Development

Bone develops in one of two ways: by (1) endochondral ossification or (2) intramembranous ossification. In endochondral ossification, bone develops on a preformed cartilaginous matrix. The long bones increase in length this way. Intramembranous ossification takes place in bands of connective tissue without any cartilaginous framework. Flat bones, such as those of the skull, form in this way. The increase in the diameter of long bones is by intramembranous ossification, which is initiated by the deeper layers of the periosteum.

Long bones have three main centers of ossification: one for the diaphysis (shaft) and one for each epiphysis (end). The cartilaginous matrices are elaborated at the growth plates and at the articular cartilages. Apophyses are accessory centers of ossification that do not contribute to the growth in the length of a bone. They are sites of attachment for muscles and ligaments. An example is the greater trochanter of the femur. Cartilage is radiolucent, and the first radiographic sign of bone formation in a long bone is the appearance of a collar of mineralized matrix around the cartilaginous shaft. Later other ossification centers appear.

Short bones, which develop by endochondral ossification, are found in the carpus and tarsus. Flat bones, which develop by intramembranous ossification, are found in the skull and pelvis. Irregularly shaped bones are found in the skull, vertebral column, and pelvis.

Sesamoid bones form in tendons where the direction of a tendon changes or where friction may develop. They have articular surfaces that are opposed to a long bone. The patella is a sesamoid bone.

The term fabella describes a small ossification in the medial and lateral heads of the gastrocnemius muscle. Fabellae may be bipartite. There is a fabella or sesamoid bone in the popliteus muscle at the caudolateral aspect of the proximal tibia. Two sesamoid bones are present proximal to the palmar (plantar) aspect of each metacarpophalangeal and metatarsophalangeal articulation. They are numbered 1 to 8, from the medial to the lateral side. The second and seventh are often bipartite with smooth, well-defined edges. This developmental anomaly, often seen in Greyhounds, should not be mistaken for fracture. In Rottweilers, fragmented sesamoids are occasionally seen in the second and fifth digits. The fragments are irregular in outline but are often of no clinical significance (Figure 4-1, C). A single sesamoid lies at the dorsal aspect of each metacarpophalangeal and metatarsophalangeal joint.

Figure 4-1 A, A marked trabecular pattern can be seen toward each end of the bone. B, A radiolucent, oblique linear shadow in the lateral tibial cortex is a nutrient foramen and should not be mistaken for a fracture. C, Bipartite sesamoids (arrows). The second and seventh sesamoids are most commonly affected by this anomaly. D, Craniocaudal view of the elbow joint showing the supinator sesamoid (arrow) on the lateral aspect of the proximal radius. This sesamoid bone is seen occasionally and is a normal anatomical variant within the supinator muscle. It should not be mistaken for a fracture fragment. E, This cat has one sesamoid (short arrow) in a normal location on the lateral aspect of the distal femur. The medial sesamoid (long arrow) is very small and lies at the medial aspect of the stifle joint.

There is sometimes a small sesamoid bone on the craniolateral aspect of the proximal radius. It lies in the supinator muscle but has also been reported to lie in the lateral collateral ligament, the ulnaris lateralis, and the annular ligament (Figure 4-1, D). In the carpus a small sesamoid lies on the distomedial aspect of the radial carpal bone proximal to the first metacarpal bone. It lies in the abductor pollicis longus muscle.

Anatomic variation in the number and location of the fabellae (which are the small sesamoids in the head of the gastrocnemius muscle) at the caudal aspect of the stifle joint can occur in dogs and cats. It is often absent or occasionally displaced distally, particularly in smaller dog breeds (see Figure 4-12, I). Fracture or displacement of sesamoid bones is sometimes seen as a result of trauma (Figure 4-1, E).

Dewclaws, or first digits, normally have a metatarsal (metacarpal) bone and two phalanges. They are always present on the forelimbs, less often on the hindlimbs. They may or may not have associated sesamoid bones. The forelimb dewclaws articulate with the carpus, whereas the hindlimbs often have a soft tissue attachment. Some breeds such as the Pyrenean mountain dog have double dewclaws on the hindlimbs as a breed feature. This may also occur in cats.

