Biomechanical Role of the Osseous, Ligamentous, and Muscular Structures of the Spine

3 Biomechanical Role of the Osseous, Ligamentous, and Muscular Structures of the Spine


E. Emily Bennett, Jeffrey P. Mullin, Rick Placide, and Edward C. Benzel


Abstract


The study of biomechanics of the spine is especially complex. The cervical, thoracic, lumbar, and sacral portions of the spine greatly affect the function of the muscular, ligamentous, and bony structures that support the spine. All elements play a role in stability and mobility of the body.


Keywords: stability, mobility, biomechanics, sagittal balance, coronal balance, kinematics, rotational motions, flexion, extension, rotation, lateral flexion, transitional motions, distraction, compression, ventral subluxation, dorsal subluxation, medial subluxation, lateral subluxation, osseous, vertebra, thoracic cage, vertebral arch, ligamentous, anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, intertransverse ligament, concentric muscle contractions, eccentric muscle contractions, isometric muscle contractions


3.1 Introduction


The study of biomechanics involving any portion of the human body is complex. This is particularly true with regard to the spine. The nonuniform anatomy throughout the cervical, thoracic, lumbar, and sacral spine is partly responsible for the changes in the biomechanical function of the muscular, ligamentous, and bony structures supporting the spine and their overall role in providing stability and mobility.


An appropriate balance between stability and mobility is required for functional efficiency of the spine. Unlike most joints in the body that provide predominantly mobility or stability with a relative sacrifice of the other, the spine and its components make important contributions to stability as well as mobility. The coordinated efforts of these spinal ligamentous, muscular, and bony structures allow the body to move in and out of sagittal and coronal balance as required for normal functional activities.


3.2 Essential Biomechanical Principles


To understand spine biomechanics, several physical and kinematic properties must also be addressed. Spinal kinematics is the study of spinal motion in normal physiological movements. A coordinate system involving three orthogonal planes and three axes has been designed to describe human movement. The axes are termed x, y, and z and are based on the Cartesian coordinate system1 (image Fig. 3.1). The three planes are the sagittal, coronal (frontal), and axial (transverse). The sagittal plane divides the body into left and right portions, the coronal plane divides the body into ventral and dorsal portions, and the axial plane divides the body into rostral and caudal portions (image Fig. 3.2).


Two specific types of motion exist between the two surfaces of a joint: rotation and translation. Rotation occurs about an axis and translation occurs along a plane. These motions can occur in positive and negative directions. This yields a total of six rotational and six translational motions (image Fig. 3.3). The six rotational motions are flexion, extension, left and right rotation, and left and right lateral flexion. The six translational motions are distraction, compression, ventral and dorsal subluxation, and medial and lateral subluxation.


The concept of motion segment, or functional spine unit (FSU), as it is sometimes called, facilitates the study of the mechanical properties of the spine and has properties with respect to spine biomechanics and motion addressed earlier. The FSU consists of two adjacent vertebrae with their intervening disc, muscles, ligaments, and facet joints.2 Spinal motion occurs within these FSUs or across a multilevel spinal unit. Another key point when discussing spine mechanics is the concept of the instantaneous axis of rotation (IAR). By definition, the IAR is an axis that is perpendicular to a plane of movement that passes through the center of rotation at a particular instant.1 The spine rotates in this axis after a moment arm is applied.


Knowledge of normal spine anatomy as it relates to coronal and sagittal balance is essential in understanding spine mechanics in healthy and pathologic states. In the coronal plane, the same vertical line should bisect all the vertebrae.3 Some have described a slight right thoracic curve as normal because of either the thoracic aorta or a predominance of right-handedness. Specifically, the central sacral line assesses coronal balance, which is a perpendicular line to the iliac crests drawn up from the center of the sacrum. If the line passes through C7, coronal balance is present. When the line is lateral to C7, coronal imbalance may occur. Although earlier evidence suggested coronal balance was important in patient function, more recent studies have shown restoring sagittal balance is more significant in improving patient outcomes.4





In the sagittal plane, several normal curves are encountered. Primary curves are present from the time of development as a fetus and are termed kyphosis (dorsal convexity). These are throughout the whole spine initially but are retained past infancy only in the thoracic and sacral spine. In the thoracic region, this is at least in part due to the intravertebral height differences with the ventral height being less than dorsal vertebral body height, thus contributing to kyphosis. Normal thoracic kyphosis is approximately 20 to 40 degrees. In the sacrum, sacral inclination is approximately 40 to 45 degrees and varies with anterior and posterior pelvic tilting and patient habitus.


Moreover, when infants begin to hold their heads up, cervical lordosis (dorsal concavity) develops. Further, as the child begins to stand and ambulate, the lumbar lordotic curve forms. The original kyphosis in the cervical and lumbar spine thus reverses during infancy and are termed secondary curves. In the cervical spine, vertebral body height is greater ventrally and the intervertebral disc is wedge shaped, resulting in an overall cervical lordosis of approximately 15 to 20 degrees. Similar vertebral body and intervertebral disc characteristics are found in the lumbar spine, resulting in an overall lumbar lordosis that ranges from 40 to 60 degrees and is approximately 30 degrees greater than the thoracic kyphosis. The amount of lumbar lordosis is influenced not only by the anatomy of the vertebral bodies and intervertebral discs but also by the angle of sacral inclination (image Fig. 3.4).


