Chapter 16 Radiologic Aspects of Orthopedic Diseases

This chapter contains a radiologist’s viewpoints and suggestions regarding the interpretation of bone roentgenograms. It is hoped this will serve as a useful and practical guide for the busy physician. Principles concerning roentgenographic positioning, anatomy, and pathology are presented by using a regional anatomic approach. Additionally, the musculoskeletal applications of specialized radiologic modalities are discussed. This includes comments on nuclear medicine procedures, diagnostic ultrasound, arthrography, and angiography. Importantly, the imaging technologies of computed tomography (CT) and magnetic resonance imaging (MRI) as they pertain to the evaluation of orthopedic problems are detailed.

General Considerations of Roentgenographic Bone Anatomy

Before beginning a detailed discussion of regional roentgenographic anatomy and pathology, a consideration of pertinent radiologic bone anatomy is important. A bone can be evaluated according to its various components (Fig. 16-1).

Fig. 16-1 Anatomy of a bone. A, Radiologic terms: (1) articular cartilage does not show in the film; (2) white outline of the subarticular margin of the epiphysis; (3) epiphysis; (4) increased density of the terminal plate, inner bone margin of the epiphysis; (5) epiphyseal line, strip of lesser density, epiphyseal plate, diaphyseal–epiphyseal gap (roentgenographically, these terms exclude the recently calcified cartilage, which appears as part of the metaphysis); (6) metaphysis, including both calcified cartilage and newly formed bone, (7) diaphysis or shaft. B, Histologic terms: (1) articular cartilage; (2) compact bone of subarticular margin; (3) epiphysis, spongy bone; (4) terminal plate; (5) physis, epiphyseal disc, growth cartilage (histologically, these terms include the calcified cartilage); (6) metaphysis, including only newly formed bone of primary ossification; (7) spongy bone of diaphysis.

From Pyle SI, Hoerr NL: Radiographic atlas of skeletal development of the knee, Springfield, IL, 1955, Charles C. Thomas Publishers. Used by permission.

EPIPHYSIS

The primary growth center of a bone is termed the epiphysis. It contributes to the growth in length of the bone. It, as well as the following components, can be affected and changed in appearance by congenital, metabolic, nutritional, traumatic, and other disease processes.

PHYSIS

The radiolucent band between the epiphysis and metaphysis constitutes the physis, or epiphyseal cartilage plate. It is formed in part by the zone of provisional calcification.

METAPHYSIS

The metaphysis is characteristically splayed or funnel shaped. It is a favorite site for the development of benign and malignant lesions.

DIAPHYSIS

The longer tubular segment of bone forms the diaphysis. The constituent parts include the central medullary cavity and spongiosa and the dense cortex lined internally by the endosteum and covered externally by periosteum.

APOPHYSIS

There are several apophyses throughout the skeletal system; the best example is the greater trochanter of the proximal portion of the femur. Apophyses do not contribute to bone growth but are considered accessory ossification centers. They tend to appear later than the epiphyses in normal bone development and form protrusions to which ligaments and tendons attach.

Epiphyses and apophyses occasionally fail to unite to the parent bone, and this results in unfused ossicles or accessory bones that are often misinterpreted as fractures. Most of these are discussed under regional anatomic variations. When confronted with such problems as accessory bones, An Atlas of Normal Roentgen Variants That May Simulate Disease (Keats, 2007) and Borderlands of the Normal and Early Pathologic in Skeletal Roentgenology (Kohler and Zimmer, 1993) are excellent reference sources.

In addition to the status of nonfusion, epiphyseal and apophyseal centers can be divided before they unite and cause further confusion with fractures. Sesamoid bones, as seen in the hands and feet, can also be divided.

BONE VESSELS

There is only indirect evidence of the presence of blood vessels on roentgenograms, and it is the nutrient canals and foramina. They enter the medullary cavity by way of foramina, most often near the midshaft of a long bone, and result in radiolucent channels. When viewed on the roentgenogram, these channels can be confused with a fracture. Prominent nutrient canals frequently are seen in the diaphyses of the femur and tibia. Y-shaped nutrient canals occur in the midportion of the iliac bones of the pelvis and in the subspinous region of the scapulae.

Analytic Approach to Bone Changes

GENERAL CONSIDERATIONS

This section discusses some basic fundamentals and principles related to bone roentgenograms, rather than going into a great deal of confusing anatomic and pathologic detail. The first and foremost rule is that every roentgenogram is abnormal until it is proved, after thorough examination, that everything is normal. It may not work all of the time, but this approach encourages as complete a search of the film as possible. This results in two important objectives. First, it eliminates the method of instantaneous pattern recognition. Second, it prods the physician to search for even the slightest, most subtle change in bone roentgenographic patterns. It is important to avoid tunnel vision; one should look at the “whole picture,” from the corners of the film to the center. This involves separate evaluations of the soft tissues, joints, bone density, architecture, and trabecular patterns. Scrutinizing the medullary cavity and spongiosa, as well as the cortex, periosteum, and endosteum, is also necessary to get the overall picture.

In other words, an analytic approach should be attempted by evaluating each specific feature of bone. Each detail must be visualized separately. An obvious fracture catches one’s attention, and the tendency is to forget the remainder of the picture, possibly missing an associated but unsuspected dislocation or even an early destructive tumor.

Taking two views of a bone at right angles is an important law in radiology. Diagnostic interpretations should never be made on the basis of one view. There should be no hesitation to obtain additional projections of the suspicious area if there is any suggestion that something is wrong. Comparative films of the opposite side are also often helpful, especially in younger patients whose growth centers cause more than enough confusion. And if the roentgenograms remain normal to the eye but clinical suspicion persists, the patient must return in 7 to 10 days for repeated films, because it is not uncommon for a stress fracture, radial head fracture, or early osteomyelitis to have delayed appearances roentgenographically.

Once a bone lesion is discovered on radiographic film, a differential diagnosis can be formed. At first, the spectrum of possibilities might be quite broad, but by correlating historical, physical, and laboratory findings with the roentgenographic discovery, the spectrum can be narrowed to a few diseases and will, in most instances, lead to the correct diagnosis.

To permit the most accurate diagnosis, the differential list should be based on certain specific bone responses that are most characteristic of the disease process. There are various distinctive visible reactions that bone may have to singular disease entities, such as the pattern of bone destruction or production, the type of periosteal reaction, and soft tissue involvement, if any.

Unfortunately, there are more diseases affecting bone than there are responses that bone can create. Consequently, there are more roentgenographic similarities than differences among the various bony lesions. For example, the periosteal reaction usually associated with Ewing’s sarcoma, characteristically described as “onion skin,” quite often occurs with osteomyelitis.

Conversely, there are specific disease processes that produce more than one kind of predictable bone reaction. Such is the case with osteomyelitis, whose periosteal response can appear benign but can also mimic malignant changes.

Any office or clinic that performs diagnostic radiographic film studies should have appropriate references available for proper positioning of the patient. Several technical books may be found to assist the nurse or technician when examining the various skeletal regions. A three-volume set by Ballinger, Merrill’s Atlas of Roentgenographic Positions (2003), is highly recommended.