Structure

During development, each long bone consists of a shaft (diaphysis), two metaphyses, and two extremities (epiphyses). The diaphysis is composed of dense, compact bone. This dense bone surrounds the medullary cavity, which contains the bone marrow. The epiphyses are centers of growth at either end of the diaphysis. Between the epiphysis and the diaphysis is the physis, or growth plate, and the metaphysis, an area of spongy bone between the physis and the diaphysis. The physis is sometimes referred to as the physeal plate or growth plate in relation to radiographs. When a bone matures, the epiphysis fuses with the metaphysis, and the physis disappears.

Immature bone, also called woven bone, does not have a lamellar structure. It is present only in early life or where new bone is being formed rapidly, such as in a healing fracture. Mature bone has a lamellar structure. Two types of bone can be recognized radiographically: compact bone is dense and radiopaque and is seen in the cortices of bones; cancellous, or spongy, bone is less dense and is seen in the metaphyses and epiphyses. Cancellous bone shows varying degrees of trabeculation. Because compact bone is radiopaque, it shows no structure. It surrounds the medullary cavity, which is less opaque.

Living bone is constantly undergoing remodeling. The bone-forming cells are the osteoblasts. They produce the matrix, often called osteoid, in which mineralization occurs. They elaborate alkaline phosphatase, an indicator of osteoblastic activity. Osteoclasts are responsible for bone resorption. Osteocytes are osteoblasts that have become surrounded by mineralized osteoid. They are found in lacunae within the bone, and they help maintain the calcified matrix. The normal functioning of bone therefore depends on the maintenance of a balance between the activities of these various cells.

The periosteum is a connective tissue layer that covers bone except at the articular surfaces. These surfaces are covered by articular cartilage. The periosteum has an outer fibrous layer that serves for muscle and ligament attachments and an inner, or cambium, layer capable of elaborating osteoblasts. The osteoblasts lay down new bone as the bone grows in width (intramembranous ossification); they also play a part in repair processes. The endosteum is a membrane lining the medullary cavity. It is composed of osteoblasts and osteoclasts. Both periosteum and endosteum elaborate the cells necessary for bone repair.

Blood vessels enter a long bone through the nutrient foramen (canal). The nutrient foramen appears as a radiolucent, sharply defined line in the caudal aspect of the cortex. It is directed obliquely cranially and distally. It is seen in the middle to proximal third of the bone diaphysis. There may be some endosteal irregularity in its vicinity. The nutrient foramen should not be mistaken for a fracture line. There is usually one in each bone (see Figure 4-1, A and B).

Radiography

At least two views, taken at right angles to one another, are required for proper evaluation of the status of a bone. It is important that studies of the skeleton be made in standard positions. The standard views for limb bones are craniocaudal (dorsopalmar, dorsoplantar) and mediolateral. Oblique, flexed, weight-bearing, and stressed studies are often useful. Stressed studies are studies made when lateral or medial leverage is applied distal to a joint under examination. They are useful in the evaluation of joint instability. The reader is referred to textbooks of radiographic technique for details of proper positioning.

Bone scintigraphy can be used to identify lesions not seen on conventional radiographs, such as subtle fractures, inflammatory foci, or metastases.

Normal Appearance

In normal bone, the diaphysis is seen as a band of compact, opaque bone surrounding the medullary cavity, which is more radiolucent. The epiphysis and metaphysis show trabeculations associated with cancellous bone. The trabeculations fade out at the diaphysis. In young animals, the physes (growth plates) appear as radiolucent bands or lines separating the epiphyses from the metaphyses (Figure 4-2). When growth ceases, the epiphyses fuse with the metaphyses and the physes are no longer seen. For some time a band of increased opacity is seen at the junction of the epiphysis and metaphysis, representing the closed physis. This band is sometimes referred to as an epiphyseal or physeal scar (Figure 4-3, A).

Figure 4-2 A normal immature shoulder joint.

Figure 4-3 A, A physeal scar (arrow) at the distal extremity of a radius. Trabeculations are seen in the spongy bone of the metaphysis and epiphysis. B, A 9-month-old Saint Bernard puppy was severely stunted in growth. A lateral radiograph of the right forelimb shows open growth plates, which are abnormal for an animal of this age. Transverse, radiopaque, linear opacities are seen in the metaphyses of the radius and proximal humerus. These are growth arrest lines. C, This Jack Russell Terrier shows radiolucent, well-marginated defects within the cortices of the radius and ulna along the attachment of the interosseus ligament (arrows). They are usually incidental findings.

