The scapula is the flat, triangular bone on each side of your upper back, commonly called the shoulder blade. It sits against the back of your ribcage, spanning roughly from the second to the seventh rib, and forms the rear portion of the shoulder girdle. Despite being easy to feel through the skin, the scapula is one of the most complex bones in the body, serving as an anchor point for over a dozen muscles and playing a central role in nearly every movement of your arm.
Where the Scapula Sits
Each scapula rests against the back of the rib cage but doesn’t connect directly to it with a true joint. Instead, it “floats” over the ribs, held in place entirely by muscles. This arrangement gives it remarkable freedom to glide, tilt, and rotate as you move your arm. The only bony connection between the scapula and the rest of your skeleton is at its outer tip, where a bony projection called the acromion meets the collarbone. That junction is the acromioclavicular joint, the small bump you can feel on the top of your shoulder.
Key Parts of the Bone
The scapula has several distinct landmarks, each with a specific job.
- Glenoid cavity: A shallow, cup-shaped socket on the outer edge of the scapula. This is where the head of the upper arm bone (humerus) sits, forming the main shoulder joint. Because the socket is so shallow, the shoulder trades stability for an enormous range of motion.
- Acromion: The flat, bony shelf that extends over the top of the shoulder joint like a small roof. It’s the highest point of your shoulder and the place where the collarbone attaches.
- Coracoid process: A hook-shaped projection on the front of the scapula, just below the collarbone. Several muscles and ligaments anchor to it, helping stabilize the shoulder from the front.
- Spine of the scapula: A prominent ridge running horizontally across the back surface. You can feel it through the skin. It divides the back of the scapula into two shallow depressions where rotator cuff muscles sit.
Joints the Scapula Forms
The scapula participates in two true joints and one functional joint. The glenohumeral joint is the main shoulder joint, where the glenoid cavity meets the humerus. The acromioclavicular joint connects the acromion to the collarbone. And the scapulothoracic joint is the gliding surface between the scapula and the ribcage. It’s not a joint in the traditional sense (there’s no cartilage or capsule), but it functions as one, allowing the scapula to slide and rotate over the ribs.
Muscles Attached to the Scapula
More than 17 muscles either originate from or insert onto the scapula, which is why it’s so central to upper body movement. The most important group is the rotator cuff: four muscles (supraspinatus, infraspinatus, subscapularis, and teres minor) that wrap around the shoulder joint and keep the arm bone centered in its socket during movement. All four originate directly from the scapula’s surfaces.
Other major muscles anchored to the scapula include the trapezius and rhomboids along the inner border, the serratus anterior along the underside, the deltoid along the spine and acromion, and the biceps, which originates from two points on the scapula (the top of the glenoid and the coracoid process). The latissimus dorsi, one of the largest muscles in the back, also sends fibers to the scapula’s bottom corner. This dense web of muscle is what holds the scapula against the ribcage and controls its position during every reaching, pushing, pulling, and lifting motion.
How the Scapula Moves With Your Arm
Your arm doesn’t move independently of your scapula. Every time you raise your arm overhead, the scapula rotates upward in a coordinated pattern called scapulohumeral rhythm. During a full overhead reach, roughly two-thirds of the movement comes from the shoulder joint itself, and the remaining one-third comes from the scapula rotating on the ribcage. This 2-to-1 ratio means that for every three degrees your arm rises, about two degrees come from the arm bone rotating in the socket and one degree comes from the scapula tilting upward.
This coordination is essential. Without scapular rotation, you’d only be able to raise your arm to about shoulder height. The scapula also tilts the glenoid socket upward as the arm rises, which prevents the rotator cuff tendons from getting pinched under the acromion.
Scapular Winging
One of the more visible scapular problems is “winging,” where the inner border of the scapula lifts away from the ribcage and becomes prominent under the skin. It typically results from nerve damage affecting the muscles that hold the scapula flat. The most common cause is injury to the long thoracic nerve, which controls the serratus anterior muscle. When the serratus anterior stops working, the inner edge of the scapula pushes outward (medial winging). Less commonly, damage to the spinal accessory nerve weakens the trapezius, or damage to the dorsal scapular nerve weakens the rhomboids, both of which cause the outer edge to lift instead (lateral winging). Scapular winging usually causes noticeable weakness when pushing or reaching overhead.
Scapula Fractures
Scapula fractures are rare, accounting for less than 1% of all bone fractures and about 6% of shoulder girdle fractures. The bone is well protected by thick layers of muscle on both sides, so breaking it typically requires significant force, like a car accident or a fall from height. When fractures do occur, the glenoid (socket) is the most commonly affected area, involved in nearly half of all scapula fractures. Body fractures, which crack the broad flat portion, make up about 40%. Fractures of the bony projections (the acromion or coracoid) are least common, representing roughly 11% of cases.
How the Scapula Develops
The scapula begins as cartilage in the developing embryo and gradually turns to bone through a process involving eight or more separate growth centers. The main body of the scapula ossifies first, while secondary centers appear at the coracoid, acromion, glenoid rim, inferior angle, and medial border during childhood and adolescence. These growth centers fuse at different ages, but the process is typically complete by age 23. The inferior angle and acromion tend to be among the last areas to fully fuse, sometimes not finishing until the mid-20s.
Imaging the Scapula
When doctors need to evaluate the scapula, the standard approach involves four X-ray views of the shoulder region: a front-facing view (anteroposterior), a Grashey view (angled to show the joint space clearly), an axillary view (shot from below, through the armpit), and a lateral scapular view often called the “scapular Y-view” because the bone’s projections form a Y shape from the side. The Y-view is particularly useful for checking shoulder dislocations and the alignment of the scapula’s bony projections. CT scans or MRI may follow if a fracture pattern is complex or soft tissue damage is suspected.