Spinal Biomechanics

According to the American Society of Biomechanics, biomechanics can be defined as "the study of structure and function of biological systems via methods of mechanics." Mechanics refers to the concurrent and mobilizing characteristics of the spine. This includes how spinal structures remain and maintain static and move with and against forces.

A general knowledge of spinal biomechanics is beneficial to understand the function of spinal anatomy when normal and healthy; spinal disorders, such as whiplash, vertebral fracture, scoliosis, and spondylolisthesis; and, effects of nonoperative and surgical treatment

The spine is the most complex and least understood musculoskeletal structure. Although the spine's anatomy and function are relatively well understood, biomechanics is not. This is due in part to the limitations of cadaveric study, in which tissues are inert.

A first step to understand spinal biomechanics is a brief overview of directional terms, anatomical planes, and anatomy. Several terms used throughout this learning opportunity will be more familiar if this preliminary anatomical review is considered.

Terms used to describe anatomical direction are: Anterior, or toward the front; posterior, or toward the back; dorsal or back; ventral or front; medial, or toward the body's midline; lateral or side, away from the body's midline; superior or upper or above; and inferior or lower or below.

Other terms refer to anatomical planes. Coronal, or front, vertically divides the front and back of the body; sagittal, or lateral, vertically divides the left and right body sides; and, axial, or transverse, horizontally divides the upper and lower body.

Anatomical planes are invisible lines or flat surfaces. The coronal, sagittal and axial planes are illustrated. These terms are extensively used by spine professionals to describe the location of a body part in relationship to another.

The histology of spinal bone is similar to other skeletal bone. Skeletal bone is composed of an outer layer of cortical or compact bone and inner core of cancellous, or trabecular bone. Cortical bone is stronger than cancellous bone.

In the image, cortical bone is shown near bottom left. Cancellous bone makes up the majority of the picture's lattice-like structures. The periosteum, not shown, is a connective tissue that overlies the outer cortical bone and provides an attachment point for ligaments and tendons.

Normal bone physiology is an active and ongoing process as osteoclasts resorb bone and osteoblasts form new bone. Healthy bone remodeling preserves calcium reserves, keeps bones dense, and maintains the spine's ability to resist stresses.

The spine is an essential part of the body's framework. Besides supporting the body against gravitational forces, the spine protects organs, produces blood, stores inorganic calcium and phosphorus salts, and provides a base of attachment for ligaments and tendons. Ligaments and tendons are important to spinal stability when static and during movement.

The four regions of the spine are: cervical, thoracic, lumbar, sacral, and coccygeal. Curves in the spine are termed lordosis or kyphosis.

Lordosis and kyphosis are measured in degrees, which vary by spinal level. Cervical lordosis is 20 to 40-degrees; thoracic kyphosis is 20 to 40-degrees, lumbar lordosis is 30 to 50-degrees; and, sacral kyphosis varies, because the sacral elements are fused and the range of kyphotic curve, or tilt, widely varies.

The spine's lordotic and kyphotic curves serve important functions. The curves provide balance, add flexibility, distribute body mass, absorb and distribute shock, and pelvic rotation, such as during gait. The benefits of normal spinal curvature are demonstrated during a golfer's swing sequence.

During fetal development, and for a time period after birth, the baby's spine is "C" shaped. This shape is a primary curve or kyphotic. When the baby begins to raise his head upright, a secondary, or lordotic, cervical curve develops. As the baby continues to grow and gain muscular strength, body weight shifts, causing lumbar lordosis. Development of lordosis enables the baby to eventually walk upright. Curves continue to develop until the child reaches skeletal maturity.

The cervical spine consists of seven vertebrae abbreviated C1 through C7. This part of the spine is divided into the upper, C1 and C2, and lower cervical regions C3 through C7.

