Paul & Juhl’s Essentials of Radiologic Imaging
7th Edition

Chapter 8
Normal Anatomic Variants and Miscellaneous Skeletal Anomalies
Lee F. Rogers
L. F. Rogers: Department of Radiology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157.
Unlike the congenital and genetic disorders described in Chapter 9, the anatomic variants and miscellaneous skeletal anomalies described here are encountered daily in the practice of radiology. A general familiarity with these structures and their variants is necessary to lessen concern, decrease confusion, and increase the accuracy and confidence of radiographic interpretation. Their importance lies in the fact that they can easily be misinterpreted as pathologic conditions by those unfamiliar with them, when in reality they are of little or no clinical significance. When misconstrued as an abnormality they may lead to costly and unnecessary additional examinations. One of the principal differences between the trained and experienced diagnostic radiologist and other physicians is the ability of the radiologist to recognize and dismiss normal variations, thereby avoiding a potentially expensive, added workup for the patient. The texts by Keats9 and by Kohler and Zimmer11 are excellent, thorough references devoted to this subject. It is difficult to conceive of practicing radiology without one or both of them close at hand. In this chapter, only the most common variants of the peripheral skeleton are described. Many are found in the skull and spine and are covered in Chapters 11 and 12, respectively.
Miscellaneous skeletal anomalies are isolated, anomalous developments. In most cases, they are sporadic and nonfamilial. They may be considered as (1) supernumerary developments, (2) a failure to develop, or (3) segmentation defects.
Supernumerary development is usually seen in the hands and feet as the development of an extra digit (polydactyly) on either side of the hand or foot (Fig. 8-1). These may be sporadic or genetically determined and, at times, are associated with malformation syndromes.
FIG. 8-1. Duplication of the thumb in the newborn. Note the two separate and distinct distal and proximal phalanges, with fusion of the intervening soft tissues.
Failure to develop is usually either hypoplasia of a structure or, less commonly, aplasia. Hypoplasia is most commonly identified in the middle phalanx of the fifth digit and is known as clinodactyly (Fig. 8-2). This condition may be sporadic, familial, or associated with a whole host of congenital disorders. Hypoplasia and aplasia are most commonly encountered in the posterior elements of the spine, particularly in the pedicle and transverse process.
FIG. 8-2. Clinodactyly in a 53-year-old father (A) and his 9-year-old son (B), an example of familial hypoplasia. The deformity was bilateral in both father and son. A: The middle phalanx of the fifth digit is slightly shortened and curved inward. B: The middle phalanx of the fifth digit is short and curved inward distally. The proximal epiphysis of the phalanx is already beginning to close. Compare with the epiphyses of the distal and proximal phalanges.
Defects of segmentation are common. They may consist of a fusion of segments, commonly encountered in the vertebrae as a partial or complete fusion of two or more vertebral bodies, which is described in Chapter 12. Fusion occurs less commonly in the carpal (Fig. 8-3) and tarsal bones. The fusion of tarsal bones is often symptomatic (Fig. 8-39 and 8-40).
FIG. 8-3. Lunatotriquetral fusion in a 13-year-old girl. The lunate (L) and triquetrum (T) are fused, and there is a small, incomplete cleft between them distally.
FIG. 8-4. Bifid anterior third anterior left rib (asterisk and arrows).
FIG. 8-5. The os tibiale externum (arrow), adjacent to the proximal pole of the navicular.
FIG. 8-6. Two accessory ossicles. A: Os peroneum. B: Os acetabuli.
FIG. 8-7. Common accessory ossicles in the foot. 1, Os trigonum; 2, os tibiale externum; 3, os peroneum; 4, os intermetatarseum; 5, calcaneus secondarius; 6, supranavicular; 7, secondary astragalus; 8, os vesalianum; 10, os sustentaculi; 12, sesamoid tibiale anterius; S, sesamoid bones. The small black dots over the metatarsal heads and proximal phalanges of first and second toes represent the most frequently observed sites of the sesamoid bones, but they may occur in other locations. There are no numbers 9 or 11 in this diagram.
FIG. 8-8. Sesamoid bones. A: Feet. Sesamoid bones underlie the head of each metatarsal (arrows). There are two sesamoids on the great toe, a medial (tibial) sesamoid, and a lateral (fibular) sesamoid. The tibial sesamoid of the great toe and that of the second metatarsal are bifid, a common normal variant. Sesamoid bones are always present on the great toe and less commonly on the fifth. They are rarely encountered on the second, third, and fourth. Although the findings in this case are unusual, they are of no clinical significance. B: Hands. Sesamoid bones are found in relation to the heads of the first, second, and fifth metacarpals (arrows). There is also a sesamoid at the base of the distal phalanx of the thumb (arrow). There are two sesamoids at the head of the first metacarpal. Sesamoid bones of the hand and feet lie within the anterior joint capsule.
FIG. 8-9. Lateral view of the proximal femur demonstrating a vascular groove in the posterior cortex (arrow). Note the slightly roughened but normal posterior surface of the cortex. This is known as the linea aspera and represents the site of insertion of the abductor muscles.
FIG. 8-10. Bone bars. A: Anteroposterior (AP) view. The bright dots (arrow) of trabecular bone are even more dense than the cortex. This is a normal finding and should not be mistaken for evidence of bone infarction or chondroid calcification associated with an endochondroma. B: Lateral view. The bright dots on the AP view are these elongated, horizontal, coarse trabeculae (arrow) seen on end. These are more commonly found in the osteopenic elderly at this site, the distal humerus, and elsewhere.
FIG. 8-11. Student’s tumor. Prominent ossification in the cartilaginous ends of the first ribs (arrow). These are easily misconstrued as tumors in the underlying lung.
FIG. 8-12. Cervical rib. A: The transverse process of C7 is usually prominent, as seen in this case on the right. The cervical rib extends from this transverse process in a manner similar to the thoracic ribs, as seen on the left (arrow). B: The cervical rib is better seen in the oblique view (arrow).
