Comparative Analysis of the Subtonsillar, Far-Lateral, Extreme-Lateral, and Far-Medial Approaches to the Lower Clivus: An Anatomic Cadaver Study

Background: The lower clivus (LC) is one of the most difficult areas to access in neurosurgery due to its ventrocaudal location and proximity to important cranial nerves and vessels. Several microsurgical approaches to LC exist in the literature, with subtonsillar (ST), far-lateral (FL), extreme-lateral (EL), and endoscopic far-medial (FM) being most commonly cited. However, there is no consensus on the optimal approach for reaching LC. In this study, quantification and comparison of exposure and surgical freedom for surgical targets at LC were undertaken in dissected cadaveric heads to permit objective analysis of approaches.

Materials and Methods: ST, FL, EL, and FM approaches were performed on 10 specimens, and metrics were acquired via neuronavigation. For surgical exposure, the lengths of intradural CN IX and CN XII and maximum accessible distances from the jugular foramen (JF) and the hypoglossal canal (HC) were measured. Area of exposure was calculated with software. For surgical freedom, the angle of attack (AA) at JF, HC, and the anterior midline in the axial planes of JF and HC were measured. Values were compared using paired t-tests.

Results:

Exposure:

1. For CN IX, EL provided the greatest length of exposure (19.3 ± 3.0 mm, p <  0.01). For CN XII, FM and EL provided greater exposure (15.9 ± 2.5 mm and 15.0 ± 2.3 mm, respectively) than the other approaches (p <  0.01; [Fig. 1]).

2. In the axial plane of JF at LC, EL reached 26.8 ± 3.9 mm medially from JF, more than FL and ST (p <  0.01 for both pairs). FM reached 15.4 ± 3.4 mm laterally from the anterior midline ([Fig. 1]).

3. In the axial plane of HC at LC, EL reached 19.8 ± 2.2 mm medially from HC, more than FL and ST (p <  0.01 for both pairs). FM reached 18.3 ± 1.8 mm laterally from the anterior midline ([Fig. 1]).

4. EL provided the greatest area of exposure of LC (p <  0.01; [Fig. 2]).

Surgical freedom:

1. The greatest AA was obtained at JF and HC through EL (p <  0.01). JF was inaccessible through FM ([Fig. 3]).

2. At the level of JF and HC, FM provided the maximum AA at the midline, more than EL and FL (p <  0.01 for both). The midline was inaccessible with ST in the axial planes of JF and HC ([Fig. 3]).

Conclusion: EL yields the greatest area of exposure at LC but carries a greater risk of neurovascular structures and time burden than other approaches surveyed. Conversely, ST yields the least surgical exposure at LC but is the most straightforward. FM approach is superior to the other approaches for more ventral exposure, and inferior for more anterolateral exposure. FL has intermediate values for most parameters and absent/poor access to the anterior midline. Although this cadaveric study is limited by the decreased elasticity of tissues and absence of space-occupying lesions, the analysis of data and secondary findings should be credible given the comparative nature of the study. For the first time, this study provides systematic comparisons and relative measurements of all four approaches, which may better inform operative planning for LC.

Fig. 1

Fig. 2

Fig. 3