Changes in mandibular growth direction during and after cervical headgear treatment.

Cervical headgear is widely used to modify sagittal growth in growing patients. However, cervical headgear can affect vertical growth as well. The purpose of this retrospective cephalometric study was to examine mandibular growth changes in 30 growing Class II patients treated with cervical headgear and full edgewise appliances and to compare those changes with the changes occurring in 26 untreated controls. Cephalometric data were obtained from lateral cephalometric radiographs representing pretreatment, posttreatment, and postretention for each patient and from chronologically comparable radiographs for the control subjects. During the 4-year treatment or observation period, the mean mandibular rotation was 0.25 degrees in the treatment group and -1.7 degrees in the control group. After an average posttreatment period of 6 years 5 months, the mandible had rotated -1.5 degrees in the treatment group and -0.7 degrees in the control group. Mean changes in the y-axis angle and the mandibular plane angle during retention were not different between the 2 groups. Mandibular rotation during retention did not show any significant inverse correlation with mandibular rotation during treatment. These findings suggest that mandibular rotation during retention reflects the inherent growth pattern of an individual that is reasserted after treatment, rather than rebound. There were no significant differences in the size of the mandible between the 2 groups during the study period. None of the variables reflecting pretreatment morphology of the face had a clinically significant bearing on mandibular rotation. Vertical changes in the maxillary and mandibular molars showed no significant correlation with mandibular rotation, which suggests that the changes in vertical dimension of the dentition are not a major determinant of the rotational change of the mandible.

Opening rotations of the mandible during and after treatment.

In this retrospective cephalometric study of mandibular rotational change, we studied 60 cases. All patients were treated with full fixed appliances and exhibited an opening rotation of at least 1.5 degrees during treatment, as measured on the basis of the angle of the Y-axis to the sella-nasion line (SN). Our objective was to determine whether this opening rotation was sustained during retention. Paired t tests were used to test the hypothesis that the treatment change or opening rotation was stable and that these patients did not return to their original mandible-to-cranium relationship in the posttreatment period. Stepwise regression analysis was used to determine which (if any) changes in the independent variables during treatment could predict the subsequent behavior of the angle of the mandibular plane to the SN and the angle of the Y-axis to the SN during retention. During treatment, the mean increase in the angle of the Y-axis to the SN was 2.43 degrees. After an average posttreatment period of 54 months, this angle was reduced on average by only 0.73 degrees. Stepwise linear-regression analysis indicated that none of the treatment changes seen in the independent variables strongly predicted the ensuing closing rotation seen during retention. The correlation coefficient between the Y-axis angle and the mandibular-plane angle during treatment was 0.67. Mandibular opening rotations as a consequence of orthodontic treatment do not invariably return to the pretreatment value, and their negative effects--although sometimes small--cannot be discounted. Because the preponderant evidence of a closing rotation occurs in the terminal pubertal growth stages, the net effect may be even more significant.

Changes concurrent with orthodontic treatment when maxillary expansion is a primary goal.

A retrospective study of dental and maxillary skeletal changes occurring during a period of orthodontic treatment was made from pretreatment and posttreatment dental casts. Sixty maxillary expansion cases were examined. Thirty cases had maxillary expansion accomplished with a fixed rapid palatal expander and 30 were expanded with a quadhelix appliance. All cases were finished with full fixed edgewise appliances. Multiple linear regression analyses were completed for both groups with upper molar width change as the criterion and age, tipping of the upper molars, palatal width change and maxillary tipping as the predictors. All predictors were included in the analysis for the quadhelix group with a significant R2 value of 0.55. For the rapid expansion group, a significant R2 value of 0.33 was achieved with the inclusion of palatal width change and age only. The other variables did not meet the level of significance for entry into the model. Although both groups demonstrated similar amounts of maxillary dental expansion, the rapid expansion group demonstrated greater average skeletal expansion. In addition, there was a significant relationship between skeletal and dental expansion for the rapid expansion group, but not the quadhelix group. Palatal depth increased more on average in the rapid expansion group suggesting that there was greater dental eruption in that group. Expansion across the mandibular molars was greater on average in the quadhelix group. There was no difference in the degree of upper molar rotation or final upper and lower arch forms between the two groups.

Bone growth and periosteal migration control masseter muscle orientation in pigs (Sus scrofa).

