MUSCULOSKELETAL IMAGING: INTRODUCTION

When Wilhelm Conrad Roentgen discovered the x-ray in November 1895, he investigated it thoroughly, testing its ability to penetrate various inanimate objects and observing its effects on fluorescent screens and photographic film. He gazed in amazement at the image of the bones of his own hand as he allowed the new rays to penetrate his flesh. He made a photographic x-ray image of a hand (reportedly his wife's) and sent prints of it together with his paper describing the new phenomenon to a carefully selected list of scientific colleagues.

By mid-February 1896 Rontgen's paper had not only been published but also reprinted in other scientific journals including the American journal Science. Scientists everywhere repeated Rontgen's simple experiments and confirmed the truth of his discovery. Within a year x-rays were in widespread use for medical purposes—chiefly for imaging of the skeleton.

Since Rontgen's time many new imaging techniques have been developed that allow radiologists to see the muscles and other soft tissues of the musculoskeletal system as well as the bones. These techniques make skeletal imaging a very exciting area of radiology and one that can enhance patients' quality of life. They can also be very expensive, however. This chapter is intended to introduce you to musculoskeletal imaging techniques and to suggest efficient ways to use them that will help you not only to make correct diagnoses but also to do so without excessive cost. Naturally, the suggestions made in these pages must be tailored to the needs of individual patients.

TECHNIQUES

Conventional Radiography

Conventional radiographs, or plain films, as they are often called, are the most frequently obtained imaging studies. They are chiefly useful for evaluation of the bones, but useful information about the adjacent soft tissues may also be obtained. Gas may be seen within the soft tissues and may be a clue to an open wound, ulcer, or infection with a gas-producing organism. Calcifications within the soft tissues can indicate a tumor, myositis ossificans, or systemic disorders such as scleroderma or hyperparathyroidism.

To get the most information possible from conventional radiographs, you should carefully choose the study to be ordered. At most hospitals and clinics standardized sets of views have been developed that are routinely obtained for evaluation of specific body areas in certain clinical settings. It is useful to know what will routinely be obtained when a certain set of films is ordered. Radiographs of the ankle, for example, usually include a straight frontal view of the ankle, a frontal view obtained with approximately 15 degrees internal rotation of the ankle (the mortise view), and a lateral view. There will be some variation among institutions, however. At a minimum, two views at right angles to one another should be obtained when a fracture or dislocation is suspected, because such injuries are notorious for being very subtle or even invisible in one projection, even when they are glaringly obvious in another view (Fig. 6–1). Radiographs should be focused on the anatomic area being evaluated, free of overlapping, extraneous anatomy (Fig. 6–2). If the knee is the site of trouble, do not order views of the entire tibia and fibula; you will be disappointed with the visualization of the knee. This principle must be abandoned more or less in young children and individuals with mental impairments who may not be able to localize their symptoms well and also in trauma victims with so many injuries that the relatively minor ones may be overlooked.

In addition, when the radiographs will be studied by a consulting radiologist, it is helpful to provide a succinct yet accurate history pinpointing your clinical concerns. Simply indicating the site of injury will improve the likelihood that a subtle fracture will be discovered.

A conventional radiograph of a normal bone will show a smooth, homogeneous cortex surrounding the medullary space. The cortex will be thicker along the shaft (diaphysis) of long bones and thinner in small, irregular bones such as the carpal and tarsal bones and at the ends of long bones (Fig. 6–3). Exceptions are the normal roughening of the cortex at tendon and ligament insertion sites and the normal interruption of the cortex at the site of the nutrient arteries. Naturally these occur at predictable places that differ from bone to bone. Within the medullary space of a normal bone are trabeculae. These are visible in radiographs as thin, crisp white lines that are arranged not randomly but in predictable patterns that enhance the stress-bearing capability of the bone. It is beyond the scope of this chapter to address the appearance of each bone.

When questions arise concerning whether a particular appearance is normal or abnormal, several solutions are possible. Two books, Keats' Normal Variants and Kohler's Borderlands (see Bibliography), are very useful in helping to distinguish the normal from the abnormal. Correlation with the results of the history and physical examination may also be helpful. Finally, comparison with the patient's prior plain radiographs or with a radiograph of the opposite extremity may also help (Fig. 6–4). Comparison views of the opposite extremity are especially helpful in children, in whom the open physes and accessory centers of ossification may vary a surprising amount from individual to individual.

