The Shoulder: Bony Anatomy, Part 2

In part one we looked at the bony anatomy of the scapula, the first of three bones that make up the shoulder.  In part 2 we will look at the clavicle and humerus.

Let’s start with the clavicle, or collar bone.  The clavicle is the first bone to start ossifying (hardening) in the human fetus at about 6 weeks.  Interestingly enough it is also the last to fully develop, this doesn’t usually happen until the late teens or early twenties.  This combined with the fact that the clavicle is quite superficial makes it one of the most frequently broken bones in the body.  When we’re talking about the clavicle it’s broken down into thirds: the lateral third (articulates with the acromion, also called the acromial end), the middle third and the medial third (articulates with the sternum, called the sternal end).  The clavicle is pretty simple, it does have a few landmarks but I won’t mention them here.  

A view of the clavicle from above.

The humerus has a bit more going on than the clavicle, and for our purposes we’ll just look at the proximal half for now (the half that’s closest to the shoulder).   A review of scapular anatomy might be helpful, you can find that here.  

A view of the right humerus from the front.

A view of the right humerus from the back.

THE HUMERUS

Head of the humerus - the head of the humerus sits in the glenoid fossa like a golf ball on a tee.  The head of the humerus is the golf ball, the shallow glenoid fossa is the tee.  This creates the glenohumeral joint, which we will get to in another post that covers joints and other structures.  

Greater tubercle - this is the attachment site for three of the four rotator cuff muscles.  It’s a bony knob on the humeral head that sits a little bit posterior (behind) and lateral (away from the midline) to its counterpart, the lesser tubercle.  

Lesser Tubercle -  a smaller bony knob that sits just medial (closer to the midline) and anterior (in front of) to the greater tubercle.  This is the insertion point for the only muscle of the rotator cuff that doesn’t insert on the greater tubercle.

Crest of greater tubercle - Crest is a Latin term meaning ridge.  It’s just distal to the greater tubercle.

Crest of the lesser tubercle - The ridge that comes down from the lesser tubercle, sits medial to the intertubercular groove.

Intertubercular groove -  this lives in between the crests of the greater and lesser tubercles and is also called the bicipital groove, as the biceps brachii muscle feeds through here.  You can imagine the crests as the peaks and the intertubercular groove as the valley.  Intertubercular means in between the grooves.  A lot of times a part of the body will tell you where it is, how big or small it is, what it does or some combination of the three based on its name.  

Deltoid tuberosity -  this is as far down the humerus as we’ll go for now.  The deltoid tuberosity is found about halfway down the humerus, it’s a little bony knob where the all the fibers of the deltoid insert.  It’s on the lateral humerus (outside).  

Familiarize yourself with this information and it will be easier to understand the muscles and what they do (and how) once we start talking about them. 

The Shoulder: Bony Anatomy, Part 1

The shoulder is made up of three bones and two joints.  This blog will break down the anatomy of the scapula.  Part two will cover the clavicle and humerus.  

FIRST… 

A fossa is a scooped out hollow or depression in the bone.

Process just means bony projection.

A tubercle is a small rounded projection on a bone.

THE SCAPULA

Lateral Border - the outer border of the scapula, away from the midline

Medial Border - the inner border of the scapula, closest to the spine

Inferior Angle - the very bottom tip of the shoulder blade 

Superior Angle - this is not the entire top of the scapula but rather just the tip top of the medial border

Spine of the scapula - bony ridge that separates the supraspinous fossa from the infraspinous fossa. The lateral end flattens out to become the...

Acromion - flat, lateral aspect of the spine of the scapula.  

Supraspinous Fossa - supraspinous means above (supra- superior- above) the spine (spinous) of the scapula. It’s smaller than the infraspinous fossa but it’s pretty deep. The supraspinatus muscle originates here. 

Infraspinous Fossa - large scooped out hollow beneath the spine of the scapula where the infraspinatus originates.

Subscapular Fossa - this is on the anterior (front) surface of the shoulder blade. The subscapularis originates here.

Coracoid Process - beak shaped bony prominence on the anterior surface of the scapula, serves as the attachment site for several muscles.  

Infraglenoid Tubercle - bony projection just below the glenoid fossa. The long head of triceps brachii starts here. 