C1 and C2 are not shaped like the other vertebral bodies. C1, the Atlas, is ring-shaped. The Atlas helps to support the skull. C2, the Axis, includes the dens (Latin for tooth), or odontoid process, which fits up into the atlas and acts like a pivot and collar enabling the atlas and skull to rotate around the dens.

C3 through C7 comprise the lower cervical spine. A key difference between C1 and C2 is C3 through C7 have a hole for the vertebral artery.

There are 12 thoracic vertebrae, abbreviated T1 through T12. Although similar in shape to cervical and lumbar vertebrae, they are characterized by small pedicles, long spinous processes, and large intervertebral foramen.

Ribs, which together make up the rib cage, attach posteriorly to the thoracic vertebrae. Rib attachment aids spinal stability and helps to protect the spinal cord, nerve roots, and internal organs such as the heart and lungs. Ribs create the chest cavity and assist with respiration.

The five lumbar vertebrae are abbreviated L1 through L5. Lumbar vertebrae are the largest and distribute most of the body's weight and biomechanical stress. The large lumbar spinous and transverse processes provide attachment points for ligaments and tendons, which help stabilize this mobile region of the spine. The neuroforamen are broader to accommodate larger nerve bundles that innervate the legs.

Below the last lumbar vertebra are the sacrum, sacroiliac joints, and coccyx. The sacrum is the large triangular-shaped bone consisting of five fused vertebrae abbreviated S1 through S5. Two sacroiliac joints join the sacrum to the pelvic bones. The coccyx consists of three to five triangular-shaped bones commonly referred to as the tailbone.

Facet joints are also termed zygapophyseal or apophyseal joints. Each vertebral body has a set of facet joints posteriorly located. One pair faces upward and is termed the superior articular facet; and, one downward, the inferior articular facet. Joints are coated with articular cartilage. The facets are synovial joints, meaning each joint capsule contains synovial fluid; a lubricant to protect joints.

The facet joints enable spinal flexion - the ability to bend forward, extension - the ability to bend backward, and rotation. Yet, the facet joints restrict excessive, or certain, movements and add stability to the spine.

Between each vertebral body; except C1, C2, the sacrum and coccyx, is an intervertebral disc. Discs make up about 20- to 33% of the length of the spinal column. Disc thickness varies from 3-millimeters in the cervical region to 9-millimeters in the lumbar.

The structure of each disc includes an annulus fibrosus, nucleus pulposus, and superior and inferior endplates. The annulus fibrosus is the tough encapsulating peripheral supporting structure that protects the nucleus pulposus, a gel-like material. Endplates are thin layers of cartilage that cover the superior and inferior surfaces of the vertebral bodies. Endplates sandwich each intervertebral disc between the superior and inferior vertebral bodies.

The annulus fibrosus, nucleus pulposus and endplates are composed of water, collagen, and proteoglycans. Proportions vary by structure. Proteogylcans attract and retain water, collagen fibers help resist compression, and disc degeneration reflects an abnormal balance between water, collagen, and proteoglycans.

Water and proteogylcan concentrations are highest in the nucleus pulposus and lowest in the peripheral annulus fibrosus. However, collagen concentration is highest in the annulus and lowest in the nucleus. Collagen fibers that make up the annulus fibrosus are arranged in concentric bands set in opposite directions adding to disc strength.

Endplates also contain water, collagen, and proteoglycans. The center of the endplate contains similar concentrations of these elements as the nucleus pulposus.

Ligaments and tendons are fibrous connective tissue made up of closely packed collagen fibers. Ligaments link bone to bone and tendons join bone to muscle. Dense collagen fibers are responsible for structural tensile strength, meaning its ability to resist tearing.

The anterior longitudinal ligament and posterior longitudinal ligament are important to spinal stability. The anterior longitudinal ligament attaches anteriorly to each vertebra. The posterior longitudinal ligament attaches posteriorly to each vertebra from within the spinal canal.

