Spine anatomy and biomechanics

The spine consists of a curved stack of 33 vertebra divided structurally into five regions (Figure 2.1). Proceeding from superior to inferior, there are seven cervical vertebrae (C1-C7), twelve thoracic vertebrae (T1-T12), five lumbar vertebrae (L1-L5), five fused sacrum vertebrae (S1-S5), and four small fused coccygeal vertebrae. The vertebrae from each region have similar parts, but the shapes of vertebrae vary considerably from region to region in the spine.

Because of structural differences and the ribs, varying amounts of movement are permitted between adjacent vertebrae in the cervical, thoracic, and lumbar portions of the spine. Within these regions, two adjacent vertebrae and the soft tissues between them are known as a motion segment. The motion segment is considered to be the functional unit of the spine (Figure 2.2).

Each motion segment contains three joints. The vertebral bodies separated by the intervertebral disc form a symphysis type of amphiarthrosis. The right and left facet joints between the superior and inferior articular processes are diarthroses of the gliding type that are lined with articular cartilage.

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Figure 2. 1: Vertebral column: Anterior, left lateral and posterior views of the major regions of the spine [15].

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Figure 2. 2: The motion segment in the lumbar spine, which composed of two vertebrae and surrounding soft tissue [16].

2.1.1. Vertebra

A typical vertebra consists of a body, a hollow ring, and several bony processes, such as the pedicle, lamina, spinous process, and transverse process, as shown in Figure 2.3(A). Each vertebral body consists of an outer shell of cortical bone and an inner core of cancellous bone.

The vertical and horizontal structure of bone in the cancellous core is called trabecular bone (Figure 2.3 B). Most of the compressive force acting down the long axis of the spine is resisted by the cancellous bone because of its dense network of trabecular bone [17]. In general, the vertebral size is progressively increased from the cervical region to the lumbar region.

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Figure 2. 3: The Shape of a human vertebra: (A) Superior view of the typical lumbar vertebra [16]. (B) The trabecular structure of a lumbar vertebral body in sagittal section [18].

2.1.2. Intervertebral disc

The intervertebral disc is composed of two parts: the nucleus pulposus and annulus fibrosus (Figure 2.4). The nucleus pulposus located in the central of each disc which is only slightly compressible and with 80 % to 88 % water content [19]. In general, the lumbar nucleus fills 30 % to 50 % of the total disc area in cross-section [20]. The annulus fibrosus consists of approximately 15-25 concentric lamellae in the circumferential around the nucleus which contain collagen fibers [21]. The collagen fibers are oriented approximately 30° angle to the horizontal plane and crisscross to each other in the adjacent lamella. The superior and inferior cartilaginous endplates cover disc and connect with adjacent vertebrae bodies.

The primary function of the disc is transfer compressive forces evenly from one vertebral body to the next, while allowing for small-amplitude twisting and sliding movements [22]. The tensile properties of the annulus are stiffer in anterior than the posterolateral regions, with the outer region being stiffer than the inner regions [23]. The outer lamellae resist excessive

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bending and twisting of adjacent vertebrae, while the innermost lamellae are deformable and normally behave like a fluid. The endplate not only helps to equalize loading of the vertebral body but also prevents rapid fluid loss from the nucleus [24].

Figure 2. 4: In the intervertebral disc, the annulus fibrosus, made up of laminar layers of criss-crossed collagen fibers, surrounds the nucleus pulposus [16].

2.1.3. Facet joint

The size and angulation of the vertebral processes vary throughout the spinal column (Figure 2.5). This changes the orientation of the facet joints, which limit ROM in the different spinal regions. In addition to channeling the movement of the motion segment, the facet joints assist in load bearing. The facet joints and discs provide about 80 % of the spine’s ability to resist rotational torsion and shear, with half of this contribution from the facet joints [25, 26].

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Figure 2. 5: Orientation of lumbar facet to the transverse plane (left) and the frontal plane (right) [16].

2.1.4. Spinal ligaments

There are a series of ligaments that are important to the stability of the vertebral column.

Important to the lumbar spine are seven types of ligaments (Figure 2.6): ALL and posterior longitudinal ligament (PLL) are associated with each joint between the vertebrae. The ALL runs along the front and outer surfaces of the vertebral bodies. It has the most significant effect for the stability under extension. However, in the ALIF surgery, it was moved out from the surgical level. The posterior longitudinal ligament runs within the vertebral canal along the back surface of the vertebral bodies. The ligamentum flavum (LF) is located on the back surface of the canal where the spinal cord or caude equina runs. The interspinous ligament (ISL) runs from the base of one spinous process (the projections at the back of each vertebra) to another. Intertransverse ligament (ITL) and supraspinous ligaments (SSL) run along the tips of the spinous processes. Joint-related structures called facet capsular ligament (CL) also play an important role in stabilization and movement.

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Figure 2. 6: The major ligaments of the spine [27].

2.1.5. Neural foramen

The segmental spinal nerve roots exit through the intervertebral foramen (Figure 2.2). The intervertebral foramen is bounded by the pedicles superiorly and inferiorly, and ventrally and dorsally by two major intervertebral articulations. It is bounded ventrally by the dorsum of the intervertebral disc and the lateral expansion of the posterior longitudinal ligament. Foraminal disc herniations can impinge on the exiting nerve root, causing radiculopathy. The joint capsule of the articular facets and the ligament flavum make up the dorsal boundary of the intervertebral foramen. The remaining space is composed of loose areolar tissue and fat.

2.1.6. Spinal cord and nerve roots

The spinal cord is a column of millions of nerve fibers that run through spinal canal (Figure 2.7). It extends from the brain to the area between the end of first lumbar vertebra and top of second lumbar vertebra. At the second lumbar vertebra, the spinal cord divides into several different groups of fibers that form the nerves that will go to the lower half of the body.

For a small distance, the nerves actually travel through the spinal canal before exiting out the

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neural foramen. This collection of nerves is called the cauda equina while it is still inside the spinal canal.

A protective membrane called the dura mater covers the spinal cord. The dura mater forms a watertight sack around the spinal cord and the spinal nerves. Inside this sack, the spinal cord is surrounded by spinal fluid.

The nerve fibers in spinal cord branch off to form pairs of nerve roots that travel through the small openings (foramina) between vertebrae and vertebrae. The nerves in each area of the spinal cord connect to specific parts of body. This is why damage to the spinal cord can cause paralysis in certain areas and not others; it depends on which spinal nerves are affected. The nerves of the cervical spine go to the upper chest and arms. The nerves in the thoracic spine go to chest and abdomen. The nerves of the lumbar spine then reach to legs, bowel, and bladder.

These nerves coordinate and control all the body's organs and parts, and body muscles.

Figure 2. 7: Spinal Cord and Nerve Roots [28].