Pediatric knee radiograph (an approach)

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.

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
• peak age
  •  8-17 years
• 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 
• more: anterior cruciate ligament avulsion fracture

Patella dislocation

• common 
• peak age
  • adolescents, females more common than males, family history, patellofemoral dysplasias
• mechanism
  • sporting activities 
• trochlea dysplasia, patella alta, lateralized tibial tuberosity 
• more: lateral patella dislocation

Patella sleeve fracture

• most common patella fracture in skeletal immaturity
• peak age
  • 8-12 years 
• 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 
• peak age
  •  12-13 years males more common than female 
• 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 
• peak age
  • 13-14 years males more common than female 
• 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) 
• more: tibial tuberosity fractures

Proximal tibial metaphyseal fracture

• rare 
• peak age
  • 4-8 years
• 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
• peak age
  • 12-15 years males more common than female 
• 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 
• peak age
  • 12-19 years male:female ratio of 4:1
• 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 
• more: ossteochondritis dissecans of the knee

Don't miss

Distal femur fracture

• uncommon
• 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 
• peak age
  • variable 
• mechanism
  • can present as acute knee pain following trauma, pathological fracture, incidental finding
• malignant
  • Ewing sarcoma 
    • 10-20 years of age
    • males more common than female
    • most common distal femur
  • primary osteosarcoma 
    • 10-20 years of age
    • males more common than female
    • predilection for the knee
• benign 
  • simple bone cyst 
  • fibrous dysplasia
  • chondromyxoid fibroma
  • osteocartilaginous exostoses
  • nonossifying fibromas
  • chondroblastomas 

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
• epiphyseal cortical irregularity 
• Harris growth arrest line
• os­teochondroma