overdentures supported by implants in the mandible [13, 26]. This value was substituted in the above equation to derive the forces in the other directions. The loading force for the horizontal direction is 10 N, for the vertical direc- tion it is 35 N and for the oblique direction it is 70 N. The horizontal force is applied in the lingual direction to sim- ulate the constant force applied by the tongue. The oblique force is applied on the buccal surface to simulate the chewing forces. As the loading condition was different in each direction, comparisons between the models at the same loading conditions were made (Tables 3, 4; Graphs 1, 2). From the results of this study it is seen that irrespective of the loading  conditions  the  stresses  were concentrated at the crest of the cortical bone. This tendency of stress concentration around the implant neck, which was evident in all the models, is consistent with other results from Finite Element Analysis of loaded implants, as well as with findings from in vitro and in vivo experiments and clinical studies, which demonstrated bone loss around the implant neck [27].

Material Properties of Implant and  Prosthesis

Implant biomaterials should have adequate strength and modulus of elasticity to withstand forces acting on them. Biomaterials like silicone, hydroxyapatite and carbon are intolerant to such forces; hence are not preferred as primary implant materials. Conversely, ceramics are avoided despite their strength due to their low modulus of elasticity. In conclusion, titanium alloys (Ti6Al4V), which offer superior strength and comparable modulus of elasticity, are prefera- ble to transfer forces acting on them. An increase in force magnitude is deleterious to Osseointegration. Hence, the above factors should be considered to plan treatment so as to minimize force magnitude.

Nature of Bone Implant  Interface

A critical aspect affecting the success or failure of an implant is the manner in which mechanical stresses are transferred from the implant to the bone. It is essential  that

8

Table 3  Stresses developed in cortical and trabecular bone in models 7

B1 and B2

6

Direction of

force

10 N (horizontal)

Stress in cortical bone

(Mpa)

Stress in trabecular

bone (Mpa) 5

4

3

35 N (vertical)

B1 2.333 0.564464

B2 2.428 0.417386

70 N (oblique)

B1 5.063 0.988089

B2 6.256 0.627586

2

1

0

B1 B2 M1 M2

Models

  Graph 1   Showing stresses in cortical bone in   Mpa

Table 4 Stresses developed in cortical and trabecular bone in models M1 and M2

1

0.9

Trabecular bone

10 N(H) 35 N (V) 70 N (O)

Direction of force

10 N (horizontal)

Stress in cortical bone (Mpa)

Stress in trabecular bone (Mpa)

0.8

0.7

0.6

M1 1.559 0.115590 0.5

M2 1.810