Spinal Anatomy and Biomechanics

Objectives

• Discuss the Curves of the Spine, their Development
• Describe the Elements and Functions of a Typical Vertebra and Intervertebral Disc
• Identify Major Ligamentous Structures of the Spine, including function
• Discuss Joints and Kinematics Associated with Global Motions of the Spine
• Describe Factors Impacting Movement of the Spine
• Describe Regional Differences Related to Structure and Function of the Spine
• Examine Arthrokinematics within the specific Regions of the Spine

Vertebral Column Regions

• The vertebral column has 5 regions: cervical, thoracic, lumbar, sacral, and coccygeal.

Spinal Curvature

• Normal spinal curvature angles exist when in an ideal posture.

Vertebral Column Curves

• Curves develop over time to adapt to changes in forces that occur as humans transition to upright positioning.
• Reciprocal curves allow for greater resistance to compressive loads.
• Primary curves retain posterior convexity throughout life, including thoracic and sacrococcygeal kyphosis.
• Secondary curves develop as a reversal of the original posterior convexity and consist of cervical and lumbar lordosis.
• The entire spine is kyphotic in the fetus and at birth.
• Cervical lordosis appears first as babies begin to orient their heads vertically.
• All 4 curves are evident in toddlers, often with an exaggerated lumbar lordosis.
• Primary thoracic and sacrococcygeal kyphoses and secondary cervical and lumbar lordoses are evident in adulthood.

Functional Components of the Spine

• The anterior components are vertebral bodies and intervertebral discs, which provide weight-bearing and shock absorption.
• The height of the intervertebral disc impacts the amount of segmental motion available.
• The posterior components are the vertebral (neural) arch, pedicles, and posterior elements.

Vertebral Body

• The vertebral body is the weight-bearing structure of the spinal column.
• It has a block-like shape with generally flat superior and inferior surfaces.
• A shell of cortical bone surrounds a cancellous-filled cavity.
• The cortical shell is reinforced by trabeculae in cancellous bone.
• Trabeculae are thin plates of calcified tissue within cancellous bone, laid down in line with the stresses placed on the bone.

Neural Arch

• Pedicles transmit tension and bending forces from the posterior elements to the vertebral body.
• The orientation of the pedicle helps define the size and shape of the vertebral canal.
• Posterior elements include transverse processes, lamina, spinous process, and articular processes.
• Articular processes form articular pillars, and each vertebra has 2 superior and 2 inferior facets.
• Superior facets articulate with the inferior facets of the vertebra above.
• Inferior facets articulate with the superior facets of the vertebra below.
• The articular pillar extends through the facet joints associated with the articular processes.
• The articular pillar provides a supportive column to the spine.
• Laminae are relatively thin, vertically oriented pieces of bone that serve as the "roof" of the neural arch, protecting the spinal cord.
• They transmit forces from posterior elements to pedicles toward the vertebral body.
• Spinous and transverse processes are sites for muscle attachments and increase the moment arm for muscles of the vertebral column.
• Pars interarticularis are subjected to bending forces and are located between superior and inferior articular processes (facets).
• Forces transfer from lamina to pedicles through the pars interarticularis, most developed in the lumbar spine.
• Greater magnitude forces will cause an increase in cortical bone.

Trabecular Systems

• These develop in response to stresses placed on vertebrae.
• Vertical systems within vertebral bodies function to support the body’s weight and resist compressive forces.
• Fan-shaped systems result from and resist bending and shearing forces.

Components of a Typical Vertebra

• Vertebral Body: resists compressive loads.
• Pedicle: transmits bending forces from posterior elements to the vertebral body.
• Lamina: transmits forces from articular, transverse, and spinous processes to the pedicles.
• Transverse Processes: serve as muscle attachments and provide a mechanical lever.
• Spinous Process: serves as muscle attachment; may also serve as a bony block to motion.
• Vertebral Foramen: forms a passage and protection for the spinal cord when combined with all segments.

Vertebral Segment

• Consists of 2 adjacent vertebrae, the intervertebral disc between them, and soft tissues connecting them.
• It is the smallest functional unit of the spine.
• Segmental motion is defined by the movement of the superior vertebra on the inferior vertebra.
• Motion is defined relative to the anterior portion of the superior vertebral body with an example of right rotation, including the movement of the superior vertebral body to the right & its spinous process to the left.
• If the movement is initiated by the pelvis, motion is still defined relative to the motion of the superior vertebra relative to the inferior vertebra

Intervertebral Foramina

• The intervertebral foramina is located between each vertebral segment in the posterior pillar.
• Spinal nerves, blood vessels, and recurrent meningeal (sinuvertebral) nerves exit the spinal canal via the foramen.
• The anterior boundary is the intervertebral disc, the posterior boundary is the facet joint
• Superior and inferior boundaries are the pedicles of the superior and inferior vertebrae of the spinal segment.