It is important to know the positions of the various centers of ossification in the young animal and the times at which the physes close. Subsidiary centers of ossification may be mistaken for abnormalities. Young animals appear to have very wide joint spaces because the cartilaginous models on which the epiphyses and the small bones of the carpus and tarsus are developing are radiolucent. The physes are wide. Growth is completed in dogs by approximately 10 to 14 months of age. However, considerable variations may occur in the times of physeal closure, even in animals of the same breed. In the long bones, the proximal humeral epiphysis is the last to mineralize. The pelvic symphysis may not fuse for several years. The physes of the cat, particularly in the neutered cat, tend to close somewhat later than those of the dog (Table 4-1). Variations occur in the appearance of bones in some breeds, such as in chondrodystrophic animals. Variations such as the irregular outline between the radius and ulna in small-breed dogs are usually of no clinical significance and are considered normal (Figure 4-3, C).

Table 4-1. Age at Appearance of Ossification Centers and of Bony Fusion in the Immature Canine

∗ Digit numbers.

Response of Bone to Injury or Disease

Bone may respond to injury or disease in a number of ways. Disease or trauma may cause any or all of the following described changes. In many instances, several reactive processes will be visible at the site of a lesion.

Periosteal Reaction

The periosteum may react to irritation by the production of new bone. The type of periosteal reaction is often indicative of the severity of the lesion provoking it. In its earliest form it appears as a fine irregular reaction giving the bone a blurred or indistinct margin at the site of the lesion. Several kinds of periosteal reaction can be identified.

Smooth and solid: This is a response to continuous low-grade trauma, subperiosteal hematoma, or remodeling of bone. It represents a chronic process. It may be seen as the final, resolved stage of other types of periosteal new bone formation.

Lamellar or “onion skin”: This is the result of repeated episodes of periosteal irritation. Sheets of new bone are laid down approximately parallel to the cortex. It is often seen in metaphyseal osteopathy (hypertrophic osteodystrophy) in young dogs.

Palisade reaction: New bone is formed extending in columns outward at right angles from the cortex. The new bone forms a solid continuum. This type of reaction is seen with hypertrophic osteopathy and sometimes osteomyelitis.

Spicular: Thin spicules of new bone are formed radiating outward from a lesion in the bone. This type of reaction is indicative of an aggressive process and is often seen with malignant bone tumors. The term “sunburst” is sometimes given to an exuberant type of this reaction.

Amorphous: This is a random or haphazard deposition of new bone in the soft tissues adjacent to a bone lesion. It is usually associated with malignancy.

Codman’s triangle: This term is used to describe a condition in which the periosteum is elevated and a triangle of new bone forms at the margin of a lesion beneath the elevated periosteum. It may be associated with malignancy or with a benign process such as osteomyelitis.

A reactive periostitis can usually be seen on radiographs 7 to 10 days after a bone has been injured. In young dogs it may appear a little earlier than in older animals. The aging of bone lesions on radiographic evidence alone is not an exact science. In general, periosteal reactions that are interrupted or that invade soft tissues suggest aggression, whereas smooth, solid, and organized reactions are likely to accompany benign lesions.

The term “aggressive” is often used to describe processes in bone that appear to be very destructive and are not being contained by an inflammatory or defensive reaction. The principal signs of aggression are a proliferative periosteal response that is interrupted in nature, a rapid destruction of bone, poor margination of the lesion, a disorganized reaction, and invasion of the surrounding soft tissues. With aggressive lesions there is a poorly defined transitional zone between affected and unaffected bone. Aggression may be associated with malignancy or with osteomyelitis (Figure 4-4).

Figure 4-4 Four kinds of periosteal reaction. The tumor elevates the periosteum, resulting in subperiosteal new bone formation, often a feature of malignancy but also seen in some cases of osteomyelitis. This palisade-like periosteal reaction is seen in hypertrophic osteopathy and sometimes in osteomyelitis. A “sunburst” type of periosteal reaction is sometimes seen associated with osteosarcoma or chondrosarcoma. A linear (lamellar or laminated) type of periosteal reaction may be seen with callus formation, or it may result from metabolic disease or irritation of the bone from a variety of causes.

Change in Size or Contour

Bones may be of an abnormal size or have an abnormal contour as a result of disease or trauma, particularly during the growth phase. Premature closure of a physis will cause a bone to be shorter than normal. The shortened bone may affect the growth and shape of bones in its immediate neighborhood and result in joint deformity. Bones may have an abnormal size or shape after fracture healing when reduction or immobilization has not been adequate. In the vertebrae, compression fractures result in foreshortening. Bone may remain thickened at the site of a healed fracture (Figure 4-5, A). The term valgus indicates an angulation away from the midline of the body and the term varus, an angulation toward the midline.

