# Reverse Guided Bone Regeneration (R-GBR) Digital Workflow for Atrophic Jaws Rehabilitation
## 1 Introduction  
Guided bone regeneration (GBR) with titanium meshes enabled clinicians to treat three-dimensional and extended defects due to the stiffness of their structure, which contains in situ the particulate graft and contrasts the collapse of the overlying soft tissues [1, 2, 3]. Digital technology improved this approach, allowing the design and printing of a scaffold that fits the defect to be reconstructed, correlating the bone grafting to what is needed to support the fixtures and reducing surgical timings and discomfort [1, 4, 5, 6]. Hence, the dehiscence, the significant shortcoming of this technique, seems to be lowered [4, 6, 7], with >80% bone volume regeneration predictability of what was virtually planned [4, 7, 8, 9]. Nevertheless, some doubts about the actual standardization of this approach were raised, and the reliability of the digital systems in realizing what is virtually planned is still unclear [2, 4]. Indeed, the literature is scarce on patients and the methodology of the studies, with 97-100% implant survival rate up to 30 months of mean follow-up after implant loading [2, 4, 7]. When the exposure complication is manageable, appearing after about a month since the intervention and without signs of infection, the CAD/CAM GBR approach is reliable in fulfilling what was pre-operatively planned, homogenizing the outcomes [2].  
The precision of the virtual project, comparing the starting clinical situation with the ideal prediction of the final result, is a crucial issue. The forecasting of peri-implant mucosa thickness and shape, and its interaction with the abutment and crowns to be reached at theend of treatment, can allow for predicting the surgical or prosthetic correction of the soft tissues before or after the bone reconstruction with a more accurate implant positioning [10]. Static computer-aided implantology (s-CAI) entails coupling the tissue surfaces data from intra- or extra-oral scanning with the radiographic hard tissues information from the CT to guide the seating of the fixtures. This approach obtained better results regarding insertion accuracy than the free-hand one [11]. The starting bone defect extension and the soft tissue characteristics were crucial in conditioning the GBR with membranes and no customized titanium meshes [12], even with a customized protocol [2; 13]. Atrophic sites present an impaired mucosa, raising the muscle insertions toward the residual bone ridge and losing keratinized epithelium. The soft tissues can be irregular with different thicknesses, scars due to previous lesions and treatment, or particularly thin and fragile. A correct soft tissue evaluation can condition the flap design, reducing the exposure rate [13]. Furthermore, the passivation of the flap to cover the graft after the muscle detachment and the periosteum releasing cuts entails a vestibule reduction and a lifting of the mouth pavement with the necessity of soft tissue grafting procedures to obtain keratinized peri-implant marginal tissues [2; 4; 13]. According to a so-called fully digital protocol, two mono-edentulous sites were treated with implant insertion contextual to the bone reconstruction after pre-surgical planning of the ideal soft tissue architecture [14; 15]. Another clinical case was managed by evaluating the vestibular mucosa thickness to resolve the horizontal deficit of an edentulous site with surgical soft tissue modification in combination with prosthetics tricks [10].  
This paper proposes a modified workflow for manufacturing a mesh/membrane to reconstruct two different extended alveolar defects. This protocol was called \"reverse\" since the operative phase started after the digital prediction of the final ideal relationship between dental crowns and gingiva, adding the pre-operative clinical situation.