SPECIFIC PARAMETERS

Certain specific parameters of bone need careful attention. In the process of evaluating these, using a systematic approach can lead to a logical conclusion. Among the criteria to be analyzed are an increase or decrease in bone density, alterations in osseous texture (trabecular pattern), periosteal reactions, and the conditions of the cortex, endosteum, and medullary spongiosa. If any changes of these are observed, then the abnormality should be studied in terms of its size and configuration and the sharpness of its margins (transition zone). The specific bone involved and the position of the lesion within that bone (epiphysis, metaphysis, or diaphysis) should also be noted. For example, leukemia, metastatic neuroblastoma, a benign simple cyst, and Brodie’s abscess have a predilection for the metaphysis, whereas a chondroblastoma typically involves the epiphysis.

This is a simple, general outline, and many more particulars come into play. By developing this type of approach, the many clues offered will help to determine the nature of the pathologic process and place the lesion into a certain category (for example, benign, malignant, infectious, traumatic, or metabolic). Table 16-1 lists some of the specific characteristics of different bone lesions.

Table 16-1 General Characteristics of Different Bone Lesions

Characteristics of benign bone lesions

Sclerotic margins (narrow transition zone)

Homogeneous periosteal reaction

Expansion of an intact cortex

Characteristics of solitary malignant bone lesions

Permeative or moth-eaten destruction (wide transition zone)

Irregular, sometimes spiculated periosteal reaction

Preferential metaphyseal location

Extraosseous extension with soft tissue mass and occasional fluffy calcifications

Characteristics of metastatic lesions

Absence of periosteal reaction

Moth-eaten destruction of medulla and cortex

Preferential diaphyseal location

Pathologic fractures

Multiple bone involvement

Characteristics of infection

Often irregular periosteal reaction, no spiculation

Bone destruction variable

Diaphyseal involvement, often involving long segments

Destruction of adjacent cartilage, crossing joints (majority of malignant neoplasms lack this ability)

Sequestration and involucrum formation

Two parameters require more expanded discussion: patterns of bone destruction and periosteal reaction.

PATTERNS OF BONE DESTRUCTION

Three roentgenographic patterns of bone destruction have been described, according to Lodwick: geographic, moth-eaten, and permeative.

1 Geographic bone destruction is distinguished by single or multiple sharply marginated, relatively large, punched-out holes. Multiple myeloma and histiocytosis X (eosinophilic granuloma, Letterer–Siwe disease, and Hand–Schüller–Christian disease) tend to fit this pattern.

2 Moth-eaten bone destruction involves a lesion containing multiple coalescing holes of moderate size that suggests a somewhat aggressive process. Osteomyelitis and less pathogenic tumors may exhibit this form of destruction.

3 Permeative bone destruction is seen when the bony alterations are characterized by multiple tiny holes that become smaller and fewer in number near the periphery of the lesion; as a result, there is a wide transition zone from abnormal to normal bone. This type of destruction is seen in very aggressive tumors and poorly localized infections.

Regardless of the type of osseous resorption that is taking place, as much as 50% of the bone must be destroyed before it becomes evident roentgenographically. Radionuclide bone scans constitute a much more sensitive examination for determining the presence of bone replacement by the tumor or infection. The more advanced nuclear technology of single proton emission computed tomography (SPECT) imaging further enhances this lesion detection sensitivity. In one comparative study, the accuracy of isotope imaging with technetium 99m phosphate complexes for skeletal lesion detection was 98%, as compared with 28% for standard roentgenography (Siberstein, Saenger, Tofe, 1973). Because of this fact, the use of skeletal roentgenographic surveys has declined in favor of nuclear scans. Nevertheless, although the sensitivity of scanning is high, its specificity is low, and therefore roentgenographic studies of the abnormal isotope areas are mandatory. CT and MRI can be a helpful adjunct in the determination of the presence of marrow replacement by metastatic lesions.

TYPES OF PERIOSTEAL REACTIONS

When the outer periosteal membrane of bone is irritated, its constituent osteoblastic cells react by producing new bone. This new bone can take on either a solid or an interrupted appearance, and the specific form can assist in identifying the inciting cause (Table 16-2). Solid periosteal reactions are usually indicative of a benign process. The new bone is consistently uniform in density. As seen in Table 16-2, the thickness and marginal characteristics vary according to the precipitating factor.

Table 16-2 Roentgenographic Types of Periosteal Reactions *

Types	Examples
Solid periosteal reaction
Thin	Eosinophilic granuloma, osteoid osteoma
Thin undulating	Hypertrophic pulmonary osteoarthropathy
Dense undulating	Vascular
Dense elliptical (with destruction) osteoid osteoma
Cloaking	Long-standing malignancy
Chronic infection
Interrupted periosteal reaction
Perpendicular (spiculated or sunburst)	Osteosarcoma, Ewing’s sarcoma, infection
Lamellated (onion skin) Osteosarcoma,	Ewing’s sarcoma, infection
Amorphous	Malignant tumors
Codman’s triangle	Malignant tumors, infection, hemorrhage

* From Edeiken J, Hodes PJH: Roentgen diagnosis of diseases of bone, Baltimore, 1967, Williams & Wilkins. Used by permission.

The interrupted forms of periosteal reactions are more commonly found in malignant disease, although benign lesions such as infection are occasionally responsible. The classic spiculated or sunburst pattern of osteogenic sarcoma and the lamellated or onion skin, periosteal reaction of Ewing’s sarcoma come under this heading (see Table 16-2).

Of the various types of periosteal reactions, Codman’s triangle is of special interest. This form is produced by a lifting of the periosteum, which causes a break in its continuity and forms an angle. Thought to be pathognomonic of malignancy at one time, it is now known to develop in benign diseases as well. The sign can be induced by underlying infiltrating tumor cells, pus, or hemorrhage.

Another interesting periosteal condition is hypertrophic pulmonary osteoarthropathy. The uniform, thin, but solid undulating new bone, often associated with bone pain, is a relatively infrequent roentgenographic finding in individuals suffering from benign and malignant thoracic tumors, chronic obstructive pulmonary disease, and chronic lung infections. The exact mechanism is uncertain, but it may be related to decreased arterial oxygen tension, or an unrecognized humoral substance that mediates connective tissue proliferation. The long bones of the forearms and lower extremities and the distal ends of the metacarpal and metatarsal bones are most frequently involved. Clubbing of the fingers and toes may occur, and arthritic symptoms may be manifested.

Regional Anatomic and Pathologic Roentgenography

CERVICAL SPINE

ROENTGENOGRAPHIC EXAMINATION

The standard roentgenographic views of the cervical spine include anteroposterior, lateral, and both posterior oblique views, as well as an open-mouth anteroposterior projection of the odontoid process and the first two cervical vertebral segments (Fig. 16-2). The majority of cervical spine studies are performed for the evaluation of trauma. In the more severely injured individual, the most important views are a cross-table horizontal beam lateral image and an anteroposterior supine film. By keeping patient movement to a minimum, potentially fatal spinal cord injury is prevented.

Fig. 16-2 Normal roentgenographic study of the cervical spine. A, Anteroposterior view. B, Lateral view. C, Right oblique view. D, Odontoid view.

