Variations in Musculoskeletal System

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Osseous Variations - Skeletal Surprises

  • Accessory Ossicles: Extra bones, often asymptomatic.
    • Os trigonum: Posterior talus; causes posterior ankle impingement.
    • Accessory navicular (Os tibiale externum): Medial navicular; can cause pain.
    • Os peroneum: Near cuboid, in peroneus longus tendon.
  • Sesamoid Bones: Bones within tendons.
    • Fabella: Lateral gastrocnemius head. Common.
    • Bipartite Patella: Unfused ossification; usually superolateral.
  • Coalitions: Failed bone segmentation.
    • Tarsal coalition: e.g., talocalcaneal; causes rigid flatfoot.
  • Cervical Rib: From C7 vertebra; may cause Thoracic Outlet Syndrome (TOS).
  • Spina Bifida Occulta: Mild vertebral arch defect (L5/S1); skin tuft.
  • Wormian Bones: Small, irregular bones in cranial sutures.

Os Trigonum Syndrome: Posterior ankle pain with plantarflexion, common in dancers/kickers, due to os trigonum impingement between posterior tibia and calcaneus.

Muscular Variations - Mighty Muscle Mutants

  • Definition: Deviations from typical muscle anatomy (origin, insertion, belly, number, innervation).
  • Clinical Significance: Can be asymptomatic, predispose to injury, or cause compressive neuropathies/vasculopathies.
  • Common Examples:
    • Palmaris Longus: Absent in ~15% of population (variable by ethnicity).
    • Psoas Minor: Absent in ~40-60%.
    • Sternalis Muscle: Accessory chest wall muscle; present in ~3-8%.
    • Axillary Arch (Langer's Arch): Variant muscle in axilla; can compress axillary structures.
    • Accessory Soleus: Can cause tarsal tunnel-like symptoms.
    • Peroneus Tertius: Often absent.
    • Flexor Digitorum Superficialis (FDS) of Little Finger: May be absent or fused.

Poland Syndrome: Congenital absence of pectoralis major (sternocostal head) & often ipsilateral syndactyly/brachydactyly. Unilateral, more common in males, right side.

Neurovascular MSK Variations - Nerve & Vessel Ventures

  • Nerve Plexus Variations:
    • Brachial: Prefixed (C4 input) / Postfixed (T2 input). Musculocutaneous nerve anomalies.
      • Median nerve: Variable root contributions.
      • 📌 Martin-Gruber (Median→Ulnar forearm), Riche-Cannieu (Ulnar→Median hand) anastomoses.
    • Lumbosacral: Sciatic nerve & piriformis (piercing/splitting). Accessory obturator nerve (~20%).
  • Arterial Variations:
    • Upper Limb: High origin radial artery. Persistent median artery (risk for carpal tunnel).
    • Lower Limb/Pelvis: Profunda femoris origin variations. Corona mortis (obturator-external iliac anastomosis).
  • Venous Variations:
    • Highly variable superficial veins (cephalic, basilic). Duplication of deep veins common.
  • Clinical Relevance: Impacts entrapment neuropathy diagnosis, surgical approaches (e.g., avoiding Corona Mortis), and flap viability.

⭐ The sciatic nerve typically passes inferior to the piriformis muscle; however, in up to 15-20% of individuals, it may pierce or split the muscle, predisposing to piriformis syndrome.

Clinical Significance - Variant Impact Zone

  • Diagnostic Pitfalls:
    • Mimic pathology (e.g., os acromiale vs. fracture).
    • Obscure true disease.
  • Surgical Hazards:
    • ↑ Risk of iatrogenic injury (nerves, vessels).
    • Altered landmarks: careful preoperative planning.
  • Functional Impact:
    • Altered biomechanics: ↑ injury risk (tendinopathy).
    • Accessory muscles: impingement, nerve compression.
  • Symptom Origin:
    • Direct cause of pain/dysfunction (cervical rib → TOS).
  • Radiological Interpretation:
    • Differentiate variant vs. pathology; avoid misdiagnosis.

⭐ Accessory navicular (os tibiale externum), in 10-14%, causes medial foot pain (tibialis posterior tendinopathy/irritation).

High-Yield Points - ⚡ Biggest Takeaways

  • Sprengel's Deformity: Undescended scapula, linked to Klippel-Feil syndrome and omovertebral bone.
  • Cervical Rib: May cause Thoracic Outlet Syndrome by compressing neurovascular structures.
  • Accessory Navicular: Common cause of medial foot pain, especially in young athletes.
  • Bipartite Patella: Often asymptomatic, superolateral fragment, differentiate from fracture.
  • Os Trigonum: Leads to posterior ankle impingement, seen in dancers.
  • Madelung Deformity: Characterized by volar carpal subluxation and dorsal ulnar displacement.
  • Poland Syndrome: Unilateral absence of pectoralis major, often with ipsilateral hand anomalies.

Practice Questions: Variations in Musculoskeletal System

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Tardy ulnar nerve palsy is most commonly associated with which of the following fractures of the humerus?

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Flashcards: Variations in Musculoskeletal System

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Tortuosity of the _____ artery is an anomaly present in front of the origin of the cystic duct known as caterpillar turn or Moynihans hump.

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Tortuosity of the _____ artery is an anomaly present in front of the origin of the cystic duct known as caterpillar turn or Moynihans hump.

hepatic

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