FIG. 8-13. Hypoplastic first ribs. The first rib is hypoplastic bilaterally (arrows). It is often difficult to determine whether there is a hypoplastic first rib or a cervical rib. The distinction is made by simply counting the ribs on both sides.
FIG. 8-14. Fusion of the first and second ribs bilaterally (arrows).
FIG. 8-15. Ununited apophyses of the transverse processes of L1. The transverse processes are not united with the main body of the vertebra (arrows). Note that the medial borders of the transverse processes are slightly rounded and faintly sclerotic, as are the opposing margins of the base of the transverse processes. The sclerotic margin distinguishes this condition from a fracture. These may be either unilateral or bilateral.
FIG. 8-16. Sprengel’s deformity. The left shoulder is affected. There is an associated abnormality of ossification of the cervical and upper thoracic vertebrae, with irregular segments fused together (Klippel-Feil deformity). These deformities frequently coexist.
FIG. 8-17. The omovertebral bone in association with Sprengel’s deformity. The bone forms an articulation with the scapula (arrow) and the arch of one of the cervical vertebrae.
FIG. 8-18. Pseudocyst of the humeral head. A radiolucency is present within the greater tuberosity because of the relative absence of bone trabeculae. This is a frequent normal finding that is easily mistaken for evidence of metastatic disease or other abnormalities (see Fig. 8-34).
FIG. 8-19. Proximal humeral epiphysis. Anteriorly the growth plate is chevron shaped, whereas posteriorly it is transverse (arrow) and could be misconstrued as a fracture.
FIG. 8-20. Rhomboid fossa. The scalloped inferior margin of the medial clavicle (arrows) represents the site of insertion of the pectoralis muscles. It is present bilaterally in this case but may be seen unilaterally.
FIG. 8-21. Supraclavicular nerve foramen. The small lucency in the superior cortex of the middle third of the clavicle (arrow) represents a foramen for the supraclavicular nerve.
FIG. 8-22. Supracondylar process of the humerus. There is a hook-like projection of cortical bone arising from the medial surface of the distal shaft of the humerus. This is the characteristic location and position of the supracondylar process, which should not be mistaken for an exostosis.
FIG. 8-23. The most frequent accessory ossicles in the hand and wrist. 1, os centrale; 3, os radiale externum; 4, os triangulate; 5, epilunatum; 6, os vesalianum manus; 7, epipyramis; 8, os styloideum; S, the most common sites for sesamoid bones in the hand. Number 2 is not included.
FIG. 8-24. Pseudoepiphyses and supernumerary epiphyses. The former are represented by incomplete clefts in the distal ends of proximal phalanges and proximal end of the fifth metacarpal. The latter are present at the proximal end of the second and fifth metacarpals and the distal end of the first metacarpal.
FIG. 8-25. Madelung’s deformity A: Oblique, B: Lateral, and C: PA projections demonstrate characteristic findings. The radius is curved and shortened in comparison to the ulna. The radial joint surface is tilted ulnarward and anteriorly. The hand is displaced anterior to the long axis of the forearm (B). The corpus assumes a triangular configuration (C).
FIG. 8-26. Carpal boss with os styloideum (arrow). A: Lateral view. B: Posteroanterior (PA) view. Bony prominence dorsally at the base of the second metacarpal as seen on the dorsum of the wrist (arrow) is not easily identified on the PA view. It is often difficult to determine on the plain films whether this is simply a bony protuberance or a separate ossicle, the os styloideum.
FIG. 8-27. Pubic synchondrosis in a 12-year-old. This represents the junction of the inferior ischial and pubic rami. The pattern of ossification is highly variable, often asymmetrical, and easily misinterpreted.
FIG. 8-28. Herniation pit of the femoral neck. Note the circular sclerotic radiodensity on the lateral margin of the femoral neck (arrow). In some cases, the pit may be smaller and the rim of sclerosis thicker.
FIG. 8-29. Linea aspera—pilaster complex. A: The two long, roughly parallel lines (arrows) represent the margins of the pilaster complex on the posterior surface of the femur. This represents the site of insertion of the adductor muscles of the thigh. Calcification is present in the femoral artery (open arrow), and distally there is the conglomeration of calcium representing either an enchondroma or the residuals of previous bone infarct. B: Lateral view. Note the roughened surface of the posterior femoral cortex. This is the appearance of the pilaster complex or linea aspera when seen in profile. It is normal but could be misconstrued as evidence of periosteal new-bone formation. Note the fabella (arrow).
FIG. 8-30. Physiologic bowlegs. A: Initial views demonstrate moderate bowleg deformity. B: Approximately 1 year later, the bowing has largely disappeared. Note that the bowing involves both femur and tibia.
FIG. 8-31. Blount’s disease. There is bilateral involvement, with an angular deformity at the physis. The tibial shafts are straight and the femurs are uninvolved.
FIG. 8-32. Pellegrini-Stieda disease. A thin, shell-like calcification is seen at the superior margin of the medial femoral condyle (arrow).
FIG. 8-33. Cortical desmoid. Note the irregularity of the surface of the cortex of the distal femur (arrows) at the site of the insertion of the adductor magnus and gastrocnemius muscles.
FIG. 8-34. Tibial tubercle and soleal line. A: Anteroposterior view. Prominent ossification of the interosseous membrane on the lateral surface of the tibia (arrows) is known as the tibial tubercle. This is commonly present but is of variable size. A similar ossification is present on the apposing margin of the fibula. The soleal line is seen on edge as an oblique line projecting in the medullary canal (open arrow). B: Lateral view. The soleal line is seen on the posterior surface of the tibial cortex (arrow). This may be misconstrued as evidence of periosteal new-bone formation associated with infection or stress fracture. The tibial tubercle cannot be seen on the lateral projection.
FIG. 8-35. Apophysis of the base of the fifth metatarsal. The secondary ossification center parallels the lateral aspect of the proximal end of the metatarsal. In this case, there is also an undisplaced transverse fracture (arrow) at the base of the fifth metacarpal.
FIG. 8-36. Bifid sesamoids. Both the medial and lateral sesamoids of the great toe are bifid (arrows).
