Zygapophyseal Joints

The lumbar spine has two superior and inferior articular processes that form the zygapophyseal or “facet” joints. The first lumbar vertebra has superior articular processes (which articulate with the 12th thoracic vertebrae) located primarily in the frontal plane and inferior articular processes which orient largely in the sagittal plane. Orientation of the superior and inferior articular processes continues generally to be in the sagittal plane, with a moderate transition toward the frontal plane through L5, the lowest lumbar vertebra (see Fig. 1-1). The inferior articular processes of L5 articulate with the sacral isthmus.^3,6 The facet joints are true synovial joints, and therefore each joint has an independent joint capsule, articular cartilage, and synovial fluid.^3,5 The primary roles of the zygapophyseal joint are to guide and limit movement in the lumbar spine, as discussed further in the later section on lumbar biomechanics.

Intervertebral Foramen

The intervertebral foramen is formed by the posterior aspect of the vertebral body, the adjacent pedicle, and the anterior aspect of the articular process.⁷ This space is further defined when two adjacent vertebrae come together, creating a space to allow the lumbar nerve roots to exit (Fig. 1-2). Because movement influences the intervertebral foramen space, persons with foraminal stenosis (i.e., narrowing of the intervertebral foramen) may experience symptom changes with positional variations.⁸ As the cephalic vertebra flexes, the intervertebral foraminal space opens up approximately 12%; if the cephalic vertebra extends, this space will narrow (i.e., close down) approximately 11%.^5,8 The intervertebral foramen also narrows with lateral flexion to the same side and opens up with lateral flexion to the opposite side. Moreover, ipsilateral rotation narrows the foraminal space on the same side.⁸ In addition, the posterolateral aspect of the intervertebral disk may encroach on this space, particularly if a defect exists in the annular fibers and subsequent herniation occurs (i.e., lateral directed herniation).

Vertebral Foramen

The vertebral foramen in the lumbar spine is a large, triangular space formed by the vertebral body anteriorly, the pedicles laterally, and the lamina posteriorly; it functions to protect the spinal cord and cauda equina (see Fig. 1-1).³ Similar to the intervertebral foraminal space, the vertebral foramen is influenced by movement. The vertebral foramen space tends to increase during flexion movements and becomes narrower during movements such as extension.⁵ In particular, changes in vertebral foramen space are often influenced by soft tissue changes (e.g., buckling of ligamentum flavum into the vertebral foramen with extension).

Intervertebral Disk and Vertebral End Plate

The intervertebral disks are located between the lumbar vertebral bodies (Fig. 1-3). The primary purposes of these disks are spinal mobility and shock absorption. The intervertebral disk has two primary parts: the annulus fibrosis and the nucleus pulposus.³ The annulus fibrosis consists of rings of fibrocartilage lamellae, which create the outer portion of the intervertebral disk. Each lamella contains fibers that run obliquely from one vertebra to the other and in opposite directions to adjacent lamellae (see Fig. 1-3).³ The outer third of the annulus is innervated by branches of the vertebral and sinuvertebral nerves that can transmit pain-related information to the brain in diskogenic-related conditions.^9–12 The most central portion of the intervertebral disk is referred to as the nucleus pulposus, which consists of dense mucoid material.⁵ The nucleus pulposus is avascular and aneural, and it receives nutrition by diffusion from blood vessels at the periphery of the annulus and vertebral body end plates.³ The vertebral end plates are cartilaginous structures located over the superior and inferior aspect of the articular disk. The end plates have both vascularity and nerve innervation.^13,14 They act as a structural transition between the nuclear material and the bone in the center of the vertebral body.¹⁵ Abnormal changes in the vertebral end plate have been associated with intervertebral disk herniations.^16,17

Ligaments

Ligamentous structures are extensive in the lumbosacral spine and contain numerous proprioceptors, mechanoreceptors, and free nerve endings.^12,18 These receptors play an important role in lumbopelvic sensory awareness, and some mechanoreceptors may contribute to pain perception.^19–23 Ligaments of the lumbosacral spine can be considered segmental or continuous. Segmental ligaments have local attachments between individual vertebrae. This segmental attachment suggests that the proprioceptive or nociceptive input will be localized.²⁴

