What type of bone is the scapula

27.5 Scapular avulsion

The scapula is connected to the body wall by both deep and superficial muscles. The deep muscle group holds the body up towards the scapula when the cat is standing, and is composed of the serratus ventralis, trapezius, and rhomboideus muscles. These insert at the cranial angle and dorsal border of the scapula. Rupture of the deep muscle group causes the scapula to shift dorsally, and prevents the cat maintaining normal posture during weight-bearing. Jumps, falls, and bite wounds are common traumatic causes (6). There is one case report of feline scapular avulsion in the literature (7). Fracture of the body of the scapula may coexist.

The characteristic dorsal displacement of the scapula is easily observed and palpable (Fig. 27-8). The proximal part of the scapula displaces laterally if the distal limb is adducted. The function of the limb is initially impeded but with time the cats start to use their leg again, although the scapula usually remains dorsally dislocated without surgical treatment. A Velpeau sling may be a successful treatment option for acute dislocation (8).

The goal of surgery is temporarily to stabilize the scapular body to the body wall in its anatomic position until fibrous healing has occurred. Repair can be achieved by reattaching the bone to the serratus ventralis muscle with non-absorbable sutures secured through small holes in the scapula. If the soft-tissue repair is tenuous, a wire can be placed carefully around an adjacent rib (Box 27-4).

The Scapula

The scapula bears two marked processes: the acromion and, rather less obvious, the coracoid. The acromion springs like the head of a golf club from the spine of the scapula, extends anterior to the glenoid fossa and carries a small subsidiary process, the metacromion. The scapula provides the glenoid fossa (glenoid=socket): both the scapula and the coracoid process contribute to the articular surface.

The muscles of the scapula arise from the anterior and posterior spinous fossae on either side of the spine, and from the deep surface of the bone. The deep surface is ridged by the attachments of subscapularis and serratus anterior. The scapula ossifies from two major centres, one for the scapula and one for the coracoid process. In the rat, the borders of the scapula long remain cartilaginous.

Muscles that Move the Shoulder

The scapula, a triangular bone, articulates solidly only with the small clavicle. Because the scapula is not held firmly in place, it is free to move in several directions. The muscles that move the scapulae are extrinsic muscles – they attach from the neck and/or shoulder to the limb (Figures 11(a) and 11(b)). The trapezius is a large flat muscle that covers much of the upper back. Its fibers extend in several directions, and it elevates, retracts, and rotates the scapula freely. The rhomboideus and the levator scapulae muscles also retract and elevate the scapula. Acting antagonistically to these muscles, the pectoralis minor and the serratus anterior protract the scapula forward, as when a person reaches for something. The subclavius muscle attaches to and acts indirectly through the clavicle to pull the scapula toward the sternum.

Levator scapulae

Arises from transverse process of C1–C2, posterior tubercles of transverse processes of C3–C4 vertebrae and attaches to the superior part of medial border of the scapula. It elevates and rotates the scapula (Moore et al 2002).

The Clinical Syndrome

Winged scapula syndrome is an uncommon cause of musculoskeletal pain of the shoulder and posterior chest wall. Caused by paralysis of the serratus anterior muscle, winged scapula syndrome begins as a painless weakness of the muscle with the resultant pathognomonic finding of winged scapula. As a result of dysfunction secondary to paralysis of the muscle, musculoskeletal pain often results. Winged scapula syndrome is often initially misdiagnosed as strain of the shoulder groups and muscles of the posterior chest wall because the onset of the syndrome often occurs after heavy exertion, most commonly after carrying heavy backpacks. The syndrome may coexist with entrapment of the suprascapular nerve.

Trauma to the long thoracic nerve of Bell is most often responsible for the development of winged scapula syndrome. Arising from the fifth, sixth, and seventh cervical nerves, the nerve is susceptible to stretch injuries and direct trauma. The nerve is often injured during first rib resection for thoracic outlet syndrome. Injuries to the brachial plexus or cervical roots also may cause scapular winging, but usually in conjunction with other neurological findings.

The pain of winged scapula syndrome is aching and is localized to the muscle mass of the posterior chest wall and scapula. The pain may radiate into the shoulder and upper arm. The intensity of the pain of winged scapula syndrome is mild to moderate, but it may produce significant functional disability, which, if untreated, continues to exacerbate the musculoskeletal component of the pain.