Mammographic Techniques

Any soft-tissue area that can be pulled away from the skeleton and placed between the compression paddle and film may be imaged with the mammographic technique. In extremity imaging, the mammographic technique is occasionally used to search for small calcifications or foreign bodies in the soft tissues.

Fluoroscopy

Fluoroscopy plays an important role in evaluation of joint motion. It is often used by orthopedic surgeons to monitor placement of hardware. It may also be of assistance in positioning patients for unusual conventional radiographic views.

Computed Tomography

Tomography has two major uses in skeletal imaging. The first is evaluation of fracture fragment position. CT with good multiplanar reconstruction provides excellent delineation of fractures (Fig. 6–5). With multiplanar reconstruction, fracture may be evaluated in planes of section that cannot be achieved in the primary view (Fig. 6–6). Even with multiplanar reconstruction, MR imaging may give better results for fractures running primarily in the axial plane. The decision to use advanced imaging beyond conventional radiographs should be based on whether the study will change treatment or will be of sufficient help in operative planning to justify the additional radiation and expense. Scapular fractures, for example, are often treated conservatively, but orthopedic surgeons differ on the treatment of fractures that extend into the glenoid or involve the scapular spine. Some believe these benefit from internal fixation; others do not. Tomographic evaluation of these structures will be more useful to a surgeon who would use internal fixation selectively than to one who would use conservative therapy on all scapular fractures.

Three-dimensional reconstruction is available with many CT scanners. Extra technologist time is required to perform the scan and more films as well, so there is usually an extra charge. This may be justified if it helps the orthopedic surgeon to plan operative intervention and thus decrease the time required for surgery. Three-dimensional images are also useful for teaching purposes, because they can often be understood by less experienced individuals. They do not, however, contain information beyond that available in tomographic images.

The second major use of CT is in evaluation of bone tumors or tumor-like diseases. For this purpose, MR imaging is the principal competing technique. CT is more sensitive than MR imaging in demonstrating small amounts of calcium and can show early periosteal new bone formation or small amounts of matrix calcification before they may be seen with conventional radiography. This finding can be helpful in narrowing the differential diagnosis of a tumor. Before MR imaging was developed, CT was also used widely for determining the extent of bone and soft-tissue tumors. MR imaging, however, is now more often used for staging. Its superior contrast resolution greatly eases the task of determining tumor extent within bone marrow and muscle or other soft tissues (Fig. 6–7).

Magnetic Resonance Imaging

The exquisite contrast of MR imaging makes it ideal for evaluation of soft tissues. Its most frequent use in skeletal imaging, therefore, is for diagnosis of injuries to muscles, tendons, or ligaments about joints. This superb contrast resolution also makes it very useful for evaluating disorders of the bone marrow including neoplasm, marrow-packing diseases such as Gaucher's disease, osteomyelitis, fractures that are occult on conventional radiographs, and avascular necrosis. Unfortunately, although MR imaging is very sensitive to these abnormalities, it is also very nonspecific. Many diseases of marrow cause similar signal alterations. One must then narrow the differential diagnosis based on the distribution of the abnormalities together with the clinical history.

Nuclear Medicine

Several nuclear medicine studies are used for skeletal disease. The two most common are the technetium bone scan and the technetium- or indium-labeled white blood cell scan. One of several phosphate compounds of ^99m Tc is selected for use in a bone scan. Methylene diphosphonate is used most frequently. If there is a specific anatomic area of interest, images may be acquired over that area at the time the radionuclide is injected, as well as 3 to 4 hours later. The immediate images reflect the amount of blood flow to the area; the delayed images reflect the amount of bone remodeling occurring there.

Bone scintigraphy is a sensitive but nonspecific technique. Most osseous abnormalities of clinical significance will cause an increase in radiolabeling. Exceptions are destructive lesions that incite little reparative reaction in the host bone or that destroy bone so quickly that it cannot remodel.

Because of their sensitivity and because they provide physiologic rather than anatomic information, bone scans can be used to find abnormalities before they are detectable by conventional radiography. In particular, they are often used for screening for bone metastases in patients with known malignancy. Both multiple myeloma in adults and Langerhans' cell histiocytosis in children, however, are notorious for causing no increased accumulation on bone scans. Therefore, in these diseases conventional radiographs or skeletal surveys are better than bone scans for screening for osseous involvement. MR imaging is sometimes also used for screening for metastatic disease.