During growth the muscles of mastication alter their lines of action. Research on long bones indicates that the apparent migration of muscle attachments is due to the movement of the periosteum relative to the underlying bone. To assess whether the pig masseter muscle follows the periosteum during growth, implants of titanium granules in a gelatin matrix were placed simultaneously in various parts of the masseter muscle and its periosteal and bony attachments. Growth movements of these tissues were followed radiographically for 2 months. Granule position was verified histologically. Periosteal movement was the dominant growth process at the insertion of the masseter. All implants migrated caudally relative to the mandible. However, a strong position effect was seen dorsoventrally: implants placed high in the ascending ramus migrated dorsally as well as caudally; low implants migrated only caudally. This differential migration, ascribed to the influence of the condyle, accounts for the increasing horizontal orientation of dorsal fibers. A similar differential was seen along the rostrocaudal axis of the ramus. In contrast to the insertion, the origin of the masseter from the zygomatic arch shows no periosteal movement. Rather, the entire bone-muscle complex becomes displaced by sutural growth, leading to increasing vertical orientation of the masseter. Thus two different aspects of skull growth are responsible for the change in muscle anatomy.

Estimation of tongue volume from magnetic resonance imaging.

Magnetic resonance imaging was used to estimate the volume of the tongue, oropharynx, and oral cavity in 19 adults. Each subject was imaged and had volume estimations made from the coronal and sagittal orientations. Volume was found by measuring area from a series of images and then multiplying by the thickness of each slice and the gap between each slice in the series. Mean tongue volumes of 71.2cc (coronal) and 79.3cc (sagittal) were found. The estimated volumes were found to be reproducible and each orientation was equally good for defining the anatomy of the tongue and oropharynx. Tongue volumes were found to correlate well with subject body weight, r = 0.86 for the coronal and r = 0.82 for the sagittal orientations. To test the reliability of this technique, tongue volume was estimated for ten New Zealand white rabbits by the same method. The rabbit tongues were then removed and their actual volumes were determined. The estimated tongue volumes from imaging were found to compare closely to the actual volumes but, on average, slightly underestimated actual size. When converted to a percentage, 95% confidence intervals for the estimation of rabbit tongue volume by MRI are -4.3 +/- 25.9% for the coronal and -5.9 +/- 16.5% for the sagittal.

Videofluorographic analysis of tongue movement in the rabbit (Oryctolagus cuniculus).

The movement of the entire tongue and intermolar eminence during mastication is described in the domestic rabbit (Oryctolagus cuniculus). Tongue movement and jaw position were analyzed videofluorographically from separate lateral and dorso-ventral views in six rabbits. Metallic markers were inserted into the tongue so that its movement was visible on the fluorographic image. Frame-by-frame analysis of the videofluorographic tape recordings demonstrates that tongue movement in all animals was identical in direction during each part of the chewing cycle. In the lateral view the forepart of the tongue moves down and forward during the opening stroke, whereas the intermolar eminence moves up and forward to appose the palate. During the closing stroke, as the tip of the tongue moves up and back, the intermolar eminence lowers from the palate and retracts. During the power stroke the forepart of the tongue is at its most elevated and retruded position, while the intermolar eminence is its lowest and most retruded. The dorso-ventral view showed that lateral movement of the tongue and mandible are highly synchronous. The intermolar eminence decreases in width during the power stroke, possibly twisting to place or keep food on the teeth. An anterior to posterior undulating movement of the entire tongue occurs throughout the chewing cycle. As the intermolar eminence elevates to appose the palate during the opening stroke, it may replace the bolus on the teeth on the chewing side. The intermolar eminence also appears to be twisting during the closing and power strokes to place or maintain food on the teeth.

Timing and character of reciprocal temporomandibular joint sounds in an asymptomatic orthodontic sample.

The timing and character of opening and closing temporomandibular joint sounds were evaluated in a sample of 33 asymptomatic subjects with reciprocal clicking. Using stereophonic headphones and videotape recordings, three consecutive reciprocal sounds were recorded and analyzed. The timing of the three consecutive opening and closing sounds was consistent in the majority of subjects, and occurred generally on late opening and middle to late closing. When the timing, character (frequency, amplitude, and duration), and wave patterns were evaluated, a notable amount of variation was observed in the overall sample. This suggested a variety of intra-articular reasons for the reciprocal sounds in this particular sample of subjects.

Muscle spindles in the digastric muscle of the rabbit.

The digastric muscle of the rabbit was studied for the presence of muscle spindles. One of the 19 adult digastric muscles we examined contained in excess of 12 spindles scattered throughout the belly. No spindles were found in the contralateral muscle, or in any of the digastric muscles from other animals. Although jaw depressor muscles of most mammals contain few, if any, muscle spindles, their occasional presence suggests that these muscles have a potential for spindle formation.

