The aim of the present study is to evaluate the biting force of upper removable partial denture constructed from two different flexible thermoplastic materials.

The patient will be treated in visits designated as follows:

Visit 1: Preoperative records, clinical, radiographic examination (panoramic and periapical x-rays for evaluation the crown-root ratio, the apical condition of the abutment and their alveolar bone support of abutment) and primary impression will be performed with irreversible hydro-colloid impression material (alginate), The impressions will be poured with type IV dental stone to obtain diagnostic casts .

Visit 2: Maxillary Face-bow will be recorded. The diagnostic casts will be mounted on semi adjustable articulator in centric occluding relation to evaluate the interarch distance and occlusal plane.

Visit 3: primary surveying is performed. Special trays will be constructed and Mouth preparation will be performed by preparing guiding planes and rests seats will be prepared opposing to the edentulous area. Final impression will be taken by elastomeric impression material. The impression will be poured into type IV dental stone in order to obtain master cast. The master cast will be surveyed. The proposed design will be as follows: metal framework base saddle for edentulous areas on both sides connected with palatal strap. Aker's clasp on all abutments with buccal retention.

Visit 4: the metal framework is tried into the patient's mouth. Jaw relation is recorded.

Visit 5: The denture base will be tried in with acrylic teeth in patient's mouth.

Visit 6: the denture base of the partial denture will be processed incorporation with the metal framework into the two different materials to be evaluated. The first group will receive the partial denture in which the metal framework incorporated with the denture base fabricated from PEEK material. The second group will receive the partial denture in which the metal framework incorporated with the denture base material fabricated from BRE-FLEX.

Visit7,8,9: clinical measurement will be performed for one month in three visits.

Inclusion Criteria:

• i. All patients must have Kennedy class III modification I upper partially edentulous ridges.

ii. The remaining teeth have good periodontal condition, with no signs of attrition or gingival recession.

iii. Male or female patient with age range (45-55) and in good medical condition iv. All patients have skeletal Angle's class I maxillo-mandibular relationship and have sufficient interarch distance.

v. Free from any systemic or neuromuscular disorder that might affect chewing efficiency of masticatory muscles.

vi. Free from any tempro-mandibular joint disorder. vii. The patients have good oral hygiene and low caries index.

Exclusion Criteria:

• i. Patients having abnormal habits as bruxism or clenching ii. Patients having hormonal disorders as diabetes, thyroid or parathyroid hormonal diseases were not included.

iii. Teeth with compromised bone support. iv. Patient with xerostomia or excessive salivation. v. Patient with abnormal tongue behavior and/or size.