The five primary segmental ligaments classified in the lumbosacral spine are the interspinous ligament, supraspinous ligament, intertransverse ligament, iliolumbar ligament, and ligamentum flavum (Fig. 1-4). The interspinous ligament attaches to the adjacent spinous process by filling in the space between processes. The ligament fibers run diagonally and become taut during flexion and slack during extension.²⁵ The supraspinous ligament connects the tips of adjacent lumbar spinous processes. The supraspinous ligament is also taut during lumbar flexion and slack during extension. Both the supraspinous ligament and the interspinous ligament are the first posterior ligaments to fail with extreme flexion tension forces.^25,26 The intertransverse ligament attaches to the adjacent transverse process and becomes taut predominantly with lateral flexion to the opposite side and to a lesser degree with rotation.⁵ The iliolumbar ligament is found in the lower lumbar segment and consists of a superior and inferior band. The superior band attaches from the transverse process of L4 to the ilium, and the inferior band connects the transverse process of L5 to the ilium. The iliolumbar ligaments become taut during spinal flexion and rotation because of their fiber orientation.²⁷ They also play a significant role in preventing anterior translation of L4 on L5 and of L5 on the sacral base by restricting motion.⁵ The ligamentum flavum connects the lamina of adjacent vertebrae and receives the highest strain when it is stretched during flexion.²⁸ The ligament is under constant tension and creates a continuous compressive force to the intervertebral disks.⁵

Continuous ligaments attach at multiple locations along the length of the lumbosacral spine. This multisegmental attachment and broader length suggest that injury to the continuous ligaments may contribute to diffuse symptoms.²⁴ The primary continuous ligaments include the anterior longitudinal ligament and the posterior longitudinal ligament (see Fig. 1-4). The anterior longitudinal ligament has attachments along the anterior portion of the vertebral bodies and extends down to the sacral base. The anterior longitudinal ligament tends to be a broad, thick ligament that increases in thickness and width from the lower thoracic area to L5 and S1. The ligament covers a large portion of the anterior lumbosacral vertebral bodies and helps to reinforce the anterolateral portion of the intervertebral disk.^5,29 The ligament is relaxed in flexion and taut in extension.⁵ The posterior longitudinal ligament runs within the vertebral canal along the posterior aspect of the vertebral bodies. In the lumbar spine, the posterior longitudinal ligament narrows as it courses downward towards the sacrum. The narrowed ligament provides little support for the lumbar intervertebral joints, but it does reinforce the posterior surface of the intervertebral disk.³⁰ The ligament is taut in flexion and relaxed in extension.⁵

Sacrum and Coccyx

The sacrum consists of five fused vertebrae (Fig. 1-5).³ The most cephalic sacral vertebra has superior articular facets referred to as the sacral isthmus. These processes articulate with the inferior facet joints of L5 and are largely present in the frontal plane. The sacrum not only articulates with L5 superiorly but also articulates caudally with the coccyx and laterally with the ilia to form the sacroiliac (SI) joint.⁵ The coccyx is a series of three to five fused vertebral rudiments that articulates with the inferior aspect of the sacrum (see Fig. 1-5).³ The sacrum is discussed further later, in conjunction with the SI joint.

Nerves of the Lumbar Spinal Cord

The nerves from the lumbar spinal cord pass through the intervertebral foramen below the corresponding vertebra. These nerves subsequently divide into dorsal and ventral primary rami and contain both sensory and motor fibers.³ The lumbar plexus is a network of nerves composed of the ventral rami of L1 to L4 located anterior to the lumbar transverse processes and behind the psoas major muscle (Fig. 1-6).^31,32 The major branches of the plexus are discussed later. Table 1-1 also provides a summary of these nerves.³

Blood Supply to the Lumbar Region

The abdominal aorta provides arterial blood supply to the lumbar region. The aorta begins at the aortic hiatus of the diaphragm at the approximate level of T12 and descends down the left side of the vertebral bodies to the L3 and L4 vertebrae (umbilicus region), where it bifurcates into the right and left common iliac arteries.^3,7,36,37 The aorta lies anterior to the vertebral bodies of T12 to L4, and it supplies blood to the vertebrae through parietal branches called the lumbar arteries. The inferior vena cava provides venous return of deoxygenated blood from the legs and lumbopelvic region back to the heart. The inferior vena cava begins anterior to L5, ascends on the right side of the bodies of L5 to L3 up to caval foramen of the diaphragm, and passes through to the thorax.^3,7

Muscles of the Trunk: Abdominal Core

The trunk or “abdominal core” is a general term that describes the structures of the lumbopelvic hip complex. The abdominal core can be described as a muscular box with four walls. The anterior wall consists of the abdominal muscles, the posterior wall is made up of the gluteal and spinal muscles, the superior wall would be represented by the diaphragm, and the inferior wall comprises the pelvic floor and hip girdle muscles.³⁸ The lumbopelvic hip complex has a total of 29 pairs of muscles that work together to perform patterns of functional movements.³⁸ This section discusses specific muscles in the lumbosacral region (anterior and posterior walls), followed by a classification system for the abdominal core. The subsequent sections discuss specific muscles of the pelvis and hip region.

