Anatomical Considerations for Resection of Chordomas and Chondrosarcoma of the Sacrum




Abstract


The posterior approach to sacral sarcoma resection can be compartmentalized into eight parts. Sequentially accomplishing these eight steps offers a convenient way to study the surgical anatomy of the sacrum, spine, and posterior pelvis. This outline will be used as a guide, orienting the reader to the surgical anatomy of this region.




Keywords

Anatomy, Chordoma, Sacrospinous, Sacrotuberous, Sacrum, Sarcoma

 





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The sacrum is contained within one of the most anatomically complex and surgically inaccessible areas of the human body. It is wedged behind and between both iliac wings and is bound by ligaments to the mobile spine and mid-pelvis. The nearby pelvic viscera and neurovascular structures challenge the surgeon to avoid serious morbidity during surgery. The sacral nerve roots possess significant functional importance for both peripheral and autonomic nervous systems. Large sacral tumors can be radically removed if one is prepared to sacrifice normal urogenital and anal–rectal function. When not extirpated, these neoplasms eventually impair the function of the pelvic viscera and disrupt the structure of the pelvic ring.


The most common primary sacral tumors are chordomas, chondrosarcomas, giant cell tumors, and osteosarcomas. Chordomas and chondrosarcomas are relatively radioresistant and must be treated with surgery. Although osteosarcomas are responsive to multi-agent chemotherapy regimens, cure is not possible without tumor-free margins of resection. Giant cell tumor of bone treated with intralesional resection often recurs locally.


Patients with sacral tumors often have a long history of vague, nonspecific pains. Routine physical examination frequently does not detect their etiology. A rectal exam provides the only reliable assessment of sacral neoplasms. Tenderness of a presacral mass is evident on transrectal palpation. Proximal, cranial extension of the presacral component of a sacral neoplasm can be estimated on rectal exam by attempting to reach over the top of the mass. Commonly, the index finger of a hand fitting comfortably into a size eight glove can reach the S2 sacral foramen. Thus, if one can feel the top of the mass, then the tumor is located inferior or caudal to S2.


Plain radiographic imaging of the pelvis and sacrum belies the true amount of bone destruction and extraosseous mass present with many sacral neoplasms. Three-dimensional scanning with CT scan or MRI often reveals a complex bone and soft-tissue neoplasm.


The soft-tissue mass is often disproportionately larger than the area of bone destruction.


The oncologic surgeon is at a distinct advantage compared to the pathologist, internist, or radiologist in understanding pathologic anatomy. The surgeon is able to correlate a patient’s symptoms and exam with the radiographic image, histopathology, and surgical anatomy, and to integrate these into a comprehensive understanding of the disease. To do so requires repeated analysis of the correlation between anatomy and radiographic images. The precise three-dimensional localization of the tumor and its biologic behavior predicted from its histopathology will help elucidate the optimal surgical exposure, the oncologic dissection plane, and the expected postoperative functional deficits.


Fig. 28.1 is an AP projection of a plain radiograph of the normal human pelvis. Figs. 28.2–28.4 illustrate a dried-bone human pelvis. Fig. 28.2 examines gross morphologic pelvic anatomy. Landmarks are highlighted in color. Surgically significant relationships are emphasized. Fig. 28.3 is the plain radiographic image of the dried pelvis from Fig. 28.2 . True anatomy is not always represented radiographically. For example, the arcuate line or pelvic brim serves as the entrance to the true pelvis (pelvic inlet). This line extends from the symphysis around laterally to end posteriorly at the sacroiliac joint. The true posterior-midline continuation of the arcuate line lies at the S1 foramen level. Radiographically, however, the AP radiographic projection reveals that the pelvic brim lies at the S2 level ( Fig. 28.1 ). The visual discrepancy occurs at the posterior one-fourth of the arcuate line. The condensation of bony trabeculae in the sciatic buttress combined with the beam projection angle and sacral tilt create an artificial posterior pelvic brim line. Fig. 28.4 allows direct comparison of prior images of the pelvis with a CT scan. The identical landmarks can now be visualized in the transverse plane.




Figure 28.1


Anteroposterior plain radiograph of a normal pelvis to be compared with the labeled pelvis in Figs. 28.2–28.4 .



Figure 28.2


Color pictures of a dried human pelvis. The differing views are labeled: (A) anterior projection, (B) inlet view, (C) outlet view, (D) lateral view, (E) Judet view at 45 degrees to the right of midline relative to the coronal plane and (F) posterior projection. Seven structures are highlighted: (i) The right pelvic brim, one side of the anatomic brim of the true pelvis, is delineated by white wire and gray putty. It is synonymously called the iliopectineal or arcuate line. (ii) The right sacral ala has an orange metal strip on its surface. In the upright position the ala slopes to the floor at approximately 30 degrees caudad to a line perpendicular to the spine. (iii) The right posterior-superior iliac spine has blue clay over it, anchored to a small nail lodged in the bone. (iv) The left anterior and posterior S1 foramina have red clay in them. A large nail protruding from the posterior surface traverses both foramina in an inferiorly angled trajectory. (v) The right S2 anterior and posterior foramina are marked with green clay. A small nail extending anterior to posterior connects the anterior and posterior S2 foramina. (vi) A transverse red line connects the anterior right and left S1 foramina. It lies directly over the fused disc space between the S1 and S2 vertebral bodies. (vii) A transverse reinforcement bar posteriorly connects both ilia to the sacrum just below the S1 foramina. It is a nonanatomic structure.



