The effect of denture stability on bite force and muscular effort.

This study tested the hypothesis that denture instability limits the amount of voluntary muscular effort generated by denture wearers. Seventeen edentulous subjects (seven men, 10 women; mean age 60·3 ± 13·0 years) with newly acquired implant-retained mandibular overdentures and a conventional maxillary denture participated. Maximum bite forces and corresponding electromyographic (EMG) activity from the temporalis and masseter muscles (bilaterally) were recorded under two experimental conditions: (i) Unilateral premolar and molar bites without additional support, and (ii) premolar and molar bites with bite block support on the opposite side. In addition, EMG values alone were recorded during maximum clenching without any transducer between the upper and lower dentures. The level of muscular effort was significantly higher with greater denture support. These results indicate that denture instability probably prevents denture wearers from using the full potential of their jaw muscles, especially during unilateral biting and chewing, even with two implants supporting the mandibular dentures.

Does long-term use of unstable dentures weaken jaw muscles?

Although it is well known that conventional denture wearers have lower maximum bite forces than dentate subjects, no previous studies have compared the strength of the jaw muscles between these two groups. This study compared maximum bite forces, electromyographic (EMG) activity and estimated jaw muscle strength among three groups: (i) 17 edentulous subjects using newly acquired implant-retained overdentures (seven men, 10 women; mean age 60.3 +/- 13.0 years); (ii) 10 age-matched, fully dentate subjects (five men, five women; mean age 57.9 +/- 11.0 years); and (iii) 39 young, fully dentate subjects (19 men, 20 women; mean age 24.4 +/- 3.5 years). Electromyographic activity was recorded from subjects' bilateral superficial masseter and anterior temporalis muscles while they generated maximum voluntary bite forces at the right central incisor, right first premolar and right first molar positions. Jaw muscle strength was estimated as the ratio of average EMG activity for all four muscles to the maximum bite force. At all three bite positions, edentulous subjects produced maximum bite forces that were less than half that of dentate subjects. Edentulous subjects also produced significantly less EMG activity and had significantly lower estimated jaw muscle strength. Our results suggest that weakened jaw muscles are one factor contributing to lower maximum bite forces among users of conventional dentures.

Calibration of T-Scan sensors for recording bite forces in denture patients.

Although measurement of maximum voluntary biting force is an important indicator of health of the masticatory system, few commercially available transducers have been validated for routine clinical use. The T-Scan III system records distribution of relative forces around the tooth row during clenching, but not absolute forces. This study assesses the reliability of T-Scan sensors with and without protection materials and develops calibration curves that allow measurement of absolute forces with the T-Scan III.

Reducing within-subject variation in chewing cycle kinematics--a statistical approach.

OBJECTIVES: High levels of within-subject variability have limited the use of chewing cycle kinematics in the experimental and clinical context. The purpose of this study was to validate a new strategy for reducing within-subject variability in chewing cycle kinematics, based on the 10 most representative cycles from a chewing sequence. METHODS: This prospective study included 25 young subjects, with normal class I occlusions. An optoelectronic recording system was used to track chin movements of subjects chewing gum (2.5 g). Computer programs provided estimates for duration and movement as well as the 3D coordinates of the chin point. The total output files were further processed for selection of 10 representative cycles based on standard scores for total duration, excursive ranges in the lateral, vertical and antero-posterior directions. Multilevel modelling procedures were used to test for significant differences. RESULTS: There were no significant differences in cycle duration or excursions between the estimates for all cycles versus the 10 most representative cycles. Cycle shapes were very similar. There were no statistically significant differences in between-subject variances. All within-subject variances were smaller when using data from the 10 most representative cycles. The reduction of variance was approximately 33% for total duration and 75% for total 3D excursion. CONCLUSIONS: The results validate the pre-processing strategy that selects the 10 most representative cycles from a sequence without altering cycle duration, excursions or shape or affecting between-subject variation but reducing within-subject variation substantially.

Bolus size and unilateral chewing cycle kinematics.

