The Occipitalis Muscle as an Adjunct Superficial Landmark for the Transverse Sinus and Transverse-Sigmoid Junction: An Anatomical Study With Application to Posterior Cranial Fossa Surgery

Introduction: Although neuronavigation systems are widely used for identifying deep intracranial structures, additional superficial anatomical landmarks can be useful when this technology is not available or is not working properly. Herein, we investigate the potential of the occipitalis muscle (OM), rarely mentioned in neurosurgical literature, as a superficial landmark for the transverse sinus (TS) and transverse-sigmoid sinus junction (TSJ). Methods: Eighteen adult cadaveric heads underwent dissection. The borders of the OM were identified and measured. The muscle was then removed and the bone underlying the muscle was drilled. The relationships between the OM and the underlying dural venous sinuses were then investigated by using a surgical microscope. Results: The OM is a quadrangular-shaped muscle, that invariably crosses the lambdoid suture, showing relationships with the TS inferiorly and the TSJ laterally. The medial border was located a mean of 2.7 cm from the midline and its lower edge was a mean of 1.6 cm above the TS. The inferior border was found between the lambdoid suture and the superior nuchal line in all the specimens. The medial half of the inferior margin was placed on average 1.1 cm superiorly to the TS while the lateral margin ran just above or over the TS. The lateral border was located a mean of 1.1 cm medially to the asterion and approximated the mastoid notch, being within 1-2 cm from it. The TSJ was between 2.1 and 3.4 cm lateral to OM lateral border. Conclusion: A combination of superficial anatomical landmarks can be useful for surgical planning. We found that the OM represents a valuable aide for neurosurgeons and is a reliable landmark for the deeper -lying TS and TSJ.