Hans Nägerl

Masticatory musculature under altered occlusal relationships--a model study with experimental animals.

In a study with experimental animals, the occlusal relationships of male Wistar rats were altered by the bilateral extraction of lateral teeth; the bolus function was eliminated by feeding a soft diet. Both steps led to relief of strain on the temporomandibular joint and thus also in the muscular system itself. The masticatory muscles adapted to the new experimental conditions. We observed a reduction in muscle dry weight and a shift in muscle fiber composition in favour of the IIb fibers, which indicates that less masticatory strength was required. Adaptation thus occurred equally on the macroscopic and microscopic levels.

Centers of rotation with transverse forces: An experimental study

A new noninvasive method for simultaneously applying a known force system and measuring tooth movement was developed and tested. The instruments proved to be highly accurate and reliable. Using a model of an upper canine with a simulated periodontal ligament, we located the center of rotation and the center of resistance and found that the product of two distances--a, from the center of resistance to the line of application of force (F) and, b, from the center of resistance to the center of rotation (Co)--yields a constant that represents the parallel transverse forces in the same plane. Knowing this constant enables the clinician to determine the appropriate force position for a given center of rotation. We also found that the occlusoapical position of the center of resistance varies somewhat, depending on the transverse direction of loading around the long axis. Differences in values of the constant were found also to depend on the direction of loading in a given plane. Thus a transverse force exerted in a mesiodistal direction at the level of an orthodontic bracket through the long axis produces different effects from those produced when an identical force is applied in a labiolingual direction.

Morphology of the Interphalangeal Joint Surface and Its Functional Relevance

PURPOSE To study and to clarify the curvature morphology of the articular surfaces of the proximal interphalangeal (PIP) joint and to relate joint morphology and joint kinematics. METHODS The radii and centers of curvature of 40 PIP joints were determined by sagittal and transverse intersections of highly precise replicas that were prepared by dental methods. RESULTS The PIP joint is proved to be a nonconforming joint: the articular surface of the proximal end of the middle phalanx has lesser curvatures than the condyles of the proximal phalanx. In intersections through the apex of the radial and ulnar condyles, the measured differences of the radii between the articular surfaces of the PIP joint were sagittally about 30% and transversely about 49% of the respective radii of the condyles. Incongruity of the joint results in 2 morphologically given axes for extension respective to flexion: (1) an axis given by the articular surfaces of both condyles of the proximal phalanx; and (2) a second axis given by the articular surface of the proximal end of the middle phalanx. Both articulating surfaces have 2 contact points in the transverse plane, one each, central to the apex of radial and ulnar condyles, respectively. In the middle of the joint, in the intercondylar groove, a small joint cavity was present in 37 of 40 joints. CONCLUSIONS The physiological incongruity of the 2 articular surfaces of the PIP joint was defined quantitatively. This allows the derivation of a theoretical model for PIP joint function that explains the kinematics and mechanical stability of the joint as well as the lubrication and nutrition of the cartilaginous structures.

How do spinal segments move

PURPOSE To study and clarify the kinematics of spinal segments following cyclic torques causing axial rotation (T(z) (t)), lateral-flexion (T(x) (t)), flexion/extension (T(y) (t)). METHODS A 6D--Measurement of location, alignment, and migration of the instantaneous helical axis (IHA) as a function of rotational angle in cervical, thoracic, and lumbar segments subjected to axially directed preloads. RESULTS IHA retained an almost constant alignment, but migrated along distinct centrodes. THORACIC SEGMENTS: IHA was almost parallel to T(z) (t), T(x) (t), or T(y) (t), stationary for T(x) (t) or T(y) (t), and migrating for T(z) (t) along dorsally opened bows. IHA locations hardly depended on the position or size of axial preload. LUMBAR SEGMENTS: IHA was also almost parallel to T(z) (t), T(x) (t), or T(y) (t). In axial rotation IHA-migration along wide, ventrally or dorsally bent bows depending on segmental flexional/extensional status. Distances covered: 20-60mm. In lateral-flexion: IHA-migration to the left/right joint and vice versa. In flexion/extension IHA-migration from the facets to the centre of the disc. CERVICAL SEGMENTS: In flexion/flexion IHA was almost stationary for and parallel to T(y) (t). In axial rotation or lateral-flexion IHA intersected T(z) (t)/T(x) (t) under approximately -30 degrees /+30 degrees. CONCLUSIONS Generally joints alternate in guidance. Lumbar segments: in axial rotation and lateral-flexion parametrical control of IHA-position and IHA-migration by axial preload position. Cervical segments: kinematical coupling between axial rotation and lateral-flexion. The IHA-migration guided by the joints should be taken into account in the design of non-fusion implants. FE-calculations of spinal mechanics and kinematics should be based on detailed data of curvature morphology of the articulating surfaces of the joint facets.

