The Supracondylar Triangle: An Anatomic Guide in Drilling the Occipital Condyle and Accessing the Hypoglossal Canal—Description of Microsurgical Anatomy

Introduction: Access to the posterior fossa via the presigmoid or far lateral approaches often requires removal of segments of the occipital condyle (OC), resulting in a potential need for spinal fixation to prevent craniocervical instability and the need to locate the hypoglossal canal to prevent damage to the hypoglossal nerve. Organization of the relative relationships of osseous and neurovascular structures in this region may assist the surgeon in dictating the extent of resection of the OC.

Objective: To design an anatomical triangle that will serve as a guide in accessing and resecting the OC.

Methods: Twenty-five dry skulls (50 total sides) were used to measure the following anatomical correlates (ventral plane): (1) MT (median margin) to the OC, (2) OC to the osseous impression/foramina of the IEV, (3) IEV to the MT, (4) length of HC within OC, (5) ventral surface area of OC, (6) length of OC, (7) segmental surface area of OC relative to position of HC, (8) percentage of OC to be resected relative to position of IEV, and (9) angle of MT, HC, and IEV relative to each other. CT scans were used to correlate with the anatomical findings. Four cadaveric dissections were undertaken to provide sequential exposure of the MT, OC, HC, articular facet of C1, digastric groove, intercondylar emissary vein (IEV), hypoglossal canal, and pontomedullary cistern from both surgical and ventral views. A clinical case example describing this anatomical triangle will also be described.

Results: Dissections revealed that a direct trajectory can be made from the mastoid tip to the OC, which correlates with the entrance of the HC. An anatomic triangle was designed: MT–OC–IEV (supracondylar triangle). The osseous location of the IEV consisted of either an impression or a foramen (72 and 28%, respectively). The MT-HC-IEV triangle consisted of the digastric groove, intercondylar emissary vein, jugular bulb, spinal accessory nerve, jugular tubercle, and the pontomedullary cistern (containing the 11th nerve). In the skull specimens, the average distances from MT to HC, HC to IEV, and IEV to MT were the following (in cm): 4.3 ± 0.3, 1.4 ± 0.26, and 3.4 ± 0.31, respectively. The average length of the HC within the OC, from medial to lateral, was 1.4 cm ± 0.15. The average length of the OC was 2.4 cm ± 0.2. The average surface area of OC toward the IEV was 0.88 cm ± 0.29. Within the MT–HC–IEV triangle, angles connecting the MT, HC and IEV were 37.80 ± 8.1, 126.10 ± 10.6, and 15.10 ± 3.8, respectively.

Conclusion: The MT–OC–IEV triangle has several benefits: (1) for a frame of reference, a straight line can be connected from the MT to the entrance of the HC via the OC; all OC anterior to this line, toward the IEV can be safely removed, without the need for spinal fixation; (2) drilling of this portion of the OC provides direct access the jugular bulb; (3) knowledge of this triangle may prevent damage to hypoglossal nerve; and (4) direct access to the spinal accessory nerve via the jugular tubercle.

No conflict of interest has been declared by the author(s).