Cartesian Coordinate System

The right-handed Cartesian coordinate system is routinely used in spine biomechanics as a system of reference. This system consists of three axes: the x-axis, the y-axis, and the z-axis. Translational and rotational movement can occur along or about these three axes. Translational movements are positive if they occur in the positive direction of the axis and negative if they occur in the negative direction. Rotational movements are considered positive in the clockwise direction when viewed from the origin and negative when counterclockwise from the same viewpoint. The right-handed Cartesian coordinate system as applied to the spine places the x-axis in the dorsal-to-ventral plane (axial plane), the y-axis in the right-to-left plane (sagittal plane), and the z-axis in the rostral-to-caudal plane (coronal plane). This placement results in positive moments with flexion, left-to-right lateral bending, and right axial rotation. Using these coordinates, any point can be defined by both its location and its rotation.

Vectors, Scalars, Bending Moments, and the Instantaneous Axis of Rotation

A vector is a force oriented in a fixed direction in three-dimensional space (as defined by the right-handed Cartesian coordinate system previously discussed). It possesses both magnitude and direction (i.e., force and velocity), and it may be broken down into its component vectors. A scalar is a quantity that is direction independent and possesses only magnitude (i.e., time, volume). A bending moment occurs when a force acts on a lever or moment arm. A bending moment, when applied to a point in space, results in a tendency to rotate about an axis. When these principles are applied to a vertebral body, the three-dimensional axis about which a bending moment causes the body to rotate; this is known as the instantaneous axis of rotation (IAR). The IAR is the axis about which a vertebral segment rotates at any given instant, and the IAR itself does not move during this instant of rotation or translation. It may be considered a fulcrum about which a segment moves. When a body moves in a plane, there is a point in the body or some extension of it that does not move, and the axis that passes perpendicular to the plane of motion and through this point is the IAR (as defined by White and Panjabi¹). The IAR becomes the center of the Cartesian coordinate system used to define the motion. The IAR is dynamic, and as spinal movement occurs, the IAR of each involved segment moves. The location of the IAR depends on the manner in which it is determined and the theoretical foundation on which it is based, and it is therefore subject to error.

Avulsion Fracture

These occur as a result of the strong ligamentous attachments to the spine. The ligaments, which are strongest in tension due to their high collagen content, can avulse bony fragments (Table 136-1). An example of this is the bony avulsion seen with Jefferson burst fractures, where the transverse ligament avulses a portion of the lateral mass of C1.

TABLE 136-1 Spinal Ligament Forces

ALL, anterior longitudinal ligament; ISL, interspinous ligament; PLL, posterior longitudinal ligament; SSL,supraspinous ligament.

From White AA, Panjabi MM: Clinical biomechanics of the spine, ed 2, Philadelphia, 1990, Lippincott, p 22.

Anterior Wedge Compression Fractures

These occur with direct axial load to the spine, most commonly in osteoporotic cancellous bone. For these fractures, the force vector is anterior to the IAR and causes compression failure of the anterior aspect of the vertebral body. The result is a wedge-type fracture, often seen with low-energy injuries in osteoporotic bone.

Flexion-Distraction Fracture (Chance Fracture)

These fractures occur with a significant flexion moment, with the axis of rotation anterior to the spinal unit. This large moment (mass times acceleration times moment arm) results in a tremendous extension force to the spine. The mode of failure is tensile through the posterior elements and then progresses through the vertebral body. The classic example of this type of injury was described by Chance in 1948 and bears his name.⁴ The bony or ligamentous component of the fracture is determined by the difference between the tensile strength of the bone and ligaments, and the force travels through the path of least resistance.

Burst Fracture

The burst fracture typically has both axial load components as well as a flexion moment. Comminution is a function of the magnitude of force that was transmitted to the bone. It can be graded on the amount of the vertebral body that is involved and the amount of displacement. These grades have been used to understand the ability of that bone to support the anterior spine. Gaines described this load-sharing classification and the necessity for long-segment fixation or supplemental anterior support based on the concept of vertebral body comminution and fracture displacement.⁵ In healthy bone, this energy results in a burst pattern rather than a compression pattern. With failure of the anterior support, the spine falls into flexion with continued energy. The resultant disruption of the dorsal tension band or posterior bony fractures is usually the primary predictor of stability. Most classification systems recognize this, and posterior stability is a primary predictor of necessity of operative stabilization.⁶

Torsion

Complex fracture patterns often have a torsion component and can be responsible for fracture-dislocations with complete disruption of the anterior, middle, and posterior columns. Torsion injures are also common in the more flexible cervical spine. Typically, the failure is through the ligamentous structures because the oblique nature of the cervical facets allows for significant rotation. Torsional forces are the primary forces responsible for unilateral facet dislocations.⁷

Vertebral Body

The vertebral body is the bony cylinder that makes up the ventral aspect of each vertebra, except C1, which has no vertebral body. Each vertebral body consists of an outer rim of cortical bone surrounding a core of softer cancellous bone. The vertebral bodies resist much of the compressive loads placed on the spine in physiologic situations.

