Objective: To evaluate the influence of shape of four different short implants on the stress characteristics of jaw bone around implants in the posterior maxilla.
Methods: Finite element models of the jaw bone and crown were established based on computed tomography (CT) data and implant geometries were obtained from manufacturers' catalogues. The whole models included jaw bone, four different short implants (A: torpedo-shape implant with fin threads; B: taper implant with triangle threads; C: cylindrical implant with fine threads; D:cylindrical implant with sparse threads), implant abutments, abutment screws and crowns. Three-dimensional hexahedral and wedge-shaped brick elements were used to mesh the finite element models. Assumptions made in the analyses were: linear elastic material properties for bone, type IV jaw bone, and 50% osseointegration between bone and implant. Inclined load of 200 and 1 000 N at 45° to the axis of the implant were applied to the cusps of ceramic crown respectively. The von Mises stresses in cortical bone and cancellous bone around the implants were calculated. Measure lines were designed along the crest in cortical bone and nearly 0.2 mm distance away from thread tips in cancellous bone for stress comparation and discussion.
Results: All short implants showed the same stress distribution characteristics in the cortical bone. Different stress distribution characteristics in cancellous bone were found in different implant system. From the measure line the maximum stress were located at the level of the neck of implant A, C and D. Under inclined load of 200 N, the maximum stress were 6.60, 6.50 and 7.79 MPa for implant A, C, and D respectively. For implant B the maximum stress was located at the periapical area (5.84 MPa)instead of the level of neck of implant (5.72 MPa). The stress along A and C was more evenly distributed compared with implant B and C. Under inclined load of 200 N, the maximum stress around implant A and C in the periapical area were 5.02, 4.96 MPa; for implant B and D they were 5.84, 7.52 MPa respectively. Under inclined load of 1 000 N, the maximum stress around implant A and C in the periapical area were 25.96, 24.06 MPa; for implant B and D they were 29.52, 38.53 MPa respectively.
Conclusions: Torpedo-shape implant with fin threads and cylindrical implant with fine threads are recommended to be choosed in type IV jaw bone with limited height.