding of the progression of periodontitis, which is of great significance for clinical treatment.

Several research groups have analyzed the gingiva through different methods [6], [7], [8], [9], [10], [11], [12], [13]. Chang SV et al. applied atomic force microscope to evaluate the structural and biomechanical properties of human free gingiva collagen fibrils, where the Young’s modulus was achieved by using mechanical stretching [6]. Goktas et al. carried out a study to explore biomechanical properties of the porcine attached gingiva and surrounding tissues by using tensile testing, dynamic compression, and stress relaxation analysis [13]. The resulting Young’s modulus indicated that the attached gingiva is significantly stiffer than regions of non-keratinized oral mucosa. Although the elastic modulus of the gingiva was obtained in both studies, drawbacks like the invasive nature and the requirement of thin sectioning limited their clinical application. Moreover, the localized quantification could not be realized by using the methods mentioned above, which is a key limitation to early diagnosis. Therefore, in order to achieve in vivo elasticity measurements of the gingiva, there is an urgent need for a noninvasive technology with localized sensitivities.

Elastography is an efficient way to evaluate biomechanical properties of biological tissues with features of noninvasive measurement and localized imaging, which includes an excitation mechanism and a detection system. The excitation unit is used to induce deformation, vibration, or vibration propagation, which are detected by an imaging unit [14]. In this area, ultrasound elastography and magnetic resonance elastography are the most widely used methods due to the advantages such as deep imaging [15], [16], [17], [18], [19]. However, the relatively low resolution limits their clinical application, particularly for detecting slight changes in tissue morphology and elasticity, which is important for early diagnosis [20].

Due to the limitations of the above-mentioned methods, alternative elastography techniques have been developed [21], [22], [23], [24], [25]. Among them, optical coherence elastography (OCE) is an emerging imaging modality for charactering tissue elasticity, which holds great potential for clinical application owing to its noninvasive nature, high resolution, high sensitivity, high acquisition speed, and real-time processing, which can be attributed to the advantages of optical coherence tomography (OCT) because OCE uses OCT to detect sample deformations [25]. Since the first introduction of the OCE technique in 1998, with the rapid development of system devices and detection algorithms, it has been widely applied in ophthalmology, cardiology and dermatology, and the related results have proved the significance of the biomechanical properties for disease diagnosis and treatment [26], [27], [28], [29], [30].

Based on the previous studies, the feasibility and reliability of the OCE technique for evaluating biomechanical properties of biological tissues have been confirmed. However, to date there is not much information available regarding the gingiva. In this paper, we performed a pilot study to quantitatively investigate gingival elasticity by using a shaker-based OCE system, where a shaker was used to mechanically induce the deformation in the gingiva because the shaker-based OCE can induce sufficient elastic wave propagation with high resolution and large field of view when compared with other excitation methods, such as acoustic radiation force [31], [32], [33], air puff [34], laser pulse [35], acoustic micro-tapping [36] and piezoelectric transducer [37], which is good for clinical translation [38]. Additionally, a spectral domain (SD) OCT system was used to track the mechanical deformations, on account of the higher resolution and better phase-stability when compared with the swept source OCT system [39].

To validate the feasibility of the proposed method, we first performed experiments on an agar phantom. The visualization of the elastic wave propagation enabled us to achieve the two dimensional (2D) and three dimensional (3D) elastic images and to quantify the elastic wave velocity and elastic modulus. Based on this, we applied our system to quantify biomechanical properties of an in vivo rabbit gingiva. As expected, the OCT structural images and elastic wave propagations were achieved, the corresponding shear modulus and Young’s modulus were also obtained, indicating that our system has the capability to evaluate gingival elasticity and may have the translational potential for clinical diagnosis.