Posterior Muscles

The posterior back muscles in the region of the thoracic and lumbopelvic spine have three layers. The superficial group of muscles includes the latissimus dorsi and trapezius, which have an intimate attachment to the thoracolumbar fascia that covers the deep muscles of the back.³⁹ The thoracolumbar fascia spans laterally from the spinous processes with a narrow muscular overlay in the thoracic region and a thicker covering in the lumbar spine (Fig. 1-7).^7,39 The intermediate group of muscles includes the erector spinae group: iliocostalis (lateral), longissimus (intermediate), and spinalis (medial). The group has a common origin of attachment at the posterior iliac crest, posterior sacrum, SI ligament, and lower lumbar and sacral spinous processes. The muscles travel superiorly and attach to the various vertebrae of the lumbar, thoracic, and cervical spine. Because of their position, they are commonly classified by each region of the spine (e.g., iliocostalis lumborum or thoracis; Fig. 1-8). The deep layer of muscles includes the transversospinalis group: semispinalis (superficial), multifidus (middle), and rotatores (deep). The semispinalis muscle is also named by its regional attachments in the spine (i.e., spinalis thoracis) (Fig. 1-9).³

Other posterior muscles include the quadratus lumborum, psoas major, and iliacus (Fig. 1-10). The quadratus lumborum spans distally from the iliolumbar ligament and iliac crest up to the twelfth rib and lumbar transverse processes.³ The psoas major originates on the anterior bodies and intervertebral disks of T12 to L5 and inserts into the lesser trochanter of the femur. The iliacus originates with the iliac fossa, ala of the sacrum, and anterior SI ligament and distally joins the psoas major, inserting into the lesser trochanter of the femur.⁷ Table 1-2 provides a summary of muscle groups involved in motion of the trunk.

TABLE 1-2

Lumbopelvic Movements With Corresponding Muscles

Movement	Muscles
Rotation (unilateral)	Unilateral: rotatores, multifidus, external oblique, opposite internal oblique, semispinalis
Side bending (unilateral)	Iliocostalis, longissimus, multifidus, external oblique, internal oblique, quadratus lumborum
Flexion	Rectus abdominis, psoas major
Extension	Erector spinae, multifidus, semispinalis

Anterolateral Muscles

The anterolateral trunk region contains four muscles that help stabilize and control movement (Fig. 1-11). First, the rectus abdominis is a long, straplike muscle that originates on the pubic symphysis and pubic crest and inserts superiorly on the xiphoid process and costal cartilages of the ribs 5 to 7. Each side of the rectus abdominis is connected centrally by the linea alba, which is a vertical fibrous band that spans the length of the muscle.³ It also has attachments with the oblique and transversus abdominis. Second, the external oblique originates on the external surface of ribs 5 to 12 and inserts into the linea alba, pubic tubercle, and anterior half of the iliac crest. Third, the internal oblique originates on the thoracolumbar fascia, anterior two thirds of the iliac crest, and lateral half of the inguinal ligament. The muscle inserts into the inferior border of ribs 10 to 12, linea alba, and pecten pubis (through the conjoint tendon).³ Fourth, the transversus abdominis is the deepest of all the abdominal muscles (see Fig. 1-10). The fibers run in a transverse fashion, originating on the internal surface of the costal cartilage of ribs 6 to 12, thoracolumbar fascia, iliac crest, and lateral third of the inguinal ligament with insertion anteriorly onto the linea alba, aponeurosis of the internal oblique, pubic crest, and pecten pubis (through the conjoint tendon).^3,7 The transversus abdominis is a key muscle for local feed-forward stabilization and rehabilitation of the lumbosacral spine.^40,41 The anterolateral abdominal wall may be susceptible to hernias in the umbilical and inguinal region, as discussed further in Chapter 5. See Table 1-2 for a summary of muscle groups involved in motion of the trunk. Appendix A provides more information about the muscles.

Classifying the Abdominal Core

To understand the abdominal core fully, one must understand how all the structures work as a system. Clark and colleagues⁴² created a system that classifies the structures of the lumbopelvic hip complex into two main systems: stabilization and movement. Each system has various subsystems that define the functional anatomy of that system (Fig. 1-12).

Stabilization System

The stabilization system is divided into two subsystems: local and global. The local muscle subsystem includes those structures that have direct influence on specific segments of the lumbosacral spine,⁴³ because of the proximal or distal attachment of a given muscle or the close proximity of these muscles to the vertebrae. These muscles are primarily composed of type I muscle fibers with a high density of muscle spindles.^1,42 The muscles work together to influence orientation of the individual vertebrae, increase segmental spinal stability through tension created in thoracolumbar fascia, decrease biomechanical forces (e.g., compression, shear), and assist with proprioception and postural control.^39,42,44 The muscles of the local system include the multifidus, internal oblique, pelvic floor muscles (see Chapter 9), and diaphragm.