The purpose of this study was to determine how bolus size alters the human chewing cycle. This prospective within-subject design evaluated chewing cycles of 38 young adults between 20 and 38 years of age (21 males and 17 females). An optoelectric jaw tracking system was used to record movements of the chin during unilateral (right sided) chewing of four randomly ordered bolus sizes (1, 2, 4 and 8 g) of gum. Using each subject's 10 most representative cycles, multilevel statistical procedures were used to evaluate jaw kinematics. The results showed that bolus size has no consistent effect on opening, closing or total cycle duration. Cycle excursions increased significantly with increasing bolus size. With increasing bolus sizes, chewing cycle excursions along the three axes increased 52-115%. The greatest differences between bolus sizes occurred when the jaw was changing direction (i.e. passing from opening to closing and from working to balancing sides). However, the increases were proportionate and the shape of the chewing cycle was maintained. In order to maintain cycle duration while increasing excursive ranges, jaw velocities increased significantly, with the greatest differences occurring at approximately 70% of opening and 30% of closing. We conclude that humans adapt to larger bolus sizes by increasing chewing cycle perimeter and by increasing cycle speed, while maintaining cycle shape and duration.

Occlusal phase of gum-chewing strokes.

The occlusal phase of chewing is especially interesting because food particles are being pulverized in this phase. For efficient chewing the upper and lower teeth must come together in a congruent fashion with less variation than in other phases. To examine this expectation the chewing motions of 28 women were recorded optoelectrically, and their frontal and sagittal angles of the closing and following opening strokes were measured at 3.0 mm (3-D linear distance) of opening. Closing strokes were more stable than opening strokes. The frontal angle was correlated with the sagittal angle during closing. The opening and closing sagittal angles were moderately correlated, and the opening and closing frontal angles were negatively correlated at the intersubject level. No direct association was found between the closing strokes and following opening strokes at the intra-subject level. These results suggest that closing strokes are more stable than opening strokes, resulting in efficient mastication.

Masticatory performance: a protocol for standardized production of an artificial test food.

A protocol for standardizing the production of an artificial test food was established and its reliability was determined. An artificial test food was selected based on its superior properties compared with natural foods. The primary emphasis during production was the incorporation of all 3 cm of paste-hardener and thoroughly kneading the material for 30 s. The curing process was studied to determine overall setting time. After at least 30 min, the physical properties of the test food were ready to be measured. The overall mean hardness of the test food was 489 +/- 60 load grams. Random error between operators was +/-16 load grams while batches differed by +/-13 load grams and individual tablets differed by +/-15 load grams. This protocol provides a reliable standardized method for future masticatory performance studies.

The effects of bolus hardness on masticatory kinematics.

This study investigated how jaw kinematics, including cycle duration, three-dimensional (3-D) excursive ranges and velocities, and cycle shape, changed with increasing hardness of chewing gum. Twenty-six subjects (13 males and 13 females; mean age 23.6 +/- 2.5 years) with Class I normal occlusion were asked to chew two brands of gum with differing hardness. Jaw motion during chewing was tracked with an Optotrak camera at 100 Hz, and all movements were recorded as pure 3-D mandibular movements relative to Frankfort horizontal. Cycle duration did not change significantly with harder gum, but 3-D excursive ranges and velocities increased, except during the occlusal phases of the chewing. Cycle shape was similar for hard and soft gum, but the overall size of the cycle was larger with hard gum. These results suggest that greater muscular effort when chewing harder gum produces a greater acceleration of the mandible in all phases except when the harder gum slows the mandible during the occlusal phases.

Does malocclusion affect masticatory performance?

This purpose of this study was to evaluate the largely untested assumption that malocclusion negatively affects masticatory performance. A sample of 185 untreated subjects (48% male and 52% female) from 7 to 37 years of age, representing subjects with normal occlusion (n = 38), Class I (n = 56), Class II (n = 45), and Class III (n = 46) malocclusion, were evaluated. Masticatory performance was evaluated objectively using artificial (CutterSil, median particle size and broadness of the distribution) and real foods (number of chews for jerky and almonds), and subjectively using a visual analog scale. The results showed no significant differences in age or the body mass index (Wt/Ht2) between the occlusion groups. Subjects with normal occlusion had significantly smaller particle sizes (P = .001) and broader particle distributions (P < .001) than subjects with malocclusion. Compared with the normal occlusion group, the median particle sizes for the Class I, II, and III malocclusion groups were approximately 9%, 15%, and 34% larger, respectively. There were also significant group differences in their subjective ability to chew fresh carrots or celery (P = .019) and firm meat (P = .003). Class III subjects reported the greatest difficulty, followed by Class II subjects, Class I subjects, and subjects with normal occlusion, respectively. We conclude that malocclusion negatively affects subjects' ability to process and break down foods.

