Anterior and posterior nasal fontanelles.

According to Dorland's Illustrated Medical Dictionary, a fontanelle (Fr., dim. of fontaine spring, filter) is "a soft spot, such as one of the membrane-covered spaces (fonticuli cranii [NA]) remaining in the incompletely ossified skull of a fetus or infant." [1] The anterior fontanelle is the unossified area of the skull situated at the junction of the frontal, coronal, and sagittal sutures. The posterior fontanelle is the unossified area of the skull at the junction of the sagittal and lambdoidal sutures. Dorland's makes no mention of nasal fontanelles.

Nasal fontanelles are the areas of the lateral nasal wall that are absent of bone. They are usually found immediately above the insertion of the inferior turbinate. Thus, the mucosa of the maxillary sinus and the middle meatus are separated only by a fibrous layer of periosteum. The anterior fontanelle is inferior and anterior to the uncinate process and fuses with the medial wall of the maxillary sinus. [2] The boundaries of the posterior fontanelle include the posterior end of the uncinate process anteriorly and the palatine bone posteriorly. [2] The fontanelles are common sites of accessory maxillary sinus ostia (figure, A and D). Wigand reported that about one-quarter of the specimens he studied demonstrated secondary accessory ostia of the antral cavity through these membranes, especially posteriorly. [3] In the absence of accessory ostia, palpation with a ball-tipped probe (Lusk probe) can confirm a bony dehiscence at the membranous portion of the lateral middle meatal wall.

The anterior fontanelle has an important relationship to the nasolacrimal apparatus. When performing a middle meatal antrostomy through a retrograde uncinectomy, the surgeon must take care not to inadvertently injure the nasolacrimal duct. [4,5] When using the back-biting forceps to enlarge the natural ostium of the maxillary sinus in an anterior direction, the surgeon should remember that the mean distance from the maxillary ostium to the nasolacrimal duct has been reported to be approximately 0.9 cm. [5]

Unlike the anterior fontanelle, the posterior fontanelle serves as a relatively safe surgical landmark during a middle meatal antrostomy. The antrostomy window should be widened in a posterior direction by excising the posterior fontanelle.

A maxillary sinoscopic view of the medial wall of the left maxillary sinus will show the relationship of the natural ostium and the posterior fontanelle (figure, C).

In order to differentiate the fontanelles of the skull from those of the nasal cavity, the latter should be referred to as nasal fontanelles.

From the Southern New England Ear, Nose, Throat, and Facial Plastic Surgery Group, New Haven, Conn.; the Section of Otolaryngology, Hospital of St. Raphael, New Haven; and the Section of Otolaryngology, Yale University School of Medicine, New Haven (Dr. Yanagisawa); and the Memorial Sloan-Kettering Cancer Center, New York City (Dr. Joe).

References

(1.) Dorland's illustrated Medical Dictionary. 29th ed. Philadelphia: W.B. Saunders, 2000.

(2.) Stammberger HR, Kennedy DW. Paranasal sinuses: Anatomic terminology and nomenclature. The Anatomic Terminology Group. Ann Otol Rhinol Laryngol Suppl 1995;167:7-16.

(3.) Wigand ME. Endoscopic Surgery of the Paranasal Sinuses and Anterior Skull Base. New York: Thieme Medical Publishers, 1990.

(4.) Christmas DA, Yanagisawa E, Joe JK. Transnasal endoscopic identification of the natural ostium of the maxillary sinus: A retrograde approach. Ear Nose Throat J 1998;77:454-5.

(5.) Calhoun KH, Rotzler WH, Stiernberg CM. Surgical anatomy of the lateral nasal wall. Otolaryngol Head Neck Surg 1990;102: 156-60.