Mucous retention cysts (MRC) of the maxillary sinus are benign and self-limiting lesions that originate from the accumulation of mucus within the sinus mucosa as a result of ductal obstruction of the seromucous glands.^(1,2) True MRCs have a thin epithelial lining, whereas pseudocysts lack an epithelial wall and are caused by diffuse subepithelial accumulation of inflammatory exudate.^(1,2)

Most cases are incidental discoveries during radiography, and are observed in up to 14% of the adult population.^(1-3) They are normally asymptomatic, although they may occasionally cause headaches, periorbital or facial pain, and may exceptionally even predispose to development of recurrent rhinosinusitis and cause nasal obstruction.^(1-3) They are classically described as dome-shaped or rounded lesions that originate in the mucosa of the floor of the maxillary sinus, though they may appear in other locations within the sinus. Their size is variable, and growth is slow. Regarding evolution over time, in 60% of cases size does not change, in 30% they may decrease in size or even disappear, and will only increase in volume in 10% of cases.⁽¹⁾

In radiology, they reveal a typically radiodense, uniform, domed or rising sun -shaped image, with clearly defined edges, perfectly shadowing the underlying bone contour^(1-3) (Table 1 and Figure 1).

Except in such cases where there are symptoms, the cysts do not require treatment, but it is absolutely essential proper diagnosis is made of all cases.^(1-3) CBCT (Cone Beam Computed Tomography) is a fundamental tool for establishing diagnosis.^(1-3) Differential diagnosis should be made against other benign pathologies such as maxillary sinus mucocele or sinonasal inverted papilloma, and even malignant pathologies such as maxillary sinus squamous cell carcinoma^(4-5) (Tables 2 and 3).

MRC AND ORAL SURGERY

Various sinus lift techniques have shown reliability in the treatment of atrophic posterior maxilla.⁽⁶⁾ There has been controversy in the past regarding whether or not to perform sinus lift techniques and implant placement in patients with MRC without prior removal of the lesion. Nowadays however, several studies^(7-12) have shown similar rates of success and complication in patients with and without MRC when performing sinus lift surgery and implant placement, both immediate and delayed (Table 4).

Treatment for this type of patient requires programming and a detailed study of the case, as well as a broad knowledge of the anatomical and pathological characteristics of the sinus, more than in any other kind of intervention in this area.

Most authors^(7-12) recommend aspiration and decompression of cysts during sinus lift surgery through the lateral wall. In larger lesions, or when there is any doubt of a diagnostic nature, enucleation and a subsequent histopathological study is indicated. Suctioning is generally thought to help decrease internal sinus pressure by removing fluid and reducing the size of the lesion, thereby decreasing the risk of perforation of the sinus membrane.^(7-9)

The low frequency of sinus membrane perforation and postoperative sinusitis, as well as the implant success rates published, suggest that sinus lift techniques in patients with MRC are safe. However, the literature on the subject is extremely heterogeneous and limited and further studies may be necessary to confirm these observations.

The discovery of any lesion in the maxillary sinus requires a detailed study to establish proper diagnosis. The presence of an MRC is not currently considered a contraindication for sinus lift surgery and implant placement.^(7-12) However, it is essential proper diagnosis is assured in all cases, and that results are monitored.