Objectives: In partial edentulous cases with distal extensions, an implant-associated removable partial denture (IARPD) may be used to support the denture base. However, until now, most studies or case reviews have involved long implants in IARPD. Information on short implants is lacking. Therefore, the purpose of this study was to evaluate the effects of implant location and length on stress distribution and displacement in osseointegrated implants that were associated with mandibular distal extension removable partial dentures (DERPD) with a two-dimensional finite element analysis.
Material and methods: A sagittally cut distal extension edentulous model with the #33 and #34 teeth and a removable partial denture of the left mandible was used as a basic anatomic model. Seven models were designed with NX 9.0 (Siemens AG, Berlin, Germany). Models A, B, and C had implants with lengths of 11 mm, 6 mm, and 4 mm, respectively, under the denture base of the #37 artificial tooth. Models D, E, and F had implants with lengths of 11 mm, 6 mm, 4 mm, respectively, under the denture base of the #36 artificial tooth. Model G did not have any implants and was used as a control. The models were loaded with 250 N of axial force on the central fossa of #36. The finite element analysis was performed with MSC Nastran (MSC Software Corporation, Newport Beach, CA, USA). von Mises
stress maps were plotted to visualize the results. The point values of displacement and von Mises stress were acquired quantitatively.
Results: The decreasing length of implants tended to increase the stress on the bones near the implant. The models of #37 implant placement showed much lower stress concentration on the surrounding bone of the implant compared with #36. Especially, the stress of Model C (#37 position, 4-mm implant) was smaller than that of Model D (#36 position, 11-mm implant). Regardless of the implant length, the displacement values were reduced largely by implant placement. The displacement values of edentulous alveolar bone and the bone surrounding the implant increased with shorter implant length. The #36 implant position tended to reduce displacement more than #37.
Conclusions: When an IARPD is designed for a distal extension edentulous case, the distal positioning of implant placement has more advantages than mesial positioning in the edentulous bone of DERPD on the prognosis of short implants and the stress distribution of edentulous alveolar bone. Using implants with longer lengths are important for stress distribution. However, Additional studies are necessary of the effects of length on implant survival for various periods and conditions.