Intervertebral Disc (IVD)

• Regions of the spine has varying structures.
• The IVD increases in size from the cervical to lumbar regions.
• IVD size is related to the amount of motion and the magnitude of loads transmitted in the region.
• The functions are to separate adjacent vertebral bodies to increase segmental motion and to absorb/transmit compressive and shear forces between vertebrae.
• The amount of motion between adjacent vertebrae correlates to the ratio of disc thickness to vertebral body height.
• Greater ratios lead to >intersegmental mobility.
• The ratio is greatest in the cervical region, then the lumbar region, and finally the thoracic region.
• The 3 components of the IVD are the nucleus pulposus, the annulus fibrosus, and the vertebral (cartilaginous) endplate.
• The nucleus pulposus is a gelatinous center mass of the disc.
• The annulus fibrosus is a fibrous outer ring.
• The vertebral endplate is a cartilaginous layer covering superior & inferior surfaces of the disc and separating it from cancellous bone of vertebral bodies above & below.
• The IVD is composed of water, collagen, and proteoglycans in varying concentrations.
• Fluid & proteoglycan concentration is highest in the nucleus & lowest in the outer annulus fibrosus & outer vertebral endplate.
• Collagen concentration is highest in the vertebral endplate & outer annulus & lowest in the nucleus pulposus.
• The annulus fibrosus is composed of multiple, dense layers of predominately type I collagen fibers oriented in alternating directions (criss-cross pattern).
• It is firmly attached to adjacent vertebrae, layers bound to one another and annular rings enclosing the nucleus pulposus.
• Provides a mechanism for dissipating compressive forces.
• The nucleus pulposus is generally located centrally in the disc except in lumbar discs where it is offset posteriorly.
• Contains of a high concentration of proteoglycans and up to 80% water.
• Depends on age and time of day.
• Fluid mechanics of the confined nucleus function to distribute pressure evenly throughout disc
• The nucleus pulposus absorbs H2O when disc pressure is reduced.
• It squeezes H2O out under compressive loads, providing nutrient transport to maintain healthy disc tissue.
• It contains a higher % of type II collagen, with greater ability to withstand compressive force.
• Loosely aligned type II collagen fibers merge with inner layer of the annulus fibrosus.
• Vertebral endplates cover the NP superiorly & inferiorly, strongly attached to AF but weakly attached to vertebral body
• The Vertebral endplates function is to allow nutrients to diffuse from marrow of vertebral bodies to disc via end-plates and to prevent nucleus from migrating superior or inferior.
• Only the outer 1/3 to ½ of the annulus fibrosus is innervated by branches from the vertebral & sinuvertebral nerves.
• There is no blood supply from major arterial branches.
• The outer surface of AF receives blood supply from the capillary plexus in base of endplate cartilage and subchondral bone.
• Majority of IVD receives nutrition via diffusion.
• In a healthy disc, slight distortion may occur with movement to redistribute the load through the disc.
• During extension of a vertebral segment, there is compression of posterior disc, tension in anterior disc, and the nucleus pulposus shifts anteriorly.
• During flexion of the vertebral segment, there is tension of the posterior disc, compression of the anterior disc and the NP shifts posteriorly.
• Asymmetrical loading in flexion distorts the Nucleus Pulposus toward the opposite posterolateral corner

Articulations of the Spine

• Articulations of the Spine which include the following:
  • Intervertebral (Interbody) Joints: Joint formed b/w Adjacent Vertebral Bodies, Cartilaginous Joints, Formed by 2 Vertebral Bodies & IVD Separating Them
    • Facet (Zygapophyseal or Apophyseal) Joints: Synovial Joint, Formed b/w Facets of the Articular Processes
  • Motion varies by Spinal Region and is related to Structural Differences and Facet Orientation. Also includes the following Available Movements: -Translation (Sliding): Anterior/Posterior, Side to Side, Axial Rotation (Torsion) -Tilting: Anterior/Posterior, Lateral (Side to Side) -Distraction and/or Compression
  • Zygapophyseal (Facet) Joints are also Synovial joints that are formed b/w the Inferior & Superior Articulating Facets and feature Accessory Structures, Fibroadipose Meniscoids; which also protect Articular Surfaces that are exposed during Flexion/Extension.