On the lateral exposure, C7 and occasionally C6 often are excluded, especially in heavy, short-necked individuals. In such instances, it becomes necessary to depress the shoulders by gentle but firm downward traction of the arms. If the cervicothoracic junction still has not been adequately visualized and severe injury to the neck has been excluded, a so-called swimmer’s view may be obtained. The patient lies in a prone-oblique position with the higher tube-side arm above the head and the lower table-side arm beside his or her body. The x-ray tube is angled 15 to 20 degrees toward the feet.

Other special projections can occasionally be of assistance. Flexion and extension lateral views often reveal minor degrees of subluxations resulting from damage to ligaments.

Dr. Don Weir of Saint Louis University developed the “pillar view” for better evaluation of the lateral articulating masses, the superior and inferior articulating facets, and their intervening joints. Moreover, this view brings into focus the anterior and posterior margins of the lamina (Fig. 16-3). The film is produced with the patient supine. Each side is done separately. The head is turned very slightly to one side, with the roentgenograph tube angled toward the feet 35 to 45 degrees and centered over the middle to lower vertebrae. The opposite side is then examined in a similar manner.

Fig. 16-3 Pillar views of the cervical spine. Right (A) and left (B) projections.

When available, CT with three-dimensional (3-D) reconstruction can eliminate plain film radiographs altogether for assessment of cervical spine trauma, and this can be performed quickly with minimal patient movement.

ROENTGENOGRAPHIC ANATOMY

A thorough understanding of the roentgenographic anatomy of the cervical spine is an absolute prerequisite to its proper evaluation. Each component of the individual vertebrae should be appraised separately on all views.

Odontoid View

Of the seven cervical vertebrae, the first two are anatomically distinct. The odontoid represents a superior extension of the body of C2 and is actually the vestigial body of C1. On the anteroposterior open-mouth film, the odontoid should be analyzed in terms of its position between the two lateral articulating masses of C1 (see Fig. 16-2, D). The spaces between the lateral edges of the odontoid and the medial borders of the C1 articulating masses should be equal. However, minor degrees of rotation can produce spurious inequality of these interval distances. How can this be determined? The alignment of the densities of the spinous processes of C1 and C2 can be seen. If the C2 spinous process is to one side or the other, then rotation is present.

Also on viewing the odontoid film, the transverse processes and lateral borders of the articulating masses of C1 and C2 should not be overlooked. The horizontal joints between the atlantoaxial articulating masses should be symmetric (see Fig. 16-2, D).

Confusing artifacts superimposed on the odontoid can be misleading and can result in misinterpretation of fractures of this structure. The inferior margin of the anterior or posterior arch of C1 can overlie the base of the odontoid and create a “Mach” effect, a radiolucent line produced by overlap of the edges of two bones. In a similar fashion, the space between the two incisor teeth may lay over the odontoid and yield an artifactual vertical cleft (Fig. 16-4).

Fig. 16-4 Artifacts over the odontoid process simulating fractures. A, Anterior arch of C1. B, Cleft of the incisor teeth.

It is not unusual for the odontoid process to be completely obscured by the base of the occipital bone if the head is held too far in extension. In such circumstances, have the view repeated with the patient’s head slightly more flexed.

Anteroposterior View

The straight anteroposterior film of the cervical spine demonstrates several specific structures. The vertical alignment of the spinous processes, as well as the lateral margins of the articulating masses, should be followed. The uncinate processes are the small triangular projections arising from the posterolateral margins of the vertebral bodies that by their apposition form the uncovertebral joints (also termed joints of Luschka). These establish the anterior boundaries of the intervertebral foramina. As synovial joints, they can be involved by degenerative osteoarthritis and produce the spurs that encroach on and narrow the foramina so as to result in impingement of the cervical nerve roots. In addition, being lined with synovium, they are subject to the alteration of rheumatoid arthritis and rheumatoid spondylitis. However, the foramina are best evaluated on the oblique films.

Several important points must be remembered when viewing the lateral projection of the cervical spine (see Fig. 16-2, B). Alignments of the anterior and posterior borders of the vertebral bodies, alignment of the lateral articulating masses, and alignment of the spinolaminar line are studied. The latter is formed by the anterior margins of the spinous processes, which also describe the posterior surface of the spinal canal. The superior extension of this line is in direct alignment with the posterior margin of the foramen magnum.

The distance between a line drawn through the posterior borders of the vertebral bodies and the spinolaminar line provides the anteroposterior width of the spinal canal. This should measure no less than 13 mm from C3 through C7, being normally wider above C3. Any measurement less than this suggests the possibility of spinal stenosis.

The clivus at the base of the skull, which is a dense line continuous with the dorsum sella, is a helpful indicator for confirming normal craniovertebral alignment and should be included, at least in part, on all lateral studies of the cervical spine. A line drawn along its margin will pass through the posterior third of the odontoid.

The superior and inferior articulating facets ordinarily are well visualized on the lateral film. The posterior borders of the lateral articulating masses should form a straight line (see Fig. 16-2, B). If there is a slight offset, a facetal dislocation must be considered, although slight rotation can give a similar appearance. The view should be repeated if there is any confusion, and if the problem persists, CT scanning should be considered.

Measurement of the space between the anterior border of the odontoid and the posterior margin of the anterior arch of C1 is mandatory. This is an indicator of a possible transverse ligament tear and should measure no more than 5 mm in children and 3 mm in adults in neutral, flexion, and extension positions.

Finally, evaluation of the lateral cervical spine film is incomplete without an analysis of the prevertebral soft tissues. There are minor differences in the measurement from physician to physician and from patient to patient, but the width of the soft tissues at the level of the inferior margin of the C3 body should not exceed 5 to 7 mm. Any increase in this measurement indicates swelling from hemorrhage or infection.

Oblique Views

The intervertebral foramina and surrounding elements, including the pedicles and uncinate processes, are the most important components demonstrated on the oblique views. The foramina on the right side are visualized on the left posterior oblique projection and those on the left on the right posterior oblique film (see Fig. 16-2, C).

TRAUMA

Recognition of an abnormal cervical spine should be a relatively simple task once normal roentgenographic anatomy has been mastered. After a traumatic lesion has been identified, the primary objective is the establishment of whether the condition is stable. The stability of a fracture is best determined by grouping the type of injury according to the mechanisms of trauma, which are outlined in Table 16-3. From this classification, a statement of the instability of the injury can be made (Table 16-4).

Table 16-3 Classification of Cervical Spine Injuries According to the Mechanisms of Injury *

Flexion

Subluxation

Bilateral interfacetal dislocation

Simple wedge fracture

Flexion teardrop fractures

Clay-shoveler’s fracture

Flexion-rotation

Unilateral interfacetal dislocation

Vertical compression

Bursting fractures

Jefferson’s C1

Bursting fractures, other levels

Extension

Posterior neural arch fracture

Extension teardrop fracture

Hangman’s fracture

* From Harris JH Jr: Acute injuries of the spine, Semin Roentgenol 13:53–68, 1978. Used by permission.

Table 16-4 Classification of Cervical Spine Injuries Based on Stability *

Stable

Subluxation

Simple wedge fracture

Unilateral interfacetal dislocation

Bursting fracture, except Jefferson’s fracture of C1

Clay-shoveler’s fracture

Unstable

Bilateral interfacetal dislocation

Flexion teardrop fracture

Jefferson’s bursting fracture of C1

Hangman’s fracture

Extension teardrop fracture, unstable in extension but stable in flexion

* From Harris JH Jr: Acute injuries of the spine, Semin Roentgenol 13:53–68, 1978. Used by permission.

