Despite the high survival rate of dental implants inserted in maxillary sinuses that have undergone sinus floor elevation (SFE) with bone grafting, complications still occur [1–3]. Sinus membrane perforation is reported to be the most common complication [4, 5]. Postoperative maxillary sinusitis is less common (0–22%) [6, 7]; nevertheless, it could potentially compromise the outcome of SFE and affect the overall well-being of the patient [8]. Developing postoperative sinusitis is often associated with a reduction in the patency or complete obstruction of the ostium due to inflammatory edema in the sinus or preexisting chronic sinusitis [8–12].

Various cyst-like pathologies can be found in the maxillary sinus, including a pseudocyst and a surgical ciliated cyst [13]. However, a thickened mucous membrane can be physiologic or benign without presentation of symptoms. In contrast, cyst-like entities may need surgical removal due to their pathologic progression [14]. A surgical ciliated cyst is defined as a posterior maxillary cyst found after the surgical treatment of maxillary sinusitis [15]. Pseudocysts are diagnosed as dome-shaped, noncorticated soft tissue opacities with a well-defined border in the maxillary sinus [16].

In the dental literature, sinusitis has most commonly been identified on radiographs as thickening of the sinus membrane [12]. Mucosal thickening >2 mm is considered a threshold for pathological thickness [17]. Although mucosal thickening is the most common radiographic finding observed regarding sinus pathology [18], the knowledge regarding its clinical significance on the outcomes of dental implants and grafting in the maxillary sinuses is still limited [19].

The purpose of this retrospective study was to evaluate the outcomes of dental implant placement in sinus augmented areas with preexisting mucosal thickening of more than 2 mm. The aims of this study were to (1) determine the success rate of dental implant placement in augmented maxillary sinus areas with mucosal thickening, (2) evaluate the effect of gender, age, and smoking on the dimensions of sinus mucosal membranes, and (3) based on the overall findings, develop a written protocol to guide dental practitioners regarding cases that exhibit mucosal thickening (the threshold of safety).