Comparative Analysis of Subtonsillar, Endoscope-Assisted Subtonsillar, and Far-Lateral Approaches to the Lower Clivus: An Anatomic Study

Background: Far-lateral approach (FL) has become a mainstay for skull base surgeries involving the lower clivus (LC). Suboccipital approach has also recently been resurrected as possible in modified, subtonsillar form (ST), and offers several advantages over FL. It is possible that endoscopic assistance might help in overcoming disparity in surgical overview and freedom between ST and the more commonly used FL. In this study, quantification and comparison of exposure and surgical freedom for various surgical targets at LC were undertaken in dissected cadaveric heads to permit objective comparison of ST and FL, and exploration of any objective benefit of endoscopic assistance to the former.

Materials and Methods: ST, Endoscope-assisted subtonsillar approach (EST), and FL were performed on 10 specimens, and metrics of interest acquired via neuronavigation. For exposure, lengths of intradural CN IX and CN XII and maximum accessible distances from jugular foramen (JF) and hypoglossal canal (HC) were measured. Area of exposure was calculated. For the 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. Greater length of exposure of CN IX was obtained via FL (16.2 ± 1.9 mm) and EST (14.8 ± 2.4 mm) than ST (10.5 ± 1.9 mm; p <  0.01). Greater length of exposure of CN XII was also obtained via FL (11.4± 1.9 mm) and EST (10.5 ± 2.3 mm) than ST (7.4 ± 2.2 mm; p <  0.01) ([Fig. 1]).

2. In the axial plane of JF at LC, FL reached 20.1 ± 3.2 mm medially from JF, more than EST (17.1 ± 2.2 mm) and ST (13.7 ± 2.7 mm; p <  0.01) ([Fig. 1]).

3. In the axial plane of HC at LC, FL reached 13.5 ± 2.4 mm medially from JF, more than EST (10.8 ± 1.8 mm) and ST (8.7 ± 1.6 mm; p <  0.01) ([Fig. 1]).

4. FL provided the greatest exposure area (p <  0.01), and EST provided greater exposure than ST (p <  0.01) ([Fig. 2]).

Surgical freedom:

1. FL and EST afforded greater AA at JF than ST (p <  0.01). The greatest AA was obtained at HC via FL, with EST permitting greater AA than ST ([Fig. 2]).

2. The midline was inaccessible with EST or ST in the axial planes of JF and HC. The midline was inaccessible with FL in the axial plane of JF, but accessible in 40% of cases (4/10 sides) in the axial plane of HC.

Conclusion: It seems unlikely that the benefits conferred by a posterior midline approach over the more commonly used FL renders this technique preferable for LC, with a significant (albeit reduced) disparity of surgical overview and freedom persisting despite endoscopic assistance. Although, because of the absence of elasticity of tissues and space-occupying lesions, this cadaveric study and cannot completely simulate the intraoperative situation, analysis of data and secondary findings should be credible given its comparative nature.

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