## 4 Discussion  
Our previous study demonstrated that jaw-opening forces in TMD participants were significantly lower than in TMD-free participants [20]. Furthermore, the study also showed no statistically significant differences in jaw-opening forces between the three TMD subtypes (myofascal pain, disc displacement, and myofascal pain and disc displacement combined) (Supplementary Materials) [20]. The current study investigated maximum jaw-opening forces at baseline and again after a 6-month follow-up in patients who underwent treatment for TMDs. Specifically, we examined the measurement's reliability and potential fatigue effects, how well it discriminated at baseline between TMD-free participants and TMD patients (and their subtypes), and whether jaw opening forces could be used as a quantitative measure to assess clinical improvement following treatment. TMD is one of the common pain disorders in the orofacial region [25]. Studies have shown that chronic TMD pain tends to persist in most patients, although some chronic TMD patients show improvement with time [26; 27; 28]. In this study, the jaw-opening forces of TMD patients increased significantly after receiving conventional treatment for six-months, in comparison to their baseline values (geometric mean 18.6 N at baseline and 32.4 N at the six-month follow-up). In contrast, jaw-opening forces in the TMD-free group were relatively similar (49 N at baseline and 48.3 N after six months). The increase in jaw-opening forces in the TMD group suggests that patients improved following treatment, demonstrating that the treatment received by the TMD patients may have assisted in pain relief and, therefore, improved muscle function. The relevant muscles involved in TMDs are the muscles of mastication, including the masester, temporalis, and medial and lateral pterygoid muscles [29]. The improvement of muscle activity might be related to the restoration of the lateral pterygoid muscle function, as the superior and inferior lateral pterygoid muscles are involved in jaw opening and closing. However, there was a reduction in jaw-opening forces for four patients following their six-month follow-up appointment (Figure 3). All four patients were later diagnosed with osteoarthritis by clinical examination and MRI, suggesting that it is likely that continuing pain in the temporomandibular joint (TMJs) due to this degenerative condition is responsible for the lack of improvement in the reduction of jaw-opening forces. Temporomandibular joint osteoarthritis affects the cartilage, subchondoral bone, synovial  
Figure 4: Spaghetti plot illustrating the jaw-opening forces of the TMD subgroups and TMD-free group at baseline and after 6-month follow-up. Red line indicates the mean increase in jaw-opening forces in the TMD subtype. membrane, and other hard and soft tissues, thus causing TMJ deterioration, which results in severe pain, stiffness, and limited movement [30; 31]. In contrast, three participants in the TMD-free group were diagnosed with TMD during their six month-follow-up after their jaw-opening forces were measured, indicating that the device may be useful in diagnosis. Intra-articular disorders are common TMJ conditions which occur due to abnormal positioning between the disc and condyle, articular eminence, or articular fossa [30; 32]. Disc displacement with reduction may occur in asymptomatic individuals and is related to TMJ noise [33]. However, the range of mandibular movement is not limited, and patients with disc displacement with reduction usually do not have pain [33; 34]. In disc displacement without reduction, the patient often complains of a limitation in jaw opening, with or without pain [32]. In the three TMD subtypes investigated in this study, the disc-displacement group had a smaller improvement (27.8% improvement) compared to patients with myofascal pain (93.5% improvement) and both myofascal pain and disc displacement (91.1% improvement) after treatment.