الفهرس 
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Abstract
Maxillofacial defects may be the end result of; congenital malformations, developmental, acquired, traumatic or, surgical involving resection of tumors. Maxillary defects are the most common among all intraoral defects. They can cause connection between the oral cavity and maxillary sinus or nasopharynx.
These defects can be treated either by surgical approaches with plastic surgery or by prosthetic appliances called obturator. Obturators are designed to lock the gap between the residual hard and/ or soft palate and nasopharynx.
Retention of obturator prostheses is generally a problem. For this reason, hollowing of the obturator is essential for better retention and stability. A hollow-bulb obturator constructed by the removable lid approach is lighter in weight and effectively extending into the defect region.
Thermoformed resin material has the advantages of being light in weight, flexible and more convenient to the patient. They are metal-free, making them biocompatible, nonirritant, nontoxic, and biologically inert, with splendid esthetics.
This study was conducted on 30 identical stone casts obtained from a single silicone mold of a patient cast. These 30 models were randomly assigned to the two test groups and the control group, 10 in each one. Autopolymerizing acrylic resin was used as an adhesive between the removable lid and the bulb portion of a thermoformed obturator in one group, while macromechanical holes and undercuts was used in the other group along with the adhesive, the test groups were compared to the standard HCAR obturators. After construction and sealing of the obturators, each one was attached firmly to its respective cast. The obturator-cast complex was attached to the machine and gradual load was applied until failure or fracture of the samples. This load at failure was registered and used in statistical testing of the groups.
Obturators made of HCAR showed the statistically significantly highest mean tensile strength. Thermoformed resin obturators supplied with mechanical undercuts showed bond strength values that were 40% that of HCAR obturators and 244% that of thermoformed resin obturators bonded only with APAR. Thermoformed obturators bond only with adhesive showed statistically significantly lower values than HCAR group.
It was clear that the chemical bond in HCAR obturators in general was greater in strength than the mechanical bond in thermoformed obturators with undercuts and holes, and that the thermoformed obturators bonded with adhesive only showed the lowest values. So modifying the surface for bonding between the lid and obturator by mechanical undercuts may enhance their bonding together.
Conclusion:
1- Obturators made of HCAR have the highest bond strength among the tested groups.
2- Thermoformed obturators sealed by APAR resin and mechanical undercuts have bond strength nearly 40% of that of HCAR group. On the other hand, their bond strength is more than 240% of that of thermoformed obturators sealed by APAR only.
3- The use of mechanical undercuts improved the bond strength of thermoformed resin greatly. That may recommend its clinical use.