The Thoracic Wall

Valerie F. Samii

The thoracic wall is composed of skin, fat, subcutaneous and intercostal musculature, parietal pleura, blood vessels, nerves, and lymphatics. The spine, ribs, costal cartilages, and sternum provide rigid support for the thoracic wall soft tissues. Thoracic wall abnormalities are often overlooked on initial appraisal of thoracic radiographs. Careful inspection of the extrathoracic soft tissue and bony structures is always warranted and can provide information critical to the appropriate diagnosis and treatment.

Normal Radiographic Appearance

The soft tissues of the thoracic wall are normally homogeneous in opacity. In particularly obese animals, curvilinear soft tissue opacities, representing extracostal musculature outlined by fat, may be seen paralleling the lateral curvature of the ribs on dorsoventral (DV) and ventrodorsal (VD) projections (Fig. 28-1). Thirteen pairs of ribs and eight sternebral segments are normal. On the lateral projection, the first few ribs are oriented vertically but become progressively more caudoventral in orientation, from rib head to costochondral junction, in the mid to caudal aspect of the thoracic spine (Fig. 28-2). On the DV or VD projection, the first few ribs are oriented perpendicular to the spine. In the mid to caudal thoracic spine, ribs curve in a caudolateral direction from their respective vertebrae to their most lateral extent, then continue caudomedially (Fig. 28-3).

Fig. 28-1 DV thoracic radiograph of an obese cat. Note the bilateral, curvilinear, soft tissue opacities peripheral to the lateral rib margins caused by extracostal muscles interposed between fat *(black arrows).*

Slight differences in thoracic wall conformation are common among various canine breeds (Fig. 28-4). Breed-associated costochondral conformation leading to a misdiagnosis of pneumothorax or pleural effusion is discussed in Chapter 31.

Fig. 28-4 Right lateral **(A)** and DV **(B)** thoracic radiographs of an adult Boston terrier. There are multiple wedge-shaped vertebrae and hemivertebrae in the midthoracic spine. The congenital vertebral anomalies in this dog have resulted in cranioventral angulations of the ribs on the lateral radiograph and a radiating appearance to the rib cage on the DV projection. Scoliosis of the midthoracic spine is evident. Extensive mineralization of the costal cartilages is present; this is a normal finding seen commonly in young and old dogs.

Mineralization of the costal cartilages may be seen in young dogs and cats and is nearly always present in older animals (Fig. 28-5). Movement at the costochondral and costosternal joints increases as the costal cartilages stiffen from mineralization. This in turn results in osseous proliferation at the costochondral and costosternal joints. The opacities resulting from enlargement of these joints may be confused with lung nodules on DV or VD radiographs. Excessive costochondral or costosternal mineralization may also be confused with aggressive processes such as infection or neoplasia.

Fig. 28-5 Right lateral radiograph of a middle-aged German shepherd with extensive costal cartilage mineralization and sternebral and costochondral degeneration.

Pedunculated soft tissue opacities on the thoracic wall, such as nipples, papillomas, or engorged ticks, may summate with the pulmonary parenchyma and be misdiagnosed as pulmonary nodules. Inspection and palpation of the thoracic wall surface usually clarify the significance of this finding. The inability to see the presumed pulmonary nodule in the lung on an orthogonal radiograph increases the likelihood of the suspicious opacity being caused by summation created by a superficial structure. Application of positive contrast medium, such as barium paste, on the thoracic wall nodule followed by repeat radiographs can be performed if localization is still in question (Fig. 28-6).

Fig. 28-6 Right lateral thoracic radiographs before **(A)** and after **(B)** application of barium paste to a nipple. In A, a nodular soft tissue opacity summates with the seventh costal cartilages *(black arrow).* In B, barium paste was applied to a palpable nipple on the skin, confirming that the nodular opacity was created by the nipple *(black arrow)* and not a lung nodule.

Congenital and Developmental Abnormalities

Anomalies of the ribs and sternum are fairly common. Rudimentary ribs are sometimes present on the seventh cervical vertebra (Fig. 28-7) or the thirteenth thoracic vertebra (Fig. 28-8). Ribs may be hypoplastic or absent on the thirteenth thoracic vertebral segment (Fig. 28-9). These anomalies may be unilateral or bilateral (Fig. 28-10). The clinical significance of rib asymmetry at the thoracolumbar junction with regard to their use as a surgical landmark was discussed in the “Incidental Factors” section of Chapter 7.

Fig. 28-7 Right lateral radiograph of a Pekingese dog with a left-sided rudimentary rib on the seventh cervical vertebra, forming a pseudoarthrosis with the first thoracic rib *(black arrows). T1,* First thoracic vertebra.

Congenital sternal deformities, such as reduction in number of segments, fusion of neighboring segments (Fig. 28-11), pectus carinatum (ventral displacement of the sternum), and pectus excavatum (funnel chest or dorsal displacement of the sternum) may be incidental findings, but some sternal anomalies have also been reported in animals with peritoneopericardial diaphragmatic hernia (Fig. 28-12).^1,2 Peritoneopericardial diaphragmatic hernias are discussed in Chapter 29.

Fig. 28-11 Right lateral radiograph of a young dog. There is fusion of the fourth and fifth sternebral segments, likely because of congenital malformation. The dorsal concave margin of the fused segments is smooth, and no evidence of degenerative disease is present to suggest trauma.

Pectus excavatum (Fig. 28-13) results in dorsal to ventral narrowing of the thorax and is often associated with respiratory and cardiovascular anomalies. Although pectus excavatum may be congenital or acquired in human beings, all reports in animals describe congenital malformations.³ The etiology is unknown; however, a hereditary component has been suggested.⁴