Stresses from loading distal-extension removable partial dentures

Implant-assisted removable partial denture: An approach to switch Kennedy Class I to Kennedy Class III

The Kennedy Class I and II distal extension situation poses a challenge to the prosthodontist as it inherently possesses a lack of stability, which may be attributed to the difference in compressibility of the mucosa and the periodontal ligament surrounding the distal-most abutment tooth. This results in a rotational tendency of the prosthesis around the line connecting its terminal abutments. Placement of osseointegrated dental implants in the posterior edentulous regions, distal to the terminal abutment provides improved vertical support to the distal extension removable partial denture, effectively converting its intraoral performance from a Kennedy Class I to a Class III situation, thereby resulting in improved stability of the prosthesis and consequently, enhanced patient satisfaction. This case report describes such an approach to the restoration of a Kennedy Class I partially edentulous situation.

Photoelastic analysis of stress distribution in oral rehabilitation

The purpose of this study was to present a literature review about photoelasticity, a laboratory method for evaluation of implants prosthesis behavior. Fixed or removable prostheses function as levers on supporting teeth, allowing forces to cause tooth movement if not carefully planned. Hence, during treatment planning, the dentist must be aware of the biomechanics involved and prevent movement of supporting teeth, decreasing lever-type forces generated by these prosthesis. Photoelastic analysis has great applicability in restorative dentistry as it allows prediction and minimization of biomechanical critical points through modifications in treatment planning.

Influence of length and diameter of implants associated with distal extension removable partial dentures

PURPOSE

The aim of this study was to evaluate the influence of the length and diameter of the implant incorporated under the saddle of a distal-extension removable partial denture, acting as support.

MATERIALS AND METHODS

Six hemi-mandibular models were made with the presence of left inferior cuspid and first bicuspid, with the following differences: model A, without removable partial denture; model B, removable partial denture only; model C, removable partial denture and implant of 3.75 x x mm; model D, removable partial denture and implant of 3.75 x x3 mm; model E, removable partial denture and implant of 5 x x mm; and model F, removable partial denture and implant of 5 x x3 mm. These models were designed with the aid of AutoCAD 2000 (Autodesk, Inc., San Rafael, CA) and processed for finite element analysis by ANSYS 5.4 (Swanson Analysis Systems, Houston, PA). The loads applied were 50 N vertical on each cuspid point.

RESULTS

It was noted that the presence of the removable partial denture overloaded the supporting tooth and other structures. The introduction of the implant reduced tensions, mainly at the extremities of the edentulous edge. Both the length and diameter tended to reduce tensions as their dimensions increased.

CONCLUSIONS

Increasing the length of the implant had a great influence on the decrease of displacement and von Mises tension values. Increasing the diameter of the implant had a great influence on the decrease of von Mises tension values, but did not influence the displacement values. According to the results of this study, it is a good choice to use the greater and larger implant possible in the association between implant and distal extension removable partial denture.

Initial stress induced in periodontal tissue with diverse degrees of bone loss by an orthodontic force: tridimensional analysis by means of the finite element method

This study was undertaken to determine the stress that appears in tooth, periodontal ligament and alveolar bone, when a labiolingual force of 100 gm is applied in a labiolingual direction in a midpoint of the crown of an inferior digitalized canine, and its changes depending on the degree of loss of the supporting bone. The analysis of tensions was carried out by means of the finite element method (FEM) for a normal case and after reducing the periodontal support bone 2, 4, 6, and 8 mm. Three-dimensional images in false color in which intensity of tensions and its areas of extension are generated. Special attention was paid to changes at level D (apical transversal section) to which maximum, mean, minimum, and Von Mises tensions are calculated. After applying the labiolingual force in the canine, a progressive increase of the stress in the labial and lingual zones of the tooth, periodontal membrane and alveolar bone was observed when the alveolar bone was reducing. In the mesial and distal zones, no compensating forces appear, which could provoke a tooth rotation during the tipping movements.

