Biomedical subjects

J H Koolstra

Publications and source records attributed to J H Koolstra.

35 records · Page 2Linked to original sources

PubMed1998

A model for mylohyoid muscle mechanics.

The purpose of the present study was to develop a mathematical model of the mylohyoid muscle allowing to analyze the complex mechanics of the muscle during jaw movement. The model was based on muscle morphology and physiological properties. Bending of fibers was incorporated into the model by pulleys located along the upper lateral border of the anterior belly of the digastric muscle. The dynamical properties of the muscle portions, i.e. force length and force velocity relationships, were related to sarcomere length changes. In addition, the effective force component produced by each portion in the sagittal plane was calculated. The model provided information on the geometrical changes of the muscle portions and the concomitant effect on sarcomere length, dynamical properties and effective force component as a function of jaw opening angle. Muscle configuration changed drastically and non-uniformly during jaw opening. However, sarcomere length changes were relatively small and differed but slightly between the muscle portions. The muscle portions all operated near optimum length regarding to their force-length relationship. In all muscle portions effective muscle force was the smallest (10-20% of maximum isometric force) in the beginning of the opening movement. With an increase of jaw angle effective muscle force gradually increased to 60-70% of maximum isometric force in the posteriormost muscle portion and to 20% in the anteriormost portion. Muscle fiber bending appeared to increase the sagittal plane component of the muscle force substantially.

Biomechanical Phenomena↗

PubMed1997

Dynamics of the human masticatory muscles during a jaw open-close movement.

The movements of the human jaw are controlled by the forces produced by the masticatory muscles. As the jaw moves, these muscles change in length and their force producing units, the sarcomeres, change in length simultaneously. The lengths and length changes of the sarcomeres are determinants for the forces they are able to produce. Hence, masticatory muscle force and jaw movement influence each other which makes it difficult to study their mutual relationship. In this paper, lengths and contraction velocities of the sarcomeres of the human jaw-opening and jaw-closing muscles are presented as well as the consequences for force production during jaw open-close movements simulated with a biomechanical model. Jaw-opening muscles acted almost synchronic in terms of sarcomere length, contraction velocity and force production. They were able to produce the largest isometric forces at relatively small jaw openings at the cost of reduced force production capabilities in wide open positions. In contrast, the jaw-closing muscles acted more differently. They were able to sustain active muscle force throughout a large range of the closing movement. Within this group the masseter and medial pterygoid contracted excentrically during a short time. The lateral pterygoid muscle portions behaved differently with respect to both groups. The jaw-opening muscles produced negligible passive forces during jaw closing. The passive forces of the jaw-closing muscles, however, contributed significantly to a limitation of the jaw-opening movement.

Humans↗

PubMed1997

The jaw open-close movements predicted by biomechanical modelling.

The aim of this study was to analyse unloaded jaw-opening and jaw-closing movements in humans. For this purpose a dynamical 6-degree-of-freedom mathematical model of the human masticatory system was developed. It incorporated morphology, muscle architecture and dynamical muscle properties. Various symmetrical jaw-opening and jaw-closing movements were simulated based upon different muscle activation schemes. It was found that the balance between swing and slide of the mandibular condyle at the onset of a jaw-opening movement was predominantly dependent on the level of activation of the digastric and inferior lateral pterygoid muscles. The level of activation of the temporalis muscle parts was of critical importance for the jaw-closing movements. The amount of jaw opening was limited by the passive forces of the jaw-closing muscles. In contrast, the influence of the passive forces of the jaw-opening muscles on the jaw-closing movement was neglectable. Throughout the movements the temporomandibular joints remained loaded. The average torques generated by the jaw-opening or jaw-closing muscles with respect to the centre of gravity of the lower jaw had similar orientations and can be considered to be responsible for joint stabilization. The average direction of their lines of action, however, was about opposite, and this can be considered as the major discriminant between a movement in opening or closing direction.

Biomechanical Phenomena↗

PubMed1997

Relationships between the orientation and moment arms of the human jaw muscles and normal craniofacial morphology.