The effects of isometric exercise on maximum voluntary bite forces and jaw muscle strength and endurance.

The effects of training and exercise on the strength and endurance of limb muscles has been investigated extensively, but the response of the jaw muscles to exercise remains poorly known. The purpose of this study was to determine whether short-term isometric training increases strength and endurance of the superficial masseter and anterior temporalis muscles. Maximum and submaximum voluntary bite forces and corresponding electromyographic (EMG) activity were measured in 28 young adults, randomly divided into exercise and non-exercise (control) groups. Subjects in the exercise group performed isometric clenches against a soft maxillary splint for five 1-min sessions per day over a 6-week period. After exercise, subjects increased their maximum bite forces by 37%, but control subjects' bite forces also increased by 25%. After exercise, EMG levels per unit of bite force generally decreased, but similar decreases were also seen in the non-exercised controls. Masseter muscle activity levels during standardized 10-kg bites decreased after 6 weeks of exercise. Fatigue resistance increased significantly with exercise but did not differ significantly from control values after 6 weeks of exercise. The results of this study indicate that increases in maximum bite force can be easily produced with training, but that actual strengthening of the jaw muscles is more difficult to achieve.

Morphological and positional asymmetries of young children with functional unilateral posterior crossbite.

This prospective clinical study evaluated the morphological and positional mandibular asymmetry of young patients with functional unilateral posterior crossbite. The sample included 9 girls and 6 boys (8.8 +/- 1.0 years of age), evaluated at the initiation of treatment and approximately 6 months after the retention phase (1.1 +/- 0.2 years after initiation of treatment). Each patient had a complete unilateral posterior crossbite involving 3 or more posterior teeth, a functional shift from centric relation-intercuspal position, and no signs or symptoms of temporomandibular disorder. A bonded palatal expansion appliance was used to rapidly expand the maxilla (1 month) and retain the treatment changes (6 months). Zonograms were used to assess articular joint spaces, and submental vertex radiographs were used to assess morphological and positional asymmetry. The results showed that the mandible was significantly longer on the noncrossbite side than it was on the crossbite side. The asymmetry was most evident for the ramus and involved both the condylar and the coronoid processes. The posterior and superior joint spaces were larger on the noncrossbite side than they were on the crossbite side. After treatment and retention, the mandible showed no significant morphological asymmetries. Mandibular growth was greater on the crossbite side than it was on the noncrossbite side, and the mandible had been repositioned; the crossbite side had rotated forward and medially toward the noncrossbite side. We concluded that unilateral posterior crossbites produce morphological and positional asymmetries of the mandible in young children, and that these asymmetries can be largely eliminated with early expansion therapy.

Changes in the masticatory cycle following treatment of posterior unilateral crossbite in children.

In a previous study, we established that young children with unilateral posterior crossbite have a longer mandibular ramus and more superiorly and posteriorly positioned condyles on the crossbite side. In this study, we evaluated chewing cycle shape and duration in 14 of the patients before treatment, and we looked for changes in cycle shape and duration 6 months after treatment with rapid palatal expansion. Mandibular kinematics was recorded while chewing gum using an optoelectric recording system at 100 Hz. Subjects were asked to chew normally for 20 cycles, chew on the crossbite side only for 20 cycles, and chew on the noncrossbite side only for 20 cycles. A special computer program selected the 10 most representative cycles from each series and computed an average duration and an average maximum excursion along 3 orthogonal axes. Multilevel linear models were used to generate an 8th-order polynomial describing average cycle shape and to test for statistically significant differences in shape between the patients and the controls and between the patients before and after treatment. Before treatment, the patients chewed more slowly than did the controls. Treatment shortened their cycle duration to equal control values. Before treatment, the patients also had larger maximum excursions than did the controls and exhibited a reverse-sequence cycle shape when chewing on the crossbite side. Treatment did not alter the patients' abnormal cycle shape. These results suggest that some features of the masticatory kinematics respond to orthodontic treatment alone, but others do not.

Mandibular rest position: a reliable position influenced by head support and body posture.