Timing of temporomandibular joint sounds in orthodontic patients.

The consistency of occurrence and also the timing of TMJ sounds during jaw opening and closing were studied by means of an audio-visual sound recording system in an attempt to address the possible causes of temporomandibular joint (TMJ) sounds. From a group of 347 orthodontic patients, 104 were found to have medium- or high-amplitude TMJ sounds during jaw opening or closing. Most patients (53%) had reciprocal clicking--that is, a single sound on opening and on closing; another 12% had multiple sounds on opening or closing; 22% had a single closing sound; and 13% had a single opening sound. Sounds occurred at all degrees of jaw opening throughout this sample, but in most patients opening sounds tended to be closer to maximum opening, whereas closing sounds tended to occur in the middle of the closing movement. No statistically significant association was found between the timing of the opening and closing sounds. In 42.3% of patients, the sound was inconsistent in its occurrence on successive opening and closing cycles. Twenty-three percent of patients reported pain, jaw locking, or limitation of movement, but these were not associated with the timing of the opening sound. The findings suggest that the reciprocal click, widely associated with anterior disc displacement with reduction, was relatively common, but that other explanations for the joint sounds should also be considered. Conversely, a large variation may exist in the timing and the occurrence of sounds in patients with anterior disc displacement in the absence of pain and limitation of movement.

The effects of tenotomy on the morphology of the rabbit digastric muscle.

The digastric muscles of 17 New Zealand White rabbits were subjected to tenotomy. A sham operation was performed on six animals. Groups of three or four animals were killed at one hour, one day, 10 days, 30 days, and 60 days after the tenotomy. There was evidence of tendon regeneration by 10 days, and by 30 days the tendon appeared normal macroscopically. The tendon was normal in microscopic appearance in the animals examined at 60 days. Muscle belly length and fascicle length decreased after the tenotomy, and the angle of pinnation increased. Sarcomere lengths underwent a transient decrease within one hour of the tenotomy, but then were as long as or longer than those in the sham-operated group. A biomechanical analysis suggests that the net result of the morphological changes produced by the tenotomy leads to a reduction in force capability of only about 12%. The shorter belly and fascicle lengths, however, may diminish the effective range over which effective force can be exerted.

The force-velocity relation of the rabbit digastric muscle.

In 30 animals, the digastric was made to pull actively against a slide loaded by a servo-controlled linear motor. Force and velocity were recorded at the end of active shortening to the in-situ (jaw-closed) muscle length. Passive and active force-length relations were also determined in 17 of the rabbits. The empirical force-velocity data were fitted to a hyperbolic equation. The average speed of muscle shortening at zero load was 14.67 cm/s. Mean maximum isometric force at in-situ length (P0) was 1267 g, and the mean ratio a/P0 was 0.18. The average time-to-peak twitch tension was 31.8 ms under isometric conditions. In-situ muscle-belly length was about 3 per cent less than optimum length for isometric force. Maximum muscle force was positively correlated with animal size, but maximum velocity showed no relation to force or length. The estimated maximum speed of sarcomere shortening was 26 micron/s, which is slightly slower than in fast limb muscles of the cat, and may indicate the presence of both histochemical type I and II fibres. The isometric force after shortening had ceased was less than P0, and was correlated with the velocity during shortening. This depression of isometric force may result from an alteration of the excitation-coupling system during activation. These observations suggest a role for the digastric in the rapid acceleration and deceleration of the mandible near the jaw-closed position during opening and closing.

The effects of digastric muscle tenotomy on jaw opening in the rabbit.

The digastric and geniohyoid muscles of the rabbit both produce jaw-opening torque. Anatomic and biomechanical analysis, and electromyography of normal chewing, are not wholly adequate in determining the roles of these two synergists. Cinematographic and electromyographic records of pellet and carrot chewing were obtained before and after tenotomy of both digastric muscles. After tenotomy, jaw opening occurred more slowly and maximum gape was reduced for both foods. However, the overall frequency of chewing was unchanged, and the jaw muscles did not change their contraction patterns. Changes in opening speed and amount of gape result from loss of functional digastric muscles, not fully compensated for by the synergistic geniohyoids. The changes in opening speed and maximum gape are consistent with a biomechanical analysis which predicts a maximal contribution to jaw-opening torque by the geniohyoid muscle of about 25 per cent at the start of opening, and a substantial reduction of this torque in the course of the opening movement.

The influence of periosteum on tendon and ligament migration.

