Biomechanical Evaluation of the Craniovertebral Junction After Anterior Unilateral Condylectomy: Implications for Endoscopic Endonasal Approaches to the Cranial Base

BACKGROUND: Endoscopic endonasal approaches to the craniovertebral junction and clivus, which are increasingly performed for ventral skull base pathology, may require disruption of the occipitocondylar joint. OBJECTIVE: To study the biomechanical implications at the craniovertebral junction of progressive unilateral condylectomy as would be performed through an endonasal exposure. METHODS: Seven upper cervical human cadaveric specimens (C0-C2) underwent nondestructive biomechanical flexibility testing during flexion-extension, axial rotation, and lateral bending at C0-C1 and C1-C2. Each specimen was tested intact, after an inferior one-third clivectomy, and after stepwise unilateral condylectomy with an anterior approach. Angular range of motion (ROM), lax zone, and stiff zone were determined and compared with the intact state. RESULTS: At C0-C1, mobility during flexion-extension and axial rotation increased significantly with progressive condylectomy. ROM increased from 14.3 +/- 2.7 degrees to 20.4 +/- 5.2 degrees during flexion and from 6.7 +/- 3.5 degrees to 10.8 +/- 3.0 degrees during right axial rotation after 75% condyle resection (P<.01). At C1-C2, condylectomy had less effect, with ROM increasing from 10.76 2.0 degrees to 11.7 +/- 2.0 degrees during flexion, 36.9 +/- 4.8 degrees to 37.1 +/- 5.1 degrees during right axial rotation, and 4.3 +/- 1.9 degrees to 4.8 +/- 3.3 degrees during right lateral bending (P = NS). Because of marked instability, the 100% condylectomy condition was untestable. Changes in ROM were a result of changes more in the lax zone than in the stiff zone. CONCLUSION: Lower-third clivectomy and unilateral anterior condylectomy as would be performed in an endonasal approach cause progressive hypermobility at the craniovertebral junction. On the basis of biomechanical criteria, craniocervical fusion is indicated for patients who undergo.75% anterior condylectomy.