Clay-Shoveler’s Fracture

This lesion is named for the injury acquired by people in occupations that require heavy lifting. This injury consists of a fracture through the spinous process of either C6 or C7 (Fig. 16-6). There is no significant ligamentous damage, and therefore a stable condition exists. Because C7 is sometimes difficult to project on the lateral film, the fracture can be missed. A clue might be apparent on the frontal film, where an extraspinous process fragment is sometimes evident as the result of inferior displacement. An unfused apophysis of a spinous process must not be misinterpreted as a fracture (Fig. 16-7).

Fig. 16-6 Clay-shoveler’s fracture of the cervical spine. A, A lateral view defines spinous process fractures of C6 and C7 (arrows). B, An AP projection demonstrates a “double” spinous process of C6, suggesting a fracture (arrow).

Fig. 16-7 An unfused apophysis of the spinous process of C7 simulates a fracture (arrow).

Flexion-Rotation Injuries

When the cervical spine is subjected to a rotational force in addition to flexion, a unilateral interfacetal dislocation may be observed (Fig. 16-8). An inferior articular facet of one body is displaced over the adjacent superior articular facet of the next inferior vertebra on one side only. The dislocated facet is more or less locked in place, but because of associated ligament damage, the lesion may be unstable; therefore, flexion and extension views are contraindicated. Oblique projections are most productive in identifying the displaced facet, but it may be suggested on the lateral film when the margins of the facets of a single vertebra do not superimpose on one another as the result of rotation. Additionally, a line drawn through the posterior borders of the bodies is offset at the level of the suspected injury.

Fig. 16-8 Unilateral interfacetal dislocation. A, Lateral view. B, Oblique view.

Extension Injuries

Various extension injuries are described as follows.

Posterior Neural Arch Fractures

The neural arch of one vertebra may be compressed between the posterior elements of the two adjoining vertebrae during maximal forced extension. A unilateral or bilateral fracture may be sustained. If bilateral, the fragment can be displaced posteriorly, yet there will be no encroachment on the neural canal; thus, the situation is a stable one (Fig. 16-9).

Fig. 16-9 Posterior neural arch fracture.

Extension Teardrop Fracture

Similar to the flexion variety, an extension teardrop fracture demonstrates a triangular-shaped fragment at the anterior-inferior margin of the body, most often involving C2, although other levels are affected (Fig. 16-10). However, the injury is not quite as severe, because there is no posterior involvement. When the neck is held in flexion, there is stability of the spine because the posterior ligament complex is intact. However, instability occurs during extension because the minor fragment remains attached to the anterior longitudinal ligament but not to the parent body.

Fig. 16-10 Extension teardrop fracture of C2 (arrow).

Hangman’s Fracture-Dislocation of C2

This lesion consists of vertical disruption of both pedicles of C2 and is created by flexion forces against the extended vertebrae. The body of C2 is thrust forward over C3, with concomitant rupture of both the anterior and the posterior longitudinal ligaments, giving rise to an unstable injury (Fig. 16-11).

Fig. 16-11 Hangman’s fracture-dislocation of the second cervical vertebra.

Vertical Compression Fractures

A considerable force directed through the vertical axis of the spine (such as a large object falling on top of the head or a diving injury) can produce the so-called bursting fractures of the cervical vertebrae. The least common type is the Jefferson fracture of C1. The anterior and posterior aspects of the ring are fractured on both sides, with bilateral lateral displacement of the fragments. This unstable injury can be apparent on the open-mouth anteroposterior film but more reliably is distinguished on the lateral roentgenogram where the fractures are seen to extend through the posterior arches.

Vertical compression or bursting fractures more frequently involve the middle and lower cervical vertebral segments. The longitudinally oriented pressures cause the intervertebral discs to impact against the end plates of the bodies. Ordinarily, there is no instability because the posterior elements remain intact. When viewed on the anteroposterior film, a vertical fracture line is identified within the involved body.

Although vertebra plana is not usually related to trauma, it might be mistaken for a compression fracture. This very rare condition, in most instances related to eosinophilic granuloma (histiocytosis X), has the appearance of a pancake with extreme, uniform flattening of the body but with preservation of the posterior elements. Some other possible causes for this unusual finding are metastasis, multiple myeloma, osteochondritis of the primary ossification center of the body (Calvé disease) and postradiation osteitis.

THORACIC AND LUMBAR SPINE

ROENTGENOGRAPHIC EXAMINATION

The thoracic spine is ordinarily examined by means of anteroposterior and lateral films. However, because of the superimposition of the shoulders, the upper three to four vertebrae are omitted from the lateral view, and if clinical symptoms point to this region, an oblique swimmer’s projection should be obtained. It is generally agreed that the above films will offer optimum information about the anatomy of the thoracic vertebrae. However, either or both oblique views of no more than 10 to 20 degrees can be obtained and provide a different aspect of a questionable or subtle change in the vertebral bodies or articulating facets.

Routine roentgenographic examination of the lumbar spine must be composed of anteroposterior, lateral, and both oblique projections as well as a cone-down lateral film of the lumbosacral joint.

Roentgenographic Anatomy

Anteroposterior View

On the anteroposterior study, the 12 thoracic and 5 lumbar bodies show a slight increase in size from top to bottom. The height, width, and alignment of the bodies and their cortical margins, trabecular architecture, and density need to be scrutinized carefully.

Subsequently, the round-to-oval pedicles need to be evaluated separately. They overlie the superolateral corner of each body and, like them, become progressively larger inferiorly. The density and sclerotic cortical margins should be noted for any alterations, because they are involved in a number of pathologic processes.

The interpedicular distance, a line drawn between the inner boundaries of a vertebral body’s pedicles, is an important observation throughout the spine. It affords an indirect evaluation of the size of the neural canal.

The transverse processes in the thoracic spine become smaller from superior to inferior, are oriented posterolaterally, and are located behind the intervertebral foramina. The normal articulations of the ribs should not be overlooked. In the lumbar spine, the transverse processes are more laterally oriented and are much larger, the processes of the third vertebra usually being the largest.

On the anteroposterior projection, the spinous processes provide another guide for the evaluation of alignment. Rarely, they may be absent because of aplasia or malignant or infectious destruction.

Additionally, on the anteroposterior view, the paravertebral soft tissues should be studied. An increase in the width of the soft tissues alerts the clinician to a possible fracture, neoplasm, or infection in the spine, the result of hematoma, infiltrating tumor cells, or bacterial extension, respectively. The psoas margins in the paralumbar region are fairly sensitive indicators to similar changes when they become obliterated. This abnormal change can also reflect lymphadenopathy, hematuria, or urinary lesions such as a perirenal abscess. In fact, when viewing the lumbar spine, the clinician should always attempt to delineate the psoas and renal margins as well as search for urinary tract calculi as causes of back pain.

