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Orthopedic radiography in exotic animal practice
ORTHOPEDICS
1094–9194/02 $15.00 .00
ORTHOPEDIC RADIOGRAPHY IN EXOTIC ANIMAL PRACTICE Jamie Williams, MS, DVM, Diplomate, American College of Veterinary Radiology
The radiographic examination of orthopedic disease is relatively straightforward and follows guidelines for obtaining and evaluating radiographs of axial and appendicular skeleton that already are established for common pet species.1, 3–12, 14–16, 24–28 Orthopedic diseases common in routine pet species (dog and cat) also are found in avian and exotic species; however, avian and exotic species also exhibit rather unique orthopedic diseases and unique responses to routine orthopedic maladies. This article describes some of the common orthopedic insults that are suffered by exotic animals and discusses some notable anatomical differences and exceptions in response to injury when appropriate. Knowledge of normal radiographic anatomy is critical when evaluating patients for orthopedic disease. When faced with the prospect of obtaining and then evaluating radiographs of avian or exotic pet species, the practitioner may be intimidated by a lack of knowledge of normal radiographic anatomy or disease entities of a particular species. As with dogs and cats, however, there should be a bilateral symmetry of skeletal structures. Published references that detail the normal radiographic anatomy of avian and exotic species may be used by the less-experienced practitioner when reviewing the radiographs of birds, snakes, lizards, rabbits, and turtles.3, 13, 16–24
From the American College of Veterinary Radiology, Department of Veterinary Clinical Science, School of Veterinary Medicine, Louisiana State University School of Medicine, Baton Rouge, Louisiana VETERINARY CLINICS OF NORTH AMERICA: EXOTIC ANIMAL PRACTICE VOLUME 5 • NUMBER 1 • JANUARY 2002
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RESTRAINT AND POSITIONING Patient malpositioning during radiographic exposure is the most common reason for nondiagnostic radiographs or misdiagnosis of disease, especially for those practitioners who either are new to exotic diagnostic imaging or see only a few cases per year. Care and attention must be paid to accurate patient positioning so that symmetry is maximized. Manual restraint may be sufficient in many exotic pets, especially for smaller, relatively calm birds, turtles, tortoises, and some lizards. Chemical restraint (sedation or anesthesia), however, is extremely beneficial for larger uncooperative patients, such as large birds or raptors, snakes, ferrets, and even rabbits. Chemical restraint not only improves radiographic quality by reducing patient motion but also allows more precise positioning for the intended views and increases the safety of the patient and technical staff. Sedation may be accomplished via injectable products; anesthesia may be accomplished via injectable products or the tank or mask administration of gas agents (e.g., Isoflurane) (Fig. 1). Patient monitoring (e.g., pulse, respiration, heart rate, body temperature) is essential during sedation or anesthesia. Positioning is improved or aided with the use of cloth tape; the patient can be taped either directly to the radiographic cassette (Fig. 2) or to a positioning device (bird board) made of acrylic (Fig. 3). Because whole-body radiographs often are obtained, making the radiographic exposure at peak inspiration is important, especially in species, such as rabbits and prairie dogs, that have a disproportionately small thorax compared with the abdomen. The vast majority of avian and exotic patients are small in truncal diameter, and, therefore, films are often exposed without the use of a grid. High-detail film and screen combina-
Figure 1. Mask administration of Isoflurane (Abbott Laboratories, North Chicago, IL) to a ferret before radiography. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 2. Avian patient taped directly to radiographic cassette in ventrodorsal position. Tape on the feet allows retraction during exposure. Note the mask administration of Isoflurane to eliminate stress or movement in the patient during exposure. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
tions provide the best radiographic detail but require higher radiographic techniques (kVp and mAs) for proper radiographic exposure. Motion artifact is relatively more common in smaller patients because of their more rapid respiratory and heart rates; therefore, exposure times
Figure 3. A, Acrylic positioning device (bird board). B, Ventrodorsal radiographic image of an egg-bound avian patient using this device. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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should be kept at 1/60 second or less. As with the more common pet species, two orthogonal (90 apart) radiographic views are necessary to evaluate any body structure completely. For the truncal body, this is accomplished with lateral and either ventrodorsal or dorsoventral views. With small mammals (e.g., ferret, rat, hedgehog), orthogonal radiographic views of the appendicular skeleton are obtained in much the same way as for a dog and cat. The lateral view is relatively easy to obtain in avian species; however, the orthogonal view of the wing is more difficult and requires that the patient be held vertically in a headdown position with the wing extended against the radiographic cassette and aligned so that the radiograph photons penetrate the wing from caudal to cranium. Radiographic examination of extremely small structures may be improved with nonscreen film. Dental film is one example of nonscreen film; however, other single emulsion detail radiographic film placed in a light-tight envelope may be used. Radiographic detail is improved greatly with nonscreen film because there is no phosphor bloom to blur the edges of the structures; the film is exposed directly to the radiograph photons rather than to the light from the glowing or activated phosphors of the radiographic screen. Unfortunately, this improved radiographic detail requires higher exposure factors. It is well known that radiographic film is five times more sensitive to room light than radiograph photons; therefore, exposure factors must be raised significantly when using nonscreen radiographic film. Usually, the milliamperage (mA) can be increased greatly while maintaining a short exposure time to avoid motion artifact. Many cage birds are relatively small, as are some of the exotic pet species (e.g., gerbils, hamsters, rats, mice). Magnification radiography can be employed to enlarge the radiographic images; however, there is an inherent loss of edge sharpness. Magnification radiography is accomplished by increasing the distance between the patient and the film, in effect raising the patient off the film cassette and placing it closer to the tube head of the radiography machine. Placing a thick nonradiopaque device (e.g., square sponge) between the patient and the radiographic cassette effectively results in magnification radiography. It is imperative that the routine orthogonal radiographic views be obtained first, with the patient placed directly against the radiographic film cassette. Magnified images can be added to the basic study in the smaller species, if desired, to enlarge the image of the smaller skeletal structures. A better alternative is to purchase an inexpensive magnifying glass, which decreases the number of radiographic exposures because the magnified images are no longer necessary. Additionally, reviewing routine films with a magnifying glass effectively enlarges the radiographic image without the loss of edge sharpness. Turtles and smaller lizards may be placed on a small, rectangular, nonradiopaque support (e.g., a sponge) during radiography. The tube head is extended and rotated horizontally (parallel to the table surface), with the radiographic cassette positioned vertically against the patient to obtain the lateral or craniocaudal radiographic image (Figs. 4 and 5).