Scapula (Fig. 14.2-3)

The scapula is a broad, flat bone bisected on the lateral surface by the scapular spine. Noteworthy anatomic landmarks include the scapular cartilage, scapular spine, supraglenoid tubercle, coracoid process, and glenoid cavity. Unlike cats and dogs, the clavicle is absent in the horse. Horses also lack an acromion on the distal part of the scapula.

The scapula has 4 centers of ossification: the body of the scapula and the scapular cartilage (fused at birth), the cranial portion of the glenoid cavity of the scapula (fused at 5 months), and the supraglenoid tubercle (fused between 12 and 24 months).

The most common fracture of the scapula is through the supraglenoid tubercle, usually seen in younger animals (<  2 years old) associated with direct trauma and a concurrent avulsion injury from the biceps. The goal of surgery to repair supraglenoid tubercle fractures is to restore congruity of the glenoid cavity. Depending on the size of the fragment, it can be removed or repaired with cortical bone screws. The supraglenoid tubercle is the origin of the biceps tendon and can place considerable tensile forces on fixations of these fractures. Other fractures seen are stress fractures of the body of the scapula in young racehorses, complete scapular body fractures, and fractures of the scapular spine.

Dorsal Scapular Luxation

The scapula is attached to the thoracic body wall by the serratus ventralis, rhomboideus, and trapezius muscles. Trauma incurred by falling from a height can cause partial or complete rupture of these muscles, which will displace the scapula, especially during weight-bearing activities. If these cats are presented acutely there will be a non–weight-bearing lameness accompanied by pain and swelling between the dorsal scapula and the body wall. After a few days, this inflammation will subside and the cat will begin bearing weight. This produces an unusual gait because the scapula is dorsally displaced with each step. Most cats will regain a reasonable degree of mobility, and if the cat is not especially active, no further treatment need be undertaken. The cat will, of course, retain the distinctive gait abnormality. If the owner is unsettled by the gait or feels that it represents a disability for the cat, surgical stabilization can be undertaken. This consists of exploration of the dorsal aspect of the scapula and primary suturing of the torn muscles, where possible. In addition, two holes are drilled in the dorsocaudal area of the scapula and the scapula is attached in a normal position to an adjacent rib with nonabsorbable suture or surgical wire. Care must be taken that the thoracic cavity is not entered while passing the wire. The limb is then bandaged against the chest so it cannot be used for 2 weeks.48

Scapula – anterior aspect

The scapula (Fig. 2.14), whose mobility is essential to facilitate the wide range of movements of the shoulder, is rarely fractured as the bone is almost completely encased in muscles. From the anterior aspect of the scapula (the costal surface) which covers the thoracic cage the subscapularis muscle originates. The surface is marked by ridges for the attachment of fibrous septae of this multipennate muscle. The serratus anterior muscle which moves the scapula forward (protraction of scapula) is inserted on the medial margin on the costal surface. The glenoid fossa, seen at the lateral aspect in the upper part, faces forward as well as laterally. Above this is the acromion which is the uppermost bony point in the shoulder region. The coracoid process projecting anteriorly receives the attachments of three muscles, i.e. short head of biceps, coracobrachialis and the pectoralis minor. The coracoclavicular and the coracoacromial ligaments are also attached to the coracoid process.

2.1 Embryonic and fetal scapula development

The scapula is forms from multiple progenitor cell populations from different tissue layers—a rare characteristic among appendicular bones, most of which derive from only one tissue layer or progenitor cell population. The scapula has contributions from the dermomyotome (a dorsolateral derivative of the somite), somatopleure (somatopleuric derivative of the lateral plate mesoderm plus inner ectoderm), and neural crest (migratory pluripotent cells delaminated from the dorsal neural tube) (Durland, Sferlazzo, Logan, & Burke, 2008; Huang, Christ, & Patel, 2006; Huang, Zhi, Patel, Wilting, & Christ, 2000; Matsuoka et al., 2005). The dermomyotome gives rise to the scapula blade and proximal scapula spine, whereas the somatopleure gives rise to the glenoid fossa, coracoid, acromion, structures of the scapula neck, head, and spine, although the extent to which each of these progenitor tissues contribute to the scapula varies across species (Durland et al., 2008; Ehehalt, Wang, Christ, Patel, & Huang, 2004; Huang et al., 2000; Malashichev, Christ, & Pröls, 2008; Valasek et al., 2010). Matsuoka et al. (2005) additionally demonstrated in mice that post-otic neural crest cells contribute to the scapular blade's superior border, spine/acromion, and coracoid process.

