The scapula is a large, flat, triangular-shaped bone positioned over the posterior thorax between the second and seventh ribs. The scapula has three borders (superior, medial, and lateral) and two important angles (superomedial and inferomedial) that primarily serve as sites for muscle attachment (Fig. 9.1). According to Lewitt [2], the three-dimensional resting position of the scapula is defined as being tilted anteriorly between 10° and 20° and medially rotated in the coronal plane between 30° and 40° (in other words, the glenoid faces more in the superior direction). As discussed in Chap. 2, the scapula is also angled anteriorly between 10° and 20° from the coronal plane. This position of anterior angulation is often referred to as the “scapular plane” when evaluating the shoulder.

While the general shape of the scapula is fairly consistent across the population, there exist several known topographical variations that may predispose some individuals to certain pathologic conditions (such as scapulothoracic bursitis or snapping scapula syndrome). For example, Aggarwal et al. [3] performed measurements in 92 dried scapulae and found that the costal (anterior) surface of the scapula “undulated” and also varied in depth between 10.5 and 26.5 mm. The investigators noted that the thickness of the superomedial angle ranges between 2 and 4 mm whereas the inferomedial angle had a thickness between 5 and 8 mm. Anterior angulation of the superomedial angle also varied between 124° and 162° in the majority of their specimens. In addition, the investigators also identified an anterior “horn-like” projection along the lateral border of at least one scapula. Several researchers have described other osseous abnormalities that may predispose some individuals to painful scapular snapping. These include the superomedial “bare area,” [4] the “Luschka tubercle” (bony protuberance at the superomedial angle) [5], the teres major tubercle (located at the insertion of this muscle) [6], and anterior “hooking” of the superomedial angle [7].

The suprascapular notch is located near the junction of the lateral third of the superior scapular border, just medial to the confluence of the coracoid process with the scapular body [8]. The anatomy of the suprascapular notch is also known to have various morphological features that may predispose some individuals to suprascapular nerve entrapment [9–12]. The transverse scapular ligament travels mediolaterally between the crests of the suprascapular notch. In most cases, the suprascapular nerve is found below the ligament and within the notch whereas the suprascapular artery is found above the ligament and outside of the notch (Fig. 9.2). The transverse scapular ligament is also known to have significant anatomic variations that can also generate symptoms related to suprascapular nerve entrapment [13, 14].

The scapulothoracic articulation is unique in that its motion is not dictated by osseous constraints. Rather, the scapula is positioned through the dynamic, coordinated action of surrounding periscapular muscles (see Chap. 3). Therefore, disruption or dysfunction of any one of these muscles can result in scapular malposition or dyskinetic motion which can lead to disordered shoulder function.

Bursae are fluid-filled sacs lined with synovial-like cells that facilitate gliding of opposing surfaces over one another. In the case of the scapula, there are several periscapular bursae that allow the scapula to glide smoothly over interposed muscle layers. These bursae are commonly defined as either “anatomic” or “adventitial” bursae, depending on their propensity to cause periscapular pain [15]. Anatomic bursae are typically thought to represent normal, physiologic bursae that allow smooth gliding over the posterior thorax. The infraserratus and supraserratus bursae lie on either side of the serratus anterior muscle along the medial scapular border and are the most frequently recognized anatomic bursae [15, 16]. Adventitial bursae are most often located at the superomedial or inferomedial scapular angles and are thought to be significant pathological pain generators [17, 18]. Some researchers have suggested that pain near the superomedial angle can be due to pathologic infraserratus or supraserratus bursal tissue [19, 20], pain near the inferomedial angle is most likely the result of pathologic infraserratus bursal tissue [21, 22], and pain near the confluence of the scapular spine may be caused by a pathologic scapulotrapezial bursa which is most commonly located deep to the trapezius and superficial to the scapular spine (Fig. 9.3) [23].

Knowledge of pertinent neurovascular anatomy around the scapula is necessary to fully evaluate any patient with a condition related to disordered shoulder motion (see Chap. 3). The spinal accessory nerve, which innervates the levator scapulae muscle, travels with the transverse cervical artery along the levator scapulae muscle which is situated deep to the trapezius muscle. In some cases, the spinal accessory nerve may penetrate through the central portion of the levator scapulae [24]. As the transverse cervical artery travels distally, it becomes the dorsal scapular artery which, in turn, travels with the dorsal scapular nerve beneath the rhomboid musculature a few fingerbreadths medial to the medial scapular border (see Fig. 9.2) [25]. The long thoracic nerve is relatively protected as it travels along the anterior aspect of the serratus anterior muscle. The suprascapular nerve arises from the superior trunk of the brachial plexus and courses towards the suprascapular notch with the suprascapular artery. As mentioned above, the suprascapular nerve passes beneath the transverse scapular ligament whereas the suprascapular artery travels above the ligament (see Fig. 9.2).

