Biomedical subjects

L V Christensen

Publications and source records attributed to L V Christensen.

At least 55 records · Page 3Linked to original sources

PubMed1991

An electrognathographic study of aspects of 'deprogramming' of human jaw muscles.

Three-dimensional electrognathography of an incisor point was used to detect peripheral correlates of deprogramming of the jaw elevator muscles. Putative deprogramming was attempted through the clinically recommended use of a leaf gauge, placed for 10-15 min between the maxillary and mandibular anterior teeth and disoccluding the posterior teeth by about 2 mm. Studied mandibular displacements were those that occurred during voluntary elevation of the mandible from the postural to the intercuspal position. Use of the leaf gauge did not affect the displacement patterns. Within a freeway space of about 0.2 mm3, the incisor point moved about 2 mm vertically, about 1 mm sagittally, and about 0.2 mm laterally.

Adult↗

PubMed1991

Postural contractile activities of human jaw muscles following use of an occlusal splint.

As induced by an occlusal splint over a period of 1 week, this study monitored surface electromyographic changes in the postural contractile activities of jaw elevator and depressor muscles in six healthy adults. The immediate effect of the occlusal splint was to increase the postural contractile activities of the suprahyoid muscles. All postural muscle activities showed wide-ranging biological variation, but the activities induced by the splint tended to stabilize within 1 week, with decreased postural activities in the masseter and anterior temporalis muscles, and increased postural activities in the suprahyoid muscles.

Adult↗

PubMed1991

Activity index and isometric contraction velocity of human jaw muscles.

To determine the relative contribution of the masseter and anterior temporalis muscles to global isometric bite force, activity indices were constructed on the basis of the peak mean voltage (microV) and the integrated voltage (microV.s) of bipolar surface electromyograms obtained during brief maximum voluntary teeth clenching (MVC). The index that was based on integrated myoelectrical activity showed that the masseter muscle contributed the major part of the isometric MVC force. The index that was based on instantaneous peak myoelectrical activity also showed that activity in the masseter muscle predominated over that in the anterior temporalis muscle. In addition, the latter index showed a negative linear association with the initial (0-50% MVC) isometric contraction velocities of the masseter and anterior temporalis muscles. Both indices appear to be promising clinical diagnostic tools.

Action Potentials↗

PubMed1990

Observations in the time and frequency domains of surface electromyograms of experimental brief teeth clenching in man.

Maximum voluntary teeth clenching was performed for about 1 s to study the interactions between subjective sensory-motor events and changes in the time and frequency domains of surface electromyograms of the masseter and anterior temporalis muscle. Isometric jaw muscle contractions were examined for their speeds of contraction, the total energy content and median frequency of their power density spectra, and for the specific rate of change and the efficiency of their voluntary and involuntary activation efforts. The observations suggested that, in general, brief maximum isometric contractions were not preprogrammed, but rather were regulated by a subconscious proprioceptive feedback mechanism; the mechanism, possibly a transcortical loop, appeared to have a low gain and to be based on recruitment/decruitment of motor units.

Bite Force↗

PubMed1990

An electromyographic study of aspects of 'deprogramming' of human jaw muscles.

Surface electromyograms from the right and left masseter and anterior temporalis muscles were used to detect peripheral correlates of deprogramming, also known as programming and reprogramming, of jaw elevator muscles. Putative deprogramming was attempted through the clinically recommended use of a leaf gauge, placed for 15 min between the maxillary and mandibular anterior teeth and disoccluding the posterior teeth by about 2 mm. Studied contractile activities were those of postural activity (subconscious, semi-isometric, minimal activity) and intercuspal teeth clenching (conscious, isometric, maximal activity). Use of the leaf gauge did not affect normalized postural activity (about 4%), the duration (about 900 ms) and static work efforts of clenching (about 1200 microV.s), the time to peak mean voltage of clenching (about 400 ms), and the peak mean voltage of clenching (about 300 microV). Activity and asymmetry indices showed that the studied motor innervation patterns were not changed by the leaf gauge.

Adult↗

PubMed1989

Relationship of the maxillary canines to the incisive papilla.

The positioning of denture teeth in a completely edentulous patient is dependent on many factors. Anatomic landmarks are frequently valuable in complete denture fabrication. The topography of the palatal soft tissue in relationship to tooth position in dentate patients may be an aid in the placement of denture teeth. By using casts formed from irreversible hydrocolloid impressions, 50 subjects were examined to determine the orthographic distance from the posterior of the incisive papilla to a line intersecting the distal contact points of the maxillary canines. In 92% of subjects the posterior point of the incisive papilla was approximately 3 mm anterior to the line between the distal points of the canines. Neither gender, age, nor maxillary tooth arch form affected this distance.

Adolescent↗

PubMed1989

Reliability of maximum static work efforts by the human masseter muscle.