THE BIOMECHANICAL RELATION BETWEEN INCISAL AND CONDYLAR GUIDANCE IN MAN

The arrangement, form and movement of incisal and condylar guidance allow representation of the stomatognathic system by a gearing with positive drive and closed linkage. Because of problems of stability, this gearing has to be a throttle crank. Therefore, the stomatognathic elements have to fulfill the condition of Grashoff. The analysis of 14 Class I patient data confirms this condition and yields that their stomatognathic systems seem to be determined by only two parameters. The properties of these systems are largely independent of the jaws growth if the momentary growth takes place in the direction of the initial inclination of hinge-axis movement out of centric relation.

Biomechanics of the dental arch and incisal crowding.

Abstract.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.Zusammenfassung.Der Zahnbogen stellt im horizontalen Kontakt eine Gelenkkette dar, in der sich zwei konvexe Gelenkflächen berühren und die durch den dentogingivalen und dentoalveolären Faserapparat verspannt sind. Gelenke mit konvex-konvexen Flächen sind äquivalent zu gestreckten dimeren Gelenkketten, deren Kettenglied sich unter Kompression in einer mechanisch instabilen Position befindet. In Modellversuchen wird gezeigt, dass ein Gelenk-(Zahn-)Bogen erheblich formstabiler ist, wenn sich in den Gelenkkontakten eine konkave und eine konvexe Gelenkfläche berühren, da diese Gelenke äquivalent zu einer überschlagenen dimeren Gelenkkette sind, deren Kettenglied sich unter Kompression in einer stabilen Lage befindet.In den Abrasionsgebissen von Menschen der Frühgeschichte (Steinzeit, Eisenzeit ...) berühren sich selbst bis in den Frontzahnbereich hinein im horizontalen Schnitt solche konkav-konvexen Zahnoberflächen. Ein Stabilitätsgewinn für den Zahnbogen und eine Reduktion der Engstandproblematik kann erwartet werden, wenn in jedem horizontalen Zahnkontakt durch eine zarte approximale Schmelzreduktion morphologisch eine überschlagene dimere Kette hergestellt wird.

[A novel knee endoprosthesis with a physiological joint shape. Part 1: Biomechanical basics and tribological studies].

The natural tibiofemoral joint (TFJ) functions according to a roll-glide mechanism. In the stance phase (0-20 degrees flexion), the femur rolls backwards over the tibia plateau, while further flexion causes increased gliding. This kinematics is based on the principle of a quadruple joint. The four morphological axes of rotation are the midpoints of the curvatures of the medial and lateral femoral condyles and the medial and lateral tibia plateau. In addition, the medial and lateral compartments are shifted a few millimetres in a sagittal direction, the medial tibia plateau being concave and the lateral plateau convex. In most knee arthroplasties, these factors are not taken into account; instead they are equipped with symmetrical medial and lateral joint surfaces. Thereby, the midpoints of the curvatures of the sagittal contours of the lateral and medial joint surfaces, on the femoral as well as on the tibial sides, create a common axis of rotation which does not allow a physiological roll-glide mechanism. The goal of this study was therefore to report on the biomechanical basis of the natural knee and to describe the development of a novel knee endoprosthesis based on a mathematical model. The design of the structurally new knee joint endoprosthesis has, on the lateral side, a convex shape of the tibial joint surface in a sagittal cross section. Furthermore, from a mathematical point of view, this knee endoprosthesis possesses essential kinematic and static properties similar to those of a physiological TFJ. Within the framework of the authorization tests, the endoprosthesis was examined according to ISO/WC 14243 in a knee simulator. The abrasion rates were, thereby, lower than or at least as good as those for conventional endoprostheses. The presented data demonstrate a novel concept in knee arthroplasty, which still has to be clinically confirmed by long term results.

