Biomechanics of the Spine in Sport

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CHAPTER 125 Biomechanics of the Spine in Sport

Lisa M. Bartoli, Robert S. Gotlin

INTRODUCTION

The spine is a complex design of bony and soft tissues, allowing the human form to move through various ranges of motions and positions. It serves both to transfer forces between the upper and lower extremities and to actively generate forces.^1,² Both the biomechanical design of the spine and the biomaterial composition of the various tissues determine its response to external stressors and forces generated within the body. The design and composition also provide an incredible ability to adapt to these stressors; however, there is a finite amount of adaptation that can occur before structural failure and injury can result.³ The epidemiology and biomechanical analysis of sports-related spinal injuries has allowed for the identification of the precise mechanism of various spinal injuries. This has resulted in sporting rules changes, equipment modifications, and changes in skill techniques, all of which have lowered the incidence of catastrophic spinal injury.⁴^–⁷

FUNCTIONAL ANATOMY

The osseous spine consists of 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral vertebrae and the coccyx. The ligaments, muscles, and intervertebral discs connect the vertebrae to form the four curves of the spine: the two lordotic curves of the lumbar and cervical spine and the two kyphotic curves of the thoracic spine and sacrum. This structural property helps the spine absorb forces of compression and also contributes to the coupled motions that occur in the spine.^1,³

The functional spinal segmental unit is defined as two adjoining vertebrae, the disc between them, and their surrounding soft tissues.^1,^3,^8,⁹ The functional spinal segment, however, may be further divided into anterior and posterior segments. The anterior segment consists of the anterior longitudinal ligament, the vertebra, the disc, and the posterior longitudinal ligament. The anterior segment with its osseous vertebral body serves as the primary load-bearing portion of the spinal segment.^1,³ The posterior segment contains the neural arch, beginning at the pedicles and extending posteriorly to the spinous process. This area is responsible for guiding and restraining the motion that occurs between the spinal segments.³ The vertebral bodies of the anterior motion segment are composed of cancellous bone, surrounded by a thin layer of cortical bone. Various studies have shown that cancellous bone contributes as much as 50% to the strength of the bone during compression. The surrounding cortical bone provides approximately 10% of the strength.^1,⁹ As the spine progresses from C3 to L5, the size of the vertebral bodies increase in size and mass. This is an adaptation of the spine to allow for more load-carrying capacity in the lower lumbar segments. The loss of cancellous bone seen in osteoporotic patients explains the high incidence of fractures occurring with increases in compressive loads.^1,³

Specific to each area of the spine are the zygapophyseal joints (Z-joints) and their orientation in space. The Z-joints are located in the posterior functional segment and restrict motion at each spinal segment. Their orientation determines the amount and direction of movement that can occur in each segment of the spine. In the cervical vertebrae (C2–7), the Z-joints are oriented 45 degrees from the horizontal axis (x axis) and parallel to the vertical axis (y axis).^3,⁹ The exception in the cervical spine occurs with the atlas and axis, which have the Z-joints oriented nearly parallel to the horizontal axis.⁹ In the thoracic spine the Z-joints are oriented 60 degrees from the horizontal plane and 20 degrees from the vertical axis. The lumbar spine Z-joints are oriented 90 degrees from the horizontal axis and 45 degrees from the vertical axis. The inferior articulating processes of lumbar spine Z-joints are convex while the superior articulating processes have a concave surface.⁹ Articulations with the skull, ribs, and sacrum also influence motion in the spine.

