A laboratory investigation of the role of guide planes in the retention of cast cobalt-chromium alloy partial denture frameworks.

OBJECTIVES: This in vitro study examined the resistance to displacement offered by guide planes and the displacement mechanics of a bilateral bounded saddle cast cobalt chrome alloy removable partial denture framework under conditions simulating an average initial fit. METHODS: A specially designed model mouth jig fitted with strain gauges to monitor the forces across the guiding plane/guiding surface interfaces was used. Guiding planes of enamel, silver tin amalgam and dental composite with seven combinations of guide planes and three angles of withdrawal were examined. Guiding plane size was also examined for dental enamel. RESULTS: A statistically significant difference in retention was found for all factors examined. Retention increased with increasing angle of withdrawal. Size effects were found but considered unreliable. CONCLUSIONS: It was concluded that with a tight fit guiding plane retention may be readily predicted for any given material, combination of mesial, distal and lingual guiding planes and angulation conditions by the application of mechanical principles providing the basic parameters are known. Attention is drawn to the reasons for further work to fully understand the clinical situation.

A simplified method of attachment for the quadhelix and transpalatal arch.

A modification to the standard quadhelix transpalatal expansion arch is presented in which the wire passes distal to the most posterior molar and enters the mesial end of the headgear tube. Laboratory measurements were carried out on small, average and large sized appliances with this modification using a special rig fitted with strain gauges. For the three sizes of appliance, the modified quadhelix is more flexible than the standard quadhelix for both displacement and rotation of the molar arms. Also it is less time consuming to fit and remove it is concluded that this appliance is capable of producing clinically useful forces and couples with increased precision.

The distortion of cast cobalt-chromium alloy partial denture frameworks fitted to a working cast.

PURPOSE: This in vitro study examined the distortion of 10 identically designed cast cobalt-chromium alloy frameworks, individually constructed, and adjusted by 10 technicians to fit one of 10 replicated stone working casts. MATERIAL AND METHODS: The distortion was measured with a specially developed strain gauge apparatus when the frameworks were seated on the maxillary metal analog, the stone working cast, or a stone duplicate cast. RESULTS: A statistically significant difference between the distortion of each of the frameworks was observed. A significant difference in the distortion was also observed for the frameworks when placed on each of the three casts. The distortion was the least when the frameworks were seated on the stone working cast and the greatest when seated on the metal analog. CONCLUSION: This study demonstrated that the distortion of a framework is not related to its closeness of fit as observed by the presence or absence of gaps between the framework components and the teeth.

The prediction of distortion in formed orthodontic appliances.

The theoretical prediction of the onset of distortion within a retraction U-loop as derived from a knowledge of the maximum bending moment, the wire characteristics and the loop dimensions was found to be in good agreement with the experimental measurements carried out on large scale models of the U-loops. The ability to predict distortion in appliances should be of use in any comparison of the properties of the different designs of U-loop or in the evaluation of different designs of orthodontic appliance.

A comparison of the forces required to produce tooth movement ex vivo through three types of pre-adjusted brackets when subjected to determined tip or torque values.

Friction in fixed appliance systems has received considerable attention in recent literature, although that attributable to varying second order (tip) and third order (torque) adjustments in either the bracket or the archwire has not been fully investigated. The ex vivo study of 0.022 x 0.028-inch slot Minitwin, Activa, and Standard Straight Wire brackets investigates friction when known values of tip or torque were applied to 0.018 x 0.025-inch stainless steel wires. The resistance to sliding of the wire through the ligated brackets was measured on a vertically-mounted Instron testing machine. The results showed that the self-ligating Activa brackets consistently produced less friction than the other conventionally tied brackets. Minitwin brackets were slightly more resistant to movement than the Standard brackets during torquing, but the converse was found when tip was applied. Increasing tip and torque (ranges tested 0-6 degrees and 0-25 degrees, respectively) produced almost linear increases in friction for all brackets, although increasing tip had the more profound effect on friction, particularly in Activa brackets.

The characteristics of super-elastic Ni-Ti wires in three-point bending. Part I: The effect of temperature.

The load-deflection behaviour of a number of commercially available superalloy nickel-titanium orthodontic wires has been examined in three-point bending over the temperature range 5-50 degrees C. The loading and unloading curves and plateau regions are found to be closely related to temperature with the stiffness decreasing quite dramatically over a narrow temperature range. The position of this range depends on the material being tested as there are marked differences due to the differing processing methods of the manufacturer. Force values at mouth temperature can differ by 600 per cent for wires of the same nominal diameter made by different manufacturers.

The characteristics of super-elastic Ni-Ti wires in three-point bending. Part II: Intra-batch variation.

