Terry Whittle

Functional Heterogeneity in the Superior Head of the Human Lateral Pterygoid

The activity of the superior head of the human lateral pterygoid muscle (SHLP) is controversial. Given the non-parallel alignment of some SHLP fibers, the SHLP may be capable of differential activation. The aims were to clarify SHLP activity patterns in relation to location within SHLP. In 18 subjects, SHLP single motor units were intramuscularly recorded at computer-tomography-verified sites during horizontal (e.g., protrusion) and vertical (e.g., opening) jaw tasks (recorded by a jaw-tracking device) and at resting postural jaw position. None of 92 units was active at the resting postural position. Medially located units (21) showed activity during contralateral movement, protrusion, and opening; 5 were also active on jaw closing. There was a significant association between unit location and the number of units active during vertical tasks (i.e., jaw closing and clenching). Analysis of the data suggests differential activation within SHLP and raises the possibility of functional heterogeneity within SHLP.

Masseter Motor Unit Recruitment is Altered in Experimental Jaw Muscle Pain

Some management strategies for chronic orofacial pain are influenced by models (e.g., Vicious Cycle Theory, Pain Adaptation Model) proposing either excitation or inhibition within a painful muscle. The aim of this study was to determine if experimental painful stimulation of the masseter muscle resulted in only increases or only decreases in masseter activity. Recordings of single-motor-unit (SMU, basic functional unit of muscle) activity were made from the right masseters of 10 asymptomatic participants during biting trials at the same force level and direction under infusion into the masseter of isotonic saline (no-pain condition), and in another block of biting trials on the same day, with 5% hypertonic saline (pain condition). Of the 36 SMUs studied, 2 SMUs exhibited a significant (p < 0.05) increase, 5 a significant decrease, and 14 no significant change in firing rate during pain. Five units were present only during the no-pain block and 10 units during the pain block only. The findings suggest that, rather than only excitation or only inhibition within a painful muscle, a re-organization of activity occurs, with increases and decreases occurring within the painful muscle. This suggests the need to re-assess management strategies based on models that propose uniform effects of pain on motor activity.

A method for standardizing jaw displacements in the horizontal plane while recording single motor unit activity in the human lateral pterygoid muscle

The normal function of the lateral pterygoid muscle is not well understood although this muscle is thought to play an important role in the control of jaw and jaw-joint function and is implicated in temporomandibular disorders (TMD). The lack of a validated method for standardization of jaw movement in studies of lateral pterygoid function has contributed to the lack of understanding of the normal function of this muscle. An improved understanding of normal function will allow valid comparisons to be made with TMD patients in order to identify whether purported differences in activity actually exist. This paper describes a methodology for standardizing command jaw movements in the horizontal plane, together with reliable recordings of single-motor-unit (SMU) activity. In six human participants, jaw movements were standardized by having participants track a linear bank of light-emitting diodes (LEDs) aligned on a monitor displaying the mid-incisor point (MIPT). In all participants, the MIPT target (i.e. an illuminated LED) could be tracked, according to a pre-determined criterion, during single- and multiple-step displacements at different rates (1.3--6.5 mm/s at MIPT) and magnitudes (0.65--12 mm) of movement. SMU activity from the superior (SHLP) or inferior (IHLP) head of the lateral pterygoid muscle could be reliably discriminated during repeated trials of these defined tasks. This methodology establishes a reliable technique for characterizing the firing properties of SMUs within the lateral pterygoid, and has implications for analogous studies in other jaw muscles.

The role of the inferior head of the human lateral pterygoid muscle in the generation and control of horizontal mandibular force.

The aim was to test the hypothesis that the inferior head (IH) of the human lateral pterygoid muscle (LP) is involved in the generation and fine control of horizontal isometric mandibular force. Although previous studies provided some evidence for this, they had limitations that necessitate a re-examination. In eight participants, electromyographic (EMG) activity was recorded from the IHLP unilaterally, as well as bilateral surface recordings from the masseter (M) and anterior temporalis (AT), and the submandibular group of muscles (SUBM), during the generation of horizontal isometric mandibular force in a direction contralateral to the side of the IHLP recording. Isometric force at 5-8 mm open from the intercuspal position was exerted on a transducer (attached by a bar to the upper teeth) by a rod attached to the lower teeth. Participants tracked a target on a video screen that required 5-s holding periods at each 100 gwt (0.98 N) between 400 gwt (3.92 N) and 800 gwt (7.84 N). The mean of multi-unit EMG activity from all muscles during the most stable 2-s force-holding periods increased significantly with each force increment (GLM repeated measures: P<0.0001). When normalized, the multi-unit data from the IHLP exhibited the steepest rate of increase. The mean firing rates of 21 IHLP single motor units (SMUs) significantly increased with force (GLM repeated measures: P<0.0001). Two SMUs fired in advance of force onset, which suggests a role in force initiation. There were close associations between fluctuations in force and in IHLP SMU firing rates and multi-unit activity, but a similar correspondence was not as clear for the other recorded jaw muscles. These findings suggest that the IHLP is important in the generation and fine control of contralaterally directed, horizontal jaw forces.