An intricate and layered muscular system, combined with ligament and tendon support is important to spinal stabilization. Anterior and posterior muscles are the primary groups, which include the superficial, middle and deep layers. The posterior muscles create a tension band important to sagittal and coronal plane balance.

The brain and spinal cord comprise the central nervous system. The spinal cord starts below the base of the skull and ends near L1 where it becomes the cauda equina; a bundle of nerve roots resembling a horse's tail. The Spinal Accessory Nerve, abbreviated CNXI, innervates muscles that support the spine.

Thirty-one pair of spinal nerve roots branch beyond the cord through neuroforamen and form the peripheral nervous system. There are eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal pair of spinal nerves.

Functions common to most regions are related to spinal stability, movement, and structural protection. Keep in mind that whether static or in motion, movement at any spinal level affects adjacent structures.

The spinal column protects the spinal cord, nerve roots, and internal organs; is a base of attachment for ligaments, tendons, and muscles; provides structural support; connects the upper and lower body; is key to balance and weight distribution; and, allows flexibility and mobility.

The cervical spine supports the skull, allows a large amount of varied movement, protects the spinal cord, protects the vertebral arteries, and maintains airway patency.

The thoracic spine is a base for rib attachment and protective framework for respiration. The thoracic spine protects internal organs, the aorta and vena cava, and sympathetic chain. The sympathetic chain is part of the autonomic nervous system. The autonomic system regulates homeostasis through involuntary control.

The lumbar spine supports upper body weight, protects the cauda equina, and allows movement of the rigid pelvis and pelvic twisting for biped ambulation.

The sacral spine supports the pelvis, protects the reproductive organs, and stabilizes the sacroiliac joints. The sacroiliac joints support much of the body's weight. When standing erect, much of the body's weight is disbursed through L5 and S1.

This section explains balance and posture; describes a motion segment; mechanisms of movement; direction of motion; bending, compression, creep; force, load, moment; and, shear, stiffness, tension, and torque.

Coronal, sagittal, and axial spinal balance is illustrated. Good posture or neutral spine occurs when the head and upper body are aligned over the pelvis when standing.

Good posture benefits the spine during static and active movement. Benefits include correct alignment of spinal structures and reduced wear of joint surfaces, joint, ligament and tendon stress, and muscle fatigue. Movement is more efficient, which may help to prevent posture-related spinal curvature. Efficient movement may help reduce overuse injury and good posture offers a pleasing appearance.

Each motion segment, or functional spinal unit, is made up of two adjacent vertebrae, an intervertebral disc, two posterior facet joints, and supporting ligaments. The fused sacrum and coccyx are not considered motion segments. Although small in comparison to the entire spine, each structure contributes to spinal balance.

Vertebrae move in relation to one another through pivots, levers, actuators, and restraints. Discs and facet joints are pivots and allow rotation. Levers provide an actuator base of attachment and include the anterior vertebral body, posterior arches, and spinous and transverse processes. Ligaments, facet joint capsules, and discs are restraints that help provide spinal stability and restrain motion.

Flexion, extension, lateral flexion, and rotation are types of spinal motion. The amount and direction of motion varies by spinal region. For example, the cervical spine allows the greatest magnitude of flexion, extension, lateral flexion, and rotation. By contrast, the thoracic spine is the least mobile because of its attachment to the rib cage.

Flexion involves the vertebrae, discs, ligaments, facet joints, and muscles. The anterior superior flexes over the anterior posterior. The amount of forward tilt is controlled by the size of the disc, and tension created in the posterior ligaments and facet joints.

Extension involves the vertebrae, discs, spinous processes, ligaments, and facet joints. The amount of extension is limited by disc size, bony contact of the spinous processes, and tension in the anterior longitudinal ligament.

Lateral flexion involves the vertebrae, discs, ligaments, facet joints, and muscles. The superior vertebra tilts and rotates over the adjacent inferior vertebra. Tension in the ligaments, muscles and facet joints limit lateral flexion.