Figure 4-5 A, Thickening of the cortex (arrows) at the site of a healed fracture in a young dog. B, Mineralization (arrow) within the stifle joint of a cat is an incidental finding in this case. C, This cat has a calcified meniscal cartilage (arrow). There is remodeling on the proximal and distal poles of the patella and on the cranioproximal aspect of the tibia consistent with osteoarthritis.

Anatomy

A synovial (diarthrodial) joint consists of two apposing bone surfaces, each covered by articular cartilage and surrounded by a joint capsule. The term enarthrodial is used to describe a ball-and-socket joint, allowing movement in all directions. The inner layer of the joint capsule, or synovial membrane, secretes fluid, a thin layer of which separates the opposing articular cartilages. Some synovial joints contain intraarticular ligaments, menisci, fat pads, or synovial projections. Other types of joints are synarthrodial, meaning a fibrous joint, and amphiarthrodial, meaning a cartilaginous joint.

Radiography

Radiography of the joints of very young animals is often unrewarding because of the large amount of radiolucent tissue present. Even severe abnormalities may not be demonstrated.

Arthrography—injection of contrast medium into a joint—is not widely practiced in veterinary radiology. Its use has been confined mainly to the shoulder joint to demonstrate defects in the articular cartilage or cartilage flaps in osteochondrosis. It can also be used to demonstrate defects in the joint capsule and abnormalities in the bicipital tendon sheath. A positive contrast medium is preferred. A nonionic, low-osmolar, iodine-based contrast material is used. Iohexol or iopamidol may be used in a concentration of 100 mg/mL. Synovial fluid is first aspirated from the joint, and contrast medium is then injected. The contrast should be diluted to 50% with sterile saline. From 2 to 9 mL is used, depending on the size of the animal. To enter the shoulder joint, a 20- to 22-gauge short bevel or spinal needle is inserted approximately 1 cm below and lateral to the acromion process. The needle is directed distally, medially, and caudally. The joint is manipulated to disperse the contrast material. Radiographs should be taken after 5 minutes. General anesthesia is required, and aseptic procedures are mandatory (see Figure 4-15, A).

Computed tomography, if available, gives a cross-sectional representation of a joint and enables small fragments or lesions to be identified and localized (see Figure 4-15, H and I).

Normal Appearance

The articular cartilages, synovial fluid, and joint capsule are not visible on radiographs. The subchondral bone (i.e., the bone just beneath the articular cartilage) is visible and merges smoothly with the cortex of the metaphysis in mature animals. The infrapatellar fat pad in the stifle joint can be seen on a lateral view as a triangular radiolucency lying caudal to the patellar ligament. The patellar ligament is seen as a soft tissue band attaching to the tibial tuberosity forming the cranial boundary of the fat pad. The fat pad provides contrast so that joint effusion or capsular thickening may be identified. In cats a small triangular mineralized opacity is often seen between the distal femur and proximal tibia (see Figure 4-5, B). The joints in young animals appear to be much wider than those in adults. This is because the immature, largely cartilaginous epiphyses and small cuboidal bones are not fully seen. In very young animals the incompletely ossified epiphysis often has an inhomogeneous mineralization and a stippled, ragged, and irregular margin particularly obvious in large breeds. It should not be mistaken for an abnormality.

Fascial planes are identified as linear radiolucencies because of fat interposed between muscle masses. These planes have characteristic contours around various joints.

Fat in the fascial planes adjacent to the caudal pouch of the shoulder joint may be displaced if there is moderate or severe joint swelling. The fascial planes at the caudal aspect of the stifle joint are oriented in a proximal to distal direction from the fabellae to the caudal aspect of the tibial plateau. They are displaced caudally by swelling of the joint.

Abnormalities

Congenital, developmental, metabolic, and various other conditions can affect bones and joints.

Luxations

Luxations usually present no problems of diagnosis, provided adequate radiographic studies are available. At least two standard views, made at right angles to one another, are necessary for proper evaluation of the degree and direction of the displacement. Luxations may easily be missed if only one view is relied on. A careful search should be made to detect small fracture fragments associated with a luxation because they may interfere with attempts at reduction. Sometimes small avulsion fractures are associated with collateral ligament damage. Intracapsular swelling will displace adjacent fascial planes. Luxations, as a rule, reduce the normal range of movement of a joint.

