The periodontal ligament (PDL) is a special connective tissue that connects the alveolar bone and teeth. When teeth are subjected to mechanical loads, the periodontal ligament experiences various mechanical stimuli, such as pressure, tension, hydrostatic pressure, and shear forces (Shen et al., 2014, Yang et al., 2015). Previous studies on the biomechanical effects of the periodontal ligament have mainly focused on tension and pressure (Jia et al., 2020, Li et al., 2022, Liu et al., 2017, Mayr et al., 2021) and treated the periodontal ligament as a solid material with low elastic modulus (Kang et al., 2023, Minch, 2013). However, the basic components of the periodontal ligament include fibers, cells, blood vessels, and interstitial fluid primarily composed of water and water-soluble glycosaminoglycans, with water constituting about 70 % of the periodontal ligament mass (Najafidoust et al., 2020). Therefore, in rigorous mechanical simulations, the periodontal ligament should be considered as a biphasic mechanical framework involving solid-liquid interactions. The solid phase consists of elastic medium formed by tightly arranged collagen fiber bundles, containing compressible, interconnected porous structures (Bergomi, Wiskott et al., 2010), while the liquid phase fills the pores (Li et al., 2018).

With the development of biomechanical research, several studies have reported on the viscoelastic properties of the periodontal ligament through mechanical experiments and numerical simulations (Fill et al., 2012, Wu et al., 2023, Zhou et al., 2021, Zhou et al., 2021). This is mainly attributed to the interstitial fluid flow in the periodontal ligament under mechanical load, providing a hydraulic damping effect that plays a crucial role in the mechanical feedback of periodontal tissues and tooth movement (Bien, 1966, Li et al., 2018). In vitro mechanical experiment conducted by Bergomi et al. revealed significant fluid flow and exchange at the interface between the periodontal ligament and alveolar bone under cyclic loading (Bergomi, Cugnoni et al., 2010). However, due to limitations in existing research methods, the specific fluid dynamics feedback within the periodontal ligament during occlusal load remains challenging to observe in vivo. Finite element analysis with the hydro-mechanical coupling model, i.e., the biphasic physical framework model consisting of porous elastic solid matrix and interstitial fluid, provides an effective method for simulation predictions. The hydro-mechanical coupling model was initially used for simulation in cartilage, intervertebral discs, and bone tissues (Freutel et al., 2014, Klika et al., 2016, Schmidt et al., 2013, Trivedi et al., 2023; M. Zhou et al., 2021, Zhou et al., 2021, Zhou et al., 2021) before being applied to periodontal ligament. Multiple studies have compared the hydro-mechanical coupling model with in vitro and in vivo experiments, validating its effectiveness in simulating periodontal ligament mechanics (Bergomi et al., 2011, Natali et al., 2002, Ortún-Terrazas et al., 2018, van Driel et al., 2000, Wei et al., 2014).

Most previous studies conducted mechanical simulation analyses of periodontal tissues using models with normal alveolar bone height (Begum et al., 2015). However, periodontitis is one of the most common oral diseases, with a global prevalence of severe periodontitis estimated at around 10 % (Kwon et al., 2021). Alveolar bone resorption stands as the primary pathological alteration in periodontitis, with changes in bone morphology potentially exerting a significant impact on biomechanical responses. Therefore, it is necessary to perform mechanical analyses on periodontal tissues with bone defects. In this study, we used the hydro-mechanical coupling model to simulate the mechanical response of periodontal tissues with or without intraosseous defects under occlusal loading. The study aims to predict tooth displacement and periodontal tissue strain while analyzing magnitude and distribution of fluid velocity and fluid movement pattern within periodontal tissues. Simultaneously, the periodontal mechanical feedback between intact periodontium and periodontium with intraosseous defects was compared.