The imaging diagnosis by CBCT allows the clinician to reduce the risk of damaging vital structures in mandibular areas with limited residual bone. The surgical placement of implants in the anterior edentulous mandible for overdenture treatment is a predictable option with long-term successful results. However, implant-guided surgery in the atrophic anterior mandible presents several anatomic challenges owing to the vascular and neurologic structures related to this region. The incorporation and development of CBCT have resulted in a better appreciation of the risks involved with surgery in this area [14,15].

Moreover, in the anatomical diagnosis of the edentulous mandible, the assessment of the bone volume and density of the mandible by CBCT allows a practitioner to select the best implant design and surface to achieve good primary stability and speed osseointegration. All patients of the present study were evaluated, by CBCT, before the surgery to assess the specific bone characteristics, as cortical density and trabecular spaces, choosing the best location for the placement of implants, also according to prosthodontic approach with overdentures [12].

In the present study, based on the clinical and CBCT findings and the patient’s expectations, treatment planning was established to place two implants in the anterior mandible by using flapless surgery. Fourteen patients received 28 implants, inserted through a flapless guided surgery. No implants were lost, during the mean clinical follow-up of 44.7 ± 31.4 months. The cumulative implant survival rate was 100%. Flapless implant surgery is a minimally invasive surgical approach that has several important advantages for both the clinician and the patient [14,15]. Flapless guided implant surgery increases the ability to insert implants more precisely, specifically in fully edentulous cases, with an important reduction of surgery duration, better clinical conditions after surgery, and the possibility of placing restorations for immediate loading. Computer-guided dental implant systems also provide necessary information for the prosthetic evaluation of mandibular edentulous patients [18,19,20,21,22,23,24].

Evidence provided by implant-supported mandibular overdenture research on different loading protocols is important. Several systematic reviews have been published about the clinical applicability of conventional-, early-, and immediate-loaded implants for overdenture treatment. Among the studies, the mandibular overdenture design included the use of two, three, and four implants and different attachment systems as locator, bar, and balls [26,27,28].

The mean MBL found by us (1.25 mm) is higher than that reported by Galindo et al. in 2015 [29], who published a 0.6 mm loss (mean distal and mesial) at 6 months; and 1.11 mm at 18 months (mean distal and mesial). Our study has a mean follow-up of 44.7 months (±31.4), and if we analyze other studies, such as the one by Palacios-Garzón et al. [30], they accept a loss of less than 2 mm in the first year. However, we must not forget that losses greater than 0.5 mm in the first year increase the risk of presenting peri-implantitis in the long term by 5.43 times [31]. Both in the cases of peri-implantitis, and with regard to the MBL data, we have not been able to relate them to the height of the locator-attachment system used, all of which is that the literature refers to a higher bone order in the lower systems [32,33].

Computer-guided surgery in the rehabilitation with overdentures, by immediate loading of four implants, of edentulous mandibular patients has been investigated in several studies [18,19,24,28]. During a follow-up of 2 years, 10 consecutive patients were restored with implant-supported overdentures [18]. Patients were treated with four intraforaminal implants using a computer-guided flapless approach. No implant was lost. Patients demonstrate the ability of oral hygiene for maintenance of peri-implant tissue health. The satisfaction of patients was very high. These results reported that computer-assisted implant dentistry can be a predictable protocol for treating elderly edentulous patients with a mandibular overdenture [18]. These satisfying results were confirmed by a recent one-year randomized controlled clinical trial [19]. Thirty mandibular edentulous patients were rehabilitated using overdentures supported by four implants, inserted by guided surgery in canine and second premolar position. These patients were randomly distributed into two groups, performing an immediate loading protocol using resilient stud (locator) or stress-free implant bar (SFI-Bar) attachments. After a one-year follow-up, implant survival in the locator group was 96.6%, while in the group using SFI-Bars, it was 98.3%. The locator showed significantly higher overall satisfaction, satisfaction with retention, comfort, and cleanliness of overdentures compared to bar-overdenture-rehabilitated patients. [19].

Immediate loading of two implants in overdentures by guided surgery, in edentulous mandibular patients, has been investigated in some clinical cases [22,23]. In these clinical situations was necessary high primary implant stability measured by resonance frequency analysis. Implant stability quotient values over 65, which was a prerequisite for the immediate loading, should be accomplished. Locator abutments were inserted on the implants and the metal housings were seated on the locators [23]. Another type of used retention system of overdentures was magnetic attachments [22].

In the present study, after guided placement of implants, the treatment planning was the early delivery of implant-retained overdentures at 6 weeks. No implants were lost, during the clinical follow-up. The cumulative implant survival rate was 100%. Early loading of two implants placed in edentulous mandibular patients can be a reliable and predictable technique for implant-supported overdentures [26,27]. Although all three loading protocols (conventional, early, immediate) provide high survival rates of treatment with overdentures, early- and conventional-loading protocols are still better documented than immediate loading and seem to result in fewer implant failures during the first year of clinical follow-up [26].

Marginal bone loss in overdentures by guided surgery implants has been reported in a recent study [19]. The marginal bone loss of immediately loaded implants ranged from 0.68 to 0.83 mm after a 1-year clinical follow-up. Similar mean values were obtained in locator (0.83 ± 0.10 mm) and bar retention systems (0.87 ± 0.13 mm). Marginal bone loss was significantly higher at 12 months compared with 6 months. This increased marginal bone loss can be due to the bone response to overdenture loading and bone maturation combined with functional occlusal forces [19]. In the present study, the mean marginal bone loss was 1.25 ± 0.94 mm during a follow-up period of 44.7 ± 31.4 months. Clinical outcomes showed a higher significant marginal bone loss in patients for more than 70 years and followed more than 5 years. Additionally, a common history of medical conditions (64.3%), smoking (35.7%), and periodontitis background (28.6%) can explain the higher mean values of marginal bone loss in the patients of the present study.

The global satisfaction of patients with computer-guided surgery and prosthetic rehabilitation with overdentures is very high because the postoperative pain and discomfort is very low and improves the compliance in the functional and aesthetic outcomes of prosthodontic treatment [5,18,24]. However, biological and technical complications had also been reported in several studies of treatment with implants placed using computer-guided surgery [28,34]. In the present study, four implants (14.3%) in two patients (14.3%) were associated with peri-implantitis. Peri-implantitis was more frequent in those patients with a previous history of periodontitis (50%) and smoking patients (40%). Additionally, the locators abutments can show an important vertical bone loss with higher plaque and gingival scores and increased probing depth with the progress of time that can explain the development of peri-implantitis in susceptible patients (i.e., periodontitis background and smoking habits) [19].

Prosthodontic complications were frequent in the present study. Four patients (28.5%) showed some kind of technical complications. Two patients (14.2%) showed resin fracture of the prosthesis, and two patients (14.2%) needed the change of locator-attachment system. The matrix resiliency among different locator systems can eventually be compromised by insertion and removal of the overdenture. This prosthetic complication may require reactivation or even replacement of the matrix; however, this maintenance procedure can be easily provided by the clinician in clinical practice [27].