To study the reliability of nonfatiguing maximum voluntary static work efforts by the masseter muscle, six healthy subjects exercised teeth clenching in centric occlusion. Maximum voluntary teeth clenching was performed for 10 seconds on 2 different days, each with two trials, and maximum static work efforts (without artificial feedback-control) were quantified by integrated surface electromyography. Reliability was determined by factorial analyses of variance and intraclass correlations. Data reduction showed that maximum voluntary static work efforts were reproduced reliably during the four different trials.

Adult↗

PubMed1989

Experimental teeth clenching in man.

The thesis develops an electromyographic (EMG) method to quantify maximum voluntary teeth clenching (MVC), studies the onset and endurance of jaw muscle fatigue and pain from MVC, and explores the prevention of the discomforts through pharmacological and physical means. MVC, or maximum voluntary static work efforts by the elevator muscles of the mandible, was quantified by continuous (integral) functions of variations in both time and recruitment/rate coding of motor units in the masseter muscle. Fatigue was felt in the masseter muscle after about 30 seconds of MVC; differential calculus suggested that the appearance and disappearance of fatigue was associated with primarily recruitment and decruitment of masseteric motor units, respectively. About 60 seconds of MVC elicited a mild pain in the masseter and temporalis muscles; about 120 seconds of MVC induced a moderate pain and complete exhaustion of the isometrically contracting muscles. Although pain releasing maximum static work efforts are stable variables they cannot predict the pain magnitude of brief and prolonged MVC, probably because of modulations (recruitment/decruitment/rate coding) of masseteric motor units. It is suggested that the modulations begin with the onset of fatigue, are practically complete with the onset of pain, and are absent or negligible with an experience of exhaustion. A single oral dose of 1000 mg of ibuprofen did not affect the onset, endurance, and magnitude of pain from MVC. By contrast, 30 minutes of cooling (ice) of the masseter muscle effectively prevented the onset of pain; it also increased the masseteric EMG, credibly because of modulations of myoelectrical signals and, possibly, increased MVC efforts in the absence of pain.

Cryotherapy↗

PubMed1988

Comparison of cast positions by using four face-bows.

The positions that the maxillary right and left first molars occupy when a maxillary cast is mounted in the three-dimensional space of a semiadjustable articulator (Hanau H2) were studied. Mountings were made by using four face-bows, the Hanau Kinematic Face-bow, the Hanau Facia-bow, the Hanau 159 Earpiece Face-bow, and the Hanau Twirl Earpiece Face-bow. A kinematically determined hinge-axis was used arbitrarily as the baseline for comparisons among the three other bows. Deviations from the baseline were measured along the X, Y, and Z axes by using a contour meter (Fig. 1). The deviations showed great variability (Table I), and because maxillary casts are mounted in relation to anatomic landmarks that differ from subject to subject, it was not possible to establish clinical superiority of one face-bow over another.

Adult↗

PubMed1988

Accuracy and comparative stability of three removable die systems.

A study was performed to evaluate three removable die systems: the Accu-Trac, the Pindex, and the conventional brass dowel pin technique. The stability of the dies in the mesiodistal, bucco-lingual, and gingivoocclusal directions was compared. Under the conditions of this investigation, the following conclusions were drawn. 1. No significant difference was found in the shift of the removable die in a horizontal direction between any of the three systems. 2. No significant difference was found in the displacement of the removable die in the vertical direction with the Pindex system and the conventional dowel pin techniques. The slight displacement that did occur was in an upward direction. 3. A significant vertical displacement of the removable die in a downward direction was found with the Accu-Trac system compared with the other two systems.

Denture Design↗

PubMed1988

Effect of topical cheek surface anesthesia on isometric contractions of the human masseter muscle.

To study the possibility of interactions between buccal cutaneous sensory receptors and voluntary maximum isometric contractions of the masseter muscles, six adult subjects exercised maximum teeth clenching before and after spraying the right cheek surface with aerosol containing 20% benzocaine. The right cheek and masseter muscle served as the experimental side, the left cheek and masseter muscle as the control side. Isometric motor outputs, on the right and left sides, were monitored by integrated surface electromyography over periods of 10 seconds. Topical surface anesthesia provided no evidence of motor modulation by cutaneous tactile receptors. Before and after anesthesia, the two muscles showed nearly identical and well-coordinated motor innervation patterns. It is suggested that the cortical motor commands of maximum isometric contractions, with recruitment of practically all available motor units, overrule all modulatory inputs except those of fatigue.

Adult↗

PubMed1988

Quantitative observations on maximum static work efforts and associated pain of the human masseter muscle.

