Temporal summation (TS) or wind-up is cited as a central spinal (trigeminal) mechanism in which repetitive painful stimulation results in a slow temporal summation that causes increased pain reports [13, 35]. Several pieces of evidence strongly suggest that TS and wind-up share a common central mechanism [36]. It is a widely applicable quantitative sensory test method that invokes neural mechanisms related to the pain-promoting process, which is believed to be the result of C-fiber-induced dorsal horn neuron responses[24]. It can serve as both an amplification and maintenance mechanism for pain and central sensitization[37]. Nevertheless, as wind up is mediated by central mechanisms [38], it can be used in human studies to determine the degree of CNS excitability to nociceptive stimulation[37, 39]. Several studies have found evidence of abnormal wind-up and slower dissipation of painful after-sensations in patients with fibromyalgia, a widespread pain condition that is comorbid with painful temporomandibular disorders (TMDs) [15, 40, 41].
TMDs are common pain problems in the population with uncertain pathophysiology and mechanisms but with good evidence for increased sensitivity to mechanical stimulation[31, 42]. The pressure pain threshold (PPT) was used to test deep pain sensitivity mediated through C- or Aδ-fibers[26]. Mechanical pain threshold (MPT) was used as a test for Aδ-fiber mediated hyper- or hypoalgesia to pinprick stimulation. This study indicated that patients with painful TMJs might be more sensitive to sharp mechanical stimulation with lower MPTs on painful TMJs compared to healthy controls (P = 0.005, Table 1).
Significant differences still existed between the TMJs in patients and controls in terms of NRS ratings for repetitive stimulation (P < 0.05, Table 2). Indeed, it is commonly reported that somatosensory abnormalities can be detected in patients with painful TMJs both inside and outside the area of primary pain, which strongly indicates disturbances in the central processing of somatosensory stimulation[31]. We hypothesized the patients with TMJ pain would demonstrate abnormal wind-up of second pain as an indication of central sensitization and it was confirmed by the results. Pain catastrophizing could be elicited by repetitive gentle palpation (0.5 kg) stimulation on the non-painful TMJs (increased NRS10 scores), which may indicate that repetitive stimulation should be avoided during a clinical TMJ examination. In addition, the WURs of repetitive pressure stimulation at higher intensities (1.0 kg) at the painful TMJs were similar to the control TMJs (P = 0.684, Table 2) while on the non-painful TMJs the WUR were significantly lower than the control TMJs (P = 0.013, Table 2). This finding may be related to the high NRS scores for single pressure stimulation, which therefore decreases the ratio, i.e., the WUR has inherent problems when both NRS scores to single and repeated stimuli are increased.
Manual palpation is of great importance in clinical examination of TMD and other musculoskeletal pain conditions in assessment of deep pain sensitivity in muscles and joints [2]. It was suggested that palpation-induced pain in the masticatory muscles might lead to different diagnosis among painful TMDs, primary headaches and bruxism[43]. Previous research showed that manual palpation at light force levels (0.5 kg, 1.0 kg) is related to a tendency to “overshoot” the pressure[30]. In order to get a high level of reliability, two standardized palpometers (0.5,1.0 kg) were used to deliver a more accurate pressure stimulation during palpation. Interestingly, when standardized mechanical stimulation was applied to the non-painful TMJs, the patients were more sensitive to pressure stimulation (0.5 kg and 1.0 kg) and single mechanical stimulation, which might be associated with a sensitization in patients with chronic pain in line with several other studies [44, 45]. In addition, the patients’ painful TMJs were more sensitive to the stimulations described above except the repetitive sharp stimulation and the more intense (1.0 kg) blunt stimulation than the controls, which demonstrated that pressure of 0.5 kg might be appropriate to examine the TMJs in clinical work and repetitive palpations should be avoided during examination of painful TMJs.