The Shoulder

Now that the spine and ribs are complete, you can finally attach the upper extremity to it. Begin with the shoulder, which really means four different joints:

  1. Sternoclavicular Joint
  2. Acromioclavicular Joint
  3. Scapulothoracic "Joint"
  4. Glenohumeral Joint

While each one of these are relatively simple, in total you are looking at about 18 different drill holes to make it work. Let's start at the most proximal and work out from there.


The Sternoclavicular Joint

On each side (left/right) of the manubrium,  drill two paths:

  • A1: runs from anterior to posterior
  • B1: runs from the superior surface of the manubrium in a posterior/inferior direction to the posterior surface, exiting above A1

The green zip ties will give you a sense of the path these two should take

The next set of holes will be through the proximal end of the clavicle:

  • A2: from anterior to posterior (and a bit inferior)
  • B2: from the articulating surface of the head of the clavicle, running laterally toward the posterior surface of the clavicle

Again, the green zip ties provide a sense of the location and orientation of the drill holes. Hold off on attaching anything just yet as it will be easier to do the following steps without the clavicle affixed to the sternum.


The Acromioclavicular Joint

Click to enlarge

There are a couple steps to provide the necessary stability here.

Drill two holes each in the distal end of the clavicle, and the adjoining section of the scapula:

Click to enlarge

  • A1: distal clavicle
  • B1: distal clavical
  • A2: scapula side of AC joint
  • B2: scapula side of AC joint

Bind them together with some cord.

With the bones joined, you can now better visualize where the clavicle sits over the coracoid. Drill two more holes through the clavicle so that they equally straddle the coracoid process, as well as one through the base of the coracoid itself. By binding these together with a zip-tie (here in yellow) the  coracoclavicular ligament will be simulated. Without this in place, the AC joint would be far too unstable.

  • C: at the "bend" of the clavicle
  • D: just distal to it on the other side of the coracoid
  • E: through the base of the coracoid

Scapulothoracic

Click to enlarge.

Stabilizing the scapula against the rib cage can help with setting the posture of the model's shoulder girdle, though some anterior tilt seems inevitable.

Begin with a drill hole along the spine of the scapula at (A). Anchor a bungee to (A), and then to the bungee that traverses the transverse foramen of the cervical spine. By adjusting the tension you can help to improve the elevation of the shoulder girdle. This is largely following the line of pull of the upper trap.

The pull of the pec minor can be mimicked with two drill holes:

Click to enlarge

  • B: through the coracoid process
  • C: through the third rib

 


The Glenohumeral Joint

Two cords will bind together the glenohumeral joint, which means you will have 4 holes to drill in total. The holes should be roughly at right angles to each other. One of the trickiest aspects at this stage is making sure that they do not intersect each other midway, which makes threading the bungee cords difficult, if not impossible. 

On the humerus, drill A1 and B1 at right angles to each other near the anatomical neck of the humerus. On the scapula, drill A2 and B2 at right angles to each other in such as a way as to match up with the humerus in the anatomical position. 


At this stage, tie the untying the glenohumeral joint in order to facilitate the elbow and wrist. But before you can move on to that phase, it's best to complete the hand.

Which will be next time...

 

Anatomical Sketch:

By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body, Gray's Anatomy, Plate 411, Public Domain