RADIOGRAPHIC PATTERNS OF BONE RESPONSE TO DISEASE OR TRAUMA

Bone responds to disease or injury in limited ways. When osteoblastic activity predominates, as it often does with infection, neoplasia or following trauma, a productive response is radiographically evident and is expressed as periosteal or endosteal new bone formation. When osteoclastic activity predominates or when bone calcium mobilization increases, a destructive or osteolytic response results. A decrease in bone mineral may be generalized as with certain metabolic bone diseases such as 2° hyperparathyroidism, regional as occurs with disuse atrophy, or focal as occurs with many neoplasms. A mixed radiographic pattern may develop with osteomyelitis, bone tumors and to some extent, normal bone healing. Another response is an alteration in normal bone shape or architecture without a change in bone opacity. This pattern may be seen with developmental or inactive traumatic disorders that affect bone modeling, malunion fractures and some benign bone tumors. Despite the limited responses of bone to insult, lesions can often be classified by disease category which can, in turn, be used to establish a definitive or differential diagnosis (Table 1).

Table 1. Categories of Skeletal Disease

1.  Degenerative

2.  Anomalous developmental, congenital

3.  Metabolic, endocrine, nutritional

4.  Neoplasia

5.  Inflammatory, infectious, idiopathic

6.  Traumatic

AGGRESSIVE VERSUS NONAGGRESSIVE BONE LESIONS

A useful step in establishing a differential diagnosis is to characterize a bone lesion as being aggressive or nonaggressive based on radiographic features (Table 2). For example, aggressive features are frequently associated with inflammatory and neoplastic bone disorders whereas non-aggressive features are often found with developmental and degenerative disease. It is important to remember that the terms aggressive and nonaggressive define radiographic features. They are not equivalent to the terms malignant and benign nor should they imply a specific diagnosis.

Table 2. Radiographic Features Used to Characterize Bone Lesions as Aggressive or Nonaggressive

1.  Lesion distribution

2.  New bone production

3.  Bone destruction

4.  Zone of transition

5.  Duration and progression

Lesion Distribution: A lesion may be focal, multifocal, regional or generalized. Often, but not always, a regional or multifocal distribution is considered more aggressive than a solitary focal lesion. A generalized lesion distribution, one that involves the entire skeletal system, is often associated with abnormalities that affect bone metabolism. A generalized pattern may be considered aggressive if it is present in combination with other aggressive bone lesion features.

New Bone Production: New bone formation often occurs as a reactive periosteal and/or endosteal productive response to disease or trauma. Osteoblastic activity at or near the lesion site results in a productive periosteal new bone response that will vary in appearance depending on the duration, extent of periosteal activity and the inciting insult. Periosteal productive responses may be smooth, uniform and dense (nonaggressive) or non-uniform, interrupted, irregular, spiculated or amorphous (aggressive). Endosteal productive responses may occur either alone or combined with periosteal production. New bone may form in neoplasms arising from bone forming cell lines such as osteosarcoma and chondrosarcoma. This tumor bone can be dense and well-defined but more often is amorphous and often not associated with underlying normal bone (aggressive).

Bone Destruction: Bone destruction, particularly destruction of dense cortical bone, is almost always considered an aggressive characteristic. Bone destruction, or osteolysis, can be classified into one of three broad patterns, focal, moth-eaten and permeative. Focal bone loss represents a well-margined, destructive lesion that may be surrounded by a rim of sclerotic reactive new bone suggesting slow or no lesion progression. Moth-eaten bone loss is represented by multiple punctate lucencies of varying size and definition causing a non-uniform bone opacity. Moth-eaten bone loss is often more aggressive than focal bone loss. Permeative bone loss, considered the most aggressive form, has a more uniform appearance. Bone opacity is diminished more uniformly across the lesion site due to minute punctate lucencies distributed across the region of bone destruction and discrete lesion margins are not evident.

Other Forms of Bone Loss: Revascularization of bone following an ischemic event may result in localized bone resorption (nonaggressive). This may occur at fracture sites and can occasionally be seen associated with some forms of nonunion. Regional bone resorption may also occur from disuse. This represents a nonaggressive, and often reversible, form of bone loss. Also, soft tissue masses may cause erosion of the surface of an adjacent bone due to direct pressure. The affected bone might be considered an innocent bystander in this nonaggressive form of bone loss. Finally, active bone loss or osteolysis must be differentiated from an absence of bone formation (nonaggressive) that is seen with certain types of developmental bone disease such as osteochondrosis.

Transition Zone: A transition zone represents the junction between normal and abnormal bone. Generally, the longer and less well-defined the transition zone, the more aggressive the lesion. Conversely, nonaggressive lesions typically have a short and well-defined transition zone. The transition zone concept is most easily applied to long bones but can also be used to characterize axial skeletal lesions.

Duration and Progression: It is intuitively obvious that bone lesions that change for the worse over a short period of time are more aggressive than those that remain unchanged or change very little. Sometimes serial radiographic examination is the best means of determining the aggressiveness of a lesion.