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Figure 4. Depiction of horizontal beam radiograph to obtain a craniocaudal image of this tortoise. The tube head is extended and rotated such that the radiograph beam is oriented parallel to the tabletop, the patient (tortoise) is placed upon a non-radiopaque sponge, and the radiographic cassette is held in place vertically behind the patient using sand bags. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
Using this positioning aid often allows a more true projection to be obtained compared with manually holding the patient on its side for vertical overhead tube exposure. Additionally, providing a small-diameter stick to which smaller lizards can cling during the radiographic procedure may be helpful. This may decrease their activity level, allow
Figure 5. Lateral radiographic image of a small lizard using the same equipment orientation as seen in Figure 4, with the patient positioned to obtain a lateral radiographic image. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 6. Lateral radiographic image of a small chameleon obtained with horizontal beam orientation. Note that the chameleon is grasping the small stick used to aid positioning. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission)
them to feel more secure, and provide optimal positioning for horizontalbeam radiography (Fig. 6). UNIQUE SKELETAL FEATURES Avian bones exhibit much thinner cortices than those normally found in mammals.7, 14, 23, 24 It is well known that certain avian species possess pneumatic humeri and femora.7, 8, 23, 28 Extensions of the interclavicular (humerus) and caudal ceolomic (abdominal) air sacs invade the medullary portion of these bones, decreasing their medullary opacity (Fig. 7). When a fracture involves the pneumatic bones, subcutaneous emphysema results, which can make the determination of an open versus closed fracture more difficult. More importantly, because of this free communication, it is possible for osteomyelitis to result from the direct extension of air sacculitis or, conversely, for air sacculitis to result from the extension of osteomyelitis.8, 23 The avian axial skeleton is also rather unique, with three fused vertebral segments that are thought to provide increased stability during flight. From an orthopedic standpoint, the most clinically important of these regions is the synsacrum, which is formed by ankylosis of the last several thoracic vertebrae, all the lumbar and sacral vertebrae, and the first few caudal vertebrae. Reproductively active female birds store calcium salts in the ulna and tibia prior to beginning egg laying which results in significantly increased opacity or hyperostosis of these particular bones (Fig. 8).14, 28 The increased opacity is diffuse, uniform, and seasonal. Avian tuberculosis (Mycobacterium species) also has been reported to result in focal, increased medullary opacity and periosteal proliferation, predominantly in the long bones, as well as lameness.7, 8, 14
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Figure 7. Ventrodorsal radiograph of a large avian patient demonstrating the decreased opacity of the humeri and femurs typically found in avian species possessing pneumatic bones. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
FRACTURES OF THE APPENDICULAR SKELETON Fractures in avian or exotic species are classified in the same manner as that used for routine pet species. Fractures may be open (i.e., the bone protrudes beyond the dermal surface or there is air within the soft tissue of the fracture site) or closed, and complete (i.e., both cortices of bone are involved) or incomplete (i.e., only one cortex is broken [e.g., folding fracture, stress fracture, greenstick fracture]). Fractures are further classified as simple (i.e., one fracture line results in two fracture fragments) or comminuted (i.e., multiple fracture lines result in more than two fracture fragments). Recognizing whether the fracture involves the articular surface of a bone is important, because proper realignment of the fracture fragments and stabilization of the fracture site is imperative. Significant degenerative changes, however, are likely to result. Displacement of the fracture fragments may affect the treatment approach. The distal segment or fragment is always considered in the wrong location when describing fracture displacement. In other words, the location of the distal segment is always described relative to the
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Figure 8. Close-up ventrodorsal view of the pelvic limbs of a female hawk. Note the significant increase in osseous opacity of the tibiotarsal bones indicative of seasonal hyperostosis in reproductively active female birds. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
proximal segment. A cranially displaced fracture has its distal segment cranial to or in front of the proximal segment. Evaluating all fracture segments for the presence of fissure fractures is critical. Fissure fractures are incomplete fractures that involve only one cortex and are found extending from the original fracture site. Evaluation for fissure fractures can be especially trying in avian and exotic animals because their skeletal structures are relatively small and thin, necessitating the use of a magnifying glass and careful scrutiny of the cortical margins adjacent to the fracture site on orthogonal radiographic views. Failure to recognize fissure fractures may result in a previously nondisplaced piece of bone becoming a complete fracture secondary to implant placement. Fractures that involve the immature physis (Salter–Harris fractures) risk asynchronous growth and limb deformity.6 Animals with open fractures are at risk for osteomyelitis (Fig. 9),9 and therefore a delayed or nonunion fracture. All gunshot-induced fractures are considered open; however, the heat involved in this trauma often decreases the likelihood of osteomyelitis. Fractures that involve the pneumatic bones of avian species are at greater risk for osteomyelitis and for extension to the associated air sacs (air sacculitis). Avian have a rather unique healing process in fracture repair: Periosteal proliferation (callus) is reduced greatly compared with that of mammalian species, and healing is predominantly through endosteal callus7, 8, 28 and fibrous tissue proliferation. This is also true in reptiles which demonstrate a similar response to bone healing.16 Based on mammalian criteria for fracture healing, reptiles and other exotic animal species are considered slow healers. In fact, it is not uncommon to visualize the radiolucent fracture line, even though the bone has healed, because of the predomi-
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Figure 9. Lateral radiograph of the fractured right wing of a bird. Note that the distal aspect of the proximal segment of the radial fracture extends beyond the skin surface dorsally (open fracture) and the comminuted fracture of the mid-diaphysis of the ulna. Soft tissue swelling is associated with the fractures. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
nantly fibrous callus. As a result of this phenomenon, radiographic assessment is recommended at 12 to 16 weeks after the injury to assess fracture healing.16 Periosteal reaction is often more prominent in snakes than in lizards or turtles. Entanglement or entrapment is the biggest cause of traumatic fracture in caged birds, most often resulting from inappropriate cage toys, chains, wires, and so forth. Evaluating turtles for fracture is difficult because of their propensity to retract their head, tail, and appendages into their shell. Occasionally, self-exteriorization of a particular limb may suggest trauma to or an abnormality of that limb (Fig. 10A, B, and C). Tape or rolled gauze may be used to extract the limbs gently during positioning for radiography, allowing a more thorough evaluation for potential fracture disease (Fig. 11A, B, and C). This same technique is used to evaluate the joints for effusion or sepsis. Traumatic fractures of the tibia are relatively common in chinchillas, presumably because the tibia is longer than the femur. Chinchillas possess on extremely thin and fragile bone structure, which undoubtedly predisposes them to fracture. Commonly reported causes for these fractures include grasping the animal by its rear limb or it having caught its rear limb in its cage.15 Nonunion fracture may result from these traumatic fractures because applying external fixators to the fracture site is difficult and soft bandages generally do not provide excellent stabilization of these thin and fragile bones.15 FRACTURES OF THE AXIAL SKELETON Fracture, subluxation or luxation of the vertebral column are relatively uncommon in exotic species; however, vertebral fractures sec-
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Figure 10. A, A tortoise with the self-exteriorization of the left rear leg. B, Dorsoventral radiograph shows that the left rear leg remains exteriorized and that there is a fracture of the proximal left femur. C, Close-up radiograph illustrating the normal posture of the right rear limb and the abnormal posture of the left rear limb caused by the proximal femoral fracture. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.) Illustration continued on opposite page
