Basic Biomechanics of Lumbar Spine & TMJ

Introduction to Spine Biomechanics

• Primary functions of the human spine
  • Protect the spinal cord
  • Transfer and dissipate loads between head, trunk and pelvis
  • Permit multiplanar movement while providing intrinsic & extrinsic stability
• Stability contributors
  • Intrinsic: intervertebral discs, facet joints, vertebral end-plates, spinal ligaments
  • Extrinsic: trunk muscles (erector spinae, abdominals, quadratus lumborum, pelvic floor, diaphragm)
• Normal sagittal curvatures (cervical lordosis – thoracic kyphosis – lumbar lordosis) act as a spring, increasing load-bearing capacity compared with a straight column

Functional Spinal Unit (Motion Segment)

• Definition: smallest biomechanical unit that exhibits characteristics of the entire spine
• Constituents
  • 2 adjacent vertebrae (superior & inferior)
  • Intervertebral disc (IVD)
  • Longitudinal ligaments (anterior & posterior)
  • Facet (zygapophyseal) joints with joint capsule
  • Transverse & spinous processes with associated ligaments
• Motion characteristics
  • 6 degrees of freedom: \pm rotation & translation about transverse (x), sagittal (y) and longitudinal (z) axes
  • Typical lumbar segment primary (coupled) motion 4^{\circ} – 6^{\circ}; accessory motion 2^{\circ} – 3^{\circ}
  • Instant centre of rotation (Reuleaux method): migrates posteriorly during flexion, anteriorly during extension → load transferred from facets to disc and back

Anterior Portion: Vertebral Bodies & End-plates

• Vertebral bodies withstand mainly compressive forces; cross-sectional area enlarges caudally to support rising loads
• Superior end-plate thinner and weaker than inferior; fails first under axial compression → end-plate fracture redistributes load (↓ pressure in nucleus pulposus, ↑ stress on posterior annulus fibrosus)

Intervertebral Disc (IVD) Biomechanics

• Nucleus pulposus (NP)
  • Gel–like, high glycosaminoglycan (GAG) & water content in youth; progressive desiccation with age
  • Slightly posterior to disc center in lumbar levels
  • Avascular; nutrition by diffusion ⇒ cyclic loading/unloading essential; sustained compression impairs diffusion
• Annulus fibrosus (AF)
  • Concentric fibrocartilaginous lamellae oriented alternately \pm 30^{\circ} to horizontal; resists bending & torsion
  • Degenerative annular tears ↑ rotational laxity and moments ⇒ segmental instability
• Mechanical loading patterns
  • Flexion/extension & lateral flexion → combined tensile (contralateral) & compressive (ipsilateral) stresses
  • Axial rotation & translation → shear stresses
  • Pure axial compression → disc bulge + circumferential tensile stress in AF (can reach \approx 5 \times applied compressive load)
  • Thoracic discs have larger diameter-to-height ratio ⇒ lower circumferential stress
• Degeneration cascade
  • ↓ proteoglycans → ↓ hydration → ↓ elasticity & load distribution; ↑ facet loading; risk of herniation

Posterior Portion: Facet Joints & Posterior Ligamentous Complex

• Facet orientation dictates permitted motion
  • Cervical: 45^{\circ} to transverse plane, parallel to frontal ⇒ favors rotation & flexion/extension
  • Thoracic: 60^{\circ} transverse, 20^{\circ} frontal ⇒ permits rotation, limits flexion/extension
  • Lumbar: 90^{\circ} transverse, 45^{\circ} frontal ⇒ resists rotation, allows flexion/extension
• Ligaments (high collagen limits extensibility)
  • Anterior/Posterior longitudinal, ligamentum flavum, interspinous, supraspinous, intertransverse, facet capsular
  • Ligamentum flavum under constant tension (prestress) due to posterior location; hypertrophies with degeneration, spondylolisthesis, osteophytes → canal narrowing

Kinematics of the Whole Spine

• Segmental motion adds to provide functional range (e.g., lumbar flexion \approx 40^{\circ} derives from 5–7^{\circ} per motion segment × 6 lumbar segments)
• Coupled motions: lateral flexion often linked with axial rotation (direction depends on region & posture)

Lumbar Spine Kinetics: Posture & Loading

• Static compressive load at L3-L4 during relaxed upright standing ≈ 2 \times weight of body superior to that level
• Loads vary with posture (values relative to relaxed standing =100\%)
  • Well-supported reclining \approx 20–48\% (during night rest)
  • Relaxed sitting w/o backrest 92\%
  • Active erect sitting 110\%
  • Maximum trunk flexion sitting 166\%
  • Standing bent forward 220\%
  • Lifting 20 kg with round back 460\% vs. back-school technique 340\% vs. holding same mass close 220\% vs. at 60 cm reach 360\%
• Influencing factors
  1. Object position relative to lumbar motion centre
  2. Object size, shape, mass, density
  3. Spinal flexion/rotation magnitude
  4. Rate of loading (dynamic vs. static)