It has been suggested that subjects with increased vertical craniofacial dimensions have relatively oblique orientated jaw muscles with a reduced possibility to restrain the vertical component of craniofacial growth. To test this hypothesis, relationships were investigated between the spatial orientation of the jaw muscles and the craniofacial morphology. Computer reconstructions of the external shape of the jaw muscles of 30 adult males with a normal skull were made with the use of serial magnetic resonance imaging (MRI) scans. The orientation of the jaw muscles was defined by a regression line through the centroids of the serial cross-sections. Sagittal and frontal projections of the moment arms of the muscles were measured with respect to the centre of the ipsilateral condyle. Craniofacial morphology was analysed three-dimensionally using lateral head films and coronal MRI scans. The cephalometric data were analysed statistically using regression and factor analyses. Six cephalometric factors with Eigen values higher than 1 were correlated with jaw muscle orientation and moment arm data, using a multiple regression analysis. The anterior face height factor was significantly correlated with the orientation of the jaw opening muscles in the sagittal plane but was not significantly correlated with the orientation of the mandibular elevators. The sagittal moment arms of the mandibular elevators showed significant correlations with the factors describing the gonial angle and the posterior face height. It was concluded that the variation of spatial orientation of the human jaw closing muscles is predominantly associated with the variation of mandibular morphology (expressed by the gonial angle) and the posterior face height. The orientation of the jaw opening muscles shows significant relationships with anterior vertical craniofacial dimensions. The hypothesis that persons with an increased anterior face height have relatively oblique orientated jaw elevators was rejected.

Adult↗

PubMed1997

[Masticatory muscles. Part III. Biomechanics of the masticatory muscles].

The masticatory muscles are able to produce forces. These forces may cause movements of the lower jaw. Furthermore, they can be applied by the teeth for the generation of bite or chewing forces. During these kind of processes the temporomandibular joints will be loaded also. The interaction between forces and movements in the masticatory system is complex but obeys the relatively simple laws of mechanics. By application of these laws the development of joint loading, force patterns and movements during masticatory function and dysfunction can be understood. This is illustrated by a few examples of both statical and dynamical masticatory performance.

Biomechanical Phenomena↗

PubMed1996

Three-dimensional structure of the human temporalis muscle.

BACKGROUND: The maximal force a muscle is capable of producing is proportional to its physiological cross-sectional area and its excursion range to the length of the muscle fibers. The length of the sarcomeres is a major determinant for both force and excursion range. The human temporalis muscle is an architecturally complex muscle, and little is known regarding the possible heterogeneous distribution of these parameters throughout the muscle. The objective of this study was to determine this distribution for different muscle portions and to examine the functional consequences. METHODS: In eight cadavers, sarcomere lengths, fiber lengths, and physiological cross-sectional areas were measured for the closed mouth position in six different anteroposterior portions of the temporalis muscle. To determine the spatial position of the muscle portions, the three-dimensional coordinates of attachment sites of a number of fiber bundles were registered. These parameters were used as input for a mathematical model with which sarcomere length changes and the consequences for the production of active force at different open positions of the jaw were estimated. RESULTS: At the closed-jaw position, average sarcomere length ranged between 2.26 and 2.34 microns and did not differ significantly among the muscle portions. Average fiber bundle length ranged between 21.7 and 28.9 mm and differed significantly among the muscle portions. The physiological cross-sectional area ranged between 1.82 and 2.93 cm2; the smallest values were found posteriorly, and the largest values anteriorly. The line of pull of the anteriormost muscle portion was slightly inclined anteriorly and medially, whereas the posteriormost portion was relatively strongly inclined backwardly and laterally. The model predicted that during jaw open-close movements a nonuniform change in length of the sarcomeres would occur; sarcomere excursions were smaller posteriorly than anteriorly. Different muscle portions seemed to function along different parts of the active length-force relationship. CONCLUSIONS: The temporalis muscle is an architecturally heterogeneous muscle. Different muscle portions are capable of producing different maximum force and excursion range, and the portions have the capability of performing different mechanical actions.

Aged↗

PubMed1996

Influence of the dynamical properties of the human masticatory muscles on jaw closing movements.

A dynamic six degrees-of-freedom mathematical model of the human masticatory system has been developed in order to study the contribution of the different masticatory muscles and their dynamical properties to the closing movement of the jaw. Muscles were included as forces acting according to their lines of action and the temporomandibular joints were modelled by linear elastic surfaces. Ligaments were not included. The geometry of the model was derived from a human cadaver. With this model symmetrical jaw closing movements were simulated. It was found that the normally observed jaw closing movement which includes a swing-slide movement of the condyle along the articular eminence, can be generated by various separate pairs of masticatory muscles. Among the masticatory muscles the different parts of the masseter as well as the medial pterygoid muscle appeared to be the most suitable to complete this action. The dynamical muscle properties appeared to provide a mechanism causing the movements to pass off more smoothly. The force/length relationship of muscle fibres, which also introduced a limit for protrusive excursions of the lower jaw, turned out to play a predominant role in this mechanism.

Biomechanical Phenomena↗

PubMed1995

Architecture of the human pterygoid muscles.