ondary to metabolic bone disease may occur. There are three fused or ankylosed regions of the vertebral column in avian species, which results in rigid sections, that are thought to stabilize the vertebrae during flight. In terms of orthopedic maladies or disease, the most important of these regions is the synsacrum (Fig. 12), which is formed by ankylosis of the last several thoracic vertebrae, all the lumbar and sacral vertebrae, and the first few caudal vertebrae. Birds are known to fly toward uncovered windows, which results in craniocaudal impact when they encounter a window that is closed. The most common site for traumatic vertebral fracture that results from this impact is immediately cranial to the fused synsacrum7, 8; therefore, this particular region must be evaluated thoroughly for fracture or malalignment in all avian patients with suspected vertebral trauma. Fractures that involve the cranium are always difficult, even in routine small animal patients. The difficulty is augmented in exotic species owing to their reduced size, which results in a smaller radiographic image. The normally thin cortical margin of the osseous structures of the skull makes determination of small fractures even more difficult. Numerous air-sac extensions are found within the skull of avian patients, which further complicates the radiographic recognition of fracture.13 Skull fracture is easier to recognize if there is an obvious malalignment or depression in the normal contour. In small exotic species, such as lizards and ferrets, occlusal (intraoral) radiographs provide increased radiographic detail, allowing closer scrutiny of the rostral maxilla and mandible for fracture or lytic change (Fig. 13). Intraoral radiographs are obtained by placing a nonscreen detail film in the mouth
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Figure 10. See legend on opposite page
of the anesthetized animal. Because no intensifying screen is used, mAs must be increased substantially; however, the detail that is obtained is better than that provided by a detail film and detail screen. Masses or soft tissue swelling of the head should be investigated radiographically
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Figure 11. A, Close-up image of a dorsoventral radiograph of a tortoise. Tape applied to the forelimb was used to gently extract the limb from superimposition of the shell during radiographic exposure. B, A closer view of following demonstrates the thin radiopaque linear structures extending from either side, secondary to the tape extraction aid, resulting in a craniocaudal or dorsopalmar view of the forelimb. C, A lateral view of the limb again visualizes the tape surrounding the distal aspect of the forelimb. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 12. Close-up of a lateral radiographic of an avian patient. The white bracket indicates the synsacrum. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
to determine the character of the associated osseous structures (i.e., evaluate for lysis or proliferation of adjacent bony structures) (Fig. 14). The avian pelvis is fused to the synsacrum dorsally but is open ventrally,8, 17–23 to allow passage of large, mineralized eggs. Traumatic
Figure 13. Mandibular occlusal (intraoral) radiograph of a Water Dragon Lizard (small species) with rostral swelling. Note the expansile nature of the rostral right mandible suggestive of osteomyelitis. This particular radiograph was made with nonscreen dental radiographic film. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 14. Lateral radiographic view of the skull of a rabbit presented for firm swelling of the rostroventral mandibular region. There is no evidence of osseous destruction. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
pelvic fractures may be difficult to recognize in avian patients, especially if axial rotation is present during positioning for the ventrodorsal radiograph. If careful attention is paid to accurate alignment and position, then the practitioner need only compare the right side to the left and evaluate for asymmetry (Fig. 15). Vertebral fractures in exotic patients may result from metabolic bone disease. Traumatic fracture or luxation of the vertebral column is the most common cause of acute-onset posterior paresis in otherwise healthy rabbits. The hindquarters of the rabbit are heavily muscled as a part of its fight-or-flight apparatus. Lack of proper support during patient handling is the most common reason for fracture or luxation of the vertebral column, with the most common site being L7.4 Therefore, it is essential that the hindquarters of the rabbit be supported securely during handling to prevent explosive extension of the rear limbs, potentially resulting in fracture or luxation of the vertebral column. INFECTION, OSTEOMYELITIS, AND SEPSIS Radiographic changes may not be readily apparent in the early stages of infection or osteomyelitis.10, 14 Later in the course of disease, however, soft tissue swelling, bone lysis, and aggressive periosteal proliferation often are found. Local bone destruction is noted by lytic lesions in a focal area, often at or near the site of fracture or penetrating wound (Fig. 16A and B). Bone destruction and soft tissue swelling are
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Figure 15. Close-up ventrodorsal avian radiograph depicting abnormal orientation of the pelvis consistent with fracture of the right ileum. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
recognized more easily in the lower extremities, which normally have reduced soft tissue thickness. Septic arthritis is seen most commonly in the intertarsal joints of caged birds.28 Open fractures, penetrating wounds, or iatrogenic causes may result in osteomyelitis.8 Osteomyelitis also has resulted from hematogenous dissemination or extension from air sacculitis [pneumatic bones], septic arthritis, and pododermatitis.4, 7, 8, 15, 16, 23 Because avian species do not produce much periosteal proliferation during fracture healing, similarly it does not occur extensively with infection either. In fact, lysis is the predominant radiographic osseous abnormality with both infection and neoplasia.8, 14 Increased periosteal proliferation and increased medullary opacity, however, have been noted in cases of fungal osteomyelitis or tuberculosis (Mycobacterium species).8 Osteomyelitis in reptiles also produces a primarily lytic reaction or response, with little periosteal proliferation (Fig. 17A and B).6 Pododermatitis (bumblefoot) is common in guinea pigs, and osteomyelitis is possible in the distal extremities via direct extension.15 Ulcerative pododermatitis occasionally may result in osteomyelitis or septice-
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Figure 16. A, Lateral radiograph of an open gunshot fracture of the ulna of a hawk. The small metallic opacities are indicative of gunshot, which is always considered an open fracture. The significant periosteal reaction associated with the comminuted ulnar fracture, along with the palpable increased temperature of the soft tissues is indicative of osteomyelitis in the avian patient. B, Close-up craniocaudal radiograph of the swollen foreleg (elbow) of an iguana. Note the loss of subchondral bone in the humeral condyle, proximal radius and proximal ulna; the periosteal proliferation extending proximally on the distal humerus; and the significant soft tissue swelling. These radiographic findings are indicative of osteomyelitis and septic arthritis. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
mia via direct extension in rabbits4; however, rabbits are prone to abscessation (from P. multocia or Staphylococcus aureus). These abscesses may affect the joints, long bones, or skull, resulting in osteomyelitis (Fig. 18A and B).4 PATHOLOGIC FRACTURES Pathologic fractures are simply fractures of bone that had a preexisting disease (e.g., neoplasia, osteomyelitis (bacterial or fungal), metabolic bone disease). Bone is a living tissue that is resorbed and redeposited constantly. Natural stresses on existing bones determine where bone salts are deposited; bone is remodeled according to Wolff’s law (i.e., according to the lines of stress). Metabolic bone disease is suggested radiographically by generalized bone changes and osteopenia of multiple bones of axial and appendicular skeleton. Exotic species seem to be more susceptible to nutritional secondary hyperparathyroidism (fibrous osteodystrophy secondary to nutritional imbalance). Animals that suffer from nutritional secondary hyperparathyroidism demonstrate abnor-
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Figure 17. Dorsoventral (A) and lateral (B) close-up radiographs of a snake with a perivertebral abscess and subsequent vertebral osteomyelitis. The perivertebral abscess (confirmed by way of fine needle aspiration) is seen as the soft tissue mass adjacent to the vertebrae, while the osteomyelitis is suggested by the focal demineralization in the vicinity of the soft tissue abscess. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
mally thinned cortices of long bones with decreased overall osseous opacity (medullary and cortical) and also may exhibit pathologic fractures (e.g., folding fractures, incomplete fractures) in these bones (Fig. 19A and B). It is not uncommon to recognize several fractured ribs along with fractures of the appendicular skeleton in animals with subjectively thinned cortices and a loss of normal bone opacity. Metabolic bone disease with secondary pathologic fracture is more common than traumatic fracture in cage birds, according to McMillan.8 Guinea pigs are genetically susceptible to vitamin C deficiency. Radiographic changes noted in scurvy (hypovitaminosis C) include enlargement of the epiphyses of long bones and the costochondral junctions of the ribs. Pathologic fractures also may result.15 Pathologic fractures of the mandible may indicate renal secondary hyperparathyroidism, especially if the cortical opacity of the long bones appears relatively normal or at least not as osteopenic as the mandibular cortices.