Muscle Activity & Flexion–Relaxation Phenomenon

• Flexion-relaxation: at end-range trunk flexion superficial erector spinae EMG falls silent; deep fibers + quadratus lumborum maintain passive tension
• Forced or prolonged flexion → superficial erectors reactivate; viscoelastic creep lowers stiffness, increases ROM, impairs sensorimotor control
• Sit-to-stand & active sitting require timely trunk muscle recruitment to protect spine

Pelvic Mechanics & Standing Posture

• Sacroiliac joint studied using new tri-axial description; early force studies invasive ⇒ limited replication today
• Maintenance of upright posture relies on continuous, low-level neuromuscular feedback correcting minute perturbations
• Altering sacral angle modifies lumbar lordosis & thoracic kyphosis to balance head over pelvis with minimal muscle effort

Loading During Gait & Everyday Activities

• Walking
  • Normal speed: peak spinal compression \approx 1.8 \times BW (between heel strike & toe-off)
  • Very fast gait: up to 2.5 \times BW
  • Limited arm swing increases compressive load; forward trunk flexion raises loads
• Jogging (tennis shoes on hard surface): 70–190\% of relaxed standing disc pressure
• Stair negotiation: 60–240\% depending on ascending/descending and step-at-a-time vs. two-at-a-time
• Walking considered low-load, highly scalable exercise to promote disc nutrition & muscular endurance

Exercise Prescription Considerations

• Strengthening of erectors & abdominals can generate high spinal loads; must match individual capacity & rehab goals
• Emphasis on neutral spine control, graduated loading, avoidance of end-range flexion under heavy resistance in degenerative discs

Mechanical Stability & Surgical Instrumentation

• Fusion or instrumentation changes segment stiffness; may increase motion & stress at adjacent levels → risk of Adjacent Segment Disease (ASD)
• Stability should be evaluated regionally, not only at operated level

Intra-Abdominal Pressure (IAP) & Trunk Co-contraction

• Synergistic contraction of diaphragm, pelvic floor & abdominal wall creates a “pressurized balloon” enhancing spinal stiffness
• Expected loading: pre-activation elevates IAP → controls flexion moment; unexpected loading without warning ↑ peak muscle response by \approx 70\% (esp. in flexed posture)
• External devices
  • Back belts aim to augment IAP yet evidence inconclusive; NIOSH does not recommend for injury prevention
  • Rigid orthoses may limit motion but cause disuse atrophy
  • Industrial exoskeletons under investigation to off-load lumbar spine during manual tasks

Temporomandibular Joint (TMJ) Biomechanics (overview)

• Bilateral synovial joints between mandibular condyles & temporal bone articular eminence
• Functional anatomy: fibrocartilaginous disc, retrodiscal tissue, capsule, lateral/medial collateral ligaments, muscles of mastication (masseter, temporalis, medial & lateral pterygoid)
• Kinematics
  • Depression/elevation: early rotation (hinge) around transverse axis, late anterior translation of condyle-disc complex
  • Protrusion/retrusion: pure translation along articular eminence
  • Lateral excursion: ipsilateral condyle pivots, contralateral condyle translates forward/down
• ROM norms: depression 40–50\text{ mm}, protrusion 6–9\text{ mm}, lateral excursion 8–12\text{ mm}
• Mandibular “gait” involves cyclical opening/closing with coordinated muscle firing; dysfunction alters disc translation & loading

Key Clinical & Ethical Implications

• Prolonged static flexion postures (e.g., sitting, forward bending occupations) foster disc creep & instability → ergonomic redesign & movement breaks imperative
• Degenerative changes shift loads posteriorly → facet arthropathy, stenosis; early detection guides preventative strengthening & posture education
• Surgical decisions must weigh adjacent level biomechanics; over-zealous fusion can precipitate new pathology
• Industrial policies on back-belt use should rely on evidence; misplaced reliance may give false security → higher injury risk

Summary

• Motion segment = 2 vertebrae + disc; discs provide hydrostatic load distribution, facets guide motion
• Segmental ROM small; functional movements require multi-segment contribution
• Trunk muscles supply extrinsic stability; ligaments & discs intrinsic
• Any departure from upright neutral increases lumbar disc load; keep lifted object close, minimize trunk flexion, engage IAP and balanced co-contraction
• Walking offers low-load, disc-nourishing exercise adaptable for most populations; progressive, fatigue-aware protocols advised