Lateral View

One of the goals for studying lateral views of the thoracolumbar spine is to ensure normal alignment of the anterior and posterior borders of the vertebral bodies. Furthermore, the cortical outlines and the internal structure of the bodies must be mentally recorded. Any narrowing of the intervertebral disc spaces can be established. In the thoracic spine, the intervertebral foramina can be seen on end but require oblique views for their proper visualization in the lumbar area.

The inferiorly directed spinous processes, as seen on the lateral film, may require a “hot” light to be adequately seen but should not be neglected, for reasons mentioned previously.

Oblique Views

Important anatomic features are introduced on the oblique views of the lumbar spine. These details have been covered more extensively in Chapter 8. On the left posterior oblique film (left side down against the film), the left half of the posterior neural arch elements, including the intervertebral foramina, are identified, whereas the right half of these components are viewed on the opposite projection. In the cervical spine, the reverse condition exists.

DEVELOPMENTAL VARIATIONS AND CONGENITAL ABNORMALITIES

Confusing anatomic variations are seen in the developing and mature spine. In infancy, the vertebral bodies are egg-shaped on lateral view. The upper and lower anterior corners are beveled until the apophyseal vertebral rings appear about each end plate. They fuse with the body by the fifteenth year, but occasionally they remain ununited and, except for the presence of a complete sclerotic border, often are confused with a corner fracture (Fig. 16-12). These are sometimes referred to as limbus vertebrae.

Fig. 16-12 An unfused ring apophysis of a lumbar vertebra as seen here can be confused with a fracture.

On the anteroposterior film, a vertical radiolucent cleft is seen superimposing on the vertebral bodies (Fig. 16-13). This represents the normal uncalcified portion of the posterior neural arch and spinous process and is not to be confused with spina bifida. At approximately 3 to 5 years of age, these will ossify and fuse. When they do fail to fuse completely, the cleft persists as a spina bifida occulta. This can occur anywhere in the spine, but is most common at L5. These are of no clinical importance.

Fig. 16-13 Normal thoracic spine of an infant. Note the vertical cleft of the neural arch.

True spina bifida is the result of a very wide defect in the posterior neural arch, usually involving multiple vertebrae and commonly found in the lumbar region. The widened spinal neural canal is evidenced by an increased interpedicular distance. These may be associated with a meningocele that produces a prominent posterior soft tissue density on the roentgenogram.

This wide gap of the dorsal arch and increased interpedicular width should also bring to mind the possibility of diastematomyelia, a condition in which the cord is divided by an intraspinal cartilaginous, fibrous, or bony spur. This has a predilection for the thoracolumbar junction. Syringomyelia can produce a similar appearance in the cervical spine where there is cystic dilation of the cord. The same type of appearance can also be seen with intraspinal tumors, such as lipomas or dermoids.

Vascular channels may give rise to confusing appearances. Blood vessels perforate the anterior cortical borders of each body and form a radiolucent line, termed the clefts of Hahn. A similar vascular defect that becomes more apparent in adulthood may exist along the posterior margin of the body.

Hypoplasia of the vertebrae, fusion or block vertebrae, and hemivertebrae constitute a few of the other possible spinal anomalies (Fig. 16-14).

Fig. 16-14 Anomalies of the spine. A, Hemivertebra of the lumbar spine. B, Fusion or block vertebrae.

TRAUMA

The focus of attention will now be shifted to some of the traumatic lesions involving the thoracolumbar spine. The majority of injuries disturb the muscular and ligamentous structures surrounding the spine. Just as in the cervical spine, the only visible roentgenographic findings may be scoliosis and/or straightening of the normal curvatures, which indicates muscle spasms. When a fracture is present, the usual appearance is a wedge-shaped compression deformity, the result of hyperflexion forces. Avulsion-type corner fractures of the anterosuperior body result from extension forces. They appear as a very sharp, nonsclerotic fragment as opposed to an unfused apophysis.

Quite frequently, the dilemma of determining the age of a fracture presents itself. In long-standing compression fractures, degenerative changes with hypertrophic spurs arise along the articular margins, and there may be an unpredictable degree of eburnation. Yet at times, these changes are absent, and the determination of age may be difficult, if not impossible. Radioisotopic bone scans that use ^99mTc phosphate compounds then become a valuable aid. Within 2 to 5 days of the injury, a recent collapse shows an increased uptake of the isotope, whereas older lesions show no activity. However, the abnormal uptake may remain detectable for 18 to 24 months or longer, depending in part on the extent of the fracture and its location.

Wedge-shaped deformities of the vertebral bodies necessitate differentiation from other causative processes such as Scheuermann disease. This represents a form of osteochondritis or aseptic necrosis that involves the end-plate ring apophyses and is seen in adolescents. Described as juvenile kyphosis, it predominates in the thoracic region. Schmorl’s nodes commonly accompany this spinal affliction. These are rounded-to-oval protrusions of the anterior margins of the end plates and develop from focal intrabody herniation of the nucleus pulposus.

Biconcave deformities of the bodies, or so-called fish vertebrae, have a predisposition for the thoracic and lumbar location. They represent a chronic progressive form of compression characteristically seen in postmenopausal patients with osteoporosis. Vertebral body compression fractures with a history of minimal or no trauma should alert the physician to the possibility of a metastatic or primary malignant process.

Another important traumatic lesion, particularly of the lumbar spine, that can be easily overlooked is the transverse process fracture (Fig. 16-15). The importance of its recognition rests with the fact of frequently related renal injury. It is important not to mistakenly call the unfused apophysis of the transverse process a fracture.

Fig. 16-15 Transverse process fractures of the lumbar spine in two different patients. A, Fracture of L4 on the left on plain film (arrow). B, CT in another trauma patient demonstrating a fracture of the transverse process (arrow).

A seldom-seen injury to the lumbar vertebrae is the Chance fracture (Fig. 16-16). An extreme flexion force, often related to seat belt injuries, causes a fracture through the spinous process, across the neural arch or lamina, and into the posterosuperior aspect of the vertebral body. An isolated fracture of the spinous process is uncommon in the thoracic and lumbar regions.

Fig. 16-16 Chance fracture of the lumbar spine that extends through the body into the posterior neural elements and can be seen in individuals wearing seat belts. A, Anteroposterior view demonstrating lateral displacement of the pedicles (arrows). B, Lateral view showing a fracture extending through the body (curved arrow) and involvement of the neural arch (straight arrow).

One more pitfall to watch for in the assessment of spinal trauma is the unfused ossification center of the articular facets, which can resemble a fracture (Fig. 16-17). In evaluating the posterior elements, it is important to keep in mind the entities of spondylolysis and spondylolisthesis (see Chapter 8).

Fig. 16-17 Unfused apophysis of the superior articular facet of L5 (arrow).

TUMORS

Except for metastatic neoplasms to the spine, tumors—both benign and primary malignant lesions—are very infrequent. The vertebrae are the most commonly affected portion of the skeleton for metastasis and account for approximately 40% of all secondary tumors of bone. A few of the common and not-so-common tumors are discussed.

Hemangioma

Hemangiomas of the vertebral body may be the most common benign tumor, although few have been documented pathologically. Typically, their appearance is that of increased vertical trabeculations producing a somewhat striated pattern (Fig. 16-18). They should not be confused with Paget’s disease or osteoblastic metastasis.

