Biomechanical Analysis Of Distal-extension Removable Partial Dentures Retained By Attachments

Restoration of Distal-extension partially edentulous presents a number of design challenges. Abutments are on one side of edentulous area, leaving the other side free end. Restoration is difficult due to inherent problems of retention and stability. As attachment could help to improve denture's stability and retention in this kind of situation, it has ben used more in distal-extension removable partial denture (RPD).Attachment is a kind of modern retention style for denture. It is superior to conventional RPD in aspect of retention, stability and esthetics, which could be applied in more fields. Many patients are satisfied with external shape, function and physiological characteristic of attachment denture. Different kinds of attachment have their own clinical indication according to construction and characteristic. Attachments are divided into rigid and non-rigid according to movement between female and male parts. And the non-rigid style is more applied in clinical, for the resilient effect could induce stress around the distal abutment during occlusal loading. Most of the existing researches about attachment-retained distal-extension RPD are focused on the effect of denture design and clinical effects for a certain kind of edentulous situation, and evaluation among different retention style. However, for the ERA attachment which is being used more and more in domestic, there are not so many studies that systematically evaluated its mechanism and the effect of factors, as well as different design for different kinds of distal-extension edentulous situation.The purpose of the present study is to investigate mechanical interaction between female and male parts of ERA attachment, and its effect on supporting tissue of the distal abutment and alveolar under denture base, and optimize designs in different kinds of unilateral ERA attachment-retained distal-extension RPD. Therefore, the movement mechanism and loading transfer in attachment denture is well understood. The result can provide some suggestion in clinical design in unilateral ERA attachment-retained distal-extension RPD.The total study was divided into two parts.The first part includes three experiments:In experiment 1, model of three-dimensional (3D) finite element was obtained through helix CT scanning, CT images reconstruction, converse engineering technology and Abaqus CAE program. The model includes abutment, periodontal ligament, alveolar bone and denture part retained by ERA attachment. In Abaqus program, interaction setting was considered between female and male parts of ERA attachment, and between denture base and mucosa. Then the accuracy of the models was evaluated, and the assembled models provide the technical basis for further analysis in mechanical behavior of ERA denture.In experiment 2, structure of ERA attachment with both rigid and non-rigid connection between female and male parts was analyzed, and stress distribution in supporting tissue of distal abutment and alveolar under denture base were investigated. Results showed that: ERA attachment could transfer occlusal loading to residual alveolar through denture moving towards the arch and rotation of ERA configuration. However, the move and resilient extent was little and finite.In experiment 3, different direction and magnitude of loading was applied on the removable part of denture. The alteration of stress in supporting tissue was evaluated. Results showed that: ERA attachment had resilient effect on different loading condition. The effect was in relation with ERA mechanism, and the magnitude of loading; the partial anatomical shape of arch and alveolar could bring stress increase under each loading direction, especially under higher buccal-lingual load. The result suggested that indirect retainer should be applied on other side of mandible arch to sustain a balanced state of supporting tissue in distal abutment under loading.The second part includes four experiments:In experiment 1, photoelastic models of alveolar bone were established based on standard model of tooth and dentition, using two-step impression and two-cure processes. Abutment tooth were simulated using PMMA material. And silicon material was used to simulate the periodontal ligament and mucosa. Dentures were fabricated by a factory. Results: the whole model system was constructed, including abutment tooth, PL, denture and mandible alveolar.In experiment 2, rectangular photoelastic specimens were prepared according to give size, and divided into five groups. Tensile test was performed to test fringe order of the material used in this part. Loading was increased 1kg each time, and different fringe colors appeared. Results: An order of fringe altered when every 2kg weight was added. Each color alteration was accompanied with increase of 0.25 fringe order. After calculating and statistical analysis by size and loading datum, a certain material fringe value was obtained.In experiment 3, restorations on different edentulous situation were investigated using stress frozen method by three retention styles, including ERA attachment, Mini-Dalbo attachment and RPI clasp. Results showed that: uniform stress distribution appeared in these two attachment groups, which was more in accordance with biomechanical characters of natural teeth; there was no significant difference among groups with three kinds of retainer in second molar defection; directed hinge tend to exert more lateral force on distal abutment than the universal hinge style; in attachment-retained restoration with directed hinge, it is suggested that more consideration should be forced on setting indirect retainer on the other side of arch.In experiment 4, one of the most common situation in Kennedy class II was investigated. Design factors included major connector and abutment number. Design optimization was carried out through evaluating stress in supporting tissues of the distal abutment. Results showed that: the second abutment could help to induce fringe order in distal abutment, but two-abutment and three-abutment splinting made no difference on stress distribution on the distal terminal abutment teeth; compared with abutment splinting, a major connector on the other side of arch could be more helpful to improve stress distribution in supporting tissues; there didn't exist interactions between these two factors of major connector and abutment number.Based on the results above, it can be briefly concluded that: ERA-retained unilateral distal-extension RPD can help to transfer some occlusal loadings from distal abutment to the residual alveolar, through the movement and rotation of the attachment parts; However, the resilient effect is finite; ERA-retained RPD can cushion occlusal forces to the distal abutment and the effect is in correlation with attachment mechanism and force magnitude; under all loading conditions, posterior area of mandible arch and anatomical figure may influence the position of stress distribution and increase magnitude of stress value, especially in buccal-lingual situation; uniform stress distribution appeared in these attachment-retained denture; there was no significant difference among groups with three kinds of retainer in second molar defection; directed hinge tend to exert more lateral force on distal abutment than the universal hinge style; two-abutment splinting is enough on the side of edentulous area in Kennedy class II; major connector could help more in inducing the force on distal abutment; there was no interaction between the factors of abutment number and major connector design .