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Mechanics and Pathomechanics of the Lumbosacral Spine

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1. Functional Anatomy of the Motion Segment

Structural Components

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2. The Intervertebral Disc

• Annulus fibrosus - outer ring of collagen surrounding fibrocartilage arranged in a lamellar (concentric laminar) configuration. Fibers in alternate lamellae run at roughly 30 degrees in opposite directions. This arrangement limits rotation and resists tensile and shear stresses. Tensile stresses are highest here.
• Nucleus pulposus - central gelatinous mass, highly hydrated (up to 80% water in youth). Absorbs compressive loads and distributes them uniformly as hydrostatic pressure to the annulus and end plates. Compressive stresses are highest here.

• Creep: deforms progressively under a sustained load (explains why we are slightly shorter at the end of the day)
• Hysteresis: absorbs energy during repeated axial loading (acts as a shock absorber), but this capacity decreases with repeated loading

• Lowest when lying supine
• Higher when standing
• Highest when sitting
• Increases further with bending and torsional stresses
• Loads are lowest when carried objects are close to the body

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3. Lumbar Facet (Zygapophysial) Joints

• 90 degrees to the transverse plane
• 50 degrees to the frontal plane ("wrapped" configuration)
• They progressively tilt upward (transverse) and inward (frontal) from L1 to L5
• This orientation permits flexion/extension and some lateral bending but severely limits axial rotation

• Facets contribute 40%
• Disc contributes 40%
• Ligamentous structures contribute 20%

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4. Range of Motion - Lumbar Spine

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5. Sagittal Alignment

• Normal lumbar lordosis: 20-80 degrees (average ~60 degrees)
• Lordosis is created primarily by disc height and shape (wedge-shaped discs), not the vertebral bodies themselves
• Most lordosis occurs between L4 and S1
• The sacral base angle is approximately 30 degrees to the horizontal; the lumbosacral angle is approximately 140 degrees

1. Shifts load posteriorly to the facets under extension loading
2. Acts as a spring to absorb and distribute compressive forces

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6. Ligamentous Support

• Anterior longitudinal ligament (ALL) - strong, broad; resists hyperextension and anterior disc herniation
• Posterior longitudinal ligament (PLL) - narrower, weaker posteriorly; resists flexion and posterior disc herniation (but its weakness posterolaterally is why most disc herniations occur in that direction)

• Ligamentum flavum (yellow ligament) - high elastin content; resists flexion; thickens with degeneration and contributes to canal stenosis
• Interspinous and supraspinous ligaments - resist flexion
• Capsular ligaments of facets - resist flexion; cervical disc injury commonly compromises these

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7. Pathomechanics

A. Disc Herniation

1. Repeated torsional loading (shear forces) separates the nucleus pulposus from the annulus and end plate
2. A tear develops in the annulus fibrosus (begins in the innermost lamellae and extends outward)
3. Nuclear material is forced through the annular tear, particularly posterolaterally (the PLL is thinner and weaker here)
4. The herniated material compresses adjacent nerve roots or the dural sac

• L4-L5 herniation - typically compresses the L5 nerve root (foot drop, weak dorsiflexion)
• L5-S1 herniation - typically compresses the S1 nerve root (diminished ankle jerk, weak plantarflexion)
• Central large herniation - can compress the cauda equina (cauda equina syndrome - a surgical emergency)

B. Lumbar Spinal Stenosis

• Disc degeneration causes loss of disc height
• Loss of height shifts load toward facet joints (normally facets bear ~20% of axial load; with disc degeneration this increases)
• Facet joint arthropathy, osteophyte formation, and hypertrophy of ligamentum flavum follow
• Central canal, lateral recess, or foraminal stenosis results
• Neurogenic claudication: symptoms (buttock/leg pain, heaviness, paresthesias) reproduced by extension (canal narrows in extension) and relieved by flexion (canal widens) or sitting

