Objectives The FTO gene has been reported as an obesity-associated gene and is also considered a risk gene for osteoarthritis (OA). However, its exact function is unclear, and there is conflicting evidence on the involvement of FTO polymorphisms in OA via obesity. The purpose of this study was to determine the effects of FTO polymorphism rs8044769 alleles on OA in the temporomandibular joint (TMJ), which is minimally affected by body weight. Materials and methods A total of 324 TMJs (113 with OA and 211 without OA, serving as controls) from 162 Japanese patients with temporomandibular disorders and undergoing MRI examination were analyzed. Genotyping was conducted, and multivariate analysis was performed after adjusting for the effects of age, sex, body mass index, and TMJ disc abnormalities. Results Mean age, BMI, and sex did not differ between the TMJs with OA and the TMJs without OA, but a significant difference was found for positional and dynamic disc abnormalities (P < 0.05). The allele frequency of FTO polymorphisms also differed significantly between the TMJs with OA and the TMJs without OA (P = 0.011). Moreover, logistic regression analysis showed no significant association between BMI (P = 0.581) and the occurrence of TMJOA but also indicated that the CC allele of rs8044769 is a risk factor for TMJOA (P = 0.040). Conclusions Our results show that rs8044769 in the FTO gene might be involved in TMJOA. Clinical relevance The present study provides a basis for a deeper understanding of the mechanism underlying degenerative skeletal diseases and the more effective selection and development of treatment strategies based on the patients’ genetic characteristics.

Temporal summation of second pain (TSSP) has been suggested as a psychophysical index for central sensitization, one of the critical mechanisms in the chronification of pain. However, there is no gold standard for protocols to measure TSSP. The purpose was to establish the stimulus intensity for measuring TSSP. Female patients with chronic myofascial temporomandibular disorders pain (n = 16) and healthy female volunteers with no pain (n = 15) participated. Pain thresholds (PT °C) were measured, and repetitive heat stimuli at three stimulus intensities (PT °C, PT + 1 °C, PT + 2 °C) were applied. TSSP parameters were quantified as TSSP magnitude (TSm) and TSSP frequency (TSf). In healthy female volunteers, pain ratings significantly decreased at PT °C (p < 0.050), besides TSm and TSf at PT + 2 °C were significantly higher than those at PT °C (p < 0.025). In chronic pain patients, pain ratings significantly increased at PT + 1 °C and PT + 2 °C (p < 0.050). At PT + 2 °C, TSm and TSf in chronic pain patients were significantly higher than those in healthy volunteers (p < 0.050). It could be helpful to measure TSSP with the stimulus intensity adjusted individually to the patient’s pain thresholds + 2 °C for assessing central sensitization.

Subchondral cysts (SCs) are critical for diagnosing osteoarthritis of the temporomandibular joint (TMJ) [1]. SCs are also termed subcortical pseudocysts, which occur inside the articular bone and are rarely observed on imaging [1,2]. SCs typically present as rounded or oval areas with low signal intensity on T1-weighted and proton-densityweighted magnetic resonance imaging (MRI) and high signal intensity on T2-weighted MRI. SCs are defined as well-circumscribed radiolucent areas with sclerotic borders beneath the articular cartilage on computed tomography (CT) images [3,4]. In previous histological studies, necrotic bone fragments with dead denuclearized cells were identified with surrounding fibrous connective tissue layers with adipocytes and osteoblasts, and no evidence of epithelial lining was detected [3,5]. CT and MRI investigations conducted to clarify the pathophysiology of SCs[5-9] have led to two main theories regard-ing the etiology of SCs. The synovial fluid intrusion theory postulates that elevated intra-articular pressure leads to the infiltration of synovial fluid into the subchondral bone through fissured or ulcerated cartilage, with subsequent development of cystic cavities [10,11]. The bone contusion theory postulates that SCs result from traumatic osteonecrosis after the impact of two contralateral articular surfaces [12,13]. However, the pathophysiology of SCs remains unclear [4].Although SCs in the TMJ are a critical imaging finding and likely have clinical significance, the presence of SCs is not very useful as a criterion for treatment selection. Few studies have characterized the size, position, and number of SCs using CT[14], and there is limited evidence of their pathogenesis. The articular disc is a unique tissue of the TMJ. In a normal TMJ, the articular disc is interposed between the articular fossa and condyle and plays an essential role in protecting these hard tissues and lubricating joint movements. Articular discs in the TMJ are frequently displaced due to trauma and oral parafunctions [15]. In the early stages, the displaced discs at mouth closing are repositioned to their normal position at mouth opening (disc displacement with reduction [DDwR]). However, in the later stages, they remain displaced even at mouth opening (disc displacement without reduction [DDwoR]). DDwoR is a significant risk factor for degenera-

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Background: Central sensitization (CS) of the nociceptive system is linked to a disparity between the perceived pain intensity and the severity of the tissue damage in many chronic pain conditions. Temporal summation of second pain (TSSP) has been suggested as a psychophysical index for CS. However, there is no golden standard for protocols to measure TSSP. The purpose was to establish the stimulus intensity for measuring TSSP to obtain the best assessment of CS in healthy individuals and patients with chronic pain. Methods: Women patients with chronic myofascial temporomandibular disorders pain (n = 16) and healthy women volunteers with no pain (n = 15) participated. First, TSSP response patterns in healthy volunteers with no pain were tested. Second, differences in TSSP between groups were investigated. Pain thresholds (PT °C) were measured, and repetitive heat stimuli at three stimulus intensities (PT °C, PT+1 °C, PT+2 °C) were applied. TSSP parameters were quantified as TSSP magnitude (TSm), TSSP frequency (TSf). Generalized linear mixed-effect models with repetitive stimuli on pain ratings were conducted. Results: In healthy women volunteers, pain ratings significantly decreased at PT ℃ (P < .050), besides TSm and TSf at PT+2 ℃ were significantly higher than those at PT ℃ (P < .025). In chronic pain patients, pain ratings significantly increased at PT+1 ℃ and PT+2 ℃ (P < .050). At PT+2 ℃, TSm and TSf in chronic pain patients were significantly higher than healthy volunteers (P < .050). Conclusion: The stimulus intensity should be individually adjusted to 2 ℃ above the patient’s pain thresholds to assess TSSP clearly.