Somatosensory function following painful repetitive electrical stimulation of the human temporomandibular joint and skin

Temporomandibular disorders (TMD) are common pain problems in the population with uncertain pathophysiology and mechanisms. The aim of this experimental study was to: (1) Establish an experimental pain model using electrical stimuli to describe characteristics of nociception from the human temporomandibular joint (TMJ) and overlying skin. (2) Test the hypothesis that there would be sex-related differences in TMJ sensitivity. Forty-three healthy subjects (24 men and 19 women) participated. Using two unipolar needle electrodes into the skin (above the TMJ) in one session or into the TMJ in the other session, sensory detection threshold (SDT), pain detection threshold (PDT), and summation threshold (SumT) were measured, before and after repetitive electrical stimulation. Painful repetitive electrical stimulation was applied for 20 min with individually adjustment of the intensity of the stimuli to keep the pain rating around five on a 0-10 cm visual analogue scale (VAS). Sensitivity to tactile and pin-prick stimuli were assessed at 11 sites around the TMJ using two von Frey nylon filaments (5.16 and 84.96 g), as well as pressure pain threshold (PPT) and pressure pain tolerance (PPTOL) before the stimulation, after 20 min of stimulation and finally 15 min after the end of stimulation. Numerical rating scale (NRS) from 0 to 100 was used to rate the intensity of applied von Frey filaments. SDT, PDT, and SumT were higher in the TMJ than in the skin. These three measures increased after painful repetitive stimulation for 20 min (de-sensitization). In contrast to this effect, a hypersensitivity to pin-prick stimuli was detected around the TMJ area on the stimulated side after 20 min of electrical stimulation in the TMJ, but not in the skin. A bilateral hyposensitivity to tactile stimuli was detected after skin and TMJ stimulation. PPT and PPTOL did not show a significant change over time. Except for lower TMJ PPTOLs in women than men there were no significant sex-related differences in mechanical or electrical measures. The present findings indicate differences in the elicitation of hypersensitivity following repetitive electrical stimulation of skin and deep tissues. The mechanisms underlying these findings are not clear but differences in the induction of long-term potentiation and depression is a possibility. From a clinical point of view, the lack of sex differences in most of the used measures indicates that the higher prevalence of women than men amongst patients with persistent TMJ pain problems not entirely can be ascribed to a higher sensitivity of the TMJ. Further studies will examine the somatosensory sensitivity of patients with TMJ pain problems.