Figure 28.3


Plain radiographs of the same pelvis displayed in Fig. 28.2 . Radiopaque markers (i–vii) correlate the true anatomy with the radiographic image. The small case Roman numerals refer to the landmarks defined in Fig. 28.2 . (A) AP projection. The posterior one-fourth of the arcuate line (i) deviates from the radiographic image of the pelvic brim. The arcuate line extends toward the S1 foramen while the image of the brim blends with the S2 foramen. The radiological pelvic brim is formed by a surface 1–2 cm below the arcuate line corresponding to the trabecular condensations of the sciatic buttress. The radiographic image of the brim varies in its relationship to the arcuate line, depending upon the angle of the incoming X-ray beam and the obliquity of the pelvis relative to the ground. If the beam is parallel to the ground, then the more vertical the pelvis (inlet view in which the coronal plane of sacrum approaches parallelism with the beam), the more the brim image deviates toward S1. The verticality of the sacrum relative to the pelvis and spine also can distort the image. The S1, S2 disc space (vi) connects left and right S1 foramina. (B) An inlet view depicting the same landmarks. The pelvis is in a vertical position and the sacrum is more parallel with the X-ray beam. (C) The right iliac oblique projection (45 degrees = Judet view). The posterior border of the right acetabular posterior column is well demonstrated as it blends with the buttress over the sciatic notch. The arcuate line (i) and pelvic brim are noted joining the sacral ala (ii) at the sacroiliac joint. (D) Lateral projection of the posterior pelvis. The declination angles of the superior ala surface (ii) and the first (iv) and second (v) sacral foramina are observed. The projection illustrates the acute inferior direction taken by the nerve roots. The sacral osteotomy must parallel the paths of the roots to avoid injuring them. A common surgical error is to place the osteotomy perpendicular to the lumbar spine rather than to the sacrum. Such an osteotomy traverses two sacral levels, rather than one, and jeopardizes the oncologic margin of the resection. The level of the posterior-superior iliac spine (iii) serves as an intraoperative landmark for identifying the posterior S1–S2 space.



Figure 28.4


CT scan of the same pelvis as in Fig. 28.2 . The pelvis lies in the supine position. The CT scan slices are 5 mm apart and obliquely oriented to the table so that they are parallel to the transverse plane of the pelvic inlet or brim, as is routine. The CT scan’s main advantage is clear depiction of sacral bone destruction and soft-tissue extension of tumor. The radiopaque anatomical landmarks (i–vii) are visualized in the transverse or axial plane. The oncologically appropriate level of sacral osteotomy is often best identified on the coronal or midline sagittal MRI. Cranial tumor extension is most pronounced in the canal. (A) CT scan at the level of the anterior-superior and posterior-superior iliac spines. Markers (ii) (right sacral ala strip), (iii) (posterior-superior spine), and (iv) (left S1) and the sacral canal are identified. (B) Markers (i) (right pelvic brim), (v) (right S2), and (vii) (reinforcement bar). (C) CT scan at the level of the anterior-inferior iliac spine. Marker (i) (right pelvic brim) is identified above the open sciatic notch.


Male and female pelvises differ significantly. The gynecoid pelvis in the upright position has the sacral ala aligned more parallel with the floor. This creates a wider and flatter surface of the pelvic wings. Surgical access to the larger pelvic inlet is easier. Measurements about the pelvic inlet include the following: the conjugate diameter of the pelvic inlet, measured from the midline sacral promontory to the pubic symphysis; the transverse diameter measured from right to left pelvic brim; and the oblique diameter, measured from the inferior aspect of the sacroiliac joint to the contralateral ischial spine. In the gynecoid pelvis these measurements average between 11 and 13 cm. In comparison to those of the gynecoid pelvic inlet, the android pelvis measurements are slightly shorter, reflecting a taller, narrower pelvic structure. The pelvic inlet is oriented more vertically so that the sacral ala forms an acute angle with the floor. The iliac wings are steeper and the true pelvis is deeper. The ischial tuberosities are closer together while the pubic symphysis is deeper and taller. Consequently, anterior approaches to deep pelvic structures are more difficult in the male than in the female.


Most pelvic surgeons agree that primary sacral neoplasms at or below the level of S3 can be safely excised through a single posterior approach. Fourney et al. classified sacral resections into two main groups—those for midline tumors and those for eccentric tumors. The midline group included low, middle, and high sacral amputations, total sacrectomy, and hemicorporectomy (hemicorpectomy). These authors based the level of sacral amputation on the nerve root sacrificed: low sacral amputation involved sacrifice of at least one S4 nerve root, midsacral involved sacrifice of at least one S3 nerve root, and the high sacral amputations involved at least one S2 nerve root. Li et al. further modified this and proposed a surgical classification system for en bloc resection of primary malignant sacral tumors. They divided the sacrum into three regions of upper sacrum, middle sacrum, and lower sacrum by S1–S2 and S2–S3 junctions. En bloc tumor resections were classified into five types based on tumor extension. Type I resection involves regions of upper sacrum ± involvement of middle and/or lower sacrum. Type II resection involves middle and lower sacrum resection. Type III resection involves lower sacrum resection. Type IV resection is a sagittal hemisacrectomy usually along with adjacent ilium. Type V resection occurs when the tumor involves the fifth lumbar vertebra. The authors recommended an anterior and posterior resection for types I, IV, and V resections.


The posterior approach to sacral sarcoma resection can be compartmentalized into eight parts. Sequentially accomplishing these eight steps offers a convenient way to study the surgical anatomy of the sacrum, spine, and posterior pelvis. This outline will be used as a guide, orienting the reader to the surgical anatomy of this region.

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Feb 21, 2019 | Posted by in NEUROSURGERY | Comments Off on Anatomical Considerations for Resection of Chordomas and Chondrosarcoma of the Sacrum
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