الفهرس 
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Abstract
This invitro study was conducted to evaluate retention, release period and wear of two implant retained mandibular overdentures, with 0° implant inclination (control group), 17.5° and 35° labial implant inclinations (study groups) using Ti Si snap attachments with different retention silicone 400 g/ 4 Newton and 600 g/ 6 Newton. after 540 cycles (6 months) and after 1080 cycles (1 year) of insertion and removal on a chewing simulator.
A layer of autopolymerized resilient silicone soft lining material that was about 1.5 mm thick was applied to each of the 3 ready-made epoxy models that were completely edentulous in order to simulate resilient ridge mucosa.
CADCAM Surgical guides were used as a guide for implant placement sites and angulations (at canine bilaterally) on each epoxy model using surgical kit for implants placement according to labial inclination: group A 0°, group B 17.5°, group C 35°.
36 acrylic experimental overdentures were fabricated over the epoxy models (12 for each group) and connected to it with retention silicone 400gm/4N and 600gm/6N.
Each model was placed on the universal testing machine and secured in position with the occlusal plane of the overdenture in a horizontal position.
To perform the cyclic loading test, a programmable logic-controlled equipment; ROBOTA chewing simulator integrated with thermo-cyclic protocol operated on servo-motor simulating the vertical and horizontal movements simultaneously in the simulating thermodynamic condition. Each of the chambers consists of an upper Jackob’s chuck as a holder for the vertical screw that can be fixed to the load applicator.
Each sample was then placed on the corresponding cast while Jakobe’s chuck of the upper part of machine was fixed through inverted circular flat end-shaped plastic load applicator.
The load applicator centrally positioned between 2nd premolar and 1st molars to facilitate the aligning with the loading axis of machine and proper load distribution.
Peak load to dislodgement was recorded in order to ascertain the initial force of retention of each attachment system.
The Retention Properties of each group were measured using universal testing machine with a load cell of 5 kg and crosshead speed of 50.8 mm/min, data were recorded in Newton using computer software.
The measurement was repeated after 540 cycles (6 months) and after 1080 cycles (1 year) of insertion and removal cycles on a chewing simulator with artificial saliva.(25)
The chewing simulation cycles was repeated 75000 and 150000 times to clinically simulate the 6- and 12-months chewing condition, according to previous studies.
The load required to totally dislodge sample was recorded in Newton, the dislodging forces generated measurements of the peak load, break load, and displacement at peak load and break load. Release periods were calculated using difference between the peak load and break load and the cross-head speed
Attachment housing placed in the overdentures were examined under the electron microscope with magnification of 500x. Observing the change in surface after 540 cycles (6 months) and after 1080 cycles (1 year) of insertion and removal on a chewing simulator indicating the amount of wear.
The results of this in vitro study were summarized as follows:
A) Retention:
- Comparing the three subgroups with retention silicone 400 gm/ 4N
In each group, repeated measures analysis showed a statistically significant change in the retention (N) among the different periods of measurement in the inclination zero degree group (p= .002), in the inclination at 17.5 degrees group (p=.009), and in the inclination at 35 degrees group (p=.006).
Pairwise comparisons revealed that the retention was statistically significantly lower in the inclination at 35 degrees group (p<.001) compared with the inclination zero degree group.
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination zero degree group.
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination 35 degree group.
- Comparing the three subgroups with retention silicone 600 gm/ 6N
In each group, repeated measures analysis showed a statistically significant change in the retention (N) among the different periods of measurement in the inclination zero degree group (p= .009), in the inclination 17.5 degrees group (p=.002), and in the inclination 35 degrees group (p=.002).
Pairwise comparisons revealed that the retention was statistically significantly lower in the inclination 35 degrees group (p<.001) compared with the inclination zero degree group.
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination zero degree group.
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination 35 degree group.
- Comparing the three groups with retention silicone 400 gm/ 4N and with retention silicone 600gm/6N
The retention was statistically significantly higher in tisi retention sil 600 group than in tisi retention sil 400 at all periods of measurement in inclination 0 degree (p=.004), in inclination 17.5 degree (p=.004), and in inclination 35 degree (p=.004)
B) Release Period
- In retention silicone 400 gm/ 4N subgroups and in retention silicone 600 gm/ 6N subgroups
There was a statistically significant difference in the release period among the three studied groups at all periods of measurement (p=.001).
Pairwise comparisons revealed that the release period was statistically significantly lower in the inclination at 35 degrees group (p=.004) compared with the inclination at zero degree group.
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination zero degree group
No statistically significant difference was found between the inclination at 17.5 degrees group (p >.001) compared with the inclination 35 degree group
C) Wear Qualitative analysis:
All of the final SEM micrographs after insertion and removal revealed material wear at the silicone retentive matrices fitting surfaces of specimens for the three groups.
For the comparison between the 0, 17.5, 35 degrees labial inclination it was concluded that surface changes were higher at 35 degrees followed by 17.5 degrees and the 0 degree had the least changes in surface comparing to other groups.
For the comparison between retention silicone 600gm/6N and 400gm/4N it was concluded that retention silicone 600 had more vigorous changes.