The aim of this randomized clinical trial was to compare

I- The clinical behavior of implant retained removable overdenture using telescopic retention system fabricated using CAD\CAM techniques where the effects of the different material combinations were examined in regard to:

1. Retention of the mandibular over-denture.
2. Plaque accumulation, presence of calculus, bleeding tendency, health of peri-implant mucosa and probing depth.

II - The radiographic behavior regarding marginal bone level and bone density.

This study compared implant-retained telescopic mandibular overdentures reinforced with milled metal framework (Co-Cr) and milled polymer (PEEK) in terms of retention, periimplant mucosal health, and radiographic changes in twelve patients.

Initially, all patients received conventional complete dentures. A CBCT was conducted prior to the implant surgical phase using a radiographic stent. Two implants were inserted interforaminally using a flapless method. The implants were then left for three months to allow for osseointegration, and the patients were instructed to wear their complete dentures during the healing process.

Following the osseointegration period, the prosthetic phase began, with all patients obtaining primary crowns constructed from titanium alloys. The patients were then randomly divided into two groups, 6 patients in each group. Group A received a COCR framework that included secondary crowns while group B received a PEEK framework and secondary crowns.

Retention was tested for each group (baseline, 6 months, 9 months, and 12 months) using a digital force gauge. The pulling force gradually increased until the denture was dislodged. The retentive force was measured during the initial separation of the telescopic crown.

Soft tissue conditions was assessed using single coded probe for each group (baseline, 6 months, 9 months, and 12 months).Bleeding tendency on probing, plaque accumulation, calculus deposition, perimplant mucosal health were the parameters for each group evaluation.

While the radiographic assessment was performed for each group (baseline, 6months, 9 months and12 months) using digital periapical radiographs by the means of XCP instrument and digital sensor. The marginal bone level was measured by evaluating the difference of bone level at the neck of the implant. The bone density was evaluated at three areas surrounding the implants.

Inclusion Criteria:

• Completely edentulous patients
• Absence of any systematic contraindications to implant surgery,
• Presence of sufficient bone height and width to accommodate implants with length 14mmm and diameter 3.6mm.
• Inter-arch distance more than or equal 10 mm.
• Four months since the last extraction had taken place.
• No previous bone augmentation procedures.

Exclusion Criteria:

• psychological disorders prohibiting implant treatment.
• Heavy smoking (10 cigarettes/day)
• Alcohol and/or drug abuse,
• Physical disability that may interfere with the ability to perform oral hygiene.
• Patients were receiving drugs which impair healing .