Early detection of avascular necrosis, historically an indication for bone scanning, has largely been usurped by MR imaging because it is more sensitive and provides greater anatomic detail. Legg-Calvé-Perthes disease (spontaneous avascular necrosis of the capital femoral epiphysis) in children is an exception, however; in this diagnosis, bone scans may be more sensitive than MR imaging.

White cell scans vie with MR imaging and bone scans as a method of detecting osteomyelitis. Which technique or combination of techniques is best is far from certain. Some studies have favored nuclear medicine; others have favored MR imaging. Both bone scan and MR imaging are sensitive but lack specificity. White cell scans are relatively specific for infection but often cannot distinguish cellulitis from osteomyelitis with certainty.

Biopsy

When tumor or infection is suspected, it is often useful to obtain a tissue sample for cytologic or histologic analysis or for culture. This may be accomplished by means of an "open" procedure in the operating room or a percutaneous needle puncture of the lesion to obtain a cellular aspirate or slender core of tissue. Needle biopsies of palpable lesions need no radiologic intervention. When the lesion is not palpable, however, biopsy may be accomplished under fluoroscopic, CT, or sometimes ultrasound guidance.

When a skeletal lesion should be biopsied and by whom are important questions that can have a tremendous impact on the patient's outcome. For example, sarcomas have been reported to grow along the surgical or needle tracks after diagnostic biopsies. Therefore, when planning biopsies of suspected musculoskeletal sarcomas great care must be taken to approach the lesion through a track that can be resected en bloc with the tumor at the time of ultimate excision. These biopsies should be carried out in close consultation with the surgeon who will be performing the definitive surgery.

When systemic disease such as metastatic carcinoma is the primary consideration, percutaneous needle biopsy is the most efficacious means of making a diagnosis if the lesion is amenable to this procedure. In this setting, the yield of needle biopsy is very good (90% or more of such biopsies yield a positive diagnosis when tumor is truly present) and a negative result is less likely to lead to open biopsy than it would in some suspected primary tumors. Nonetheless, biopsy should still be performed in consultation with the oncologist or other physician giving overall care.

TECHNIQUE SELECTION

In general, as in most other organ systems, the radiograph is the initial imaging test after history and physical examination. The selection of subsequent (often more expensive) imaging tests depends not only on medical need but also on a variety of other factors, including availability, expense, and the preferences of the radiologist, clinician, and patient.

Trauma

Rely primarily on conventional radiography. When a strongly suspected fracture is not identified, you may choose among repetition of conventional radiographs in 7 to 10 days, nuclear medicine bone scanning, and MR imaging. If a fracture is noticed and more information is needed concerning the location of fragments, CT is useful.

Bone or Soft-Tissue Tumors

For local staging of both bone and soft-tissue neoplasms, MR imaging is the best technique. When a bone tumor is suspected but is not discovered with conventional radiographs, MR imaging is a useful secondary screening tool.

Metastatic Tumors

Symptomatic sites suspected of being involved by metastatic neoplasm are best evaluated initially with radiographs. An overall survey for osseous metastases may be performed by nuclear medicine bone scan. Conventional radiography is then used to evaluate sites of possible tumor involvement. Suspected soft-tissue metastases are best evaluated by MR imaging.

Osteomyelitis

Conventional radiographs should be obtained first. If these are normal or inconclusive, MR imaging, nuclear medicine bone scan, or white blood cell scanning may be helpful.

EXERCISE 6–1: TRAUMA

Clinical Histories:

Case 6-1. On the first day of your medical school rotation in orthopedic surgery, the resident and attending physician send you to the emergency room to see a 26-year-old man with a broken leg (Fig. 6–8).

Case 6-2. Infant with low-grade fever. You obtain a chest radiograph to "rule out pneumonia" (Fig. 6–9).

Case 6-3. While moonlighting in the emergency department of a small community hospital you examine a 25-year-old man who fell on an outstretched hand and now complains of elbow pain. You obtain a lateral view of his elbow (Fig. 6–10).

