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Biomedical subjects

A G Hannam

Publications and source records attributed to A G Hannam.

At least 19 recordsLinked to original sources
PubMed1992

Three-dimensional finite element stress analysis of the dentate human mandible.

The biomechanical events which accompany functional loading of the human mandible are not fully understood. The techniques normally used to record them are highly invasive. Computer modelling offers a promising alternative approach in this regard, with the additional ability to predict regional stresses and strains in inaccessible locations. In this study, we built two three-dimensional finite element (FE) models of a human mandible reconstructed from tomographs of a dry dentate jaw. The first model was used for a complete mechanical characterization of physical events. It also provided comparative data for the second model, which had an increased vertical corpus depth. In both cases, boundary conditions included rigid restraints at the first right molar and endosteal cortical surfaces of the articular eminences of temporal bones. Groups of parallel multiple vectors simulated individual masticatory muscle loads. The models were solved for displacements, stresses, strains, and forces. The simulated muscle loads in the first model deformed the mandible helically upward and toward its right (working) side. The highest principal stresses occurred at the bite point, anterior aspects of the coronoid processes, symphyseal region, and right and left sides of the mandibular corpus. In general, the observed principal stresses and strains were highest on the periosteal cortical surface and alveolar bone. At the symphyseal region, maximum principal stresses and strains were highest on the lower lingual mandibular aspect, whereas minimum principal stresses and strains were highest on its upper labial side. Subcondylar principal strains and condylar forces were higher on the left (balancing or nonbiting) side than on the right mandibular side, with condylar forces more concentrated on the anteromedial aspect of the working-side condyle and on the central and lateral aspects of the left. When compared with in vivo strain data from macaques during comparable biting events, the predictive strain values from the first model were qualitatively similar. In the second model, the reduced tensile stress on the working-side, and decreased shear stress bilaterally, confirmed that lower stresses occurred on the lower mandibular border with increased jaw depth. Our results suggested that although the mandible behaved in a beam-like manner, its corpus acted more like a combination of open and closed cross sections due to the presence of tooth sockets, at least for the task modelled.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer Simulation↗
PubMed1992

Task dependence of human masseter motor unit reflex behaviour.

Motor unit (MU) firing frequency is an important determinant of reflex inhibition in the human jaw muscles. Masseter MUs may be driven steadily by various intraoral tasks, but their lowest sustainable firing frequency varies according to task. In this study we examined the effect of task on masseter MU reflex behaviour under controlled conditions, in which the prestimulus MU firing frequency and stimulation were constrained. All MUs tested were inhibited by a non-noxious electrical stimulus applied to the oral mucosa, but there were significant differences in the magnitude of single MU inhibition depending on the task employed to drive the MUs. It appears that single masseter MU reflex behaviour can alter according to task, even when the prestimulus excitation of the masseter motoneuron pool is apparently constant. This suggests that masseter MU reflex behaviour may be modulated by task-related peripheral afferent input.

Adult↗
PubMed1992

Task-related behavior of motor units in different regions of the human masseter muscle.

Muscle activity patterns appear to vary regionally in the human masseter. However, studies of motor-unit (MU) behaviour in the masseter have been hampered by the absence of a reliable technique for locating needle-electrode recording sites. Here, voluntary MU behaviour patterns were examined in verified regions of the muscle. Activity was recorded from 50 stereotactically mapped masseter MUs. Initially, the task specificity of each MU was determined. Then for each task, the lowest sustainable firing frequency (LSFF) was reached by slow increases and decreases in voluntary firing rate, followed by sustained firing at the lowest possible rate. Pulse-discrimination and digital sampling of consecutive interspike intervals were used to measure the LSFF for each task to which the MU contributed. All MUs fired continuously during the performance of 2-6 separate tooth-contact and postural tasks. There were significant differences between LSFFs for the tasks performed by 47 units. Masseter MU task profiles appear to vary regionally, and are dependent on jaw position, the bite point along the tooth row, and the direction of effort. Descending neural drive to masseter MUs thus seems to be highly task dependent, even when the unit firing rate is controlled voluntarily at its LSFF.

