The optimal three-dimensional positioning of the implant is a critical determinant for the success of implant restoration and influences the biomechanical factors affecting its longevity. Adequate alveolar bone volume holds considerable significance for achieving this ideal three-dimensional position. The patient’s alveolar bone must not only meet the required height and lateral width for implantation but also ensure a minimum of 1.5–2 mm bone plate thickness on both buccal and lingual aspects of the implant (Chappuis et al., 2018; Chiapasco and Casentini, 2000). The study conducted by Gemma Rubio et al. utilized T-Scan measurements, and demonstrated that the posterior teeth contributed to approximately 82.4% of the total biting force, further highlighting their pivotal role as the primary functional area for biting and bearing the majority of this force (Rubio-Ferrer et al., 2024). Therefore, in implant restoration, posterior implants typically have a larger diameter than anterior ones, demanding greater alveolar bone width. However, tooth loss can result in varying degrees of alveolar ridge resorption, and clinically, more than half of patients fail to meet the criteria for direct implantation. Hence, horizontal bone augmentation surgery is required.

Currently, commonly used and relatively stable bone augmentation techniques in clinical practice include GBR, bone splitting technique, and autogenous bone grafting techniques such as Onlay grafting. Among them, GBR is characterized by lower operational difficulty and easier availability of the grafting material. However, due to the limited spatial support capacity of the collagen membrane, GBR is effective for only modest bone augmentation, making it suitable mainly for mild horizontal ridge deficiencies (Wang et al., 2024). Compared to GBR, the indications for the bone splitting technique are more restricted, as a specific amount of cancellous bone between the buccal and lingual bone plates is required to achieve effectiveness (Atef et al., 2019). Currently, autogenous bone grafting techniques, such as Onlay grafting, are continuously considered the benchmark for ridge reconstruction owing to their exceptional biocompatibility, robust bone guiding, and inducing effects, as well as their osteogenic properties (Sánchez-Labrador et al., 2021). Bone blocks, specifically the mandibular symphysis, lateral ramus, and ramus of the mandible, or those obtained from the ilium, are procured for Onlay grafting, a widely employed technique in addressing horizontal defects within the alveolar bone (Fluegge et al., 2021; Sakaguchi et al., 2023; Wang et al., 2022). However, conventional Onlay bone grafting techniques necessitate the establishment of an additional surgical site, thereby posing potential risks. These risks include wound dehiscence, graft necrosis, infection, hemorrhage, nerve impairment, and sensory aberrations. Therefore, optimizing clinical outcomes in Onlay bone grafting hinges on maximizing graft survival, minimizing resorption, and reducing surgical trauma, which requires careful wound closure. The rapid establishment of blood supply, appropriate microstructural features of the bone fragment (including the composition of cortical and cancellous bone), optimal fixation of the bone fragment, and minimal mechanical stress from soft tissue sources are essential requirements (Sanz-Sanchez et al., 2000; Bayram et al., 2024a).

In the study, the coronal ridge necessitated a horizontal bone augmentation to achieve a quadrilateral bone block with a thin superior aspect and thick inferior aspect. Following slight modification, the block was rotated counterclockwise by 180° and anchored in the original surgical site using titanium pins, in conjunction with conventional GBR techniques. This facilitated the transfer of ample bone volume from the basal region of the coronal ridge to its crest area, eliminating the need for an additional surgical site and minimizing surgical trauma. Meanwhile, it also promoted direct contact between the graft material and recipient cancellous bone, enhancing the stability of fixation and accelerating angiogenesis (Rosenthal and Buchman, 2003).

The GBR assisted in situ Onlay bone grafting provides a minimally invasive alternative to traditional Onlay bone grafting in the posterior mandible. In recent years, several scholars in clinical practice have utilized autologous bone ring as an alternative to tent nails for horizontal alveolar bone augmentation, achieving promising outcomes. The primary distinction from Onlay bone grafting lies in the use of smaller-volume autologous bone rings as osteogenic and supportive structures, which are inherently constrained by the dimensions of the bone block. Further clinical evidence is required to substantiate their supportive and osteoinductive effects (Chandra et al., 2019; Durrani et al., 2023). In contrast to traditional autogenous block bone grafting for horizontal bone augmentation, which often utilizes the mandible or ilium as donor sites for autogenous bone, this method presents a unique advantage (Elsayed et al., 2024; Manfredini et al., 2022; Mittal et al., 2023). While studies have demonstrated the significant osteogenic potential of this approach, it is frequently accompanied by complications at the donor site, including pain, hemorrhage, infection, nerve injury, and sensory aberrations (Pang et al., 2024; Zhang et al., 2024; Bayram et al., 2024b). In recent years, a relatively abundant bone volume in the basal bone area of the maxillary anterior teeth region has been observed during autogenous bone grafting procedures for vertical bone augmentation (Wang et al., 2022; Yuan et al., 2020; Yang et al., 2021; Wang et al., 2016; Korsch et al., 2022). Horizontal bone augmentation in the anterior region can be accomplished by mobilizing the bone ring or block from the nasal ridge of the maxillary anterior teeth or the mandibular medial suture to the buccal aspect of the alveolar ridge. Specifically, to achieve horizontal bone augmentation on the buccal side of an adjacent edentulous alveolar ridge, Wang et al. implemented a multidisciplinary approach by horizontally transplanting a bone block extracted from an affected tooth’s apical region during endodontic surgery (Wang et al., 2021). While these methods aim to minimize the number of incisions, there still exists a disparity between the bone harvesting area and the implantation site, posing potential risks of injuries and complications associated with the former. To this end, strictly speaking, this technique, which involves transferring relatively abundant bone mass from the basebone area to the crest of the alveolar ridge through bone block displacement, reduces the number of incisions required, effectively condensing the procedure into fewer surgical sites. Furthermore, piezoelectric surgical instruments were hereby employed for bone block harvest and separation, which not only safeguarded soft tissue and nerves but also effectively controlled bleeding while providing an enhanced operative field (Maihemaiti et al., 2023; Rolek, 2024). Furthermore, it could harness the potential of fluid vibration waves to effectively mitigate bacterial levels and yield a desired disinfection outcome (Pavlikova et al., 2011). Additionally, minimizing bone damage during the process of bone collection proves to be more efficacious compared to the application of a ring drill or a bur (Robiony et al., 2007).