FIG. 8-37. Pseudocyst of the calcaneus. The relative radiolucency beneath the tuber angle (arrow) represents an area devoid of trabeculae. This is a normal finding and is similar to the pseudocyst of the humeral head seen in Fig. 8-18.
FIG. 8-38. Congenital radioulnar synostosis with congenital dislocation of the head of the radius. There is bony fusion between the proximal radius and the ulna.
FIG. 8-39. Tarsal coalition, calcaneonavicular bar (arrow). Note also the thin, white line paralleling the distal tibial joint surface. This is an epiphyseal scar, the bony residua of the physis.
FIG. 8-40. Tarsal coalition, talocalcaneal fusion. A: Lateral view of the foot demonstrates pes planus, a prominent anterior beak of the talus (open arrow), and poor definition of the posterior facet of the talocalcaneal joint. There is a suggestion of fusion of the medial facet in the region of the sustentaculum tali (arrow). B: Normal side for comparison. C and D: Computed axial tomograms. Normal side (C) demonstrates normal appearance of sustentaculum tali (arrow). Affected side (D) demonstrates fusion of the medial facet of the talocalcaneal joint (arrow).
The failure of segments to fuse is most commonly encountered in the spine, involving the laminae and spinous processes. This failure of fusion is known as spina bifida. When seen as an isolated radiographic abnormality, it is known as spina bifida occulta. It may be associated with other congenital abnormalities of the spine, as described in Chapter 12.
Bifid structures may also occur. These may develop from a natural growth, as in the bifid anterior margin of ribs (Fig. 8-4), or from a failure of fusion of structures that arise from more than one center, most commonly in the sesamoids of the great toe (see Fig. 8-36). Certain apophyses and epiphyses arise from multiple centers of ossification, which eventually fuse in most cases. Multiple centers of ossification are seen in the proximal humerus and elbow. The trochlear center of ossification of the elbow itself may arise from separate centers, as may the epiphyses of the proximal phalanx of the great toe. Less commonly, individual carpal or tarsal bones arise from separate centers of ossification.
Accessory centers of ossification and accessory bones are found rather frequently in the skeleton (Figs. 8-5 and 8-6). An accessory bone represents either a supernumerary ossicle not ordinarily found in the skeleton or a secondary center of
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ossification that has failed to fuse and remains as a separate structure (Fig. 8-7). On occasion, they may predispose to injury or degenerative change and be responsible for symptoms.13, 15, 23 This is often associated with a positive bone scan in the region of the abnormality. Sesamoid bones (Fig. 8-8) arise in tendons, particularly those of the feet, and are very similar in appearance to accessory centers of ossification. These small accessory bones and sesamoids may be mistaken for pathologic conditions, particularly fractures, and knowledge of their distribution and frequency is therefore important.
DIFFERENTIATION OF ANOMALOUS BONES FROM FRACTURES
A fracture line is ragged along its margin, irregular, and poorly defined; anomalous ossification centers and sesamoids are characterized by smooth cortical margins (see Figs. 8-5, 8-7, and 8-8). An avulsion or small fracture has an irregular, uncorticated surface at the line of fracture and a defect in the adjacent bone that corresponds to the avulsed fragment. Fresh fractures are accompanied by swelling of the contiguous soft tissue, which should not be present about an accessory center. Accessory centers and anomalous bones are commonly bilateral. Examination of the corresponding part of the opposite extremity is helpful in doubtful cases, but it is usually unnecessary. The precise diagnosis can usually be determined by reference to standard charts and diagrams (see Figs. 8-7 and 8-23).4
Normal Radiographic Findings Confused with Pathology
Nutrient Canals and Foramina
Nutrient canals are present in all the long and short tubular bones. These are fine, sharply marginated radiolucencies that extend obliquely
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through the cortex, and they should not be mistaken for a fracture (Fig. 8-9). Nutrient canals are less radiolucent than a fracture and have a characteristic course. Nutrient foramina generally occur at the ends of bones, appearing as a small circular radiolucency, and are seen most commonly at the intercondylar notch of the knee.
Interosseous Ridges
Ossification of interosseous membranes occurs between the tibia and fibula and between the radius and ulna. These are generally thin, slightly undulant, and smooth flanges of bone that may be mistaken for periostitis or periosteal new-bone formation (Fig. 8-34). They are characteristically located on the apposing margins of the bones, although they may be more pronounced on one bone than the other. Ossification is more common on the diaphysis of the ulna and fibula and the proximal metaphysis of the tibia.
There are numerous osseous ridges and grooves at the sites of muscular and ligamentous attachments. Many of these bony prominences are commonly encountered daily and dismissed without equivocation, such as the deltoid eminence of the humerus and the linea aspera of the femur (Fig. 8-29).
Irregular and Bifid Epiphyses
Certain ossification centers are often irregular in outline at some point during the course of development. This is particularly true of the distal femoral epiphysis (see Fig. 1-4B) in the child younger than 5 years of age. The trochlear ossification center at the elbow is similarly irregular. Clefts may be seen in the epiphysis.9 The basal epiphysis of the proximal phalanx of the great toe is the most frequent site, but clefts may occasionally be seen elsewhere and should not be mistaken for fractures. Characteristically they are well marginated, undisplaced, and have no surrounding soft-tissue swelling, allowing the distinction to be made with confidence.
Metaphyseal Spurs
Small spur-like projections are often encountered at the periphery of the metaphyses of both long and short bones in infants.10 These are normal but may be mistaken for subtle evidence of injury or even child abuse by the unwary.
Epiphyseal Scars
For variable periods after closure of a physis, a fine radiodense line is present at the site, referred to as an epiphyseal scar (see Fig. 8-39). This is eventually resorbed and is usually no longer evident after the age of
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40 years. Just after closure of a physis, there may be a slight irregular margin at the peripheral edge of the epiphysis, possibly suggesting a fracture. This is most commonly encountered at the lateral edge of the distal radius.