Case 6-4. A week later you are once again moonlighting in the same small emergency department when a 29-year-old man is carried in complaining of ankle pain after a twisting injury. His ankle is swollen and ecchymotic, and he is tender to palpation along the medial malleolus. He has no other complaints. You order frontal, lateral, and oblique views of the ankle (Fig. 6–11).

Case 6-5. A 15-year-old boy complains of ankle pain after a fall (Fig. 6–12).

Questions:

Radiologic Findings:

6-1. Figure 6–8 shows comminuted fractures of the distal tibia and fibula with intra-articular extension of the tibial fracture. Gas density (white arrows) indicates that air has penetrated into the soft tissues through a skin wound, so this is an open fracture. A fracture line extends to the tibial articular surface (black arrow).

6-2. In Fig. 6–9 a row of rounded opacities (arrows) in the left chest represents posterior healing rib fractures.

6-3. This patient's anterior fat pad is pushed away from the bone, creating a small triangular "sail" (Fig. 6–13,arrowheads). The posterior fat pad (arrow) is visible when it should not normally be visible at all. There is no visible fracture or dislocation.

6-4. Figure 6–11 indicates a transverse fracture of the distal medial malleolus with widening of the medial aspect of the ankle joint.

6-5. Abnormal lucencies run vertically through the epiphysis on the frontal view (Fig. 6–12) and obliquely through the metaphysis on both the frontal and the lateral views. The lateral aspect of the distal tibial physis or growth plate is widened.

Discussion:

Your mission on Case 6-1 is to describe accurately and succinctly the features of this fracture that will affect treatment and outcome. You should discuss the alignment of the largest tibial and fibular fragments. Address both displacement and angulation. Displacement is always described in terms of the position of the distal fragment relative to that of the proximal fragment. The lateral view shows that the distal tibial fragment is displaced 1 cm posteriorly.

On the frontal view there is obvious angulation (Fig. 6–8A ). Angulation may be described either in terms of the direction of shift of the distal fragment or in terms of the direction in which the apex of the angle points. In either case, it is better to give a measurement than to use subjective modifiers like "slight" or "moderate." This angulation may correctly be described as "30 degrees of varus angulation of the distal fragment" or "30 degrees lateral apical angulation" (Fig. 6–14). (C is an incorrect statement and therefore the correct answer to Question 6-1.)

In Case 6-2, rib fractures in a young child suggest child abuse. (D is the correct answer to Question 6-2.) Because most rib fractures in infants are caused by nonaccidental injury, you should reexamine the child for other stigmata of child abuse, such as bruises, welts, burns, or retinal hemorrhages; notify protective services; and obtain a skeletal survey. If you are unsure of your diagnosis or desire confirmation of the radiographic findings, you should obtain a radiology consult. Discharging the patient could place the child in serious danger.

Figure 6–15 from the skeletal survey reveals the classic metaphyseal "corner" (large arrows) and "bucket handle" (small arrows) fractures virtually pathognomonic of infant abuse. The astute observer will also note a healing fracture of the superior pubic ramus. In summary, radiologic findings with moderate to high specificity for infant abuse include posterior rib fractures, metaphyseal fractures, multiple fractures, and fractures at differing stages of healing.

When examining a radiograph for a suspected fracture, as in Case 6-3, it is important to evaluate not only the bones themselves but also the adjacent soft tissues. In a number of areas of the body there are normal deposits of fat, termed fat pads, which may be displaced by accumulation of blood or fluid in the underlying tissues. The fat pads of the elbow are particularly helpful. (C is the correct answer to Question 6-3.)

Displacement of the elbow fat pad is a nonspecific sign that indicates distension of the joint. Effusions due to rheumatoid arthritis, an infected joint, or hemorrhage, especially in a patient with a bleeding disorder, could all cause the "fat pad sign" seen in this patient. In an otherwise healthy person who has suffered trauma, however, a radial head fracture should be suspected since it is the most common elbow fracture in an adult. This patient's frontal view did actually demonstrate a small lucent fracture line in the radial head. Even without that, however, the most prudent course is to treat the patient as though he had a radial head fracture and also arrange for follow-up care with a physician accustomed to caring for fractures.

