Awkward Postures, Pain & Biomechanics of Spine

Examples of Awkward Postures and Pain Areas

• Mechanical low back discomfort is the most common complaint, trailed by neck pain
• Shoulders, hands, and feet are other common areas of pain
• Pain serves as a protective mechanism associated with muscle spasms to immobilize the affected area
• Mechanical pain results from increased stress on the body structure due to deviations from the neutral position, affecting the balance between external and internal muscular forces

Neutral Position and Force Imbalance

• Any deviation from the neutral position causes an imbalance between external and internal forces, leading to increased muscle exertion
• Increased work-related stresses amplify the load on the body's structure, triggering pain as a protective mechanism and indication of overuse
• The spine is the primary body structure exhibiting deviation and pain

Biomechanics of the Spine

• The spine supports the individual from birth, and is a complex mechanical structure
• The spine includes semi-rigid bodies separated by soft fibro-cartilaginous cushions
• 33 vertebrae are bound together by ligaments and intervening cartilages

Vertebrae Classification

• The vertebrae are divided into five areas
• Seven flexible cervical vertebrae support the head, and allow motion in three planes
• Twelve relatively immobile thoracic vertebrae have ribs attached
• Five large flexible lumbar vertebrae support the weight of the upper body, also allowing movement of the trunk in three planes
• Five fused sacral vertebrae are a triangular structure connecting the spine with the pelvis
• Four small fused coccygeal vertebrae

Spinal Curves

• From the frontal view the spine appears straight and symmetrical
• From the sagittal view, the spine has a curved nature
• There are four normal spinal curves
• Two curves are convex posterior at the thoracic and sacral regions, called primary kyphotic curves, and start at birth and retain convexity throughout life
• Two curves are convex anterior at the cervical and lumbar regions, which are called secondary or lordotic curves, that develop to facilitate standing and walking

Motion Segment of the Spine

• A motion segment of the spine is known as the functional unit of the spine
• It consists of two vertebral bodies (above and below) and an intervertebral disc in between
• It allows six degrees of freedom in three planes: flexion, extension, right lateral bending, left lateral bending, rotation to the right, and rotation to the left

Weight-Bearing Properties of a Motion Segment

• Each motion segment contains two vertebral bodies and an intervertebral disc
• The structure of a vertebra includes an anterior bony block and posterior bony ring
• The anterior part includes a cylindrically shaped vertebral body, which carries 70%-80% of the load on the vertebra
• The posterior ring includes an irregularly shaped vertebral arch, which carries 20%-30% of the load on the vertebra

Vertebral Body

• The the vertebral body gets its strength from its dynamic composition of an anterior block and posterior ring
• The anterior bony block contains cancellous bone covered by a layer of cortical shell
• Cancellous bone on the vertebral body contains a matrix of trabeculae (vertical, horizontal, and oblique), giving a large contribution to vertebral bone strength
• Cortical shell is a thin layer of compact bone, therefore less contribution to the bone strength
• Posterior bony ring or neural arch contains two transverse processes, two pedicles, two laminae, and a spinous process
• Processes of a vertebra articulate with the vertebra below via synovial (apophyseal) joints, in a structural layout that allows small facet joint movements in different directions
• The arrangement of the trabeculae with in the vertebra may be vertical, horizontal, or oblique, corresponding with the stresses placed on the vertebra
• Vertical trabeculae help sustain body weight and resist compression forces
• The horizontal and oblique trabeculae helps resist tearing and stretching
• Trabeculae collect in area of strength, while trabeculae decrease in areas of weakness

Load Transmission Through Vertebrae

• Facets carry less of the compressive load compared to the vertebral body, but they carry >50% of shear load on the vertebra in a normal situation.
• If the load falling on spine is 500 N, then vertebral bodies carry 400 N compressive force (80% of total compressive force) and the bony rings carry 100 N compressive force (20% of total compressive force), while facets carry 250 N of shear force (50% of total shear force); the other 50% of shear force is carried by other motion segment structure

Intervertebral Disc

• The intervertebral disc is a soft, fibro-cartilaginous cushion between vertebrae
• 23 discs between 24 movable vertebra account for about 4th of spinal column's length (25-30%)
• Disc thickness varies from ~3 mm in the cervical region to ~9 mm in the lumbar region; it is a weight bearing portion that acts as a shock absorber and aids flexibility in 3 planes
• Each disc is spongy in its center (nucleus pulposus) and is surrounded by a tougher outer fibrous ring (annulus fibrosus)

Anatomy of the Intervertebral Disc

• It has two parts
• The central portion is known as the nucleus pulposus
• The peripheral portion is called annulus fibrosus
• Both are similar made up of water, collagen, and proteoglycans
• Fluid and proteoglycan concentrations are highest in the nucleus and lowest in the outer annulus, while collagen concentrations are highest in the outer annulus and lowest in the nucleus pulposus
• Fluid content varies during the day as it loses fluid when loaded continuously for a number of hours
• Fluid returns back at rest, returning discs to their original state
• People are taller in the morning than in the evening

Annulus Fibrosus

• The annulus fibrosus is a strong tire-like structure in the peripheral part of the intervertebral disc
• Collagen fibers are arranged in sheets called lamellae, and each lamella measures 1mm thick
• Lamellae are arranged in concentric rings that enclose the nucleus and keep it under constant pressure
• Collagen fibers in adjacent rings orient in opposite directions at angle 120° to each other
• The arrangement of fibers provides structural strength against compression, torsion, and other stresses that a person is subjected to

Lumbar Flexion

• During flexion, compressive force increases anteriorly on the annulus and the tensile force increases posteriorly

Nucleus Pulposus

• The nucleus pulposus is the central spongy part of the intervertebral disc, and is 80% fluid
• It resists compression, but can be reformed
• Any pressure applied at a point is equally transmitted in all directions (Pascal's law)
• It functions as a hydraulic system that can be deformed by a compressive force
• From above when weight is applied the nucleus loses height, and produces swelling pressure that expands outward towards the annulus and endplates
• It distributes pressure in all directions
• Newton's 3rd law states "for every action, there is an equal and opposite reaction“
• As a result, tensile stress is created in the annulus fibrosus in circumferential direction and a central compression loading occurs on the vertebral endplates, which is a reaction and resistance to the swelling pressure in the nucleus to reach equilibrium
• Pressure exerted on the end plate is transmitted to the superior and inferior vertebral bodies

Weight Bearing Theory

• The nucleus pulposus imbibes water, then develops internal pressure and stores energy
• The nucleus exerts pressure to distribute forces in all directions
• Lateral forces are against the annulus fibrosus
• The superior and inferior directed forces are against the endplates
• All these forces are then transmitted to the superior and inferior vertebral bodies