Single-implant Versus Two-implant Mandibular Overdentures

Inclusion Criteria: * Men and women ≥40 years. * ASA I-II; favorable general health or adequately controlled systemic disease (e.g., hypertension, diabetes). * Skeletal Class I relationship. * Completely edentulous mandible and maxilla. * Technically acceptable complete dentures in both jaws for ≥3 and ≤24 months; dissatisfaction limited to mandibular retention/stability. * Adequate anterior mandibular bone for implant ≥3.5×10 mm confirmed on CBCT; no bone augmentation needed. * Normal mucosal resilience (\~2 mm displacement). * Implant sites healed ≥4 months since extraction. * Willingness to be randomized and attend follow-ups; written informed consent. * Primary stability target ≥25 N-cm. Exclusion Criteria: * Uncontrolled systemic disease; neurodegenerative disease; osteoporosis requiring antiresorptives. * Chronic systemic antibiotics/steroids; bleeding disorders; active cancer therapy; metabolic bone disease; HIV. * Investigational drug use within 30 days. * Alcohol dependence/illicit drug use; xerostomia (stimulated flow \<0.7 mL/min). * Heavy smoking \>20 cigarettes/day. * TMD/orofacial pain; severe bruxism; moderate/severe cognitive impairment. * Prior implant therapy. * Non-adherence risk or inability to attend longitudinal visits. * Insufficient prosthetic space for attachment housing: inter-arch vertical distance \<8.5 mm at parasymphyseal/canine region. * Concurrent participation in another interventional study. * Erosive lichen planus or other significant oral mucosal disease; head-and-neck irradiation history. * Mandibular pathology on CBCT (e.g., cyst, tumor). * Ridge dimensions \<6 mm width or \<10 mm height at planned sites. * Active intraoral infection.