Action Potentials↗
PubMed1992

Regional 31P magnetic resonance spectroscopy of exercising human masseter muscle.

Changes in fibre structure and function associated with exercise have been quantified ultrastructurally and biochemically in selected limb muscles, but the biochemical events associated with contraction are rarely studied in the human jaw muscles. Here, 31P NMR spectroscopy, or MRS, was used to examine the multipennate masseter in six adult men at rest and while performing isometric clenching exercises. NMR spectra were acquired from three locations within the muscle with a 2 x 3 cm, single-turn, copper receiver coil. The spectra, corrected for partial saturation effects, were quantified on the basis of relative peak area and position. The inorganic phosphate (Pi) to creatine phosphate (PCr) ratio (Pi/PCr), which has been shown to be indirectly related to the phosphate potential and hence the metabolic activity, as well as pH, were calculated for each site and exercise. The mean resting Pi/PCr ratio was greater for the deep part of the muscle than for the superficial and intermediate parts; these differences were significant to p less than 0.01. The mean pH was similar in all parts of the muscle at rest. During exercise, a significant increase in mean Pi/PCr was found in the superficial and intermediate parts of the muscle; both these differences were significant to p less than 0.05. An accompanying decrease in mean pH was observed in all parts of the muscle during exercise. In the superficial part, this decrease was significant to the p less than 0.05 level, and in the deep part, to the p less than 0.001 level. No significant differences were found for these measures between left and right molar clenching.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Estimation of tendon-plane orientation within human masseter muscle from reconstructed magnetic resonance images.

The human masseter is a powerful multipennate jaw elevator with complex internal architecture. The three-dimensional disposition of tendon planes within the muscle is thought to be an important determinant of function. We selected five adult subjects and used cephalometric radiography, magnetic resonance imaging and graphical, three-dimensional reconstruction to describe the organization of these planes within the muscle. Putative tendon planes were fitted to the surfaces of the three-dimensional reconstructions, and these were related to the mid-sagittal plane in the coronal and transverse views. To confirm whether putative planes disclosed by magnetic resonance represented true anatomical entities, a fresh human cadaver head was imaged and the magnetic resonance slices were compared with cryosections obtained in the same planes. Tendon-plane angulation appeared to be related to ramal length and lower face height measured cephalometrically. In the transverse view, the tendon planes appeared roughly to follow the angulations of the zygomatic arch and the lateral face of the mandibular ramus. These findings suggest that the angulation of tendon planes, and possibly pennation angles, are different depending on the viewing angle. Rather than reporting pennation angle as a single angle, alpha, which has been the convention, it may be more appropriate to express it as a three-dimensional angle relative to the normal of a particular tendon plane. The inference is that muscle fibres inserting on either side of a central tendon may need to develop different tensile forces if translation is to occur directly along the tendon axis.

Adipose Tissue↗
PubMed1991

Motor-unit territory in the human masseter muscle.

Motor-unit territories in human masseter are reportedly focal and related to putative subvolumes of muscle. However, in the absence of a reliable method of locating needle-electrode recording sites within the muscle in three dimensions and due to inherent weaknesses in electromyographic recording techniques, the limits of motor-unit territory in the masseter may have been underestimated. Single motor-unit responses were recorded as time-locked events from 32 paired-needle recording sites throughout the masseter muscles of three subjects. Recording sites were located stereotactically with an optical system, magnetic resonance imaging, and a common reference, then displayed graphically in three dimensions. The mean linear separation of the paired recording sites was 8.8 +/- 3.4 mm. The putative territories had a preferred orientation in the antero-posterior axis. Motor-unit territories were larger than described previously and appeared to be related to anatomical compartments. The restriction of these territories to discrete regions of the muscle provides an anatomical substrate for selective regional motor control of the human masseter muscle.

Action Potentials↗
PubMed1990

The estimation of motor unit twitch tensions in the human masseter muscle by spike-triggered averaging.

