Radiography and Radiology

Chapter 15 Radiography and Radiology

Radiography is the acquisition of radiographic images; radiology is the study and interpretation of those images. Radiography is an important part of the diagnostic armamentarium in the evaluation of lameness. Its most important role is to give information about bones and joints. However, it also can provide information about soft tissues, most particularly tendon, ligament, and joint capsule insertions. If radiography is to be used properly, then the area under investigation must be evaluated comprehensively and appropriately. A sufficient number of views, all which have been appropriately centered and exposed, should be obtained.

Obtaining high-quality radiographic images requires attention to detail. The horse must be correctly positioned and adequately restrained or sedated. For most weight-bearing examinations the horse should stand with the cannon bone of the limb to be examined in a vertical position. The horse should be standing on all four limbs, not resting a limb. The area under investigation should be cleaned to remove any surface dirt. For examinations of the foot, the shoe should be removed (if possible) to facilitate proper paring of the sole and frog, to ensure it is adequately clean, and to avoid superimposition of a radiodense shoe over the distal phalanx and navicular bone. The tail should be tied to facilitate correct positioning of the cassette or imaging plate when the stifle and hock regions are examined.

Radiographic Detail

The aim is to obtain as highly detailed radiographic images as possible. The detail that can be obtained is influenced by a number of factors, including those listed in Box 15-1.

BOX 15-1 Factors That Influence Radiographic Images

Response of Bone to Stimuli: Wolff’s Law

Correct interpretation of radiographic images requires knowledge of the ways in which bone responds to various stimuli. Bone models according to Wolff’s law: it models according to the stresses placed on it so that it can be functionally competent with the minimum amount of bone tissue. The use of the terms modeling and remodeling creates considerable confusion because usage differs in histology and radiology. Histologically, bone remodeling refers to resorption and formation of bone that is coupled and occurs in basic multicellular units. This regulates the microstructure of bone without altering its shape and is a continuous process, replacing damaged bone with new bone. Thus remodeling cannot be appreciated radiologically. Radiologically, the term has been used to describe reshaping of bone to match form and function (e.g., after fracture repair). Strictly speaking, the term modeling should be used to describe the change in the shape of a bone as it adapts to the stresses applied to it.

Bone is a dynamic tissue, constantly reacting to the stimuli that it receives both internally and externally. However, it takes time to respond, and a 40% change in bone density must occur before changes are evident radiologically. Therefore radiographic images, although anatomically accurate, are relatively insensitive in the early stages of a disease process. This is known as the radiographic latent period. It is critical to appreciate these limitations when interpreting radiographic images. Bone can be undergoing abnormal modeling without identifiable structural change. Once radiological abnormalities have developed, some will persist over the long term without necessarily being associated with ongoing pain. Thus in effect these changes remain as scars reflecting previous injury. Aging of such lesions is impossible, and assessing clinical significance must be evaluated in the light of the clinical signs.

Bone can react to stimuli in only a limited number of ways. Bone can produce new bone, such as periosteal new bone, endosteal new bone, cortical thickening, increased thickness of trabeculae, callus formation, osteophyte and enthesophyte formation, and the palisading periosteal new bone typical of hypertrophic osteopathy. New bone often results in what has been described radiologically as sclerosis: increased opacity of the bone, caused by either new bone being laid down within the bone or superimposition of new bone on the surface of the bone. More than one radiographic image usually is required to determine why a structure appears to have increased opacity (i.e., is sclerotic). Strictly speaking, however, sclerosis is a localized increase in opacity of the bone caused by increased bone mass within the bone. Unless this can be determined with certainty it is preferable to use the term increased radiopacity.

Osteolysis is resorption of bone resulting in a radiolucency. Again, a lag period, usually of at least 10 days, occurs between the onset of osteolysis and its radiological detection. Osteolysis occurs for a variety of reasons, including pressure, infection, as part of early fracture repair, and as part of the disease process in osteoarthritis, osteochondrosis, osseous cystlike lesions, and subchondral bone cysts. Bone destruction and resorption usually are seen more easily in cortical bone rather than cancellous bone because of the greater contrast.

Generalized demineralization, or osteopenia, of bone throughout the body rarely occurs in the horse. Localized demineralization in a single limb usually is the result of disuse and is characterized by thinning of the cortices and a more obvious trabecular pattern. The proximal sesamoid bones are particularly sensitive indicators of disuse osteopenia in the horse.