Rotation involves the vertebrae, discs, spinous and transverses processes, ligaments, facet joints, and muscles enabling twisting movement.

Bending, compression, and creep are terms used to describe types of movement or force.

Bending causes compression and tension and may distort the shape of the disc. During flexion, intervertebral disc shape is altered. The tensile strength of the disc resists bending, therefore helping to restore the disc to normal shape when compression stops.

Compression is a force, such as gravity or axial load. Creep is a gradual alteration in shape. Ligaments sustaining a load elongate and creep results as the dense fibrous bands are put at risk for injury. Distraction force is similar to tension, usually associated with flexion or extension, and refers to the pulling apart of bony or soft tissues.

Force, load, and moment are terms used to explain and measure movement and force.

Force is expressed in newtons - units of force, or pounds. The result of force depends on whether the object is at rest or in motion. Forces can be described by magnitude of force, such as acceleration, and direction, such as anterior or posterior. Whiplash is an example. Load is the application of force, such as compression or rotation. Moment is a measurement of torque or rotation.

Shear, stiffness, tension, and toque are terms used to define and measure movement or force.

Shear is a parallel force. Spondylolisthesis, where a superior vertebral body slides forward over the adjacent inferior vertebral body, typifies a shear force disorder. Stiffness is the ability to resist load. Tension is a force causing extension or elongation. Torque or torsion is paired but, opposing rotational forces.

Different disorders disrupt normal biomechanics and cause spinal instability. Spinal instability may develop instantly, such as from injury causing vertebral fracture, or gradually, as in some cases of spondylolisthesis. The next slides introduce spinal instability and examine whiplash, vertebral fracture, scoliosis, and spondylolisthesis.

Whiplash, vertebral fracture, scoliosis, and spondylolisthesis are examples of conditions that can cause instability. Biomechanical instability can result from loss of normal relationship between bony and soft tissues, an alternation of normal function, dislocation and / or misalignment of bony structures, or disruption of soft tissues.

Commonly caused by being rear-ended in a motor vehicle accident, whiplash is a flexion / extension injury. Such sudden forces insult cervical ligaments, tendons, and muscles causing hyperflexion and hyperextension. Whiplash may be termed a soft tissue disorder. Other activities that can cause cervical motion strains include riding a roller coaster, being punched or shaken, and contact sports such as football.

More serious effects from whiplash include: severe forward and backward bending of the neck or trunk, excessive flexion and extension; axial loading or compression possibly causing a vertebral compression fracture; blunt trauma to the face and abdomen; vertebral compression caused by muscle contraction; potential shear and torque stresses; dislocation of facet joints; rupture or distraction of discs; tearing or dissection of blood vessels; and, traction injury to the spinal cord and nerves.

Spinal fractures include burst, wedge, and compression types. Fractures are often caused by loaded crushing forces or a single force. Osteoporosis, a metabolic disease that causes loss of bone density, is a common cause, or contributing cause to vertebral fracture. Osteoporosis erodes bones' ability to withstand compressive and tensile stresses. Bending and mechanisms of movement that incorporate tension and torque may cause vertebral fracture.

Vertebral fracture requires prompt medical attention. Left untreated, the effects of the original fracture can cause a cascade of problems that include loss of vertebral body height, spinal instability, subsequent vertebral fracture at adjacent levels, deformity, such as a hunched back, and neurological problems.

Many spine specialists use the three-column concept to describe and diagnose the severity of a spinal fracture. The spinal column is viewed as three columns - anterior, middle, and posterior.

The anterior column consists of the anterior longitudinal ligament and the anterior half of the vertebral body, disc, and annulus. The middle column consists of the posterior longitudinal ligament and the posterior half of the vertebral body, disc, and annulus. The posterior column consists of the posterior elements, facet joints, ligamentum flavum, and interconnecting ligaments.

