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- Anatomy:
    - three posterior divsions of brachial plexus form posterior cord: (C5 , C6 , C7, C8, T1)
        - upper subscapular nerve to subscapularis
        - thoracodorsal to latissimus dorsi
        - lower subscapular nerve to subscapularis  and teres major
        - axillary nerve
        - radial nerve (C5, C6, C7 , C8 , and T1 ) is largest & most frequently injured branch of brachial plexus.

- Injury:
    - injury to the brachial plexus below the clavicle usually involves cords cords or peripheral nerves;
    - damage to the posterior cord weakens elbow, wrist, and MP joint extension  (radial nerve), and shoulder abduction (from axillary nerve)

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Injury to the brachial plexus by a fragment of bone after fracture of the clavicle.