Design and microsurgical anatomy of the retrosigmoid-retrocondylar keyhole approach without occipital condyle removal

Objective: The goal of this study was to design a new retrosigmoid-retrocondylar keyhole approach based on the minimally invasive keyhole idea and to explore its feasibility and indications, which can be regarded as the base of this keyhole approach in clinical use. Methods: 8 adult cadaveric heads fixed in formalin and with intracranial vessels perfused by colored latex were used in this study. To search for the most suitable length and shape of the skin incision, we examined two kinds of incision (a longitudinal "S" shape and a straight one) and two lengths (5 cm and 7 cm, respectively). Due to the complexity and thickness of the suboccipital muscles, two ways of muscle dissection were compared: 1) the muscles were incised perpendicularly in layers; 2) the muscles were detached and reflected in layers. A 3-cm diameter retrosigmoid-retrocondylar bone flap was made with a craniotome. Many anatomic structures could be observed under the microscope when the cerebellar hemisphere was retracted. After comparing and balancing the above steps in all specimens, a feasible, duplicable retrosigmoid-retrocondylar keyhole approach was devised. Results: The proper incision of the retrosigmoid-retrocondylar keyhole approach was a longitudinal "S" shaped skin incision about 7 cm in length with its superior border 2 cm behind the middle point of mastoid and inferior margin at the level of C-2. The method of detachment and reflection of occipital muscles was superior to the method of cutting them perpendicularly. By means of adjusting the head position and the angle of microscope, the ipsilateral vertebral artery, posterior inferior cerebellar artery, anterior inferior cerebellar artery, VII, VIII, IX, X, XI, XII cranial nerves and the ventral lateral aspect of medulla oblongata were exposed via this keyhole approach. Conclusions: The novel retrosigmoid-retrocondylar keyhole approach has practical value for clinical applications. With the techniques of modern microsurgery, several diseases such as an aneurysm situated at the vertebral artery or the posterior inferior cerebellar artery, a small hypoglossal neurinoma and tumor located at the ventral lateral aspect of the medulla oblongata, may be operated via this retrosigmoid-retrocondylar keyhole approach without drilling the occipital condyle.