The Pectoral Girdle 2: Joints and Ligaments

Next we will discuss some of the joints and ligaments associated with the clavicle and scapula.  A little background is necessary first, though truly getting into joints and ligaments is not the focus of this, if you are interested in more information I am sure google can help you more than me. A joint can have one of two properties, it can be stable or it can be mobile.  These properties are inverses, when one goes up, the other goes down. There are various ways to group joints and most of them deal with the histological characteristics, meaning what types of tissues and cells they present under a microscope.  Three main categories are fibrous, cartilaginous, and synovial joints.
 

Fibrous joints are joined by dense fibrous connective tissue (DICT), a histological term, and usually do not permit movement; an example would be the Lamboid Suture found in the skull between the parietal and occipital bones, think of how the skull is made to fit tightly together and not permit movement between the bones.  Cartilaginous joints are joined by cartilage, at least initially, but may ossify, that is, turn into bone, osseo- is the Latin prefix meaning bone.  This joints allow a tiny bit of movement but not much.  An example is the sternocostal joint between the sternum and the of the first rib.  A related term that means about the same thing is synchondrosis; note the chon in the middle which denotes cartilage (more depth here is cellular and the field of histology). A synovial joint, also known as a diarthrosis, is the most movable and most common type of joint.  An example is the glenohumoral joint (the one that connects your arm to your shoulder and lets you move it around with a high degree of mobility).  The main characteristic of the synovial joint is an encapsulated space filled with synovial fluid which acts like oil in a car engine keeping everything lubricated and running smoothly and without friction.  Again these are just the basics and should be discussed further in histology but they will serve for our purposes.  And onward to the first big joint...

The sternoclavicular joint is classified as a synovial joint but is unique because unlike the standard mobile  synovial joints, this one does not want to move.  It joins exactly what it is named for, the sternum (specifically the manubrium which is the upper part of the sternum) and the proximal end of the clavicle (specifically an indent on the end of the bone called the sternal facet (facet comes from the french word facette meaning little face) .  The sternoclavicular joint has six major components:

1.    Articular Capsule

2.    Anterior Sternoclavicular Ligament

3.     Posterior Sternoclavicular Ligament

4.     Interclavicular Ligament

5.     Costoclavicular Ligament

6.     Articular Disk
Refer to picture below as you read the following...

It is helpful to think of this joint as a very strong junction.  In fact, when subjected to extreme forces, the clavicle will usually fracture before this joint gives out, but there is an sternoclavicular joint separation it can be very dangerous as the clavicle will displace either anteriorly, not great, or posteriorly, much worse due to all the important structures located generally behind it.  An articular capsule forms a sealed off area where the two bone meet.  The clavicle bone wants to push out anteriorly due to the forces created by the arm and this joint must resist that.  So, it developed very thick strong ligaments in the front and back, which we call the anterior and posterior sternoclavicular ligaments.  The interclavicular ligament sits directly on top of each clavicle running along the top of the sternum and connecting both.  It can be palpated (felt by touch) as it sits atop the jugular notch of the manubrium, that little dip of bone on the top of your chest that dips downward.  The costoclavicular ligament attaches the clavicle to the first rib, which sits directly below it.  It serves as an extra anchor attaching the clavicle to the rid and giving it extra support.  The articular disk sits in the articular capsule between the bones and acts as a shock absorbing pad.

On the opposite end is the acromioclavicular joint which again connects exactly what it says (see a pattern?), the acromion process of the scapula (a piece of bone that sticks out) with the clavicle.  The joint consists of 6 major components:

1.       Articular Capsule

2.       Articular Disk

3.       Superior Acromioclavicular Ligament

4.       Interior Acromioclavicular Ligament

5.       Coracoclavicular Ligament

a.       Conoid

b.      Trapezoid

Similarly to the sternoclavicular joint, there is an articular capsule and articular disk serving similar functions.  It is technically a synovial joint, due to the capsule and disk, but serves as more of a strut helping to raise the arm above the head. At this distal end of the clavicle the bone is trying to push up and down due to the forces placed on it, so the body strengths the 'up' and 'down' ligaments to prevent that, we name them the superior and inferior acromioclavicular ligaments, and both are strong and thick.  The coracoclavicular ligament spans from the coracoid process (another small piece of bone that sticks out) to the clavicle and is divided into two parts.  The most medial part is the Conoid Ligament and the lateral part, closest to the arm, is the trapezoid ligament.  They are named due to their shape that they generally appear in the body and function to anchor the clavicle to the scapula, similar to the costoclavicular ligament.

When someone has a separated shoulder it happens here, at the acromioclavicular joint.  This is different than a shoulder dislocation, which occurs at the glenohumeral joint.

The third and last big joint is the glenohumeral joint.  Glene is greek for socket or eyeball (among other things) while the suffix -oid means 'form of' and humerus is Latin for shoulder, though you will mainly encounter it as the name of the upper arm bone, the humerus.  The glenohumeral joint is a highly mobile joint that has 5 main components:

1. Articular Capsule

2. Glenohumeral Ligaments

3. Coracohumeral Ligament

4. Coracoacromial Ligament

5. Transverse Humeral Ligament (of Biceps Brachii)

The articular capsule is different than the first two joints discussed, it is indeed encapsulated but there is not an articular disk.  The humerus sits within the glenoid cavity of the scapula, much like a ball in a socket.  This allows it to be freely movable.  It is also much more involved with, and reliant upon, muscles than the first two, you have probably heard of the rotator cuff, a group of muscles involved with moving it.  The glenohumeral ligaments are a large group of ligaments that blend together to form a capsule around the ball and socket joint.  They consist mainly of a superior, middle, and inferior regions but going into more depth is the area of medical specialties.  

The coracohumeral ligament attaches the coracoid process with the humerus (it is also a part of the blending of the glenohumeral ligament).  The coracoacromial ligament is a very interesting ligament, it is an intra-scapular ligament going from the coracoid process to the acromion process.  It provides separation and cushioning for two muscles that travel here, the deltoid on top and the supraspinatous underneath (more later when we cover muscles), and helps prevent them from rubbing against each other (usually rubbing causes friction and bad things in the body).  The transverse humeral ligament holds the a piece of the biceps brachii muscle, the one you can see when you flex your arm, called the tendon of the long head, more later, in place against the bone.  As I previously stated, if the arm pops out it becomes dislocated.  A term often associated is luxation, which simply means it popped out but not all the way.