Stresses induced by different loadings around weak abutments

Stresses and deflections of abutments induced by various loadings were analyzed with a two-dimensional finite element model. The biomechanic system consisted of the mandibular posterior three-unit fixed partial denture (FPD). Four different loading types were analyzed: (1) a distributed force of 600 N; (2) concentrated nonaxial and (3) axial 300 N forces at the marginal ridge of the molar; and (4) a concentrated vertical 300 N force at the center of the pontic. All computations were conducted for three different alveolar bone levels. The premolar exerted a greater pressure during occlusal loadings (except axially) on the alveolar bone than the molar. According to the stresses induced in the alveolar bone, the most critical loading was the distributed force. With diminishing periodontal support, stresses elevated in the biomechanic system and critical increases were noted for the concentrated nonaxial load on the molar.

Longitudinal study on occlusal force distribution in lower distal-extension removable partial dentures with circumferential clasps

In this study, longitudinal changes of the occlusal force distribution ratio were examined in lower distal extension removable partial dentures with cast circumferential clasps. Occlusal force applied to the denture base and forces transmitted to the retainers were measured on several separate occasions from the insertion of new dentures to about 4 months after. Two rates of loading were chosen. One was simulated mastication (fast loading rate) and the other was 10 Ns-1 (slow loading rate). Location of the loading points were first premolar (P1), second premolar (P2) and first molar (P3) of the denture. The occlusal force distribution ratio to the retainers was calculated when a load of 20N was applied to the loading point. The results are summarized as follows: (1) The occlusal force distribution ratio at fast loading rate on P1 and P2 was changed until 1 or 1 1/2 months after the insertion of the new dentures, and then became constant. This constant value was 30% on P1, 20% on P2 and 10% on P3. (2) Slow loading rate produced a greater ratio than the fast loading rate on P2 and P3 while there were no remarkable differences in the ratio between both loading rates on P1.

Three-dimensional finite element stress analysis of a cantilever fixed partial denture

A three-dimensional mathematical model was generated, representing a three-unit cantilever fixed partial denture and its supporting mandibular structures. First and second premolars were used as abutments with one posterior cantilever pontic. A 5 lb vertical load was applied to the pontic. Vertical and horizontal stresses were analyzed by means of a three-dimensional finite element stress analysis technique. The results showed that a cantilever pontic creates considerable compressive stress on the abutment nearest to the pontic and produces tensile stress on the abutment farthest from the pontic.

Effect of loading on a substitute oral mucosa and underlying bone

Some mechanical effects of stresses on a bone and oral mucosa model were investigated. A substitute oral mucosa on a substrate of plano-parallel sections of bovine cortical bone modified indenter load/penetration characteristics such that the effects of loads on bone were deferred. In the case of the silicone rubber used, approximately five times the load could be withstood by the bone before it was indented than if no overlay was used. The thicker the overlay the more of the energy applied was absorbed in deforming the intermediary layer. Experimental results confirm that oral mucosa is a modifier of direct loads falling on bone and give some indication of their magnitude.

Three-dimensional analysis of orthodontic tooth movement

A three-dimensional finite element model was used to investigate the biomechanical response of an upper canine tooth. The physical model was developed from ceramic replicas and X-rays, and consisted of cancellous and cortical bone, the periodontal ligament, dentine and pulp chamber. Horizontal forces were applied at the tip of the crown and at the cervical margin and a rotational force was applied at the cervical margin of the tooth crown. The resulting displacements and stress field for each load case are presented with particular emphasis being placed on the response of the periodontal ligament. The investigation shows that quantitative information on initial tooth movement can be accurately predicted and used to evaluate the response of orthodontic treatment.

The effect of partial denture design on abutment tooth and saddle movement

A laboratory investigation designed to simulate the effect of loads applied to a free-end saddle partial denture was carried out. Abutment tooth and also saddle movement were monitored using a photogrammetric method. Loads applied to the denture saddle were transmitted through the occlusal rest and also the clasp components causing movement of both denture saddle and the abutment tooth. The direction of abutment tooth movement was towards the mesial, whether the occlusal rest was situated on the mesial or distal aspect of the tooth crown. It was also evident that the design of the clasp unit used affects the magnitude and direction of abutment tooth movement.

Stress analysis of a tooth restored with a post and core

An idealized axisymmetric finite element model of a second premolar restored with a post and core was used to study the distribution and magnitude of stresses as a function of the following parameters: the diameter of the post, the length and the shape of the post, and finally the interface characteristics between post and cement. Emphasis was directed toward the cement layer interposed between the post and the tooth. Bonding between the post and the cement appeared to be the most important parameter to achieve optimal mechanical behavior of the tooth-prosthesis combination.