Sandro Palla

Stress-field Translation in the Healthy Human Temporomandibular Joint

Movement over the surface of the temporomandibular joint (TMJ) disc produces tractional forces. These forces potentially increase the magnitude of shear stresses and contribute to wear and fatigue of the disc. Theoretically, tractional forces in all synovial joints are the result of frictional forces, due to rubbing of the cartilage surfaces, and plowing forces, due to translation of the stress-field through the cartilage matrix as the joint surface congruency changes during motion. For plowing forces to occur in the TMJ, there must be mediolateral translation of the stress-field as the condyle moves dorsoventrally during jaw function. To test whether mediolateral stress-field translation occurs in the intact TMJ, we measured stress-field position and translation velocities in ten normal individuals during rhythmic jaw opening and closing. Magnetic resonance imaging and jaw tracking were combined to animate the three-dimensional position of the stress-field between the articulating surfaces. This allowed for mediolateral translation velocity measurements of the centroid of the stress-field. The results showed that during jaw opening and closing at 0.5 Hz, the average peak mediolateral translation velocity was 35 ± 17 mm/sec. When opening and closing increased to 1.0 Hz, the average peak velocity was 40 ± 19 mm/sec. Theoretical model estimates of the work done during such translation ranged from 6 to 709 mJ between the individual joints studied. The potential clinical importance of this measure is that long-term exposure of the TMJ disc to high work may result in fatigue failure of the TMJ disc.

Description of Mandibular Finite Helical Axis Pathways in Asymptomatic Subjects

Despite wide use of systems to record jaw motion with six degrees of freedom, most studies have analyzed only the movement of a single mandibular point. The finite helical axis (FHA) is a mathematical model which can be used to describe comprehensively the movements of a rigid body. The aim of this investigation was to describe the FHA of the mandible during habitual jaw movements. Thirty subjects (13 females, 17 males; mean age, 26 years; range, 18 to 34 years) without myoarthropathies of the masticatory system participated in the study. Opening and closing movements, performed at 1-Hz frequency, were recorded with the optoelectronic system Jaws-3D. Three opening and closing movements were recorded from the right side and three from the left side of the jaw. The movement data were low-pass-filtered for noise reduction prior to the computation of the finite helical axis by means of a software program developed in our laboratory. The following parameters were calculated: the rotation of the FHA, its spatial orientation, and the translation along it, as well as its position and distance relative to an intracondylar point. In addition, methodological errors of the model were calculated. During opening and closing, the group mean FHA rotation was 24.3° ± 4.2°. The group mean of the maximum total translation along the FHA was 0.9 ± 0.7 mm. The group mean distance between the FHA and the intracondylar point was 48.9 ± 9.9 mm. The FHA pathways were smooth and varied between individuals. Furthermore, the finite helical axes were never localized within the condyle, and often were located outside of the mandible. The analysis of the FHA pathways yields more information on whole mandibular movements than simply the movements of a single condylar point.

Effect of Occlusal Interference on Habitual Activity of Human Masseter

It has been suggested that occlusal interference may increase habitual activity in the jaw muscles and may lead to temporomandibular disorders (TMD). We tested these hypotheses by means of a double-blind randomized crossover experiment carried out on 11 young healthy females. Strips of gold foil were glued either on a selected occlusal contact area (active interference) or on the vestibular surface of the same tooth (dummy interference) and left for 8 days each. Electromyographic masseter activity was recorded in the natural environment by portable recorders under interference-free, dummy-interference, and active-interference conditions. The active occlusal interference caused a significant reduction in the number of activity periods per hour and in their mean amplitude. The EMG activity did not change significantly during the dummy-interference condition. None of the subjects developed signs and/or symptoms of TMD throughout the whole study, and most of them adapted fairly well to the occlusal disturbance.