Spike-triggered averaging (STA) has been used to extract twitch profiles of single motor units (SMU) within the human masseter muscle. However, the reported twitch tensions may have been biased by the voluntary firing frequency of the SMUs, the complex architecture of the muscle, and by the biomechanical linkage of the jaw. In this study, a rigid STA paradigm was used to record spike-triggered "measured tensions" (STMTs) for 32 SMUs in the masseter muscles of four subjects. STMTs were recorded at two different orientations of a force transducer placed between the incisor teeth. The STMTs produced by each unit were used to calculate jaw torque. STMTs were also recorded in 11 units with differing degrees of muscle coactivation. STMTs for each unit varied according to the orientation of the force transducer. However, no systematic changes in STMTs occurred with reciprocal changes in the jaw moment arm. STMTs could be altered significantly by different degrees of muscle co-activation. The use of STA as a method for determining SMU tension in the human masseter muscle appears to be highly task-dependent and in the presence of co-activation may be inappropriate.

Action Potentials↗
PubMed1990

Effect of bilateral asymmetric tooth clenching on load distribution at the mandibular condyles.

The effects of balancing-side tooth contacts on temporomandibular joint loads are unclear. We used a 3-D computer model to calculate the magnitude and direction of temporomandibular reaction forces during simulated clenching on interocclusal acrylic resin shims and between natural teeth. Muscle tensions were proportioned according to the task modeled. Working-side tooth contacts included the canine alone, as well as group function, and occlusal loads were progressively shifted toward a posterior contralateral simple balancing contact. In the acrylic resin shim experiments, group function with simple balancing contact yielded the highest forces at the load point and at both temporomandibular joints. Movement of the occlusal load toward the balancing side produced greater, anteriorly oriented forces on the working condyle. For natural teeth, changes in the angle of resultant tooth force (simulating facet angulation) greatly influenced condylar forces. As the occlusal load moved toward the balancing side, greater and more laterally oriented forces were produced on the balancing condyle. Unilateral clenching on the canine produced the least condylar and bite forces. The simulation involving natural teeth offers a possible explanation for deviations in form and osteoarthrosis at the temporomandibular joints.

Acrylic Resins↗
PubMed1989

Relationships between the size and spatial morphology of human masseter and medial pterygoid muscles, the craniofacial skeleton, and jaw biomechanics.

The relationship between human craniofacial morphology and the biomechanical efficiency of bite force generation in widely varying muscular and skeletal types is unknown. To address this problem, we selected 22 subjects with different facial morphologies and used magnetic resonance imaging, cephalometric radiography, and data from dental casts to reconstruct their craniofacial tissues in three dimensions. Conventional cephalometric analyses were carried out, and the cross-sectional sizes of the masseter and medial pterygoid muscles were measured from reconstituted sections. The potential abilities of the muscles to generate bite forces at the molar teeth and mandibular condyles were calculated according to static equilibrium theory using muscle, first molar, and condylar moment arms. On average, the masseter muscle was about 66% larger in cross section than the medial pterygoid and was inclined more anteriorly relative to the functional occlusal plane. There was a significant positive correlation (P less than 0.01) between the cross-sectional areas of the masseter and medial pterygoid muscles (r = 0.75) and between the bizygomatic arch width and masseter cross-sectional area (r = 0.56) and medial pterygoid cross-sectional area (r = 0.69). The masseter muscle was always a more efficient producer of vertically oriented bite force than the medial pterygoid. Putative bite force from the medial pterygoid muscle alone correlated positively with mandibular length and inversely with upper face height. When muscle and tooth moment arms were considered together, a system efficient at producing force on the first molar was statistically associated with a face having a large intergonial width, small intercondylar width, narrow dental arch, forward maxilla, and forward mandible. There was no significant correlation between muscle cross-sectional areas and their respective putative bite forces. This suggests that there is no simple relationship between the tension-generating capacity of the muscles and their mechanical efficiency as described by their spatial arrangement. The study shows that in a modern human population so many combinations of biomechanically relevant variables are possible that subjects cannot easily be placed into ideal or nonideal categories for producing molar force. Our findings also confirm the impression that similar bite-force efficiencies can be found in subjects with disparate facial features.