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Figure 18. Dorsoplantar (A) and lateral (B) radiographic images of the swollen tarsus (3week duration) of a rabbit. Note the asymmetrical (extra-capsular) soft tissue swelling located at the tarsocrural joint and distal tibia, but lack of significant periosteal reaction. The asymmetrical distribution and irregular margination indicates the swelling is extracapsular rather than within the joint proper; and the lack of significant osseous reaction suggests the lesion is somewhat benign. Fine needle aspiration confirmed abscessation. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
Spinal lymphoma is reported rarely in ferrets but may result in pathologic fracture of the vertebrae.2 JOINTS: LUXATION, SUBLUXATION, AND DEGENERATIVE CHANGES Joint instability is not tolerated by the body; however, the response of bone or joint tissue is rather limited. The body’s natural response to joint instability is to produce more bone in an effort to increase stability, resulting in the periarticular osteophytes seen commonly in degenerative joint disease. Instability often results in increased synovial mass, either via joint effusion or hypertrophy of the soft tissue of the joint. This increased synovial mass stretches and distends the joint capsule, putting stress on the osseous attachments of the joint capsule. Specialized fibers (Sharpey’s fibers) in the joint capsule allow its attachment to bone and form an enthesis (i.e., a point of soft tissue insertion into bone). When stretched, Sharpey’s fibers mineralize in an effort to restore stability and tighten the joint capsule. This mineralization results in enthesophytes, which are seen in degenerative joint disease. Spontaneous cartilage degeneration has been reported in guinea pigs, resulting in degenerative changes and arthritis of the stifle joint.15 Additional radiographic signs
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Figure 19. Ventrodorsal (A) and close-up (B) abdominal radiograph demonstrating the folding fracture of the distal right femur in a young raccoon. The cortices of all bones, especially the long bones, are significantly thinned, suggesting metabolic bone disease (fibrous osteodystrophy). (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
of joint disease commonly evaluated in dogs and cats are more difficult to ascertain in exotic or avian species. Joint effusion may be particularly difficult to evaluate in smaller exotic species such as hamsters, mice, and gerbils simply because the overall size of the joint is so small; the presence of joint effusion is easier to note in rabbits and other larger species. In exotic or avian species, the radiographic evaluation of joints for enthesophytes is difficult owing to the small area of interest. Joint laxity results in either subluxation or luxation, depending on whether any portion of the articulating surfaces is still in contact or not (i.e., luxation is a more severe disruption of normal joint articulation than is subluxation). Luxation and subluxation certainly have been documented in avian and other exotic animal species. Although luxation is deemed uncommon,8 it seems to be most prevalent in the coxofemoral joints, stifles, and digits of birds.8, 14 EXOSKELETON Animals that possess an exoskeleton or a hard external shell (e.g., tortoise, turtle, armadillo) present yet another opportunity for radiogra-
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phy. If external trauma involving the shell is noted, radiographs are indicated to determine the extent of internal damage as well. Turtles and tortoises exhibit an intimate attachment of the vertebral column with the shell; therefore, damage to the shell may result in damage to the vertebral column. Furthermore, depending on the type or force of trauma, internal hemorrhage or lung contusion may result (Fig. 20A, B, and C). SUMMARY Radiographic examination of exotic animal species provides significant diagnostic information to the practitioner so long as attention is paid to positioning and radiographic detail and if variations in normal
Figure 20. Lateral (A) and dorsoventral (B) radiographs of a tortoise presented for fracture of the caudal left aspect of the shell. There is no radiographic evidence of fracture or subluxation involving the vertebral column, the pelvis, or the left femur. The craniocaudal radiograph (C) confirms pulmonary contusion secondary to the initial trauma. Note the increased soft tissue opacity of the lungs in the dorsal aspect of the coelom. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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radiographic anatomy are contemplated. Fracture of pneumatic bones (humerus or femur) in avian patients often results in subcutaneous emphysema, much like an open fracture. If fracture alignment is anatomical, there is generally little callous formation visible in avian and reptile patients; however, if malunion occurs, then certainly callous can be visualized. Aggressive or excessive periosteal proliferation in exotic species often predicts osteomyelitis. Practitioners can increase their diagnostic capabilities and thereby decrease any preconceived anxiety by applying the same general interpretive concepts used in the radiography of more routine small animal patients and by evaluating the resultant radiographs in a systematic manner. With the exception of certain anatomical differences and alterations in response to injury, exotic animal species suffer the same types of orthopedic disease do dogs and cats.
References 1. Allan G: Radiographic signs of joint disease. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 169–188 2. Antinoff N: Musculoskeletal and neurologic diseases. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 126–130 3. Gabrisch K, Grimm F, Isenbu¨gel E, et al: Atlas of Diagnostic Radiology of Exotic Pets: Small Mammals, Birds, Reptiles, and Amphibians. Philadelphia, WB Saunders, 1991 4. Gentz EJ, Carpenter JW: Neurologic and musculoskeletal disease. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 220–226 5. Konde LJ: Aggressive versus nonaggressive bone lesions. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 37–43 6. Konde LJ: Diseases of the immature skeleton. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 131–141 7. Krautwald-Junghanns M-E, Trinkaus K: Imaging Techniques. In Tully TN, Lawton MPC, Dorrestein GM (eds): Avian Medicine. Boston, Butterworth Heinemann, 2000, pp 52–73 8. McMillan MC: Imaging techniques. In Ritchie BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application. Lake Worth, FL, Wingers Publishing, 1994, pp 246–326 9. Morgan JP, Leighton RL: Fracture description. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 8–20 10. Morgan JP, Leighton RL: Radiology of osteomyelitis. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 43–52 11. Morgan JP, Leighton RL: Joint injury. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 67–69 12. Morgan JP, Leighton RL: Pathologic fractures. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, p 70 13. Paulmurphy JR, Koblik PD, Stein G, Pennick DG: Psittacine skull radiography: Anatomy, radiographic technique, and patient application. Vet Radiol 31:218–224, 1990 14. Quesenberry K: Disorders of the musculoskeletal system. In Altman RB, Clubb SL, Dorrestein GM, et al (eds): Avian Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 523–539 15. Schaeffer DO, Donnelly TM: Disease problems of guinea pigs and chinchillas. In
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Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 260–279 Silverman S, Janssen DL: Diagnostic Imaging. In Mader DR (ed): Reptile Medicine and Surgery. Philadelphia, WB Saunders, 1996, pp 258–264 Smith BJ, Smith SA, Spaulding KA, et al: The normal xeroradiographic and radiographic anatomy of the cockatiel (Nymphicus hollandis). Vet Radiol 31:226–234, 1990 Smith BJ, Smith SA, Flammer K, et al: The normal xeroradiographic and radiographic anatomy of the orange-winged amazon parrot (Amazona amazonica-amazonica). Vet Radiol 31:114–124, 1990 Smith SA, Smith BJ: Normal xeroradiographic and radiographic anatomy of the redtailed hawk (buteo-jamaicencis), with reference to other diurnal raptors. Vet Radiol 31:301–312, 1990 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the mallard duck (Anas platyrhynchos), with reference to other anserine species. Vet Radiol 32:87–95, 1991 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the bobwhite quail (Colinus virginianus), with reference to other galliform species. Vet Radiol 32:87–95, 1991 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the great horned owl (Bubo virginianus), with reference to the barn owl (Tyto alba). Vet Radiol 32:6–16, 1991 Smith SA, Smith BJ: Radiographic evaluation of general avian anatomy. In Smith SA, Smith BJ (eds): Atlas of Avian Radiographic Anatomy. Philadelphia, WB Saunders, 1992, pp 5–18 Stefanacci JD, Hoefer HL: Small mammal radiology. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 358–360 Thrall DE: Introduction to radiographic interpretation. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 20–30 Thrall DE: Bone tumor versus bone infection. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 161–168 Toal RL: Fracture healing and complications. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 142–160 Walsh MT: Radiology. In Harrison GJ, Harrison LR (eds): Clinical Avian Medicine and Surgery. Philadelphia, WB Saunders, 1986, pp 201–233 Address reprint requests to Jamie Williams, MS, DVM Department of Veterinary Clinical Sciences School of Veterinary Medicine Louisiana State University Baton Rouge, LA 70803