The cervical spine (C3–7) can move through flexion, extension, lateral flexion, and rotation. Motion in the thoracic spine consists mostly of transverse plane rotation, lateral bending, and a smaller component of flexion and extension. The lumbar spine allows a small amount of rotation, but a much larger amount of flexion, extension, and side-bending. Slightly more rotation occurs at the lumbosacral junction due to the more obliquely oriented Z-joints at this segment.^3,⁹ To summarize, the majority of flexion and extension occurs in the cervical, lower thoracic, and lumbar spine. The majority of rotation occurs in the upper thoracic and cervical spine, and lateral flexion is greatest in the cervical spine.^3,⁹

The combination of the orientation of the Z-joints with the spinal curves results in ‘coupled motions’ occurring at the spinal segment.^1,^3,⁹ Assuming a neutral position, cervical lateral flexion will cause rotation of the spinous process towards the convexity of the curve. For example, cervical side-bending left causes the spinous process to rotate towards the right. The same is true in the upper thoracic spine, but not in the lumbar spine. In the lumbar spine, lateral flexion causes rotation of the spinous process towards the concavity of the curve, i.e. side-bend right, rotation of the spinous process right.^1,^3,⁹

The exceptions to this motion occur between C1 (atlas) and C2 (axis). The skull articulates with C1, which lacks a vertebral body, and because it has superiorly oriented Z-joints, it is limited to flexion and extension of approximately 10–15 degrees between the occiput (C0) and C1 and approximately 8 degrees of side-bending.^3,⁹ There is almost no rotation at this segment. Rotation occurs at the C1–2 articulation.^3,⁹ Approximately 40 degrees of axial rotation occurs at the C1–2 articulation. This constitutes approximately 50% of total axial plane rotation seen in the cervical spine.

The curves of the thoracic and sacral spine are fairly rigid in comparison to the more flexible curves of the cervical and lumbar spine. It is the transition points of the spine, the cervicothoracic, thoracolumbar, and lumbosacral junctions, where the spine is subjected to the greatest amount of stress. This is due to the differences in mobility that occur at these transitions.¹

In general, the cervical spine demonstrates the greatest amount of range of motion in all planes and consequently is less stable and more vulnerable to injury. Also vulnerable to injury are the cervicothoracic and thoracolumbar transition regions. The combination of relatively mobile cervical and lumbar spines with the relatively immobile thoracic spine causes these transition zones to experience more torque. Because the spinal cord is located at both these levels, the potential for catastrophic spinal cord injury is of great concern.¹

The biomaterial make-up of the spine is an important consideration when attempting to understand spinal biomechanics. The bony structures, especially the cancellous bone, is very strong when compressive forces are applied, while the ligaments are stronger in tension and buckle under compression.^1,^3,⁹

The ligaments of the spine provide stability and store energy during motion. The primary ligaments of the spine are the anterior and posterior longitudinal ligament, the supraspinous ligament and the interspinous ligament. These ligaments have high collagen content, whereas the ligamentum flavum has high elastin content. The capsular ligaments of the Z-joints contribute to their stability.^3,⁹

The ligaments protect the spine by restricting motion to physiologic ranges, as well as helping to absorb external forces applied at high speeds.⁹ The ability of the ligaments to withstand deformation and their energy-absorbing characteristics were found to diminish with age.⁹ The high elastin content of the ligamentum flavum bestows a protective effect on the spinal unit. The elasticity allows for rapid flexion–extension motions and prevents permanent deformation of the ligamentum flavum, thereby limiting subsequent impingement of the spinal cord.⁹

When looking at injuries to the different spinal ligaments, one can identify the likely direction of the motion which caused the injury. For example, a hyperflexion motion would likely cause disruption to the supraspinous ligament. Axial rotation of the spine can cause damage to the capsular ligaments; rotation right would injure the right capsular ligament.⁹ The ligamentum flavum is stressed with lateral bending. If the intervertebral disc is degenerated, the ligaments are at higher risk for injury.⁹

The function of the intervertebral discs is to resist compression, help distribute some of the compressive load placed on the vertebral column, and to resist tensile and torsional loads as well.^3,⁹ The disc is composed of an outer covering of fibrocartilage, the anulus fibrosus. The anulus is organized in concentric layers, with the fibers oriented ±30 degrees to the horizontal axis, and 120 degrees to the adjacent layer. The inner layers are attached to the cartilaginous endplates of the vertebral body, while the outermost layers, known as Sharpey’s fibers, attach directly to the osseous tissues. This outer vertebral attachment is stronger than the attachment to the vertebral endplate.^1,^3,⁹