Transverse medial malleolar fracture (Case 6-4) usually accompanies eversion of the ankle and is often associated with a fibular fracture. The fibular injury may occur at any level from the ankle to the knee. When there is no apparent distal fibular fracture, the remainder of the bone should be imaged. (C is the correct answer to Question 6-4.) In this case, there is, indeed, a fracture of the proximal fibula (Fig. 6–16.) This fracture indicates rupture of the intraosseous membrane all along its course from the ankle to the fibular fracture, so this is an unstable injury that many orthopedic surgeons will treat with open reduction and internal fixation of the malleolar component and of the syndesmosis.

Fractures of the proximal fifth metatarsal may accompany ankle inversion and may be difficult to distinguish clinically from other ankle injuries; therefore, that part of the foot should always be included on at least one view of the ankle. If it is not, then it is prudent when possible to obtain one more view, but this should not ordinarily be considered a separate study or incur an additional charge. CT is not necessary in this case.

The adolescent patient of Case 6-5 has suffered a relatively common growth plate injury with a typical but somewhat complex fracture pattern. It is called a triplane fracture because it has components that run, more or less, in all three primary planes of section. It travels in the sagittal plane through the epiphysis, in the axial plane through the unfused portion of the physis or growth plate, and in the coronal plane through the metaphysis. (D is the correct answer to Question 6-5.) It is a Salter-Harris type IV fracture (Fig. 6–17). It can be easily diagnosed on the conventional radiographs. Overlapping of several bones in the ankle region, together with the inferiorly concave shape of the articular surface of the distal tibia, the tibial plafond, complicates evaluation of fracture fragment position, and so a CT scan was obtained (Fig. 6–18).

EXERCISE 6-2: LOCAL DISEASE

Clinical Histories:

Case 6-6. A 12-year-old girl comes to your pediatrics office complaining of 2 weeks of knee pain. There is no history of trauma. Her proximal fibula is slightly swollen, tender, and erythematous. You obtain frontal and lateral views of the tibia and fibula (Fig. 6–19).

Case 6-7. This 5-year-old girl has been limping off and on for 2 months. Her knee is warm and swollen (Fig. 6–20).

Case 6-8. A 35-year-old man complains of a lump in the soft tissues of the right arm. He first noticed the lump 6 months ago after hurting his arm in a fall from a bicycle (Fig. 6–21).

Case 6-9. A 10-year-old girl complains of a lump on the inside of her thigh near the knee. It has been there as long as she can remember but has been annoying her since she recently took up horseback riding (Fig. 6–22).

Questions:

Radiologic Findings:

6-6. Focal lytic lesion in the proximal fibular metadiaphysis with an intact shell of new cortex and a well-defined, short zone of transition between itself and adjacent normal bone (Fig. 6–19).

6-7. Figure 6–20 shows a well-defined lytic lesion in the proximal tibia. Its edges are slightly sclerotic. It extends across the physis to involve portions of both the metaphysis and epiphysis.

6-8. A well-defined ossified mass projects in the musculature of the posterolateral arm (Fig. 6–21). It has a thin but distinct cortex (arrows) surrounding trabeculae.

6-9. Arising from the medial cortex of the femur is an ossified mass topped by a cauliflower-like thin shell of cortex (Fig. 6–22). The cortex of the remainder of the femur is continuous with the cortex of the tumor (arrowheads), and the trabecular bone of the femoral metaphysis blends imperceptibly with that of the mass. The mass has grown away from its metaphyseal place of origin and points toward the diaphysis and away from the joint.

Discussion:

The radiographs of Case 6-6 (Fig. 6–19) demonstrate a focal lytic lesion in the proximal fibular metadiaphysis. The cortex appears intact around the lesion, and the bone is widened. Cortex is not pliable; it will not stretch to accommodate a growing lesion. Instead it will slowly remodel by resorption of endosteal bone and deposition of periosteal new bone. The intact cortex implies a slow growth rate for this lesion. Another indication of a slow growth rate is the sharp demarcation or short zone of transition between the lesion and adjacent normal bone.

In general, osteomyelitis will not cause apparent expansion of bone the way this lesion has. Stress fractures are usually linear lesions and usually are oriented transversely across the bone, though there are exceptions. Stress fractures may be lucent, if a gap in cortical bone is their primary manifestation, or sclerotic, either due to compression of trabeculae with resultant overlap or to healing. The periosteal reaction that they engender may cause them to be mistaken for bone tumors, but they will not look like this particular lesion (Fig. 6–23).