Nocturnal Masseter EMG Activity of Healthy Subjects in a Natural Environment

Facial pain of patients with craniomandibular disorders might be caused by muscle overload. However, the activity of masticatory muscles of healthy individuals is still unknown. The aim of this study was therefore a first attempt to clarify this question by recording the masseter muscle activity of healthy subjects during sleep by means of portable recorders. The study was performed on 21 healthy subjects selected after telephone and questionnaire screenings and clinical examination from among randomly selected inhabitants of Zurich. The masseter EMG was recorded during seven nights in each subjects natural environment with the electrodes in reproducible position. The signal was analyzed for number, amplitude, and duration of contraction periods defined as signal portions above a threshold which could contain sub-threshold signal portions shorter than the standby time of 5 sec. The signal amplitude was expressed in percent of the amplitude recorded during maximum voluntary clenches (%MVC). An average of 71.9 ± 28.7 contraction episodes per night (men, 74.7 ± 30.1; women, 65.0 ± 23.8; p = 0.043), i.e., of 10.5 ± 3.8 per hour (men, 11.0 ± 4.0; women, 9.3 ± 3.0; p = 0.005), was found. The average mean amplitude was 26.2 ± 6.4 %MVC (men, 27.0 ± 6.8; women, 24.4 ± 4.5; p = 0.009). The duration of the episodes had a mode of 0.5 sec, and the group mean of the integral of the amplitude over time was 123.7 ± 157.9 %MVC (men, 138.9 ± 184.0; women, 85.9 ± 28.2; p = 0.005). Healthy subjects showed intermittent periods of masseter activity during sleep which, on average, were of rather low intensity and short duration.

Do illness perceptions predict pain-related disability and mood in chronic orofacial pain patients? A 6-month follow-up study

In our study, we investigated the predictive value of illness beliefs as measured by the revised illness perception questionnaire (IPQ‐R) in the context of other clinical predictors in patients with chronic orofacial pain over a 6‐month follow‐up period. Consecutive patients (152) referred to the interdisciplinary orofacial pain service at the Centre for Dental and Oral Medicine and Cranio‐Maxillofacial Surgery, University of Zurich received questionnaires to assess pain and pain‐related disability, anxiety, depression as well as physical and mental quality of life at three time points: prior to treatment, 3 and 6 months after beginning of treatment. Results: significant improvement was found over time for all outcome measures except mental quality of life. Results of the regression analysis indicated that believing pain could have serious consequences on ones life (IPQ subscale consequences) is one of the most important predictors for treatment outcome. The belief in low personal control and in a chronic timeline is also shown to be predictive for outcome, though explaining a smaller proportion of variance. These results provided evidence that beliefs about pain are important predictors for treatment outcome even when controlled for pain and mood. They therefore need to be considered in the management of patients with chronic orofacial pain. Assessing patients’ illness beliefs can provide essential information on these important psychological determinants of adjustment to chronic pain and may be specific targets for individualised treatment approaches.

Reliability of scoring EMG orofacial events: polysomnography compared with ambulatory recordings.

SUMMARY The study of sleep bruxism is usually based on clinical history, signs and symptoms. The recording of electromyographic signals with either ambulatory portable home recorders or with polysomnographic techniques in the laboratory environment allows collection of objective data. The present study showed a 100% agreement with clinical evaluation in the recognition of bruxism episodes from the masseter electromyogram recorded with portable recorders and using the polysomnographic technique. On the contrary, scorers had difficulties in discriminating between different types f episodes (phasic, tonic and mixed), the between‐scorers agreement varied between 62% and 63% and the κ‐values between 0.43 and 0.33. The ideal time base at which electromyographic signals should be integrated to allow for a good discrimination of bruxism patterns is 0.06 s. The results indicate that portable electromyography recorders are a valuable complement to polysomnographic recordings of orofacial motor activities as they provide a very good recognition rate with adequate time base data collection.

