Your shoulder blade keeps knitting itself together until about age 25
The scapula, named from the Latin for a trowel, starts forming in the second month of fetal life, yet parts of it stay cartilage for years. It grows from seven or more separate centres of bone, and the last of these pieces usually fuse only around the 25th year.
This roughly triangular flat bone sits on the back of the rib cage and links the upper arm bone to the collarbone. Doctors use the prefix omo- for it, from the Ancient Greek word for shoulder, a relative of the Latin humerus. Anatomists describe it as having two surfaces, three borders, three angles and three processes. In places it is surprisingly flimsy: parts of the hollows above and below its ridge are often so thin as to be semitransparent, and occasionally the bone is missing there altogether, leaving only fibrous tissue between the muscles.
Three families of muscle anchor to it. The rotator cuff group, subscapularis, supraspinatus, infraspinatus and teres minor, turns the shoulder inward and outward and helps lift the arm sideways. The biceps, triceps and deltoid pull on its processes and tubercles to move the shoulder joint. A third set, including the trapezius, serratus anterior and rhomboids, holds the blade steady and rotates it, which is why its lowest corner slides forward round the chest when the arm is raised.
The joint itself is shallow. The glenoid cavity, a cartilage-lined socket on the thickest part of the bone, receives the head of the humerus, and a fibrocartilage rim called the glenoidal labrum deepens it. Overhead, the acromion forms the point of the shoulder, curving forward to overhang the socket, while the superior angle of the blade lies at about the level of the second thoracic vertebra.
Growth follows a long timetable. The spine of the bone rises from the main plate around the third fetal month. At birth the socket, coracoid process, acromion and lower edges are still cartilage. The coracoid begins to ossify between 15 and 18 months and usually joins the rest by about 15 years, the upper third of the socket has its own centre from age 10 or 11, and the remaining pieces harden in quick succession between 14 and 20.
Source: Scapula
