Methods: Altered TMJ loading was induced with molar dental splints placed unilaterally in skeletally mature, female rabbits for 6 weeks. Changes in nociceptive threshold were assessed with a mechanical probing of the TMJ region. C-Fos in the nucleus caudalis was assessed with standard immunohistochemical techniques. Retrogradely labeled TMJ afferents were studied with patch clamp electrophysiological techniques. Remodeling of TMJ condyles was assessed by H&E staining on paraffin sections.
Results: While variable, there was an increase in mechanical sensitivity in splinted rabbits. The increase in c-Fos+ cells in splinted rabbits was also significant: (86±8cells/section vs. 64±15cells/section, p<0.05). In contrast, rheobase (364±80pA) and action potential threshold (-31.2±2.0mV) were higher in TMJ afferents from splinted rabbits compared to control (99±22pA and -38.0±2.0mV, p<0.05). There was significant remodeling in the condylar fibrocartilage layers as manifest by a change in gag and collagen II distribution and a loss of defined cell layers.
Conclusions: Behavioral and c-Fos data were consistent with an increase in nociceptive signaling in response condylar remodeling driven by altered TMJ loading. Changes in excitability and action potential waveform were consistent with compensatory changes of TMJ afferents for an overall increase in afferent drive associated with joint degeneration. These changes are in contrast to clear increases in TMJ afferent excitability observed in other preclinical models of TMD, and raise the possibility that failure to engage such compensatory mechanisms may contribute to greater pain and disability observed in a subpopulations of TMD patients.