Biomechanics of different thread designs of dental implants assisting unilateral free end mandibular partial dentures

Abstract

Background: Dental implants assisting unilateral free end mandibular partial dentures (RPDs) improve their performance and prognosis, however, no consensus exists on the type of thread used in these implants. The current work studied the effect of dental implant thread design on stress distribution around dental implants assisting unilateral free end RPDs using strain gauges and finite element analysis to select the best performing thread design.

Methods: Twenty-four custom made titanium-aluminum-vanadium (Ti-6Al-4 V) implants were designed and milled 4 thread designs; V-shaped, buttress, reverse-buttress and trapezoid, and were inserted in the approximate locations of tooth number 36 in 6 polymethyl methacrylate Class II Kennedy models, which had teeth number 36, 37, and 38 missing, and unilateral removable partial dentures were constructed to fit each model, with a metal housings and O-rings in their fitting surface attaching to the ball abutment. Surface strains were measured with strain gauges, and mean stresses around the implants, and principal abutments in each tested model were compared using one way analysis of variance (ANOVA). The finite element analysis, recorded stresses, around each dental implant thread design, in the form of colorcoded maps using Von Mises stress analysis.

Results: The recorded micro-strains around V-shaped threads and their related abutments were higher than those recorded around the buttress threads and their related abutments, reverse buttress threads and their related abutments, and trapezoid threads and their related abutments in descending order as determined by one‑way ANOVA (F = 284.489, p < 0.001), and Tukey post hoc pairwise comparison (p < 0.001). FEA results presented the stresses generated around each thread design, under vertical load, the highest stress concentration values were observed around V-shaped threads, followed by the buttress threads, the reverse buttress threads, and finally the least stresses were observed around the trapezoid threads. Under oblique load, more stresses were observed than those under vertical load, being also greatest around V-shaped threads, then decreasing around buttress, reverse buttress, and trapezoid threads.

Conclusions: The strain gauge and finite element analysis revealed that the trapezoid threads demonstrated least stress concentration at the bone implant interface, followed by the reverse buttress, buttress, and finally the V-shaped threads.

Keywords: Dental implant; Finite element analysis; Strain gauges; Thread design; Titanium.

Fig. 1

Design features and dimensions common to all implants and ball abutments used in this study with the only difference being the thread design

Fig. 2

(a) and (b) the implant and the ball abutment assembly, design and manufacturing of (c) V-shaped thread implant, (d) buttress thread implant, (e) reverse buttress thread implant, (f) trapezoid thread implant, and (g) ball abutment

Fig. 3

(a) Class II Kennedy polymethyl methacrylate model used in this study with the strain gages attached to the lingual side of the terminal abutment and the implant place at the location of the tooth 36, (b) Unilateral RPD manufactured and seated in place, with the strain gages attached to the buccal aspect of the terminal abutment and implant

Fig. 4

(a) the universal mechanical testing machine, (b) the custom-made loading jig, (c) the jig being adjusted to load the RPD occlusal surface

Fig. 5

Dental implants lattice design, (a) V-shaped thread, (b) Buttress thread, (c) reverse buttress thread, (d) trapezoid thread

Fig. 6

(a) The meshing of the dental implant finite element model, showing the direction of the vertically applied load (white arrow), direction of the oblique applied load (yellow arrow), with the von Mises color map of stress distribution within the body of the implant, (b) The reconstructed 3D model of the edentulous mandible, (c) Occlusal view of the meshing of the 3D model of the edentulous mandible, (d)Th the von Mises stress distribution in the edentulous mandible

Fig. 7

(A)Sagittal sections in the 3D virtual models of the implants FEA, showing von Mises stress distribution under vertical load, the highest stress concentration values were observed around V-shaped threads (a), followed by the buttress threads (b), the reverse buttress threads (c), and finally the least stresses were observed around the trapezoid threads (d). Under oblique load, more stresses were observed, also, being greatest around V-shaped threads (e), then decreasing around buttress (f), reverse buttress (g), and trapezoid threads (h). (B) Horizontal sections in the 3D virtual models of the implants FEA, showing von Mises stress distribution under vertical and oblique loads related to the same threads in (A)

Fig. 8

Maximum and minimum principal stresses for vertical and oblique loads observed around each dental implant thread design