1094–9194/02 $15.00 .00
ORTHOPEDIC RADIOGRAPHY IN EXOTIC ANIMAL PRACTICE Jamie Williams, MS, DVM, Diplomate, American College of Veterinary Radiology
The radiographic examination of orthopedic disease is relatively straightforward and follows guidelines for obtaining and evaluating radiographs of axial and appendicular skeleton that already are established for common pet species.1, 3–12, 14–16, 24–28 Orthopedic diseases common in routine pet species (dog and cat) also are found in avian and exotic species; however, avian and exotic species also exhibit rather unique orthopedic diseases and unique responses to routine orthopedic maladies. This article describes some of the common orthopedic insults that are suffered by exotic animals and discusses some notable anatomical differences and exceptions in response to injury when appropriate. Knowledge of normal radiographic anatomy is critical when evaluating patients for orthopedic disease. When faced with the prospect of obtaining and then evaluating radiographs of avian or exotic pet species, the practitioner may be intimidated by a lack of knowledge of normal radiographic anatomy or disease entities of a particular species. As with dogs and cats, however, there should be a bilateral symmetry of skeletal structures. Published references that detail the normal radiographic anatomy of avian and exotic species may be used by the less-experienced practitioner when reviewing the radiographs of birds, snakes, lizards, rabbits, and turtles.3, 13, 16–24
From the American College of Veterinary Radiology, Department of Veterinary Clinical Science, School of Veterinary Medicine, Louisiana State University School of Medicine, Baton Rouge, Louisiana VETERINARY CLINICS OF NORTH AMERICA: EXOTIC ANIMAL PRACTICE VOLUME 5 • NUMBER 1 • JANUARY 2002
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RESTRAINT AND POSITIONING Patient malpositioning during radiographic exposure is the most common reason for nondiagnostic radiographs or misdiagnosis of disease, especially for those practitioners who either are new to exotic diagnostic imaging or see only a few cases per year. Care and attention must be paid to accurate patient positioning so that symmetry is maximized. Manual restraint may be sufficient in many exotic pets, especially for smaller, relatively calm birds, turtles, tortoises, and some lizards. Chemical restraint (sedation or anesthesia), however, is extremely beneficial for larger uncooperative patients, such as large birds or raptors, snakes, ferrets, and even rabbits. Chemical restraint not only improves radiographic quality by reducing patient motion but also allows more precise positioning for the intended views and increases the safety of the patient and technical staff. Sedation may be accomplished via injectable products; anesthesia may be accomplished via injectable products or the tank or mask administration of gas agents (e.g., Isoflurane) (Fig. 1). Patient monitoring (e.g., pulse, respiration, heart rate, body temperature) is essential during sedation or anesthesia. Positioning is improved or aided with the use of cloth tape; the patient can be taped either directly to the radiographic cassette (Fig. 2) or to a positioning device (bird board) made of acrylic (Fig. 3). Because whole-body radiographs often are obtained, making the radiographic exposure at peak inspiration is important, especially in species, such as rabbits and prairie dogs, that have a disproportionately small thorax compared with the abdomen. The vast majority of avian and exotic patients are small in truncal diameter, and, therefore, films are often exposed without the use of a grid. High-detail film and screen combina-
Figure 1. Mask administration of Isoflurane (Abbott Laboratories, North Chicago, IL) to a ferret before radiography. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 2. Avian patient taped directly to radiographic cassette in ventrodorsal position. Tape on the feet allows retraction during exposure. Note the mask administration of Isoflurane to eliminate stress or movement in the patient during exposure. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
tions provide the best radiographic detail but require higher radiographic techniques (kVp and mAs) for proper radiographic exposure. Motion artifact is relatively more common in smaller patients because of their more rapid respiratory and heart rates; therefore, exposure times
Figure 3. A, Acrylic positioning device (bird board). B, Ventrodorsal radiographic image of an egg-bound avian patient using this device. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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should be kept at 1/60 second or less. As with the more common pet species, two orthogonal (90 apart) radiographic views are necessary to evaluate any body structure completely. For the truncal body, this is accomplished with lateral and either ventrodorsal or dorsoventral views. With small mammals (e.g., ferret, rat, hedgehog), orthogonal radiographic views of the appendicular skeleton are obtained in much the same way as for a dog and cat. The lateral view is relatively easy to obtain in avian species; however, the orthogonal view of the wing is more difficult and requires that the patient be held vertically in a headdown position with the wing extended against the radiographic cassette and aligned so that the radiograph photons penetrate the wing from caudal to cranium. Radiographic examination of extremely small structures may be improved with nonscreen film. Dental film is one example of nonscreen film; however, other single emulsion detail radiographic film placed in a light-tight envelope may be used. Radiographic detail is improved greatly with nonscreen film because there is no phosphor bloom to blur the edges of the structures; the film is exposed directly to the radiograph photons rather than to the light from the glowing or activated phosphors of the radiographic screen. Unfortunately, this improved radiographic detail requires higher exposure factors. It is well known that radiographic film is five times more sensitive to room light than radiograph photons; therefore, exposure factors must be raised significantly when using nonscreen radiographic film. Usually, the milliamperage (mA) can be increased greatly while maintaining a short exposure time to avoid motion artifact. Many cage birds are relatively small, as are some of the exotic pet species (e.g., gerbils, hamsters, rats, mice). Magnification radiography can be employed to enlarge the radiographic images; however, there is an inherent loss of edge sharpness. Magnification radiography is accomplished by increasing the distance between the patient and the film, in effect raising the patient off the film cassette and placing it closer to the tube head of the radiography machine. Placing a thick nonradiopaque device (e.g., square sponge) between the patient and the radiographic cassette effectively results in magnification radiography. It is imperative that the routine orthogonal radiographic views be obtained first, with the patient placed directly against the radiographic film cassette. Magnified images can be added to the basic study in the smaller species, if desired, to enlarge the image of the smaller skeletal structures. A better alternative is to purchase an inexpensive magnifying glass, which decreases the number of radiographic exposures because the magnified images are no longer necessary. Additionally, reviewing routine films with a magnifying glass effectively enlarges the radiographic image without the loss of edge sharpness. Turtles and smaller lizards may be placed on a small, rectangular, nonradiopaque support (e.g., a sponge) during radiography. The tube head is extended and rotated horizontally (parallel to the table surface), with the radiographic cassette positioned vertically against the patient to obtain the lateral or craniocaudal radiographic image (Figs. 4 and 5).