C. Spondylolysis and Spondylolisthesis

• Spondylolysis is a stress fracture of the pars interarticularis (isthmus of the neural arch between superior and inferior facets)
• Mechanism: repetitive hyperextension with axial loading (e.g., gymnastics, fast bowling, weight lifting) - the L5 pars is the most vulnerable
• Fatigue loading (repetitive minor stresses) leads to pars fractures; this is distinct from acute traumatic fracture
• If bilateral spondylolysis disrupts the posterior arch, the anterior vertebral body can slip forward = spondylolisthesis
• Most common at L5-S1; forward slip (anterolisthesis) of L5 on S1

• Grade I: 0-25% slip
• Grade II: 25-50% slip
• Grade III: 50-75% slip
• Grade IV: 75-100% slip

D. Instability and Flatback Syndrome

• Iatrogenic flatback syndrome results from distraction forces applied to the lumbar spine (e.g., over-distraction during fusion instrumentation)
• Loss of normal lordosis increases anterior moment arm, increasing compressive load on discs and facets
• Translation in the lumbar spine: normal is < 2 mm; > 4 mm suggests clinical instability

E. Osteoporotic Fractures

• Vertebral body strength correlates with bone mineral content
• Osteoporosis causes loss of horizontal trabeculae (vertical trabeculae remain intact), dramatically reducing compressive strength
• Compression fractures typically occur at the vertebral end plate (the weakest zone mechanically)
• Load concentration at the anterior vertebral body in flexion makes anterior wedge fractures most common in the thoracolumbar junction (T12-L1)

F. Degenerative Disc Disease - Mechanical Cascade

1. Dysfunction phase - disc dehydration and early annular tears; minor segmental hypermobility
2. Instability phase - progressive disc collapse, facet capsule laxity, segmental instability with aberrant motion
3. Stabilization/restabilization phase - osteophyte bridging, disc fibrosis, and spontaneous stabilization (often associated with reduced pain)

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8. Summary Table: Key Mechanical Principles

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Mechanics and Pathomechanics of Soft Tissue

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1. Fundamental Mechanical Concepts

Kinematics vs. Kinetics

• Kinematics = study of motion without regard to forces (position, displacement, velocity, acceleration)
• Kinetics = study of forces that cause motion (torques, loads, moments)
• In human movement, most motion is a combination of translation (rectilinear or curvilinear) and rotation, described as general motion

Degrees of Freedom

• A rigid body in 3-D space has 6 degrees of freedom: 3 translations (x, y, z) and 3 rotations (about each axis)
• Joint motion in clinical practice is usually simplified to describe the dominant angular displacement (flexion/extension, abduction/adduction, rotation)

Core Mechanical Concepts for Soft Tissue

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2. Tendon

Structure and Composition

• Detect strain through deflection of cell cilia
• Synthesize ECM, collagen, and proteoglycans
• Produce type III collagen in response to rupture
• Produce MMPs (matrix metalloproteinases) for remodeling

Mechanical Properties

• Anisotropic - properties vary with direction of load (strongest along fiber axis)
• Viscoelastic - properties vary with rate of load application

Stress
  |                            Failure
  |                           /
  |              Linear      /
  |             /region     /  
  |            /           /  
  | Toe       /           /  
  | region   /           /  
  |_________/___________/_____ Strain
  |   0-2%   2-4%      4-8%+

1. Toe region (0-2% strain) - collagen fibers are normally "crimped" (wavy). Load straightens the crimp. Nonlinear, low stiffness. Elastin is responsible for this region. Normal physiologic loading occurs here.
2. Linear region (2-4% strain) - collagen fibers are fully recruited and bear load. Stiffness is high and constant. Molecular cross-links are stressed.
3. Yield/Plastic region (4-8% strain) - irreversible micro-failure begins. Fibers rupture progressively. Tissue does not return to original length on unloading.
4. Failure region (>8% strain) - catastrophic rupture. Ultimate tensile strength of tendon: 50-100 MPa.

