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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
neither implant nor bone be stressed beyond the long-term fatigue capacity. Any relative motion that can produce abrasion on the bone or progressive loosening of the implant should be avoided [28]. These requirements are met by osseointegrated implants by virtue of close appo-sition of the bone to implant in the angstrom level. The close apposition of titanium and bone at the angstrom level means that under any subsequent loading the interface moves as a unit without relative motion of the bone and titanium and with the possibility of transferring stress to all parts of the interface.
The Quality and Quantity of Bone Surrounding the Implant
The most common bone density that is present in the anterior mandible is the D2 type [5]. A finite element analysis conducted by Misch had predicted 100% success rate for implants placed in this type of bone. The type of bone present around the bone-implant interface spells the type of distribution of stress seen at the interface. Cortical bone can take better stresses as compared to trabecular bone. The ultimate compressive strength of cortical bone is 140-170 Mpa, where as the compressive strength of tra-becular bone is 22-28 Mpa. In all the loading conditions the stress levels did not reach the maximum yield strength of mandibular bone, hence there would be no fracture of the bone. With the probability of excessive stresses being minimized, the focus of attention should be directed to the minimal amount of stress that is required to maintain a healthy bone-implant interface without causing bone dis¬use atrophy. The minimal stresses that is required for the deposition of the bone around the implant is about 1.3-1.7 Mpa [29]. It is observed from the studied loadings that the stress generated by the models were above this range. Hence both the Magnet as well as the Ball and O-ring attachments gives favorable stress distribution to the bone. This fact being laid down by the present study, the choice of the attachment now depends on the retention and stability that the attachment offers to the patient. Studies [30, 31] conducted on the satisfaction of the patient with implant-supported overdenture has revealed that they prefer the Ball O-ring attachment as compared to the Magnet attachment as far as retention and stability is concerned. Hence the best attachment to be used for implant-supported overdenture is the small diameter ball O-ring attachment.
The advantage of using Finite Element Analysis is that accurate representation of complex geometries can be made, the models can be easily modified and internal state of stress and other mechanical quantities can be represented [32, 33]. There were certain limitations pertaining to the present study. Finite Element Analysis is a mathematical in vitro study that may not simulate the clinical situation completely. A state of optimal osseointegration was assumed between the cortical bone, trabecular bone and the implant. This may not occur in clinical situations. All materials were assumed to be linearly elastic and homogeneous in nature whereas, bone is viscoelastic, anisotropic and heterogeneous material. The resultant stress values obtained may not be accurate quantitatively but are generally accepted qualitatively [32]. Chewing forces are dynamic in nature, but the loads applied in this study were static loads. In the present study only a segment of the mandible and overdenture was considered. Prosthesis movement, retention and stability were not con¬sidered. Due to the limitation pertaining to the study, further research regarding Three-dimensional Finite Element Analysis combined with long term clinical evaluation is required.