Of the choices given in the question, the remaining ones are benign and malignant bone tumor. For the most part, malignant bone tumors in children have a rapid growth rate. This will cause them to have poorly defined borders. In addition, where they destroy cortex, the periosteum will be unable to contain them with solidly mineralized new bone, as has occurred here. There may be gaps in the cortex where tumor has broken through (Fig. 6–24). The periosteal new bone may mineralize at 90-degree angles to the diaphysis or may be lamellated (like onion-skin) or incomplete. The intact shell of periosteal new bone seen in this patient and the short zone of transition are more typical of a benign than a malignant tumor. (A is the correct answer to the Question 6-6.)

A primary bone tumor, no matter how benign its appearance, is most appropriately handled by an orthopedic surgeon experienced with tumor patients. Because the question stipulates that you are a pediatrician, the patient should be sent to an orthopedic surgeon who specializes in treatment of tumors.

Performing a percutaneous needle biopsy has the potential to cause great harm if a poorly chosen route is taken. For example, if the needle passed close to the common peroneal nerve and then the lesion proved unexpectedly to be malignant, the nerve might have to be sacrificed in order to obtain a curative resection.

Obtaining additional imaging studies to evaluate this lesion further is not a bad idea. It is better, however, to allow the orthopedic surgeon to whom the patient will be referred (in consultation with the radiologist) to decide which imaging tests are most appropriate to evaluate the lesion more thoroughly before ordering additional tests.

The edges of malignant tumors are usually not as well defined as those of the lesion shown in Case 6-7 (Fig. 6–20). Malignant tumors may extend across the growth plate, but it is uncommon for them to do so while they are still as small as this lesion.

Osteomyelitis, on the other hand, often breaches the growth plate. (A is the correct answer to Question 6-7.) The most common organisms to cause osteomyelitis are species of Staphylococcus and Streptococcus (Fig. 6–25). The relatively long history of limping, however, should suggest a more indolent organism such as Mycobacterium tuberculosis. Skeletal tuberculosis is uncommon and thus often is overlooked as a diagnostic possibility. Because it is curable yet responds to very different drugs than would be used for pyogenic osteomyelitis, it is important to keep it in mind. It may occur at any site, but it is most common in the spine. In the extremities it most often occurs in or near the hip and knee.

Langerhans' cell histiocytosis (eosinophilic granuloma) is much less common than osteomyelitis and is thus not as likely a diagnosis. When it does occur, its favorite location is the skull.

The ossified mass of Case 6-8 (Fig. 6–21) represents myositis ossificans, also known as heterotopic new bone formation. Though often associated with trauma, it may also be seen in patients without a distinct history of trauma. When it resembles mature bone as closely as in this patient, it is not a diagnostic dilemma, and you may reassure the patient that there is a benign cause for his lump. (C is the correct answer to Question 6-8.)

Occasionally myositis ossificans warrants excision on the basis of mechanical interference with the use of a muscle or joint. Recurrence is less likely if excision is performed after the lesion has matured. A bone scan may help to distinguish between mature and immature lesions. An immature lesion that is still undergoing ossification will exhibit marked radionuclide uptake. Once ossification is complete, radionuclide accumulation will resemble that of other bones.

Myositis ossificans may be diagnosed more confidently with radiography than with histology. An immature lesion will be full of immature, rapidly proliferating cells that may be mistaken for a sarcoma by the pathologist. Radiologically, however, there is a distinct difference between the two. Myositis ossificans ossifies from the outside in. Sarcomas ossify from the inside out. (See Fig. 6–24 and notice that the central portion of the soft-tissue mass of the osteosarcoma is ossified, whereas the outer portion is not.) If it is not entirely clear from conventional radiographs where and how the ossification is occurring, a CT scan is the test of choice because of its sensitivity to calcium.

The mass of Case 6-9 (Fig. 6–22) has the characteristic appearance of an osteochondroma, the most common of all benign, cartilaginous neoplasms. Osteochondromas may be very large or very small, pedunculated or sessile (Fig. 6–26). They grow as the child grows and should cease growth by adulthood. Often asymptomatic, they may be an incidental finding. They may, however, cause a wide range of symptoms. The most common complaint is that they interfere with activities or with wearing certain clothes, such as tight blue jeans. They may be painful as a result of irritation of an overlying bursa (Fig. 6–27), and they are subject to fracture. An uncommon (1% or less) but feared complication is malignant transformation, usually resulting in a chondrosarcoma. Signs of such transformation include enlargement of the osteochondroma in an adult, thickening of the cartilaginous cap that covers the tumor, development of a soft tissue mass, and destruction of bone.