In general, the height, width, and depth of the vertebral bodies increase as the spine is descended, which seems to account for this increased strength at lower levels in the spine.¹ As previously alluded to, there is a regional variation to the shape of the vertebral body. In the thoracic spine, the anterior border is shorter than the posterior border, creating a structural kyphosis in the thoracic spine. The outer shell of cortical bone is more rigid than the softer cancellous core because it is arranged in vertical lamellae to resist compressive forces. The cancellous bone is made up of trabeculae that are arranged to resist a variety of loads. There is greater compressive deformation of the cancellous bone prior to failure than of the cortical bone because of the increased rigidity of the latter.^11,12 Numerous studies regarding the load-sharing properties of the cortical and cancellous bone have been performed showing that the load carried by the trabecular bone varies anywhere from 35% to 90%, depending on age and mineral content of the bone,^13,14 and that the strength of the vertebral body decreases with age. Bell et al.¹⁵ have shown that there is a direct relationship between the mineral or ash content of the bone and the strength of the vertebral body. A 25% decrease in mineral content resulted in a greater than 50% decrease in vertebral strength due to loss of the trabecular columns that formed the core of the vertebral bodies. In osteoporosis, the mineral content of the bone is affected and results in loss of the horizontal trabeculae within the vertebral body core, thus lengthening the trabecular columns and compromising vertebral body strength.¹ In the thoracic spine, the vertebral bodies articulate with the ribs. This articulation with the rib cage significantly augments the strength of the thoracic spine. This articulation with the rib cage and the sternum significantly augments the stiffness of the thoracic spine by as much as 110% in extension and 30% to 45% in the other planes of motion.

The vertebral end plates are formed from the concave, 1- to 2-mm thick cortical bone at the rostral and caudal surface of the vertebral body. These are fused to the cartilaginous end plates of the intervertebral disc by a calcified layer of tissue known as the lamina cribrosa, which permits osmotic diffusion of nutrients for the intervertebral disc. In 1957, Perry¹⁶ extensively studied failure of the end plates under compression and noted that one of three mechanisms of failure occurs, depending on the condition of the surrounding discs. Fractures occurred centrally or peripherally or encompassed the entire end plate. In specimens with nondegenerated discs, the fractures typically occurred centrally, and in specimens with degenerated discs, the fractures occurred peripherally. At high loads, the fractures encompassed the entire end plate regardless of degeneration. The strength of the end plate itself has been evaluated as well with regard to resistance to penetration by graft material (subsidence) or disc material (Schmorl node). Kumar et al.¹⁷ determined the greatest resistance to penetration to be within the first 4 mm of depth and the end plate to be strongest at the periphery closer to the cortical margin. In fact, the weakest portion of the vertebral end plate is at the center, where it is the thinnest. With a nondegenerated nucleus, axial compression creates a central increase in pressure and deflection of the end plate. This leads to increasing bending stresses in the center of the vertebral end plate and ultimately results in its failure.¹ In contrast, in the degenerated disc, the nucleus lacks water content, and there is no build-up of central fluid pressure. The load is transmitted primarily through the anulus at the periphery of the disc and end plate, resulting in fracture in the same location or in the underlying vertebral body.¹

Intervertebral Disc

The vertebral bodies are separated at each level by the intervertebral disc. This viscoelastic structure is made up of a central nucleus pulposus, peripheral anulus fibrosus, and cartilaginous end plate of the disc, which separates it from the vertebral bodies above and below. Viscoelasticity is defined as a property of a material that exhibits both viscous and elastic characteristics when undergoing deformation. The material then has different mechanical properties that vary depending on the different loading rates. That is, if a load is applied slowly, the disc can respond with greater deformation, whereas if a load is applied rapidly, less deformation occurs. This property also changes as people age; with increasing age, there is a corresponding decrease in the ability of the disc to deform. There are 23 intervertebral discs starting between C2 and C3 and ending at L5 and S1. They make up 20% to 33% of the total height of the vertebral column, and there are regional differences that seem to parallel those of the vertebrae. The cross-sectional area of the discs increases in the rostrocaudal direction.

The central nucleus pulposus is composed of mucopolysaccharides and mucoprotein, forming a gel with water content ranging from 70% to 90%.¹ The water content is highest at birth and decreases with age.¹⁸ The nucleus pulposus makes up approximately 30% to 50% of the cross-sectional area of the disc, and it seems to lie more posterior in the lumbar spine at the junction of the middle and posterior thirds of the disc in the sagittal plane.¹ The viscoelastic properties of the nucleus pulposus allow it to act as an effective shock absorber for the spine. Surrounding the nucleus pulposus is the anulus fibrosus, which is made up of collagenous fibers in concentric laminated bands, with each band oriented 90 degrees to the adjacent bands and 30 degrees to the disc plane (Fig. 136-5). The inner fibers of the anulus are attached to the cartilaginous end plates, and the outermost fibers, known as Sharpey fibers, are attached to the cortical bone of the vertebral body. Because of the orientation of the fibers, the intervertebral disc effectively resists rotational, tensile, and shear stresses. The anulus does not resist compressive forces.

