INTRODUCTION TO SMALL ANIMAL
MUSCULOSKELETAL IMAGING
Musculoskeletal Radiography
• Permit localization and characterization of a lesion
• Size, shape, margination, number, position, opacity
• Normal radiographic anatomy
• Diseases are often bilateral in the appendicular skeleton
• Radiographic terms – use appropriately
Fractures:
Principles of Radiographic Examination
• 2 orthogonal views
• Sedation/ anesthesia
• Include joint above and below – this can help determine if rotation is present
• Immediate post-operative repair radiographs should always be obtained
• When needed, radiograph opposite limb for comparison (esp. in young
animals)
Classification of Fractures
• External communication
– Open (compound) vs. closed
• Extent of damage
– Complete vs. incomplete
• Direction and location of fracture line(s)
– Simple, comminuted, multiple, etc.
• Stability
• Location
– Diaphysis, metaphysis, epiphysis
– Special types => Salter-Harris, pathologic
Open (Compound) vs. Closed
• Communicating wound to external surface open fracture – prognosis is more
guarded if the fracture is open
• May see gas within soft tissues or large soft tissue defects – sometimes the
bone will poke out of the skin and then go back in – so using air as a means to
see if it is open is very useful
• May see causal object (gunshot)
Extent of Damage
• Complete => both cortices fractured
• Incomplete => fracture through one cortex
– Greenstick – often seen in young animals as their bones are somewhat
“bendable”.
Direction and Location of Fracture Line
• Simple => one fracture line
– Transverse – most stabilize the fracture to prevent rotation
– Oblique
– Spiral
• Comminuted
– Multiple fracture lines usually meet at a common point
– One or more small fragments
Pathologic Fractures
• Bone is weakened by pre-existing lesion
– Fracture happens spontaneously
– No history of trauma – usually older animals
– Tumors, infection, hyperparathyroidism
– Lysis may not be obvious
• BIOPSY at the fracture site and do CHEST RADS if a fracture seems to
have occurred for no reason!!!
Salter-Harris Classification of Physeal Fractures
• Classification system developed by human physicians and adopted for animals
• Correlated with prognosis
– Higher number = higher chance of premature physeal closure (worse
prognosis)
• Premature closure
– Bone foreshortening
– Angular limb deformities
Salter-Harris Type I
• Separation of metaphysis from epiphysis
• Occurs through layer of hypertrophied cells
Salter-Harris Type II
• Most common
• Fracture line travels through growth plate for variable distance then extends
into metaphysis
Salter-Harris Type III
• Fracture line travels through growth plate for variable distance then extends
through epiphysis into articular surface
Salter-Harris Type IV
• Metaphyseal fracture line extending through the physis and epiphysis to exit
through the articular cartilage
– Distal humeral condylar fractures
Salter-Harris Type V and VI
• Type V – Compression of growth plate resulting from crushing force
transmitted through physis
• Type VI – eccentric physeal impaction resulting in transphyseal bridging
Radiographic Evaluation of Fracture Healing
• ABCDs of fracture healing
– Alignment of fracture segments
– Bone healing and callus formation
– Cartilage– implants away from joint, articular fxs
– Device– appearance of implants (adjacent lysis, positional change)
– Soft tissues– swelling, emphysema, atrophy
Radiographic Evaluation of Fracture Healing
• The A’s
– Apparatus
– Alignment
– Apposition
– Activity
Fracture Healing Complications
• Absence of callus formation
• Instability/ large fracture gap
• Zone of radiolucency around fixation devices – infection, loosening of screws
• Bending or breaking of fixation devices – if exercise is not restricted the bone
plates can break or screws will break
• Fracture-associated sarcomas
– Esp. femur (mean 5.8 yrs post-fracture)
– Implant induced??
Implant Failure
• Can see catastrophic failure with bending or breakage of implants
• Lucency around implants
– Loosening
– Osteomyelitis
Nonunion
• When all signs of repair have ceased and further healing will not occur without
surgical intervention
• Types
–
–
Hypertrophic-”elephant’s foot”
Atrophic-sharp edges – looks like the end of a pencil
Malunion
• A fracture that has healed in a position that is not anatomic – see this often
with stray animals.