The continuity of the periosteum of the hindlimb of growing rabbits was disturbed by severing it. The femoral insertion of the pectineus muscle and the tibial attachment of the medial collateral ligament of the knee joint were examined 3-4 weeks after the operation. It was hypothesised that the cut periosteum would not shift with respect to the underlying bone in the normal manner, and that the migration of the tendon and ligament attachments would be reduced. The pectineus tendon reversed its direction of migration in 3 animals, showed histological evidence of modified migration in 8 animals, and no change was discerned in 4 animals. The medial collateral ligament in the operated limb migrated only half as much as that on the sham operated, control limb. The results are consistent with the hypothesis that traction of the periosteum in growing limb bones is responsible for the migration of these tendon and ligament attachments.

Temporomandibular joint sounds related to orthodontic therapy.

A cross-sectional survey for temporomandibular joint (TMJ) sounds was conducted on 347 orthodontic patients before, during, and after treatment. Those patients who reported joint sounds, or in whom sounds were noted on clinical examination, were subjected to an audiovisual evaluation which was recorded on videotape to identify more precisely the character of the sounds during jaw opening and closing. TMJ sounds were quite common before, during, and after orthodontic treatment. There was a significant association among three variables: joint sounds, age, and treatment. It is not clear, however, whether joint sounds increased due to orthodontic treatment, age, or both. No significant associations were found between TMJ sounds and functional occlusal factors. Significantly more sounds were noted by the examiners than were reported by the patients. Medium or high amplitude sounds were evident in 32.6% of the 135 subjects who underwent the audiovisual examination.

Clinical measurement of distally directed headgear loading.

A method was developed to measure the distally directed load that is delivered to molar teeth by orthodontic headgear. Measurements on a three-dimensional model and on four patients showed that the intraoral load at the maxillary molar teeth was generally the same as or greater than the measured extraoral force. Higher loadings on the molar teeth were thought to be caused by lever arms present in the headgear assemblies. Both the power arm face-bow and the symmetrical face-bow were effective in delivering an unbalanced load when one arm was bent well outward. The unilateral effect was decreased as the extraoral force was increased. Bending of the face-bow by the stronger extraoral forces probably was responsible for the decreased unilateral effect. The effect of lever arms in the headgear assembly on the load delivered to the teeth warrants further study.

Active length-tension relation and the effect of muscle pinnation on fiber lengthening.

The active length-tension relation was determined for the left digastric muscle of seven New Zealand White rabbits anesthetized with pentobarbital. Measurements of muscle length and fiber architecture were made from photographs of resting and actively contracting muscle. There was a marked difference between length-tension curves based upon resting as compared to active muscle length. The active length-tension relation had a longer descending limb than ascending limb, whereas the length-tension relation based on passive muscle length tended to be symmetrical around optimum length. On the average, muscle fibers lengthened 0.77 mm for each 1 mm of extension of the muscle belly. Since the rabbit digastric muscle is unipinnate, this suggests that pinnation serves to enhance the range of muscle excursion in this muscle.

Change of digastric muscle length in feeding rabbits.

An experiment was undertaken to measure directly the changing length of a jaw muscle during feeding in four intact, unanesthetized New Zealand White rabbits. Metal markers were implanted to define the anterior and posterior ends of the single belly of the digastric muscle and fluoroscopic images were recorded on videotape while the animals fed on pelleted chow and carrot. Graphs of muscle length versus incisor separation were obtained by making measurements of single frames of the videotape record. The graphs revealed that when pelleted chow was being chewed the length of the digastric muscle changed by no more than 9% of its greatest length; during the latter part of the closing stroke it changed very little. Incising and chewing carrot caused the digastric muscle to change in length continuously throughout the chewing cycle; incising carrot resulted in a 13% change in the length of the digastric muscle. The velocity of shortening is slightly less than one muscle length per second.

Muscle sarcomere length following passive jaw opening in the rabbit.

Sarcomere lengths were measured microscopically in formalin-fixed jaw muscles of 14 rabbits divided into two groups; jaw open, and jaw closed. The measurements were compared by means of a nested analysis of variance. The sarcomeres of the jaw open group were longer in the masseter and temporalis muscles (jaw elevators) and shorter in the digastric muscle (jaw depressor) than were those of the jaw closed group. In the jaw closed position, sarcomeres in the deep portion of the masseter muscle become markedly shorter than those in the superficial part of the muscle. The values for sarcomere length in the masseter muscle of the jaw open group and the digastric of the jaw closed group are near the top of the ascending limb of isometric length-tension relation for the rabbit digastric muscle.