When further radiologic studies are needed, MR imaging is probably the most useful modality. It can demonstrate the cartilage cap and any associated soft-tissue mass. When the diagnosis is not as obvious as in this case by conventional radiography, MR imaging can assist in confirming the identity of the tumor by demonstrating continuity between the cortices and medullary spaces of the tumor and the host bone.

EXERCISE 6-3: SYSTEMIC DISEASE

Case Histories:

Case 6-10. As a medical student on the oncology team, you see in clinic a 45-year-old woman with a history of breast cancer diagnosed 5 years previously. She underwent surgery and since that time has been free of disease. She has come for her routine follow-up appointment and complains only of vague, aching discomfort in her left hip (no figure).

Case 6-11. A 40-year-old man complains of knee pain and swelling of 3 weeks' duration. He has no other known disease. You order conventional radiographs of the knees. You notice some periosteal elevation on both femurs and tibiae (Fig. 6–28).

Case 6-12. This chest radiograph was obtained to exclude pneumonia in a chronically ill 26-year-old woman (Fig. 6–29).

Case 6-13. In a 50-year-old man with diabetes, radiographs of the hand were obtained to exclude fracture after a fall (Fig. 6–30).

Questions:

Radiologic Findings:

6-11. A thin rim of calcium added to the bony contour of both sides of the right femoral metaphysis (Fig. 6–28arrows) is due to periosteal elevation. Similar findings were present on the left femur and both tibiae.

6-12. Figure 6–29 shows surgical changes (arrow) in the right upper quadrant. The humeral heads have an abnormal, mottled, sclerotic appearance. Many vertebral bodies, as best appreciated in the lateral view, are shaped like the letter H (arrowheads), with central depressions in the superior and inferior endplates.

6-13. In Fig. 6–30, the distal interphalangeal joint of the middle finger is held in slight flexion. A small triangular chip (arrow) of bone projects in the soft tissues dorsally and a few millimeters proximally to the proximal aspect of the distal phalanx representing an avulsion fracture of the attachment site of the extensor tendon. The tufts of all fingers are abnormal. This patient's distal phalanges are too short and too narrow at their tips. The cortex has been resorbed in many places. It is difficult to decide exactly where the edge of the bone is. Besides resorption of the most distal portion of the tuft of the index finger, there is also irregular resorption or destruction of the central portion of the distal phalanx.

Discussion:

In Case 6-10, options A, a chest radiograph, and E, mammography, are both reasonable screening examinations often obtained yearly in asymptomatic cancer patients. These would be good tests to order for this patient even without new symptoms. Indeed, mammography should be obtained in any woman of 45 years every year for screening purposes, irrespective of her history. Bone scans are also often ordered as screens for metastatic disease in asymptomatic breast cancer patients, particularly for the first 2 to 3 years after diagnosis. Because this patient is complaining of skeletal pain, both a bone scan (Fig. 6–31A ) and conventional radiographs of the affected area (Fig. 6–31B ) are indicated. A skeletal survey is not appropriate. (D is the correct answer to Question 6-10.) In general, a skeletal survey is utilized in oncology only for screening for multiple myeloma and Langerhans' cell histiocytosis.

In this patient's case, a bone scan revealed multiple areas of abnormally increased accumulation of radionuclide, including the left acetabulum (Fig. 6–31A ). The multiplicity of lesions, together with the history of breast cancer (which often metastasizes to bone and may do so after a disease-free interval of many years), is very suggestive of metastatic disease. Some oncologists would choose to treat the patient for presumed metastatic disease on the basis of the bone scan, history, and current symptoms. Others would prefer a biopsy before proceeding to further treatment. This patient underwent a CT-guided needle aspiration of the acetabular lesion, which revealed metastatic tumor (Fig. 6–31C ). To evaluate for possible impending pathologic fracture (Fig. 6–32), most oncologists would also request conventional radiographs of areas demonstrating increased activity on the bone scan, particularly those in weight-bearing bones.