The single case experimental design was used to study day-to-day variations in the onset (PL), tolerance (PT), and intensity (VAS) of masseter muscle pain. Pain was induced by maximum voluntary teeth clenching, with no artificial feedback-control of the level of isometric activity, and static work efforts were quantified by cumulative electromyography. A continual effort to produce maximum static work, about 40% increase from baseline work, elicited in 30-40 s an initial sensation of muscular pain that had an intensity of about 25% of maximum possible score. A further increase in maximum static work effort, about 60% increase from baseline work, caused in about 2 min an intolerable increase in the intensity of muscular pain, about 50% of maximum possible score. Whereas PL showed day-to-day variation, PT and the ratio PL:PT did not. Intensity (VAS) scores and maximum static work efforts showed no day-to-day variations. Onset (PL) and tolerance (PT) showed no linear associations with VAS scores, and VAS scores showed no linear associations with maximum static work efforts; if anything, the latter associations tended to resemble a cubic parabola. Two different central neural processes, not associated with maximum static work efforts in a simple linear manner, might have been instrumental in: (i) the establishment of criteria for onset and tolerance of pain, and (ii) the discrimination of variable levels of pain intensity.

Adult↗

PubMed1988

Relationship of first primary palatine rugae to the maxillary canines in man.

Fifty dentate maxillary casts, obtained from thirty-four males and sixteen females, were mounted in the three-dimensional co-ordinate system of a contour meter. A transverse line of reference (x-axis) was drawn through the distal contact points of the maxillary canines, at a right angle to the y-axis which passed through the contact points of the maxillary central incisors. Relative to the canine-to-canine baseline, the locations of the lateral borders of the right and left first primary rugae were determined. The rugae were distributed on both the anterior and posterior sides of the baseline, and the anterior and posterior distances from the baseline were on the average about 1 mm. The age of the subjects, ranging from 12 to 52 years, appeared to be unrelated to the frequencies of anteriorly and posteriorly positioned rugae. Right and left first primary rugae, located on the anterior side of the baseline, showed a minute asymmetry in their topography. By contrast, there was topographical symmetry when the rugae were located on the posterior side of the baseline.

Adolescent↗

PubMed1987

Effect of ibuprofen on isometric contractions and associated pain of the human masseter muscle.

A randomized, single-blind crossover trial tested the effect of 1,000 mg of ibuprofen on jaw muscle pain induced by maximum voluntary teeth clenching (MVC). Subjects exercised MVC until there was onset of pain in the masseter muscles (pain latency in seconds), and until pain and exhaustion of the masseter and anterior temporalis muscles could no longer be endured (pain tolerance in seconds). Pain intensity was quantified by visual analogue scores, and pain sensitivity by the pain sensitivity range and the pain sensitivity ratio. During MVC the mean voltage of the left masseter muscle was recorded by cumulative surface electromyography. Ibuprofen had no significant effect on the pain latency and the pain tolerance. Neither did ibuprofen significantly decrease the pain intensity nor significantly affect the pain sensitivity range and the pain sensitivity ratio. After intake of ibuprofen, the number of electromyograms with a decrease in mean voltage was significantly increased--credibly, an expression of increased central fatigue with voluntary decruitment of motor units, and possibly the result of increased contraction times because of an undisclosed effect of ibuprofen. There was no circumstantial evidence of impaired motor activity that could be attributed to biosynthesis of prostaglandins.

Adult↗

PubMed1986

Effects of experimental loss of teeth on the temporomandibular joint.

A review is presented of the effects of loss of teeth, with impaired occlusal stabilization, on the temporomandibular joint in experimental animals. Loss of teeth, with subsequent abnormal loading of the temporomandibular joint, leads to histomorphological, pathological and pathophysiological changes in the articular cartilages, the articular disc, the synovium, and the bony articular components. The severity of the changes increases with increasing age, and an existing arthritis of the joint is aggravated by impaired occlusal stabilization. The experimentally induced changes resemble those in human subjects with extensive loss or extensive attrition of teeth.

Animals↗

PubMed1986

Physiology and pathophysiology of skeletal muscle contractions. Part I. Dynamic activity.

An overview is presented of the physiology and pathophysiology of dynamic skeletal muscle contractions in the intact living organism. Dynamic muscle activities are divided into concentric contractions with shortening of muscle fibres and the production of positive work, and eccentric contractions with lengthening of muscle fibres and the production of negative work. In positive work, muscle tension overcomes external forces. In negative work, external forces overcome muscle tension. The latter phenomenon, with relatively few active motor units, explains the injuries induced by eccentric contractions. Both the contractile and non-contractile elements are involved in the muscle injuries and, clinically, they are referred to as myofibrositis.

Animals↗

PubMed1986

Physiology and pathophysiology of skeletal muscle contractions. Part II. Static activity.

An overview is presented of the physiology and so-called pathophysiology of static muscle contractions in the intact living organism. Static muscle activity produces no external mechanophysical work, but is used for fixation. Contingent upon the levels of the generated forces, and the duration of the isometric contractions, static activities give rise to fatigue and pains. The discomforts are viewed as somatic responses that are normally within physiological limits. That is, upon cessation of isometric contractions there is usually rapid recovery from the discomforts, and they do not normally induce chronic lesions.

Animals↗

PubMed1985

Mandibular reference positions.

An overview is presented of the mandibular reference positions of centric occlusion, centric relation and centric relation occlusion, and the position of rest. Clinical applications of the different positions are discussed on the basis of the physiology of the mandibular locomotor system.

Dental Occlusion↗