Intra-batch variation in the load-deflection characteristics of 13 superalloy nickel-titanium wires has been examined at nominal mouth temperature (35 degrees C). The results indicate that although most of the superalloy nickel-titanium wires were undersized, the variation in diameter within a batch was of the same order as that reported for 18/8 stainless steel wire. The largest coefficients of variation for the intra-batch variation for the initial slope and the unloading plateau values were found to be approximately 7.5 and 10 per cent, respectively. A high degree of correlation was found between the mean slopes of the initial load-deflection curve in three-point bending at 35 degrees C and the mean wire diameter raised to the fourth power for the 13 superalloys wires, from which a value for Young's modulus (E) of 55.4 GPa was deduced on the basis that at this temperature the wires will have an austenitic structure and virtually identical compositions.

An investigation into the characteristics of the PG canine retraction spring.

The forces and couples produced by a model of the PG maxillary canine retraction spring (Danish loop) when activated have been derived by using the complementary (strain) energy method. The theoretical results of the analysis have been verified by experiments on enlarged planar models in a special jig that, by means of strain gauges, was capable of recording the forces and couples developed when the models were activated. Good agreement between theory and experiment was obtained that enabled the analysis to be used with confidence to predict the characteristics of clinical-size loops, and to compare the behavior of the PG retractor with some other maxillary canine retraction springs.

A comparison of the forces required to produce tooth movement in vitro using two self-ligating brackets and a pre-adjusted bracket employing two types of ligation.

Friction in fixed appliance systems has received considerable attention in the recent literature, although that attributable to the type of ligation used has not been fully investigated. This in vitro study of 0.022 x 0.028 inch slot Minitwin, Activa ('A' Company, San Diego, California, USA), and SPEED brackets (Strite Industries Ltd., Cambridge, Ontario, Canada), investigates friction in two forms of self-ligating brackets and in two methods of ligating Minitwin straight wire brackets with polyurethane elastomeric ligatures. The resistance to sliding of rectangular archwires through the ligated brackets was measured on a vertically mounted Instron testing machine. The stainless steel archwires used were straight lengths of 0.016" x 0.022", 0.017" x 0.025", 0.018" x 0.025", and 0.019" x 0.025" Nubryte Gold (GAC International Inc., Central Islip, New York, USA). The results showed a significant reduction (P < 0.01) in frictional resistance in the Activa brackets compared with SPEED brackets by a factor of approximately 15. When the SPEED brackets were compared to Minitwin brackets, the reduction in friction was by 50-70 per cent (P < 0.01). The placing of 'figure-of-eight' elastomeric ties increased friction by a factor of 70-220 per cent compared to conventional elastomeric ties (P < 0.01) except for 0.016" x 0.022" archwires. The results indicate that self-ligating brackets require less force to produce tooth movement because they apply less frictional contact to the archwire than conventionally tied siamese brackets.

Factors affecting tooth movement in sliding mechanics.

The force system operating between bracket and wire in sliding mechanics has been examined analytically using simple beam theory with the object of providing further information on the cause of bracket binding. The results were checked on an enlarged model system, and satisfactory agreement between experiment and theory was obtained. It is shown that for a given bracket tip the restoring couple varies not only with the flexural rigidity (EI) of the wire, the bracket width and span length, but also with the position of the bracket along the span. In addition, as tipping occurs a net vertical force is brought into play which is initially intrusive, but becomes extrusive as retraction proceeds.

The effect of reducing the number of clasps on removable partial denture retention.

Retention to a master model of 10 nominally identical chrome-cobalt removable partial denture castings with standardized bilateral molar and premolar clasps engaging 0.25 mm (0.01 inch) undercuts and 10 castings with clasps engaging 0.5 mm (0.02 inch) undercuts was measured on an Instron testing machine. The retention of each framework was remeasured as the clasps were removed in one of two sequences. Differences between frameworks, number of clasps, and depth of undercuts were all significant. The order of removal of the clasps was not statistically significant. In a separate experiment the position of the dislodging force was found to be significant. It was concluded that the dislodging force was dependent on (1) the fit of the framework, (2) the depth of the undercut, (3) the number of clasps, and (4) its point of application.

Superelastic nickel-titanium wires.

Superelastic nickel-titanium archwires, unlike conventional wires, are capable of withstanding very large deflections and on returning to their original shape on deactivation will produce more moderate forces. The region over which there is a 'plateau' depends on the deflection and is greater the larger the deflection. With most wires both the initial wire stiffness in bending and the 'plateau' deactivation loads are very dependent on temperature. Wire size is no longer a guide to wire behaviour and force values for a given temperature and for wires of the same nominal diameter from different manufacturers can vary by a factor of six.

Dental biomechanics and the dental curriculum.

Attention is drawn to the numerous biomechanical concepts which provide the scientific basis for the many precepts taught in the various branches of clinical dentistry. It is suggested that dental biomechanics should logically be a subject area in its own right and that a practical way to achieve this would be for it to be gradually incorporated into the allied and complementary subject of dental materials science.

A rationale for the selection of orthodontic wires.