Bone Bars
In older persons, after the onset of osteoporosis and on occasion earlier, groups of horizontally oriented large bone trabeculae may be encountered. When seen on edge, usually on the anteroposterior view, they appear as a collection of punctate or increased densities; but when seen in profile on the lateral view, they are seen to represent elongated, horizontal bone trabeculae (Fig. 8-10). These are encountered in the phalanges, distal humerus, femoral shaft, proximal tibia, and occasionally elsewhere.12 They are referred to as bone bars. Their significance is unknown.
SPECIFIC ANOMALIES AND NORMAL VARIANTS
Ribs
Bifid Ribs
The sternal end of a rib may be bifid or forked. The third and fourth ribs are most frequently affected (see Fig. 8-4).
Fenestrated First Rib
Fenestration of the first rib consists of a smooth, rounded opening in the anterior end of the rib. The significance of this deformity lies in the fact that it may be mistaken for a cavity in the lung in roentgenograms of the chest.
Student’s Tumor
The anterior chondral margin of the first rib is frequently irregularly calcified and easily mistaken for a nodule or tumor mass within the lung (Fig. 8-11). An apical lordotic radiograph of the chest should be obtained in doubtful cases to rule out the possibility of a true lung tumor. The irony is that underlying small lung cancers are sometimes obscured by calcification of the first rib.
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Cervical Ribs
A small rib occasionally arises from the seventh cervical vertebra and is called a cervical rib (Fig. 8-12). It may occasionally give rise to a thoracic outlet syndrome and therefore may be of importance.
Hypoplastic Ribs
Hypoplasia of an entire length of a rib is sometimes encountered (Fig. 8-13). Hypoplasia most commonly involves the 1st or 12th rib but may be encountered elsewhere.
Fused Ribs
Occasionally the first and second ribs are fused anteriorly (Fig. 8-14). This is rarely of clinical significance.
Ununited Apophysis Transverse Process First Lumbar Vertebra
Occasionally the apophysis of the transverse process of first lumbar vertebra fails to unite (Fig. 8-15). This may occur unilaterally or bilaterally and rarely, if ever, occurs at other levels. It could easily be mistaken for a fracture or an abortive rib.
Shoulder
Sprengel’s Deformity
Sprengel’s deformity also is known as congenital high scapula or congenital elevation of the scapula. The scapula is small, high in position, and rotated so that the inferior edge points toward the spine. The deformity may be unilateral or bilateral. A fusion of the cervical and upper thoracic vertebrae, the Klippel-Feil syndrome, is present in almost all cases (Fig. 8-16). This fusion anomaly may exist, however, without elevation of the scapula. In some cases there is a bony connection between the elevated scapula and either the fifth or sixth cervical vertebra. This bony connection is known as the omovertebral bone. It may join the scapula and the vertebrae by either bony or fibrous union (Fig. 8-17).
Pseudocyst of the Humeral Head
A normal area of rarefaction or lucency may be located in the lateral aspect of the proximal humerus, within the greater tuberosity (Fig. 8-18).20 This can be prominent on occasion, and it may be mistaken for a site of metastatic disease or other abnormality.
Deltoid Tubercle
The insertion of the deltoid muscle on the lateral surface of the proximal humerus normally projects as a flat cortical elevation.
Humeral Epiphyseal Line
The proximal humeral epiphysis arises from two centers, which usually fuse by 6 years of age. When viewed in the frontal projection with the shoulder in external rotation, the anterior aspect of the growth plate is chevron shaped, whereas the posterior portion of the line is transverse and may be mistaken for a fracture (Fig. 8-19).
Apophyses of the Coracoid and Acromion
In early adolescence, a flake-like ossification center appears at the superior aspect of the coracoid and the lateral margin of the acromion. These are normal ossification centers that may be mistaken for fractures.
Rhomboid Fossa
The pectoralis insertion on the medial inferior margin of the clavicle is sometimes associated with a shallow, occasionally irregular marginal defect known as the rhomboid fossa (Fig. 8-20). This is often bilateral.
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Foramen for the Supraclavicular Nerve
A small radiolucency is sometimes seen in the superior cortex of the midclavicle (Fig. 8-21).
Elbow
Supracondylar Process of the Humerus
A hook-like, bony projection may arise from the metaphysis of the medial surface of the distal humerus, curving inferiorly (Fig. 8-22). The projection may be mistaken for an osteochondroma. It is presumed to represent an atavistic trait and is said to be found in 2% of Scandinavians.
Hand and Wrist
Accessory Centers of Ossification
There are several reported accessory centers of ossification in the wrist and hand, but they are much less common than those in the foot. The most important ones are shown in Fig. 8-23.
Pseudoepiphyses for the Metacarpals and Metatarsals
Partial cartilaginous clefts may appear in the proximal ends of one or more of the lateral four metacarpals or the distal end of the first metacarpal or metatarsal, where normally no epiphyses are found.16, 19 Less frequently the clefts are complete, in which case they are termed supernumerary epiphyses (Fig. 8-24).
Clinodactyly
Clinodactyly refers to curvature of a finger in the plane of the hand. It may involve any finger, but the usual pattern is radial deviation of the fifth finger at the distal interphalangeal joint associated with a short middle phalanx, shorter on its radial than on its ulnar side (see Fig. 8-2). Most affected persons are otherwise normal; however, clinodactyly is also found in a wide variety of disorders, including Down’s syndrome.19
Madelung’s Deformity
Madelung’s deformity is a chondrodysplasia of the distal radial epiphysis. Some investigators believe that this deformity is a minimal form of the dysplasia known as dyschondrosteosis (see Chapter 9);
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others believe that it can occur as an isolated deformity without other osseous stigmata. It causes a curvature of the shaft of the radius, resulting in a deformity of the hand at the wrist and giving the appearance of an anterior dislocation of the hand. The reverse type also is seen but is very rare. The lesion is usually bilateral and is first noticed at about the beginning of adolescence.