This prospective study was designed to establish how the positions of the molars and the condyles are related to incisor position in the mandibular rest position and how their positions are altered by changing head posture. Measurements of the mandibular rest position were taken on 24 men (age range, 23 to 35) with normal Class I occlusion, skeletal patterns, and temporomandibular joint function. The movements of 5 landmarks (lower incisor, and condyles and molars bilaterally) were tracked from maximum intercuspation into 4 independent rest positions (upright supported, upright unsupported, supine supported, and supine unsupported) using an optoelectric (Optotrak; Northern Digital, Waterloo, Ontario, Canada) computer system. The positions were based on minimal electromyographic and verbal instructions to swallow, lick the lips, and say "Mississippi." The results showed significant (P <.01) movements of the incisors, the molars, and the condyles into each of the 4 rest positions. Movements of the molars and the condyles into the supported upright posture and the unsupported upright posture differed slightly but significantly because of greater movement into the supported posture. Patterns of mandibular movement were entirely different between the upright and the supine rest positions; the mandible rotated anteriorly in the supine position and posteriorly in the upright position. We concluded that movement into the mandibular rest position from the intercuspal position is not a simple opening rotation of the mandible, and that the pattern of movement is influenced by head support and body postures.

A new method of evaluating posterior occlusion and its relation to posttreatment occlusal changes.

The purpose of this study was to assess the relationship between posterior occlusion and posttreatment changes in other occlusal variables. Pretreatment (T1), posttreatment (T2), and postretention (T3) records were obtained for 49 Class I (n = 23) and Class II (n = 26) extraction cases. Overbite, overjet, mandibular incisor irregularity, right and left molar deviations, midline deviation, and mandibular arch length were measured, and occlusal registrations were made of each set of dental casts. The proximity of posterior occlusal surfaces was measured as the contact or near-contact areas at or below 300 microm thickness, based on the optical densities of scanned images of the posterior occlusal registrations. The treatment changes for overbite, overjet, right and left molar deviation, and arch length were significantly greater in the Class II group, and the reductions in incisor irregularity were greater in the Class I group. No other class differences were found. Overbite, overjet, and incisor irregularity increased after treatment, and arch length continued to decrease, with no significant class differences. Contact and near-contact areas at or below 300 microm constituted 7% of the functional occlusal table for both classes at the end of treatment, having decreased significantly in both groups (21% in Class I and 29% in Class II) during treatment. Because contact and near-contact areas increased in some patients and decreased in others, there were no significant posttreatment changes. Negative correlations were found between contact and near-contact areas at T2 and changes in overjet from T2 to T3, and between contact and near-contact areas at T3 and changes in overbite from T2 to T3. No relationships were found between posterior contact and near-contact area and incisor irregularity. We concluded that (1) the area of actual and near contacts at or below 300 microm decreased significantly with treatment, indicating that, despite excellent treatment results by conventional standards, the proximity of posterior occlusal surfaces lessened; (2) the proximity of the posterior occlusal surfaces should not be expected to increase posttreatment; (3) posttreatment contact and near-contact areas may be factors in overbite and overjet stability; and (4) posttreatment contact and near-contact areas are not related to incisor irregularity.

Sex differences in mandibular movements during opening and closing.

This study evaluated the sex differences in maximum 3-dimensional opening and closing movements. The sample included 29 men (ages, 23-39 years) and 27 women (ages, 23-35 years), who were selected for normal Class I occlusion, temporomandibular function, and skeletal patterns. Condylar (hinge axis) translation and mandibular incisor movements, were recorded with an optoelectric jaw-tracking system; each participant performed 4 maximum opening/closing cycles. The results showed significant (P <.05) sex differences for incisor opening and closing movements, with most of the differences in the vertical component. Male incisor straight-line distances and curvilinear pathways averaged 52.1 mm and 54.8 mm, respectively. Female straight-line distances and curvilinear pathways averaged 46.0 mm and 48.1 mm, respectively. There were significant (P <.05) sex differences for condylar translation, with most of the differences in the anteroposterior component. Male condyles translated 15.4 to 17.6 mm (straight-line distances) and 20.5 to 20.7 mm (curvilinear pathways); female condyles translated 12.4 to 12.7 mm (straight-line distances) and 16.2 to 17.9 mm (curvilinear pathways). Mandibular length accounted for some of the sex difference in interincisal opening and for most of the sex differences in condylar translation. Closing movements showed the same pattern of sex differences as opening movements. Mandibular opening rotation was approximately 4 degrees larger in men than in women. The shapes of the condylar opening and closing pathways also differed significantly between men and women. For both sexes, condylar translation did not correlate with incisor opening or closing movements. It was concluded that (1) significant sex differences exist in incisor opening movements that are independent of mandibular size, (2) sex differences in condylar translation are dependent on mandibular size, (3) incisor opening movements should not be used as an indicator of condylar translation, and (4) sex differences in the shapes of the condylar pathways indicate sex differences in articular eminence morphologic features.