The complexity of the dermomyotomal origin of the scapula blade is underscored by the complex organization of the dermomyotome itself, and how it is pre-patterned and differentiates during somitogenesis. Early in somitogenesis, a partially overlapping 3′ Hox expression domain is suggested to pre-pattern or specify (scapula) pre-dermomyotomal cells in the undifferentiated somite to become blade progenitors (Huang et al., 2006). Indeed, it appears that only specific hypaxial dermomyotomal cells of the brachio-thoracic region (somites 17–24 in chick) give rise to superior/inferior domains of the scapula blade and spine (Huang et al., 2006, 2000) and that such cells apparently do not mix from one axial level to the next (Ehehalt et al., 2004; Huang et al., 2000). When these somites are ablated the scapula blade is also absent (Malashichev et al., 2008). Importantly, at this stage, dermomyotomal cells from the cervical region also appear incapable of forming scapular elements, indicating that the brachio-thoracic dermomyotome is pre-patterned with the intrinsic ability to form the scapula blade (Ehehalt et al., 2004). These findings support a pre-patterning mechanism, although evidence for a direct role of a Hox code is weak (see below) and needs further testing.

Shortly after this pre-patterning stage, the dermomyotome proper forms as a dorsal subdivision of the somite (Gilbert, 2000) and further subdivides into an epaxial domain situated dorsal-medially and a hypaxial domain located ventral-laterally (Burke & Nowicki, 2003; Cheng, Alvares, Ahmed, El-Hanfy, & Dietrich, 2004). Here, dermomyotomal differentiation occurs under distinct regulatory inputs: in the epaxial domain, regulatory signals arise from the notochord and neural tube, whereas in the hypaxial region signals arise from the adjacent and more lateral plate mesoderm (Chiang et al., 1996; Ehehalt et al., 2004; Huang et al., 2006, 2000; Rong, Teillet, Ziller, & Le Douarin, 1992; Teillet, Lapointe, & Le Douarin, 1998). The hypaxial region appears important for blade differentiation since ablation of epaxial tissues results in normally patterned scapula blades (Teillet et al., 1998), whereas inhibition of signaling from the lateral plate mesoderm blocks scapula formation (Wang et al., 2005).

Once hypaxial dermomyotomal cells have been specified as scapular progenitors, additional signaling from the ectoderm and somatopleure is critical for dermomyotomal maturation, migration, and differentiation into prechondrogenic scapular mesenchyme. Based on the work of Moeller (2003), Rodríguez-Niedenführ, Dathe, Jacob, Pröls, and Christ (2003), and Ehehalt et al. (2004), Wang et al. (2005) argued that ectodermal WNT signaling into the hypaxial dermomyotome is critical for keeping cells in an undifferentiated epithelial state and that the cessation of such signals, which occurs normally in a cranial-to-caudal sequence during somitogenesis, leads to their differentiation into mesenchyme, an initial step into forming scapula cartilages. However, while Huang et al. (2000) demonstrated that only brachio-thoracic dermomyotome cells are capable of forming scapula blade condensations, Ehehalt et al. (2004) found that the ectoderm overlying the cervical region is able to support ectopic brachio-thoracic dermomyotome to form scapular mesenchymal and cartilage condensations. This finding suggests that signaling by the ectoderm (e.g., possibly by WNTs) might be responsible for inducing later, scapular mesenchymal- and/or cartilage-like fates, or that it plays an earlier, unappreciated role in pre-patterning cells to become scapular blade tissues. Additionally, during the transition to dermomyotomal mesenchyme, bone morphogenetic protein (BMP) signaling from the somatopleure facilitates the expression of early scapula progenitor marking genes, such as Pax1 (Wang et al., 2005), while blocking or inhibiting BMP signaling in this domain in chick results in the downregulation of Pax1 and the absence of the scapula blade.

The extent to which the dermomyotome contributes to the scapula blade across tetrapods remains quite unclear. In mice, fate-mapping experiments using Prx1-Cre/Z/AP and Pax3-Cre/R26RYFP reveal that dermomyotome cells give rise to only the vertebral edge of the scapula blade and minor parts of the spine and acromion (Durland et al., 2008; Valasek et al., 2010). Most blade regions are formed from the somatopleure, as is most of the head and neck, while the spine and acromion have been shown to have a neural crest cell contribution (see below) (Durland et al., 2008; Huang et al., 2006; Matsuoka et al., 2005; Valasek et al., 2010). This difference between chick and mouse appears to reflect the differential growth of the somatopleuric portion of the blade during mouse scapulogenesis (Shearman, Tulenko, & Burke, 2011; Valasek et al., 2010).

