Dental implants represent an established treatment option to successfully replace teeth with excellent long-term survival rates (Albrektsson & Donos 2012; Buser et al. 2012), and high patient satisfaction (Wittneben et al. 2017). One of the basic requirements for successful implant treatment besides ideal 3D-positioning (Buser et al. 2004) is sufficient bone volume to allow for circumferential anchorage of the implant in vital bone at completion of healing. Both requirements create challenging situations whenever the ideal 3D-position is outside the available bone. Generally, a minimum of 1.5 mm of pristine bone should surround an implant (Fig. 1), ideally with a buccal wall thickness of >1.5 mm on the facial aspect (Buser et al. 2004; Monje et al. 2019). Taking into consideration that standard implants have a diameter of approximatively 4 mm, the minimum recommended horizontal dimension of the crestal bone is 7-8 mm, which is rather tight, especially in the anterior zone of the maxilla.

Tooth loss is associated with bone resorption, mainly affecting the buccal bundle bone (Araújo & Lindhe 2005). In general, a mean horizontal bone loss of 3.8 mm can be observed in the anterior region (Hämmerle et al. 2011). Depending on the reason for tooth loss (infections, longitudinal fracture of the root etc.), the extraction technique (osteotomy versus simple extraction) and the thickness of the buccal bone wall, the amount of bone resorption can easily exceed these mean values (Fig. 2). With regard to the predictability of the resorption process it could be shown that bone walls of >1-mm thickness show statistically significantly less resorption than bone walls of <1-mm thickness (Chappuis et al. 2017). In the anterior maxilla, this unfavorable condition dominates daily practice and is present in roughly 90% of patients (Huynh-Ba et al. 2010; Braut et al. 2011). Horizontal bone augmentation procedures become a frequently needed intervention in implant surgery to re-establish a buccal bone wall of sufficient height and thickness.

Many different procedures for horizontal bone augmentation such as bone blocks, bone rings, bone splitting, guided bone generation (GBR) with membranes or titanium meshes and individualized allograft blocks have been described in the literature (Jacotti 2006; Figliuzzi et al. 2013; Schlee & Rothammel 2013; Atef et al. 2020). Independent of which procedure and material are used, they all have the same goals: the augmentation procedure should be predictable and safe, easy to handle, associated with low patient morbidity, cost efficient and scientifically well documented. With regard to this, the GBR technique has been shown to be one of the most favorable techniques fulfilling all these requirements with the overall best scientific documentation (Aghaloo & Moy 2007; Benic & Hämmerle 2014). However, GBR also has limitations and its success strongly depends on the correct selection criteria for when to use it. These selection criteria are primarily based on the defect morphology and the size of the defect. Other techniques such as bone blocks are considered more favorable in severe defects with a challenging defect anatomy.

In order to choose the most appropriate bone augmentation procedure, it is important to understand the biology of bone regeneration in defects of the alveolar ridge. Regeneration of a bone defect basically requires the presence of bleeding bone walls with access to cancellous bone and its blood vessels. Though it is described that the periosteum has regenerative potential (Lin et al. 2014), it could be shown in histological sections from a preclinical study that bone regeneration in a membrane-protected defect takes its origin from the ingrowth of woven bone from the bone walls in a centripetal way to the middle of the defect (Schenk et al. 1994). The same observation was made roughly 10 years later for the regeneration of extraction sockets (Cardaropoli et al. 2003). In consequence, a classification system – based on the number of available bone walls – is often used in daily practice. With this, a differentiation between 3-wall, 2-wall, and 1-wall defects is applied as described in a book chapter of the recently issued 3rd edition of the GBR book (Buser et al. 2021). Another important prerequisite for successful regeneration is the location of the implant within the bony envelope leading to a 2-wall or even a 3-wall defect. Implant positioning with the exposed implant surface outside the bony envelope results in a 1-wall defect and requires alternative treatment options because bone regeneration with particulated bone grafts and fillers is very difficult and not predictable in these situations.