Pediatric knee radiographs are commonly encountered in the emergency department and vary from adult knee radiographs. Younger knees have open growth plates, ossification center development and display unique injury patterns. Growth plates are areas of weakness, susceptible to fracture and injuries can result in development deformity in leg length and alignment.
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Epidemiology
Pediatric patients have strong ligaments and often sustain avulsion injuries as appose to ligament ruptures. Extra-articular soft tissue injuries account for 82% knee injuries in children, patella malalignment or dislocation 9%, ACL avulsions/collateral avulsion/meniscus tear 4%, fractures 1% and other pathology 4% 1.
Systematic review
Ossification
Check the ossification centers are present.
distal femur: 3-6 months
patella: 3-5 years
tibial plateau: present at birth
tibial tuberosity:10-12 years
fibular head: 2-4 years
Alignment
Check the tibio-femoral alignment:
draw a line down the margin of the lateral femoral condyle
tibial should line up within 0.5 cm
if it does not, consider a tibial plateau fracture
Check patella height using the Insall-Salvati ratio:
look at lateral knee x-ray with knee 30 degrees flexed
measure the patella tendon length and divide it by the patella length
normal ratio is 0.8-1.2
if it is >1.2 consider a patella tendon rupture
alternatives: Blackburne-Peel ratio or Caton-Deschamps index
Effusion
There are two fat pads in the knee (reliably seen on the lateral view):
suprapatella fat pad
prefemoral fat pad
Soft tissue density between the two fat pads indicates an effusion.
Bone cortex
Check around the cortex of every bone on the film:
fibular head, tibia, femur and patella
Acute knee pathology
Tibial eminence fracture
14% of all ACL injuries
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peak age
8-17 years
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mechanism
knee hyperextension, sporting injury often football
they are avulsion fractures at the tibial attachment of the ACL
concomitant meniscal tear is common so MRI is recommended
Meyers and McKeevers classification system helps guide management
Patella dislocation
common
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peak age
adolescents, females more common than males, family history, patellofemoral dysplasias
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mechanism
sporting activities
trochlea dysplasia, patella alta, lateralized tibial tuberosity
Patella sleeve fracture
most common patella fracture in skeletal immaturity
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peak age
8-12 years
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mechanism
contraction of quadriceps on a flexed knee, high impact jumping activities
an x-ray does not show extent of injury often shows small avulsion of inferior pole patella and patella altar
consider an MRI as a large portion of patella articular cartilage is often attached to avulsed fragment
more: patella sleeve fractures
Tibial plateau fracture (tibial epiphyseal fracture)
uncommon 0.5-3.1% incidence
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peak age
12-13 years males more common than female
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mechanism
high-energy trauma
lateral plateau more common
lateral edge avulsions are Segond fractures
risk of vascular injury and compartment syndrome (popliteal artery is closely related to the tibial epiphysis)
more: tibial plateau fractures
Tibial tuberosity fracture
uncommon 0.4-2.7% incidence
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peak age
13-14 years males more common than female
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mechanism
force knee flexion or extension during jumping/sprinting activities
consider CT to determine the intra-articular or posterior extension
can be associated with patella tendon injury, quadriceps tendon injury, or compartment syndrome (recurrent branch of the anterior tibial artery injury)
Proximal tibial metaphyseal fracture
rare
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peak age
4-8 years
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mechanism
wide range, direct blow, motor vehicle accident, forced knee abduction/adduction/ hyperextension
commonly a medial cortex fracture with valgus angulation
usually managed non-operatively with reduction and immobilization
complications include progressive valgus deformity, tibial overgrowth and leg-lengthening discrepancy
more: Cozen fracture
Patella tendon rupture
rare
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peak age
12-15 years males more common than female
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mechanism
sporting activities
complete rupture shows patella alta with increased Insall-Salvati ratio
predisposing factors include; tendinopathy, steroids, previous ACL repair
more: patella tendon rupture
Osteochondritis dissecans
rare 9.5/100,000 incidence
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peak age
12-19 years male:female ratio of 4:1
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mechanism
unknown, can present following trauma
unknown pathology causes a softening of the cartilage leading to the detachment of a articular cartilage-subchondral bone segment from the articular surface
commonly at lateral edge of medial femoral condyle
lateral femoral condyle lesions associated with a discoid lateral meniscus
intercondylar notch view is helpful
differs from osteochondral fractures which occur secondary to trauma see osteochondral defect
Don't miss
Distal femur fracture
uncommon
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mechanism
younger children high association with non-accidental injury (corner fractures), older children significant trauma motor vehicle accidents or sporting injury
usually Salter-Harris II, displaced, and require surgery
high risk of growth arrest, continued follow up recommended
Tumor or pathological fracture
rare
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peak age
variable
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mechanism
can present as acute knee pain following trauma, pathological fracture, incidental finding
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malignant
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10-20 years of age
males more common than female
most common distal femur
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10-20 years of age
males more common than female
predilection for the knee
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benign
Other findings to consider
Normal variants
fabella: accessory ossicle typically in lateral head of gastrocnemius
bipartite patella: unfused accessory ossification center at the superolateral aspect of patella
Common incidental findings
metaphyseal lucent area - cortical fibrous defect, non-ossifying fibroma, bilateral metaphyseal lucency
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