Surgical Anatomy, Approaches, and Biomechanics in the Lumbosacral Pelvic Junction

Overview

The lumbosacral junction–the confluence of the lumbar spine, sacrum, and ilium—can be afflicted by traumatic, infectious, degenerative, deformative (scoliosis), and neoplastic processes. Surgical management of diseases that affect the lumbosacral junction requires an intricate understanding of lumbosacral anatomy that can present challenges to the surgeon unfamiliar with surgical approaches and techniques applicable to this complicated region.

Classically, the lumbosacral region is accessed by anterior or posterior surgical approaches. Anterior approaches to the lumbosacral junction, whether intraperitoneal or retroperitoneal, can be used to expose the anterior aspect of the lumbar spine and sacrum. Posterior approaches to the lumbosacral junction can be used to expose the posterior surface of the lumbar spine, sacrum, and ilium. Regardless of approach, a detailed understanding of surface, bony, neural, and vascular anatomy, as well as lumbosacral biomechanics and spinopelvic fixation, are essential for the surgeon management of diseases of the lumbosacral junction.

Anatomy

Surface Anatomy

Anterior approaches to the lumbosacral junction require understanding of the anatomy of the anterior abdominal wall. At the midline, the anterior abdominal wall, from superficial to deep, consists of the skin, subcutaneous fat, Scarpa fascia, rectus abdominis, the rectus sheath, transversalis fascia, peritoneal fat, and peritoneum. In the lateral abdominal wall, the rectus abdominis is replaced by three layered muscles: from superficial to deep, these are the external oblique, internal oblique, and the transversus abdominis. Moving lateral to medial, the investing fascia of these three muscles condense to form the rectus sheath, which surrounds the rectus abdominis ( Fig. 50-1 ).

At the midline, between the right and left rectus muscles, the rectus sheath forms a fascial condensation called the linea alba . Above the umbilicus, the internal oblique fascia splits to invest the anterior and posterior surface of the rectus abdominis; the external oblique fascia travels anterior to the rectus abdominis in the anterior rectus sheath, and the transversus abdominis fascia travels posteriorly to form the posterior rectus sheath.

Below the umbilicus—starting at the linea semicircularis of Douglas, or arcuate line of the abdomen—all three layers of fascia pass anterior to the rectus abdominis to form the anterior rectus sheath, leaving only transversalis fascia between the rectus and the peritoneum ( Fig. 50-2 ). The linea semicircularis is not to be confused with the linea semilunaris, which is the lateral border of the rectus abdominis, where the external oblique, internal oblique, and transversus abdominis fascia invest the muscle that forms the rectus sheath. The inferior epigastric vessels course between the muscles of the abdominal wall and the transversalis fascia. They travel in oblique fashion from lateral to medial and thus are rarely at risk in midline anterior approaches, although caution must be paid to these vessels in more lateral approaches. Deep to the peritoneum are the abdominal and pelvic viscera, most prominently the large and small intestine, and the urinary bladder. From the surface, the L3–L4 disk space can be found on level with the umbilicus, the L4–L5 disk space at the level of the superior iliac crest, and the L5–S1 disk space midway between the umbilicus and pubic symphisis ( Fig. 50-3 ).

Bony Antatomy of the Lumbosacral Junction

Lumbosacral iliac stability depends on the posterior bone and ligaments of the ilium and sacrum. The dorsal surface of the sacrum can be divided into three zones, as defined by bony anatomy. Moving in a lateral-to-medial direction, zone I is lateral to the dorsal sacral foramina and includes the lateral sacral crest. Zone II, the transforaminal zone, includes the bony surface that surrounds the dorsal sacral foramina. Most medial is zone 3, the region medial to these foramina, ending at the median sacral crest. In zone 3, along the midline, the distal sacrum opens at the sacral hiatus, an opening through which the final sacral nerves exit ( Fig. 50-4 ).

The sacrum has three prominent articulations: Above, it articulates with the fifth lumbar vertebral body at the lumbosacral joint. Laterally, the ilium articulates with the sacrum’s auricular process at the sacroiliac joint. Inferiorly, the sacrum articulates with the coccyx at the sacrococcygeal joint. In the anteroposterior (AP) plane, the lumbosacral joint rests anterior to the sacroiliac joint, transmitting the spine’s axial load to the superior surface of the sacrum, which is tilted forward; the result is a rotatory force that drives the sacral promontory forward and the apex backward. The axis of this rotary force passes through the center of the second sacral vertebra, and the sacrum’s rotary predisposition is restrained by ligamentous forces.

The two strongest ligaments in the sacrum are the interosseous ligament, a band of short fibers that connects the ilial and sacral tuberosities, and the posterior sacroiliac ligament, which passes from the sacral tubercles to the ilial tuberosity and posterior superior iliac spine counter to the natural forward tilt of the upper sacrum. The sacrotuberous ligament connects the ischial tuberosity to the anteroinferior sacrum and coccyx, and the sacrospinous ligament connects the ischial spine to the lateral sacrum and coccyx; these ligaments resist the natural posterior tilt of the inferior sacrum and coccyx. The iliolumbar ligament passes from the L5 transverse process to the inner lip of the iliac crest, and it resists the natural tendency of the fifth lumbar vertebra to slide anteriorly over the sacral promontory ( Fig. 50-5 ).

