Objective This study aims to determine whether the line between the digastric point and posterior edge of the occipital condyle (DC line) is a reliable surface landmark for the posterior margin of the horizontal part of the sigmoid sinus.
Methods Cadaveric specimens were used to show the relationship between the DC line and retrosigmoid craniotomy. Three-dimensional computed tomography angiography (3D-CTA) images of adult heads (56 sides) were analyzed with OsiriX software and the distance between the DC line and horizontal part of the sigmoid sinus was measured at the digastric point, posterior edge of the condyle, and midpoint of the line.
Results The DC line was roughly parallel and posterior to the posterior margin of the sigmoid sinus. The distance between the DC line and posterior edge of the sigmoid sinus at the digastric point, condyle, and midpoint of the line measured 4.7 ± 3.3 mm, 5.9 ± 2.6 mm, and 1.3 ± 2.2 mm, respectively. All sigmoid sinuses coursed anterior to the digastric point and condyle but in 17.9% the posterior edge of the sigmoid sinus (10/56 sides) extended a maximum of 4.1 mm posterior to the midpoint of the DC line. The distances from the DC line to the sigmoid sinus at the midpoint did not significantly differ between dominant and nondominant sides (dominant side: 1.2 ± 2.3 mm; nondominant side: 1.4 ± 2.2 mm; p = 0.71, unpaired t-test) or between right and left sides (right side: 1.1 ± 2.6 mm; left side: 1.5 ± 1.8 mm; p = 0.50, unpaired t-test)
Conclusion The DC line can be used as a new surface landmark for estimating the position of the horizontal part of the sigmoid sinus. The posterior edge of the sinus may extend posterior to the line at the midpoint, so exposure should begin at the digastric point and just behind the posterior edge of the condyle and carefully continue to expose the midpoint of the DC line to prevent sinus injury.