Glenoid Fossa - more commonly known as the shoulder socket, the glenoid fossa is the shallow hollowed out space in the bone where the head of the humerus sits

The Lats + Teres Major

In part two of my fascial layers blog, I wrote about aponeuroses being thick sheets of connective tissue that serve as attachment sites for some muscles.  One of those muscles that has a point of origin on the thoracolumbar aponeurosis is the very large, thin and superficial back and arm muscle known as the latissimus dorsi, or the lats.  This muscle is particularly interesting for a few reasons.  One, while most of its fibers are found covering the majority of the lower and middle back, it’s actually an arm mover.  The lats connect the pelvis to the upper arm bone to create medial rotation, adduction and extension at the glenohumeral joint, which is the ball and socket joint more commonly known as the shoulder joint.  The actions of the latissimus dorsi are easy to remember if you can remember that it’s the handcuff muscle.  If you were being arrested and handcuffed (and I hope you never are), your arms would be medially rotated, a little bit extended and also adducting across the midline behind you.  

The main actions of the lats are medial rotation, adduction and extension of the shoulder.  If you stand or sit upright with your arms relaxed heavily by your sides and turn the palm back behind you or spin your thumbs back behind you, that’s medial rotation of the shoulder.  If you reach both arms back behind you, that’s extension of the shoulder and if you then clasp your hands, bringing the arms together behind the back, that’s adduction.  (Side note: ABduction is when you take away from the midline. To abduct is to take away so it you take the arm or leg out to the side away from the body, that is abduction of the shoulder or hip.  ADduction is the opposite, it’s adding to the midline.  When the arms are out to the side and you bring them back down, that’s called adduction because you’re adding the arms back to the midline).  In addition to being able to move the arm, the lats also assist in lateral flexion of the trunk and can help extend the spine as well as tilt the pelvis anteriorly and laterally.  

HOW DOES A BACK MUSCLE MOVE THE ARM?

The lats have points of origin on the posterior iliac crest (top of the back of your hip bones), the thoracolumbar aponeurosis, the last few ribs (varies in cadavers), the spinous processes of last six thoracic vertebrae and in some people they cross the inferior angle of the scapula as well.  That is a huge origin and a lot of words.  All you really need to know is that it’s a large and superficial back muscle that originates on the pelvis, ribs, spine, fascia and sometimes the scapula so it can affect all of those things.  

The second interesting thing about the latissimus dorsi is that from its origin, it thickens around the armpit and wraps underneath to insert on the front of the upper arm bone in the space between the greater and lesser tubercles of the humerus called the intertubercular groove.  The lats form the posterior wall of the axilla, which is the fancy word for armpit.  You can feel for these more lateral fibers by taking your right hand to find the back flap of your left armpit and gently pressing your elbow in to your side to engage the fibers.  

A third interesting thing is that the lats have a helper, a smaller muscle that does exactly the same thing to help get the job done.  This muscle is called teres major, not to be confused with teres minor which is just above it and rotates the shoulder in the opposite direction.  Teres major originates on the inferior angle of the scapula (as the lats sometimes do too) and the lower third of the lateral border of the scapula.  Just like the lats, teres major wraps under the armpit to also insert on the front of the arm just distal to the intertubercular groove at the crest of the greater tubercle.  It doesn’t affect the trunk or the pelvis because it doesn’t cross them, but it does medially rotate, extend and adduct the shoulder.  Both the latissimus dorsi and the teres major are handcuff muscles.  

WHEN ARE THE LATS + TERES MAJOR CONTRACTING?

When you grab onto a pull up bar and pull yourself up, the lats and teres major are contracting (or shortening) which means the origin is working to bring the insertion toward it to complete one, some or all of the actions.  When the hands are clasped behind the back, the lats and teres major are also contracting as the shoulder is medially rotates, extended and adducted.  People with really developed lats may have a hard time clasping their hands behind their backs because of high motor tone and lots of dense muscle fibers.  

WHEN ARE THE LATS + TERES MAJOR LENGTHENING? 

Child’s pose with the arms stretched out in front is a nice stretch for the lats and teres major.  You can walk the arms and upper body off to one side to deepen the stretch on the other and vice versa.  Any time you are reaching the arms overhead, the lats are stretching.