With respect to normal shoulder kinematics, the scapula has several important functions that should be considered before evaluating any patient with a complaint related to the shoulder. First, the scapula provides a stable fulcrum against which glenohumeral motion can occur through the dynamic action of the periscapular musculature, including the rotator cuff and deltoid muscles. In fact, several authors have shown that external stabilization of the scapula may improve the contraction strength of the rotator cuff [26, 27]. Smith et al. [27] found that stabilizing the scapula in a position of retraction substantially increased external rotation strength in 20 normal subjects when compared to external rotation strength with the scapula protracted. Similarly, in a series of 20 patients with shoulder pain (but without rotator cuff tears) and ten healthy controls, Kibler et al. [26] demonstrated a 13–24 % increase in supraspinatus strength when the “empty can” test was performed with the scapulae in a retracted position (see Chap. 4 for details regarding Jobe’s “empty can” test). In 29 overhead athletes with scapular dyskinesis, Merolla et al. [28] measured significantly increased contraction forces of both the supraspinatus and the infraspinatus muscles following the completion of specially designed rehabilitation protocols designed to improve periscapular muscle balance. The same group published similar results in a series of volleyball players who also demonstrated scapular dyskinesis upon initial presentation [29]. Second, accurate positioning of the scapula through coordinated muscle contractions facilitates glenohumeral articular congruency by maintaining alignment of opposing force couples, thus preserving the so-called concavity compression mechanism of dynamic stability (concavity compression is discussed more thoroughly in Chaps. 4 and 6). Third, the scapula plays an important role in the transmission of force through the kinetic chain. In short, the scapula facilitates the transfer of kinetic and potential energy from the largest muscles of the core and trunk towards site of action [30]. Dynamic scapular stability, which is facilitated by adequate core and trunk strength, is necessary to optimize the efficiency of this complex system [31]. Perhaps one of the most well-known examples of this concept is the classic pitching motion most often utilized to deliver a high-velocity pitch in baseball.

There are numerous potential etiologies responsible for the development of scapular dyskinesis, most of which can be divided into primary and secondary causes.

Scapular dyskinesis is usually diagnosed by simple palpation of the relevant scapular landmarks while also observing both scapulae during movement of the shoulder through the various motion planes. The condition is most often characterized by prominence of the inferomedial angle and the medial scapular border (as a result of protraction in the resting position), early upward rotation of the scapula during arm elevation and/or early downward rotation of the scapula when lowering the arm back to the side (variations in dyskinetic patterns are described below for specific conditions). Recent evidence suggests that increased upward rotation may be associated with symptom compensation whereas increased downward rotation may be associated with symptom causation. Regardless, any abnormal scapular motion can compromise normal shoulder function by reducing glenohumeral articular congruency, reducing the acromiohumeral distance, increasing tension and strain across the AC joint capsule, decreasing the strength of rotator cuff contraction (which can also reduce glenohumeral stability) and shifting the arc of glenohumeral motion as which commonly occurs in overhead athletes. In addition to these changes, scapular dyskinesis can also mask or enhance the symptoms related to other concomitant shoulder pathologies, such as rotator cuff tears and labral tears, thus complicating the physical diagnosis and subsequent treatment decisions.

Clinical examination of the scapulae should begin with an assessment of posture and symmetry. In many overhead athletes, the dominant shoulder will appear to rest in a slightly lower, or depressed, position relative to the contralateral shoulder (Fig. 9.6). This abnormality often occurs in conjunction with SICK scapula syndrome (scapular malposition, inferomedial angle prominence, coracoid pain, and scapular dyskinesis) which can often be corrected by a focused rehabilitation program. As necessary components of scapular motion, AC and SC joints should also be evaluated for any evidence of pain and/or instability (see Chaps. 7 and 8 for details regarding the AC and SC joints, respectively). The clavicle should also be palpated to confirm adequate length and to identify any abnormal angulation or malrotation.

In most cases, the clinician can identify dyskinetic scapular motion by simple observation, palpation and compression of the medial scapular border as the patient elevates and lowers the affected arm (forward flexion, horizontal abduction, and scaption). Specifically, the appearance of a visible prominence of the medial scapular border with any of these motions can be considered dyskinetic motion (Fig. 9.7) [35]. Weights (e.g., 3 or 5 pounds) can also be used to increase the visibility of medial border prominence with shoulder motion (also known as the scapular dyskinesis test). Similarly, resisted external rotation can also produce same pattern of scapular dyskinesis as that which is observed during humeral elevation (i.e., a positive “flip test”; Fig. 9.8). Pain with compression of the scapular body against the thoracic wall with shoulder motion may also be an indicator of snapping scapula syndrome.

As mentioned above, there are many shoulder disorders that may have an association with scapular dyskinesis. However, we have chosen to focus on several of the more common conditions that we believe are closely related to scapular dyskinesis. The purpose of this section is to highlight the most important concepts related to disordered scapular motion that can subsequently be applied to other, less common shoulder pathologies that are not specifically mentioned below.