The global subsystem includes muscles that influence several vertebral segments and often cross multiple joints along the pelvis and spine.⁴³ These muscles help to transfer forces from the upper extremity to the lower extremity, provide stabilization, and promote eccentric control of the abdominal core during movement.⁴⁵ Because of the complex roles of these muscles, they may be involved with pathologic processes of the lumbosacral hip region. For example, trigger points have been identified in muscles of the global system.⁴⁶ These trigger points generally have distal referral patterns that may cross the lumbopelvic region and may be mistaken for pathologic conditions in other areas of the spine or hip.^47,48 The global system includes the following muscles: psoas major, rectus abdominis, portions of the internal oblique, external oblique, gluteus medius, adductors, gracilis, pectineus, and quadratus lumborum.^42,43

Movement System

The movement system includes larger muscle that link the lumbopelvic region to the lower extremities. These muscles are responsible for both force production (concentric) and deceleration (eccentric) during multiplane dynamic movements.⁴² This dynamic stability occurs from the muscles working in groups or subsystems, rather than in isolation. Efficient patterned movement is enhanced when these muscles are contracting synchronously. Four main subsystems comprise the movement system. First, the anterior oblique subsystem includes the internal oblique, external oblique, adductors, and hip external rotators. Second, the posterior oblique subsystem includes the gluteus maximus, latissimus dorsi, and thoracolumbar fascia. Third, the deep longitudinal subsystem includes the erector spinae, thoracolumbar fascia, sacrotuberous ligament, and biceps femoris. Fourth, the lateral subsystem includes the gluteus medius, tensor fasciae latae, adductor complex, and quadratus lumborum.

Inguinal (Groin) Region

The inguinal region is a junction between the trunk and the lower limbs and is considered an area of weakness of the lower anterolateral abdominal wall.³ The inguinal ligament spans the area from the anterior superior iliac spine to the pubic tubercle. Along with the pubic bones, the inguinal ligament marks the inferior border of the anterior abdominal wall and provides attachments for the transversus abdominis and internal oblique muscles.³ The inguinal region is the most susceptible to hernias among all the areas of the anterolateral abdominal wall (see Chapter 5).⁵⁰

The inguinal canal is a passage that houses the spermatic cord in males, the round ligament of the uterus in females, the blood and lymphatic vessels, and the ilioinguinal nerve (Fig. 1-13).^3,7,51 The deep (internal) inguinal ring is the entrance to the inguinal canal and is located superior to the inguinal ligament. The superficial (external) inguinal ring is the distal exit to the inguinal canal and is located between the fibers of the external oblique just superior lateral to the pubic tubercle. The superficial ring is the site that examiners often palpate to diagnose an indirect hernia.⁵² The floor is formed by the superior surface of the inguinal ligament. The anterior wall formed by the aponeurosis of the external oblique and posterior wall is created by the transversalis fascia and medial part of the wall by the conjoint tendon. The conjoint tendon (aponeurotic falx) is a common tendon that connects the pubic attachments of the internal oblique and transversus abdominis aponeuroses.^3,51

The femoral triangle is a fascial junctional inferior to the inguinal ligament. The femoral triangle is bound superiorly by the inguinal ligament, medially by adductor longus, and laterally by the sartorius. The roof of the femoral triangle is formed by the fascia lata, cribriform fascia, subcutaneous tissue, and skin.⁵³ The floor of the triangle is composed of the iliopsoas (laterally) and pectineus (medially). The contents of the femoral triangle include the femoral nerve (L2 to L4), arteries, and veins that are within the femoral fascial sheath, and each structure is housed within its own compartment (Fig. 1-14).⁵³ The lateral compartment contains the femoral artery, and the intermediate compartment contains the femoral vein, whereas the medial compartment contains the femoral canal. The femoral canal is the smallest of the compartments and contains lymph vessels, fat, and connective tissue. The femoral artery and vein bisect the triangle and enter the adductor canal, which is located in the middle third of the thigh between the vastus medialis and adductor muscles.³

The pelvis contains two innominate bones that are a fusion of the ilium, ischium, and pubis (Fig. 1-15). The female pelvis has a wider architecture and circular shape than the male pelvis (Fig. 1-16).⁵⁴ Anteriorly, the pubic bones form an amphiarthrodial, cartilaginous joint called the pubic symphysis with hyaline cartilage on the end of each pubic bone and an interpubic fibrocartilaginous disk that forms the joint.^3,55 The pubic symphysis is supported by four ligaments: the anterior pubic, posterior pubic, superior pubic, and inferior pubic.⁵⁶ This joint can be subject to overuse injury (e.g., osteitis pubis), as further discussed in Chapter 3. Along the lateral aspect of the innominate lies a deep socket called the acetabulum, which articulates with the femoral head to create the coxofemoral joint.⁵⁶ The hip joint is discussed later in this chapter.