The innovative aspect of the in situ Onlay bone grafting technique used in conjunction with GBR lies in its efficacy in establishing a spongy-bone interface within the grafting area. Research has demonstrated that this composite structure with cancellous bone on both sides provides enhanced stability for the bone block compared to conventional cortical-cortical or cortical-cancellous interfaces in Onlay grafting procedures. This enhanced stability effectively counteracts rotational forces and micro-motion of the implant, aligning with the principle of primary implant stability as a critical factor for successful bone augmentation (Wang and Boyapati, 2006). In terms of expedited blood vessel regeneration, the trabecular-trabecular interface offers an additional advantage. According to Oppenheimer et al., the local microenvironment of the transplanted bone and the extent of blood vessel regeneration are considered pivotal predictors for graft survival. Among various factors influencing non-vascularized autogenous bone graft outcomes (Oppenheimer et al., 2012), in situ bone grafting techniques involve the transfer of bone from one site to another, ensuring the comparable vascular distribution, recipient distribution, as well as immunogenic and genetic characteristics of the donor and recipient areas (Pape et al., 2010). Additionally, a multitude of studies have demonstrated an accelerated and more substantial restoration of blood supply of bone blocks containing trabecular bone compared to those composed solely of cortical bone (Gens et al., 2023; Wang et al., 2023; Lee et al., 2022). Furthermore, the abundance of osteoblasts within the bone marrow cavity, exposure to osteoinductive proteins, and the proliferation and differentiation of mesenchymal stem cells triggered by acute bone injury all contribute significantly to effective vascularization and osseointegration. In this particular case study, a blurred demarcation was observed between the transplanted bone block and alveolar bone upon 7 months of bone augmentation. The transplanted bone block exhibited a healthy coloration, while both implants achieved torque values of 35 N ·cm. Collectively, these results indicate that the fresh wound surface at the harvest site is an ideal receptor area for non-vascularized bone block transplants, offering sufficient blood supply for their viability. This approach performs better in promoting early and rapid osseointegration compared to traditional Onlay bone grafting techniques involving nutrient hole preparation.

However, the in situ Onlay bone grafting technique also has certain limitations, as the available bone volume at the in situ donor site is constrained compared to that of traditional Onlay bone grafting donor sites (Pape et al., 2010). Furthermore, achieving a precise fit between the transplanted bone block and the margins of the recipient area remains challenging. Thus, additional measures should still be necessarily explored to address this limitation. According to Thomas et al.'s study, combining autologous block bone transplantation with absorbable collagen membrane and demineralized bovine bone granules could yield highly effective and predictable levels of alveolar bone augmentation. The patients experienced an average horizontal bone thickness increase of 4.6 mm, with minimal bone resorption of 0.36 mm observed from the completion of bone augmentation to implant placement (von Arx and Buser, 2006). Similar conclusions were also drawn by Maiorana et al. They revealed that the bone resorption rate in the grafted area treated with ABBM particles was merely 9.3%, while the uncovered area exhibited a higher bone resorption rate of 18.3% (Maiorana et al., 2005). Therefore, Bio-oss demineralized bovine bone granules and collagen membrane were hereby employed to facilitate in situ Onlay bone grafting, and thus to augment the grafting efficacy. The primary role of Bio-oss bone powder granules encompasses two aspects. Firstly, they fill the defect area where adequate vascularization enables their involvement in osseointegration and contour remodeling. Secondly, once enveloped by fibers following the absorption of the collagen membrane, the Bio-oss bone powder granules covering the transplanted bone surface act as a barrier against bone resorption. This is because they cannot engage in osseointegration due to an insufficient blood supply (Zerbo et al., 2003). The application of an absorbable collagen membrane in the early phase of bone graft integration can exert a certain influence on maintaining contour and preventing the resorption of block bone graft (Chappuis et al., 2017; Cordaro et al., 2011). Additionally, in cases of limited bone volume, digital tools can enhance surgical precision and personalization. For instance, a precise bone block tailored to the size and contour of the surgical site can be obtained by utilizing preoperative CBCT data to design bone volume parameters and employing 3D printing technology to create a drilling guide. This approach effectively minimizes unnecessary surgical trauma while reducing bone loss associated with reshaping the bone block (Wang et al., 2022).

The present case report describes the utilization of GBR assisted in situ Onlay bone grafting technique for horizontal bone augmentation in the posterior region of the mandible, providing a viable and reliable alternative. By employing autogenous bone blocks in situ through trabecular-trabecular contact, this technique enhances the stability and accelerates the blood supply reestablishment, yielding satisfactory bone augmentation effects with minimal surgical trauma. Notably, it exhibits remarkable resistance to resorption during both the healing phase of the bone graft and the post-implant restoration period. Moreover, when combined with GBR technique, the horizontal bone augmentation effect and contour maintenance ability are further augmented. Compared to pre-implantation bone grafting, the surgical area exhibited an average increase in alveolar bone volume of 1,628.21 mm³ over a 7-month follow-up period in this case. However, additional clinical cases are still required to substantiate the reliability of this innovative technique.