The characteristic roentgenographic findings include shortening of the radius in comparison to the length of the ulna and a lateral and dorsal curvature of the radius (Fig. 8-25). There is early fusion of the radial epiphysis on the internal or ulnar side. This results in a tilting of the radial articular surface internally and anteriorly. The epiphysis develops a triangular shape. Because the radius fails to grow properly in length, the distal radioulnar articulation is disrupted and the lower end of the ulna projects posterior to the radius. The deformity of the radial articular surface leads to a derangement in the alignment of the carpal bones. The carpus assumes a triangular configuration, with the apex pointing toward the radius and ulna and the base formed by the carpometacarpal articulations.
Lunatotriquetral Fusion
Fusion of the lunate and triquetrum is encountered in approximately 2% of Africans and less commonly in Caucasians (see Fig. 8-3).19 It is the most common carpal fusion and, when isolated, is of no clinical significance.
Os Styloideum
The os styloideum is an accessory center of ossification arising at the base of the second metacarpal; it is visualized on the lateral view (see Fig. 8-20). Occasionally, an unmovable bony protuberance is located on the dorsum of the wrist at the base of the second and third metacarpals, adjacent to the capitate and trapezoid bones. This has
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been referred to as a “carpal boss.” It may represent either degenerative osteophyte formation at the metacarpal joint or the presence of an os styloideum (Fig. 8-26). Patients may complain of pain and limitation of motion of the hand.
Pelvis and Hip
Os Acetabuli
The os acetabuli is a round or oval ossicle lying along the upper rim of the acetabulum (see Fig. 8-6B). There is normally an apophyseal center or centers for the upper rim of the acetabulum that appear at about the age of 13 years and fuse with the acetabulum within a very short time. Failure to unite results in the formation of an os acetabuli. In other instances, a small sesamoid may be found in this area, usually situated more laterally than the apophysis but called by the same name.
Diastasis of Pubic Bones
Diastasis of pubic bones is ordinarily found in association with exstrophy of the bladder, epispadias, and other lower urinary tract anomalies. It may also be associated with cleidocranial dysostosis (see Chapter 9). However, it has also been reported in a family with no other anomalies.
Pubic Synchondrosis
The pubic synchondrosis is the site of fusion of the inferior ramus of the pubis and ischium and is located medially on the obturator foramen (Fig. 8-27). The synchondrosis is often expansile and may be confused with a pathologic condition. It is usually most prominent at about the age of 10 years.
Herniation Pit of the Femoral Neck
A round or oval radiolucency surrounded by a thin rim of sclerosis is often identified in the proximal, superior aspect of the femoral neck in adults (Fig. 8-28).18 The radiolucency represents a cortical depression or cavity formed by the herniation of capsular soft tissues through defects in the cortex. This “herniation pit” is a normal finding.
Femoral Linea Aspera-Pilaster Complex
Frontal radiographs of the femur commonly demonstrate two longitudinally oriented, thin, parallel lines projected over the middle third of the shaft (Fig. 8-29).17 These lines, called the “track sign,” represent the site of insertion of the strong adductor
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and extensor muscles of the thigh. When seen in the lateral view, the surface of the linea aspera is often rough, undulant, and irregular. This may suggest periosteal reaction but is, in fact, a normal finding.
Knee
Bipartite Patella
The patella may be divided into two or even more segments (see Fig. 2-65C, D in Chapter 2). The smaller segment or segments are usually located along the upper outer quadrant of the patella. These may be mistaken for fractures. In approximately 80% of the cases, the anomaly is bilateral. Flake-like ossification centers also appear on the anterior and occasionally on the inferior surface of the patella. These are likewise normal.
Fabella
The fabella is a small sesamoid bone that is very frequently found in the tendon of the lateral head of the gastrocnemius muscle at the level of the knee joint (Fig. 8-29B). It may become enlarged and roughened in the presence of degenerative disease of the knee joint.
Physiologic Bowlegs of Infancy
During early infancy, a mild degree of bowleg deformity is physiologic. It has been suggested that this bowing is the result of the normal internal tibial torsion that occurs during intrauterine life. This type of bowleg deformity tends to correct itself, and usually the legs have become perfectly straight by the time the child has reached the age of 4 to 5 years (Fig. 8-30).
Occasionally the bowing is accentuated to the point where it may be considered abnormal Fig. 8-31) and the result of disease, particularly rickets or Blount’s tibia vara. Differentiation from rickets can be made with assurance in most cases because the metaphyses are well ossified and none of the other findings seen in active rickets is present. It may not be possible to rule out rickets that has healed, but, because this type of bowing usually comes to the attention of the physician during the first months or year of life, there will seldom have been time for rickets to have been present and to have undergone complete healing. Differentiation from Blount’s tibia vara may be more difficult.
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Tibia Vara, Blount’s Disease (Osteochondrosis Deformans Tibiae)
Blount’s disease is an infrequent cause of bowlegs during infancy and childhood. Its cause is uncertain, but it often is classified with the osseous dysplasias. The possibility of ischemic necrosis as a causative factor has been considered by some investigators. A progressive, nonrachitic outward bowing of the legs is the characteristic clinical finding. The medial aspect of the upper tibial metaphysis is evidently the site of partial growth arrest, resulting in medial, flange-like broadening of the metaphysis in addition to shortening. This causes a rather sharp posteromedial slope of the medial tibial plateau. The amount of varus deformity depends on the angle of this slope.
The deformity actually is an angular one rather than a curved bowing. It is centered at the junction of the proximal tibial epiphysis and metaphysis (Fig. 8-31).6 Tibia vara must be differentiated from physiologic bowlegs. In tibia vara, the angular deformity is centered at the junction of the proximal metaphysis and epiphysis of the tibia. There is a broad, beak-like projection of the inner side of the metaphysis, within which are small islands of cartilage, and the tibial epiphysis tends to be triangular with the apex pointing medially. In physiologic bowing, both the tibia and the femur are affected, the femur often showing more deformity than the tibia.
Irregular Ossification of Distal Femoral Epiphyses
Before age 5, the distal femoral ossification center is often irregular in outline (see Fig. 1-4B). This is a normal variant. In later adolescence, an irregular center of ossification often appears in the posterior margin of both condyles. This is likewise normal but is readily mistaken for osteochondritis dissecans. However, the latter occurs on the lateral margin of the medial femoral condyle anteriorly.