Muscular forces and hip flexibility largely control pelvic tilting. In the sagittal plane, muscular force couples exist to help control pelvic tilt and, therefore, sacral inclination and lumbar lordosis. This muscular force includes the rectus abdominis and iliopsoas ventrally and the erector spinae and gluteal/hamstring group dorsally (see image Fig. 3.4). For example, simultaneous contraction of the abdominals and the gluteal/hamstring group results in posterior pelvic tilting with axis of rotation being the hip joint. Therefore, these muscles are important in sagittal spine balance and overall posture, and are important clinically in conditions such as flat-back syndrome. Similar muscular force couples exist in the coronal plane with the quadratus lumborum and contralateral gluteus medius. Maintaining and restoring sagittal balance has been associated with improved patient outcomes.



As stated, lumbar lordosis develops as bipedalism occurs in infancy and is critical to maintain upright posture.5 The sagittal curves in the cervical and lumbar spine help to counterbalance the thoracic curve in the sagittal plane so the head can be positioned centrally over the pelvis. In the sagittal plane, the center of gravity in humans is just ventral to S2. This, of course, depends on how the individual is proportioned. The lumbar spine bears the greatest mechanical loads of the spine and is a common site for degenerative changes resulting in back pain. With aging, decreased lumbar lordosis and increased thoracic kyphosis may also occur resulting in loss of sagittal alignment and lumbar flattening. With sagittal malalignment, knee flexion, pelvic retroversion, and thoracic hypokyphosis may occur to compensate.5



To assess sagittal balance, the sagittal vertical axis is used and a sagittal plumb line is drawn from the center of the C7 vertebrae toward the S1 vertebrae on a standing radiograph. The line should cross through C7–T1 and T12–L1, and through the dorsal aspect of the L5–S1 disc1 (image Fig. 3.5). If the plumb line falls behind the S1 vertebral body, the patient is in negative sagittal imbalance; and, if the line is in front of the S1 vertebral body, the patient is in positive sagittal imbalance. Further, when changes occur in one region of the spine, another region of the spine must undergo a compensatory change in curvature to maintain sagittal balance. As stated earlier, other measures, such as knee flexion, may also occur. The sagittal curves assist with shock absorption, the distribution of applied loads, mobility and stability, and the maintenance of normal soft tissue length–tension relationships for strength and endurance, not only of the trunk but of the extremities as well.


Lastly, the pelvis, including the pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS), is also significantly important in addition to spinal curvature in maintaining sagittal balance.5 PI is a sacral orientation fixed clinical measure which is equal to PT and SS. SS is the angle that is created between the sacral plate and a horizontal line,5 whereas PI is the angle between a line connecting the midpoint of the sacral point to the femoral head axis and a vertical line. Lumbar spine deformity is related to the difference between the PI and lumbar lordosis measured by the Cobb angle.5 Corrective surgical intervention aims at resolving differences in PI and lumbar lordosis and decreasing sagittal malalignment.5


3.3 Anatomy and Mechanical Properties: Osseous, Ligamentous, and Muscular Structures


Tissues containing relatively high amounts of collagen such as bone and ligaments have certain unique mechanical properties. They demonstrate anisotropic and viscoelastic properties. Anisotropy means that a tissue has mechanical characteristics that differ with different loading directions. This is largely due to nonuniform microanatomy. Viscoelasticity is a property that allows a tissue to behave differently under varying loading rates. For example, increasing the rate of loading of a particular tissue increases its mechanical strength.6


3.3.1 Osseous Structures


The bony elements of the spine include the vertebra and thoracic cage (see discussion later in this chapter in the Spinal Stability Schemes section). The bony structures provide the scaffolding on which ligaments and muscles act to provide stability. When injury or disease affects the bony structures of the spine, the soft tissues can maintain stability provided the bony damage is not extensive. Clinically, isolated bony pathology occurs in the setting of metastatic disease to the spine or conditions of bone such as osteoporosis. In the setting of trauma, it is difficult to injure the bony elements in isolation without injury (to some extent) of the supporting ligaments and muscles. This has led to the development of several models to describe spine stability as well as potential mechanisms of injury and treatment recommendations, which will be discussed in detail in the section on spine stability.


Vertebral anatomy is largely the same throughout the spine, especially from C3 to L5. These vertebrae have a ventrally positioned body that is roughly cylindrical (vertebral body). There is a dorsal bony arch (vertebral arch or neural arch), consisting of the pedicles, transverse processes, superior and inferior articular processes (facets), lamina, and spinous process, which serves to protect the spinal cord/cauda equina and act as attachment sites for ligaments and muscles. The rostral and caudal termini of the spine have atypical vertebrae in the form of the atlas and axis rostrally and the sacrum and coccyx caudally.