The sacroiliac joint itself is a synovial joint formed by the articular surfaces of the sacrum and ilium; the sacral articular surface is covered by hyaline cartilage, and the ilial surface is covered by fibrocartilage. The articular surface of the sacrum has a characteristic earlike shape, and its surface irregularities articulate with complementary irregularities on the surfaces of the ilial articular process. The sacroiliac joint terminates inferiorly at the S2 level.

The body’s weight is transmitted from the lumbar spine to the sacrum through the lumbosacral joint then from sacrum to ilum through the sacroiliac joint. In sitting and standing positions, the sacroiliac joint is the principal transmitter of force between the pelvis and spine. Surgical disruption or resection of the sacroiliac join, as in total sacrectomies, can weaken this link and result in sacroiliac instability with poor weight bearing in the upright position that will affect the patient’s ability to sit, stand, or walk. Sacroiliac stability is little affected by sacral resections as long as the sacroiliac joint is spared. Furthermore, sacroiliac stability can withstand resection of up to 50% or more of the sacroiliac joint, a finding corroborated by cadaveric data from Gunterberg and colleagues. Therefore during partial sacrectomies, the lower sacrum can be removed up to the S2 level—where the sacroiliac joint ends and the posterior sacroiliac ligament terminates—without introducing instability into the pelvis.

Biomechanics of Lumbosacral Junction

The lumbosacral junction represents an important point of transition between the spinal column, the pelvis, and the lower extremities. Increased attention is being paid to the interrelationship between the pelvis and the spine in maintaining appropriate sagittal balance, and many argue that isolated analysis of the spine alone is insufficient.

One of the guiding principles in analysis of spinal deformity is the concept of sagittal balance, which is influenced by several spinopelvic parameters. The most common measurements taken to assess sagittal balance are angles of thoracic kyphosis and lumbar lordosis, calculated using the Cobb method. Aside from actual measurements of the curvature of the spine, global spinal alignment can be assessed using the plumb line method, measuring on a full-length sagittal radiograph the horizontal offset from the posterosuperior corner of the S1 vertebral body to a plumb line drawn from the C7 vertebral body. This distance, known as the sagittal vertical axis (SVA) offset , has been reported in asymptomatic adults to have a normal range of 0.5 cm (± 2.5 cm; Fig. 50-6 ). SVA increases with sagittal plane “anterior imbalance.”

A more recently developed assessment of global sagittal balance is the T1–T9 global spinopelvic inclination, measured as the angle between the vertical plumb line and a line drawn from the center of the T1–T9 vertebral body and the center of the bicoxofemoral axis ( Fig. 50-7 ). Both the SVA and T1 spinopelvic inclination have been demonstrated to correlate significantly with health-related quality of life (HRQOL) measurements, with T1–T9 global spinopelvic inclination actually showing greater correlation. Thus anterior sagittal imbalance correlates with self-reported disability.

Analysis of sagittal imbalance in the spine is likely incomplete if not coupled with similar analysis of the pelvis. Analysis of the pelvis in the sagittal plane commonly uses three angular measurements: the pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). These three measurements exist in an algebraic relationship, such that PI = PT + SS ( Fig. 50-8 ).

The pelvic incidence is defined as the angle between a perpendicular line passing through the midpoint of the sacral plate and a line connecting this midpoint to the axis of the femoral head. PI is unique among the pelvic parameters in that it is a morphologic parameter, meaning that it is unique to the morphology of an individual pelvis and remains constant even with changes in pelvic position or inclination. As the only fixed value in a chain of interrelated measurements, the PI significantly influences the many positional parameters involved in sagittal balance. Most importantly, the PI has been closely linked to the degree of lordosis in the lumbar spine, such that low values of PI imply flattened lordosis, and high values imply pronounced lordosis with larger sagittal curves. Various formulas have been proposed for prediction of lumbar lordosis that use the pelvic incidence along with other pelvic parameters.

The sacral slope is defined as the angle between the horizontal and the sacral plate. Unlike the PI, the SS is a positional parameter, meaning that its value is fluid, and it varies in a given patient as that patient’s pelvis moves in space. The SS is a surrogate for the orientation of the pelvis. Lumbar lordosis is closely related to pelvic orientation and is thus influenced by the SS, which is in turn influenced by the PI, a connection that underlies the relationship between PI and lumbar lordosis as described above.

The pelvic tilt is defined as the angle between the vertical and a line from the midpoint of the sacral plate to the axis of the femoral head. PT is a positional parameter commonly thought to act in compensatory fashion; when the spine tilts forward, the body recruits compensatory mechanisms in an effort to bring the spine over the pelvis. One such mechanism is pelvic retroversion, rotation of the pelvis backward around the hips. This retroversion manifests itself as an increase in pelvic tilt. Schwab and colleagues demonstrated that PT is increased in patients with anterior sagittal imbalance—age-related changes, loss of lordosis, or increase of kyphosis—and is decreased in patients with posterior sagittal imabalance. PT is also the only pelvic parameter found to be significantly correlated with HRQOL indices, which once again underscores the importance of pelvic parameters in the clinical assessment of spinal deformity.