Fig. 16-18 Hemangioma of a lumbar vertebra. A, Plain film. B, CT image. Note incidental finding of a herniated disc (arrow).

Aneurysmal Bone Cyst

Of the benign tumors of the spine, aneurysmal bone cysts are possibly the next most frequent after hemangiomas. They are more commonly found in the extremities, especially the distal portion of the femur. In the spine, they tend to involve the posterior elements as well as the body. Possessing a thin shell of peripheral bone, they are expansile and cystic in nature. Roentgenographically, a malignant appearance can be seen with a predominant cystic pattern. Unlike other tumors, benign and malignant alike, aneurysmal bone cysts have the unique capability of crossing over the cartilaginous disc space to involve adjacent vertebrae. A malignant chordoma also has this potential.

Osteoid Osteoma

A benign tumor that can produce significant and disabling pain is the osteoid osteoma. Most frequently found in the extremities, this tumor can involve the spine on rare occasions and must be considered in the differential diagnosis of back pain. It is one of many causes of scoliosis, most likely the result of muscle splinting secondary to pain. The lesion is almost always limited to the neural arch, a position that often prevents its detection on conventional roent genographs. CT and isotopic SPECT bone scanning may be required for its recognition. The lesion is composed of a small radiolucent nidus sometimes containing calcifications. A very dense reactive sclerosis surrounds the central nidus and often obscures it.

Osteoblastoma

An osteoblastoma is a benign lesion that classically involves the spine, primarily the dorsal elements. Because of its varied histologic appearances, it has been described as a giant osteoid osteoma, and certain cases may even mimic osteogenic sarcoma.

A wide variety of primary malignant tumors may involve the spine, only a few of which are mentioned. Such tumors may be seen anywhere in the spine but have a preference for the sacrococcygeal region.

Multiple Myeloma

The most common primary malig-nant tumor of bone is multiple myeloma. The classic picture of “punched-out” lesions, particularly evident in the skull, is characteristic but not that frequent. The usual presentation is diffuse demineralization. Multiple vertebral compression deformities form in a progressive nature.

Sarcoma

Osteosarcoma, fibrosarcoma, and chondrosarcoma involve the spine only rarely and usually in the sacrum. These tumors may arise in an area of previous irradiation, Paget’s disease, or fibrous dysplasia.

Round-Cell Tumors

Ewing’s sarcoma and reticulum cell sarcoma are rarely encountered in the vertebral column. They have more of a predilection for the appendicular skeleton but must be included in the differential diagnosis of destructive lesions.

Hodgkin’s Lymphoma

Hodgkin’s disease can occasionally be seen within the bony spine and may present as an osteolytic or osteoblastic alteration or a combination of the two. However, more commonly, paraaortic lymph node involvement with lymphoma produces erosive changes of the vertebral cortical margins.

In the same way, an aortic aneurysm can cause pressure destruction of the vertebra. A massively enlarged heart such as that in mitral stenosis can produce extrinsic changes of the thoracic spine. It is of interest that elderly individuals who ordinarily demonstrate degenerative spurs of the thoracic spine only show them to any extent on the right side; apparently, the pulsating aorta prevents their formation on the left.

Chordoma and Sacrococcygeal Teratoma

There are two primary malignant tumors of the spine that are classified as developmental. A chordoma is an invasive lesion arising from remnants of the primitive notochord. It therefore can presumably occur anywhere along the vertebral column, although there are no reported cases of its occurrence in the thoracic region. The sacrum is the most popular site and accounts for approximately 50% of cases. An intracranial chordoma located along the clivus represents about 30% of cases, and the remaining 20% originate in the cervical and lumbar areas. Roentgenographically, the tumor presents as an expansile, lytic process with moderate soft tissue extension. As noted previously, it may cross the disc space cartilage, an uncommon occurrence of any tumor but typical of infections. In one third of the tumors, calcifications are observed. The second type of developmental primary malignant tumor is a sacrococcygeal teratoma. However, 60% of these lesions are benign but exhibit some degree of localized infiltration. They are found in infants, within the pelvis, around the parasacral region. Malformation of the spine can be an associated finding. The lesions appear roentgenographically as variable-sized soft tissue masses, often with calcifications or ossifications within them. The rectosigmoid colon is extrinsically displaced forward. Destructive changes are present to some extent within the sacrococcygeal bony structure. CT, MRI, or both modalities together can demonstrate these changes more accurately.

INFECTION

For obvious reasons, infections involving the spine have decreased in frequency over the years. Nevertheless, this must be considered in the differential diagnosis of a painful back and abnormal roentgenographic findings. Acute bacterial infections or pyogenic spondylitis can involve either the disc space, the vertebral body, or both. The former condition, termed pyogenic discitis, is more common in the younger population, apparently on the basis of a healthy vascularization of the cartilage and, therefore, easy access by blood-borne bacteria. This results in a closed-space infection that can secondarily extend into the bodies.

On the other hand, infection of the vertebral body, particularly its anterior two thirds, is more commonly identified in the adult. Originating in the substance of the body, the infection may then secondarily involve the interspace by extension.

An important principle in pathophysiology should be reemphasized at this point. Cartilage serves as no barrier to the extension of infection and thereby is vulnerable to destruction. Conversely, the disc cartilage is very resistant to malignant cellular infiltration, so tumors tend to remain confined to the body.

In both pyogenic discitis and vertebral osteomyelitis, the earliest roentgenographic findings may be joint space narrowing. Depending on the aggressiveness of the offending bacteria and the time of institution of therapy, the surrounding bone shows varying degrees of demineralization brought on by hyperemia. The end plates then demonstrate progressive loss of continuity with irregular destruction and focal areas of subchondral reactive sclerosis. Extension into the posterior third of the body, then into the dorsal appendages, as well as into the adjoining vertebrae, may occur. The body may eventually collapse. Some degree of surrounding soft tissue swelling is invariably present and detectable on roentgenograms and corroborated on MRI (Fig. 16-19).

Fig. 16-19 Pyogenic discitis. A, Anteroposterior and B, lateral views of the thoracic spine demonstrating pyogenic discitis (large arrows). The disc space has been destroyed with extension of the infection into the vertebral bodies with trabecular compression and reactive sclerosis. An associated paravertebral soft tissue mass (open arrows) indicates expansion of the inflammatory process. Note aorta (small arrows). C, Corresponding sagittal T-I post-gadolinium MRI images exhibit the infection extending through the posterior neural elements (pedicles and lamina) into the epidural space and producing cord compression.

Tuberculosis of the spine, or Pott’s disease, is an extreme rarity among today’s abnormal spine studies. The vertebral column remains the most common skeletal site for this chronic infection. The midthoracic and thoracolumbar junctions constitute the most prevalent sites of involvement.

Because of its insidious and chronic nature, the spinal lesions are usually advanced when first examined roentgenographically, although this is not always the situation. An occasionally prominent feature is a large paravertebral soft tissue mass that constitutes the abscess. In later phases, there may be extensive calcification within this mass (Fig. 16-20). The bony structures show a decrease in density, and there is narrowing of one or more of the disc spaces. The margins of the end plates manifest irregular destruction, and a mottled sclerotic and lytic appearance extends into the bodies, which display varying degrees of compression.