The sagittal MRI shows a T12 burst fracture that involves the anterior and middle column and displaces bone into the spinal canal. In this particular case, spinal instability is apparent.

Scoliosis is an abnormal curvature of the spine in the coronal, sagittal, and / or axial planes. Depending on its severity, it can cause spinal displacement and deformity. Most scoliosis occurs in the thoracic and lumbar spine. It is not uncommon for a compensatory curve to develop superior or inferior to the primary curve. Compensatory curves develop in an attempt to maintain spinal balance. Rotation along the axis of the spine may develop concurrent with curvature further complicating the scoliosis.

The biomechanical consequences of scoliosis include spinal instability and imbalance, curve progression, deformity, loss of body symmetry, one leg shorter than the other, and difficulty with coordination or perception. When severe, scoliosis contributes to pulmonary and neurological problems.

Scoliosis affects infants, adolescents and adults. Congenital curves, neuromuscular disorders, degenerative spinal problems, curves caused by tumor or trauma, and familial occurrence may lead to development of scoliosis. When the cause is not known, it is termed idiopathic. Some cases of adult scoliosis result from abnormal curvature not treated during youth.

Curve measurement includes analyzing curve magnitude, pattern, flexibility, and the probability for curve progression. Sex, age, skeletal maturity, puberty, and bone mineral density help to predict curve progression.

Curve Magnitude determines whether the curve or curves are nonstructural, compensatory or minor, or structural or major. Curve pattern and direction determines the spinal levels and the number of vertebrae involved, as well as the direction of the curve or curves. Direction is concave or convex.

Standard full-length anterior-posterior, posterior-anterior, lateral, and side bending radiographs are needed to classify curves. Different classification methods, such as Lippman-Cobb, King, and Lenke, use geometric calculations to measure the degree of curve angle and rotation in the coronal, sagittal, and axial planes. Information compiled from measurements is used in nonsurgical and surgical planning.

Different curve patterns are associated with scoliosis. Curve patterns vary by the cause, type, and magnitude of scoliotic curvature.

A single major lumbar curve may produce waistline asymmetry, contralateral hip prominence, and short leg on the curve side. A single major thoracolumbar curve may produce trunk imbalance and deformity. Combined thoracic and lumbar curves are also called double major curves. A deformity may be less noticeable because the curves may be of near equal size. A single major thoracic curve may produce rib prominence on the convex side of the curve, which causes rib depression on the concave side. A single major high thoracic curve may cause one shoulder to become elevated resulting in a deformed thorax. A double major thoracic curve may occur in the upper thoracic spine and cause vertebral rotation affecting the spine below the upper curve.

Degenerative scoliosis is more common in adults than adolescents. When severe, back pain can become significant. Spinal stenosis may develop at the apex of the curve. Since these patients are older, osteoporosis is a treatment consideration.

The lumbar facet joints and neural arches - and ligaments - are important to low back function and stability. Forces such as shear, compression, and torque - physical forces encountered during activities such as weightlifting and gymnastics - can cause a superior vertebra to slip forward over the adjacent inferior vertebra. This is termed spondylolisthesis, or slippage of the vertebrae. Some cases of spondylolisthesis are stable, while others are unstable.

Spondylolistheses can be progressive. A spondylolisthesis changes the position of anatomical structures and can shift the body's center of gravity. Spondylolisthesis may cause swayback, a protruding abdomen, and a waddling gait.

Using sagittal radiographic studies, the Myerding grading system is applied to measure the percentage of vertebral slip. Grade one is less than 25%, grade two is 25 to 49%, grade three is 50 to 74%, grade four is 75 to 99%, and grade five is 100% and known as spondyloptosis. To determine whether a spondylolisthesis will progress, several factors are considered including the percentage of slip, patient's age, and cause.

Not all spinal disorders cause instability. Soft tissue injuries, such as whiplash may cause stiffness and spasm. Spasms are involuntary muscle contractions that guard and stabilize the cervical spine. Hyperflexion and hyperextension injuries are common causes. Such soft tissue injuries often resolve within a few weeks.