Vascular Anatomy

Intricate understanding of vascular anatomy is important for anterior approaches to the lumbosacral region, because vascular injuries are one of the most feared complications associated with surgical interventions in this area.

The aorta most commonly bifurcates into the common iliac arteries at the caudal L4 vertebra, just left of midline ( Fig. 50-9 ). The common iliac arteries run inferolateral on the medial surface of the psoas muscle to their bifurcation into the internal and external iliac arteries at the level of the lumbosacral articulation, anterior to the sacroiliac joints. The common iliac veins run superomedial to terminate in the inferior vena cava (IVC) posterior to the right common iliac artery at the L5 level.

The right common iliac artery is longer (5 cm) than the left (4 cm) owing to its origin at the aortic bifurcation just left of the midline. Just lateral to the right common iliac artery are the IVC, the termination of the right common iliac vein, and the right psoas. Medial to the artery are the right common iliac vein and the termination of the left common iliac vein, both of which pass beneath the right common iliac artery to join the IVC.

The shorter left common iliac artery is crossed by the inferior mesenteric artery that courses from its origin in the aorta above. To the left of the left common iliac artery is the psoas muscle; on the right is the left common iliac vein.

Both common iliac arteries are crossed anteriorly by the right and left ureter, respectively, as well as the right and left ovarian arteries in women. The ureters, loosely imbedded in the retroperitoneal space, cross the common iliac arteries at the level of the sacroiliac joint. Sympathetic nerve branches cross both arteries as they descend into the superior hypogastric plexus between the common iliac vessels. Anterior to both common iliac arteries are the sigmoid colon and mesocolon.

Descending from the posterior aorta, the middle (or median) sacral artery descends on the midline down the anterior surface of the L4–L5 vertebrae and sacrum. The lateral sacral arteries arise from the distal common or internal iliac artery, posterior division. These sacral arteries contribute a significant vascular supply to presacral tumors. The middle sacral artery is often sacrificed in anterior approaches to the lumbosacral junction.

The internal iliac arteries branch from the common iliac artery at an acute angle, making a dorsocaudal descent into the pelvis, which it supplies. The anatomy of the artery is variable, but it generally divides into an anterior and posterior branch, which give rise to its many branches, all of which will not be covered in this discussion. The first major branch is the iliolumbar artery, which exits at the posterior aspect of the internal iliac artery then ascends rostrolaterally, crossing over the lumbosacral trunk. As mentioned above, one or more lateral sacral arteries branch from the internal iliac artery. Next, and most prominently, the superior gluteal artery exits the internal iliac artery as its largest branch, passing through the upper sacral plexus en route to its exit through the greater sciatic foramen, superior to the piriformis muscle ( Fig. 50-10 ). More distally, the inferior gluteal artery passes through the lower sacral plexus on its own path to the greater sciatic foramen, inferior to the piriformis muscle.

The venous anatomy of the lumbosacral junction echoes the arterial, with far more variability. Two constant variations from the arterial anatomy are the drainage of the middle sacral vein into the left common iliac vein, instead of the IVC, and the drainage of the iliolumbar veins into the common, not internal, iliac veins. These iliolumbar (or ascending lumbar) veins, which arise from the posterior aspect of the common iliac veins at the L5 level, are of particular interest to the surgeon, because they must be isolated and controlled during anterior lumbosacral approaches.

Neural Anatomy

The right and left sacral plexi are formed by the confluence of the lumbosacral trunk, which travels inferolaterally along the ventral sacrum, with the S1–S3 ventral rami emerging from the ventral sacral foramina ( Fig. 50-11 ). This confluence of nerves forms at the sacroiliac joint on the level of the S2 foramina just anterior to the piriformis muscle. The superior and inferior gluteal nerves are the most proximal braches of the plexus, and these leave the pelvis through the greater sciatic foramen above and below the piriformis muscle, respectively ( Fig. 50-12 ). The largest branch of the sacral plexus is the sciatic nerve, which forms at the level of the S3 foramina and exits the pelvis through the greater sciatic foramen inferior to the piriformis. Smaller motor and sensory branches also form in the sacral plexus and pass out of the pelvis with the sciatic nerve. Emerging in the inferomedial sacral plexus, contributions from the ventral rami of S2 to S4 join to form the pudendal nerve, which among other functions innervates the striated muscle of the rectal and urethral sphincter and the coccygeal and levator muscles of the pelvic floor. The pudendal nerve is the only nerve to both exit and enter the pelvis, leaving via the greater sciatic foramen, hugging the sacrospinous ligament, and returning via the lesser sciatic foramen. The fourth and fifth sacral roots and the coccygeal roots form an inconsistent coccygeal plexus that supplies the perianal region.