Ligaments of the Spine

• Variability is based upon Spinal Region. While major Ligaments exist along the Spine which include these structures: -Anterior Longitudinal Ligament (ALL): Runs along Anterior & Anterolateral Aspect of Vertebral Bodies. Limits the act of Extension in the Spinal Segments involved: Reinforces Anterolateral Portion of AF & Anterior Aspect of Intervertebral Joints and spans from C2 to the Sacrum -Posterior Longitudinal Ligament (PLL): Runs Along Posterior Aspect of Vertebral Bodies: Limits the act of Forward Flexion (Reinforces Posterior Portion of AF) (From C2 to the Sacrum) -Ligamentum Flavum: Connects Lamina to Lamina, Forming Posterior Surface of Vertebral Canal (From C2 to the Sacrum); also limits forward flexion -Supraspinous and Interspinous Ligaments: Serves to limit Forward Flexion in the Spinal Column. This Strong/Cordlike Structure and is connect between the Tips of Spinous Processes from the Thoracic and Lumbar Regions (From C7-L3/L4) -Intertransverse Ligaments: Run between the Transverse Processes of Adjacent Vertebrae, primarily in the Lumbar Regions and limits Contralateral Lateral Flexion.
  • These Ligamentous structures provide an optimal function for the Spine, as the ALL resists Extension and is compressed while the PLL is stretched in acts of Flexion/movement along the Spine.

Kinematics of the Spine

• Motions available to the spine as a whole: Flexion, Extension, Lateral Flexion (Side Bending), & Rotation
• Motions are often coupled at the Segmental Level. Coupling is a consistent association of one motion with another and is the predominate coupled motions of rotation & lateral flexion. Coupling patterns vary by Region of the Spine and among Individuals.
• Coupling patterns are related to:
  • The Individual, Spinal Region, Spinal Posture and Curves, Orientation of the Articulating Facets, the Integrity of the IVD, the Integrity of the Ligaments and Joint Capsules
• The most consistent Coupled Pattern in the Spine is the Lower Cervical Spine; where Lateral Flexion and Rotation to the same side are paired, especially when either motions for Lateral Flexion or Rotation are initiated.

Movement of Vertebral Column: Flexion

• Tilts & translates anteriorly on vertebra below -Disc: Compressive forces on anterior disc Tensile stress on posterior disc Nucleus pushes against posterior annulus -The Spinous processes move superiorly and the Iintervertebral foramen size increases Limitation: PLL, posterior AF, ligamentum flavum, muscles (when tight), supraspinous & interspinous ligaments, intertransverse ligaments, facet capsules

Movement of Vertebral Column: Extension

• Tilts & translates posteriorly on vertebra below
• Disc: Compressive forces on posterior disc Tensile stress on anterior disc Nucleus pushes against anterior annulus
• Spinous processes move inferiorly
• Intervertebral foramen size decreases Limitation: ALL, anterior AF, approximation of spinous processes/lamina/facet joints, musculature (when tight)

Movement of Vertebral Column: Lateral Flexion

-The Vertebral Body Tilts and Translates Ipsilaterally -Disc: Compresses Ipsilaterally Tensile stress on Contralateral Side Nucleus Distorts Contralaterally

• The Iintervertebral foramen Size Increased Contralaterally & Decreases Ipsilaterally Limitation: AF, musculature (when tight), Facet Joint Capsule, Approximation of Vert Bodies, Intertransverse Ligament

Movement of Vertebral Column: Rotation

• Vertebrae Move In Transverse Plane
  • If body rotates right, Spinous Process Moves Left
  • If Body Rotates Left, Spinous Process Moves Right
• Limitation: Facet Orientation, Capsules, AF, Muscles (When Tight)

Cervical Structures

• Craniovertebral Region also consists of an Upper C-Spine and is often associated with being:
• Composed of Occipital Condyles, the Atlas and Axis which also includes Occipitoatlantal (OA) & Atlanto-Axial (AA) Joints.
• The Lower Subaxial Cervical Region includes structures starting from C3-C7
• There is also a Transverse Foramen which is located on Transverse Processes and Serves as Passage for Vertebral Artery
• Atypical Cervical Vertebrae also include:
• C1, C2 & along C7
• Atlas (C1):* -Firm Ring situated between Occipital Condyles, Axis, Features: No Vertebral Body/Spinous Process, only Large Lateral Masses Formed by 2 Superior/Inferior Facets:
• Superior Facets are Large & Deeply Concave with Complex Occipital Condyles -Inferior Facets: -Function: For Support for the Lower C-Spine while transmitting force between Occiput to lower Structures
• Axis (C2):*
• Dens arises from Superior Surface of Body and articulates with Anterior arch of Atlas as well as featuring a Posterior Groove On Transverse Ligament Superior Facets Articulate and Inferior Facets Articulate With C3, serving to Transmit/Load down C-Spine while providing Axial Rotation of Atlas/Head.

