Human masticatory muscle forces during static biting.

Muscle forces determine joint loads, but the objectives governing the mix of muscle forces involved are unknown. This study tested the hypothesis that masticatory muscle forces exerted during static biting are consistent with objectives of minimization of joint loads (MJL) or muscle effort (MME). To do this, we compared numerical model predictions with data measured from six subjects. Biting tasks which produced moments on molar and incisor teeth were modeled based on MJL or MME. The slope of predicted vs. electromyographic (EMG) data for an individual was compared with a perfect match slope of 1.00. Predictions based on MME matched best with EMG activity for molar biting (slopes, 0.89-1.16). Predictions from either or both models matched EMG results for incisor biting (best-match slopes, 0.95-1.07). Muscle forces during isometric biting appear to be consistent with objectives of MJL or MME, depending on the individual, biting location, and moment.

Reduced utilization of Man5GlcNAc2-P-P-lipid in a Lec9 mutant of Chinese hamster ovary cells: analysis of the steps in oligosaccharide-lipid assembly.

Recently we reported that CHB11-1-3, a Chinese hamster ovary cell mutant defective in glycosylation of asparagine-linked proteins, is defective in the synthesis of dolichol [Quellhorst et al., 343:19-26, 1997: Arch Biochem Biophys]. CHB11-1-3 was found to be in the Lec9 complementation group, which synthesizes polyprenol rather than dolichol. In this paper, levels of various polyprenyl derivatives in CHB11-1-3 are compared to levels of the corresponding dolichyl derivatives in parental cells. CHB11-1-3 was found to maintain near normal levels of Man5GlcNAc2-P-P-polyprenol and mannosylphosphorylpolyprenol, despite reduced rates of synthesis, by utilizing those intermediates at a reduced rate. The Man5GlcNAc2 oligosaccharide attached to prenol in CHB11-1-3 cells and to dolichol in parental cells is the same structure, as determined by acetolysis. Man5GlcNAc2-P-P-polyprenol and Man5GlcNAc5-P-P-dolichol both appeared to be translocated efficiently in an in vitro reaction. Glycosylation of G protein was compared in vesicular stomatitus virus (VSV)-infected parent and mutant; although a portion of G protein was compared in vesicular stomatitus virus (VSV)-infected parent and mutant; although a portion of G protein was normally glycosylated in CHB11-1-3 cells, a large portion of G was underglycosylated, resulting in the addition of either one or no oligosaccharide to G. Addition of a single oligosaccharide occurred randomly rather than preferentially at one of the two sites.

Canine retraction with rare earth magnets: an investigation into the validity of the constant force hypothesis.

The objective of this study was to test the hypothesis that a prolonged constant force provides more effective tooth movement than an impulsive force of short duration. Six human subjects were selected, the main criterion being a need for extraction of their upper first premolars. Canine retraction on these subjects was executed on one side with the application of a force rapidly declining in magnitude, produced by a vertical loop, and on the other side with the application of a relatively constant force. This type of force was achieved by a similar vertical loop which was constantly activated by three parylene-coated neodymium-iron-boron (Nd2Fe14P) block magnets. The vertical loop on the control side was reactivated 6 weeks after the initial activation. No reactivation was necessary on the experimental side for the duration of the experiment. The rate of tooth movement on the two sides was compared over a period of 3 months, on the basis of maxillary impressions taken at frequent intervals during the course of the study. The canines retracted with a constant force moved statistically significantly more than the control canines (p < 0.05) during the experimental period. The average differences in the mean rates of tooth movement between the two sides were in the order of 2:1 in favor of the experimental side. There were no statistically significant differences in the changes of angulation (tipping) or rotation about the y axis between the two sides. The duration of force application seems to be a critical factor in regulating rate of tooth movement. Conversely, magnitude of the applied force did not appear to be of primary significance.

Modelling of forces in the human masticatory system with optimization of the angulations of the joint loads.

