The trephination procedure is a limited external approach for frontal sinus drainage. An incision is made above the brow and a hole is drilled through the anterior wall of the frontal sinus taking care to avoid the supratrochlear and supraorbital neurovascular bundles. Once access to the sinus is gained, aspiration for bacteriological studies and precise irrigation of the sinus can be performed. When performed in combination with endoscopic revision surgery about the frontal recess, irrigation fluid instilled via the trephination can be visualized from below guiding the endoscopic surgeon to the frontal sinus ostium.

Diagram demonstrating trephination defect in the inferior aspect of the right frontal sinus created in the anterior inferior aspect of the right frontal sinus.	Coronal CT image in a patient with prior right frontal sinusitis. Trephination defect (arrow) is seen in the anterior floor of the sinus. The frontal sinus is well-aerated.

Serial coronal CT images in a patient with chronic frontal sinusitis. Preoperative image (A) demonstrates opacification of the frontal sinuses. Trephination procedure was performed on the left side and follow-up scan (B) demonstrates trephination defect (arrow) and partial clearing of the sinus.	Coronal CT image in a patient 1 year after left frontal sinus trephination demonstrates persistent opacification of the affected sinus. Patency of the frontal recess was not maintained and a patent drainage pathway was not established. This patient subsequently underwent obliteration of the sinus.