The effects of chewing rates on mandibular kinematics.

The purpose of this study is to provide basic understanding of how the speed of chewing affects masticatory jaw kinematics. Twenty-six healthy subjects (23.6 +/- 2.5 years of age) chewed a standardized bolus of gum at fast (100 cycles s-1), habitual and slow (50 cycles s-1) rates. The rates were controlled with a metronome and the order of rates was randomized for each subject. An optoelectrical system independently recorded head and jaw movement. Special computer programs identified representative cycles for each subject and computed various aspects of jaw movement. Multilevel statistical procedures were used to compare cycle variables among the three rates, estimate variability and model jaw movements. Maximum ranges of anteroposterior (AP), vertical and lateral jaw excursions were significantly less for the fast than the habitual or slow rates. While the shape of 3-D pathway was similar for the three rates, the perimeter of the pathway was significantly shorter for fast chewing cycles. Maximum AP, vertical, lateral and total 3-D jaw velocities were significantly different among the three rates. Between cycle variation in cycle duration and jaw excursion were least during fast chewing and the greatest during slow chewing; variability in maximum velocity was similar for the three rates.

The relationship between surgical changes in dentofacial morphology and changes in maximum bite force.

PURPOSE: This study determined whether patients with greater surgical changes, and presumably greater normalization of their skeletal morphology, showed greater increases in their maximum voluntary bite forces after orthognathic surgery. PATIENTS AND METHODS: A total of 104 adult patients (32 males and 72 females) treated with 1 of 8 different orthognathic surgical procedures were examined. Patients' presurgical and postsurgical morphologic and biomechanical measurements were taken from lateral cephalograms. Measurements known to be related to maximum bite force were used in the analysis. Patients' presurgical and postsurgical maximum bite forces were measured at 8 tooth positions (ie, right and left incisors, canines, premolars and molars). RESULTS: Presurgical and postsurgical morphology and biomechanics variables were strongly correlated with each other, suggesting that orthognathic surgery produced relatively little change in patients' overall craniofacial form. Maximum voluntary bite forces were primarily correlated with variables relating to jaw size-both before and after surgery. No correlations were noted between the increases in maximum voluntary bite forces and surgically produced changes in skeletal morphology and the biomechanics variables. CONCLUSIONS: Factors other than surgically produced changes in skeletal morphology are responsible for increases in maximum voluntary bite force after orthognathic surgery.

Bite forces after open or closed treatment of mandibular condylar process fractures.

PURPOSE: This study compared maximum voluntary bite forces in patients who received either open or closed treatment for fractures of the mandibular condylar process. PATIENTS AND METHODS: One hundred fifty-five patients (127 male, 28 female) with unilateral fractures of the mandibular condylar process (91 treated closed and 64 treated open) were included in this study. Maximum voluntary bite forces were measured at 6 weeks, 6 months, and 1, 2, and 3 years after fracture. At each trial, unilateral maximum voluntary bite force was measured at 4 different tooth positions bilaterally using a standard transducer. Electromyography (EMG) of the masseter muscles was also recorded during the bite force measurements, and ratios of the working/balancing side EMG were calculated. Analysis of the data was performed using standard statistical methods. RESULTS: The only significant difference between the 2 samples was in the level of fractures on the condylar process. No patients treated open had fractures of the "head" of the condylar process, whereas there were 11 in the group treated closed. No differences were observed in maximum voluntary bite forces between the 2 treatment groups at any time period, or were there correlations between bite force magnitude and location of the fracture, displacement of the fracture, or any other variable studied. Both groups showed a significant recovery of maximum bite force from the 6-week to the 6-month testing session. For both groups, working/balancing EMG ratios were significantly greater when subjects were biting on the side opposite the fracture. When biting on that side, the working/balancing EMG ratios were higher in the closed treatment group, but the difference did not reach significance. CONCLUSIONS: Maximum voluntary bite forces in patients treated for mandibular condylar process fractures do not differ significantly when treatment is open or closed. Neuromuscular adaptations to the fractured mandibular condylar process occur in both groups.