Numerical models of the human masticatory system were constructed using algorithms which minimized non-linear functions of the muscle forces or the joint loads. However, the predicted solutions for isometric biting were critically dependent upon the modelled angular freedom of the joint loads. The most complete mathematical minimization of any objective function occurs when the joint load angles are predicted. However, the predictions have to be sensible in relation to the actual morphology of the joints. Therefore, the models were tested in terms of the angles of joint load predicted for a dry skull, using muscle vectors reconstructed from the geometry of the skull. The minimizations of muscle force were intrinsically incapable of predicting the angles of joint load. Such models must rely on constrained angles and this produces a restricted minimization and also an indeterminacy. In contrast, the minimizations of joint load predicted angles of joint load which varied appropriately with condylar position. The condylar movement was achieved with a positioning model which adjusted the angulation of the muscle vectors as the jaw was positioned. This model also generated the optimal sagittal shape of the articular eminence. Muscle predictions from the various models were not examined in detail, but the general nature of the predicted muscle force patterns was shown to be reasonable in some of the models and unreasonable in others. The results supported the hypothesis that the temporomandibular joint develops functionally to allow an approximate minimization of the joint loads during isomeric biting. This does not necessarily imply that the neurophysiological control is actually based on a minimization of joint load.

Three enzymes involved in oligosaccharide-lipid assembly in Chinese hamster ovary cells differ in lipid substrate preference.

Initial steps in N-linked glycosylation involve formation of a large oligosaccharide structure on a lipid carrier, dolichyl phosphate. We have previously characterized Chinese hamster ovary (CHO) glycosylation mutants (Lec9 cells) that utilize the polyisoprenoid lipid polyprenyl phosphate rather than dolichyl phosphate in these glycosylation reactions. Polyprenyl phosphate differs from dolichyl phosphate only in the degree of saturation of its terminal isoprenyl unit. Our goal was to determine whether the glycosylation defect of Lec9 cells could be explained simply by knowing lipid substrate preferences of the enzymes involved in the assembly of oligosaccharide-lipid (OSL) intermediates. In this study, we have used in vitro assay systems to compare the ability of dolichyl phosphate and polyprenyl phosphate to act as substrates for three glycosyl transferase enzymes involved in OSL assembly. In order to insure that we were only examining lipid substrate preferences of the enzymes and not other potential defects present in Lec9 cells, we used membranes prepared from wild-type cells in these in vitro reactions. Our results indicate that one of the enzymes, mannosylphosphoryldolichol (MPD) synthase, exhibited a significant preference for the dolichol substrate. Glucosylphosphoryldolichol (GPD) synthase, on the other hand, showed no binding specificity for the dolichol substrate, although the enzyme used the dolichol substrate at a twofold higher rate. N,N'-diacetyl-chitobiosylpyrophosphoryldolichol (CPD) synthase was able to use either lipid substrate with equal efficiency. These results suggest that not all glycosyl transferases in this pathway show a preference for dolichol derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro measurement of the stress-distribution properties of the pig temporomandibular joint disc.

An in vitro experimental technique was developed to measure the stress-distribution properties of this disc. Discs were tested for conditions of increasing load and decreasing congruity between loading surfaces. Peak stresses and pressure gradients increased linearly as load on the disc increased. For a given load, as congruity of the surfaces decreased, pressure gradients increased. The results demonstrate that the disc has only a limited capacity for stress distribution. Disc thickness was a critical factor determining the peak stresses measured under the disc.

An analysis of surface congruity in the growing human temporomandibular joint.

The influence of condylar position on congruity of the surfaces of the temporomandibular joint (TMJ) was determined. The degree of congruity between the loading surfaces of the condyle and the eminence varied depending on condylar position. Better congruity of surfaces was found in condylar positions consistent with molar biting. Incongruity between surfaces occurred more frequently at the crest of the TMJ eminence and was related to the degree of eminence development. The evidence supports an hypothesis that growth of the TMJ eminence creates incongruities between loading surfaces that predispose to stress concentrations in the anterior regions of the articular surfaces. The findings may help to explain why degenerative lesions commonly occur at the crest of the eminence.

