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This is the final stage of the procedure wherein the pre¬pared model was subjected to different parameters to simulate the field variables in different elements. Herein, the meshed model was subjected to horizontal, vertical
(along long axis) and oblique (120° to long axis) forces to analyze the stress patterns formed in the bone. The four finite models were subsequently loaded from the horizontal (lingual), vertical, and oblique (buccal) directions with the force of 35, 70, 10 N respectively [11-13]. The forces were applied on the overdenture at the surface of the modeled tooth. The model is constrained in the mesial, distal and inferior directions and was allowed movement in the bucco- lingual plane. Stress levels according to Von-Mises criteria were calculated because Von-Mises stresses are most commonly reported in Finite Element Analysis studies to summarize the overall stress state at a point.
Results
Three dimensional models of the section of the mandibular bone segment comprising the cortical and trabecular bone, having an implant supported overdenture with various types of attachment (Ball and Magnet) were constructed. Forces of 10 N, 35 N and 70 N were applied in the hori-zontal, vertical and oblique directions respectively [11-13]. Stresses generated around the implant in the bone were studied.
The four different models studied are:
(a) B1 = 2.5 mm Ball attachment
(b) B2 = 4.0 mm Ball attachment
(c) M1 = 4.0 mm Magnet attachment with 800 g mag-netic attraction
(d) M2 = 4.5 mm Magnet attachment with 910 g mag-netic attraction
The material properties of the four different models were obtained from the literature [10]. The stress distri-bution was represented with different color-coding. Red being the highest followed by orange, yellow light green, green, light blue, blue and dark blue colors representing the stresses in the descending order. With these different colors the stress distribution pattern can be analyzed in the different models. The corresponding stress values for that particular color is also given at the bottom end of the photographs.
Stresses produced in the cortical bone were greatest with the model B2 when loaded with 70 N in the oblique direction. Overall it was seen that the stresses were con-centrated near the neck of the implant in all of the sections of the cortical bone (Fig. 7). Stresses produced in the tra-becular bone were greatest with the model B1when loaded with 70 N in the oblique direction (Fig. 8). The lowest stress concentration in the cortical bone was seen with horizontal force applied on model B1 (Fig. 9) and in the trabecular bone with model B1 in the horizontal direction ^ig. 10).
Fig.7 Greatest Stress generated in cortical bone in oblique loading in B2
Fig. 8 Greatest Stress generated in trabecular bone in oblique loading in B1
Discussion
The McGill consensus statement on implant—supported overdentures was brought out in May of 2002 [1]. According to the consensus two implant- supported man-dibular overdenture was considered to be the first choice of treatment for edentulous patients. Studies have shown that clear differences exist in the way stresses are transferred to the bone in a tooth-supported overdenture and an implant-supported overdenture The load transfer at the bone implant interface depends on (1) Implant geometry, (2) The type of loading, (3) Material properties of implant and prosthesis, (4) The nature of bone implant interface, (5) The quality and quantity of surrounding bone (6) Implant surface structure [2, 14].
Fig. 9 Cortical bone showing least stresses generated by B1 on horizontal loading
Fig. 10 Trabecular bone showing least stresses generated by B1 on horizontal loading
Implant Geometry-Implant Dimensions
Cylindrical implants produce high shear stress on the bone; Increase in diameter increases the load bearing area by the square of its radius and the bending resistance by the fourth power of the radius. Studies show that stresses reach only a particular distance (approximately 10 mm in height) within the implant [5].