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Figure 4. Depiction of horizontal beam radiograph to obtain a craniocaudal image of this tortoise. The tube head is extended and rotated such that the radiograph beam is oriented parallel to the tabletop, the patient (tortoise) is placed upon a non-radiopaque sponge, and the radiographic cassette is held in place vertically behind the patient using sand bags. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
Using this positioning aid often allows a more true projection to be obtained compared with manually holding the patient on its side for vertical overhead tube exposure. Additionally, providing a small-diameter stick to which smaller lizards can cling during the radiographic procedure may be helpful. This may decrease their activity level, allow
Figure 5. Lateral radiographic image of a small lizard using the same equipment orientation as seen in Figure 4, with the patient positioned to obtain a lateral radiographic image. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 6. Lateral radiographic image of a small chameleon obtained with horizontal beam orientation. Note that the chameleon is grasping the small stick used to aid positioning. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission)
them to feel more secure, and provide optimal positioning for horizontalbeam radiography (Fig. 6). UNIQUE SKELETAL FEATURES Avian bones exhibit much thinner cortices than those normally found in mammals.7, 14, 23, 24 It is well known that certain avian species possess pneumatic humeri and femora.7, 8, 23, 28 Extensions of the interclavicular (humerus) and caudal ceolomic (abdominal) air sacs invade the medullary portion of these bones, decreasing their medullary opacity (Fig. 7). When a fracture involves the pneumatic bones, subcutaneous emphysema results, which can make the determination of an open versus closed fracture more difficult. More importantly, because of this free communication, it is possible for osteomyelitis to result from the direct extension of air sacculitis or, conversely, for air sacculitis to result from the extension of osteomyelitis.8, 23 The avian axial skeleton is also rather unique, with three fused vertebral segments that are thought to provide increased stability during flight. From an orthopedic standpoint, the most clinically important of these regions is the synsacrum, which is formed by ankylosis of the last several thoracic vertebrae, all the lumbar and sacral vertebrae, and the first few caudal vertebrae. Reproductively active female birds store calcium salts in the ulna and tibia prior to beginning egg laying which results in significantly increased opacity or hyperostosis of these particular bones (Fig. 8).14, 28 The increased opacity is diffuse, uniform, and seasonal. Avian tuberculosis (Mycobacterium species) also has been reported to result in focal, increased medullary opacity and periosteal proliferation, predominantly in the long bones, as well as lameness.7, 8, 14
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Figure 7. Ventrodorsal radiograph of a large avian patient demonstrating the decreased opacity of the humeri and femurs typically found in avian species possessing pneumatic bones. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
FRACTURES OF THE APPENDICULAR SKELETON Fractures in avian or exotic species are classified in the same manner as that used for routine pet species. Fractures may be open (i.e., the bone protrudes beyond the dermal surface or there is air within the soft tissue of the fracture site) or closed, and complete (i.e., both cortices of bone are involved) or incomplete (i.e., only one cortex is broken [e.g., folding fracture, stress fracture, greenstick fracture]). Fractures are further classified as simple (i.e., one fracture line results in two fracture fragments) or comminuted (i.e., multiple fracture lines result in more than two fracture fragments). Recognizing whether the fracture involves the articular surface of a bone is important, because proper realignment of the fracture fragments and stabilization of the fracture site is imperative. Significant degenerative changes, however, are likely to result. Displacement of the fracture fragments may affect the treatment approach. The distal segment or fragment is always considered in the wrong location when describing fracture displacement. In other words, the location of the distal segment is always described relative to the
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Figure 8. Close-up ventrodorsal view of the pelvic limbs of a female hawk. Note the significant increase in osseous opacity of the tibiotarsal bones indicative of seasonal hyperostosis in reproductively active female birds. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
proximal segment. A cranially displaced fracture has its distal segment cranial to or in front of the proximal segment. Evaluating all fracture segments for the presence of fissure fractures is critical. Fissure fractures are incomplete fractures that involve only one cortex and are found extending from the original fracture site. Evaluation for fissure fractures can be especially trying in avian and exotic animals because their skeletal structures are relatively small and thin, necessitating the use of a magnifying glass and careful scrutiny of the cortical margins adjacent to the fracture site on orthogonal radiographic views. Failure to recognize fissure fractures may result in a previously nondisplaced piece of bone becoming a complete fracture secondary to implant placement. Fractures that involve the immature physis (Salter–Harris fractures) risk asynchronous growth and limb deformity.6 Animals with open fractures are at risk for osteomyelitis (Fig. 9),9 and therefore a delayed or nonunion fracture. All gunshot-induced fractures are considered open; however, the heat involved in this trauma often decreases the likelihood of osteomyelitis. Fractures that involve the pneumatic bones of avian species are at greater risk for osteomyelitis and for extension to the associated air sacs (air sacculitis). Avian have a rather unique healing process in fracture repair: Periosteal proliferation (callus) is reduced greatly compared with that of mammalian species, and healing is predominantly through endosteal callus7, 8, 28 and fibrous tissue proliferation. This is also true in reptiles which demonstrate a similar response to bone healing.16 Based on mammalian criteria for fracture healing, reptiles and other exotic animal species are considered slow healers. In fact, it is not uncommon to visualize the radiolucent fracture line, even though the bone has healed, because of the predomi-
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Figure 9. Lateral radiograph of the fractured right wing of a bird. Note that the distal aspect of the proximal segment of the radial fracture extends beyond the skin surface dorsally (open fracture) and the comminuted fracture of the mid-diaphysis of the ulna. Soft tissue swelling is associated with the fractures. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
nantly fibrous callus. As a result of this phenomenon, radiographic assessment is recommended at 12 to 16 weeks after the injury to assess fracture healing.16 Periosteal reaction is often more prominent in snakes than in lizards or turtles. Entanglement or entrapment is the biggest cause of traumatic fracture in caged birds, most often resulting from inappropriate cage toys, chains, wires, and so forth. Evaluating turtles for fracture is difficult because of their propensity to retract their head, tail, and appendages into their shell. Occasionally, self-exteriorization of a particular limb may suggest trauma to or an abnormality of that limb (Fig. 10A, B, and C). Tape or rolled gauze may be used to extract the limbs gently during positioning for radiography, allowing a more thorough evaluation for potential fracture disease (Fig. 11A, B, and C). This same technique is used to evaluate the joints for effusion or sepsis. Traumatic fractures of the tibia are relatively common in chinchillas, presumably because the tibia is longer than the femur. Chinchillas possess on extremely thin and fragile bone structure, which undoubtedly predisposes them to fracture. Commonly reported causes for these fractures include grasping the animal by its rear limb or it having caught its rear limb in its cage.15 Nonunion fracture may result from these traumatic fractures because applying external fixators to the fracture site is difficult and soft bandages generally do not provide excellent stabilization of these thin and fragile bones.15 FRACTURES OF THE AXIAL SKELETON Fracture, subluxation or luxation of the vertebral column are relatively uncommon in exotic species; however, vertebral fractures sec-
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Figure 10. A, A tortoise with the self-exteriorization of the left rear leg. B, Dorsoventral radiograph shows that the left rear leg remains exteriorized and that there is a fracture of the proximal left femur. C, Close-up radiograph illustrating the normal posture of the right rear limb and the abnormal posture of the left rear limb caused by the proximal femoral fracture. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.) Illustration continued on opposite page
