The sternoclavicular joint (SCJ) is a synovial sellar joint and represents the only skeletal articulation between the axial skeleton and the upper limbs (Johnson 2008). The joint is completely divided by an articular fibrocartilaginous disc and comprises the sternal end of the clavicle, cartilage of the first rib, and the clavicular notch of the manubrium, as well as the fibrous capsule, anterior and posterior sternoclavicular ligaments, interclavicular ligament, and the costoclavicular ligament (Johnson 2008). These ligaments provide stability to the joint (Fig. 11.2).

The first to seventh ribs articulate to the sternum by costal cartilages (Fig. 11.1). The cartilages of the first to tenth ribs join the superjacent costal cartilage, whereas the 11th and 12th ribs have a free anterior end. Slight gliding movements occur at sternocostal joints to allow for ventilation. The sixth to ninth costal cartilages articulate via synovial joint with a fibrous capsule, and these interchondral joints are strengthened by interchondral ligaments (Gatzoulis 2008). Costal cartilages are connected to the ribs via the costochondral junction, at which no movement occurs. Posteriorly, the head of the first and tenth to 12th ribs articulates with the corresponding vertebral body by a simple synovial joint (costocorporeal joint). In the others, an intraarticular ligament bisects the joint, producing a double synovial compartment (Gatzoulis 2008). For the first to tenth ribs, the facet of a costal tubercle articulates reciprocally with the transverse process of its corresponding vertebra, forming the costotransverse joint. Strong ligaments binding the costocorporeal and costotransverse joints allow only slight gliding along the shape and direction of the articular surfaces at these joints (Gatzoulis 2008).

Muscles around the thorax cover body parts from the neck to the upper abdomen. Intrinsic chest wall muscles include the intercostal muscles, subcostal, transversus thoracis, levatores costarum, and serratus posterior, all of which play a role in rib cage movement including ventilation (Gatzoulis 2008). A bony injury of the chest wall can damage these small muscles if the injury occurs at their attachment sites. Other muscles with direct attachments to the ribs or the sternum include the pectoralis major (Fig. 11.15), pectoralis minor, rectus abdominis (Fig. 11.16), serratus anterior, latissimus dorsi, transversus abdominis (Fig. 11.17), external/internal oblique (Fig. 11.17), levator costae, quadratus lumborum, erector spinae, scalenus, and subclavius muscles (Gatzoulis 2008).

Muscle injuries can be assessed by ultrasound or MRI (Figs. 11.18, 11.19, and 11.20). Ultrasound can be performed quickly in the emergency room or an outpatient clinic, facilitating a quick and dynamic imaging diagnosis. However, the examiner has to be aware that subtle grade 1 lesions may have a normal sonographic appearance in the acute stage (Koh and McNally 2007). MRI, on the other hand, has higher sensitivity to low-grade lesions, will give more detailed anatomical information, and enables imaging evaluation of deeper and a wider range of anatomical structures. For complete or partial tears of the pectoralis major, MRI has been shown to have excellent agreement with surgical findings (Carrino et al. 2000). It is important to recognize the severity of the muscle injury (complete or partial tear), as well as the location within the particular muscle because the therapeutic strategy will depend on the nature and location of the injury.

Pectoralis major injuries are relatively rare injuries that occur primarily while lifting weights (Petilon et al. 2005), but can occur in a rugby match when a player receives a direct blow to the muscle during a tackle (Fig. 11.18). Pectoralis major injuries are known to occur following an eccentric contraction of the muscle as it is actively stretched (Connell et al. 1999) and during a fall with the arm abducted and extended (Rehman and Robinson 2005). A snap may be heard at the time of injury, followed by acute pain. A tear may be at the sternal and/or clavicular heads, intramuscular (Fig. 11.18), or at the musculotendinous junction, or the injury may be distal tendon avulsion (Petilon et al. 2005). Complete tears are most commonly avulsion injuries at or near the humeral insertion (Petilon et al. 2005), which require surgical repair, while partial tears can be treated conservatively (Connell et al. 1999). On plain films, soft tissue swelling can be seen and the pectoralis major shadow is absent; a bone fragment may be present at the humerus in case of humeral avulsion at the pectoralis insertion (Hopper et al. 2010). To achieve accurate localization of injury by ultrasound and MRI, imaging should be performed in the abduction and external rotation position to extend and improve visualization of the musculotendinous junction (Koh and McNally 2007). Ultrasound imaging longitudinal to the pectoralis muscle fibers shows fiber disruption, retraction, and possible hypoechoic or anechoic hematoma, most commonly involving the musculotendinous junction of the sternal head (Weaver et al. 2005). With MRI, a combination of sequences should be used for anatomical delineation (T1-weighted spin echo or proton density-weighted spin echo) and fluid detection (T2-weighted spin echo, T2-weighted fast spin echo with fat suppression, or STIR). Because of the robustness of STIR for homogeneous fat suppression, STIR is the sequence of choice in thoracic assessment of muscle edema (Carrino et al. 2000). Oblique coronal sequences along the long axis of the pectoralis major tendon are most useful for grading a tear (Connell et al. 1999). In chronic injuries, axial T1-weighted sequences are helpful for demonstrating scarring and atrophy (Ohashi et al. 1996).