Radiologic Signs

1. The articular surfaces are displaced and do not articulate properly with one another.

2. There is disruption of adjacent fascial planes.

3. There may be associated avulsion fractures.

4. In the case of the stifle joint, there will be a disruption of the normal intraarticular fat pad.

In young animals, anatomic abnormalities of joint surfaces suggest that a luxation may be congenital. Comparison with the opposite limb, if normal, is advisable. Anatomic abnormalities of joint surfaces may also be associated with chronic luxations.

Subluxations (partial dislocations) are more difficult to evaluate than frank luxations. Stressed views or studies made with the animal bearing weight on the affected limb, if that is possible, may show a subluxation not seen on a conventional radiograph.

Figures 4-6 through 4-12 show a selection of normal and abnormal joints.

Figure 4-6 The shoulder joint. A, Mediolateral view of a normal shoulder joint. B, Line diagram of the shoulder joint. C and D, Caudocranial views of a normal shoulder joint. The shoulder joint. E and F, Lateral and caudocranial views of the shoulder of a cat. A rudimentary clavicle is present (arrows). G, Separation of the proximal humeral epiphysis in a cat. The shoulder joint. H and I, Luxation of the shoulder joint. The humerus is luxated medially, proximal to the glenoid. The displacement can easily be missed on a lateral view.

Figure 4-7 A and B, Mediolateral view of the elbow joint. C and D, Craniocaudal view of the elbow joint. E, Flexed mediolateral view of the elbow joint of an immature dog. Note the growth plate associated with the medial epicondyle of the humerus (arrow). F and G, Mediolateral and craniocaudal views of the elbow of a cat. H and I, Craniocaudal and mediolateral views of the elbow of a dog. There is a subluxation of the elbow joint. The displacement is not visible on the lateral view. J and K, Mediolateral and craniocaudal views of the radius and ulna of a dog. J, There is type II Salter-Harris fracture of the proximal radius (short arrow) with slight cranial displacement of the metaphysis in relation to the epiphysis (long arrow). K, On the craniocaudal view two fissure fracture lines (arrow) are evident at the proximal third of the radial diaphysis. The degree of displacement of the fracture is made clear on the mediolateral view. This case illustrates the importance of at least two views to evaluate a fracture. L, Mediolateral view of the elbow showing a dislocation of the humeroradial joint with an associated fracture of the ulna (Monteggia’s fracture). M and N, Craniocaudal and mediolateral views of the elbow showing luxation of the elbow joint without an associated fracture. This may easily be missed on a lateral view and demonstrates the importance of two views. O, Mediolateral view of a normal puppy elbow. P, An 8-week-old Jack Russell Terrier presented with a history of lameness of 2 weeks’ duration. The radius and ulna are rotated 90 degrees in a medial direction in relation to the distal humerus and displaced laterally on this craniocaudal view. Diagnosis: congenital elbow luxation.

Figure 4-8 A and B, Mediolateral view of the normal carpus of a dog. C and D, Dorsopalmar view of the normal carpus (antebrachiocarpal joint). E and F, Mediolateral and dorsopalmar views of the normal carpus of a cat. G, Dorsopalmar view showing a luxation of the radiocarpal joint. H, Mediolateral view of the antebrachiocarpal joint showing severe damage caused by a high-velocity bullet.

Figure 4-9 A and B, Mediolateral view of the metacarpus. C and D, Dorsopalmar view of the metacarpophalangeal region (manus). E, Multiple fractures of the metacarpal bones in a cat are shown on this dorsopalmar view.

Figure 4-10 A, A normal ventrodorsal view of the pelvis of a dog. Note how the line of the inner aspect of the shaft of the ilium blends smoothly with the line of the sacrum (arrows). The sacroiliac joint may disrupt the continuity of the line but not its direction. B, Ventrodorsal pelvis of an immature dog. Note the wide joint spaces and the several physes. Cartilaginous ossification centers at the caudal aspect of the ischium and the cranial aspect of the ilium may not fuse with the pelvis until the dog is several years old. Closure of the pelvic symphysis may be likewise delayed. C, Normal ventrodorsal view of the pelvis of a cat. The pelvis is slightly rotated. D, Bilateral sacroiliac subluxations. The lines of continuity between the ilia and the sacrum have been disrupted. The right femur is fractured. E and F, Luxation of the right hip joint of a dog. The lateral view is necessary to determine the direction and degree of displacement in the vertical plane, which in this case is dorsal and cranial. G, Severe trauma to the pelvis of a cat. There are fractures of the pubis and ischium on the right side, and the right sacroiliac joint is subluxated. The right hemipelvis is displaced cranially when compared with the left side.