Pelligrini-Stieda Disease
Pelligrini-Stieda disease is an irregular calcification that appears on the superior margin of the medial femoral condyle. It is probably related to previous injury of the medial collateral ligament (Fig. 8-32).
Cortical Desmoid
The adductor magnus and the medial head of the gastrocnemius insert on the posterior superior junction of the condyles and metaphysis of the distal femur, often accompanied by radiographic evidence of cortical irregularity (Fig. 8-33).21 This is a normal variant that may easily be misinterpreted as something sinister, such as a malignancy or infection.
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The Soleal Line
A prominent ridge of bone along the origin of the soleus muscle in the proximal tibia, as seen on the lateral projection, may mimic periosteal reaction along the posterior margin of the proximal tibial shaft.18 The underlying cortical bone is normal. On the frontal projection, it is seen as a thin, obliquely oriented vertical band of sclerosis traversing the upper tibia (Fig. 8-34).
Tibial Tubercle
On the proximal medial metaphysis of the tibia, there is often a thin, smoothly defined flange of bone projecting into the interosseous space (see Fig. 8-34A). This could be mistaken for periosteal new-bone formation, but it actually represents ossification at the base of the interosseous membrane.
Ankle and Foot
The foot is a common site for accessory bones and sesamoids (see Fig. 8-7). As stated previously, these should not be mistaken for fractures. The most common of these are described in the paragraphs that follow.
Os Trigonum
The accessory ossicle known as the os trigonum occurs in about 10% of the general population. It is a separate center for the posterior process of the talus, to which the talofibular ligament is attached. Its shape varies from a small triangular fragment to one more rounded or oval. The division from the talus may be incomplete. A fracture of the posterior process of the talus may resemble an os trigonum.
Os Tibiale Externum
The unfused tuberosity on the medial proximal side of the tarsal navicular (scaphoid) is called the os tibiale externum (see Fig. 8-5). It is sometimes called the divided scaphoid or an accessory scaphoid. It is a common variation and is usually bilateral.
Os Peroneum (Peroneal Sesamoid)
The os peroneum is a small ossicle found in or adjacent to the tendon of the peroneus longus, just lateral to and below the os calcis and cuboid (see Fig. 8-6A). It occurs in about 8% of persons. Occasionally there may be two or even three separate ossicles, representing a bipartite or tripartite sesamoid.
Calcaneus Secondarius
The secondary os calcis is a
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small, irregular bony mass found at the tip of the anterior process of the os calcis, where it articulates with the navicular. It is seen to best advantage in oblique roentgenograms of the foot. Its frequency is about 2%.
Supranavicular
The os supranavicular is a small, triangular bone occurring at the proximal superior edge of the navicular and articulating with the talus and navicular. It is relatively common and can easily be mistaken for a fracture.
Secondary Astragalus
A small, rounded bone found just above the head of the talus, seen only in lateral views of the foot, is the secondary astragalus. It should not be confused with the os supranavicular, which lies between the talus and navicular.
Apophysis at the Base of the Fifth Metatarsal
An apophysis that appears at about the age of 13 years and unites shortly thereafter is a flat bony center found along the lateral side of the proximal end of the fifth metatarsal (Fig. 8-35). It often is irregular in shape, but its long axis parallels the long axis of the metatarsal. A fracture in this location is also common (Fig. 8-35), but the line of fracture invariably extends transversely across the long axis of the shaft. The fracture surfaces are irregular, the soft tissues overlying the area are swollen, and the proximal fragment often is displaced or rotated.
Os Subtibiale
The os subtibiale is a separate ossification center for the tip of the medial malleolus.
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Os Subfibulare
Corresponding to the subtibiale, the os subfibulare is a separate center for the tip of the lateral malleolus. It varies from a tiny, rounded ossicle to a fairly large triangular fragment. It is best seen in anteroposterior views of the ankle joint.
Some of these apparent accessory ossicles around the ankle joint may be old chip fracture fragments that have smoothed off and have united with fibrous rather than bony union. Others may be foci of ossification that have formed as a result of soft-tissue injury. It often is impossible to determine their precise origin from a single roentgenographic examination.
Bifid Sesamoids
The sesamoids of the great toe are commonly bipartite, particularly the tibial or medial sesamoid, which is bifid in 10% of cases. The fibular or lateral sesamoid is bifid in approximately 3% (Fig. 8-36).
Pseudocyst of the Calcaneus
A lucency is frequently encountered in the body of the calcaneus just beneath the tuber angle on lateral radiographs of the foot (Fig. 8-37). It is simply an area relatively devoid of trabeculae and is of no clinical significance. Rarely, a lipoma, a simple bone cyst, or other tumor arises in this region. However, in contrast to a normal pseudocyst, they are usually sharply defined by a rim of sclerotic bone.
CONGENITAL SYNOSTOSIS
A congenital synostosis consists of a fusion of two or more bones. It is a frequent anomaly in the thorax, where there may be a partial fusion of several of the ribs. This can affect any part of the rib, but it is more common in the lateral portions and at the vertebral ends (see Fig. 8-14).
The proximal ends of the tibia and fibula occasionally are fused. Another uncommon site of fusion is at the proximal ends of the radius and ulna, resulting in an inability to supinate the forearm. In some cases, there is an associated dislocation of the head of the radius (Fig. 8-38).
Carpal and Tarsal Fusions
Fusions have been found in almost every combination in the carpal and metacarpal regions and in the corresponding portion of the foot. The fusions can be fibrous, cartilaginous, or osseous.