The anulus fibrosus surrounds the nucleus pulposus. It is composed of glycosaminoglycans, which have a high water-binding capacity. Approximately 80–90% of the nucleus pulposus is water, which desiccates with age.¹⁰

Flexion and rotation of the spine results in tension, compression, and shear stress to the disc.⁹ When loads are applied slowly over a long period of time, adequate time is allowed for deformation to occur. With degeneration of the disc its viscoelastic properties are reduced, thereby decreasing its ability to absorb loads.⁹

Compression forces cause bulging of the discs and increased tensile stress in the anulus. Studies have shown that pure axial compression caused deformation of the disc, but no actual herniation of the disc. The vertebral endplate was more likely to fracture, with herniation of the disc into the vertebral body (Schmorl’s node).^1,¹¹

In flexion, the posterior portion of the disc is subjected to tensile stresses, while the anterior portion experiences tension while in extension. In a study evaluating disc injury, a side-bending motion with hyperflexion caused increased tension in the posterolateral aspect of the anulus, opposite to the induced side-bending; a sudden compressive load was then applied which resulted in disc prolapse in a posterolateral direction.^1,^9,^12,¹³

The spine consisting only of ligaments, discs, and bony vertebrae, and absent of any musculature can only resist 20 N before collapse.^1,^9,¹⁴ The rib cage acts to strengthen and stiffen the spine as well as provides energy absorption during trauma.⁹ When the rib cage is added only 70 lb can be supported.^1,⁹ It is the addition of the spinal musculature which provides additional strength, stability, and movement of the spine and protection from large externally applied forces (i.e. football tackles).^9,¹⁵ The stability of the spine comes from the osseous and ligamentous structures, which provide a more passive stiffness, as well as the dynamic or more active stiffness provided by the muscles.¹⁶

The muscles of the spine can be divided into two basic groups. Those located posterior to the spine are primarily responsible for extension of the spine, and those located anterior to the spine are primarily responsible for flexion of the spine.

The anteriorly located spinal flexors consist of the deeply located psoas, and the more superficial abdominal muscles: the internal/external abdominal obliques, transversus abdominis, and the rectus femoris.^9,^16,¹⁷ The posteriorly located spinal extensors can be divided by location into deep and superficial muscles. The deep muscles include the rotators, intertransversari, multifidi, semispinalis thoracicis, cervicis, and capitis. The superficial muscles include the more laterally located iliocostalis and the medially located longissimus and spinalis.^9,¹⁶ McGill has identified the muscles attaching directly to the vertebra as most important in spinal stabilization: the multifidi, the quadratus lumborum, the longissimus and the iliocostalis. The abdominal muscles also have an important role in spinal stabilization.¹⁸

Richardson et al. have divided the muscles into either local muscles or global muscles. The local muscles act at the spinal segmental level and function as postural or segmental stabilizers. These muscles are the multifidi, quadratus lumborum, the lumbar portions of the iliocostalis and longissimus posteriorly, and the psoas, transversus abdominis, internal oblique and the diaphragm. The global muscles are dynamic and have greater torque-production capabilities. These consist of the rectus abdominis, external oblique, internal oblique (anterior fibers) and iliocostalis (thoracic portion).¹⁹

A term used often in rehabilitation of the lumbar spine is ‘core strengthening.’ This is an approach which strengthens the muscles about the spine in order to provide better dynamic stabilization, or muscular control, on the core. This core consists of the abdominals anteriorly, the paraspinals and gluteals posteriorly, the diaphragm above and the pelvic floor musculature and hip muscles below.^16,¹⁹

