# Seven-Year Retrospective Study on Conometric Retention for Complete Fixed Prosthesis
## 1 Introduction  
Mandibular complete conventional dentures are frequently associated with stability and retention problems, soreness, pain, and further loss of function [1]. To overcome these problems and improve function, speech, and comfort of edentulous patients, different implant supported fixed and removable solutions have been introduced. Among these, a two-implant supported removable overdenture has been identified as the elective treatment to restore completely edentulous mandbles [2]. Even if implant supported overdentures reduce the overall treatment time and the overall morbidity in cases of severe atrophic jaws, fixed implant supported restorations present better stability, retention, and aesthetics with a reduced extension on the soft tissue [3].  
Recently, a systematic review compared mandibular implant supported overdentures and fixes prostheses in terms of patient reported outcomes [4]. From a patient perspective, implant supported overdentures showed greater satisfaction in terms of ease of cleaning. On the other hand, fixed implant supported prostheses were associated with an overall better quality of life, with higher scores in terms of comfort, ease of chewing, and stability [4]. Therefore, the implant supported restorative option must be widely discussed with the patient, considering anatomical and socioeconomical factors and the patient's expectation.  
In the case of implant supported fixed prostheses in the mandible, the use of four implants has been widely investigated and is now a well-established and scientifically validated protocol, with a high patient satisfaction rate [5]. Indeed, recent scientific evidence has shown that implant-supported prosthetic rehabilitations are able to improve chewingand speech through improved denture stability. Furthermore, in the same systematic review, it was stated that supporting a conventional removable prosthesis through implants is the most cost-effective solution when patients want to improve their oral health quality [6].  
Traditionally, to fix prosthetic superstructures to dental implants, two main types of connection were proposed: screw retained or cemented connection. However, both these connection types present biological and technical advantages and disadvantages. Even if screw-retained restorations are easier to remove and retrieve in case of complications or need for intervention, cemented restorations allow an easier compensation of implant angulations through individualized abutments [7]. However, both these connection types have their limitations. Screw-retained restorations have been associated with a high rate of technical complications, such as screw loosening or fracture, and ceramic chipping. On the other hand, cemented retained prostheses have been associated to biological complications, such as peri-implantitis, fistulae, or suppuration due to the possible presence of excessive cement [8].  
As an alternative to the traditional retention systems proposed to restore dental implants, concentric retention has been proposed, as a successful method to provide fixed retention to complete mandibular dentures immediately after implant placement (Figure 1).  
four implant supported mandibular complete dentures in terms of marginal bone level (MBL), and implant and prosthetic success rate.