Sacral Plexus, Sciatic Nerve & Lumbar ROM Study Notes

Sacral Plexus Overview

• Formed by ventral rami L4\,-\,S4 (sometimes noted L4\,-\,S3 for the large branches)
• Located on the anterior surface of the piriformis; appears as a «big circle of bundled nerves» in gross-lab images
• Gives rise to two especially large peripheral nerves:
  • Sciatic nerve – the single largest/longest nerve in the body
  • Pudendal nerve – key sensory/motor supply to perineum
• Ligamentous landmarks
  • Sacrotuberous ligament crosses the greater sciatic notch, subdividing it into the greater & lesser sciatic foramina

Sciatic Nerve: Origin, Course, Branches

• Roots: contributions L4 \rightarrow S3
• Exit: passes through the greater sciatic foramen
• Typical relationship to muscles
  • Inferior (below) the piriformis in ~85–90 % of individuals
  • Deep (anterior) to gluteus maximus
• Composed of TWO components traveling together within a common sheath until mid-thigh:
  1. Tibial nerve (medial division)
  2. Common fibular (peroneal) nerve (lateral division)
• Point of split varies; clinically still labelled “sciatic” until the fibers diverge

Pudendal Nerve (brief)

• Arises mainly from S2\,-\,S4
• Courses inferiorly into the lesser sciatic foramen to supply pelvic floor & perineum

Anatomical Variations & Piriformis Syndrome

• Classic anatomy (Diagram A): sciatic nerve entirely inferior to piriformis
• Variation B: common fibular portion pierces the piriformis, tibial division passes inferior → potential compression when muscle contracts
• Variation C: part of sciatic nerve travels superior to piriformis; tension can develop with hip motions (e.g.
  as hip flexes)
• Piriformis syndrome
  • Peripheral entrapment neuropathy
  • Symptoms: unilateral numbness/tingling radiating buttock → knee/foot
  • Exacerbated by piriformis contraction or lengthening (stretch)

Sciatica vs Piriformis Syndrome (Central vs Peripheral)

Clinical Reasoning: “Rule the Spine Out First”

• Central lesions jeopardize reflexes, motor & sensory supply broadly → higher stakes (possible progression to cord/disc degeneration)
• Examination sequence:
  1. Screen cervical/lumbar spine (especially with any neurovascular signs)
  2. If negative, pursue peripheral source (e.g. piriformis, pes planus → knee pain, etc.)
• Parallels trauma protocol for C-spine clearance before treating extremity injuries

Superior & Inferior Gluteal Nerves

• Both emerge from sacral plexus, remain entirely within gluteal region (do not continue down the limb)
• Superior Gluteal Nerve
  • Roots L4\,-\,S1
  • Passes through greater sciatic foramen superior to piriformis
  • Travels laterally between gluteus medius & minimus
  • Motor supply: gluteus medius, minimus, tensor fasciae latae, (some texts add portions of deep rotators)
• Inferior Gluteal Nerve
  • Roots L5\,-\,S2
  • Passes through foramen inferior to piriformis, superficial to sciatic nerve, deep to inferior gluteus maximus fibers
  • Runs with inferior gluteal artery
  • Sole motor supply: gluteus maximus

Peripheral vs Central Terminology

• Nerve root = central nervous system structure → compression = radiculopathy (e.g. classic sciatica)
• Named peripheral nerve distal to plexus = peripheral nervous system → entrapment = neuropathy (e.g. piriformis syndrome, carpal tunnel)

Lumbar Spine Range of Motion (AAOS Norms)

(Values in ^{\circ}; memorize red-flagged AAOS for exam)

• Flexion: 80^{\circ}
• Extension: 25^{\circ}
• Lateral flexion (side-bend): 35^{\circ} each side
• Rotation: 45^{\circ} each side
  Other authors (Floyd, Schultz) report differing normals (e.g. Schultz flexion 50^{\circ}; extension 30–40°) → illustrates variability & sport-specific adaptations (e.g. baseball pitcher ↑ER ↓IR)

ROM Testing: Active vs Passive vs Resistive & Gravity Debate

• Standing trunk flexion
  • Gravity assists movement → some clinicians label it “passive-like” despite patient initiating (active-assist)
  • Stabilizing musculature: quads, hamstrings, calves maintain balance; rectus abdominis minimally active
• Supine hook-lying or long-sitting double-leg raise for lumbar flexion removes gravity assist → greater demand on abdominals (clear active test)
• Lumbar extension
  • Standing back-bend: gravity assists descent
  • Prone “Superman” lift: against gravity; erector spinae actively contract
• Lateral flexion
  • Standing side-reach: down-phase assisted by gravity; up-phase requires contralateral obliques & QL
  • Document position, side tested, and whether against gravity or with gravity assist (important for muscle-grade reliability)
• Resistive ROM: manual pressure applied at end-range (e.g. hand on shoulder during side-bend) to gauge strength

Distinguishing Lumbar vs Hip Motion during Forward Bend

• Clinician observes spinal curve changes:
  1. Normal: smooth lumbar flexion then anterior pelvic rotation (hip flexion)
  2. Limited hip: early lumbar rounding, pelvis immobile → excessive lumbar strain
  3. Limited lumbar: pelvis hinges, spine remains neutral → may indicate stiff lumbar segments
• Supine/prone positions isolate lumbar motion by fixing hips (e.g. pillows under abdomen) or vice-versa

Facet Joint Kinematics

• Flexion: inferior articular facets glide superior/anterior → joint spaces OPEN
• Extension: facets glide inferior/posterior → CLOSE
• Lateral flexion (right)
  • Right (ipsilateral) facets approximate (close)
  • Left (contralateral) facets separate (open)
• Rotation couples variably with lateral flexion depending on region; thoracic motion limited by rib cage, cervical & lumbar allow larger excursion

Thoracic vs Lumbar vs Cervical Motion Highlights

• Thoracic: rib cage restricts flex/extend; rotation moderate; facets oriented in coronal plane
• Lumbar: greatest flex/extend; rotation limited due to sagittal-plane facet orientation
• Cervical: large flex/extend & rotation; facets in 45° plane encouraging mobility

Practical / Ethical Implications for Clinicians

• Mislabeling piriformis syndrome as “sciatica” can direct treatment to spine, delaying relief
• Importance of imaging & thorough neuro exam to avoid missing central pathology before peripheral treatment
• Recognize individual anatomical variants; avoid over-reliance on textbook «normal» (e.g. athlete-specific ROM)

Classroom & Lab Tips Mentioned

• Video recording of demonstrations; double-check availability for review
• Expect quizzing on AAOS numbers and gravity-influenced ROM definitions
• Faculty (Dr. Viro/Byra) may challenge students to defend active vs passive labeling – prepare rationale