FIGURE 136-5 The intervertebral disc. The nucleus pulposus (dashed circle) is contained (in nonpathologic situations) by the anulus fibers.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

With age, the water content of the nucleus pulposus decreases and the nucleus itself becomes less deformable. This desiccation results in an overall stiffer intervertebral disc that behaves differently under compressive loads. In a disc in a younger person, the nucleus is compressed, resulting in viscoelastic deformity at the central part of the end plate, but when the nucleus becomes less elastic and stiffer, the load is transmitted through the anular fibers around the periphery with very little deformation, leading to a higher load at the end plate. This can result in compression fractures, as are commonly observed in the elderly or osteoporotic population. The increased stiffness of the disc and bone results in a smaller elastic zone and lower threshold for failure.

Facet Joints

The vertebrae articulate with each other at the superior and inferior facet joints, which are diarthrodial joints with a loose synovial capsule located at the superior and inferior aspect of the pars interarticularis. In the cervical spine, the facet joints are oriented with an inclination of approximately 45 degrees from the horizontal, and this orientation changes in the thoracic and lumbar spine as the facets become larger and more vertical.¹ The shape, position, and orientation of these articulating surfaces largely determine the pattern of movements throughout the spine (Fig. 136-6). The cervical spine from C2 to C7 allows 8 to 12 degrees of rotation, whereas the entire lumbar spine only allows 2 to 5 degrees.

FIGURE 136-6 Facet joint orientation. The relative coronal plane orientation in the cervical region (A), the intermediate orientation in the thoracic region (B), and the relative sagittal orientation in the lumbar region (C). The facet joint orientation changes substantially in the lumbar region; here the facet joint angle (with respect to midline) is depicted versus the spinal level (D).

(A–C, Copyright Cleveland Clinic Foundation.)

The coronally oriented cervical facet joints allow for supple flexion, extension, rotation, and lateral bending. In the thoracic spine, the inclination of the articulating surfaces is approximately 60 degrees, with an additional rotation about the y-axis of approximately 20 degrees toward the midline. This allows for limited flexion and extension but limits rotation. The facet surfaces in the lumbar spine are at an inclination of approximately 90 degrees, with approximately 45 degrees of rotation about the y-axis directed outward. This orientation allows for restricted rotation with greater flexion and extension. The orientations of the facet joints have a great deal of variation even within the specific regions of the spine.¹ The facets act as important stabilizing structures in the spine, with resultant instability following unilateral or bilateral excision.¹⁹

In the lumbar spine, the sagittal orientation of the facet joints allows for substantial resistance to rotation, which is well documented.^19–21 As the facet orientation begins to change from about T7 to L4, the torsional stiffness of the spine increases due to these changes in facet articulation, with the peak at T12-L1.²² In the cervical spine, transection of the disc and longitudinal ligaments resulted in a 33% increase in horizontal translation with flexion. This increased by 140% following transection of the facets.²³ Removal of thoracic facets resulted in increased flexion and extension in the upper thoracic region and increased axial rotation in the lower thoracic–thoracolumbar region. The capsular ligaments also have a role in maintaining torsional strength.

The facets provide a load-bearing role when the spine is in extension. In dynamic studies with cadaver spines in various postures, the facets have been shown to carry anywhere from 0% to 33% of the load, depending on the force vector and position of the spine.²⁴

Dorsal Elements

The remainder of the dorsal elements of the spine includes the pedicle, the lamina, the transverse processes, and the spinous process. Each of these elements varies significantly in morphology between regions of the spine. The pedicle serves as the bridge between the posterior elements and the vertebral body. The importance of pedicle anatomy is due to the major role pedicles now play in stabilization constructs. In the cervical spine, the pedicles are shorter with greater diameter. From the cervical to midthoracic region, the transverse pedicle width gradually decreases, and it then increases from the midthoracic to the lumbar region (Fig. 136-7).^25–27 Sagittal pedicle width increases from the cervical to the thoracolumbar region and then decreases in the lumbar region (Fig. 136-8).^25–27

FIGURE 136-7 Transverse pedicle width versus spinal level.

FIGURE 136-8 Sagittal pedicle width versus spinal level.

From the cervical to the thoracolumbar region, the transverse pedicle angle decreases, and it increases into the lumbar region (Fig. 136-9).^25–27 The sagittal pedicle angle is a final consideration with regard to pedicle anatomy; it becomes fairly steep in the thoracic and thoracolumbar regions (Fig. 136-10).^25–27

FIGURE 136-9 Transverse pedicle angle versus spinal level.

FIGURE 136-10 Sagittal pedicle angle versus spinal level.

The laminae form the dorsal aspect of the neural canal, which contains the spinal cord. They extend from the pedicles to the spinous process dorsally and form the foundation for the spinous processes, provide dorsal protection to the dural sac, and serve as the attachment site for one of the seven spinal ligaments, the ligamentum flavum.²

The spinous processes project dorsally and, for the most part, caudally. They form attachment sites for the interspinous ligaments and supraspinous ligaments. The spinous processes project more caudally as one descends to the midthoracic spine. In the thoracolumbar and lumbar regions, the spinous processes also project more dorsally.²

The transverse processes are located at the junction of the lamina and pedicle and in general extend laterally with some variation in the rostral-caudal dimension, depending on the region of the spine. They serve as sites of attachment for the paraspinal muscles. In the cervical spine, the transverse processes are smaller and transmit the vertebral artery through the foramen transversarium from C6 to C1. The transverse processes of the middle to lower thoracic spine become more substantial, projecting in a lateral and upward direction. The lower thoracic processes become smaller and more atretic, but those of the lumbar spine become more substantial, extending laterally and ventrally.