The vertebral body is essentially a block of cancellous (trabecular) bone covered by a layer of cortical bone. The cortical layer is reinforced by vertical and horizontal trabecular “struts” that help prevent vertebral body collapse. The cortical layer extends to cover the rostral and caudal surfaces of the vertebral bodies. The periphery of this covering is thickened in the region of the epiphyseal plates, and the center is composed of a hyaline cartilage covering, termed the cartilaginous end plate. The internal architecture of the cancellous bone of the vertebral body has various trabeculae that correspond to the compressive, tensile, and torsional stresses placed on the bone1,3,6 (image Fig. 3.6). When examining this trabecular pattern, it is apparent that a weakness exists in the ventral aspect of the body. This manifests itself clinically as a compression fracture. During axial compressive loading, the cancellous bone contributes 25 to 55% of the strength of the vertebral body in those under the age of 40 years. As one ages, declining bone density causes a decrease in weight-bearing through the cancellous bone as the cortical bone carries a greater proportion of the load. A loss of approximately 25% of vertebral body bony tissue results in a loss of over 50% of the vertebral body strength.7 A recent study suggests that biomechanical age changes decrease the ventral loading causing increased bone loss in the already relatively weak ventral vertebral body.8



The vertebral arch is a bit more complex anatomically. In the area where the transverse process, articular process, and pedicle meet, the bone demonstrates a reinforcing, crossing trabecular pattern that develops secondary to the stresses applied to the area. In addition to providing ligamentous attachment sites, the transverse and spinous processes are attachment sites for the erector spinae musculature. These are relatively long levers, and the muscular forces acting on these levers are transmitted to the lamina. Because these muscles are an important component of spinal stability, any compromise in the lamina (injury or surgery) can, in turn, compromise stability. This is particularly obvious regarding postlaminectomy kyphotic deformity in the cervical spine.9 The thoracic spine, however, has been shown in cadaver studies to maintain stability following laminectomy.10


As part of the dorsal vertebral arch, the articular processes, and hence the facet (zygapophyseal) joints, play a significant role in the mechanics of the spine. The facet joints serve two particularly important roles: first, guiding and limiting the direction and range of motion (ROM) and, second, sharing in the role of weight-bearing.11 Although the majority of spinal axial loads are borne through the vertebral bodies and intervertebral discs, the facet joints share in the distribution of axial loads. The literature provides a wide range of overall load bearing by the facet joints, depending on the region of the spine studied and whether the spine was in a flexed, extended, or neutral posture.2 Furthermore, the amount of loading varies based on prior surgeries; for example, facet loading has been shown to double following nucleotomy.12 The facet joints tend to be loaded in extension and unloaded in flexion. The orientation of the facet joints is different in the cervical, thoracic, and lumbar regions, and can vary from side to side at the same level (image Fig. 3.7). This orientation is largely responsible for determining what planes of motion are available at a particular motion segment. Accordingly, the facets guide the motion of one vertebra on the next, but they also are involved in limiting ROM (see Zygapophyseal [Facet] Joints section (p.37)). Additionally, there are zones of transition of facet orientation from cervical to thoracic, thoracic to lumbar, and lumbar to sacral regions where the facet orientation gradually changes over several segments.


3.3.2 Ligamentous Structures


The six major ligaments of the spine are the following:


1. Anterior longitudinal ligament (ALL).


2. Posterior longitudinal ligament (PLL).


3. Ligamentum flavum.


4. Interspinous ligament.


5. Supraspinous ligament.


6. Intertransverse ligament.


This is, of course, excluding spinal cord and nerve root–related ligaments. The ALL, PLL, and supraspinous ligament are intersegmental, and the others are intrasegmental. These ligaments protect the spinal cord and its nerve roots by restricting motion and absorbing energy of the FSU.


The majority of spinal ligaments are made of collagen with the exception of the ligamentum flavum, which is composed primarily of elastin. Ligament strength varies between spinal regions and between types of ligaments.1,2 To understand the role a particular ligament has in providing spinal stability, the material properties or strength of the ligament and the length of the moment arm through which it functions must be considered. The moment arm is defined as the perpendicular distance from the IAR to the applied force1 (image Fig. 3.8). Ligaments with longer moment arms may have a mechanical advantage over stronger ligaments with shorter moment arms. Additionally, structures ventral to the IAR are taut in extension and shortened with flexion, and structures dorsal to the IAR are shortened in extension and taut with flexion.


Anterior Longitudinal Ligament

The ALL runs along the ventral and ventrolateral surface of the vertebral column from C2 (the axis) to the sacrum. It continues rostrally as the atlanto-occipital ligament. The ALL has attachments to the vertebral bodies and discs. There are superficial fibers that span several vertebral segments and deep fibers that run between adjacent vertebrae. The deep fibers blend with the ventral annulus of the disc and serve to reinforce the intervertebral discs. The ALL demonstrates its greatest strength in the upper cervical, lower thoracic, and lumbar regions, with its greatest tensile strength in the lumbar region. In general, the ALL is approximately twice as strong as the PLL.3 The ALL is usually ventral to the IAR and therefore provides resistance to extension.


Oct 17, 2019 | Posted by in NEUROSURGERY | Comments Off on Biomechanical Role of the Osseous, Ligamentous, and Muscular Structures of the Spine

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