## 1 Introduction  
Dental implants are nowadays considered a highly predictable treatment option to replace missing teeth with a reported long-term (10-20 years) survival rate of 89.5% and 92.7% [1, 2]. However, when bone dehiscence is present at the time of implant placement to place a dental implant, different bone regenerative techniques, such as guided bone regeneration (GBR), need to be used. GBR is a successful, well-documented and widely used dental surgical procedure used to treat various alveolar bone defects [3, 4]. GBR requires the application of a resorbable or non-resorbable membrane to exclude non-osteogenic tissues from interfering with bone regeneration and can be used as a surgical procedure preceding implant treatment [5, 6].  
Currently, a broad spectrum of grafting materials of different origins, such as allografts, xenografts, and alloplastic materials, is available [7, 8]. Similarly, materials of different origins are available in various forms ranging from blocks to bone chips, and this provides the dental surgeon and patient with many treatment options. The situation with membranes is no different; the choice between non-resorbable (including PolytetrafluoroethylenePTFE; expanded polytetrafluoroethylene e-PTFE, high-density polytetrafluoroethylene d-PTFE; Titanium-Reinforced PTFE Membranes, Titanium Mesh and Cage membranes) and resorbable membranes of different types and origins is wide [9]. Depending on their origin, resorbable membranes used in GBR can be divided into natural polymers (collagen membranes) and synthetic polymers (aliphatic polyesters (e.g., poly (lactic acid) (PLA), poly (polyglycolic acid) (PGA), poly (\(\epsilon\)-caprolactone) (PCL) [10; 11].  
Undoubtedly, the treatment options are wide, but most importantly, they require adaptation to a specific clinical situation. Since the beginning of dental implantology, implant placement in the aesthetic zone in such a way that implants imitate nature and, at the same time, maintain healthy tissues has been challenging. The perfect three-dimensional position of the implant is critical for proper prosthetic restoration, especially in the aesthetic area. When a proper prosthetic-driven implant positioning of the implant requires covering small fenestration or dehiscence caused by an exposed implant surface on its facial aspect, the GBR technique, in combination with xenogeneic bone and resorbable membranes, can be used [12]. In these situations, the GBR can be used simultaneously with implant placement since, according to [PERSON] and [PERSON] [13], guided bone regeneration procedures are a reliable means for treating dehiscences and fenestrations created during implant placement.  
The first objective of the study was to evaluate the long-term effectiveness of the applied GBR procedure in correcting small peri-implant bone dehiscence using the xenogeneic deproteinized bovine bone mineral material and a xenogeneic native bilayer collagen membrane. A secondary objective was to evaluate factors that may influence marginal bone loss.  
Hypotheses of this study were:  
1. When the GBR procedure is used for implant dehiscence treatment, it does not influence the marginal bone loss around the implant in 7 years of observation;
2. There are no gender-related differences in marginal bone loss around implants;
3. There is no difference in marginal bone loss around implants between age groups.