ondary to metabolic bone disease may occur. There are three fused or ankylosed regions of the vertebral column in avian species, which results in rigid sections, that are thought to stabilize the vertebrae during flight. In terms of orthopedic maladies or disease, the most important of these regions is the synsacrum (Fig. 12), which is formed by ankylosis of the last several thoracic vertebrae, all the lumbar and sacral vertebrae, and the first few caudal vertebrae. Birds are known to fly toward uncovered windows, which results in craniocaudal impact when they encounter a window that is closed. The most common site for traumatic vertebral fracture that results from this impact is immediately cranial to the fused synsacrum7, 8; therefore, this particular region must be evaluated thoroughly for fracture or malalignment in all avian patients with suspected vertebral trauma. Fractures that involve the cranium are always difficult, even in routine small animal patients. The difficulty is augmented in exotic species owing to their reduced size, which results in a smaller radiographic image. The normally thin cortical margin of the osseous structures of the skull makes determination of small fractures even more difficult. Numerous air-sac extensions are found within the skull of avian patients, which further complicates the radiographic recognition of fracture.13 Skull fracture is easier to recognize if there is an obvious malalignment or depression in the normal contour. In small exotic species, such as lizards and ferrets, occlusal (intraoral) radiographs provide increased radiographic detail, allowing closer scrutiny of the rostral maxilla and mandible for fracture or lytic change (Fig. 13). Intraoral radiographs are obtained by placing a nonscreen detail film in the mouth
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Figure 10. See legend on opposite page
of the anesthetized animal. Because no intensifying screen is used, mAs must be increased substantially; however, the detail that is obtained is better than that provided by a detail film and detail screen. Masses or soft tissue swelling of the head should be investigated radiographically
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Figure 11. A, Close-up image of a dorsoventral radiograph of a tortoise. Tape applied to the forelimb was used to gently extract the limb from superimposition of the shell during radiographic exposure. B, A closer view of following demonstrates the thin radiopaque linear structures extending from either side, secondary to the tape extraction aid, resulting in a craniocaudal or dorsopalmar view of the forelimb. C, A lateral view of the limb again visualizes the tape surrounding the distal aspect of the forelimb. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 12. Close-up of a lateral radiographic of an avian patient. The white bracket indicates the synsacrum. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
to determine the character of the associated osseous structures (i.e., evaluate for lysis or proliferation of adjacent bony structures) (Fig. 14). The avian pelvis is fused to the synsacrum dorsally but is open ventrally,8, 17–23 to allow passage of large, mineralized eggs. Traumatic
Figure 13. Mandibular occlusal (intraoral) radiograph of a Water Dragon Lizard (small species) with rostral swelling. Note the expansile nature of the rostral right mandible suggestive of osteomyelitis. This particular radiograph was made with nonscreen dental radiographic film. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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Figure 14. Lateral radiographic view of the skull of a rabbit presented for firm swelling of the rostroventral mandibular region. There is no evidence of osseous destruction. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
pelvic fractures may be difficult to recognize in avian patients, especially if axial rotation is present during positioning for the ventrodorsal radiograph. If careful attention is paid to accurate alignment and position, then the practitioner need only compare the right side to the left and evaluate for asymmetry (Fig. 15). Vertebral fractures in exotic patients may result from metabolic bone disease. Traumatic fracture or luxation of the vertebral column is the most common cause of acute-onset posterior paresis in otherwise healthy rabbits. The hindquarters of the rabbit are heavily muscled as a part of its fight-or-flight apparatus. Lack of proper support during patient handling is the most common reason for fracture or luxation of the vertebral column, with the most common site being L7.4 Therefore, it is essential that the hindquarters of the rabbit be supported securely during handling to prevent explosive extension of the rear limbs, potentially resulting in fracture or luxation of the vertebral column. INFECTION, OSTEOMYELITIS, AND SEPSIS Radiographic changes may not be readily apparent in the early stages of infection or osteomyelitis.10, 14 Later in the course of disease, however, soft tissue swelling, bone lysis, and aggressive periosteal proliferation often are found. Local bone destruction is noted by lytic lesions in a focal area, often at or near the site of fracture or penetrating wound (Fig. 16A and B). Bone destruction and soft tissue swelling are
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Figure 15. Close-up ventrodorsal avian radiograph depicting abnormal orientation of the pelvis consistent with fracture of the right ileum. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
recognized more easily in the lower extremities, which normally have reduced soft tissue thickness. Septic arthritis is seen most commonly in the intertarsal joints of caged birds.28 Open fractures, penetrating wounds, or iatrogenic causes may result in osteomyelitis.8 Osteomyelitis also has resulted from hematogenous dissemination or extension from air sacculitis [pneumatic bones], septic arthritis, and pododermatitis.4, 7, 8, 15, 16, 23 Because avian species do not produce much periosteal proliferation during fracture healing, similarly it does not occur extensively with infection either. In fact, lysis is the predominant radiographic osseous abnormality with both infection and neoplasia.8, 14 Increased periosteal proliferation and increased medullary opacity, however, have been noted in cases of fungal osteomyelitis or tuberculosis (Mycobacterium species).8 Osteomyelitis in reptiles also produces a primarily lytic reaction or response, with little periosteal proliferation (Fig. 17A and B).6 Pododermatitis (bumblefoot) is common in guinea pigs, and osteomyelitis is possible in the distal extremities via direct extension.15 Ulcerative pododermatitis occasionally may result in osteomyelitis or septice-
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Figure 16. A, Lateral radiograph of an open gunshot fracture of the ulna of a hawk. The small metallic opacities are indicative of gunshot, which is always considered an open fracture. The significant periosteal reaction associated with the comminuted ulnar fracture, along with the palpable increased temperature of the soft tissues is indicative of osteomyelitis in the avian patient. B, Close-up craniocaudal radiograph of the swollen foreleg (elbow) of an iguana. Note the loss of subchondral bone in the humeral condyle, proximal radius and proximal ulna; the periosteal proliferation extending proximally on the distal humerus; and the significant soft tissue swelling. These radiographic findings are indicative of osteomyelitis and septic arthritis. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
mia via direct extension in rabbits4; however, rabbits are prone to abscessation (from P. multocia or Staphylococcus aureus). These abscesses may affect the joints, long bones, or skull, resulting in osteomyelitis (Fig. 18A and B).4 PATHOLOGIC FRACTURES Pathologic fractures are simply fractures of bone that had a preexisting disease (e.g., neoplasia, osteomyelitis (bacterial or fungal), metabolic bone disease). Bone is a living tissue that is resorbed and redeposited constantly. Natural stresses on existing bones determine where bone salts are deposited; bone is remodeled according to Wolff’s law (i.e., according to the lines of stress). Metabolic bone disease is suggested radiographically by generalized bone changes and osteopenia of multiple bones of axial and appendicular skeleton. Exotic species seem to be more susceptible to nutritional secondary hyperparathyroidism (fibrous osteodystrophy secondary to nutritional imbalance). Animals that suffer from nutritional secondary hyperparathyroidism demonstrate abnor-
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Figure 17. Dorsoventral (A) and lateral (B) close-up radiographs of a snake with a perivertebral abscess and subsequent vertebral osteomyelitis. The perivertebral abscess (confirmed by way of fine needle aspiration) is seen as the soft tissue mass adjacent to the vertebrae, while the osteomyelitis is suggested by the focal demineralization in the vicinity of the soft tissue abscess. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
mally thinned cortices of long bones with decreased overall osseous opacity (medullary and cortical) and also may exhibit pathologic fractures (e.g., folding fractures, incomplete fractures) in these bones (Fig. 19A and B). It is not uncommon to recognize several fractured ribs along with fractures of the appendicular skeleton in animals with subjectively thinned cortices and a loss of normal bone opacity. Metabolic bone disease with secondary pathologic fracture is more common than traumatic fracture in cage birds, according to McMillan.8 Guinea pigs are genetically susceptible to vitamin C deficiency. Radiographic changes noted in scurvy (hypovitaminosis C) include enlargement of the epiphyses of long bones and the costochondral junctions of the ribs. Pathologic fractures also may result.15 Pathologic fractures of the mandible may indicate renal secondary hyperparathyroidism, especially if the cortical opacity of the long bones appears relatively normal or at least not as osteopenic as the mandibular cortices.
