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twelfth rib

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The twelfth rib is classified as one of the 'floating' group. Posteriorly, it articulates with the twelfth thoracic vertebra. Anteriorly, its free end is ensheathed by muscle and layers of lumbar fascia.

Key features include:

  • head has a single facet for synovial joint articulation with the body of the T12 vertbra
  • very short neck
  • lack of tubercle, angle and costal groove
  • anterior end tapers to a point, unlike superior ribs which remain at constant width or broaden
  • internal surface faces internally and slightly superiorly:
    • superior margin of neck has attachment for costotransverse ligmanent
    • posterior two thirds of shaft on superior edge has attachment for internal intercostal muscles and makes contact with pleura
    • anterior third of shaft on superior margin has attachment sites for anterior diaphragmatic slips and fibres of transversus abdominis
    • posterior two thirds of shaft on lower, medial margin is attached to quadratus lumborum
    • slightly superior to quadratus lumborum originate fibres of the lateral arcuate ligament
  • external surface faces externally and slightly inferiorly:
    • erector spinae attaches to posterior third of shaft on inferior margin
    • slightly anterior to the erector spinae attachment is a small site of attachment for fibres of serratus posterior inferior
    • on the inferior margin of the end of the rib resides the site of attachment for external oblique
    • latissimus dorsi arises from superior margin and it is anterior to the attachments of the external intercostals and levator costae
 
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