Tendon Junctions

• Actin microfilaments from the last Z-line link via sarcolemma to tendon collagen fibrils
• Site of maximum stress concentration during eccentric loading
• Most common site of muscle strains and tears

Tendon Blood Supply and Vulnerability

• Paratenon-covered tendons (Achilles, patellar): higher vascularity, better healing
• Sheathed tendons (digital flexors): blood supply via vincula; some nutrition from synovial diffusion
• Critical hypovascular zone: Achilles tendon is hypovascular 4-6 cm proximal to the calcaneal insertion - this is the site of most Achilles ruptures

Tendon Healing (3 Stages)

• Weakest at 7-10 days post-repair
• Maximum strength at 6 months = two-thirds of original strength
• Early controlled motion and mechanical loading have beneficial effects on tenocyte function
• Immobilization decreases strength at the tendon-bone interface

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3. Ligament

Structure and Composition

Ligament Injury Patterns

• Ligaments "break, not bend" - they do not plastically deform; they fail at their yield point
• Adults: midsubstance tears are more common
• Children: avulsion injuries are more common - failure typically occurs between unmineralized and mineralized fibrocartilage at the enthesis

Ligament Healing (3 Stages)

• Intraarticular injury (ACL heals poorly vs. MCL)
• Old age, smoking, NSAID use, diabetes mellitus, alcohol use
• Local corticosteroid injections

• Extraarticular injury
• IL-10, IL-1 receptor antagonists
• Mesenchymal stem cells
• Collagen-PRP hydrogel scaffolds

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4. Muscle

Structure

• Endomysium surrounds individual myofibers
• Perimysium surrounds muscle fascicles (groups of hundreds of fibers)
• Epimysium surrounds the entire muscle
• Sarcomere is the contractile unit (from Z-line to Z-line)

Types of Muscle Contraction

Muscle Fiber Types

Neuromuscular Stretch Receptors

• Muscle spindles (intrafusal fibers): detect muscle length; transmit to CNS; control muscle stiffness via the gamma motor system
• Golgi tendon organs (GTOs): at the myotendinous junction; detect tension; inhibit contraction to prevent excessive tendon lengthening (autogenic inhibition)

Pathomechanics: Muscle Strains

• Most common sports injury
• Occur primarily at the myotendinous junction (maximum stress concentration)
• Most common in two-joint muscles (hamstring, gastrocnemius, rectus femoris) with higher type II fiber content
• Precipitated by rapid eccentric contraction (highest force generation at longest length = maximum injury risk)

1. Inflammation → fibrosis mediated by TGF-β
2. Satellite cells act as stem cells - primary mechanism of muscle fiber regeneration
3. Defect can also heal by bridging scar tissue (less functional)

Delayed Onset Muscle Soreness (DOMS)

• Occurs 24-72 hours after unaccustomed eccentric exercise
• Most common in type IIB fibers
• Mechanism: edema and inflammation in connective tissue; neutrophilic response; changes in the I-band of the sarcomere
• Treatment: NSAIDs relieve DOMS in a dose-dependent manner; other modalities (ice, ultrasound, electrical stimulation) have not been shown to affect DOMS

Immobilization Effects

• Atrophy results from disuse or altered recruitment
• Single-joint muscles atrophy faster than multi-joint muscles
• Sarcomeres at the myotendinous junction are especially affected
• Immobilization in lengthened positions decreases contractures and maintains strength better
• Electrical stimulation can offset atrophic effects

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5. Articular Cartilage

Composition and Mechanical Role

Mechanical Behavior

Pathomechanics: Osteoarthritis

1. Proteoglycan loss → increased water content initially (cartilage appears "soft")
2. Collagen framework disruption → fibrillation of the surface (earliest visible change)
3. Loss of hydraulic pressurization → direct collagen-collagen contact; abrasive wear
4. Chondrocyte death → reduced matrix synthesis; failure of repair mechanisms
5. Aging effect: decreased water content of cartilage, reduced proteoglycan core proteins → reduced resiliency during joint loading

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6. Properties Comparison Summary

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7. Key Pathomechanical Principles - Clinical Summary

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