The present study focused on exploring the therapeutic efficacy of S.S. in managing adult patients with TMD, specifically targeting intra-articular joint disorders and arthralgia, with or without the influence of CSP, and assessing whether the CSP can be related to TMD using CBCT-derived quantitative/qualitative condylar measurements.
Chewing food is most challenging during the initial cycles, where side preference is more pronounced. The direct chewing gum technique is advantageous due to its low cost, ease of use, and stable size, making it easier to observe. Varela et al.14 found no significant differences between this method and kinesiography for measuring CSP.
In this study, the rationale for selecting OPCS and SPCS lies in their complementary strengths. OPCS offers objective precision in quantifying biomechanical asymmetry, while SPCS captures patient-reported behaviors (answers why) and awareness of CSP habits. Together, they enhance the validity of CSP assessments and compensate for each other’s limitations.
Concerning the quantitative outcome for pre-treatment T0 in TMD + CSP, the SJS, PJS, and CLS were narrower and significantly different on the preferred side compared to the balancing side. This suggests that the condyle may be positioned more posteriorly, laterally, and in closer proximity to the articular surface (superiorly) on that side due to increased loading or pressure during chewing.
The balancing side had wider SJS, PJS, and CMS, suggesting a more posterior-medial with a compensatory mechanism where the condyle may be more distanced from the articular surface (inferiorly). This could imply a shift in the condylar position to accommodate the asymmetry created by the preference for one side during chewing. Our results align with those of Jiang et al.23 who reported narrower posterior-superior, posterior, and lateral joint spaces on the preferred side. Moreover, Ma et al.31 found in patients with CSP, 50.7% of the condyles on the preferred side and 42.0% on the unpreferred side located posteriorly.
The findings for post-treatment T1, on the preferred side, were as follows: the averages of SJS, PJS, and CMS increased, while simultaneously, on the balancing side, AJS decreased, and SJS decreased compared to pre-treatment T0. These changes suggest that the condyle on the preferred side moved downward, forward, and medially post-treatment T1, leading to the upward and forward to a more centric position of the balancing side.
In the TMD group, a significant difference was noted between the symptomatic and contralateral sides regarding the AJS in both pre- and post-treatment, likely due to an asymmetrical disc position. However, no significant differences were found in the intra-group comparison at T1, suggesting that S.S. effectively balanced the average joint space, consistent with the findings of Musa et al.1.
Morphologically, in the TMD + CSP group, the preferred side displayed a decrease in condylar length (CL1) and width (CL2) compared to the balancing side. The TMD group (bilateral chewers) displayed an increase in condylar length (CL1), while the TMD + CSP group (unilateral chewers) showed an increase in width (CL2).
In the condylar and fossa morphology in TMD + CSP, the preferred side exhibited less steep eminence θ, along with increased FH and FW, showing statistically significant differences compared to the balancing side prior to treatment. This could be due to the mechanical loading and functional demands placed on the preferred side during chewing (greater stress and strain), leading to adaptive changes in bone morphology. Our results align with these studies22,23,32 who reported that TMD patients with CSP exhibit a deep glenoid fossa and a steepened inclination of the articular eminence associated with the preferred chewing side. These may be characteristic imaging features. In contrast, Sritara et al.33found no significant association between articular eminence inclination and chewing patterns, and34 clinical dysfunction index in patients with TMD.
Regarding the qualitative outcome, for the TMD + CSP group, the preferred side exhibited higher BMD than the balancing side, indicating CSP’s role in disturbing BMD (AS, SS, PS, MS, LS). Increased BMD on the preferred side since it is used more frequently and subjected to greater mechanical load is likely to experience an increase in BMD. Our finding is in agreement with Wolff’s Law35, which states that bone adapts to the loads under which it is placed. Conversely, the balancing side that is used less may have lower BMD, as it does not experience the same level of stress and stimulation suffered decreased BMD (weak muscles) can weaken bone layers. This imbalance can lead to structural differences in the bones or joints, potentially increasing the risk of injury or conditions like arthritis on the balancing side. Numerous studies1,2,36,37 reported thickness of the condyle cortical bone in patients with TMD after S.S. therapy supports the results of this study.
