Accuracy of fit of 3-to-3 retainers after adhesive fixation using a neodymium-iron-boron magnet chain.

BACKGROUND AND OBJECTIVE: Six-base retainers are often used to prevent tertiary crowding. To minimize shear stress on the retainer, these should be fitted as precisely as possible. The aim of this study was to compare the accuracy of fit of 6-base retainers after adhesive fixation using a neodymium-iron-boron (NdFeB) magnet chain or a resin positioning aid. MATERIALS AND METHODS: A 6-base retainer was prepared for 40 pseudo-anonymous lower jaw models (2 × 20). Temporary fixation was performed with a resin positioning aid or a NdFeB magnet chain. The adhesive fixation of the retainers was randomized. The area and vertical distance between wire and teeth were determined by histomorphometry. The results were evaluated by ANOVA. The probability of error was specified as 5%. RESULTS: With the magnetic chain method, the area was significantly smaller than with the positioning aid (p = 0.0125). The difference between the two methods was ca. 1.3 mm². Moreover, the measured distance was affected significantly by the method used (p < 0.0002). With the magnet chain, the distance between the tooth and retainer was reduced by a mean of 0.05 mm. Conclusion. The fit of the retainer wire improves somewhat when the NdFeB magnet chain is used even though the wire is not as precisely positioned as it would be with a custom-made positioning aid.

Variations in cyclic mandibular movements during treatment of Class II malocclusions with removable functional appliances.

The aim of the study was to establish whether juveniles with a Class II malocclusion change the neuromuscular control of mandibular movements during the course of orthodontic treatment with removable functional appliances (RFAs). Neuromuscular control can be indirectly evaluated by recording cyclic planar mandibular movements which were freely carried out by the patients (28 girls, 14 boys, aged 11.1 ± 1.1 years at the start of treatment) and measured with an ultrasonic device before, during, and after Class II functional appliance therapy, with either an activator or a bite jumping plate. The cyclic movements represented simultaneous rotations of the mandible around a maxillary and mandibular fixed axis (MFHA) and could be characterized by µ(α)-diagrams (µ = swing angle of MFHA, α = mouth opening angle) and path length (L) of the MFHA. The µ(α)-diagrams clearly divided into four parts: movement representing protrusion, mouth opening, and two parts of backward closing as known from Posselt diagrams. Parameters from the Posselt and µ(α)-diagrams were checked by one-factor analysis of variance on a 5 per cent significance level for group dependency. For one-third of the patients investigated, no significant changes were seen in any parameter pre- or post-therapy. However, patients showing an initially large mouth opening capacity or a very short condylar path changed their neuromuscular control to that of Class I subjects. Analysis of µ(α)-diagrams provides the possibility of assessing changes in the neuromuscular control of the mandible during Class II treatment.

Morphological structures and protrusive cranial border guidance of the temporomandibular joint of Cercopithecus mona.

Morphological parameters of the temporomandibular joint (TMJ) of Cercopithecus mona were analyzed by sagittal medial/lateral slicing of the entire joint. The slice contours of the osseous structures of the joint surfaces were approximated by circles. In this manner, the main parameter of the protrusive cranial border guidance, the protrusive dimeric Link chain (DLC), could be measured. In each joint, all slices yielded protrusive DLCs which were nearly parallel to each other. In medial/lateral direction all parts of the joints participate in force transmission in initial protrusive cranial border function.

Aspects of morphology and guidance of the human temporomandibular joint.

Examinations of the curvature morphology of the temporomandibular joints (TMJs) in macerated human skulls yielded that in initial protrusive cranial border motion, parts of the condylar articulating surfaces are only functional under force transmission. These areas were found on the lateral-central side of the condyle. In contrast to the Cercopithecus mona, a monkey species, the human TMJ apparently possesses a distinctly higher spatial performance range.

The mandibularly fixed hinge axis--investigation of patients with sound and pathological jaws.

In former works, we had proved that test persons with sound temporomandibular joints (TMJs) used a mandibularly fixed hinge axis (MFHA) and were able to pilot the mandible by solely two kinematical degrees of freedom. We wondered if we could evaluate the MFHA the same way for patients who had problems with their TMJs. Actually, the MFHA could be determined likewise. The results could provide information on the reason for the distortion of the movement of the TMJs, which cannot be yielded by X-ray radiographs.

The movement structure of the mandible and alignment of the neck.

