- Discussion: 
   - AC joint is situated between the clavicle and acromion; 
   - acromion has two ossification centers which fuse at age 22 yrs; 
   - it permits motion in three planes:
           - AP gliding of acromion during protraction & retraction of scapula;
           - tilting of acromion during abduction & adduction of arm;
           - rotation of the clavicle;
           - rotation occurs during abduction & adduction of shoulder.

- Anatomy: 
    - innervation: provided by the suprascapular and lateral pectoral nerves
    - ref: The suprascapular nerve and its articular branch to the acromioclavicular joint: an anatomic study
    - joint is reinforced by two sets of ligaments:
         - AC ligament
                - directed horizontally, and functionally the AC joints control horizontal stability;
                - palpable shallow depression between end of clavicle & acromion;
                - superior AC lig is most important ligament in stabilizing AC joint for normal daily activities;
         - coracoclavicular ligaments:
                - stronger, vertically directed contains conoid and trapezoid ligaments help to control vertical stability;
                - coracoclavicular lig are suspensory ligaments of upper limb;
                - conoid:
                        - is the most important ligament for support of the joint against significant injuries and superior displacement;
                        - cone shaped which extends between the conoid tubercle on the posterior clavicle and the base of the coracoid;
                - trapezoid:
                        - resists AC joint compression;
                        - begins anteriorly and laterally to the conoid ligament on the clavicle and inserts on the coracoid process; 
         - reference: Biomechanical study of the ligamentous system of the acromioclavicular joint. 

    - Sternoclavicular joint:
         - see S.C. joint injury in the adolescent;
         - inherently more stable than AC joint; because of this stability & its more protected medial location; 
         - it is injured less frequently than the acromioclavicular joint.

- Management of Specific Injuries:
    - AC joint arthrosis / distal clavicle excision 
          - The influence of distal clavicle resection and rotator cuff repair on the effectiveness of anterior acromioplasty. 
    - AC joint septic arthritis:
         - Septic arthritis of the acromioclavicular joint - a report of four cases
         - Septic arthritis of the acromioclavicular joint.
         - Sonographic detection, evaluation and aspiration of infected acromioclavicular joints.
         - Primary septic arthritis of the acromio-clavicular joint: case report and review of literature  
    - AC Joint Separation

- Exam: 
    - palpate the AC joint during flexion and extension of shoulder; 
    - distract the arm as it is placed in adduction
    - significant prominence of the distal clavicle indicates unstable AC injury; 
    - BvR test for DJD: resisted shoulder upward flexion with arm hyperadducted; 
    - ref: Clinical evaluation of acromioclavicular joint pathology: sensitivity of a new test

- Radiology:
    - classification of AC separation 
    - acromioclavicular joint stresses views 
           - grade I injuries remain nondisplaced; 
           - type I and type II injuries can be differentiated on stress radiographs;
           - w/ pt standing, 10 lb weight is secured to affected upper limb;
           - w/ grade II injury, suspended  wt displaces AC joint articulation, which increases distance between clavicle & acromion; 
    - zanca view
    - scapular outlet view
    - reference: Radiological evaluation of the acromioclavicular joint. 

The acromioclavicular joint in rheumatoid arthritis.

Osteolysis of the distal part of the clavicle in male athletes.