Vertebral fracture, scoliosis, and spondylolisthesis are examples of disorders than can cause spinal instability and may require surgical treatment. Depending on the severity of the disorder, all three of these spinal problems may initially be treated with physical therapy, bracing, and medications. In most cases, spine surgery is not urgent. However, some disorders that cause spinal instability may best be treated with surgery.

Nonoperative treatment can often address instability and restore spinal stability. Physical therapy or rehabilitation may reduce a soft tissue injury and pain, while improving posture, flexibility, and strength. Bracing may improve deformity, prevent curve progression and provide external support to promote healing. Muscle relaxants are drugs that ease spasm and can reduce nonstructural deformity, such as hunchback due to back strain.

Surgery is often considered for cases that do not respond to less invasive treatments. The goals of surgery are to apply loads to realign the vertebrae, provide and maintain stability, limit motion during fusion, and promote arthrodesis.

Most surgeries involve both instrumentation and fusion. Instrumentation includes implants such as rods, plates, cable, wire, and screws. Implants provide immediate stabilization, or internal fixation. Additional external bracing may be recommended while the fusion heals.

Fusion is the bony bridge that provides long-term stability and load-bearing. All metal instrumentation is prone to fatigue or pullout over time, so the active bone growth of fusion is what provides solid immobilization. Fusion, or arthrodesis, employs autograft or allograft. Autograft is the patient's own bone and allograft is donor bone. Fusion increases the load-bearing capacity of bone.

In addition to instrumentation, bone cement, such as polymethylmethacrylate, abbreviated PMMA, and bone graft may be used. Collectively called implants, these devices are available in various sizes and configurations to meet varied needs.

Some recent surgical developments include cervical and lumbar artificial discs; an alternative to spinal fusion. An artificial disc may help restore lost disc height and mimics human motion. Not only do they preserve natural mobility, but they may reduce abnormal forces that occur following fusion. Bone Morphogenetic Protein or BMP is a genetically derived substance that stimulates bone growth and fusion. Successful solid fusion is essential to spinal stability.

Solutions to translational and angular deformation of bone graft, which may cause loosening and rocking of the motion segment, are being met to reduce postoperative implant problems. Specific implants to address translational and angular deformation of bone graft. This should reduce postoperative complications including loosening and rocking of the instrumented segment.

Minimally invasive spine surgery is the use of advanced instruments and procedures with less disruption of surrounding tissue. Such procedures reduce incision length and preserve spinal musculature to shorten recovery and improve healing.

Biomechanical factors now direct surgical plans and can improve functional outcomes. Cervical kinematics, the classic mechanics of motion, has been a major component in arthroplasty. Because adjacent level disease may develop after traditional cervical fusion, the preservation of natural motion is essential. In the Journal of Spinal Disorders and Techniques, Sasso and Best reported that one particular artificial disc, "prevented or delayed adjacent level operations by retaining motion at the target level and eliminating abnormal adjacent activity."

Interbody grafts reduce the incidence of pseudarthrosis, increase fusion rate, and greatly improve axial load-bearing ability in lumbar fusion. Lumbar interbody procedures include Anterior Lumbar Interbody Fusion or ALIF, Axial Lumbar Interbody Fusion or AxiaLIF, Direct Lateral Interbody Fusion or DLIF, Posterior Lumbar Interbody Fusion or PLIF, Transforaminal Lumbar Interbody Fusion or TLIF, and Extreme Lateral Interbody Fusion or XLIF.

Kyphoplasty treats vertebral compression fractures by attempting to recreate the initial bone shape rather than simply cementing the compressed bone. Ledlie and Renfro reported outcomes of a two-year clinical cohort study revealed, "significant height restoration and normalization of morphologic shape indexes that remain stable for at least 2 years following treatment."