Fig. 16-20 Pott’s disease of the spine. The chronic tuberculous process has resulted in calcified paravertebral psoas abscesses.

SPINAL ARTHRITIS

Arthroplasty of spondylosis of the spinal column most commonly is degenerative in nature with the formation of hypertrophic bony spurs. These osteophytes are formed by recurrent stimulation of wear and tear factors, and are predominantly located along the end-plate margins of the vertebral bodies. When they bridge the interspaces and fuse, such spurs are described as syndesmophytes, which can more often be identified in specific forms of arthritis, such as diffuse idiopathic skeletal hyperostosis (DISH; Fig. 16-21).

Fig. 16-21 DISH (diffuse idiopathic skeletal hyperostosis). A, Cervical spine and B, lumbar spine radiographs with large bridging osteophytes (syndesmophytes).

Ossification of the anterior longitudinal ligament of the spine is characteristic of ankylosing spondylitis (Marie–Strümpell disease). Psoriatic arthritis, Reiter disease, and other autoimmune diseases (including inflammatory bowel diseases) may lead to exuberant osteophyte formation.

Degenerative findings of the facet joints, particularly those of the lumbar spine, can result in various degrees of spinal stenosis. This is the result of bone spurs and soft tissue hypertrophy, primarily the ligamentum flavum. These produce narrowing of the central neural canal and can compromise the cord. Extension into the subarticular recesses and neuroforamina will encroach on the descending and exiting nerve roots, resulting in radiculopathy.

Degenerative disc disease is a commonly associated manifestation of arthritis. The aging process as well as trauma (either acute or related to chronic recurrent and repetitive injury) leads to eventual deterioration, dehydration, and protrusion or herniation of the nucleus pulposus through the annulus fibrosis. Broad-based posterior protrusion of disc material, along with degenerative hypertrophic spurs and thickened soft tissue elements, initiates the symptoms of spinal stenosis.

PAGET’S DISEASE

One of the distinctive roentgenographic characteristics of spinal Paget’s disease is that the disorder involves the entire vertebra—the body and all of the posterior elements, including the transverse and spinous processes (Fig. 16-22). Generally, there is an increase in density produced by a thickened trabeculae. A picture frame appearance can be imparted to the body, the result of a dense peripheral margin of thickened cortex. Furthermore, there is an actual increase in volume of the entire vertebra. This finding, along with total vertebral involvement, distinguishes Paget’s disease from an osteoblastic metastasis.

Fig. 16-22 Spinal Paget’s disease and osteoblastic metastasis. A, Paget’s disease demonstrates increased trabeculations, a bone-within-bone appearance, and increased volume including the pedicles. B, Osteoblastic metastasis.

THE SHOULDER

ROENTGENOGRAPHIC EXAMINATION

In the majority of cases, a complete and optimal roentgenographic study of the shoulder girdle needs only include upright—standing or sitting—anteroposterior views with the humerus in internal and external rotation. These two projections usually permit adequate evaluation of the bony structures constituting the shoulder. In the uncooperative child or severely traumatized patient, these films may be obtained while the patient is in the supine position. On these roentgenograms, the complete clavicle should be visualized in addition to the entirety of the scapula and at least the proximal third of the humerus. Special attention should be given to the alignments of the glenohumeral, acromioclavicular, coracoclavicular, and sternoclavicular joints. The tuberosities of the humeral head and its articular surface should be specifically scrutinized. The acromium and coracoid processes, in addition to the borders and flat surfaces of the scapula, require attention.

The standard examination, of course, may require modification, because there are certain situations when special views must be obtained for better delineation of specific structures. In an acutely injured shoulder in which immobilization of the shoulder is required, the humerus ordinarily is held in internal rotation. To obtain roentgenograms of the humerus in external rotation without moving the arm, patients can be positioned by rotating them 40 degrees posteriorly with the affected side toward the film. Because of pain, this requires that patients be sitting or standing.

Upright exposures are also beneficial in demonstrating fat-fluid levels. This occurs when a fracture has extended into the joint through the articular cortex; this most often happens with fractures of the greater tuberosity. Such information is useful since articular cartilage damage is certain, and the patient should be informed that degenerative osteoarthritis in time is a possibility.

When there is suspicion of a fracture involving the shaft of the humerus below the neck, in addition to an anteroposterior view, a transthoracic lateral projection is necessary for evaluation of alignment (Fig. 16-23). The glenohumeral relationship can also be accessed with proper exposure, but because of superimposed ribs, fractures above the neck are difficult to see. Instead, a transscapular study is more helpful (Fig. 16-24). This view is obtained by having the patient face the film at an angle of 45 degrees with the affected side toward the film holder. The resultant picture forms a Y where the acromium, coracoid, and scapular body intersect. The humeral head will be superimposed on the Y. This is useful not only for fractures but also for posterior dislocations. This transscapular examination is always used when studying the scapula, in addition to the routine anteroposterior roentgenogram. CT or MRI imaging, particularly the latter, can provide additional valuable information especially regarding soft tissue involvement. The multiplanar capabilities of these imaging modalities augments visualization of gross pathologic findings.

Fig. 16-23 Transthoracic view of the shoulder.

Fig. 16-24 The scapular Y view. The humeral head should be centered at the intersection of the coracoid, spine, and body. This view also can visualize any bony abnormalities of the underneath surface of the acromion which could cause impingement.

Posterior shoulder dislocations are notorious for their ability to avoid detection on the standard anteroposterior views; they are frequently associated with seizure disorders. The tangential projection is an excellent means for detecting the abnormality. This is performed by angling the roentgen tube approximately 30 degrees so that the central ray passes tangentially across the glenoid articular surface. In this manner, the glenohumeral relationship is much better defined than on the conventional straight anteroposterior view.

The axillary film also provides another perspective of the shoulder anatomy (Fig. 16-25). The acromium, coracoid, glenohumeral joint, and humeral head are viewed in a different plane. By holding the film against the top of the shoulder, the picture is produced by abducting the arm and centering the roentgen tube through the axilla. The study requires a special “grid” cassette for the film. It is helpful in determining the presence of a humeral head displacement, but it is not recommended when a dislocation has just been reduced, because the required abduction may reluxate the joint.

Fig. 16-25 Axillary view of the shoulder. Note the glenohumeral relationship, the hooklike coracoid process, and the acromium behind the head of the humerus.

The clavicle itself requires two projections in the anteroposterior plane to evaluate it properly. One film should be made perpendicular to the plane of the body, whereas the other exposure is made with a 15- to 20-degree cephalad angulation.

The acromioclavicular joints are best inspected with an anteroposterior film and tilting the roentgen tube 15 degrees toward the head. Comparative films are necessary to determine the presence of a separation, and this should be performed with and without weight bearing.

A more difficult area to examine roentgenographically is the sternoclavicular joint. Because of bony superimposition, CT may be required, but because this is not universally available, both oblique and lateral views may suffice. However, if these attempts are unsuccessful, the patient can be positioned prone. The roentgen tube is then centered through the sternoclavicular joints and angled toward the head at a 35-degree tilt.

