Objectives: Extreme alveolar ridge resorption is the result of long-term edentulism, aggravated by poor bone quality, osteoporosis or inadequate complete denture (CD) wearing. Sometimes, even the inferior alveolar nerve can be exposed under keratinized oral mucosa. Mandibular alveolar ridge resorption may sometimes be so advanced that mandibular interforaminal height can be less than 10 mm (class D or E according to Lecholm and Zarb). It’s almost impossible to achieve stable and well-functioning mandibular CD in these cases. Therefore, in addition to having poor oral health-related quality of life, patients with extreme mandibular alveolar ridge resorption are usually not satisfied with their mandibular CD. In order to preserve the alveolar bone volume, the first choice of treatment for edentulous patients has shifted from conventional CD in the mandible to mandibular overdenture supported by two standard size implants (SSIs) (McGill consensus, 2002). Moreover, the ITI approved the utilization of four mini dental implants (MDIs), which are longer than 10 mm, for mandibular CD retention and support (ITI consensus, 2014). In cases of extreme mandibular alveolar ridge resorption, it is not possible to insert SSIs or MDIs longer than 10 mm. The aim was to first test short MDIs behavior using finite element analysis (FEA) and digital image correlation (DIC). The aim of the clinical part of the study was to measure marginal bone loss (MBL), post-loading survival and success of short MDIs, as well as patient centered outcomes (PROMs) of short MDIs and prosthodontic complications. Materials and methods: The FEA was modeled after constructing one average, realistic and representative model of a fully edentulous extremely resorbed mandible (based on 5 CBCTs). Four MDIs of different lengths (short MDIs: 6 or 8 mm long; standard length MDI: 10 mm long; 2 or 2.5 mm wide) were inserted in the FEA model of the mandible and loading forces were simulated (axial and off-axial 45° loads; 20N, 50N and 100N per MDI). Data for von Mises Stress and Elastic Equivalent (von Mises) Strain were generated. The model of the fully edentulous extremely resorbed mandible used for the FEA was also used for the DIC method, where six identical models were 3D printed. In each model, four MDIs (short MDIs: 6 or 8 mm long; standard length MDI: 10 mm long; 2 or 2.5 mm wide) were inserted interforaminally and CoCr reinforced overdentures (OD) were loaded using a testing machine (Inspekt 20-1 table, Hegewald & Peschke Meß- und Prüftechnik GmbH, Nossen, Germany) with 150N loads applied on the molar region of the denture bilaterally, unilaterally and on the anterior region. The precise measurement of von Mises Strain and vertical displacement was conducted using the Digital Image Correlation Method (Gom Correlate 2019, v2.0.1; GOM-Optical Measuring Techniques, Braunschweig, Germany). The Ethics Committee of the School of Dental Medicine, University of Zagreb, Croatia, approved the clinical study protocol. Patients with extreme mandibular alveolar ridge resorption (inclusion criteria: D or E class according to Leckholm and Zarb, 1985) were examined by means of panoramic radiographs and CBCTs and were included in the study after obtaining informed consents. The exclusion criteria, in addition to general medical conditions, were the height of mucosa overlying the residual ridge ≥4 mm or a flabby ridge. All patients were prescribed antibiotics prior to the surgical procedure. Each patient received four MDIs (SlimLine, Dentium, Seul, South Korea; 2.0 or 2.5 mm wide and 6 or 8 mm long). MDIs were placed according to the manufacturer’s instructions using calibrated burs, a physiodispenser (W&H Implantmed, GmbH, Austria) and a saline solution for drill cooling under local anesthesia (Ubistesine forte 4% or Mepivastesin 3%, 3M, Germany). The required procedure was done using the flap elevation technique. When the insertion torque was ≥30 Ncm, MDIs were immediately loaded, and when the insertion torque was lower, MDIs were loaded after three months. All new mandibular overdentures were reinforced with CoCr metal framework to prevent denture fractures. Primary outcomes were marginal bone loss (MBL) and MDI survival and success rates. Peri-implant bone changes were measured on panoramic radiographs. The magnification error was corrected using the formula for crestal bone height correction reported by Yoo et al. MBL survival and success rates were measured after one year and after two years. Technical complications and prosthodontic maintenance were also recorded. Patient reported outcome measures (PROMs), such as oral health-related quality of life (OHRQoL; OHIP14 questionnaire), chewing function (chewing function questionnaire – CFQ) and orofacial esthetics (orofacial esthetic scale – OES) were also observed. Questionnaires were completed four times: pre-treatment (with old complete dentures), post-treatment (with new ODs retained by MDIs), after one year and after two years. The level of postoperative pain and swelling was measured using the visual-analog scale (VAS) over the course of ten days after MDI insertion. Results: FEA simulations showed lower levels of strain and stress after axial loading than after off-axial loading. The highest strain after axial loading was observed on the cortical layer of the thinnest part of the mandible. Observing only the MDIs and its insertion site, the highest strain was on the head of the MDI and the cortical bone around the MDI. During offaxial loadings, the highest strain was observed on the neck of the MDI as well as on the cortical bone around the MDI, opposite to the applied force. Stress showed a pattern similar to the strains one. Strain and stress patterns were equally distributed throughout the mandible and MDIs. Strain and stress values did not exceed values that could cause MDI fracture or bone resorption around the MDIs. Measuring strain and vertical displacement on 3D printed mandibles and associated dentures using the DIC method showed very low strain values, with the highest values observed at the zone of bone-denture contact and at sites closer to the force application. The DIC showed, in all loading stages, identical spreading direction with the maximum strain value within the denture-bone complex. The same mean value of vertical displacement was measured for all three types of loading, amounting to 0.2 mm on all the models. It followed that the vertical displacement was the result of denture subsidence due to “o-ring” elasticity. Twenty patients with extreme mandibular ridge resorption (75% females, 25% males) participated in the clinical part of the study. The follow-up period was one year for all twenty patients and two years for ten patients. Three MDIs broke during insertion and were immediately replaced. One patient received only three MDIs due to anatomical restrictions; all others received four MDIs each. Eleven MDIs were lost, all within the first month after insertion. The MBL after one year was 0.45 ± 0.38 mm (20 patients) and 0.78 ± 0.67 mm (10 patients) after two years. MDI survival and success after one year were 86.1%. The lost MDIs were replaced using the flap elevation technique. The subsequent calculation of survival and success rates showed a survival rate of 98.6% and a success rate of 97.3% after two years. Altogether, twenty o-rings were replaced (over two years) and several denture corrections (in the first year) were made. When analyzing PROMs, the Summary scores dropped significantly for the OHIP14 and the CFQ questionnaire (p<0.001) after the treatment and remained unchanged throughout the 2-year follow-up period (p>0.05). The OES showed a significant increase in esthetics after the treatment (p<0.001) and remained unchanged throughout the 2-year follow up period (p>0.05). The insertion of short MDIs elicited low levels of postoperative pain and swelling. Conclusion: Short MDIs showed good preclinical and encouraging clinical results throughout the 2-year follow-up period when taking into account certain requirements. It is better to apply the open-flap method in patients with extremely resorbed interforaminal bone and prepare MDIs full length in cases of very dense bones (D1 type). Flapless insertion causes a low level of postoperative pain and swelling to patients. Most importantly, short MDIs show very good patient centered outcomes by improving their quality of life, chewing function and orofacial esthetics. The effects of the treatment remained unchanged throughout the 2-year follow-up period.