الفهرس 
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Abstract
Aim: The objective of the present in vitro study was to compare the retention of frameworks using milled
cobalt chromium alloys versus cast cobalt chromium alloys for implant supported mandibular
overdentures using a universal testing machine.
Methodology: An epoxy resin model representing a completely edentulous mandible was constructed
and prepared to receive four implants in the intra-foraminal region of the model corresponding to the first
premolar and lateral incisor regions bilaterally. Four pre-milled titanium abutments were screwed to the
installed implants which were considered to be the primary coping. The secondary copings were either
group C or group M. group C was cast cobalt chromium frameworks or group M was milled cobalt
chromium frameworks. Each framework was attached to the primary coping, retention was measured at
baseline, then each framework was subjected to chewing cycles of 12,500 cycles equivalent to 1 month
and then subjected to chewing cycles of 37,500 equivalent to 3 months using the chewing simulator
(robotic simulating machine). Retention was evaluated using INSTRON universal testing machine.
Results: Both the cast and the milled cobalt-chromium groups showed a statistically significant decrease
in retention when subjected to different cycles. At baseline, group C recorded retention values of (12.846 ±
1.604) while group M recorded (15.552 ± 1.408). After 12500 cycles (1 month), group C recorded (11.001 ± 1.411)
while group M recorded (14.818 ± 1.286) and after 37500 cycles (3 months), group C recorded (9.046 ± 0.696),
while group M recorded (9.319 ± 1.374). group C showed a higher change in retention from baseline to 12500
cycles that was not significant. While group M showed a higher change in retention from 12500 cycles to 37500
cycles and from baseline to 37500 cycles that were not significant.
The overall % change in retention recorded (-29.58%) in group C while it recorded (-40.078%) in group M.
Conclusion: Within the limitations of this study, it can be concluded that the retention of the milled
cobalt chromium frameworks is higher than the cast cobalt chromium frameworks up to an equivalent
period of 1 month, after which the retention of both frameworks became equivalent.
Clinical implications: Considering the ease of manufacturing, the reduced chair side time and higher
initial retention values, it can be recommended that the milled cobalt chromium frameworks rather than
the cast ones.
Implications for research: Considering limitations of the present study, Randomized control trials
are considered highly recommended to evaluate the credibility of the found results.
Key words: implant overdentures, telescopic attachments, retention, cast Co-Cr telescope, milled Co-Cr telescope, mechanical properties, surface characteristics, microstructure characteristics.