A combination of aggressive and nonaggressive features may coexist in the same lesion. When this occurs, the aggressive features should generally be considered more significant. In addition, some features are more clinically significant and specific than others. Cortical bone destruction, for instance, is usually a more ominous sign than other features such as irregular new bone formation or a long transition zone.

CATEGORIES OF SKELETAL DISEASE

Following are general descriptions of radiographic findings typical of the categories of disease included in Table 1. Of course, radiographic features of a given lesion may vary depending on its location, duration, and other factors particular to that lesion. In addition, because one disease may beget another (developmental joint disease leading to degenerative joint disease), the end result may be a hybrid of two or more conditions.

Developmental Disorders

The radiographic findings associated with developmental disease are as diverse as the etiologies of the disorders themselves. As such, an attempt to generalize typical radiographic signs of developmental lesions is not so useful as with the other categories of disease. Developmental disease may include one or more of any of the radiographic signs described previously. In particular, unlike most of the other categories of disease, developmental disorders may include a change in number of a given structure or organ. Although radiographic features of the different developmental lesions vary markedly, they will generally appear nonaggressive. Two additional points of commonality occur with developmental disease. First, degenerative joint disease is often a sequela to developmental orthopedic disease, especially when the primary lesion involves joints or causes alterations in long bone growth resulting in conformational abnormalities. Frequently the most obvious radiographic findings are these secondary degenerative changes which can often mask the original developmental lesion. From the standpoint of understanding etiology of a problem, it is important to keep separate the cause (developmental) from the effect (degenerative). Second, developmental lesions are frequently bilateral when they affect a specific bone or joint, multifocal when they are not site specific, or generalized.

Degenerative Disorders

With few exceptions, degenerative changes are exclusively productive as periarticular new bone formation (osteophytes) and subchondral bone sclerosis. Mineralization of intra-or periarticular soft tissues and enthesiophyte formation may also contribute to the productive response. Except for occasional development of subchondral bone cysts, alterations are purely productive, generally well-defined, change slowly with time and are characteristic of non-aggressive lesions. Joint effusion or synovial hypertrophy may result in joint capsule distension or thickening with a resulting increase in the size and prominence of the periarticular soft tissues. Joint effusion may occasionally result in widening of a joint on radiographic examination, however, an apparent increase in joint space width is more often artificially induced when traction is placed on a limb for radiographic positioning. As articular cartilage becomes thinner with degeneration, the joint space may appear narrower than normal; subchondral bone becomes sclerotic and occasionally subchondral bone cysts form as part of the remodeling process. In extreme cases, especially in low range of motion joints, ankylosis may occur.

Metabolic Disorders

Metabolic bone disease is relatively rare. In some instances, as with hypervitaminosis A and nutritional or renal secondary hyperparathyroidism, the metabolic abnormality is acquired. In other instances, as with such diseases as congenital hypothyroidism, mucopolysaccharidosis and osteogenesis imperfecta, an underlying developmental anomaly leads to the metabolic disorder affecting the musculoskeletal system. Although some metabolic diseases carry a grave prognosis, radiographic features are often nonaggressive. In most cases, because of the nature of these diseases, radiographic changes will be multifocal or generalized although the effects may not manifest uniformly. Typical examples are the generalized bone mineral loss associated with nutritional or renal secondary hyperparathyroidism, generalized increased medullary opacity in osteopetrosis, periarticular mineralization from Hypervitaminosis A and generalized physeal growth abnormalities and epiphyseal dysplasia associated with congenital hypothyroidism and mucopolysaccharidosis.

Inflammatory Disease

In contrast with developmental, degenerative, metabolic and traumatic disorders, radiographic features of inflammation are usually aggressive and may include bone destruction, poorly defined and irregularly margined periosteal production, long, ill-defined zones of transition and rapid progression. Inflammatory bone disease may be infectious or non-infectious. Because of the greater frequency with which it occurs, this general description of radiographic features of infectious inflammatory disease emphasizes bacterial osteomyelitis. A short description of typical fungal osteomyelitis findings is also included. In addition to inflammatory lesions within bone, infectious and noninfectious inflammatory joint disease may occur.

Bacterial osteomyelitis is somewhat arbitrarily divided into acute and chronic stages. Lesions usually affect only one bone (monostotic) and are typically the result of direct infection. Multiple bones may be affected (polyostotic) when infection is due to hematogenous spread but this is uncommon in dogs and cats. In the acute phase (days to weeks), pronounced soft tissue swelling may be seen radiographically due to cellulitis. Gas may be seen within the soft tissues when open wounds are present or when gas-forming organisms are involved. Bony changes typically appear aggressive and consist of an irregular productive periosteal response that often has a mottled, non-uniform appearance and a long zone of transition. A mixed productive/destructive response is often present resulting in a disruption of normal trabecular bone architecture and a loss of cortical definition. In the earliest phase of disease, marked bone destruction may predominate. In the chronic phase (weeks to months), in addition to the features described above, there may be a rim of irregular sclerotic bone (involucrum) within which is a well-margined, bone dense body (sequestrum). A sequestrum and involucrum fails to form in many cases and is less likely to occur with hematogenous osteomyelitis. Periosteal productive bone appears smoother and less reactive as the infection becomes more chronic. When osteomyelitis occurs in the presence of a fracture, a proliferative periosteal response may be marked but the response is typically of non-uniform density, irregularly margined (aggressive) and often associated with a concurrent bone destructive response (aggressive). The productive response often does not bridge the fracture as normal healing callus does and the productive response may appear abnormally exuberant even in regions some distance from the fracture site. Fracture margins may remain sharp and well-defined.