Periosteal elevation (Case 6-11, Fig. 6–28) is a nonspecific finding that occurs with local disorders such as fracture, bone tumors, and osteomyelitis and also with systemic or multifocal disorders such as bone infarction (Fig. 6–33), venous stasis, and secondary hypertrophic osteoarthropathy. Because this finding is bilateral, it is more likely due to a systemic or multifocal disorder than to a local one.

Of all the systemic disorders that may be associated with periosteal new bone formation, secondary hypertrophic osteoarthropathy is the most important to exclude. At one time it was called hypertrophic pulmonary osteoarthropathybecause it is usually caused by pulmonary disease. The designation secondary hypertrophic osteoarthropathy reflects current understanding that this disorder may also be due to nonpulmonary diseases such as inflammatory bowel disease or congenital cardiac anomalies. Nonetheless, pulmonary disease, specifically lung cancer, remains the most common cause. (D is the correct answer to Question 6-11.) This patient, in fact, had lung cancer (Fig. 6–34).

A bone scan could be useful if you did not notice the periosteal new bone or were not sure of its presence. Hand films could demonstrate clubbing, which may be seen with some of the same disorders that cause hypertrophic osteoarthropathy, but simple physical inspection of the patient's hands would accomplish the same thing. MR imaging of the knees will not be helpful in this case.

The patient in Case 6-12 (Fig. 6–29) has sickle cell disease. The surgical clips in the right upper quadrant of the abdomen are from a prior cholecystectomy. People with sickle cell disease are prone to early development of cholelithiasis.

The peculiar shape of multiple vertebral bodies is very characteristic of sickle cell anemia, though it may occasionally be seen in other diseases affecting the marrow cavity, particularly Gaucher's disease. It may be caused by infarction of bone beneath the endplates, with remodeling of the cortex to produce the H shape.

When red cells sickle, they clump together and may block blood vessels. In bone this leads to avascular necrosis, which may be widespread, involving many bones simultaneously. The mottled appearance of the humeral heads is due to avascular necrosis and is a common finding in patients with sickle cell anemia.

Modest enlargement of the pulmonary artery, as seen in this patient, is so common in young women that it is considered normal in that population. (B is the correct answer to Question 6-12.)

Though it was not included among the possible answers to the question, another finding of interest on this examination involves the appearance of the left upper quadrant. The gas-filled splenic flexure of the colon occupies too much of the left upper quadrant on the frontal view. There is no room for a spleen of normal size. In sickle cell patients the spleen is often infarcted so that by the time they reach adulthood, it has shrunk to a small fraction of normal size.

The patient in Case 6-13 (Fig. 6–30) has many findings of hyperparathyroidism. Primary hyperparathyroidism usually results from a hyperfunctioning parathyroid adenoma, which is usually detected and removed before the osseous findings of hyperparathyroidism develop. Secondary hyperparathyroidism is most often associated with chronic renal failure, a common complication of diabetes mellitus. (D is the correct answer to Question 6-13.)

Parathormone stimulates the action of osteoclasts and thus causes resorption of bone. Acroosteolysis (resorption of the tufts of the fingers) is one manifestation of hyperparathyroidism. Two other places where such resorption often occurs are also seen in this patient's hands. One is intracortical, where resorption is causing longitudinal striation in the cortex. The other is subperiosteal. Notice that the cortex of the lateral or radial side of each of the the middle phalanges is finely serrated (Fig. 6–35). This is caused by resorption of bone in the troughs, and the peaks are areas where the cortex has maintained a more normal thickness. Subperiosteal resorption in this location is often the earliest radiographic sign of hyperparathyroidism. It is also nearly pathognomonic of this disorder. Another common site of bone resorption in hyperparathyroidism is the distal clavicle (Fig. 6–36).

Resorption of the central portion of the distal phalanx of the index finger may also be due to hyperparathyroidism, but other causes such as osteomyelitis or tumor should also be considered. Of the two, osteomyelitis is more common and may occur in the fingers as a result of direct implantation of organisms from a puncture wound. Metastasis may occur in any bone but favors the axial skeleton and is uncommon in the hands.

Keats TE, Anderson M. Atlas of Normal Roentgen Variants That May Simulate Disease, 7th ed. St. Louis: Mosby; 2001.

Schmidt H, Kohler A, Zimmer EA. Borderlands of Normal and Early Pathologic Findings in Skeletal Radiography, 4th ed. New York: Thieme Medical Publishers; 1993.