Three bones make up the shoulder: the humerus (upper arm bone), the scapula (shoulder blade), and the clavicle (collarbone). These three bones connect through multiple smaller joints that work together to give the shoulder its extraordinary range of motion. Understanding how they fit together helps explain why the shoulder is both the most mobile and the most injury-prone joint in the body.
The Three Bones of the Shoulder
The humerus is the long bone of the upper arm. Its top end, called the humeral head, is a smooth, rounded ball that sits in the shoulder socket. Just below the head are two bony bumps, the greater and lesser tuberosities, which serve as anchor points for the muscles that rotate and stabilize your arm.
The scapula is a flat, triangular bone that sits against your upper back. It does far more than just form the back wall of the shoulder. Three key landmarks on the scapula play direct roles in the joint. The glenoid fossa is a shallow indentation that forms the socket side of the main shoulder joint. The acromion is a bony shelf that extends over the top of the joint, forming a protective roof. And the coracoid process is a small, hook-shaped projection just below the collarbone that anchors several muscles and ligaments.
The clavicle is an S-shaped bone that runs horizontally across the front of your chest, connecting the shoulder blade to the breastbone. It acts as a strut that holds the arm away from the trunk, allowing it to swing freely. Without the clavicle, your shoulder would collapse inward toward your chest.
The Glenohumeral Joint: The Main Shoulder Joint
When most people say “shoulder joint,” they mean the glenohumeral joint. This is where the ball of the humerus meets the glenoid cavity of the scapula. It’s classified as a ball-and-socket joint, the same basic design as the hip, but with one critical difference: the socket is extremely shallow. Think of a golf ball sitting on a tee rather than a ball nestled deep in a cup.
That shallow socket is what gives the shoulder its huge range of motion. You can reach overhead, behind your back, and across your body in ways no other joint allows. But that freedom comes at a cost. The bones alone don’t do much to hold the joint in place, which is why the shoulder relies heavily on soft tissue for stability.
A ring of tough fibrocartilage called the labrum wraps around the rim of the glenoid cavity, deepening the socket by up to 50% in some areas. In measurable terms, the labrum adds about 9 mm of depth from top to bottom and 5 mm from front to back. That extra depth helps keep the humeral head centered during movement and protects against dislocation.
The Acromioclavicular Joint
The AC joint sits at the very top of the shoulder, where the outermost tip of the collarbone meets the acromion (the bony point at the top of the shoulder blade). You can feel this joint as a small bump on top of your shoulder. It allows the scapula to rotate and tilt as you raise your arm, and it transfers force between the collarbone and the shoulder blade. AC joint separations, often caused by a direct fall onto the shoulder, are a common sports injury.
The Sternoclavicular Joint
The sternoclavicular joint is often overlooked, but it’s the only bony connection between the entire shoulder complex and the rest of the skeleton. It forms where the inner end of the collarbone meets the manubrium, the upper portion of the breastbone. A small disc of cartilage divides the joint into two compartments, helping absorb impact and allowing the collarbone to glide during shoulder movements. Several strong ligaments, including one that anchors the collarbone to the first rib, keep this joint remarkably stable.
How These Bones Work Together
Raising your arm isn’t a single-joint motion. The humerus rotates in the glenoid socket, the scapula tilts and slides along your ribcage, and the clavicle rotates at both of its joints. This coordinated movement, sometimes called scapulohumeral rhythm, is why shoulder problems rarely involve just one structure. A stiff AC joint or a weak rotator cuff can change how the entire system moves.
The collarbone, for example, rotates up to 30 to 45 degrees at the sternoclavicular joint during full overhead reaching. At the same time, the scapula rotates upward so the glenoid socket tilts to support the arm’s weight. If any link in this chain is disrupted by injury or stiffness, the shoulder compensates, often leading to pain or impingement over time.
Common Fracture Sites
A broken collarbone is the most common shoulder fracture, frequently caused by falls onto an outstretched hand or a direct blow to the shoulder. The clavicle’s S-shape and relatively slim middle section make it vulnerable. Proximal humerus fractures, where the rounded top of the arm bone breaks, are also common and typically result from falls or shoulder dislocations that force the humerus out of its socket. Scapula fractures are rare because the shoulder blade is protected by thick layers of muscle on both sides, but they can occur in high-energy trauma like car accidents.