الفهرس 
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Abstract
Edentulism has been a serious problem affecting people in many aspects. It affected their function and their ability to masticate and chew food, their speech, their facial esthetics and their self esteem. Complete dentures have been used since very long time and have restored some of the patients deficiencies, yet they haven’t been that satisfying for the patients.
The introduction of dental implants has solved many of these problems giving the patients a fixed or removable solution with better outcomes. The transitional phase between the dental implants insertion and their loading has been a problem too since all patients need to leave the dental office with teeth and without compromising their success rates. With the era of digital dentistry, dental implants are now pre-planned with their superstructure even before starting any surgical procedure which makes the surgery guided, takes
less time and effort with more reliable outcomes. The mostly used dental materials in the temporization phase was PMMA because of its low cost, easy manufacturing, providing good function and esthetics. On the other hand, PMMA had a serious problem which turned to be catastrophic in most of the cases which is its fracture. PMMA does not withstand high stress values without deformation and fracture. In the ”All on four” concept, there is a cantilevered portion of the prosthesis posterior to the
distal implant leading to higher stress values and more prosthetic complications when PMMA is used which made it less cost effective as the patients complain of repeated fracture causing them to
remake the prosthesis. Many dental materials were introduced to solve the current problems; Thermoplastic resins
were one of them. Acetal as a thermoplastic resin material showed same esthetics, providing function with reasonable cost and the main advantage was the high strength and withstanding the high stress values at the cantilevered part of the prosthesis.
The high fracture strength of the Acetal was due to many material properties as the Acetal is a flexible material showing high proportional limit which makes it withstands high stress without
showing deformation. As well as the resiliency as the material absorbs alot of energy and stress without reaching the proportional limit.
This study compares the Acetal and the PMMA by milling fixed dentures of the same design and tested on the same model containing four implants. Discs of both materials had the same manufacturer, shade, disc diameter and thickness to control all the other variables and the testing was done by a vertical bilateral load on the cantilevered part of the prosthesis at the mesial half of the first molars using a universal testing machine. A statistical significant difference in the results of both groups favoring the Acetal group was found.