The transverse processes of the midthoracic and lumbar spine are sizable enough to serve as sites for hook placement and fusion, but they are limited by a poor blood supply and are easily fractured.²

Ligaments

The ligamentous complex from the occiput to C2 represents a continuation of ligaments of the subaxial spine (Figs. 136-11 and 136-12). The anterior longitudinal ligament begins at the occiput as the atlanto-occipital membrane, as it passes between the occiput and the anterior arch of C1. The posterior atlanto-occipital membrane passes from the posterior arch of C1 to the occiput. When intact, these ligaments offer some advantage in preventing anterior displacement of C1 on C2. The apical ligament stretches from the tip of the odontoid process of C2 to the basion of the skull. The alar ligaments run obliquely from the rostrolateral aspect of the odontoid to the occipital condyles. The cruciate ligament has a vertical and transverse portion. The transverse portion of the ligament attaches to the tubercles located at the lateral masses of C1 and runs dorsal to the dens. The vertical portion of the cruciate ligament attaches to the occiput immediately dorsal to the apical ligament and descends to intertwine with the transverse portion. The vertical portion then descends to attach to the dorsocaudal aspect of C2. The transverse ligament is the most important ligament of the occipitocervical complex because it prevents horizontal translation of C1 on C2. The tectorial membrane is a continuation of the posterior longitudinal ligament and runs from the body of C2 over the posterior portion of the dens to attach to the anterior foramen magnum (see Fig. 136-11).

FIGURE 136-11 Axial (A) and midsagittal (B) schematic illustrations of the major ligaments involved in the clinical stability of the upper cervical spine. The anterior atlantodental ligament has been described recently.

(Copyright Cleveland Clinic Foundation.)

FIGURE 136-12 Ligaments of the spine. Besides the disc, seven ligaments connect one vertebra to the next. Contribution to the spine stability by an individual ligament is dependent on its cross section, its distance from the instantaneous axis of rotation, and its orientation in space. The anatomy of the ligaments is such as to collectively provide stability to the spine in its various physiologic motions.

(Copyright Cleveland Clinic Foundation.)

Beyond the occiput-C1-C2 complex, there are seven major spinal ligaments that work to stabilize the subaxial spine in its physiologic range of motion. These ligaments also work to restrict the motion of the spine to well-defined limits while at the same time allowing adequate motion and fixed postures to protect the spinal cord. Starting anteriorly and moving posteriorly, the ligaments are the anterior longitudinal ligament, posterior longitudinal ligament, capsular ligaments, intertransverse process ligaments, ligamentum flavum, interspinous ligament, and supraspinous ligament (see Fig. 136-12). The strength characteristics of the ligaments vary, and the effectiveness of a ligament not only depends on the morphology but also on the length of the moment arm through which it works²⁸ (Fig. 136-13). A longer moment arm gives a relatively weak ligament a mechanical advantage so that it may contribute more to overall spinal stability.

FIGURE 136-13 The ligaments and their effective moment arms. Note that this length depends on the location of the instantaneous axis of rotation (dot). An “average” location is used in this illustration. ALL, anterior longitudinal ligament; CL, capsular ligament; ISL, interspinous ligament; LF, ligamentum flavum; PLL, posterior longitudinal ligament.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

The anterior longitudinal ligament spans the entire spinal column from the clivus, where it begins as the anterior atlanto-occipital membrane, to the sacrum. It covers the ventral one fourth to one third of the vertebral body circumference with interdigitizing layers of elastin and collagen fibers. The innermost layers bind to the intervertebral discs of adjacent vertebrae. The middle layer binds the vertebral bodies and discs over three levels, and the superficial layer extends over four to five levels. Because it is ventral to the IAR, it provides resistance to extension.

The posterior longitudinal ligament starts as the tectorial membrane and also spans the entire length of the spine. It also consists of several layers, with the deeper fibers spanning adjacent vertebrae and the more superficial fibers extending over several levels. The posterior longitudinal ligament only marginally attaches to the vertebral body compared with the anterior longitudinal ligament, which blends into the periosteum at times. The posterior longitudinal ligament is closely adherent to the anulus fibrosus, whereas the anterior longitudinal ligament is not. The posterior longitudinal ligament widens in the region of the disc space and narrows as it passes over the vertebral body. It is dorsal to the IAR and has a short moment arm, and, therefore, it is only able to weakly resist flexion forces.

The capsular ligaments are located at the facet joints. These fibers attach to the superior and inferior facets of adjacent vertebrae, and they act as significant stabilizers of the spine, particularly in the cervical spine. The fibers of the facet capsules run in a direction that is perpendicular to the plane of the facet joint. The moment arm (see Fig. 136-13) is not substantial, but the ligaments are strong enough for the stresses placed on them, which consist mainly of axial rotation, ventral shear loads, and axial loads under certain conditions. Other capsular ligaments in the thoracic spine attach the ribs to the vertebral bodies. These costochondral ligaments provide additional stability to the thoracic spine.