Figure 4-11 A and B, Mediolateral view of a normal stifle. Arrows show normal fascial planes. C and D, Craniocaudal view of a normal stifle. E, Skyline (flexed cranioproximal-craniodistal oblique) view of the normal patella and trochlear groove of the femur. F and G, Mediolateral and craniocaudal views of the normal stifle (femorotibial) joint of a cat. H, Subluxation of the stifle joint caused by rupture of the cranial cruciate ligament. This injury does not always produce a visible radiographic displacement. I, This was a 2-year-old Rottweiler with a cranial cruciate ligament injury. An intraarticular soft tissue or fluid opacity (long arrow) is seen within the femorotibial joint space displacing the fat pad (short arrow) cranially. This may occur with any intraarticular lesion. J and K, Cruciate ligament rupture. Lateral and craniocaudal views of the stifle. J, There is a mineralized opacity visible in the caudal part of the joint. K, The opacity is superimposed on the joint space just medial to the midline. There is remodeling of the proximomedial tibia. This opacity is resulted from an avulsion of the attachment of the cruciate ligament to the bone. L, Luxation of the patella. The displaced patella can be seen on the medial aspect of the stifle (arrow). The stifle joint is deformed. The alignment of the femur to the tibia is abnormal. The tibial crest is rotated medially. There is an abnormal contour to the proximal tibial diaphysis, which is bowed medially. M, A skyline (tangential) view of a displaced patella.

Figure 4-12 A and B, Mediolateral view of the normal tarsus of a dog. C and D, Dorsoplantar view of the normal tarsus of a dog. E and F, Mediolateral and dorsoplantar views of the normal tarsus of a cat. G, Luxation of the talocrural joint. The tarsus is luxated lateral to the tibia. H, Avulsion fracture of the medial malleolus of the tibia. There is disruption of the skin on the medial aspect of the distal tibia. I, This puppy had been in a road traffic accident. The medial fabella (arrow) is displaced and lies at the distomedial aspect of the femur. The medial fabella on the opposite hindleg was in a similar position. This is a transverse fracture of the proximal tibia. Diagnosis: incidental finding—fabellar dsiplacement.

Luxation of the Carpus

Luxation of the carpus may occur as a result of trauma. Oblique studies are required to identify small fracture fragments associated with luxation. Carpal subluxation or luxation has been described as a syndrome in Shetland Sheepdogs and Collies. It is chronic in onset, and signs of degenerative joint disease are common (Figure 4-13, L to O).

Figure 4-13 Joint disease. A, Mediolateral stifle showing degenerative changes in the stifle joint of a dog. There is intraarticular calcification distal to the patella, and new bone formation around the fabellae and on the proximocaudal aspect of the tibia. There is sclerosis of the subchondral bone of the tibia. B and C, Osteoarthrosis of the carpal joints. There is subchondral bone sclerosis, obliteration of joint spaces, and periarticular new bone formation. There are areas of decreased opacity in the subchondral bone of the radius. A soft tissue swelling is also visible. D, This Shetland Sheepdog presented with soft tissue swellings around both antebrachiocarpal joints. Multiple subchondral radiolucent erosive lesions are seen along the margins of the carpal bones. The normal articular margins are disrupted. New bone formation is present on the distomedial aspect of the radius. A soft tissue swelling is visible. Osteoarthritic changes are seen affecting some metacarpophalangeal articulations. Diagnosis: erosive osteoarthritis. Joint disease. E and F, Mediolateral views of the elbow and stifle joints in a cat showing chondromatosis (osteochondromatosis). This condition may or may not be associated with arthritis. Parts of the synovial membrane become cartilaginous, and later they may become mineralized. The etiology is unknown. This cat had a stilted gait for approximately a year but was not disabled. Similar changes may be seen with hypervitaminosis A. Joint disease. G to I, Erosive arthritis. G and H, There is destruction of the medial radial head, subchondral bone erosion of the articular margin of the distomedial humerus, and a narrowed joint space laterally. This is severe, chronic, degenerative osteoarthritis. I, A study of the same limb made 3 months earlier shows the rapid progression of the lesion.Joint disease. J and K, Mediolateral and craniocaudal views of the elbow showing osteophyte formation on the proximal radius, proximolateral ulna, and anconeal process. This is osteoarthritis. L to O, This is a 10-year-old Rough Collie that was lethargic. There is severe bilateral carpometacarpal osteoarthritis.

