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Type of Overdenture Attachment
There are various attachments that have been used with implant overdenture, the most common being the Bar-Clip attachment, Ball/O-ring attachment and the Magnet attachment. In vitro and in vivo [10, 15-17] studies show that the ball and O-ring attachment transferred less stress to the implants than the bar-clip attachment. In vitro studies [18] have shown that constant retentive properties and low retentive energy of magnet attachment could assist abutment preservation. Ball attachment are considered the simplest type of attachment for clinical application with tooth or implant supported overdentures [19]. Generally considered resilient, the specific design of the ball attach-ment may influence the amount of ^ee movement thereby limiting its resiliency. Magnetic attachment has evolved over the years to become an additional option also avail-able for use with the implant supported mandibular over-denture [20]. The development of closed field magnets of rare earth alloys substantiated magnets as an overdenture attachment system [21]. When wear induced retentive changes are considered, studies have shown that the ball attachment were found to have lost between 32% and 50% of their initial retentive force. By contrast magnets incurred a minimal reduction in retentive force of only 1.7-5.3% this is despite the microscopical corrosion that is seen within the stainless steel magnetic case [22]. The findings from comparative studies on the retentive force of ball and magnetic attachment identified the latter as the weaker attachment system. Inspite of this the magnetic attachment reflected the tendency to relatively maintain a reproducible and consistent force under wear simulation [23, 24]. This can be contributed largely to the inherent mode of retention being magnetic rather than frictional or mechanical. Stud attachment provide varying degree of resiliency in the vertical and horizontal directions. Magnetic attachment produces no vertical resiliency while quite effectively decreasing horizontal stress transmission to the abutment [25]. In the present study a comparison was made between the Ball/O-ring and the Magnet attachment. Different diameters for the attachments were used. Both the Ball/O- ring and the Magnet attachment have shown favorable stress distribution to the surrounding bone. It was observed that when the diameter of the attachments was increased there was an increase in the stress in the cortical bone. This result was consistent for both the ball and magnet attach¬ment. This could be the result of the larger surface area of the bigger attachment, which transfers greater stresses on to the bone. Hence, if a larger diameter attachment is to be used then increasing the width of the implant will help to reduce the stresses to the cortical bone.
Type of Loading
The magnitude of the bite force is dependent on the force direction. In the present study three forces from different directions were selected: a horizontal bite force, a vertical bite force and an oblique bite force. The proportion of the force magnitude was 1:3.5:7 respectively. The vertical bite force was determined to be 35 N from studies which measured the bite force of edentulous patients with
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].