Focal demineralization and loss of bone may be caused by pressure—for example, as seen in chronic proliferative synovitis in the fetlock—resulting in erosion of the dorsoproximal aspect of the sagittal ridge of the third metacarpal bone (McIII). It may be the result of infection, invasion by fibrous tissue, or a neoplasm.

Cortical thickness changes (models) according to Wolff’s law as an immature athlete develops into a mature, trained athlete. The dorsal cortex of the McIII and the third metatarsal bone becomes thicker. If a horse has marked conformational abnormalities, such as offset or bench knees, the bones model accordingly, with the lateral cortex of the distal limb bones becoming thicker.

Periosteal New Bone

Blunt trauma to bone can lead to subperiosteal hemorrhage, resulting in lifting of the periosteum away from the bone. This process may stimulate the production of periosteal new bone (Figure 15-1). Some bones, such as the second and fourth metacarpal and metatarsal bones, seem particularly prone to such reactions. There is individual variation among horses in susceptibility to such reactions. Usually a lag period of at least 14 days occurs between trauma and the radiological detection of periosteal new bone. Such bone usually is much less dense than the parent bone; therefore soft exposures (or low kilovoltages) are essential for detection of this bone, which initially has a rather irregular outline. As the bone gradually consolidates and then models, it becomes more opaque and more smoothly outlined. Curiously, although a well-established splint exostosis would be expected to become inactive, many have increased radiopharmaceutical uptake (IRU) compared with the parent bone if examined scintigraphically.

Periosteal new bone can also develop as a result of fracture, infection, inflammation, and neoplasia. Inflammation of the interosseous ligamentous attachment between the second metacarpal bone and the McIII or the fourth metacarpal bone and the McIII caused by movement and loading can result in periosteal new bone formation and a splint exostosis. It is curious that some of these formations develop rapidly without associated pain, whereas others can cause persistent pain and lameness for many weeks, despite a similar radiological appearance. The bony protuberances that develop on the proximolateral aspect of the metatarsal regions, often bilaterally, are even more enigmatic. They are rarely associated with clinical signs, although they often have IRU despite having been present for several years.


Sclerosis is the localized formation of new bone within bone and results in increased bone mass. It is most easily identified in trabecular bone (Figure 15-3) and occurs in response to several stimuli, including the following:

Enostosis-like lesions are the development of bone within the medullary cavity or on the endosteum, resulting in a relatively opaque (sclerotic) area of variable size. They frequently occur adjacent to the nutrient foramen, and the origin is unclear. They may develop as a focal or multifocal lesion. They vary in size and generally are seen in the diaphyseal regions of long bones in the horse. These lesions have been seen most frequently in the humerus, the radius, the tibia, the McIII, the third metatarsal bones, and the femur. When these lesions develop, they have focal intense IRU and may be associated with pain and lameness. However, they also are seen as incidental findings.

Small focal opacities in the proximal metaphyseal region of the tibia have been seen. The origin and clinical significance are not known. Care must be taken in the fetlock region not to misinterpret the radiopacity caused by the ergot as an opaque lesion within the proximal phalanx.

Osteophyte Formation

An osteophyte is a spur of bone on a joint margin that develops as a result of a variety of stimuli, including joint instability, or in association with intraarticular disease, particularly osteoarthritis. Not all periarticular modeling changes at the junction of the articular cartilage and periarticular bone are associated with ongoing joint disease, but radiological differentiation between a subclinical osteophyte and a clinically significant one is difficult. Small spurs frequently are seen on the dorsoproximal aspect of the third metatarsal bone close to the tarsometatarsal joint. Some are quiescent, unassociated with articular pathological findings, whereas others are progressive. Small spurs on the dorsoproximal aspect of the middle phalanx are frequent incidental findings in Warmblood breeds. Mature horses with offset- or bench-knee conformation frequently have spurs on the lateral aspect of the antebrachiocarpal joint without associated clinical signs.

The time of development of an osteophyte after stimulus varies depending on the inciting cause and individual variation. Two weeks to several months may pass before an osteophyte may be identified radiologically. A smoothly marginated osteophyte of uniform opacity is more likely to be long-standing, whereas a poorly marginated osteophyte with a lucent tip is likely to be active.

Some joints seem to have a greater propensity than others for the development of periarticular osteophyte formation. The reason for this tendency is unknown and may in part reflect the ease with which osteophytes can be detected radiologically. Even within what is currently considered a single disease process, osteoarthritis of the distal hock joints (bone spavin), some horses develop predominantly periarticular osteophytes (Figure 15-4), whereas others have narrowing of the joint space and subchondral sclerosis. A third group develops extensive radiolucent areas (Figure 15-5).

Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Radiography and Radiology
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