Adult↗
PubMed1989

Reflex inhibition in single motor units of the human lateral pterygoid muscle.

Reflex electromyographic (EMG) responses to electrical stimulation in the human lateral pterygoid muscle are ambiguous, possibly due to previous methods of EMG analysis. In a rigorous paradigm, single motor unit (SMU) activity in 30 units was recorded in the inferior head of the lateral pterygoid muscle in 5 subjects. Single pulses of 1 ms duration at 6T intensity were delivered via bipolar electrodes attached to the gingiva near the maxillary canine. The firing frequency of each SMU was constrained to 10 or 15 Hz by biofeedback techniques. Stimuli were delivered at known positions relative to a series of preselected spikes. Successive stimuli were delivered sequentially by increasing increments of 1 ms. SMUs were activated consistently by incisal clenching and jaw opening against resistance applied to the chin. When the SMU prestimulus firing frequency was constrained at 10 Hz, each unit displayed an inhibitory effect which was highly dependent on the stimulus position. In units with a constrained firing frequency of 15 Hz, no effect was observed. These findings qualitatively resemble responses observed in the human masseter muscle.

Action Potentials↗
PubMed1989

Imaging orofacial tissues by magnetic resonance.

Magnetic resonance imaging was used to study the human orofacial complex. Two imaging methods, a fixed head coil and a surface coil, were used. Images from a database of 31 subjects revealed details of many structures including the masseter, temporal, medial and lateral pterygoid muscles, the teeth, articular condyles, and facial bones. A dentigerous cyst and a maxillary sinus "polyp" were also identified. Our study demonstrates the utility of this imaging modality in the identification and localization of soft tissue lesions. The strengths and weaknesses of the technique and its clinical potential are discussed.

Face↗
PubMed1989

Location of needle electrode recording sites in the human masseter muscle by magnetic resonance imaging.

A stereotactic method was developed for locating needle electrode recording sites within the human masseter muscle. The method combines a single motor unit (SMU) electromyographic (EMG) technique, magnetic resonance imaging, the 3-dimensional reconstruction of orofacial tissues, and a common reference systems. SMU EMG activity can be recorded from different sites in the masseter muscle, and the location of these sites displayed graphically in 3 dimensions. The technique should be a useful adjunct in future studies of the internal architecture and electrophysiological properties of the human masseter muscle.

Adult↗
PubMed1989

Relationships between the size, position, and angulation of human jaw muscles and unilateral first molar bite force.

Human subjects commonly show large variations in bite force produced at the first molar teeth. To evaluate the role of muscle cross-sectional sizes and lever arms in bite-force production, we correlated these variables in 11 healthy adults. Axial and coronal images obtained by magnetic resonance were combined with conventional lateral cephalograms and dental cast data to reconstruct the craniomandibular morphology in each subject. The cross-sectional sizes of the right masseter and medial pterygoid muscles, their lever arms, and the bite-point lever arms were measured directly from these reconstructions. Physiological recordings of bite force were made in the region of the right first molar by means of a customized transducer aligned perpendicular to the functional occlusal plane. The average bite force for the sample as a whole was 189 +/- 78 N. The coefficients of variance were greater for bite forces, and for the cross-sectional sizes of the two muscles, than for their respective lever arms. Highly significant Pearson Product Moment correlation coefficients (p less than 0.005) were found between masseter and medial pterygoid cross-sectional size, and between the cross-sectional size of each muscle and bite force. No significant correlations (p greater than 0.1) were found between muscle or bite-point lever arms and bite force. Despite the fact that craniofacial spatial morphology may differ among subjects, jaw muscle size alone seems to explain most of the variation in bite force reported by ourselves and others.