Muscle force is proportional to the physiological cross-sectional area (PCSA), and muscle velocity and excursion are proportional to the fiber length. The length of the sarcomeres is a major determinant of both force and velocity. The goal of this study was to characterize the architecture of the human pterygoid muscles and to evaluate possible functional consequences for muscle force and muscle velocity. For the heads of the lateral and medial pterygoid, the length of sarcomeres and of fiber bundles, the PCSA, and the three-dimensional coordinates of origin and insertion points were determined. Measurements were taken from eight cadavers, and the data were used as input for a model predicting sarcomere length and active muscle force as a function of mandibular position. At the closed-jaw position, sarcomeres in the lateral pterygoid (inferior head, 2.83 +/- 0.1 microns; superior head, 2.72 +/- 0.11 microns) were significantly longer than those in the medial pterygoid (anterior head, 2.48 +/- 0.36 microns; posterior head, 2.54 +/- 0.38 microns). With these initial lengths, the jaw angle at which the muscles were capable of producing maximum active force was estimated to be between 5 degrees and 10 degrees. The lateral pterygoid was characterized by relatively long fibers (inferior, 23 +/- 2.7 mm; superior, 21.4 +/- 2.2 mm) and a small PCSA (inferior, 2.82 +/- 0.66 cm2; superior, 0.95 +/- 0.35 cm2), whereas the medial pterygoid had relatively short fibers (anterior, 13.5 +/- 1.9 mm; posterior, 12.4 +/- 1.5 mm) and a large PCSA (anterior, 2.47 +/- 0.57 cm2; posterior, 3.53 +/- 0.97 cm2).(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed1995

Biomechanical analysis of jaw-closing movements.

This study concerns the complex interaction between active muscle forces and passive guiding structures during jaw-closing movements. It is generally accepted that the ligaments of the joint play a major role in condylar guidance during these movements. While these ligaments permit a wide range of motions, it was assumed that they are not primarily involved in force transmission in the joints. Therefore, it was hypothesized that muscle forces and movement constraints caused by the articular surfaces imply a necessary and sufficient condition to generate ordinary jaw-closing movements. This hypothesis was tested by biomechanical analysis. A dynamic six-degrees-of-freedom mathematical model of the human masticatory system has been developed for qualitative analysis of the contributions of the different masticatory muscles to jaw-closing movements, it was found that the normally observed movement, which includes a swing-slide condylar movement along the articular eminence, can be generated by various separate pairs of masticatory muscles, among which the different parts of the masseter as well as the medial pterygoid muscle appeared to be the most suitable to complete this action. The results seem to be in contrast to the general opinion that a muscle with a forward-directed force component may not be suitable for generating jaw movements in which the condyle moves backward. The results can be explained, however, by biomechanical analysis which includes not only muscle and joint forces as used in standard textbooks of anatomy, but also the torques generated by these forces.

Biomechanical Phenomena↗

PubMed1992

Application and validation of a three-dimensional mathematical model of the human masticatory system in vivo.

A previously described three-dimensional mathematical model of the human masticatory system, predicting maximum possible bite forces in all directions and the recruitment patterns of the masticatory muscles necessary to generate these forces, was validated in in vivo experiments. The morphological input parameters to the model for individual subjects were collected using MRI scanning of the jaw system. Experimental measurements included recording of maximum voluntary bite force (magnitude and direction) and surface EMG from the temporalis and masseter muscles. For bite forces with an angle of 0, 10 and 20 degrees relative to the normal to the occlusal plane the predicted maximum possible bite forces were between 0.9 and 1.2 times the measured ones and the average ratio of measured to predicted maximum bite force was close to unity. The average measured and predicted muscle recruitment patterns showed no striking differences. Nevertheless, some systematic differences, dependent on the bite force direction, were found between the predicted and the measured maximum possible bite forces. In a second series of simulations the influence of the direction of the joint reaction forces on these errors was studied. The results suggest that they were caused primarily by an improper determination of the joint force directions.

Adult↗

PubMed1991

Biomechanical changes in the rabbit masticatory system during postnatal development.

Using dissection, biometry, and two three-dimensional mechanical models, the postnatal changes of the rabbit masticatory muscles were studied by analyzing their three-dimensional orientation, their strength and fiber lengths, and certain functional consequences of these changes. The first mechanical model uses length-tension relationships of the muscles and predicts the maximum bite force as a function of mandibular position. It shows that young rabbits are able to generate large bite forces at a wider gape than adult animals and that the forces are directed more vertically. In spite of the postnatal changes the mechanical advantage of the system remains about equal. However, the muscles are reoriented so that they exert a larger degree of parallel action, suggesting a larger bite force magnitude but a smaller range of bite force directions. The second model-predicts this range. It shows that during postnatal development a relative gain occurs in the possibilities for the system to exert forces directed rostrodorsally. In all other directions the capability to exert force decreases. The results suggest that during development the possibility of the system to generate large bite forces is increased at the cost of a restriction in the range of jaw excursion and that a restriction takes place in the range of possible force directions that can be exerted at the molars.

Animals↗

PubMed1990

Computer-assisted estimation of lines of action of human masticatory muscles reconstructed in vivo by means of magnetic resonance imaging of parallel sections.