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Figure 18. Dorsoplantar (A) and lateral (B) radiographic images of the swollen tarsus (3week duration) of a rabbit. Note the asymmetrical (extra-capsular) soft tissue swelling located at the tarsocrural joint and distal tibia, but lack of significant periosteal reaction. The asymmetrical distribution and irregular margination indicates the swelling is extracapsular rather than within the joint proper; and the lack of significant osseous reaction suggests the lesion is somewhat benign. Fine needle aspiration confirmed abscessation. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
Spinal lymphoma is reported rarely in ferrets but may result in pathologic fracture of the vertebrae.2 JOINTS: LUXATION, SUBLUXATION, AND DEGENERATIVE CHANGES Joint instability is not tolerated by the body; however, the response of bone or joint tissue is rather limited. The body’s natural response to joint instability is to produce more bone in an effort to increase stability, resulting in the periarticular osteophytes seen commonly in degenerative joint disease. Instability often results in increased synovial mass, either via joint effusion or hypertrophy of the soft tissue of the joint. This increased synovial mass stretches and distends the joint capsule, putting stress on the osseous attachments of the joint capsule. Specialized fibers (Sharpey’s fibers) in the joint capsule allow its attachment to bone and form an enthesis (i.e., a point of soft tissue insertion into bone). When stretched, Sharpey’s fibers mineralize in an effort to restore stability and tighten the joint capsule. This mineralization results in enthesophytes, which are seen in degenerative joint disease. Spontaneous cartilage degeneration has been reported in guinea pigs, resulting in degenerative changes and arthritis of the stifle joint.15 Additional radiographic signs
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Figure 19. Ventrodorsal (A) and close-up (B) abdominal radiograph demonstrating the folding fracture of the distal right femur in a young raccoon. The cortices of all bones, especially the long bones, are significantly thinned, suggesting metabolic bone disease (fibrous osteodystrophy). (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
of joint disease commonly evaluated in dogs and cats are more difficult to ascertain in exotic or avian species. Joint effusion may be particularly difficult to evaluate in smaller exotic species such as hamsters, mice, and gerbils simply because the overall size of the joint is so small; the presence of joint effusion is easier to note in rabbits and other larger species. In exotic or avian species, the radiographic evaluation of joints for enthesophytes is difficult owing to the small area of interest. Joint laxity results in either subluxation or luxation, depending on whether any portion of the articulating surfaces is still in contact or not (i.e., luxation is a more severe disruption of normal joint articulation than is subluxation). Luxation and subluxation certainly have been documented in avian and other exotic animal species. Although luxation is deemed uncommon,8 it seems to be most prevalent in the coxofemoral joints, stifles, and digits of birds.8, 14 EXOSKELETON Animals that possess an exoskeleton or a hard external shell (e.g., tortoise, turtle, armadillo) present yet another opportunity for radiogra-
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phy. If external trauma involving the shell is noted, radiographs are indicated to determine the extent of internal damage as well. Turtles and tortoises exhibit an intimate attachment of the vertebral column with the shell; therefore, damage to the shell may result in damage to the vertebral column. Furthermore, depending on the type or force of trauma, internal hemorrhage or lung contusion may result (Fig. 20A, B, and C). SUMMARY Radiographic examination of exotic animal species provides significant diagnostic information to the practitioner so long as attention is paid to positioning and radiographic detail and if variations in normal
Figure 20. Lateral (A) and dorsoventral (B) radiographs of a tortoise presented for fracture of the caudal left aspect of the shell. There is no radiographic evidence of fracture or subluxation involving the vertebral column, the pelvis, or the left femur. The craniocaudal radiograph (C) confirms pulmonary contusion secondary to the initial trauma. Note the increased soft tissue opacity of the lungs in the dorsal aspect of the coelom. (Copyright Jamie Williams, MS, DVM, Baton Rouge, LA; with permission.)
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radiographic anatomy are contemplated. Fracture of pneumatic bones (humerus or femur) in avian patients often results in subcutaneous emphysema, much like an open fracture. If fracture alignment is anatomical, there is generally little callous formation visible in avian and reptile patients; however, if malunion occurs, then certainly callous can be visualized. Aggressive or excessive periosteal proliferation in exotic species often predicts osteomyelitis. Practitioners can increase their diagnostic capabilities and thereby decrease any preconceived anxiety by applying the same general interpretive concepts used in the radiography of more routine small animal patients and by evaluating the resultant radiographs in a systematic manner. With the exception of certain anatomical differences and alterations in response to injury, exotic animal species suffer the same types of orthopedic disease do dogs and cats.
References 1. Allan G: Radiographic signs of joint disease. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 169–188 2. Antinoff N: Musculoskeletal and neurologic diseases. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 126–130 3. Gabrisch K, Grimm F, Isenbu¨gel E, et al: Atlas of Diagnostic Radiology of Exotic Pets: Small Mammals, Birds, Reptiles, and Amphibians. Philadelphia, WB Saunders, 1991 4. Gentz EJ, Carpenter JW: Neurologic and musculoskeletal disease. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 220–226 5. Konde LJ: Aggressive versus nonaggressive bone lesions. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 37–43 6. Konde LJ: Diseases of the immature skeleton. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 131–141 7. Krautwald-Junghanns M-E, Trinkaus K: Imaging Techniques. In Tully TN, Lawton MPC, Dorrestein GM (eds): Avian Medicine. Boston, Butterworth Heinemann, 2000, pp 52–73 8. McMillan MC: Imaging techniques. In Ritchie BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application. Lake Worth, FL, Wingers Publishing, 1994, pp 246–326 9. Morgan JP, Leighton RL: Fracture description. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 8–20 10. Morgan JP, Leighton RL: Radiology of osteomyelitis. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 43–52 11. Morgan JP, Leighton RL: Joint injury. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, pp 67–69 12. Morgan JP, Leighton RL: Pathologic fractures. In Morgan JP, Leighton RL (eds): Radiology of Small Animal Fracture Management. Philadelphia, WB Saunders, 1995, p 70 13. Paulmurphy JR, Koblik PD, Stein G, Pennick DG: Psittacine skull radiography: Anatomy, radiographic technique, and patient application. Vet Radiol 31:218–224, 1990 14. Quesenberry K: Disorders of the musculoskeletal system. In Altman RB, Clubb SL, Dorrestein GM, et al (eds): Avian Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 523–539 15. Schaeffer DO, Donnelly TM: Disease problems of guinea pigs and chinchillas. In
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WILLIAMS
Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 260–279 Silverman S, Janssen DL: Diagnostic Imaging. In Mader DR (ed): Reptile Medicine and Surgery. Philadelphia, WB Saunders, 1996, pp 258–264 Smith BJ, Smith SA, Spaulding KA, et al: The normal xeroradiographic and radiographic anatomy of the cockatiel (Nymphicus hollandis). Vet Radiol 31:226–234, 1990 Smith BJ, Smith SA, Flammer K, et al: The normal xeroradiographic and radiographic anatomy of the orange-winged amazon parrot (Amazona amazonica-amazonica). Vet Radiol 31:114–124, 1990 Smith SA, Smith BJ: Normal xeroradiographic and radiographic anatomy of the redtailed hawk (buteo-jamaicencis), with reference to other diurnal raptors. Vet Radiol 31:301–312, 1990 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the mallard duck (Anas platyrhynchos), with reference to other anserine species. Vet Radiol 32:87–95, 1991 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the bobwhite quail (Colinus virginianus), with reference to other galliform species. Vet Radiol 32:87–95, 1991 Smith BJ, Smith SA: Normal xeroradiographic and radiographic anatomy of the great horned owl (Bubo virginianus), with reference to the barn owl (Tyto alba). Vet Radiol 32:6–16, 1991 Smith SA, Smith BJ: Radiographic evaluation of general avian anatomy. In Smith SA, Smith BJ (eds): Atlas of Avian Radiographic Anatomy. Philadelphia, WB Saunders, 1992, pp 5–18 Stefanacci JD, Hoefer HL: Small mammal radiology. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, pp 358–360 Thrall DE: Introduction to radiographic interpretation. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 20–30 Thrall DE: Bone tumor versus bone infection. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 161–168 Toal RL: Fracture healing and complications. In Thrall DE (ed): Textbook of Veterinary Diagnostic Radiology, ed 3. Philadelphia, WB Saunders, 1998, pp 142–160 Walsh MT: Radiology. In Harrison GJ, Harrison LR (eds): Clinical Avian Medicine and Surgery. Philadelphia, WB Saunders, 1986, pp 201–233 Address reprint requests to Jamie Williams, MS, DVM Department of Veterinary Clinical Sciences School of Veterinary Medicine Louisiana State University Baton Rouge, LA 70803