Adult↗
PubMed1986

The electromyographic activity of the inferior part of the human lateral pterygoid muscle during clenching and chewing.

The nature of activity in the two parts of the muscle is controversial. A reliable technique was developed for recording activity in its inferior part by means of an indwelling needle electrode. This part was most active during anteriorly- or contralaterally-directed intercuspal clenching, vertically-directed clenching with the jaw positioned to the contralateral side or anteriorly, and during jaw opening and least active during vertically-, ipsilaterally- or posteriorly-directed intercuspal clenching, and during vertically-directed clenches with the jaw positioned to the ipsilateral side. During chewing, activity appeared in the late intercuspal phase irrespective of the side used. It commenced earlier when chewing strokes were ipsilateral to the muscle. Activity during both ipsilateral and contralateral chewing strokes continued until maximum opening, when it ceased for the duration of the closing and crushing phases of the cycle. Thus the inferior part, with other muscles, may participate in bracing the condylar head against the articular eminence during vertical-clenching efforts involving condylar displacement, but not in the compressive or crushing phases of the cycle.

Adult↗
PubMed1986

The contribution of the deep fibers of the masseter muscle to selected tooth-clenching and chewing tasks.

Anatomically, the human masseter muscle consists of at least two portions (pars superficialis, pars profunda) with distinctly different fiber directions. The purpose of this study was to describe functional behavior in the deep fibers of the masseter muscle and to define any differences in its behavior from that of the superficial fibers. In 20 subjects, EMG activity of the superficial and the deep portions of the masseter muscle was recorded during specific parafunctional (intercuspal and eccentric tooth clenching) and functional (unilateral chewing) tests. Superficial and deep activity was measured with bipolar surface electrodes and intramuscular fine-wire electrodes. Simultaneously, displacement of a lower incisor point was recorded in three dimensions. The data were collected and stored for analysis by a disk-based computer system. The results indicated that changes in the direction of effort, in mandibular position, and in the side used for chewing all influenced activity in both parts of the muscle to different extents. The most distinct separation of activity occurred when intercuspal clenching was directed retrusively; the deep fibers of the masseter muscle response reduced to 47.5% of its maximum value while that of the superficial fibers of the masseter muscle fell to 5.5%. During chewing, activity in the deep fibers of masseter muscle was distributed evenly bilaterally, whereas that in the superficial fibers of the masseter muscle was biased significantly toward the chewing side. Differentiation of activity within the masseter muscle may be relevant to the distribution of regional tenderness in the muscle when it is involved in parafunctional activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1985

The influence of altered working-side occlusal guidance on masticatory muscles and related jaw movement.

The effect of four different occlusal situations (group function, canine guidance, working side occlusal interference, and hyperbalancing occlusal interference) on EMG activity in jaw elevator muscles and related mandibular movement was investigated on 12 subjects. With a computer-based system, EMG and displacement signals were collected simultaneously during specific functional (unilateral chewing) and parafunctional tasks (mandibular gliding movements and various tooth clenching efforts) and analyzed quantitatively. When a naturally acquired group function was temporarily and artificially changed into a dominant canine guidance, a significant general reduction of elevator muscle activity was observed when subjects exerted full isometric tooth-clenching efforts in a lateral mandibular position. The original muscular coordination pattern (relative contraction from muscle to muscle) remained unaltered during this test. With respect to unilateral chewing, no significant alterations in the activity or coordination of the muscles occurred when an artificial canine guidance was introduced. Introduction of a hyperbalancing occlusal contact caused significant alterations in muscle activity and coordination during maximal tooth clenching in a lateral mandibular position. A marked shift of temporal muscle EMG activity toward the side of the interference and unchanged bilateral activity of the two masseter muscles were observed. The results suggest that canine-protected occlusions do not significantly alter muscle activity during mastication but significantly reduce muscle activity during parafunctional clenching. They also suggest that non-working side contacts dramatically alter the distribution of muscle activity during parafunctional clenching, and that this redistribution may affect the nature of reaction forces at the temporomandibular joints.

Adult↗