Quantitative description of the far-lateral transcondylar transtubercular approach to the foramen magnum and clivus

Object. The purpose of this study was to evaluate the far-lateral transcond ylar transtubercular approach (complete FLA) based on quantitative measurem ents of the exposure of the foramen magnum and petroclival area obtained af ter each successive step of this approach. Methods. The complete FLA was reproduced in eight specially prepared cadave ric heads (a total of 15 sides). The approach was divided into six steps: 1 ) C-1 hemilaminectomy and suboccipital craniectomy with unroofing of the si gmoid sinus (basic FLA); 2) partial resection of the occipital condyle (up to the hypoglossal canal); 3) removal of the jugular tuberculum; 4) mastoid ectomy (limited to the labyrinth and the fallopian canal) and retraction of the sigmoid sinus; 5) resection of the lateral mass of C-1 with mobilizati on of the vertebral artery; and 6) resection of the remaining portion of th e occipital condyle. After each successive step, a standard set of measurem ents was obtained using a frameless stereotactic device. The measurements w ere used to estimate two parameters: the size of the exposed petroclival ar ea and the size of a spatial cone directed toward the anterior rim of the f oramen magnum, which depicts the amount of surgical freedom available for m anipulation of instruments. The initial basic FLA provided exposure of only 21 +/- 6% of the petrocliva l area that was exposed with the full, six-step maximally aggressive (compl ete) FLA. Likewise, only 18 +/- 9% of the final surgical freedom was obtain ed after the basic FLA was performed. Each subsequent step of the approach increased both petroclival exposure and surgical freedom. The most dramatic increase in petroclival exposure was noted after removal of the jugular tu berculum (71 +/- 12% of final exposure), whereas the least improvement in e xposure occurred after the final step, which consisted of total condyle res ection. Conclusions. The complete FLA provides wide and sufficient exposure of the foramen magnum and lower to middle clivus. The complete FLA consists of sev eral steps, each of which contributes to increasing petroclival exposure an d surgical freedom. However, the FLA may be limited to the less aggressive steps, while still achieving significant exposure and surgical freedom. The choice of complete or basic FLA thus depends on the underlying pathologica l condition and the degree of exposure required for effective surgical trea tment.