A caveat concerning center of resistance.

The center of resistance is a concept in theoretical orthodontics used to describe tooth movement under loads. It is commonly used to qualitatively predict tooth movement without recourse to complex equations or simulations. We start with a survey of the historical origin of the technical term. After this, the periodontal ligament is idealized as a linear elastic suspension. The mathematical formalism of vector and tensor calculus will clarify our reasoning. We show that a point such as the center of resistance basically only exists in two dimensions or in very special symmetric spatial configurations. In three dimensions, a simple counterexample of a suspension without a center of resistance is given. A second more tooth-like example illustrates the magnitude of the effects in question in dentistry. In conclusion, the center of resistance should be replaced by a newer and wider mathematical concept, the “center of elasticity,” together with a limiting parameter, the “radius of resistance.”

Quantified Contours of Curvature in Female Index, Middle, Ring, and Small Metacarpophalangeal Joints

PURPOSE To study and quantify the morphology of the curvature of the surfaces of metacarpophalangeal metacarpophalangeal joints and to relate joint morphology to joint function. METHODS Forty metacarpophalangeal joints of the index, middle, ring, and small fingers from 5 right and 5 left hands were taken from female cadavers. The articulating surfaces of the metacarpal head and the base of the proximal phalanx were copied in a true-to-scale fashion. The hard plaster models were sliced in 7 sagittal and 7 transverse planes. The curvatures of the section contours were determined with circular gauges. Statistical analyses were performed by analysis of variance and paired Student t-tests. RESULTS In the sagittal plane, the cartilaginous surface of the metacarpal head is divided into 2 functional regions and a third dorsal region that does not articulate with the base of the proximal phalanx. The articulating surface of the base of the proximal phalanx approximates a circle in the midsagittal plane. The mean median sagittal radius of curvature of the dorsal articulating aspect of the metacarpal head (6.9 mm) is 33% smaller than that of the base of the proximal phalanx (10.3 mm). The palmar articulating aspect of the metacarpal head (5.8 mm) is 44% smaller than that of the base of the proximal phalanx (10.3 mm). In the median transverse section, the mean radius of curvature of the metacarpal head (7.3 mm) is 18% smaller than that of the base of the proximal phalanx (8.9 mm). CONCLUSIONS The data demonstrate the highly significant incongruity in the curvature of the articulating pair. This incongruity provides a joint space with its greatest dimension in the sagittal plane. From a mechanical perspective, the metacarpophalangeal joint mechanically represents a joint with 5 kinematic degrees of freedom: 2 for flexion and extension, 2 for abduction and adduction, and 1 for axial rotation.

The retention catalogue: an instrument for quality management.

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.ZusammenfassungZiel und Material:Die klinische Anwendung des Retentionskataloges wurde mit Hilfe eines Untersuchungsbogens evaluiert.Ergebnisse:Die statistische Auswertung des Untersuchungsbogens zeigte: Mit dem vorgestellten Retentionskatalog (RK) ist für den Kieferorthopäden eine effektive Patientenaufklärung über die Ursachen und die Vermeidung von Engstandrezidiven in der täglichen Praxis des Kieferorthopäden gegeben.Schlussfolgerungen:Eine routinemäßige Verwendung des Retentionskataloges ist zu empfehlen. Er stellt neben dem Qualitätsbeitrag im Retentionsmanagement einen forensischen Schutz für den Behandler dar.