We read the article by Bassi et al. entitled “Maxillary sinus lift without grafting, and simultaneous implant placement: a prospective clinical study with a 51-month follow-up” with great interest. The authors presented a highly important clinical study of maxillary sinus lift procedures in the posterior region of the maxilla, using no graft material. The sinus mucosa was lifted together with the anterior wall of the osteotomized maxilla, and supported by dental implants. Most importantly, the blood clot was the only filling material around the inserted implants and, as such, the authors claimed a high success rate of bone regeneration. The degree of synthesis and deposition of extracellular collagen matrix followed by bone mineralization was also observed to increase over time and was ascribed to the recruitment and migration of osteogenic cells and their differentiation into osteoblasts. Based on previous studies, Bassi et al. postulate that the cells that migrated to the blood clot filling the elevated sinus membrane originated from the bone marrow of the alveolar bone, as well as from the periosteum of the elevated membrane.

Although we consider the data provided in the reported article to be of great importance, the claimed source of the cells seems somewhat speculative. Therefore, we would like to further support the study concerned with our own data. With regard to the bone-regenerating cells, we have demonstrated that these cells originate from the maxillary sinus Schneiderian membrane (MSSM), and based on other studies, from the sinus maxillary floor. The osteoprogenitor cells present within the MSSM confer inherent osteogenic potential to the membrane, culminating in bone formation around the inserted implants. More specifically, we have shown by histological techniques and flow cytometry analysis, that MSSM-derived cells express many osteogenic markers and can support extracellular matrix mineralization.

The osteogenic potential of MSSM-derived cells was also confirmed in vivo, as evidenced by bone structures formed at ectopic sites. Furthermore, new bone formation was observed subcutaneously after transplanting MSSM folded around a fibrin clot. These studies strongly indicate the importance of both human MSSM and its constituent cells, as well as the fibrin clot, in understanding bone regeneration in graftless sinus lifting procedures. The combination of fibrin and the MSSM osteoprogenitor cells is thus necessary and sufficient to provide an optimal microenvironment for bone formation. In addition, cultures of cells isolated by enzymatic digestion from the membrane showed high alkaline phosphatase (ALP) enzyme activity and mRNA expression of classical osteogenic markers (e.g., ALP, osteocalcin, osteonectin, and bone sialoprotein), providing evidence of the osteogenic potential of the cultured cells.

In a recent study performed by our group, 18 patients underwent 30 graftless sinus lifting procedures with immediate insertion of 72 dental implants. Fifteen bone biopsies from the newly formed bone near the basal floor and 15 bone biopsies from newly formed bone near the elevated membrane were collected at 6 months postoperative. Based on histological techniques and standard histomorphometric data, we demonstrated that the bone tissue taken from the two types of biopsy were fairly similar. These findings provide strong support of the hypothesis that the bone formation was initiated within the fibrin scaffold or clot at the same rate and at the same density, indicating that the cells had migrated from the sinus membrane and basal floor at the same rate and were distributed evenly to form the arranged woven bone tissue.