In nongrafted sinus-lifting procedure, several devices such as titanium [12–14], hollow hydroxyapatite [15], and bioresorbable materials [16] have been used for space retention to maintain the lifted sinus membrane and the results of predictable bone formation have been reported in addition to histological examination. In this case, a mesh plate device consisting of HA/PLLA materials was applied for space maintenance under the elevated sinus membrane in combination with dental implant placement. PLLA materials are generally used for bone fixation in the extremities and maxillofacial region [17]. Since the PLLA materials can stay firm and stable in vivo for several years, it probably has enough strength to resist air pressure in the sinus. In addition, the flat surface of the mesh device could provide wide elevation of the sinus membrane and provides a favorable condition to facilitate bone formation in nongrafted sinus lift procedure.

In conventional sinus lift with grafting materials, the materials are usually transplanted to the secluded space under the lifted sinus membrane covering the entire length of the implants [1], and in the initial stage after sinus augmentation, the grafted sinus floor is consistently located above the implant apex [11]. However, the augmented bone volume has a tendency to decrease postoperatively. After 2–3 years, it would generally reach the level same as or slightly below the implant apex [11]. In addition, such tendency for bone loss seems to be consistent in the sinus lift procedure without grafting materials [18] and might reflect the influence of physiological stimuli by implant loading and maxillary sinus ventilation [11]. Conversely, a few studies on nongrafted sinus lift procedure showed a contentious increment of intra-sinus bone volume [7, 13]. Nedir et al. [7], in their sinus lift procedure using the crestal approach without grafting materials, reported postoperative bone gain around the implant apex that increased slightly over the course of 2 years after surgery. In our case, the bone regeneration surrounding the protruded implant was confirmed at 6 months after implant insertion; thereafter, during a follow-up period of about 3 years, additional increase of new bone occurred above the implant apex and new bone was continuously formed in the sinus. In the follow-up CT taken at 42 months after the implant insertion, there was no significant change in the situation of the HA/PLLA mesh device 6 months after the implant insertion and the condition of lifted sinus membrane was stable. Therefore, the intra-sinus augmented site under the HA/PLLA mesh plate was not affected by pneumatization due to air pressure associated with respiration, which could contribute to the maintenance of bone volume after sinus lift [11]. In nongrafted sinus lift, blood clots are the only filling material around the inserted implants, and the recruitment and migration of osteogenic cells to the blood clot that fills the defect under the elevated sinus membrane generally occur from the bone marrow of the alveolar bone, the periosteum of the elevated sinus membrane, or both [18–20]. Recent studies demonstrated that the maxillary Schneiderian membrane presents osteogenic potential and contributes to the process of bone regeneration, as evidenced by in vivo and in vitro studies [19, 20]. Interestingly, in our case, bone formed from the intra-sinus dense soft tissue under the HA/PLLA mesh plate device. Soft tissue density could be regarded as postoperative hyperplasia of sinus membrane. Therefore, it may reflect the osteogenic property of the maxillary Schneiderian membrane, and new bone deposition induced by the migration of osteogenic cells originating from the Schneiderian membrane might improve vertical bone height around the implant apex. Whereas although PLLA materials are biodegradable and complete degradation in vivo normally occurs over 5 years [17], intra-sinus air pressure could affect the augmented sites again after degradation of materials, and bone loss above implant apex and pneumatization may occur in the sinus thereafter. In addition, several clinical studies of orthognathic and fracture surgery in the maxillofacial unit using PLLA device reported adverse events such as wound infection after surgery [17]. Therefore, prudent observation is required to prevent the development of complications such as sinusitis or implantitis over the long term.