Incisor and mandibular condylar movements of young adult females during maximum protrusion and lateratrusion of the jaw.

This study evaluated the correlations between condylar translation and incisor movements during maximum protrusion and lateratrusion. The sample was 27 adult females (23--35 years old), selected for normal temporomandibular function, occlusion, and skeletal patterns. Condylar and mandibular central incisor movements [linear distances (LD) and curvilinear pathways (CP)] were recorded in three dimensions for 20 s with an optoelectric (Optotrak) jaw-tracking system while each participant performed multiple maximum protrusive and lateratrusive cycles. Masticatory analysis and multilevel statistical programs computed the three-dimensional movements of the incisors and condylar hinge axis during protrusion and lateratrusion. CP of the incisor point averaged 12.0 mm (9.3 mm LD) during protrusion, 13.0 mm (11.5 mm LD) during right excursion and 12.3 mm (11.0 mm LD) during left excursion. CP of the condyles averaged 11.9--12.9 (9.2--9.5 LD) mm during protrusion. During lateratrusion the contralateral condyles moved anteroinferiorly 11.6--14.1 mm (9.5--10.2 mm LD); the ipsilateral condyles moved posterolaterally 5.8-6.8 mm (2.3--2.5 mm LD). The left condyles demonstrated more movement than the right condyles during protrusion and than the contralateral condyles during laterotrusion. Relative variation, as measured by the coefficient of variation, was greater for the movements of the ipsilateral than contralateral condyles. Incisor movements were only moderately related to condylar movements between individuals and between replicates; LDs showed stronger correlations than CPs; and correlations were stronger for lateratrusion than protrusion. While incisor and condylar movements were not affected by repeated protrusion, incisor CP (approx. 0.2 mm/cycle) and LD (approx. 0.1 mm/cycle) increased significantly with repeated excursive movements to the left and right. It was concluded that (1) incisor protrusion and lateratrusion provide moderately reliable measures of condylar translation; (2) the linear distances that the incisors move during lateratrusion provide the best measure of contralateral condylar translation; and (3) condylar movements are not affected by repeated protrusion or lateratrusion.

Morphologic and biomechanical correlates with maximum bite forces in orthognathic surgery patients.

PURPOSE: The purpose of this study was to determine which factors of craniofacial morphology best predict maximum bite forces and jaw muscle strength (based on [electromyogram] EMG/force slopes) in patients selected for various orthognathic surgical procedures. These factors were then compared for their ability to separate orthognathic surgery patients by their clinical diagnosis. PATIENTS AND METHODS: Standard lateral cephalograms were taken of 121 orthognathic surgery patients (before treatment) and 80 control subjects to establish multivariate sagittal and biomechanical factors of craniofacial form. Maximum and submaximal bite forces were recorded at 8 tooth positions for each subject. EMG activity was recorded for 3 pairs of muscles (anterior temporalis, posterior temporalis, and superficial masseter) during the isometric bites. The EMG and bite force measurements were used to calculate EMG/force slopes as a measure of jaw muscle strength. The study looked for significant correlations between the morphologic factors and maximum bite force or jaw muscle strength. RESULTS: Factor analysis determined 12 sagittal and 6 biomechanical factors. However, only 3 of the sagittal and 4 of the biomechanical factors were significantly correlated with maximum bite force or jaw muscle strength. Factors reflecting jaw size were correlated with maximum bite forces and jaw muscle strength but generally did not separate patient groups. The factor most strongly correlated with maximum bite forces separated patients by their relative difference between anterior and posterior facial height. The factor for anteroposterior maxillomandibular discrepancies was not correlated with maximum bite force or jaw muscle strength. CONCLUSIONS: Many cephalometric measurements used to diagnose craniofacial deformities and to assign patients to particular orthognathic surgical procedures are not correlated with maximum bite forces or jaw muscle strength. Only measurements reflecting relative differences between anterior and posterior facial height are both strongly correlated with maximum bite force and reflect assignment of surgical procedures.