The temporomandibular joint (TMJ) is one of the most complex joints in human body, consisting of a pair of connected synovial joints located between the temporal bone and the mandibular condyle and assisting chewing, swallowing and speaking. The movement of TMJ is influenced by the interaction of bones, muscles, ligaments, and dental occlusion, which forms a dynamic balance. The osseous morphology of TMJ might be related to this dynamic balance (Fan et al., 2021).When patients are accompanied by the free loss of posterior teeth, the change of occlusal state breaks the dynamic balance，followed by the influence on TMJ bone structure, such as the morphological changes of mandibular condyle and articular eminence(Vîrlan et al., 2021; Zheng et al., 2023). The loss of posterior teeth causes the change of occlusal force(Lekaviciute & Kriauciunas, 2024; Tabatabaei et al., 2024). Researches indicated that occlusal force had something to do with the condylar morphology and low occlusal force was correlated with the smaller mandibular condyles(Denes et al., 2018; Kurusu et al., 2009). Some investigators noted that loss of posterior teeth resulted in shallower glenoid fossa and flattened articular eminence(Vîrlan et al., 2024; Y. Zhang et al., 2025). While other studies reported that there was no significant difference in TMJ bone structure morphology between those with missing teeth and those with complete dentition(Chandran Kana Veettil et al., 2022; Zabarović et al., 2000).

Clinically, the common types of free loss of posterior teeth include the bilateral free loss of posterior teeth (BFLP) and the unilateral free loss of posterior teeth (UFLP), which can also be called Kennedy Class I and Kennedy Class Ⅱ respectively. The loss of posterior teeth led to the decrease of occlusal support and masticatory performance，which can also cause the unilateral chewing (Asher et al., 2025, Iwashita et al., 2014). Some researchers found that the BFLP and UFLP patients tended to chew on one side more predominantly than healthy dentate subjects(Kinoshita et al., 2021; Oki et al., 2021). It is commonly accepted that unilateral chewing has a high potentially traumatic effect on dentition, jaw muscles and the TMJ(Jurt et al., 2020; Zamanlu et al., 2012), resulting in asymmetry of TMJ structure on the two sides, which also has been reported to be the cause of temporomandibular disorders(TMD) (Diernberger et al., 2008, Rawat et al., 2023). This may be because the changed occlusal conditions result in altered stress environment of TMJ, and condyle and articular eminence have strong functional adaptation and reconstruction, which lead to TMJ anatomical structural changes and functional disorders(Chen et al., 2022). However, other scholars were suspicious of the relationship between occlusion and TMJ morphology or TMD, since they found that the morphological changes of condyle can appear in adults without TMD and loss of posterior teeth(Jiang et al., 2015; Rosado et al., 2021). Thus, it is still a controversial issue.

During masticatory movements, the load on TMJ mainly concentrated on the condyle, and articular eminence (Jurt et al., 2020). As mentioned, patients with free loss of posterior teeth prefer to chew on the side with more teeth(Olivieri et al., 2025). Due to the relatively strong remodeling capacity, the bone structures of TMJ, especially the condyle and articular eminence undergo significantly morphological changes under the pressure of complex bio-mechanical force during unilateral mastication. TMJ loads were greater on the non-missing teeth side than on the missing teeth side (Jurt et al., 2020, Lai et al., 2020), which may lead to the difference in the morphological changes of TMJ bone structure on the two sides(Suenaga et al., 2015). Fang et al.'s study found the articular eminence inclination(AEI) of the non-missing teeth side was slightly steeper than that of the missing teeth side(Fang et al., 2020). Nevertheless, there were obvious differences in the AEI and glenoid fossa depths (GFD) between the two sides in the dentate group, and the degree of asymmetry between left and right TMJ was not related to the number of teeth (Jasinevicius et al., 2006).

However, by reason of the high expense of dental implantation and the lower comfort of removable partial denture restoration, patients with free loss of posterior teeth sometimes abandon the restorative treatment, which has a potential risk of developing into TMD. To date, literatures with regard to the influence of free loss of posterior teeth on TMJ osseous morphology are somewhat obscure. Cone Beam Computed Tomography (CBCT) could not only qualitatively but also quantitatively analyze the bone structure of TMJ, such as joint space, condyle, glenoid fossa and articular eminence, and is currently widely used in clinical practice to observe the osteological morphology of the TMJ with higher reliablility for morphologic measurements(Tsai et al., 2020). The purpose of this cross-sectional study was to examine the association between free loss of posterior teeth and the osseous morphology of TMJ, and assess whether the relationship between free loss of posterior teeth and TMJ morphology varies by sex and age using CBCT imaging. Our hypothesis, therefore is that free loss of posterior teeth is associated with changes in TMJ osseous morphology.