The orientation of these lines of action was estimated in 9 healthy subjects, by reconstructing the muscle shape from a series of parallel sections obtained by MRI. In order to gain insight into sources of error, the lines of action of the masseter and medial pterygoid were estimated from two mutually perpendicular series of sectional images. Average results were compared with anatomical data from the literature. The results indicated that the accuracy of the estimate was principally dependent on the reliability of the reconstructions; the average accuracy of the estimated orientations was about 5 degrees.

Adult↗

PubMed1989

An iterative procedure to estimate muscle lines of action in vivo.

A method is described to estimate the line of action of muscles in the three-dimensional space from serial images of parallel muscle sections obtained in vivo by means of CT or MRI scanning. The external shape of a muscle, reconstructed from the series of parallel sections, is mathematically divided into a series of imaginary slices directed arbitrarily in the three-dimensional space. The line of action is estimated initially as a regression line through the centroids of these mathematical slices. A new series of mathematical slices is constructed perpendicular to the regression line and a new estimate of the line of action is obtained from their centroids. This procedure is repeated until the estimated line of action is perpendicular to the mathematical slices; it can then be considered as a reliable estimate of the line of action. The accuracy of the method has been tested for various reconstruction parameters and muscle shapes. The results of these tests show that the accuracy is relatively independent of the direction in which the sectional images have been made and that, except for relatively short and thick muscles, the estimated lines of action deviated less than about 2 degrees from the theoretical one. The presented method is a relatively simple mathematical technique which can be used easily for muscles reconstructed in vivo from routinely obtained sectional MRI or CT images.

Biomechanical Phenomena↗

PubMed1989

[Loading of the masticatory system].

The masticatory system has the capability to produce biteforces in many directions, the magnitude of the maximum possible biteforce varies with the biteforce direction and depends on several parameters, such as the length-tension relationship, the physiological cross-section and the orientation of the jaw muscles, the shape of the temporomandibular joint and the location of the bite point. The loading of the left and right temporomandibular joint varies, among others, with the bite-force direction.

Bite Force↗

PubMed1988

Mechanical capabilities of the human jaw muscles studied with a mathematical model.

The human muscles of mastication have complex shapes with large attachment areas. This suggests a variety of bite force directions and magnitudes. The possible range of these and the concomitant joint force of each individual muscle were determined by a mathematical model describing static equilibrium conditions in the sagittal plane. The range of force directions for each muscle was defined by the action lines of the most anterior and most posterior (for the lateral pterygoid, most superior and most inferior) muscle fibre bundles. Calculations from the various directions of the reaction force in the temporomandibular joint demonstrated that each muscle can produce a unique variety of bite force directions. Except for the lateral pterygoid and posterior temporalis, the range and orientation of possible bite forces was closely related to the orientation of the joint force. In general, at the canine tooth the bite forces were directed more posteriorly than at the second molar. Within a muscle, distinct portions may produce considerably different bite force magnitudes; the largest bite forces are produced at horizontal and vertical joint force directions. The posterior portions of the deep masseter and temporalis muscles and the lateral pterygoid muscle have the largest mechanical advantage. In the majority of muscles the magnitude of the joint reaction force is smallest at an oblique joint force direction.

Aged↗

PubMed1988

A three-dimensional mathematical model of the human masticatory system predicting maximum possible bite forces.

A three-dimensional mathematical model of the human masticatory system, containing 16 muscle forces and two joint reaction forces, is described. The model allows simulation of static bite forces and concomitant joint reaction forces for various bite point locations and mandibular positions. The system parameters for the model were obtained from a cadaver head. Maximum possible bite forces were computed using optimization techniques; the optimization criterion we used was the minimizing of the relative activity of the most active muscle. The model predicts that at each specific bite point, bite forces can be generated in a wide range of directions, and that the magnitude of the maximum bite force depends on its direction. The relationship between bite force direction and its maximum magnitude depends on bite point location and mandibular position. In general, the direction of the largest possible bite force does not coincide with the direction perpendicular to the occlusal plane.

Biomechanical Phenomena↗

PubMed1988

A feedback method to determine the three-dimensional bite-force capabilities of the human masticatory system.

A feedback procedure is described that enables a subject to exert bite forces in certain specified directions during static contraction of the human jaw muscles. The output of a three-dimensional transducer is fed to a computer. The magnitude and direction of the resultant force are computed and visualized by a cross on the screen of the computer terminal. In a bite experiment, the subject is instructed to match this cross with a point on the screen, representing the desired bite-force direction. The procedure allows for determination of the range of possible bite-force directions and magnitudes for various locations on the dental arch and study of the concomitant recruitment patterns of the jaw muscles. Some examples of measurement are given.

Adult↗