Carpal fusions may be sporadic or hereditary. Lunatotriquetral fusion is the most common (see Fig. 8-3). In general, fusions between bones in the same carpal row are of less significance than those that occur between the carpal rows. The latter frequently are associated with other, often clinically significant, congenital abnormalities and are encountered in congenital malformation syndromes.19
Congenital fusion of tarsal bones is commonly referred to as a tarsal coalition. The unusual rigidity of the fused joints may cause pain. The condition is often referred to as “peroneal spastic flatfoot” or “rigid flatfoot.” The latter term is preferred, because the rigidity of the tarsus is the result of a bony fixation and not spasm. In many cases, the clinical presentation suggests the correct diagnosis. Fusion may occur at any point between two bones but is most common between the calcaneus and navicular.
Radiographic verification is important. Calcaneonavicular coalition often can be recognized on conventional radiographs of the foot (Fig. 8-39). However, talocalcaneal coalition is frequently difficult to demonstrate radiographically, and special views supplemented by bone scintigraphy and CT may be necessary to demonstrate the site of coalition (Fig. 8-40B).14
Talocalcaneal fusion is often associated with a prominent beak on the anterior, superior margin of the head of the talus (Fig. 8-40A). Irregularity and a lack of definition of the
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posterior subtalar joint are indirect signs of talocalcaneal coalition. The coalition almost invariably occurs at the medial facet, between the talus and sustentaculum tali of the calcaneus, a joint not easily demonstrated by routine radiography. Patients who have clinical signs and radiographic findings suggestive of coalition should have a CT scan performed. Bone scanning may demonstrate a focal increase in radionuclide activity in the region of the coalition or in the talar beak and posterior facet of the subtalar joint. Coronal CT in the axial plane demonstrates the site and nature of the fusion quite satisfactorily and at the same time allows comparison with the opposite side (Fig. 8-40C, D).
DEVELOPMENTAL DISLOCATION OF THE HIP (DDH)
The hip is the most frequent site of congenital dislocation. It is 6 to 10 times more common in girls than in boys, the left hip is involved more often than the right in the ratio of 3:2, and it is much more common in whites than in blacks. It is unusual for dislocation to be present at birth, displacement occurring gradually during the first year of life. It was formerly believed that faulty development of the hip joint and associated structures was responsible for the dislocation, referred to as “acetabular dysplasia.” Most investigators now consider the fault to be in the supporting soft tissues of the hip joint, with the primary abnormality being a relaxation of the joint capsule. Others consider shortening or tightening of the muscles that cross the joint to be the primary cause.
Diagnosis of the predislocation stage during the newborn period is important, since early treatment prevents the ultimate dislocation and results in a normal hip joint. The Ortolani maneuver of 45° of abduction and internal rotation of the leg is useful in detecting hips that are susceptible to
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dislocation. With this maneuver, the examiner feels a “click” as the hip dislocates.
Roentgenographic Features
Roentgenographic examination of the hips for a suspected hip joint dislocation should include an anteroposterior roentgenogram of the pelvis (Fig. 8-41), obtained with the patient’s legs straight or slightly flexed at the knee and with the toes pointing forward. A so-called “frog-leg” view is also included (Fig. 8-42). In this position, the thighs are flexed, externally rotated and maximally abducted, with the feet brought together in the midline. Careful positioning is necessary to make certain that the hips are symmetrically placed so that one side can be compared with the other.
FIG. 8-41. Congenital dislocation of the right hip in an infant. A: Roentgenogram of the pelvis. B: Tracing illustrating the method for determining the acetabular angle. Line A, drawn along the upper margin of the acetabulum, represents the bony roof of the fossa, although in an infant the acetabulum is composed largely of cartilage. Line H is drawn through the centers of the triradiate cartilages of the acetabular fossae. The vertical lines P, or Perkin’s lines, are drawn through the outer limits of the bony margin of the acetabular roof of either side, perpendicular to the H line. The acetabular angle is larger on the right than on the left; however, this difference is not entirely diagnostic. The right capital femoral epiphysis is displaced laterally and very slightly superiorly. The curved broken line S, or Shenton’s line, is disrupted on the right and normal on the left.
FIG. 8-42. Congenital dislocation of the hip illustrated in the frog-leg position, with the thighs abducted and externally rotated. Note the absence of an ossified center for the right capital epiphysis, the poorly developed acetabular roof on the right side, and the increased acetabular angle. The position of the femoral neck indicates the subluxation, even though the femoral head is not visible. The patient also had an extensive spina bifida in the lower lumbar and sacral spine. A: Roentgenogram of the pelvis. B: Tracing of the roentgenogram.
Increased Acetabular Angle
The acetabular angle is a measurement of the slope of the upper half of the acetabular wall. The method of measurement is shown in Fig. 8-41. The normal angles vary widely, and the upper limit of normal should be close to 40°. The acetabular angle for the left hip is usually slightly larger than for the right. The normal angle decreases considerably between birth and the age of 6 months and to a lesser degree between the ages of 6 months and 1 year. These observations indicate that considerable caution should be exercised in the diagnosis of hip joint dysplasia based only on the finding of an acetabular angle greater than 30°. When one hip is affected, the acetabular angle is a more useful indicator than when both are involved, and a definite discrepancy in the angles on the two sides is an important finding.
Lateral Displacement of the Femur
Lateral displacement of the femur in relation to the acetabulum is an important finding. Because the ossification center for the head of the femur is not present at birth and does not appear normally
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until the age of 3 to 6 months, the neck of the femur must be used for this determination in the newborn. Perkin’s line, as shown in Fig. 8-41, is useful when either one or both hips are involved. This consists of a vertical line drawn from the upper outer edge of the iliac portion of the acetabulum to intersect at a right angle the transverse line drawn through the centers of both acetabula. The beak of the femoral neck normally falls medial to this line in practically every case, whereas in the majority of abnormal hips (60%) the femoral neck is situated lateral to this line.13
Disruption of Shenton’s Line
Shenton’s line is a smooth, curved imaginary line formed by the inner margin of the femoral neck and the inner surface of the obturator foramen, as shown in Fig. 8-41. Lateral displacement of the femur disrupts the smooth curve. Some degree of upward displacement is usually required before a significant break in the curve is seen.