The ligamentum flavum has the highest percentage of elastic fibers of any tissue in the body. The ligamentum flavum is actually a pair of broad ligaments that connect the spinal laminae. They arise from the ventral surface of the caudal lamina and attach to the dorsal border of the adjacent rostral lamina, and they extend laterally to become confluent with the joint capsules. These ligaments begin at C1-2 and continue caudally to L5-S1. White and Panjabi noted that the ligament is in a state of resting “pretension” or tension, which is present with the spine in neutral position.¹ This is postulated to stabilize the spine by keeping the intervertebral discs under compression and keeping the ligament from impinging on the spinal cord when the spine is in extension.

The interspinous and supraspinous ligaments connect the adjacent spinous processes. The interspinous ligament extends from the base to the tip of each spinous process, and this ligament is present from C2 to S1. The supraspinous ligament begins at the dorsal aspect of C7, attaching to the tip of the spinous process, and extends to the lumbosacral region. It is thought to be a continuation of the ligamentum nuchae associated with the dorsal cervical spine. These ligaments use the long moment arm of the spinous process to provide an effective resistance to flexion (see Fig. 136-13). These ligaments are extremely important in determining the stability of the spine, both for their importance in providing stability and for their inability to heal effectively.

Musculature

One final consideration regarding the basic biomechanical anatomy of the spine is the surrounding musculature, which lends significant support and stability to the spine.

In the cervical region, there is a fibrous intermuscular septum previously mentioned known as the ligamentum nuchae, which serves as an attachment to many of the occipitocervical muscles. The splenius capitis muscle arises from the lower ligamentum, cervical, and upper six thoracic transverse processes to attach to the occiput. The splenius cervicis muscle arises from the upper six thoracic transverse processes to insert on the posterior tubercles of C1-3. The semispinalis cervicis muscle occupies the next deeper layer and arises from the facets and transverse processes of the upper thoracic spine to insert on the spinous processes of the cervical spine. The semispinalis capitis muscle, occupying the same layer, is more lateral, arising from the transverse processes of C3-6 to insert on the occiput. Deeper muscles include the rectus capitis and the capitis obliquus, which serve as head extensors. Ventrally, the sternocleidomastoids arise bilaterally from the clavicles and insert on the mastoid processes of the occipital bone. The longus colli occupy a deeper layer in the anterior cervical region, arising from the atlas to insert on the transverse processes of C3 to C6.

In the thoracic region, the superficial musculature attaches to the thoracic spinous processes, and in the rostral thoracic region, it is made up principally of the trapezius muscle, which inserts laterally on the scapula and medially on the ligamentum nuchae. Lower in the lower thoracic and thoracolumbar region, the superficial musculature is made up principally of the latissimus dorsi muscle, which arises from the transverse processes of the lower thoracic spine and spreads ventrally to the axilla. The rib cage, which articulates with the thoracic spine and lends stability to this region, also possesses its own intrinsic musculature. The intercostal muscles and serratus posterior muscles run between ribs but in opposite directions. In the lower thoracic and thoracolumbar region, the lateral muscle groups include the psoas, intertransverse, and quadratus lumborum muscles. The psoas muscles arise from the lateral aspect of the vertebral bodies and insert on the femurs. The intertransverse muscles arise and insert between transverse processes throughout the spine, and the quadratus lumborum muscle arises from the transverse processes and inserts at the lateral ileum.

Ventrally, the lumbar spine is supported by those muscles surrounding the abdominal region, which include the internal and external oblique, transversus abdominis, and rectus abdominis muscles. Dorsally, lumbar support is principally from the very prominent paravertebral muscles. These make up the erector spinae muscle group, which begins as a broad tendon attached to the sacrum and iliac crest and extends the entire length of the spine. It is composed of three columns, each of which is composed of shorter fascicles. The lateral column is the iliocostalis muscle, the middle column is the longissimus muscle, and the medial column is the semispinalis muscle. The iliocostalis arises from the iliac crest to insert on the angles of each rib and the cervical transverse processes. The longissimus muscle is the largest and arises from the transverse processes at the lower spinal levels and inserts onto the transverse processes rostrally and onto the mastoid processes of the occipital bone. The semispinalis arises from the spinous processes of the sacrum and inserts onto the rostral spinous processes. Deeper to the erector spinae muscle group is the transverse muscle group, which principally arises from transverse processes and inserts onto the spinous processes. The multifidus muscle differs in the cervical and lumbar regions, where it arises from the articular joint as opposed to the transverse processes.

Weakness of the muscle groups that support the spine can lead to deformity or pain. The musculature plays a significant role in spine stability. Disuse or denervation atrophy can result in deformity, degeneration, or other painful pathology. Biomechancially, toned musculature can contribute to maintaining the load balance along the spinal column. Furthermore, in the degenerating spine, toned musculature aids in offloading the detrimental forces and disruption in load balance arising from the degeneration.

Implant Properties

Implants can be made of various materials, including metals, nonmetals (i.e., ceramic, glass), and bone. Each material has intrinsic properties that make it more useful for certain operations, and these specific properties must be kept in mind. An instrumented fusion is essentially a race between fusion formation and instrument failure, and, therefore, certain implants are more suited for this. Metallurgists have produced many alloys, which are now being used clinically in spinal implants.² Stainless steel alloys include 316L stainless steel and 22-13-5 stainless steel, which has the same modulus of elasticity and tensile strength as 316L but twice the ultimate tensile strength.⁶ Titanium is available in pure, unalloyed forms, as well as various alloyed forms.