Soft Tissue Abnormalities
• Intra-capsular soft tissues
– Enlargement of soft tissue within the joint
• Stifle, tarsus and carpus easiest to evaluate
– Swelling usually conforms to joint margins
– Can be caused by:
• Effusion
• Soft tissue proliferation
• Tumor
Bone Abnormalities
• Bones response
– Bone production - osteoblast
• Periosteal reaction and sclerosis
• Takes 12-14 days after insult
– Bone loss – osteoclast
• Lysis
• 30-50% bone loss required to be seen on radiographs
Bone Loss
• Determining Aggressiveness
– Zone of transition
– The less distinct the margin  the more aggressive the lesion
Focal Bone Loss
• Geographic Lysis
– Large area of lysis
– Usually less aggressive
– If destroys the cortex  aggressive
• Geographic lysis
– Expansile appearance
– Expansion of the cortex around an enlarging mass  less aggressive
– Note the intact cortex in the picture
Bone Cyst
Focal Bone Loss
• Moth Eaten lysis
– Multiple smaller areas of lysis
– Areas may become confluent
– More aggressive than geographic lysis
Permeative Lysis – is the worst form of bone loss – kinda reminds me of a
sponge with multiple small holes
Primary Bone Tumors
• Radiographic Signs:
– Lesion may be primarily productive, lytic or both
– Lytic or productive lesions usually have an aggressive appearance
– Away from the elbow and toward the knee
• Radiographic Signs:
– Typically mono-ostotic
– Typically located in the metaphysis
– Lesions typically do not cross joints
Fungal Osteomyelitis
• Radiographic Signs:
– Typically lesions are seen in the metaphysis
– Appear similar to primary bone tumor
– Often extensive destruction when a joint is infected (septic arthritis)
– Often is poly-ostotic
• Etiological Agents:
• Blastomyces dermatitidis
– Southern states, mid-west and south-west
• Coccidioides immitis
– Western states
• Histoplasma capsulatum
– Mid-western states
• Cryptococcus neoformans & Aspergillosis
– Throughout the US
Bacterial Osteomyelitis
• Usually secondary to:
– Gunshot wound
– Penetrating wound ( dog or cat bite)
– Previous surgery (implants)
– Open fracture
• May be seen secondary to septicemia in young animals or animals which are
immuno-compromised
• Radiographic Findings
– Early = ST swelling
– May take 10-14 days before periosteal reaction is seen
– Periosteal reaction is typically solid and extends along the shaft of the
diaphysis
Synovial Cell Sarcoma
• Early in the disease there is intra-capsular and/or peri articular swelling
• Swelling then turns to a mass effect
- Common sites are the elbow and stifle
• Later there is bone lysis of multiple bones of the joint
Cruciate Ligament Rupture
• Cranial displacement of the infra-patellar fat pad
• Caudal displacement of the fascial stripe
• DJD
– Base and apex of the patella
– Proximal aspect of the trochlear ridge
– Medial and lateral aspects of the distal femur and proximal tibia
– Fabellae
Developmental MS Diseases
• OCD
– Shoulder
– Elbow
– Stifle
– Tarsus
• Retained Cartilage Core
• Fragmented Medial Coronoid Process
• Ununited Anconeal Process
• Panosteitis
• Hypertrophic Osteodystrophy
• Hip Dysplasia
• Legg-Calve-Perthes
Osteochondrosis
• Subchondral defect – flattening
• Surrounding sclerosis as time progresses
• Joint mice
• Secondary DJD
• Locations: shoulder, elbow, stifle, tarsus
Shoulder OCD
• Subchondral defect on the caudal aspect of the humeral head
•
•
•
•
May see a joint mouse
May just be flattened
Secondary DJD
May need arthrogram or explore
Elbow OCD
• Subchondral defect present on the distal medial aspect of the humerus
(humeral condyle)
• Surrounding sclerosis
• large osteophytes on the anconeal process – this is often times one of the
earliest changes seen with DJD in the elbow
Stifle OCD
• Subchondral defect on the distal aspect of the lateral femoral condyle
• Mineralized flap rarely seen
• Joint effusion common
• DJD develops
• Do not confuse the extensor fossa for OCD
Tarsus OCD
• Rotts!
• Medial trochlear ridge of the talus
• Often see small mouse
• Joint effusion
• DJD
• See best on oblique view or flexed lateral
Ununited Anconeal Process
• Forms from a separate center of ossification
• Should fuse in all dogs by 6 months
• Lucent line – best seen on flexed lateral
Panosteitis
• Multiple leg involvement is likely – Shepard’s are over represented
• Shifting leg lameness – disease will regress as the dog reaches maturity
except in Shepard’s
Hypertrophic Osteodystrophy
• Radiographs:
– A thin band of radiolucency in the metaphyseal portion of the bone
– Double physis
– Cheeseburger sign
– Sclerosis seen adjacent to lucency
Hip Dysplasia
• Extended VD view
• Used for OFA
• Legs pulled down and rotated inward
• Must include the entire pelvis and stifles
Normal Anatomy - Coverage
• There should be at least 50% coverage of the femoral head by the dorsal
acetabular rim