The frontal sinus drainage pathway and related structures.

The purpose of this pictorial essay is to provide a visual guide to the frontal sinus drainage pathway (FSDP), associated anatomic structures, and normal variations in sinus anatomy (Figs 1–9). Suggested readings provide more detailed descriptions of paranasal sinus anatomy and terminology for further study (1–9). Sinus nomenclature may be both redundant and confusing (10, 11). Originally, the terms sinus, antrum, and recess all referred to a cavity, then a cavity within a bone. Now an osseous recess is an air space with more than one drainage ostium (as distinct from an osseous air cell, which has only one drainage ostium), whereas the terms maxillary sinus and maxillary antrum may be used interchangeably. Both cribriform and ethmoid mean “sievelike,” and both bulla and agger signify a “projecting anatomic structure,” but convention has limited usage of each term to specific structures. For these reasons, and because anatomic variations are frequent, the exact terminology employed to designate paranasal air spaces may be subjective. Herein one coherent set of names for the structures of the FSDP is used that specifically excludes the term “frontal recess” because that term is difficult to define anatomically (6). Figure 1 provides an overview of the drainage pathways of the paranasal sinuses that are based upon the skull illustrated in Figures 3 and 4. The frontal sinus has the most complex and variable drainage of any paranasal sinus. Each frontal sinus narrows down to an inferior margin designated the frontal ostium (Figs 1 and 3D-F ). The frontal ostium extends between the anterior and posterior walls of the frontal sinus, is demarcated by a variably shaped ridge of bone on the anterior wall of the sinus, and is oriented nearly perpendicular to the posterior wall of the sinus. It may be difficult to define when air cells marginate the ostium (Fig 8). The FSDP has superior and inferior compartments. The superior compartment of the FSDP is formed by the union of adjacent air spaces at the anteroinferior portion of the frontal bone and the anterosuperior portion of the ethmoid bone (Fig 2). Its upper border is the frontal ostium. Its size and shape vary with the variable anatomy of the frontoethmoid air cells (8; Figs 6–9). The superior compartment communicates directly with the inferior compartment. The inferior compartment of the FSDP is a narrow passageway formed by either the ethmoid infundibulum or the middle meatus (6, 7). When the anterior portion of the uncinate process extends superiorly to attach to the skull base, the inferior compartment of the FSDP is the ethmoid infundibulum (Figs 3D–F , 4E, 5). This then communicates with the middle meatus via the hiatus semilunaris; however, when the anterior portion of the uncinate process attaches to the lamina papyracea instead of the skull base, the inferior compartment of the FSDP is then the middle meatus itself (Fig 5). Together, the frontal sinus and the superior compartment of the FSDP resemble an Erlenmeyer flask, with the neck of the flask representing the frontal ostium. The inferior compartment of the FSDP could then be conceptualized as directly communicating with a small opening in the base of the flask. The basal lamella of the middle turbinate is a thin bony plate that forms part of the ethmoid infrastructure. It has three portions (6, 7). The vertical portion of the basal lamella attaches to the cribriform plate. The middle and posterior portions extend laterally to join the lamina papyracea, thereby dividing the anterior ethmoid air cells from the posterior ethmoid air cells and the posterior margin of the basal lamella attaches to the perpendicular plate of the palatine bone (Figs 3C and 3F ). The basal lamella is best displayed in three-dimensional models and sagittal and coronal CT images. Its vertical portion is seen best on coronal CT images, whereas the middle and posterior portions are seen best on sagittal CT images (Figs 3C , 3F , and 4F–H). Figure 2 separates the frontal bone from the ethmoid bone to illustrate how they join to form the superior compartments of the FSDPs. In Figure 2B, the paired air spaces at the anteroinferior aspect of the frontal bone are not the frontal sinuses, but porReceived October 17, 2002; accepted after revision February 14, 2003. From the Division of Neuroradiology, Department of Radiology, (D.L.D., M.M.S., W.D.B., J.L.U., L.H-B., J.M.S.) and the Department of Otolaryngology (T.L.S.), Medical College of Wisconsin, Milwaukee, WI; the Division of Neuroradiology, Department of Radiology (T.P.N), Mount Sinai Medical Center, New York; Department of Radiology (M.F.M.), University of Illinois Medical Center, Chicago, IL; and Department of Pediatric Surgery (W.E.B.), Uniformed Services University, Bethesda, MD. Address correspondence to David L. Daniels, M.D., Department of Radiology, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226.