Knee radiographs are common and often a quick and easy diagnostic exam in the emergency setting. An efficient approach to them requires a good understanding of anatomy with review strategies to ensure an accurate diagnosis.

Systematic review

Choosing a search strategy and utilising it consistently is a helpful method to overcome common errors seen in diagnostic radiology. The order in which you interpret the radiograph is a personal preference. A recommended systematic checklist for reviewing musculoskeletal exams is: soft tissue areas, cortical margins, trabecular patterns, bony alignment, joint congruency, and review areas. Review the entire radiograph, regardless of perceived difficulty. Upon identifying an abnormality, do not cease the review, put it to the side and ensure to complete the checklist.

Soft tissue

Assess all soft tissue structures for any associated or incidental soft tissue signs. In the case of the knee, it will involve the detection of secondary signs such as effusion or soft tissue swelling. 

Check for an effusion on the horizontal beam lateral:

• peripatellar fat pads should sit next to each other

  • anterior suprapatellar fat pad

  • prefemoral fat pad

• soft tissue density between them indicates an effusion

  • if simple effusion (haemarthrosis), think of severe ligamentous, meniscal or intra-articular bony injury

  • if fat-fluid level (lipohaemarthrosis), think of an intra-articular fracture

Bone cortex

Trace the cortex of each bone paying particular attention to regions that are superimposed such as the fibular head or patella. 

Plateau review

• carefully look for a proximal tibial fracture

• pay particular attention to:

  • tibial spine: avulsion

  • lateral tibial plateau: small avulsion (Segond fracture)

  • areas of increased density may point to underlying fracture

• medial epicondyle: don't overcall calcification adjacent to the medial femoral epicondyle (Pellegrini-Stieda lesion)

Patella

• fractures are usually easy to spot, often transverse

• don't call a bipartite patella or tripartite patella fractures: well-corticated unfused centre(s) at the superolateral pole

Femoral condyles

• subtle avulsion fractures can be hard to spot ensure to check the origins of the:

  • medial collateral ligament

  • lateral collateral ligament

  • anterior cruciate ligament

  • posterior cruciate ligament

• trace the articular surface keeping in mind the chance of an osteochondral defect

Bony alignment and joint congruency

One should inspect for smooth, concurrent bony alignment in all views.

• tibiofemoral alignment

  • draw a line down the lateral margin of the lateral femoral condyle

  • if >5 mm tibia is observed outside the line, think tibial plateau fracture

• check for patella tendon disruption

  • patella tendon: inferior pole of patella to tibial tuberosity

  • patella tendon length = patella length ± 20%

    • there are multiple techniques to measure this

      • Insall-Salvati ratio

        • modified Insall-Salvati index

      • Blackburne-Peel ratio

      • Caton-Deschamps index

      • de Carvalho index 

  • if increased, think patella tendon rupture

Review areas

Small avulsion fractures of the knee more often than not are associated with instability and internal derangement. Careful scrutinisation of the origins and insertion points of ligaments is a must. From lateral to medial, superior to inferior, ensure to inspect ¹:

• origin of the lateral collateral ligament at the lateral femoral condyle

• origin of the anterior cruciate ligament at the posterior-lateral portion of the intercondylar notch femoral condyle

• insertion of the lateral capsule at the lateral tibia (at the joint line)

• insertion of the arcuate ligament at the fibular styloid

• insertion of the lateral collateral ligament and the bicep femoris tendon (conjoint tendon) at the fibular head

• insertion of the iliotibial band at Gerdy tubercle of the tibia

• origin of the medial collateral ligament at the medial femoral condyle

• origin of the posterior cruciate ligament at the anterior middle portion of the medial condyle

• insertion of the deep meniscofemoral ligament of the medial collateral ligament at the medial tibia (at the joint line)

• insertion of the superficial fibres of the medial collateral ligament at the anteromedial tibia 5 cm distal to the tibiofemoral joint

• anterior cruciate ligament insertion at the medial portion of the tibial spine

• posterior cruciate ligament insertion at the posterior medial portion of the tibial plateau

It is worth spending extra time scrutinising areas of superimposition such as the fibular head and patella. 

Common pathology

Lipohaemarthrosis

• fat and blood from bone marrow collect in suprapatellar bursa

• a fat-fluid level may be the only sign of intra-articular fracture

• associated with tibial plateau or distal femoral fractures

• more: lipohaemarthrosis

Tibial plateau fracture

• 80% involve the lateral plateau

• fall from height or car bumper impact

• associated significant cruciate and medial collateral ligament damage

• more: tibial plateau fracture

Segond fracture

• avulsion fracture; bony fragment adjacent to lateral tibial plateau

• internal rotation and varus stress; falls or sports

• 75% associated with anterior cruciate ligament injury

• more: Segond fracture

Intercondylar eminence fracture

• typically avulsion fracture of tibial attachment of anterior cruciate ligament

• mechanism: rapid deceleration or hyperextension of the knee

• most common in adolescents

• more: intercondylar eminence fracture

Patella fracture

• majority transverse, also vertical or comminuted

• direct blunt force or violent contraction of quadriceps

• oblique or skyline views will confirm fractures

• more: patella fracture

Distal femoral fracture

• 6% of all femur fractures

• bimodal distribution

• high energy blunt trauma; falls in elderly

• more: distal femoral fracture

Proximal fibula fracture

• typically occur with lateral tibial plateau fractures, but may be isolated

• varus force

• associated with lateral collateral ligament damage

• more: proximal fibula fracture

Don't miss...

Pellegrini-Stieda lesion

• post-traumatic soft-tissue calcification adjacent to medial epicondyle of femur

• ossification following injury to medial collateral ligament

• do not misdiagnose as a fracture

• more: Pellegrini-Stieda lesion