Infectious (Septic) Arthritis

Infectious arthritis may result from wounds or spread of an infectious process from neighboring structures. It may also result from invasion of the joint by blood-borne agents. Clinically, severe lameness, distention of the joint capsule, heat, and pain on palpation occur. Joint movements are limited. Arthrocentesis is the diagnostic method of choice.

Radiologic Signs

1. There may be little radiologic evidence of infectious arthritis in early cases.

2. Distention of the joint capsule may displace adjacent fascial planes. Normal periarticular radiolucencies associated with periarticular fat in fascial planes are lost because of inflammatory exudate within the joint and edema around the joint.

3. In the stifle joint, the infrapatellar fat pad becomes obscured or lost.

4. Periosteal reaction occurs on the bones surrounding the joint, usually at the attachments of the joint capsule.

5. Spread of the infectious process will produce changes in adjacent bones.

6. There may be subchondral and perichondral bone destruction once the disease process destroys articular cartilage and spreads to the subchondral bone.

7. In longstanding cases there will be changes of secondary degenerative joint disease. The joint space becomes narrower (Figures 4-13 and 4-14, A and B).

Figure 4-14 A and B, Gross destruction of this carpal joint in a cat is the result of septic arthritis. The metacarpal bones are luxated, and the normal carpal bone alignment is disrupted. There is a large associated soft tissue swelling. C, This Collie has a slight soft tissue swelling around the calcaneus. The close-up view shows a smooth mineralization (arrow) within the gastrocnemius (Achilles) tendon. New bone formation is also seen on the plantarodistal aspect of the calcaneus and dorsal aspect of the tarsus. Diagnosis—calcifying tendinopathy.

Developmental Anomalies

Osteochondrosis

Osteochondrosis is an abnormality in endochondral ossification. Articular cartilage becomes thickened in the affected area, and chondrocytes in the deeper layers die. The surrounding cartilaginous matrix then fails to ossify. Fissures appear in the articular cartilage. In some cases normal endochondral ossification is reestablished, and the lesion regresses. More frequently, an area of devitalized thickened cartilage develops and overlies a defect in the subchondral bone, the defect being an area in which normal endochondral ossification has failed to occur. The area of dead cartilage may remain attached at the junction of cartilage and subchondral bone, or it may become separated, forming a flap or a free fragment. The condition is then termed osteochondritis dissecans. Cartilaginous flaps may become mineralized and are visible on radiographs as a thin curvilinear mineral opaque structure. Sometimes part of the flap becomes detached and floats free in the affected joint. Such a loose body may grow larger after it has become detached. When such bodies lie loose within the joint or adhere to the synovium, they are often called joint mice.

Osteochondrosis occurs in larger dog breeds, usually between 4 and 9 months of age. Affected animals show lameness. Manipulation of an affected joint is resented. A definitive diagnosis is usually made radiographically. The most common site in the dog is the caudal third of the humeral head, but the trochlea of the humeral condyle, the femoral condyles, the medial and lateral malleoli of the tibia, the medial and lateral trochlear ridges of the talus, and the cranial endplate of the sacrum (see Chapter 5, p. 521) may also be affected. Ununited anconeal process and fragmented medial coronoid process of the ulna are believed to be manifestations of osteochondrosis. The condition is frequently bilateral.

If osteochondrosis occurs at an apophysis, it may result in apophyseal separation such as has been reported in the tibial tuberosity.

Computed tomography is particularly useful in the location and identification of small osteochondral fragments in joints, particularly the elbow and tarsus (Figure 4-15, H and I).

Figure 4-15 A, Positive contrast arthrogram on a mediolateral study of the shoulder joint of a dog. The articular cartilages are clearly outlined as radiolucent lines between the contrast material and the subchondral bone. Contrast material is seen in the tendon sheath of the biceps brachii muscle cranially (open arrows

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