Regardless of the extent to which various embryonic tissues contribute to the scapula, the resulting cell populations become integrated into one mesenchymal condensation by embryonic day (E) 11.5 in mice (human E43), and into a clearly recognizable pre-cartilaginous scapula by E12.5 (human E44) located adjacent to the forelimb bud at the brachio-thoracic-axial level (Durland et al., 2008; Hita-Contreras et al., 2018; Huang et al., 2006, 2000; Young & Capellini, 2015). Recent analysis by Hita-Contreras et al. (2018) has shown that this mesenchymal condensation has multiple outgrowths: one each for the scapular body, coracoid process, and the acromion and spine. These findings agree with Capellini et al. (2010) assessment using 3D Optimal Projection Tomography of the boundaries of Sox9 expression, a marker of mesenchymal condensation formation. Subsequently, this mass shifts caudally to its final position posterior at the thorax, such that by human embryonic day 44 (E12.5–13 in mouse) it is positioned lateral to or behind the ribs, depending on the species (Bardeen & Lewis, 1901; O’Rahilly & Gardner, 1972).

Following this patterning stage, the chondrogenesis stage of bone formation begins at E12.5 in mice or the sixth gestational week in humans (Monkhouse, 1996). Like nearly all appendicular elements, the scapula forms via endochondral ossification (Karsenty & Wagner, 2002), in which the mesenchymal condensation that prefigures scapular morphology begins to differentiate into chondrocytes by secreting a collagen matrix consisting of types II, IX, and XI collagen, as well as proteoglycans (Long & Ornitz, 2013). The chondrocytes divide and differentiate, forming growth plates that ultimately elongate to form a nearly complete chondrocyte replica of the osseous scapula. During later embryonic gestation in mice (E15.5) or the second gestational month in humans (Andersen, 1963; Mall, 1906; Monkhouse, 1996; unpublished results from the Capellini laboratory), this cartilaginous scapula is invaded by blood vessels and osteoblasts allowing for the appearance of a single primary ossification center in the scapular neck region, which then facilitates bone formation (Long & Ornitz, 2013). Ossification typically extends bidirectionally across the blade medially and toward the glenohumeral joint laterally (Scheuer, Black, & Christie, 2000), reaching the base of the spine by gestational week 9 in humans (equivalent to E18.5 in mice) and the glenoid by gestational week 12 (P02 in mice) (Andersen, 1963; Ogden & Phillips, 1983).

During this period, post-otic neural crest cells also contribute to the scapula, specifically to endochondral and perichondrial tissues of the superior border, spine/acromion, and coracoid process, as well as to the connective tissues of the muscles inserting into these structures (Matsuoka et al., 2005). Sox10-Cre ROSA GFP + cells contribute specifically the endochondral and perichondrial tissues of these structures and by doing so they provide, and help anchor, major attachment regions for the branchial muscles (e.g., the trapezius). This process occurs after the initial patterning of scapula progenitors in early fetal development.

Scapula

The scapula, or shoulder blade (Figure 7.17a–b), is a flat, triangular bone. Its medial surface is nearly flat, whereas its lateral surface has a prominent scapular spine. Examine a mounted skeleton, and note that the apex of the scapula is directed ventrally. Identify the scapula’s anterior, dorsal, and posterior borders. The glenoid fossa is the smooth, concave surface at the apex for articulation with the humerus. The delicate coracoid process projects medially from the anterior margin of the glenoid fossa and is the site of origin for the coracobrachialis muscle.

The medial surface bears the subscapular fossa. It is relatively flat, with a few prominent scar ridges indicating tendinous muscular insertions. Note the prominent ridge near the posterior border that demarcates a narrow, slightly concave surface for muscular attachment. On the lateral surface, the scapular spine rises prominently and separates the supraspinous fossa anteriorly from the infraspinous fossa posteriorly, both of which are fairly smooth surfaces. Ventrally the spine ends in the acromion process. Just dorsal to the acromion process is the posteriorly projecting metacromion process.