The most common mycotic diseases of bones and joints in North America are coccidioidomycosis, blastomycosis and histoplasmosis. Although solitary fungal osteomyelitis lesions do occur, polyostotic lesions and multisystem disease often occur due to the hematogenous spread of fungal organisms. Radiographic findings of fungal osteomyelitis typically consist of an aggressive productive/destructive pattern. Although the productive response is often more pronounced than that seen with bacterial osteomyelitis, a marked underlying destructive component most often exists that may be partially obscured by the proliferation. Fungal lesions often increase in size with time; sequestra and involucra rarely develop.

Neoplasia

Malignant primary bone tumors occur with greater frequency than benign tumors and osteosarcoma is by far the most common, accounting for approximately 85% of all malignant bone tumors. Osteosarcomas are almost always monostotic and tend to occur in long bones metaphyses but can also arise in the axial skeleton. As with inflammatory bone lesions, radiographic features of neoplastic disorders typically appear aggressive and include bone destruction, aggressive periosteal and/or tumor new bone production, a variable appearance to the zone of transition and rapid progression. Osteosarcomas rarely extend into or across joints. Changes produced by osteosarcomas run the radiographic gamut from purely destructive to predominantly productive and the prevailing pattern can change over time. In general, findings for osteosarcomas consist of a mixed productive/ destructive. Cortical and medullary bone destruction often occurs in conjunction with an irregular, often spiculated or sunburst pattern, productive periosteal response. The periosteal response generally has an intermediate to long zone of transition and may either be well-defined or poorly defined.

Metastatic bone lesions most often arise in the metaphysis and diaphysis of long bones, the ribs, and the vertebral bodies. Lesions may be either monostotic or polyostotic. A combined productive/destructive response or a predominantly destructive bone response may be present.

Traumatic Disorders--Healing Response of Bone

Although fractures and luxations appear quite dramatic, radiographic features of acute trauma are nonaggressive. In uncomplicated bone healing, new bone production should predominate and zones of transition should be well-defined unless the fracture is complex or extensive periosteal or soft tissue injury was sustained. As with acute trauma, radiographic features of normal healing response are generally nonaggressive. Primary bone healing is rare and occurs by direct extension of osteons across the fracture site. This requires anatomical realignment of bone fragments and rigid fixation. There is minimal bony bridging callus at the fracture site but periosteal new bone may be formed due to periosteal injury from initial trauma or surgery. Secondary bone healing proceeds with formation of a fracture site hematoma which is replaced by a fibrous or fibrocartilaginous callus. This, in turn, is replaced by a bony bridging callus. Radiographic findings are variable and depend on fracture type and location, patient age, fracture fixation stability and adjacent soft tissue viability. Callus tends to be larger in skeletally immature patients or when the fracture fragments are unstable. In a reduced and stabilized diaphyseal fracture, there is typically a decrease in definition and opacity of the fracture margins after the first few days. Periosteal and endosteal bony callus is visible by 10-14 days and often bridges the fracture site by 3-6 weeks. Bone remodeling continues over many months, reducing the callus size, smoothing bone contours and reestablishing the medullary canal.

Malunion occurs when bone healing takes place in the face of abnormal alignment of fracture fragments. When malunion occurs in long bones, it may result in either angular or rotational limb deformity which can eventually lead to secondary degenerative joint disease. Delayed union occurs when the progression of fracture healing is slower than anticipated. This is a relative term and one that is sometimes difficult to apply accurately. Delayed union most often occurs due to persistent instability or from vascular compromise at the fracture site or in the surrounding soft tissues. The typical radiographic pattern of fracture margin resorption and bony callus formation is delayed, less prominent than normal or absent on serial radiographic examinations. Nonunion is present when fracture healing has ceased and the fracture ends remain ununited. Nonunions are typically classified as atrophic or hypertrophic but these terms represent two extremes and some nonunion fractures may have characteristics of both forms. In atrophic nonunion, a gap persists between fracture fragments and there is no evidence of bridging callus formation. This type of nonunion most frequently occurs when blood supply to the fracture ends is poor and stabilization is inadequate. Hypertrophic nonunion is the more common form and has a well-defined fracture gap associated with prominent non-bridging bony callus arising from the fracture ends. The fracture ends are often irregularly shaped but smoothly margined and sclerotic. Hypertrophic malunion is typically the result of persistent fracture instability.