For the TMD + CSP group, the preferred side showed reduced CV and CSA, demonstrating statistically significant differences compared to the balancing side prior to treatment. These reductions indicate a smaller condyle, which increases the risk of disc displacement.
When comparing the TMD + CSP group to the TMD group, the TMD group had a higher CV and CSA, in the study by Gao et al.38, both condylar volume and surface area were significantly reduced in the TMD group compared to the asymptomatic group.
During chewing, the condyle on the working side remains positioned more posteriorly than that on the balancing side throughout all phases of closure39. This position is associated with increased muscle force and functional load within the joint. Individuals with CSP consistently maintain this posterior position on their preferred side instead of alternating with the balancing side during mastication. Balcioglu et al.40 Found that patients with a dominant chewing side had significantly larger volumes of both the inferior and superior heads of the lateral pterygoid muscle on the affected side compared to the unaffected side (Fig. 2(A, B, C, D)), (Fig. 7), (Supplementary Information 4) which can lead to remodeling within the joints. Furthermore, the reduction of occlusal force leads to decreased bone density of the alveolar bone around the teeth, whereas an increase in occlusal force results in enhanced bone density, with higher values observed on the chewing side41.
Cortical Bone Changes in the Condyle: Before treatment (A, C, E, G), the cortical bone exhibited discontinuity and rough edges. After treatment (B, D, F, H), the bone appeared continuous and smooth, with a significant increase in bone mass in the cortical layer. Additionally, the joint position shifted downward and became more coordinated.
When this remodeling surpasses the joint’s ability to regenerate, it may result in degenerative joint diseases (DJD) due to excessive stress on the joint disc and the bone structures of the TMJ42. The preferred side of DJD patients with CSP presented a higher prevalence of osteoarthritic changes in the developing and advanced stages43.
It is worth mentioning that a study by Arai et al.44suggests that orthodontic treatment can alter CSP, with significant influences from being under 20 years of age, notable lateral mandibular movement, and expected changes in occlusal canting. Negishi et al.45, Believe that chewing exercises can change the chewing pattern. In addition. A study by Bae et al.46 Suggests that transitioning to bilateral chewing (using an intraoral device or unilateral mastication with gum) can enhance cognitive functions, particularly memory.
The study revealed significant differences in S.S. therapeutic outcomes between the TMD + CSP and TMD groups, as well as between the preferred and balancing sides. Our findings reject the null hypotheses, demonstrating that TMD and CSP synergistically exacerbate condylar deterioration and that S.S. therapy outcomes vary significantly based on CSP presence and location.
Limitations of the Study: The study’s small sample size, retrospective design, and short-term follow-up limit generalizability and causality assessment. The confounding variables (e.g., demographics, anatomy) and measurement variability (e.g., segmentation methods, scan resolution) may influence outcomes. Additionally, limited comparison with existing literature stems from the novelty of investigating CSP in TMD patients pre- and post-S.S. therapy, as no prior studies have examined CSP’s impact on TMD and S.S. therapeutic outcomes.
Implications: For TMD patients with CSP, raising awareness of these habits and restoring balanced masticatory function are crucial therapeutic strategies. Early detection of CSP during routine dental exams and modifying CSP can significantly reduce future TMD risks. Condylar position (joint spaces), morphology, and BMD changes are vital for TMD diagnosis/management. Narrowed spaces indicate cartilage degeneration; widened ones suggest instability. Asymmetry reflects CSP-related uneven loading. Severe morphology (e.g., resorption) may require surgery; in mild cases, conservative care. Reduced BMD signals TMJ-OA risk; decreased CV/CSA increases disc displacement risk. These radiological markers guide personalized treatments, enhancing precision and improving patient outcomes and quality of life.
Future Research Directions: While our study identifies significant differences in measured variables, the authors raise the question of whether observed changes persist or revert to baseline as patients return to their habitual MIP after discontinuing S.S., warranting the evaluation of the TMJ at a subsequent time point (T2, e.g., 6–12 months post-treatment). It raises questions about whether CSP can contribute to TMD recurrence and whether these outcomes hold clinical significance, necessitating further investigation into their implications for patient care. Additionally, it casts doubt on the effectiveness of attempts to convert unilateral chewers into bilateral chewers.