A method for studying jaw muscle activity during standardized jaw movements under experimental jaw muscle pain

This paper describes a method for studying superficial and deep jaw muscle activity during standardized jaw movements under experimental jaw muscle pain. In 22 healthy adults, pain was elicited in the right masseter muscle via tonic infusion of 4.5% hypertonic saline and which resulted in scores of 30-60 mm on a 100-mm visual analogue scale. Subjects performed tasks in five sessions in a repeated measures design, i.e., control 1, test 1 (during hypertonic or isotonic saline infusion), control 2 (without infusion), test 2 (during isotonic or hypertonic saline infusion), control 3 (without infusion). During each session, subjects performed maximal clenching and standardized jaw tasks, i.e., protrusion, lateral excursion, open/close, chewing. Mandibular movement was recorded with a 6-degree-of-freedom tracking system simultaneously with electromyographic (EMG) activity from the inferior head of the lateral pterygoid muscle with fine-wire electrodes (verified by computer tomography), and from posterior temporalis, the submandibular muscle group and bilateral masseter muscles with surface electrodes. EMG root mean square values were calculated at each 0.5 mm increment of mandibular incisor movement for all tasks under each experimental session. This establishes an experimental model for testing the effects of pain on jaw muscle activity where the jaw motor system is required to perform goal-directed tasks, and therefore should extend our understanding of the effects of pain on the jaw motor system.

Functional Activity of Superior Head of Human Lateral Pterygoid Muscle during Isometric Force

There is controversy as to the jaw tasks for which the superior head of the human lateral pterygoid muscle (SHLP) becomes active. The aim was to describe the functional activities of SHLP single motor units (SMUs) during horizontal isometric force tasks. In 11 subjects, 48 SMUs were recorded from computer-tomography-verified SHLP sites during generation of horizontal isometric force in the contralateral (CL), protrusive (P), and ipsilateral (IL) directions and intermediate directions (CL-P, IL-P). In eight subjects, SHLP SMUs were active in CL, CL-P, and P. Qualitatively, SHLP EMG activity increased with increased isometric force. Forty-two SMUs were active in directions other than IL; 6 exhibited activity at IL and other directions. The similarity of these data to previous human lateral pterygoid (IHLP) data supports the notion that SHLP and IHLP should be regarded as a single muscle, with activities shaded according to the biomechanical demands of the task.

Rehabilitation of children with ectodermal dysplasia. Part 1: an international Delphi study.

PURPOSE An international Delphi study was undertaken to determine by consensus an agreed approach to the management of children with dental manifestations of ectodermal dysplasia, including the use of dental implants. This was done using a questionnaire developed by an interdisciplinary team. MATERIALS AND METHODS The Delphi study questionnaire was built around 19 areas of clinical relevance and included 90 items. Topic areas included dental disability; initial diagnosis; global disability; oral health aspects of dental treatment (orthodontics, hypodontia, anodontia, implants); and case studies of selected treatment options. Eleven teams from six countries contributed to three iterations of the questionnaire. An algorithm was designed to standardize analysis of the questionnaire answers, all of which were blinded to ensure anonymity. The second and third rounds of the questionnaire excluded previously agreed-upon items but included the responses to the questions from the earlier rounds. The nonconsensus items inquired about the use of radiographs at initial diagnosis; sedation of an uncooperative child; use of a pretreatment questionnaire; the age range for specific treatments (eg, dentures, orthodontics, implants); specific uses of implants (eg, partial prostheses, overdentures, cantilevered prostheses); and case study 2. The residual nonconsensus questions were subsequently discussed at a 2-day meeting. RESULTS Among the 90 questions and partial questions, there was progressive consensus, with agreements in rounds 1, 2, and 3 of 61%, 21%, and 8%, respectively. At the conclusion of round 3, there was 90% agreement and it was considered that the nonconsensus items required in-depth face-to-face discussion at a consensus meeting, which is described in part 2 of the study. CONCLUSION The Delphi study provided an opportunity to engage specialist teams in recognized centers to integrate their clinical knowledge and draw on published data to develop a consensus of evidence-based responses.

