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Biomedical subjects

N E Waters

Publications and source records attributed to N E Waters.

At least 19 recordsLinked to original sources
PubMed1992

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.

Bicuspid↗
PubMed1992

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.

Chromium Alloys↗
PubMed1992

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.

Biomechanical Phenomena↗
PubMed1992

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.

Dental Stress Analysis↗
PubMed1992

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.

Activator Appliances↗
PubMed1992

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.

Dental Alloys↗
PubMed1992

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 Alloys↗
PubMed1991

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.

Denture Design↗
PubMed1990

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).

Biomechanical Phenomena↗
PubMed1989

The quadhelix maxillary expansion appliance: Part I. Mechanics.

The mechanics of the quadhelix maxillary expansion appliance have been derived using the complementary (strain) energy approach, and the results checked experimentally on enlarged models. Good agreement between theory and experiment was obtained, for both the lateral stiffness and the magnitude of the molar couple induced. The analysis shows that the spring characteristics are defined by three parameters, the lateral stiffness, and two other parameters M1 phi and M1 y which are the rate of change of the molar couple with phi and y respectively, where phi is the angle the molar arm is rotated on engagement and y the lateral displacement.

Dental Stress Analysis↗
PubMed1989

The quadhelix maxillary expansion appliance: Part II: Clinical characteristics.

In a previous paper (Jones and Waters, 1989) a theoretical model of the quadhelix was developed and tested against enlarged models. Good agreement was obtained. In this paper the theoretical model has been used to examine the effect, on the lateral stiffness and molar couple characteristics, of altering the size of the appliance's various components within the clinical size range. It is suggested that the wide variation in the force-deflection characteristics found by previous authors could be explained by differences in appliance dimensions.

Biomechanical Phenomena↗
PubMed1989

Fractography of a bis-GMA resin.

The fracture behavior of a bis-GMA resin was studied by means of the double-torsion test. The fracture parameter measured was the stress-intensity factor. Fracture occurred in either a stick-slip (unstable) or continuous (stable) manner, depending upon the test conditions. When stick-slip propagation occurred, the fracture surfaces showed characteristic crack-arrest lines. The fracture surfaces were examined by use of a reflected-light optical microscope. The stress-intensity factor for crack initiation was found to be related to the size of the crack-arrest line which, in turn, could be related to the Dugdale model for plastic zone size. The evidence supported the concept that the behavior of the crack during propagation was controlled by the amount of plastic deformation occurring at the crack tip.

Bisphenol A-Glycidyl Methacrylate↗