title: Effects of rigid and nonrigid extracoronal attachments on supporting tissues in extension base partial removable dental prostheses: a nonlinear finite element study.
abstract: BACKGROUND
  Resilient (nonrigid) and non-resilient (rigid) attachments are used in extension base partial removable dental prostheses for retention. However, the biomechanical effects of these 2 types of retainers on the terminal abutment and supporting tissues, which may influence clinical treatment planning, have not been compared.
  
  
  OBJECTIVE
  The purpose of this study was to compare the mechanical effects of 2 types of extracoronal attachments (rigid and nonrigid) in distal extension removable partial prostheses on the alveolar ridge and abutment tooth periodontal ligament.
  
  
  METHODS
  A finite element model of a human left mandible edentulous arch distal to the second premolar was fabricated. The second premolar was the terminal abutment for an attachment-retained denture. Two types of attachments (rigid and nonrigid) were modeled in the study. For the nonrigid attachment, there was movement between the patrix and matrix component of the attachment, but there was no movement between the 2 component parts for the rigid attachment. Six levels of loading (100, 150, 200, 250, 300, and 350 N) were applied from 3 directions (axial, buccolingual, and mesiodistal) on the central fossa of the first and second molars. Denture motion and stress distributions of denture supporting tissues were observed. Maximum equivalent stress values (SEQV) were recorded for 6 regions (cervical bone, cervical and apical periodontal ligaments, mesial and distal ridges, and mucosa). The data were divided into 2 groups according to the attachment type. Paired t tests were used to compare the values of the 2 groups. Factorial ANOVA was used to test the difference between the loading directions (α=.05). Multiple linear regression was used to analyze the interactions among the factors of region, direction, and level (α=.05).
  
  
  RESULTS
  Stress distributions in the rigid and nonrigid attachment models were similar but the magnitudes were different. For all 3 loading directions, significantly different stresses in the alveolar ridge and periodontal tissue of the terminal abutment were found between the rigid and nonrigid groups (P<.05). There were significant differences among the 3 loading directions (P<.05). In the nonrigid group, the stress ratio of the mesial to the distal area was higher than that of the rigid group from axial and mesiodistal loading (P<.05). Linear interactions were found between the direction and level and region and level combinations (P<.05). Movement between the patrix and matrix components increased as loading increased. The most obvious movement of attachment occurred when loading was in the buccolingual direction.
  
  
  CONCLUSIONS
  Stress on the terminal abutment can be reduced by the use of an extracoronal resilient attachment that allocates more loads onto the distal edentulous ridge. The level of loading influenced the extent of reduction. A resilient attachment with a universal hinge had the most movement when loading was in the buccolingual direction. Interactions were found between direction and level, as well as region and level combinations (P<.05).
authors.affiliation: Department of Prosthodontics, School of Stomatology, The Fourth Military Medical University, Xi'an, China.
authors.forename: Hui-yuan
authors.initials: HY
authors.lastname: Wang
authors.forename: Yu-mei
authors.initials: YM
authors.lastname: Zhang
authors.forename: Dong
authors.initials: D
authors.lastname: Yao
authors.forename: Ji-hua
authors.initials: JH
authors.lastname: Chen
mesh_terms: D000704:Analysis of Variance; D000044:Dental Abutments; D003799:Dental Stress Analysis; D003819:Denture Bases; D003779:Denture Design; D003834:Denture Precision Attachment; D003781:Denture Retention / Q000295:instrumentation*; D003832:Denture, Partial, Removable; D020342:Finite Element Analysis; D006801:Humans