Regional properties of the superior head of human lateral pterygoid muscle.

The aim of this study was to investigate functional heterogeneity within the superior head of the human lateral pterygoid (SHLP) muscle by comparing the functional properties (e.g. firing rates) of single motor units (SMUs) between different arbitrarily defined regions of the SHLP, namely, medial, middle or lateral; origin or insertion; and superior or inferior regions. Jaw movement and electromyographic (EMG) activity was recorded from computed tomography-verified locations within the SHLP of 27 asymptomatic human subjects during goal-directed contralateral, ipsilateral, and protrusive jaw movements. The SMU firing rates for protrusion in the medial, origin, and inferior regions were significantly lower than, respectively, the firing rates in the middle, insertion, and superior regions. For contralateral movement, the firing rates were significantly greater in the medial and middle regions than those in the lateral region. The data provide additional evidence that the SHLP is functionally heterogeneous and, together with previous evidence for functional heterogeneity within the inferior head of the lateral pterygoid, support the proposition that both heads should be regarded as functionally heterogeneous.

The effect of food bolus location on jaw movement smoothness and masticatory efficiency

Masticatory efficiency in individuals with extensive tooth loss has been widely discussed. However, little is known about jaw movement smoothness during chewing and the effect of differences in food bolus location on movement smoothness and masticatory efficiency. The aim of this study was to determine whether experimental differences in food bolus location (anterior versus posterior) had an effect on masticatory efficiency and jaw movement smoothness. Jaw movement smoothness was evaluated by measuring jerk-cost (calculated from acceleration) with an accelerometer that was attached to the skin of the mentum of 10 asymptomatic subjects, and acceleration was recorded during chewing on two-colour chewing gum, which was used to assessed masticatory efficiency. Chewing was performed under two conditions: posterior chewing (chewing on molars and premolars only) and anterior chewing (chewing on canine and first premolar teeth only). Jerk-cost and masticatory efficiency (calculated as the ratio of unmixed azure colour to the total area of gum, the unmixed fraction) were compared between anterior and posterior chewing with the Wilcoxon signed rank test (two-tailed). Subjects chewed significantly less efficiently during anterior chewing than during posterior chewing (P = 0·0051). There was no significant difference in jerk-cost between anterior and posterior conditions in the opening phase (P = 0·25), or closing phase (P = 0·42). This is the first characterisation of the effect of food bolus location on jaw movement smoothness at the same time as recording masticatory efficiency. The data suggest that anterior chewing decreases masticatory efficiency, but does not influence jerk-cost.

Jaw movements in patients with a history of pain: an exploratory study.

The aims were to determine whether individuals with a past history of pain exhibit (i) altered jaw movement (e.g. reduced amplitude, increased jaw movement variability) in comparison with matched asymptomatic controls, and (ii) correlations between psychological measures (e.g. catastrophising) and altered jaw movement variables. Sixteen participants with a history of trigeminal neuropathic pain (TNP) and 15 age- and gender-matched healthy controls had jaw movements recorded during open/close, free gum chewing and chewing at standardised rates. All completed the Pain Catastrophising Scale (PCS), the Pain Self-Efficacy Questionnaire (PSEQ), and the Depression, Anxiety and Stress Scales (DASS). Velocity and amplitude for open/close and chewing, as well as variability, bias and mean square error for open/close jaw movements were compared between groups. Spearmans rank correlation coefficient was used to relate kinematic variables with psychological variables. Statistical significance: P < 0·05. There were no significant differences in mean jaw velocity and amplitude between the TNP and control groups during the open/close jaw movements or free or standardised chewing. In comparison with control, the TNP participants exhibited significantly greater variability, bias and/or mean square error during slow and/or fast opening, and significantly greater variance in velocity and/or amplitude during free and standardised chewing. There were significant negative correlations between PCS scores and velocity and/or amplitude of free and/or standardised chewing. This exploratory study suggests that individuals with a history of pain have altered patterns of jaw movements in comparison with asymptomatic control participants and that catastrophising may play a role in the manifestation of these altered jaw movements.