A logical basis for the sequence of wires chosen during orthodontic treatment with fixed appliances is presented. This utilizes a new more realistic model for the radial deflection of a segment of the archwire in which the wire is restrained by the ligatures on the misaligned tooth, contacts the inner edges of the brackets either side, and is restrained by the outer ligatures on these brackets. The model, analysed using simple beam theory, allows the determination of the upper and lower bounds for the range of deflection over which orthodontic forces within the optimum range are produced without distortion for any given wire providing its flexural rigidity (EI) and yield stress in bending (sigma o) are known. The same model can be applied to vertical deflections. Some wire sequences recommended on clinical experience appear to conform to this more pragmatic approach, although to cater for even moderate misalignment the forces predicted by the model are higher than normally considered desirable.

Investigation into the behavior of symmetrically and asymmetrically activated face-bows.

The distal and lateral forces and also the couples (in a transverse plane) delivered by the inner terminals of a face-bow have been derived analytically according to the complementary (strain) energy method for various face-bow configurations on the assumption that the terminals are rigidly held by the molar tubes and that the external forces acting are in the plane of the inner bow. The results of the theory have been checked experimentally with a specially designed apparatus in which two mild steel transducer pegs that carry three pairs of strain gauges were used to measure the force system delivered by the inner bow under a range of loading, both symmetrical and asymmetrical. Good agreement with the theory was obtained, confirming the validity of the theoretical analysis. It is concluded that force systems are dependent on bow geometry and that with increasing asymmetry of the external loading transference of the distal load to the power-arm side is accompanied by a shift of lateral force and couple to the opposite side.

The characteristics of the Ricketts maxillary canine retractor.

The characteristics of the Ricketts maxillary canine retraction spring (Ricketts, 1974) have been derived using the complementary (strain) energy method. The theoretical predictions of the analysis have been confirmed by experiments on large planar models in a jig fitted with strain gauges capable of measuring the forces and couples developed when the springs were activated in a way simulating clinical use. The verified analysis was used to compare the horizontal stiffness and anti-tilt couple to retraction force ratios of the Ricketts with other simpler retraction components. The behaviour of the Ricketts mandibular sectional, as predicted by the analysis was also examined.

Value of guide planes in partial denture retention.

The retention of five nominally identical partial denture casting fitted to a master model was measured on an Instron testing machine in the direction of the guide planes and also at angles inclined to them. It was found that: the maximum retention was considerably reduced when the clasps were removed; the guide planes offered some frictional resistance to displacement, the magnitude being critically dependent on the fit; for frameworks with or without clasps a significant increase (P less than 0.01) in retention occurred when the pull was at 22 degrees but not at 12 degrees to the guide planes, and that there was no statistically significant difference between the retention in the forward and backward directions for either a 12 degrees or a 22 degrees tilt of the dislodging force.

Accuracy of dental surveyors.

Maximum lateral forces as measured by strain gauges mounted on the analysing rod of a Nesor surveyor for four operators during surveying procedures ranged from 81 to 110 cN (mean 92.4 cN). Maximum lateral forces used when trimming blockout and inlay wax ranged from 159 to 184 cN (mean 169 cN) and 137 to 196 cN (mean 172 cN), respectively. Mean tilting errors produced in the vertical surveying arm by lateral loads of 98 and 196 cN (i.e. weights of 100 and 200 g) were determined to be 0.29 and 0.42 degrees for five Nesor surveyors and 0.21 and 0.36 degrees for five Krupps surveyors, respectively. All surveyors had been in constant use for at least 12 years. Play in the bearings of the horizontal arm due to loads of the same magnitude applied obliquely produced an additional mean error of approximately 0.5 degrees in the Krupps surveyors. Finally, discrepancies of up to approximately +/- 0.04 mm were found in 10 undercut gauges in each of the nominal sizes 0.25, 0.50 and 0.75 mm. It is concluded that surveyors and their accessories should be checked and serviced regularly, and possible sources of error pointed out to potential users.

The mechanics of asymmetric retraction loops used in fixed appliance therapy.

In order to move a tooth bodily without tipping, both a force P and an anti-tilt couple N of a suitable constant magnitude are required. Although many different spring components are available to orthodontists for this purpose, a comparison of the merits of different designs has not been possible because analytical solutions to the deformation behaviour have only been generally available for the atypical case in which the loop is centrally positioned. The asymmetric case, with two redundancies, for open and closed U-loops both with or without helices has been solved using the complementary (strain) energy method on the assumption that the effect of a longitudinal tension or compression on the bending of a cantilever beam by lateral forces (the tie-bar effect) is small. Unless the arms are of equal length it is then found that unequal couples M and N act at the posterior and anterior ends of the appliance, and in addition equal but oppositely directed forces Q are induced at either end. Check measurements were carried out on enlarged model components using a special jig utilizing a mixed dead-load and strain-gauge system. Agreement with theory was generally within the known errors of measurement. The main characteristics of these retraction components in the clinical situation can consequently be predicted. The characteristics examined are the lateral and vertical stiffnesses (S and dQ/dy, respectively), the N/P and Q/P ratios, the self-righting ability (SRA) and dN/d beta, the change in anti-tilt couple per degree change in gable angle (the angle of the mesial arm with respect of a horizontal in the unactivated appliance).