Ideal scapular function reflects its complex anatomy and in turn is foundational for all shoulder function. The scapula plays a multitude of roles. Anatomically, it is the “G” of the glenohumeral (GH) joint and the “A” of the acromioclavicular (AC) joint. Physiologically, it is the “S” of scapulohumeral rhythm (SHR), the coupled and coordinated movement between the scapula and arm that allows the arm to be placed in the optimum position and motion to accomplish tasks. Biomechanically, it provides a stable base for muscle activation, a moving platform to maintain ball-and-socket kinematics, and an efficient link between the core, which develops force, and the arm, which delivers the force. Critical to these roles is normal scapular motion.

To comprehend the complex biomechanics of the scapula, it is critical to have a deep knowledge of the anatomy. It is not surprising that all types of shoulder pathology demonstrate altered motion. Frequently, assessment of scapular muscular attachments, innervation, motion, and position can provide key information on treatment options and guide rehabilitation. This chapter will concisely address pertinent aspects of anatomy of the scapula as it pertains to normal scapular function and clinical implications.

The bony anatomy is predicated on the developmental advantages of mobility, such as prehension and overhead use. This is reflected in several primary changes noted through time in the hominid scapula. First, the acromion has broadened and lateralized to allow mechanical advantage for the deltoid muscle [1]. The coracoid process (meaning “like a crow’s beak”) enlarged in a manner theorized to assist in the prevention of anterior dislocation at 90° of abduction [2, 3]. Finally, broadening and alteration in the force vector of the infraspinatus and teres minor are postulated to increase both external rotation strength and humeral head depression [4].

The scapula is a large flat bone which forms from a collection of mesenchymal cells [5]. It shows signs of ossification by the fifth week of embryologic development [5]. The scapula follows a predictable course in descending from the paracervical region to the thorax. Failure of this process leads to Sprengel’s deformity [6]. By the seventh week, the scapula has descended to its final position, and the glenoid is easily identified.

The scapula is primarily formed through intramembranous ossification. The body and spine are ossified at birth and subsequently follow an expected pattern. However, there are several notable exceptions with clinical implications. The coracoid forms from two centers of ossification and is generally united by age 15. Rarely, a third ossification center at the tip can persist and present confusion with a fracture [7]. The glenoid also forms from two separate ossification centers, one at the base of the coracoid and another with a horseshoe contour inferiorly [7]. These are usually fused by 15 years of age as well. Finally, os acromiale may be noted in up to 8% of the population and is the result of two or three ossification centers which arise in puberty and fail to unite by the expected age of 22 [8]. The variable failures of fusion may result in the following abnormalities, from anterior to posterior, pre-acromion, meso-acromion (most common), meta-acromion, and basi-acromion [1, 8].

Grossly, the scapula is a thin sheet of bone which serves as a critical site of muscle attachment. The blood supply is primarily through a network of periosteal vessels which take origin from muscular insertions. Thickening of the bone is notable at the lateral border and superior and inferior angles. Ventral concavity creates a smooth articulating surface against the ribs. Small oblique ridges exist ventrally for the tendinous insertions of the subscapularis [5]. Similarly, small fibrous septa are present dorsally to attach and separate the infraspinatus, teres minor, and teres major. The dorsal surface is traversed by the scapular spine which divides two concavities, the supraspinatus and infraspinatus fossae. The medial two thirds of these fossae give rise to the supraspinatus and infraspinatus muscles. The spine contains two important notches. The suprascapular notch at the base of the coracoid contains the suprascapular nerve, and compression at this location will affect both the supraspinatus and infraspinatus muscles [3, 9]. Second, the spinoglenoid notch is present at the lateral border of the spine [3]. Various causes can lead to compression of the suprascapular nerve here as well, producing isolated atrophy of the infraspinatus.

Anatomic interest in the scapula is frequently directed at the coracoid, acromion, or glenoid. The name coracoid derives from the Greek word korakodes meaning “like a crow’s beak” [3]. The bent shape resembles a finger pointed toward the glenoid. From the Greek word “akros” for point, the acromion is often referred to as the point of the shoulder. The morphology of the acromion is among the most studied in the body. Considerable cadaveric research has been directed at the relative frequency and postulated causes of the types 1 through 3 acromion, as described by Bigliani [1]. However, the relationship between acromial shape and “impingement syndrome” or rotator cuff tear has not borne out in literature. Similarly, the glenoid has been the subject of intensive study in an effort to define bony anatomy in shoulder instability [7, 10–13]. Average values for size include a height of 35 mm and width of 25 mm, but considerable variability exists. Comparison to the contralateral side may be required to precisely define bone loss. Glenoid version may also range widely. Retroversion, up to 6°, is most common, as seen in 75% of the population, but anteversion up to 2° is reported [14–18].