In vitro measurement of the frictional properties of the temporomandibular joint disc.

The integrity of the articulating surfaces of the temporomandibular joint (TMJ) is likely to be promoted by the control of stresses due to frictional forces between moving joint surfaces. Using a pendulum designed to measure the friction on the surface of the pig TMJ disc, factors such as duration of loading and degree of hydration of the disc were found to influence the amount of friction and the time-dependent changes in friction on the disc surface. The tests provide evidence in support of the hypothesis of 'weeping lubrication' on the surface of the TMJ disc.

Mammalian glycosyltransferases prefer glycosyl phosphoryl dolichols rather than glycosyl phosphoryl polyprenols as substrates for oligosaccharyl synthesis.

We have studied the effectiveness of polyprenyl-P-mannose and polyprenol-P-glucose as donor substrates for the dolichyl-P-mannose:Man5(GlcNAc)2-PP-dolichol mannosyltransferase and the dolichyl-P-glucose:Man9(GlcNAc)2-PP-dolichol glucosyltransferase, respectively. The polyprenol moiety differs from dolichol only in the unsaturation of the terminal isoprene unit of the molecule. Based on the kinetics of the reactions, we have found that both glycosyltransferases have higher apparent Kms and lower apparent Vmaxs using polyprenyl-P-monosaccharides as substrates rather than the dolichyl-P-monosaccharides. The products formed with the polyprenyl-P-sugars were the same as those formed by the dolichol-linked sugars, indicating that the polyprenol substrates could be utilized by the glycosyltransferases in vitro. The results also indicate that the dolichyl-P-sugars and the polyprenyl-P-sugars compete for the same binding site on the enzyme. These findings are significant in terms of understanding the glycosylation phenotypes of Chinese hamster ovary cell mutants of the Lec9 complementation group, which lack the ability to convert polyprenol into dolichol.

Mutants in dolichol synthesis: conversion of polyprenol to dolichol appears to be a rate-limiting step in dolichol synthesis.

Chinese hamster ovary (CHO) cells of the Lec9 recessive complementation group display a distinctive profile of resistance to a variety of toxic lectins. In addition, they accumulate cis-alpha-unsaturated polyprenol and use mainly polyprenol rather than dolichol to synthesize the glycosylated lipids used in asparagine-linked glycosylation of proteins. The primary defect in these cells is thought to result from a deficiency in polyprenol reductase activity. Three new mutants were isolated and determined to have qualitatively, although not quantitatively, similar lectin resistance profiles to Lec9 cells. Two of these mutants (AbrR and RicR) also contained polyprenol rather than dolichol. The lectin resistance profile of an independent mutant which accumulates polyprenol, F2A8, was also found to be qualitatively similar to the Lec9 pattern. The relationship among these mutants was analysed in more detail by construction of cell-cell hybrids. Lectin resistance profiles of the hybrids demonstrated that AbrR, RicR and F2A8 fell into the Lec9 complementation group. Analysis of prenols in the hybrids also showed that F2A8 was a member of the Lec9 group. Surprisingly, a significant fraction of the prenols found in Lec9 x Parent hybrids was polyprenol (up to 30% of the neutral fraction), whereas the prenols found in Parent x Parent hybrids were nearly exclusively dolichol (97% of the neutral lipid fraction). Therefore, reduction of polyprenol to dolichol appears to be a rate-limiting step in the synthesis of dolichol since hybrids with differing numbers of wild-type alleles can be biochemically distinguished.

Substrate specificity of N-acetylglucosamine 1-phosphate transferase activity in Chinese hamster ovary cells.