ROENTGENOGRAPHIC ANATOMY AND DEVELOPMENTAL VARIATIONS

A considerable degree of confusion can be created by the ossification centers of the shoulder. The proximal humeral epiphysis does not become visible, as a rule, until the fourth to eighth month of life. There are two, occasionally three, separate centers. By the age of 20, the proximal epiphysis fuses to the shaft of the humerus. Before complete closure of the epiphyseal suture, the lucent line has a peculiar angulated appearance (Fig. 16-26). Overlap of the suture line occurs no matter what projection is used and consequently produces an image simulating a fracture.

Fig. 16-26 Normal proximal humeral epiphysis in a child.

A deceptive appearance of the proximal humeral shaft is the deltoid tuberosity, which, incidentally, receives the tendinous attachment of the deltoid muscle. The thickened cortex in this area may bring to mind the periosteal reaction of infection or tumor.

On a congenital basis, there may be a complete absence of the clavicle or at least partial underdevelopment of its lateral end. This represents hereditary cleidocranial dysplasia, a disease that also affects the skull, pelvis, hips, and other skeletal regions.

Normally, the outer end of the clavicle appears less dense than the middle and sternal aspects, the result of a lesser thickness of overlying soft tissue. Soft tissues are also responsible for a discrete line density paralleling the superior border of the clavicle that measures no more than 4 mm in width and is described as the “accompanying shadow.” This shadow may be thickened in cases of subtle fractures.

The rhomboid fossa is a familiar anatomic variation of the clavicle and is found along the inferior border at its medial aspect as a notch-like depression. This represents the location of the insertion of the costoclavicular (rhomboid) ligament, which secures the first rib to the clavicle. Its irregular appearance has been misinterpreted as a destructive process.

TRAUMA

Of all the joints in the body, the shoulder is the most frequent site of dislocations. More than 97% are anterior and can be described as subglenoid or subcoracoid, depending on the location of the humeral head. Invariably, this form of dislocation can be visualized by a single anteroposterior roentgenogram (Fig. 16-27). Whenever a dislocation occurs, it is not unusual to have an associated fracture; this should be searched for on the film. With an anterior dislocation, a fracture of the greater tuberosity may exist; the lesser tuberosity is vulnerable in posterior dislocations. The lower glenoid margin is also susceptible in either type of dislocation. An infraction of the articular cartilage without a visible fracture is also possible in dislocations and may require MRI to demonstrate.

Fig. 16-27 Anterior dislocation of the shoulder, AP.

Not infrequently with intraarticular extension of a fracture in the absence of true dislocation, an intact joint capsule may become progressively distended with blood, and if enough blood accumulates, there will be inferior displacement of the humeral head away from the glenoid. This condition is termed pseudosubluxation, or hanging shoulder.

With chronic recurring anterior dislocations, a defect becomes apparent along the superolateral aspect of the humeral head. This cortical infraction, present because of impaction against the anteroinferior rim of the glenoid, is commonly referred to as Hill–Sachs deformity. This abnormality is usually best demonstrated on an internally rotated anteroposterior view (Fig. 16-28).

Fig. 16-28 Hill–Sachs deformity in chronic recurring anterior shoulder dislocation.

The less frequent posterior dislocation is much more difficult to visualize roentgenographically. The problem arises because the humeral head on the standard anteroposterior projections shows no apparent separation from the glenoid fossa when in fact it is separated. The posterolateral orientation of the glenoid articular surface accounts for this to some extent. Ordinarily, in the normal shoulder the head of the humerus overlaps about three fourths of the glenoid fossa. When it becomes posteriorly dislocated because of lateral displacement of the head, this overlap is less, but the change may be very subtle. This perplexing situation can usually be solved by using the 30-degree tangential view, the axillary projection, or the transscapular film (Fig. 16-29). With recurrent posterior dislocations of the shoulder, a defect of the inferoposterior cartilagenous labrum may not be visible by stand-ard radiography. This is termed a Bankart lesion, which can be diagnosed by MRI or CT arthography.

Fig. 16-29 Posterior shoulder dislocation. A, Anteroposterior view showing some discrepancy in the glenohumeral joint. B, A posterior fracture-dislocation.

A very rare form of dislocation of the shoulder joint is luxatio erecta, a condition where the humeral head is located under the glenoid rim and the humeral shaft is directed above the head in fixed abduction (Fig. 16-30). A lateral fall on an elevated arm produces such an abnormality. The acromium process of the scapula acts as a fulcrum pushing the head of the humerus down and out of the glenohumeral joint. The dynamics of the applied forces can result in a fracture of the acromium, the lower lip of the glenoid, or the greater tuberosity of the humerus. Often, there is an associated tear of the rotator cuff.

Fig. 16-30 Luxatio erecta of the shoulder. Note the superior orientation of the humeral shaft and the head of the bone locked under the glenoid.

Fractures involving the proximal portion of the humerus are classified according to involvement of the head, either tuberosity, or the surgical or anatomic neck. Additionally, a fracture-displacement of the yet unfused epiphysis may occur.

For descriptive purposes, anatomists describe the surgical neck as that portion of the humerus just inferior to the tuberosities where there is a normal narrowing. The constriction or shallow groove at the articular margin of the head of the humerus is referred to as the anatomic neck.

Fractures of the proximal portion of the humerus must be evaluated by at least two views at right angles to one another. An anteroposterior view alone with a transthoracic lateral projection fulfills this criterion, but obliquely directed x-ray films may be necessary to best demonstrate displacement and/or angular deformity.

Transverse subcapital fractures of the surgical neck along with avulsions of the greater tuberosity are the most commonly encountered injuries to the shoulder. A fracture-displacement of the ununited epiphysis creates a more difficult diagnostic challenge. The normal epiphyseal line itself, as already mentioned, makes interpretation troublesome. The normal epiphysis has a uniform width and dense margins, whereas a fracture demonstrates varying thickness with a sharp, nonsclerotic edge. Often, there is impaction or distraction of the fragments causing some degree of overlapping, which makes the diagnosis somewhat simpler. Of course, if the presence of an abnormality is uncertain or indeter minate, an accurate comparison with the opposite uninjured shoulder should be performed.

The infraglenoid area of the scapula is the most common location of fractures of this bone. The normal lucent nutrient canal in this region with its dense margins should not offer much concern. Instead of a discrete fracture line, only a zone of increased density offers any clue to the injury. The clinician should also search for underlying rib fractures.

A complete fracture of the clavicle in most circumstances will exhibit overriding of the fragments when they involve its midportion. Because of the forces applied by the pectoralis minor muscle, as well as the weight of the arm, the lateral fragment is displaced inferiorly in most cases. An incomplete or nondisplaced fracture may be so subtle as to avoid detection initially, but the two standard views, including straight anteroposterior and cranially angulated projections, will in most instances reveal the fracture.

TUMOR

The proximal aspect of the humerus is a relatively common location for a benign solitary cyst, which may appear as either simple or multiloculated (Fig. 16-31). Such cysts are found within the metaphysis and exhibit destruction of the medullary spongiosa with more or less expansion of the bone. Thinning of the cortex can attain paper thickness, and pathologic fractures are a very common event, even in the absence of significant trauma. When initially discovered, the cysts extend to the epiphyseal line but do not involve the growth center itself. When followed serially until fusion of the epiphysis, the cyst appears to “migrate” toward the middle of the shaft as growth of the bone ends progresses.