The thoracodorsal fascia is an important structure with regards to lumbar stabilization and maintenance of correct spinal mechanics.^2,^16,²⁰ This fibrous connective tissue encases the spinal extensors and is made up of three layers. These have been identified as the anterior, middle, and posterior layers. It extends inferiorly from the posterior thoracic spine, to the sacral and ilial attachments of the hip musculature. It also expands anteriorly to enmesh with the fibers of the internal oblique, and transversus abdominus, and extends superiorly to the serratus posterior inferior.^2,^16,²⁰^–²² The superficial laminae of the posterior layer have fibrous connections with the latissimus dorsi and the gluteus maximus. This is a key link between the upper and lower extremities.^16,^20,²³

This posterior layer is the most important layer with regards to providing support to the lumbar spine and abdominals.¹⁶ The two lamina of the posterior layer have fibers which are directed inferiomedially and laterally. The middle and deep layers of the thoracolumbar fascia form connections with the transversus abdominus.^2,^16,^19,²⁰

The thoracolumbar fascia functions as a site of muscular attachment and aids in trunk rotation. It also works with the spinal ligaments to increase stiffness of the spine.^20,²¹

The thoracolumbar fascia is not the only fascia present in the spine. The dorsal fascia of the cervical spine extends from the ligamentum nuchae, forming attachments to the spinous process of the cervical spine, with attachments made to the sternocleidomastoid, trapezius, acromion, scapular spine, and even as far anterior as the manubrium.²⁰ This forms a functional connection between the head, neck, and shoulder girdle.^20,²⁴

The quadratus lumborum has three fascicles, the inferior oblique, superior oblique, and the longitudinal fascicles. The inferior oblique works isometrically to stabilize the spine and laterally flex the spine.^16,¹⁸ The other fibers of the quadratus lumborum actually work during stabilizing the twelfth rib.¹⁶

The abdominal muscles, internal/external obliques, transversus abdominus, and rectus abdominus make up the anterior portion of the ‘core.’ The act of ‘hollowing out’ the abdomen selectively activates the transversus abdominus, while isometrically bracing the abdomen activates the transversus abdominis, and external/internal obliques. The pelvic floor muscles also fire with activation of the transversus abdominis.^16,¹⁸ The rectus abdominus is activated by doing curl-ups of the trunk, and in essence flexes the lumbar spine.

The obliques work with the transversus abdominis to increase intra-abdominal pressure. The connection of these muscles with the thoracolumbar fascia, as noted previously, transfers contractile stresses in a ‘hooplike’ direction about the trunk, thereby increasing stability to the lumbar spine.^16,¹⁷ The external oblique helps to prevent anterior pelvic tilt, and eccentrically controls lumbar extension and torsion.¹⁶

The diaphragm and pelvic floor serve as the roof and floor of the core, respectively. Akuthota and Nadler noted poor muscular coordination in both in those individuals with sacroiliac pain. As noted earlier, firing of the transversus abdominis also causes activation of the pelvic floor, and may be a way to improve muscular control of the pelvic floor. Instruction in diaphragmatic breathing may be a way to improve recruitment of the diaphragm.¹⁶

Regarding the cervical musculature, the muscles responsible for flexion of the cervical spine when firing bilaterally are the longus colli, scalenes, and the sternocleidomastoid. The muscles of extension, again firing bilaterally, are the splenius capitis, semispinalis capitis and cervicis. Muscles producing lateral flexion are the iliocostalis cervicis, longissimus capitis and cervices, splenius capitis and cervicis, trapezius, sternocleidomastoid, and the scalenes. Rotation is produced by the unilateral action of the rotators, semispinalis capitis and cervicis, the multifidi, and the splenius cervicis.^9,²⁴ In general, the muscles posterior to the spinal column are stronger than those located anteriorly (Table 125.1).²⁵

Table 125.1 A summary of the cervical paraspinal muscles based on location

Anterior cervical muscles	Posterior cervical muscles	Lateral cervical muscles
Longus colli	Superficial	Trapezius
Longus capitis	Splenius capitis and cervicis	Sternocleidomastoid
Rectus capitis anterior and lateralis	Longissimus capitis and cervicis	Scalenes – anterior, middle, and posterior
	Iliocostalis cervicis
	Deep
	Semispinalis cervicis and capitis
	Rotatores