Delayed Ossification of the Femoral Epiphysis
The ossification center for the head of the femur normally appears between the ages of 3 and 6 months. In the presence of hip joint subluxation or dislocation, the center may be delayed in appearance, and, when it does appear, its growth lags behind normal (see Fig. 8-42).
Later Stages
In older children and adults, gross displacement is usually present and the diagnosis is made without difficulty (Fig. 8-43). In untreated subjects, the head and neck of the femur do not develop properly, remaining small and hypoplastic. The acetabular fossa is very shallow, never having accommodated the femoral head. The head often impinges against the outer pelvic wall above and behind the shallow acetabulum and forms a shallow pseudoacetabular cavity.
FIG. 8-43. Congenital dislocation of the left hip in an older child. The capital epiphysis has not developed an ossification center. The left acetabulum is hypoplastic, with a marked increase in the acetabular angle.
Recognition in the Newborn
Diagnosis of developmental dislocation of the hip is based principally on clinical
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findings. In the newborn, the standard radiographic features described previously for the older infant are not applicable. At this stage, it is necessary to obtain an anteroposterior examination of the pelvis and hips with the legs in the Ortolani position—that is, abducted 45° and internally rotated. In this position, a line bisecting the femoral shaft should pass through the acetabulum and the lumbosacral articulation. In the presence of a dislocation, the line passes laterally to both structures. Care should be taken that the line bisects the shaft and not the femoral neck, because this would give a false reading.
Ultrasonography
Ultrasonography has a distinct advantage in that it uses no ionizing radiation. Real-time examination from the lateral projection allows visualization of the unossified cartilaginous portion of the acetabulum and the cartilaginous femoral head to determine the presence or absence of a dislocation (Fig. 8-44).1, 22 Ultrasonography is of particular value in the screening of newborns and young infants for developmental dislocation of the hip.5,8
FIG. 8-44. Sonograms of two hips in coronal plane. The transducer is positioned lateral to the hip in both examples. A: Normal hip. The cartilaginous femoral head (white asterisk) lies within the acetabulum with more than 50% of the head covered by the acetabular roof (arrows). B: DDH. The femoral head (white asterisk) is subluxed and lies superior and lateral to a shallow acetabulum (arrow). Note the hypertrophied, echogenic, fatty pulvinar (black asterisk). I = iliac bone, G = gluteal muscles. (Cases courtesy of T. David Cox, MD, Winston-Salem, NC.)
Computed Tomography
CT is a useful technique in the study of developmental dislocation of the hip, particularly in those cases in which there has been a failure to obtain or maintain a reduction of the dislocated hip.2, 11 In such cases, the iliopsoas tendon can interpose between the femoral head and the acetabulum, producing an infolding of the capsule and labrum. In other cases, there may be a hypertrophy of the pulvinar: a collection of fibrofatty tissue in the center of the acetabulum that decreases the capacity of the acetabulum and prevents relocation of the femoral head. The CT examination may be combined with hip arthrography to better visualize the unossified femoral epiphysis.
Magnetic Resonance Imaging
MRI has proved to be a
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valuable adjunct in the assessment of congenital dislocation of the hip because of its superior visualization of cartilage and soft tissues.9 It is of particular value in the demonstration of hip position and the source of obstruction to relocation.
OTHER CONGENITAL DISLOCATIONS
Congenital dislocations affecting joints other than the hip are infrequent. Dislocation of the radial head is seen occasionally in the elbow joint. In these cases, the radial head is displaced forward on the humerus. In some cases there is an associated congenital fusion of the dislocated radius with the proximal part of the ulna, the latter bone maintaining a normal relationship with the humerus (see Fig. 8-38). This lesion may be unilateral, but more often it is bilateral. With the passage of time, the head of the radius fails to develop properly and the proximal end of the bone is smaller than normal.
Traumatic dislocations caused by birth trauma are decidedly rare. Most prove to be separations of the epiphyses. These are more likely to occur in high-weight babies of diabetic mothers during the course of a difficult delivery. The proximal femur and proximal and distal humerus are the most common sites of injury.
CLUBFOOT (TALIPES EQUINOVARUS)
Clubfoot is one of the more common birth defects. It may be sporadic and possibly is caused by intrauterine abnormalities including severe oligohydramnios, a constriction in the uterus, or the amniotic band syndrome. There is also an increased incidence in some families, and it may be associated with other congenital abnormalities, including cleft palate and congenital heart disease. Clubfoot is also a feature of certain malformation syndromes (e.g., Gordon and Pierre Robin syndromes).
The three principal components of clubfoot are adduction of the forefoot, inversion, and cavus foot. In many cases the condition is bilateral. Everything demonstrated radiographically is better seen and evaluated by clinical methods. The radiographic findings are confirmatory and secondary. Elaborate radiographic procedures are unnecessary.
On the radiograph, there is medial angulation of the forefoot, revealed by medial displacement of the navicular and cuboid in relation to the talus and calcaneus (Fig. 8-45). The inversion deformity is shown by an inward rotation of the calcaneus under the talus. Cavus foot is associated with posterior displacement of the calcaneus. The superior surface of the posterior segment of the calcaneus lies near the tibia. The talus overhangs the calcaneus, projecting well beyond it anteriorly. Regional hypoplasia of the tarsal bone and soft tissues of the foot often accompanies the deformity.
FIG. 8-45. Clubfoot, talipes equinovarus in a newborn. A: Anteroposterior view of the foot demonstrates inversion of the foot. The varus deformity of the foot is obvious. Inversion has displaced the calcaneus beneath the talus in this projection. B: Lateral view demonstrates the associated equinus deformity with severe plantar flexion of the foot.
The diagnosis has been made in utero by ultrasonography.2, 3 When diagnosed in utero, it should suggest the possibility of other related abnormalities and syndromes, as listed previously.
The diagnosis should be made clinically in the newborn. It is imperative to differentiate the rigid clubfoot from the flexible clubfoot, which needs minimal or no treatment. This differentiation can be made by clinical evaluation of the foot.
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Radiographic demonstration of spina bifida, dislocation of the hip, or amyotonia congenita (arthrogryposis) indicates a poor prognosis.
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