Upper Cervical Region Characteristics

• Articulations with the Upper C-Spine include Occipitoatlantal (OA) Joints which have also associated Lateral Synovial joints. Structures include:
• Atlanto-Axial (AA) Joints that serve 3 Synovial Joints, with around ~2 Synovial Joints Between Axis/Atlas, and with Pivot Joint Between Dens/Anterior Arch along Transverse Ligament. However remember kids: no disc is between the OA & AA joints! The Posterior Atlantooccipital and Atlantoaxial Structures are Fibroelastic Membranes; Superior Extensions Ligamentum Flavum and Allow Greater ROM Then Ligamentum Flavum. Same with Anterior Extensions of ALL, which are called Anteriol Atlantooccipital and Atlantoaxial Membranes, featuring: A structure between Anterior body of Axis to Basilar Groove that Features Broad and Strong Membranes.

Ligaments of the Cranial Region

• Serves to Maintain Cervical Stability and Proper Movement:
• Ligamentum Nuchae can be Described Variably, but it is as with it is a Thin and Triangular Structured Sheet. It:
• Functions To Stop Flexion of The Head Neck and as a Space For Transverse Ligament attachments. Stretches Across Ring Of Atlas -This then Holds Dens Against Atlas To Ensure Pivot Action During:
• Rotation/Transverse Ligament and also Prevents Anterior Displacement Of:

-Alar Ligaments -Arise from Axis on Either side Of Posterior Dens and Extend Laterally, Superiorly and Attach to Occipital Condyles -Rotation here is Restrained by Contralateral Movement While Flexion Can Also Pull Ligament structures taut -Passive Structure that will stop Contralateral Axial Rotation and Side-Bending which maximizes around: 90 degrees

Typical Cervical

• Vertebrae includes typical Vertebral Bodies that feature Transverse Diameter That Has a Greater Height and increases slightly starting from: (C2-C7):
• With its bodies creating a Concave shape within the Frontal plane across the vertebral structure.
• Their Spinous Processes are Short, Slender and Bifid.

Movement:

• Primarily allows for:
• Nodding and Flexion/Extension With:
• Some Limited movement on Lateral Flexion/Rotation:
• -Coupled Motion:
• -Coupled With Contralateral Rotation* and an Oblique Facet Orientation will allow for overall GREATEST ROM along the Spinal Column

Thoracic

Lower Cervical

Flexion to Extension as Mobility:

• Allows for increasing amounts of Flexion/Extension With SBing and Rotation being Greater with Upper T-Spine and Reduce Lower T-Spine
• Has orientation that transitions into a Sagittal Plane, with Tighter Capsules within the Cervical and Lumbar Regions.

Thoracic Vertebrae Rotation

• Allows for Associated Rib Pairs -Also features the amount of Rotation that can determine itself dependent by: -Distortion In Vertebrae in the Costovertebral and -Costotransverse Joints:

Lumbar

• Has Mobility Between Flexion/Extension which features a Primary Motion Along The Spine itself:

- And orientation For:

• Motion to increase along the: :L5-S1 Portion

-Facets Do Less Sliding on the Articular Structures here and that the Patterns Are Variable to Ipsilateral Flexion and Internal Rotation

Arthrokinematics

• These can be characterized on a Motion Base as relating facets motion by Superior Vertebrae while:
• -Facet Motion in Movement:

Flexion

   -Features Occipital Condyles that Will Anteriorly Roll With Sliding:

-Posteriorly + Tilting Structures (Minor Motions) Along The :

• -C1_C2 Axis

Extension

   - Occipital Structures:

• Rolling Structures:

-Posteriorly With Sliding Anteriorly on Atlas that Tilted Posteriorly (While this a minor Motion Along, it is Along C1_C2)**:*

*Side Bending Characteristics:

• Also features little range to perform with what is being called (The Motion On Superior Vertebrae along Side which slides. It also features Ipsilateral Facets With Facet Movement where what is Downward Movement along the joint and Opposing Contralateral Facets With Joint Upward Motion*).

During lumbar

Rotation:

  -With Facets To Do less On One Another Due to the Orientation While The -Ipsilateral Structures To Distract While Opposing with (Contralateral Approximation).

Arthrokinematics: Thoracic, & Lumbar Region also possess similar movement patterns to Cervical such that.

• These structures share very similar facets movement where Inferior components will perform a superior/elevated (upslide) during Flexion. -*Note/Reminder: with a "Lumbar Rotation", and there will a, an increased: -*Facet Joint with and in return decreased: /compress (In the Proximal).

Cervical

• (Togethor w/Thoraic and Lumbar segments also feature facets that will inferior facet will perform an elevated action (With Upward or superior).

Extension- -Facet Joint: Inferior Structures Here Will Slide (Downwards or inferior) -Side Bending; Features

-Facets as Joint structures as The Slide in With What An “Downward” Shift, “Inferior With Upwards Structures In Opposing Directions .

-Rotation Joints