The assembly pathway of the oligosaccharide chains of asparagine-linked glycoproteins in mammalian cells begins with the formation of GlcNAc-PP-dolichol in a reaction catalysed by the enzyme N-acetylglucosamine 1-phosphate transferase. We have investigated the efficiency of two lipid substrates for the transferase activity in an in vitro assay using Chinese hamster ovary (CHO) cell membranes as an enzyme source. Experiments were carried out with varying concentrations of dolichyl phosphate or its precursor, polyprenyl phosphate. We determined that enzyme activity was optimal at pH 9, where the enzyme exhibited a 3-fold higher Vmax and a 2-fold lower Km for the dolichol substrate. At pH 7.4, the Km and Vmax differences between the two lipids were 10-fold. Under all assay conditions tested, we found that GlcNAc-PP-lipid was the only product formed. We conclude from these results that dolichyl phosphate rather than polyprenyl phosphate is the preferred substrate for the transferase enzyme in CHO cells. This observation is significant in light of the fact that we have previously isolated CHO glycosylation mutants which fail to convert polyprenol into dolichol, and hence utilize polyprenyl derivatives for glycosylation reactions. Thus, these results contribute to our understanding of the glycosylation defects in the mutant cell lines.

Eminence development of the postnatal human temporomandibular joint.

The objective of this study was to analyze postnatal eminence development quantitatively, as a first step in defining the relationship between loading of the TMJ and eminence development. A sample of human osteological remains provided the temporal bones of forty-nine (49) individuals of ages between birth and twenty years. An angular measurement technique permitted quantification of the degree of eminence development of each individual. It was concluded that: (i) the TMJ eminence reaches more than 50% of mature size, and exhibits mature morphology, by the time of completion of eruption of the primary dentition; (ii) the maximum velocity of development of the eminence takes place before three years of age; and (iii) the velocity of development of the eminence is reduced at about five years of age, and slowly diminishes to zero by the middle to late teens.

A theoretical model of loading and eminence development of the postnatal human temporomandibular joint.

The objectives of the present study were to characterize the loading of the immature TMJ, and to develop a theoretical model to explain the relationship between joint loading and development of the eminence of the human TMJ. The osteological remains of forty individuals, ages ranging from birth to twenty years, were used to provide metric coordinates of the three-dimensional relationships of the anatomy of the biting apparatus. The data were used, in a numerical model of TMJ loading (Smith et al., 1986), to calculate the magnitudes and directions of condylar loading. The following conclusions were drawn: (i) static equilibrium cannot be satisfied unless the immature TMJ is loaded; (ii) in the neonate, the direction of condylar loading is approximately vertical but, as the child matures, the angle of condylar loading becomes more oblique; and (iii) evidence is given in support of the hypothesis that early development of the eminence is consequent upon the stimulation of bone growth by the appropriate position and timing of loading of the immature condyle on the temporal component of the joint.

A numerical model of temporomandibular joint loading.

A numerical model of the mandible, its articulating surfaces, and the forces exerted by the primary masticatory muscles has been developed for the purpose of investigating loading of the temporomandibular joint. Evidence is presented which shows that the temporomandibular joint. Evidence is presented which shows that the temporomandibular joint is a load-bearing joint over the normal functional range of bite-force positions and angles. In this investigation, temporomandibular joint loads were found to vary from a maximum appositional force of 60% of the bite force (when bite forces were applied to the incisors) to a distracting force of about 5% of the bite force (when applied to the distal surfaces of the third molars). TMJ loads tended to reach a minimum as a result of vertically directed bite forces positioned at the second molars. A range of conditions in which bite forces were directed parallel to or within approximately 20 degrees of the mid-sagittal plane was found to be conducive to stability of the temporomandibular joint. This stability included symmetry in the direction and in the magnitude of condylar loads as well as the presence of small forces tending to oppose the condyle and articular eminence. TMJ loads tended to reach a maximum in response to mediolaterally directed bite forces. This result is consistent with the fact that no muscle of mastication exhibits a spatial orientation in which the muscle fibers are predominantly mediolateral in direction.