Posted on Fri, Feb 15, 2013 @ 09:09 AM by Courtney Smith
Think of how many things you do during the day; you stretch first thing in the morning, lift your coffee cup, wave at people, type, write, text, play Pokémon Go—no matter what it is you do, chances are you're using your arms to do them. And you know what's incredible about that? You wouldn't be able to use your arms without the shoulder girdle!
I'm fascinated by the shoulder girdle, to be honest. It's such an interesting structure and presents an even more interesting joint. Let's take a quick look, shall we?
Bones of the Shoulder Girdle
The shoulder girdle is made up of two posterior scapulae and two anterior clavicles. These bones make an incomplete ring around the upper thoracic cage. The medial end of each clavicle articulates with the manubrium and scapulae; each scapula connects to the thoracic cage by muscle only. How crazy is that? The only thing that connects the upper limbs and the scapula to the axial skeleton are the clavicles. That's a pretty big job, wouldn't you say?
The scapulae are flat, triangular bones that serve as the attachment sites of many muscles, including the deltoids. Muscles of the back and thorax connect the scapulae to the thoracic cage.
Here's a quick run-down of the girdle's articulations:
Scapula, manubrium, cartilage of the first rib
Glenoid Cavity and the Shoulder Joint
Hold one of your arms out and rotate it. Your arms have more motility than your legs, despite the fact that the shoulder and hip joints are the same type of joint!
Each scapula has a concave, articular surface called the glenoid cavity (or glenoid fossa), which articulates with the head of the humerus. The cavity is covered with cartilage. The ball-like head of the humerus articulates with the glenoid cavity, creating a ball-and-socket joint that allows the upper limb great motility. A capsular ligament is part of the articular capsule that surrounds a synovial joint (freely moving joint); capsular ligaments reinforce joints and provide stability. The capsular ligament of the shoulder joint provides more motility than the hip joint—in fact, it allows the scapula and the humerus to separate more than 2.5 cm, which means your upper limbs have a fantastic range of motion!
Watch the range of motion of a ball-and-socket joint here:
Landmarks of the Scapula
The scapula is such an interesting bone. I mean, look at it! It's such a cool shape that it's also known as the shoulder blade. You are walking around with two knives permanently strapped to your back. How awesome is that?
Landmarks, or characteristics of the bone, have different functions: muscle attachment sites, bone and ligament articulation sites, passage for nerves and vessels, and more. In the image, all bone landmarks are highlighted in different colors.
For example, the acromion and the coracoid process (in light pinkish purple) are the two major projections of the scapula. The acromion is an oblong process that hangs over the glenoid cavity, while the coracoid process is a thick, curved process that runs almost parallel to the acromion. They are connected by the coracoacromial ligament. This ligament supports proper movement of the joint by arching over the superior portion of the shoulder, protecting it.
The scapular spine (in pale orange-red) is a prominent plate of bone that serves as the attachment site for portions of the trapezius and the deltoid. At the distal end of the spine is the acromion.