Background <p>Implants offer superior physiological and functional outcomes; however, their success depends on the availability of adequate local bone volume. In cases of severe maxillary resorption, this approach is often challenged by anatomical limitations such as expanded maxillary sinuses or other features that compromise the residual bone volume.</p> Objective <p>This study aimed to assess histological outcomes of lateral window sinus augmentation performed with a composite graft comprising a 2:1 mixture of allograft (corticocancellous bone; HC Biologics, USA) and a synthetic bone substitute (30% HA/70% β-TCP) combined with autologous platelet-rich fibrin (PRF).</p> Materials and methods <p>This prospective registered histomorphometric study analyzed 60 biopsy specimens obtained from sinus augmentation sites in a Vietnamese cohort with severely atrophic posterior maxilla (residual bone height ≤ 4&#xa0;mm). Bone regeneration was evaluated following lateral-window sinus augmentation using a composite graft composed of a 2:1 mixture of allograft (corticocancellous bone; HC Biologics, USA) and a synthetic bone substitute (30% HA/70% β-TCP; Osteon II, Dentium, Korea) combined with autologous platelet-rich fibrin (PRF). Histomorphometric analysis was performed on three non-consecutive sections per specimen using ImageJ. The standardized protocol and relatively large number of biopsies provide clinically relevant histological evidence.</p> Results <p>The mean proportion of newly formed bone was 31.28% (95% CI: 26.70%–35.85%), while the mean residual graft material accounted for 15.48% (95% CI: 12.29%–18.68%). Histological evaluation revealed no evidence of inflammatory infiltrates or tissue necrosis. The presence of a well-organized lamellar bone architecture, with Haversian canals and osteocytes embedded within lacunae, indicated advanced bone maturation. At the graft periphery, numerous osteoblasts and osteoclasts were identified, reflecting ongoing bone remodeling. Residual bone height averaged 2.31 ± 0.93&#xa0;mm. Newly formed bone accounted for 31.28 ± 17.71%, while connective tissue represented 53.23 ± 18.75%. Primary implant stability (ISQ1) was 58.20 ± 6.47 and increased to 74.85 ± 7.68 at six months (ISQ2). ISQ1 correlated strongly with residual bone height (<i>r</i> = 0.626) and inversely with connective tissue (<i>r</i> = − 0.627), and moderately with newly formed bone (<i>r</i> = 0.405) (all <i>p</i> ≤ 0.001).</p> Conclusions <p>Within the limitations of this prospective single-arm study, sinus augmentation using a composite graft composed of a 2:1 mixture of allograft (corticocancellous bone; HC Biologics, USA) and a synthetic bone substitute (30% HA/70% β-TCP; Osteon II, Dentium, Korea) combined with autologous platelet-rich fibrin (PRF) resulted in favorable histological and histomorphometric outcomes after six months of healing. The regenerated tissue provided a biocompatible osteoconductive environment supporting bone formation within the augmented sinus. Nevertheless, because the study lacked a comparator group, the individual contribution of each graft component cannot be determined, and the findings should be interpreted as descriptive rather than causal evidence.</p> Trial registration <p>ClinicalTrials.gov NCT07186166, 30 August 2025. Retrospectively registered.</p>