The motion patterns of mandibular points were recorded in vivo in closed free movements of the mandible in the sagittal-vertical plane. The points ran along closed loops, which were evaluated by their area and length. All points whose loops showed areas of the same size regarding the sense of circulation formed straight lines. When the absolute area of the loops was taken into account, a valley with two minima was found in the function "absolute area versus position of the point", the point which showed the deepest minimum tallied with the position of the neuromuscular mandibular rotation axis. The points with loop lengths of same size formed elliptical lines, the perimeter of which was minimal for a point below the condyle. Morphological relations: the row of teeth in the upper jaw was found to be located below the line of minimal path lengths on the straight lines with constant areas, and the cervical spine was found to be arranged along the valley of the minimal absolute areas where the path lengths have their maximum.

Functional state of the mandible and rolling-gliding characteristics in the TMJ.

For the physiological intact stomatognathic system, the three main functional states (occlusat articular functions, free mandibular movements, and ideal bolus function) were biomechanically discussed concerning the structure of movement, rolling-gliding characteristics, and force transfer in the temporomandibular joint (TMJ). In all three cases, rolling is not possible in the TMJ since the instantaneous rotational axis is positioned outside of the joint-rolling is not necessary because the TMJ is not loaded by appreciable forces. In the aged stomatognathic system with a lost discus and considerable Loads in the TMJ, however, the attrition of the joint is eased by rolling movement at the articulating surfaces. The destruction of the discus can be seen as a physiological adaptation which brings back the joint to an original odontogen condition.

Biomechanical aspects of mandibular growth.

The subject of this study was to analyse how functional parameters of stomatognathic systems are influenced by growth. For this purpose, two cephalometric radiographs of 65 patients with class-II-relation treated with functional appliances were superimposed on the occlusal plane. The two patient groups consisted of 32 open bite and 33 deep bite cases. The direction of the condylar growth significantly differed for both cases. Nevertheless the hypothesis could be confirmed that the original functional structure was hardly affected by growth.

The influence of functional orthodontics and mandibular sagittal split advancement osteotomy on dental and skeletal variables--a comparative cephalometric study.

Lateral head films of 200 Class II patients (106 females, 94 males) with a mean pre-treatment age range of 9.9-10.25 years successfully treated with functional orthodontics were analysed before (T1) and after (T2) treatment. The resulting data and findings were compared with lateral head films (T1, T2) of 20 patients (15 females, five males) with a mean pre-treatment age of 25.75 years whose Class II malocclusion and antero-posterior jaw discrepancy had been corrected by a mandibular sagittal split advancement osteotomy. The median and interquartile distances were calculated for every variable, at T1 and T2. The difference between the medians (T2-T1) was analysed using a signed rank test. The changes in scattering (T2-T1) were assessed by means of a F-test. Significant differences regarding the therapeutic influence on skeletal [ANB, Wits, Index, mandibular line-nasal line (ML-NL)], functional (beta', mu), and dental (1-NA degrees , 1-NB degrees ) variables were found. In the group initially treated with functional appliances in order to enhance mandibular prognathism, the antero-posterior (A-P) jaw discrepancy was reduced (ANB, Wits). The vertical skeletal pattern (Index) changed towards a more skeletal open relationship, whereas the ML-NL angle was reduced, which indicates a deepening of the bite. The comparison between biomechanical incisor position analysis (beta', mu) and dental variables (1-NA degrees , 1-NB degrees ) revealed different changes in incisor inclination depending on the type of analysis used. The findings for the dental variables (1-NA degrees , 1-NB degrees ) showed a protrusion of both upper and lower incisors after therapy. The results for the functional variables (beta', mu) showed a retrusion of the upper and a protrusion of the lower incisors. This change in incisor inclination is a dental compensation of the remaining sagittal jaw discrepancy. This effect is most clearly reflected by the functional analysis and the changes of the biomechanical variables beta' and mu. For the orthognathic surgery group, a clear improvement in the dental and skeletal relationship was observed: the skeletal discrepancies in the A-P plane were completely corrected (ANB, Wits) and the inclination of the incisors according to biomechanical and functional aspects was optimized (beta', mu).The alteration in both the Index and ML-NL angle in this group indicated an increase of the open bite components.

The problems of borderline cases: an aid for the analysis of orthodontic therapies.

Our aim in this analysis was to create a conceptual aid for clinicians for use in dealing with and understanding the problems of borderline cases in general, and in making it easier to make medical and orthodontic treatment decisions. The doctor and patient now have a range of measures available for making a decision.

