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Anatomical Considerations for Resection of Chordomas and Chondrosarcoma of the Sacrum
© 2018 Elsevier Inc. All rights reserved. Please note that the copyright for the original figures submitted by the contributors is owned by Contributors.
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.
Anteroposterior plain radiograph of a normal pelvis to be compared with the labeled pelvis in Figs. 28.2–28.4 .
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.
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.
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.
The Soft Tissues
The skin incision for sacral resection begins in the posterior midline. The track of any prior biopsy must be excised as it likely harbors malignant cells capable of causing local recurrence. An oncologically ideal resection mandates removal of a wide margin of potentially contaminated soft tissue. There is often a soft spot in the caudad sacrum corresponding to the midline sacral hiatus. This typically begins at the posterior S3 foramen and widens as it approaches the coccyx. The cranial apex of the hiatus is several millimeters wide. It enlarges caudally as it reaches the coccygeal component. It may be 2 cm wide caudally.
Either the cranial or caudad tip of the incision is angled in the shape of a hockey stick to increase lateral exposure. The angled portion can be upright or inverted, single or double limbed. Lateral incisions extend exposure to the sciatic notch and the soft tissues near the posterior column of the acetabulum and hip joint. Frequently, a sacral tumor, especially the extraosseous mass of a chordoma, can reach laterally to the greater trochanters, necessitating an angled incision. Adding a limb onto the midline incision also facilitates mobilization of a gluteus maximus myocutaneous flap that can be rotated to fill the defect caused by excising the biopsy track. Since this added limb can complicate wound healing, it should avoid bony prominences and exposed dura. It has proven wise to place the lateral incision limb caudal, more often than not. Should the wound heal incompletely, any open portion of the incision lies over “dead” space rather than the exposed spinal stump.
Three main muscles originate on the sacrum. They must be divided to enter the skeletal dissection plane. The sacrospinous muscles (1) or erector spinae muscles lie on each side of the spinous processes. Their superficial fascia blends with the posterior thoracolumbar fascia, attached laterally to the iliac crest. The erector spinae muscles form an anatomic compartment. The deepest or anterior-most portion of this compartment is composed of the interspinalis lumborum and multifidi muscles. Its fascia forms the anterior layer of thoracolumbar fascia. The thoracolumbar fascia envelops the abdominal wall muscles laterally. It attaches to the transverse processes and iliac crest, and divides to incorporate between its anterior and posterior layers of the external and internal oblique abdominal muscles and transversus abdominus muscle. Deep to the anterior layer of the thoracolumbar fascia, the quadratus lumborum muscle extends from the 12th rib to the iliac crest.
Once the erector spinae muscles are divided, the sacrum, L4, and L5 lamina are identified. The piriformis muscle (2) originates from the anterior aspect of the three central sacral vertebral bodies. The muscle belly converges to exit the pelvis through the greater sciatic notch. This muscle needs to be divided to expose the sciatic nerve exiting inferior to it. The superior and inferior gluteal vessels and nerves exit above and below this muscle belly, respectively. The gluteus maximus (3) is the largest and most superficial muscle that needs to be mobilized widely in exposing the sacrum. Most of the muscle originates from the lateral margin of the sacrum and the posterior-superior iliac crest. Less arises from the coccyx and sacrotuberous ligament. The width of this quadrilateral shaped muscle is maintained throughout its course as it passes anterior to the greater trochanter and iliotibial tract. The muscle inserts more extensively into the iliotibial tract than into the greater trochanter. It is the strongest external rotator and extensor of the hip. In its superior half, there is a distinct plane between the gluteus maximus and the gluteus medius muscles. In the lower half of the gluteus maximus, the plane between it and the hamstrings becomes less distinct. The muscle fibers of the maximus are laterally oriented, while those of the medius and hamstrings are vertically oriented.
The gluteus maximus is an important muscle in sacral surgery. It is frequently used to cover decubitus ulcers and fill the large dead space created by sacral removal. The gluteus maximus has a dual blood supply from the superior and inferior gluteal vessels. A neurovascular leash carrying the superior gluteal vessels and superior gluteal nerve can be predictably located at a point 3 cm lateral to the sacrum, approximately 5 cm below the posterior-superior iliac spine. It lies in the superior and most posterior aspect of the sciatic notch and often adheres to the iliac bone. It passes above the piriformis muscle. Although it is advantageous to preserve the superior gluteal nerve so as to maintain abductor function to the hip, this neurovascular leash must be frequently sacrificed to expose and control the sciatic notch. The larger, inferior gluteal vessels pass beneath the piriformis muscle. They lie just medial to the sciatic nerve at a point 5 cm from the sacral edge. A duplicate pair of inferior gluteal vessels may pass to the caudal half of the gluteus maximus. Since this vascular redundancy preserves blood flow to the maximus when the superior gluteal vessels must be sacrificed, it is important to preserve the inferior gluteal neurovascular bundle. Furthermore, the inferior gluteal nerve, which commonly arises from roots L5, S1, and S2, is the motor nerve to the gluteus maximus. In combined anterior and posterior approaches, ligation of the hypogastric artery and vein poses even greater risk to perfusion of the gluteus maximus muscle.
The Sciatic Notch
The sciatic buttress is the thick bone overlying the arch of the sciatic notch. It is commonly referred to as the strongest portion of the skeleton. The posterior arch constitutes the lateral aspect of the sacroiliac joint. The anterior aspect of the arch forms the posterior column of the acetabulum. The superior portion of the notch or buttress communicates with the pelvic brim or arcuate line on the interior pelvis. The lateral approach must expose and control the notch. Cutting the piriformis muscle and exposing the sciatic nerve allows one to enter the interior pelvis from the outside.
The strong posterior sacroiliac ligaments are preserved and act as oncologic barriers to the tumor. The sacroiliac joint is oblique in its orientation. It is more medial posteriorly than it is anteriorly. It approximates a 45 degrees angle preceding anteriorly and laterally from the midline sagittal plane. This means that the posterior 5 cm of iliac wing overlies the sacral ala and sacroiliac joint. The superior sacrum is firmly attached to the ilium by the confluence of the iliolumbar and sacroiliac ligaments which can be visualized posteriorly or anteriorly. These strong ligaments attach the transverse processes of L4 and L5 to the iliac crest and blend in with the anterior sacroiliac ligaments. Posteriorly, they blend with the posterior-superior iliac spine and posterior sacroiliac ligaments. This ligament complex maintains pelvic spinal continuity when high (S1 body) sacral osteotomies and sacral amputations are performed. The posterior ligament complex is the stronger of the two.
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