Pure titanium occurs in several grades (1–4) based on purity and degree of contaminants, and as the oxygen content increases, so does the strength of titanium.² The modulus of elasticity remains unaltered, but the ultimate strength and 0.2% tensile strength (stress that causes 0.2% deformation) increase with the grade.²⁹ Titanium has several advantages over other metals. Its modulus is very similar to bone, allowing for easy incorporation and limiting stress shielding.^2,30,31 In addition, it also is resistant to glycocalyx formation, which may prevent latent infections.³² However, it is notch sensitive. This refers to the early fatigue failure of titanium with the addition of a notch or defect in its surface.²⁹ Care must be taken when bending these materials. Titanium may also be alloyed with other elements such as aluminum, vanadium, niobium, and zirconium in specific concentrations, resulting in specific properties such as increased resistance to wear.² The stainless steel alloys 316L and 22-13-5 are stiffer than titanium, resulting in a smaller transfer of stress from the implant to the bone, termed stress shielding. Cobalt chrome is now being used for spinal applications. It has the advantage of the imaging characteristics of titanium with the strength of stainless steel. Elements such as chromium and molybdenum present in these alloys provide resistance to corrosion.²

Deformability and the ability to resist fatigue are important characteristics of spinal implants. Implants do not fail because a static load exceeds the ultimate strength of the implant; they fail because of cyclical loading and fatigue. Fatigue refers to the cumulative damage of cyclical loading that occurs in the form of cracks and corrosion. Titanium performs better than steel at lower frequencies of loading.³³

Other implant materials include synthetic nonmetals such as polymethylmethacrylate (PMMA), acrylics, ceramics, and glasses. PMMA is most commonly used but only in very specific situations. It has a high modulus of elasticity and is, therefore, very brittle. It has been shown that wire reinforcement, especially with the cobalt alloy Vitallium, yields a stronger construct.³⁴

Finally, bone is commonly used as an implant. Bone graft harvested from the patient (i.e., fibula, iliac crest) is termed an autologous graft, whereas bone graft harvested from a cadaver is termed an allograft. It can be harvested as a strut graft with both cortical and cancellous bone, which can be cut and shaped to fit into a disc space or vertebrectomy cavity. Morselized bone chips may also be processed from cadavers and used as a graft extender. Bone is used to create a fusion, which ultimately lends stability to the spine, and instrumentation will ultimately fail unless fusion and stability are achieved.² Bone can be placed ventrally or dorsally in the spine to create a fusion. Ventrally, a bone graft is placed in the weight-bearing part of the spine as an interbody graft, which results in compression and load sharing by the graft. This leads to faster and better healing as dictated by Wolff’s law. Wolff explained that internal and external changes in bone architecture occurred according to mathematical laws along with functional changes of bone.^2,3 It has also been shown that electrophysiologic changes occur when bone is placed under compression (i.e., a negative charge is generated).³⁵ Dorsally, bone is not exposed to the same fusion-enhancing forces as ventral weight-bearing grafts. The healing and fusion rates in dorsal fusions are less than in ventral fusions, probably because there is no compression that seems to encourage ventral, interbody fusion.^2,36 The dorsal fusion mass will diminish in volume with time, and it has been shown that a larger volume of initial graft is required for a larger fusion mass at 18 months.³⁷

The interface between the implant and bone is important in ensuring stability. There are several ways in which implants and bone contact each other. According to Benzel,² these are (1) abutting, (2) penetrating, (3) gripping, (4) conforming, and (5) osseointegration. Abutting implants typically include the interbody bone. The role of the interbody implant is to resist axial loads, and this is done most effectively through a large surface area contact between the ends of the bone graft and the adjacent vertebral bodies. Placement of the graft in the IAR is ideal for resisting axial loads, and placement ventral to the IAR results in a moment arm and some degree of distraction.² Penetrating implants include nails, screws, and spikes. The role of the penetrating implant is that of an anchor, and important considerations with regard to these implants include pullout resistance and overall strength when they function as fixed-arm cantilevers.² Pullout resistance of screws is enhanced by engaging cortical bone, which markedly resists pullout, and through triangulation (Fig. 136-14) or placement of converging or diverging screws optimally at 90 degrees to one another.³⁸ As discussed previously, the bending strength is proportional to the cube of the inner diameter of the screw (section modulus). The outer diameter and thread depth are important in pullout resistance (Fig. 136-15). Hooks and wires are examples of the gripping implants. Wire may be looped around the laminae and spinous processes of adjacent vertebrae and secured via twisting the loops together, and hooks may be placed at the laminae, pedicles, or transverse processes to augment dorsal instrumentation, especially screws placed in osteoporotic bone.² They contact cortical bone over a greater surface area, and thus may perform better in the setting of osteoporotic bone. Hooks may also be used to augment screw pullout resistance within the same construct.² Conforming implants have been mentioned briefly and at this point include PMMA and acrylics. These implants have the unique property of conforming to surrounding bone and are used by some surgeons to augment the screw-bone interface.^39–41 The bonding of bone to nonbiologic material is known as osseointegration, and when this occurs, the load transfer from implant to bone is distributed over a much larger surface area, reducing stress risers (focal concentrations of stress).² Titanium and a titanium alloy, Ti-13Nb-13Zr (niobium, zirconium), have exhibited potential for osseointegration.^2,30,31

FIGURE 136-14 The triangulation effect is proportional to the shaded area subtended by the screw (A). The shaded area can be increased by lengthening the screws (B) or by altering the trajectory (C).

