The infratentorial supracerebellar approach to the pineal region presents special challenges during patient positioning. The head must be flexed and the body positioned to allow an operative trajectory under the straight sinus. Image guidance is not useful during positioning because registration and navigation take place after the head is fixed in its final position. Therefore, a reliable method of positioning based on external, easily identifiable landmarks to estimate the surgical trajectory along the straight sinus toward the pineal region is needed. Based on observation, the authors hypothesized that a line between 2 palpable external landmarks, the inion and the bregma, often approximates the surgical trajectory along the straight sinus. They tested this hypothesis by quantifying the relationship between the straight sinus and the bregma, and describe a method for estimating the working angle during patient positioning.
The midsagittal, Gd-enhanced, T1-weighted MR images of 102 patients were analyzed. Demographic data and the presence or absence of tentorial pathological entities was recorded. The slant of the straight sinus was classified as common, high, or low, based on a previously described classification system. A line along the bottom of the straight sinus (that is, the straight-sinus line) was extended superiorly to its intersection with the calvaria, and the distance from this intersection point to the bregma was measured.
The intersection point of the straight-sinus line and the calvaria was on average 2 ± 8.2 mm (these values are expressed as the mean ± SD throughout) anterior to the bregma (range 19.9 mm anterior to 19.1 mm posterior). The distance from the intersection point to the bregma was not statistically significantly different in younger or older patients, or in patients with or without tumors involving the pineal region. In patients with a low slant of the straight sinus, the intersection point was 5.3 ± 6.3 mm anterior to the bregma, whereas in patients with a high slant of the straight sinus, the intersection point was 0.21 ± 9.1 mm posterior to the bregma (p = 0.015).
The straight-sinus line, which defines the working angle for the supracerebellar infratentorial approach, intersects the calvaria very close to the bregma in the majority of patients. Therefore, ideal patient positioning can be achieved by flexing the patient's head to optimize the working angle defined by an imaginary line connecting the torcula (inion) to the bregma.