Cortical Activation Resulting from Painless Vibrotactile Dental Stimulation Measured by Functional Magnetic Resonance Imaging (fMRI)

There have been few investigations on hemodynamic responses in the human cortex resulting from dental stimulation. Identification of cortical areas involved in stimulus perception may offer new targets for pain treatment. This initial study aimed at establishing a cortical map of dental representation, based on non-invasive fMRI measurements. Five right-handed subjects were studied. Eight maxillary and 8 mandibular teeth were stimulated after the vibratory perception threshold was determined for each tooth. Suprathreshold stimulation was repeated thrice per session, in a total of three sessions performed on three consecutive days. Statistical inference on cluster level identified increased blood-oxygen-level-dependent signal during vibratory dental stimulation, primarily in the insular cortex bilaterally and in the supplementary motor cortex. No significant brain activation was observed in the somatosensory cortex with this stimulation protocol. These results agree with previous findings obtained from invasive direct electrical cortical stimulation of the human insula.

Mandibular Helical Axis Pathways during Mastication

Condylar and incisor trajectories are often used for the study of mandibular movements. Condylar trajectories, however, depend on the location of the reference point and can be interpreted erroneously. In contrast, the helical axis analysis yields an unequivocal description of rigid body kinematics. The aim of this study was to analyze the mandibular helical axis during mastication. Seven subjects without signs and symptoms of craniomandibular disorders and with class I occlusion were recorded by means of the optoelectronic system Jaws-3D during unilateral mastication of bread cubes (2-cm side). The helical axis was computed every 14 ms with a rotation threshold of 1°. Parameters describing its spatial orientation and position relative to the condyles were calculated. The helical axis changed orientation and position more pronouncedly during the closing than during the opening phases of mastication. The orientation varied significantly from beginning to end of closing but not of opening, indicating less fluctuation of the helical axis on opening than on closing. Also, the distance d CP between helical axis and reference condylar point varied more significantly (p < 0.05) on the working than on the balancing side: On the working side, d CP decreased during both opening and closing, whereas on the balancing side, d CP increased only for closing. Furthermore, the helical axis pathway often showed a bowing ventrally to the balancing condyle, indicating that, during closing, the balancing condyle still translated backward while essentially only rotation occurred around the working condyle. Thus, the helical axis changed its position and orientation continuously during mastication.

Mechanical Work during Stress-field Translation in the Human TMJ

The pathomechanics of degenerative joint disease of the temporomandibular joint (TMJ) may involve fatigue produced by mechanical work on the articulating tissues. This study tested the hypotheses that mechanical work in the TMJ (i) varies with the type of mandibular activity, and (ii) is evenly distributed over TMJ surfaces. Ten healthy human participants were recorded with Magnetic Resonance Imaging (MRI) and jaw tracking. The data were used to reconstruct and animate TMJ activity. Aspect ratios, instantaneous velocities, and distances of stress-fields translation were used to calculate work (mJ). The results were analyzed by least-squares polynomial regression and ANOVA. Work magnitudes were related to peak velocity (R2 = 0.92) and distance of stress-field translation (R2 = 0.83), and were distributed over the joint surfaces (p < 0.03). During mandibular laterotrusion, average mechanical work was 1.5 times greater in the contralateral joint. Peak magnitudes of work (> 3000 mJ) were 4 times that previously reported.

Enhanced negative feedback sensitivity of the hypothalamus–pituitary–adrenal axis in chronic myogenous facial pain⋆

Dysregulations of the hypothalamus–pituitary–adrenal (HPA) axis, as a physiological substrate of stress, have been observed in patients with different stress‐related and chronic pain disorders. In this study, we investigated possible dysregulations of the HPA axis in patients with masticatory muscles pain. In 20 patients with myogenous facial pain and 20 healthy controls, awakening cortisol responses, i.e. cortisol rise in the first hour after awakening, as well as a short circadian free cortisol profile, i.e. four cortisol samples over 12h during the day, were assessed before and after administration of 0.5mg dexamethasone. Results: In comparison to controls, chronic myogenous facial pain patients showed enhanced and prolonged suppression of cortisol after the administration of 0.5mg dexamethasone. Unstimulated cortisol response (before dexamethasone‐intake) to awakening and cortisol levels during the day did not differ between the groups. Dysregulation in terms of enhanced negative feedback suppression exists in chronic myogenous facial pain. These results are in line with a multifactorial etiology of chronic facial pain, shifting the perspective away from a local towards a more central etiology with dysregulations in the stress and pain modulating system.