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

FIGURE 136-15 Screw core (minor) and outside (major) diameters, thread depth, and screw pitch.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

Application to Implants

Newton’s third law of motion, which states that for every action there is an opposite but equal reaction, governs the placement of spinal implants. Because there is no net movement in the spine when the implant is placed, the forces applied to the spine must occur in pairs such that the net result is zero. No spinal implant is placed in a truly neutral mode because once upright posture is assumed, the implant is under some degree of stress, which it then must resist. Implants, when placed dorsally or ventrally, resist these loads dynamically, resulting in load sharing or rigidly resulting in stress shielding. The type of implant is determined by the surgeon when designing the construct, and the surgeon must take into account the region of the spine, the site of fusion (ventral or dorsal), and the forces that will be applied at the site of the implant when the spine is loaded.

Spinal implants impart complex forces on the spine that can be broken down into basic component vectors and that may be considered separately. Rarely is any one force applied in isolation, and spinal implants function through several of the basic mechanisms of force application. Benzel lists six basic mechanisms of force application: (1) simple distraction, (2) three-point bending, (3) tension-band fixation, (4) fixed moment arm cantilever, (5) nonfixed moment arm cantilever, and (6) applied moment arm cantilever.² These represent the basic mechanisms by which implants impart force to the spine, and a single implant often functions using several mechanisms at once.

Anterior spine surgery frequently incorporates simple distraction and tension band fixation through a nonfixed moment arm cantilever fixation. Three-point bending is also used in longer constructs to stabilize the plate and prevent translation. Because surgery in the anterior spine is located ventral to the IAR, a bending moment is applied to the instrumented segments. Simple distraction may be applied through ventral placement of an interbody graft (ventral to the neutral axis/IAR) or by placement of a fixed moment arm cantilever in the form of a plate and screws. Plates secured to the anterior spine by screws in distraction are more frequently applied in the cervical spine. Placement of ventral distraction results in extension of the spine, and implants placed in this manner resist axial loads (Fig. 136-16). Placement of an anterior graft allows for distraction of the vertebral bodies, although the graft is left in compression. Typically isolated distraction of implants is avoided because this places the implant in a load-bearing rather than load-sharing mode. These implants bear much of the axial load; thus, failure may occur typically at the screw-plate interface. These same implants resist extension of the spine, and, therefore, act as tension band fixators when the spine is placed in extension. Distraction and compression are forces applied perpendicular to IAR resulting in torque (Fig. 136-17). Three-point bending is applied parallel to the IAR and often is accompanied by distraction or compression forces² (Fig. 136-18). Three-point bending occurs when a fulcrum directs a force vector in a direction opposite to the two terminal force vectors. The force at the fulcrum is equal to the sum of the terminal forces.²

FIGURE 136-16 A, A spinal implant placed in distraction (short arrows). B, A spinal implant placed in compression (short arrows) shares the axial load (orange arrows) with intrinsic spinal elements (long arrows). The spinal implant is thus unloaded during weight bearing. If enough compression were applied, the spinal implant might conceivably bear no load compression if the force applied by the implant were equal to the weight of the torso above the implant itself; that is, the case of zero weight bearing.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

FIGURE 136-17 A, Ventral spinal distraction (straight arrows) can cause spinal extension (curved arrows) if the distraction forces area is applied ventral to the instantaneous axis of rotation (IAR) (neutral axis). B, Conversely, the application of distraction forces (straight arrows) dorsal to the IAR (neutral axis) results in spinal flexion (curved arrows) (i.e., in tension-band distraction).

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

FIGURE 136-18 Three-point bending constructs are commonly applied in combination with distraction. These two components, three-point bending (A) and distraction (B), are independent of each other with respect to the forces they apply to the spine.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

Three-point bending requires an intermediate point of fixation between the terminal points of fixation, and when used in a ventral construct will resist translational deformation (Fig. 136-19). Ventral compressive implants do not resist axial loads, and the axial load-resisting ability of the spine must be intact or augmented (usually by an interbody graft) when these implants are used. Dynamic ventral systems allow controlled subsidence and therefore enhanced load sharing by the spine to augment bone healing and fusion of interbody grafts in the cervical spine.

FIGURE 136-19 Versatile implants (i.e., implants that can resist a variety of deformations) are optimal. For example, a ventral cervical cantilever beam device (A) can resist axial loads (straight arrows) via distraction (B) and extension (curved arrows) via tension-band fixation (C). If an intermediate point of fixation is used, it can resist translation via a three-part bending mechanism (straight arrows) as well (D).