Biomechanical features of the periodontium: an experimental pilot study in vivo.

INTRODUCTION: The orthodontic application of a force or moment leads to a strain distribution in the periodontal ligament. The corresponding local deformation at the interface periodontal ligament-alveolar bone causes a distribution of normal and shearing tension that is thought to be the local stimulus for bone remodeling leading to orthodontic tooth movement. Although researchers have examined initial tooth movement and the biomechanical properties of the periodontal ligament, few have reported human, in-vivo studies. For such examinations, very small tooth displacements must be measured with extremely high resolution and accuracy. METHODS: By a highly sensitive goniometer (resolution <0.001 degrees ), rotations of premolars in 22 subjects with natural spacing and 14 subjects with pronounced anterior crowding were measured in vivo. The rotations followed sets of pure moment-time functions starting at zero and reaching specified final values within +/- 1.43 Ncm. RESULTS: Usually, thresholds were seen that had to be overcome by moment to produce measurable rotations of the tooth in the mesial or distal direction. The subjects with pronounced anterior crowding showed distinct asymmetry: Thresholds for rotations to the distal direction were significantly higher than for those to the mesial direction. CONCLUSIONS: The cases of symmetric thresholds indicated a shear thinning viscosity in the periodontium under increasing moment. In the case of asymmetric thresholds, the findings suggested a mesially directed pretension of the gingival fiber apparatus that might be connected with the mesial drift.

The retention catalogue: an instrument for quality management.

UNLABELLED: GOAL AND MATERIAL: We evaluated the clinical use of the "retention catalogue" employing a standardized questionnaire. RESULTS: Statistical evaluation of the standardized questionnaire demonstrated that the retention catalogue (RC) is an effective method in daily practice for orthodontists to convey patient information on the causes and prevention of relapses in crowding. CONCLUSIONS: Routine utilization of the retention catalogue is recommended. It both contributes to successful retention management and offers the orthodontist a certain degree of legal protection. CONCLUSIONS: Routine utilization of the retention catalogue is recommended. It both contributes to successful retention management and offers the orthodontist a certain degree of legal protection.

Biomechanics of the dental arch and incisal crowding.

During horizontal contact, the dental arch represents a link chain in which two convex articular surfaces are in contact and are tensioned by the dentogingival and dentoalveolar fibrous tissue. Joints composed of convex-convex surfaces are equivalent to stretched dimeric link chains whose links are in a mechanically unstable position under compression. Experiments on plaster models show that the dimensional stability of an articulated (dental) arch is considerably increased when a concave and a convex articular surface are in contact, as these joints are equivalent to an overlapping dimeric link chain whose links are in a mechanically stable position when under compression. In the abraded denture of Stone Age man, horizontal interdental contacts of transversally concave-convex dental surfaces may be found extending even into the incisal region. Increased stability of the dental arch and reduced problems of incisal crowding are to be expected if an overlapping dimeric chain is produced morphologically in each horizontal contact by means of slight interproximal enamel reduction.

An experimental method for in vivo analysis of biomechanical asymmetries of the periodontium.

For analysis of the initial tooth movement in vivo, an optical measuring apparatus with a resolution of 0.001 degree for rotation or 0.5 micron for translation respectively has been developed. Its design is described in detail. Because of its high sensitivity it is experimentally feasible to investigate the biomechanical properties of the periodontium under force application and to determine if it makes any difference whether a tooth is rotated mesially or distally out of its neutral position. For that purpose moments with defined time responses are buccally applied to premolars by means of a cardan shaft and the corresponding rotational angle-time characteristics are recorded. As expected, first measurements show sign symmetry for widely spaced teeth. In the case of pronounced anterior crowding, however, asymmetry seems to occur in such a manner that the force system has to overcome a threshold in order to rotate the tooth distally. Additionally, the periodontium may be subject to a sign-dependent rigidity. In all cases, the movement is limited to some degree.

Curvature morphology of the mandibular dentition and the development of concave-convex vertical stripping instruments.

Radii for concave-convex vertical stripping instruments can be derived from measurements of the natural curvature morphology in the horizontal contact area of the mandibular dentition. The concave-convex adjustment of contacts in the anterior dental arch with a newly developed set of concave-convex stripping instruments should enable orthodontic crowding problems to be alleviated biomechanically.