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

Interbody grafts may be placed in the neutral axis of the spine or the IAR. These implants include autologous bone (i.e., rib, fibula, iliac crest), allograft, acrylic and wire constructs, and cages filled with morselized bone. These implants are usually placed to restore the axial load-bearing ability of the spine, and the principal forces are distraction and compression. Removal of a vertebral body (i.e., spondylectomy) or removal of a disc (i.e., discectomy), which incorporates the neutral or weight-bearing axis of the spine, usually requires placement of a graft to restore stability. There are several important considerations in placing interbody grafts. Migration of the bone graft into the adjacent vertebral body is known as subsidence. This can be avoided by ensuring that the abutting interbody graft occupies a large surface area and is able to withstand axial loads (Fig. 136-20). The cortical end plates of the vertebral body may be left intact to protect against subsidence as well. The cortical margins of the vertebral body are good buttresses for instrumentation and interbody grafts. Stress shielding occurs when the implant bears more of the axial load than the interbody graft, reducing compressive forces and the chance for fusion by increasing bone resorption. Stress shielding is avoided by placing the interbody graft under compression to incorporate load-sharing principles, which can be achieved by ventral tension band fixation. A final important consideration with regard to interbody grafts is that intact surrounding ligaments add stability when a graft that is sufficiently large is placed to distract the end plates and therefore ligaments are placed under tension.

FIGURE 136-20 The surface area of the contact surface of interfaces between abutting implants and bone correlates with weight-bearing capacity. A smaller-diameter implant penetrates farther (left), whereas a larger-diameter implant withstands axial loading more effectively (right). Arrows depict load applied to bone (hatched area) by the implant.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

Dorsal instrumentation incorporates distraction, tension band fixation, and three-point bending to reduce and stabilize the spine. Often dorsal instrumentation accompanies ventral instrumentation to augment the stability of the spine, especially in flexion. This is an important concept. Dorsal instrumentation must be very rigid and is load bearing in dorsal-only constructs. This consideration is critical when determining the need for additional fixation and the levels of fixation. Loss of anterior column support (i.e., fracture) leads to implant failure unless the anterior support is restored, or longer, more biomechanically rigid instrumentation is used.⁵ The implants act as a cantilever beam in these dorsal-only constructs. Dorsal fixation techniques are numerous and include wires, hooks, screws, and rods. Cross fixation may be added to stabilize the construct and prevent a coronal or sagittal parallelogram deformity (Fig. 136-21). Dorsal tension band fixation can be accomplished with laminar wires, clamps, springs, and rigid constructs consisting of lateral mass screws and rods in the cervical spine and pedicle screws, rods, laminar wires, hooks, and cross fixators in the thoracic and lumbar spine. Isolated dorsal compressive constructs are rare. Instrumentation over a number of segments often uses intermediate points of fixation and three-point bending to resist translational deformity (Fig. 136-22). Multisegmented instrumentation ensures that the load is distributed over a number of segments. Intact anatomy can be exploited such as in the cervical spine. Dorsal tension band fixation with engagement of the facets results in increased translational stability. At the same time, fusion of the cervical spine can be achieved ventrally and augmented by disruption of the facet joint and subsequent fusion of the lateral masses. As previously stated, axial load bearing by the facet joints is increased when the cervical spine is extended and the joints are engaged. The orientation of the facets resists translational deformity as well.

FIGURE 136-21 The parallelogram-like effect of lateral translational deformation (A) can be prevented by the toeing-in of the screws of the constructs (B), providing a cross-fixator (C), or increasing the length of the construct to incorporate an additional spine segment (D). The latter provides resistance to three-point bending fixation forces in a plane that is lateral to the spine (i.e., coronal plane), as well as in the sagittal plane.

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

FIGURE 136-22 A parallelogram-like translational deformation of the spine in the sagittal plane can occur with nonfixed moment arm cantilever beam constructs (A). This untoward occurrence can be minimized by the use of more rigid constructs or the use of a nonfixed moment arm construct over additional caudal segments (B).

(From Benzel EC, editor: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons.)

Pedicle screws are better able to control the vertebral body by virtue of the improved fixation. Often, the pedicle screw is at or beyond the IAR, giving it excellent three-dimensional control of the vertebral body. In this way, surgeons are able to perform derotation procedures in complex deformity cases and push the vertebral body to a more desirable position. Pedicle screws, however, are not the “be all and end all.” Careful attention to spinal alignment and balance must be taken into account. The spine must be reduced properly, and the SVA must be returned to within 2 cm, to prevent increased stress on the vertebral levels. If residual imbalance remains, this may lead to pseudarthrosis⁴² and worse clinical outcomes.^8,43 Patient factors are also critical; osteoporosis and obesity may also play a role in adjacent-level disease or fracture.⁴⁴ Often, posterior osteotomies must be performed, with or without anterior support, to shorten the posterior column and pull the spine to a more neutral alignment. In this way, the spinal instruments can change from load bearing to load shearing and support the spine while the fusion occurs.

Long moment arms may also play a role in adjacent-level degeneration and fracture. After long fusions, the fused spine imparts increased biomechanical loads to the adjacent segments. Long, rigid fusion constructs do not allow for adequate posterior elongation or stress dissipation; thus, the stress may be transferred to adjacent segments. This is particularly true at the distal segments. Several authors have advocated against long fusions ending at the L5 level, due to high revision rates at the L5-S1 disc.^45–48 They noted high rates of degeneration and revision of that level. Additionally, for fusions to the sacrum, most authors recommend iliac fixation to prevent distal failure or sacral fracture.⁴⁹ The L5-S1 level, due to the high stress, is also the most likely level for pseudarthrosis. This can be prevented with iliac fixation and/or interbody grafting for structural anterior support.^50,51