Imaging

KNEE IMAGING

Deciding to Image

  • The Ottawa Knee Rule states that patients presenting with an acute knee injury only require an x-ray if:
    • The patient is over 55 years of age OR
    • There is tenderness over the fibular head OR
    • There is isolated tenderness over the patella OR
    • The patient is unable to flex their knee to 90 degrees OR
    • The patient is unable to weightbear for 4 steps (regardless of limp) both initially and in the Emergency Department

  • For non-acute injuries, clinical judgement needs to be applied.
  • NB Patients with PFJ dislocations and  suspected ACL ruptures may also require imaging to exclude pathology which may require urgent attention (eg avulsion # of patella, # of trochlear groove, avulsion of tibial spine)

Imaging for Suspected Ligament or Cartilage Damage

  • Plain x-rays will not show damage to ligaments or cartilage.
  • Ultrasound may show damage to the collateral ligaments and peripheral menisci, however this is sonographer dependent and a thorough clinical examination would provide more useful information to dictate management.
  • For suspected cruciate ligament rupture, meniscal damage or posterolateral corner injury, MRI is the test of choice, however this usually has no impact on acute management. Imaging is only indicated in the acute setting if the result would change immediate management for the clinician. MRI is about 95% sensitive for ACL ruptures, which is slightly better than experienced clinicians assessment.
  • If a patient has a significantly unstable knee (for example, has significant instability on varus +/- valgus testing on an extended knee), then they should be referred to the orthopaedics department.
  • CT scans are useful for further examining the bony component of a knee injury.

Knee X-rays

  • If you request a knee x-ray, you will usually be presented with an AP and a lateral (which may be a “horizontal beam” shot, which can aid in identifying a lipohaemarthrosis
  • There is also a skyline view for evaluating the patellofemoral joint. There are also oblique patella views for further evaluation of the patella (not pictured)
  • Note that standard knee films will not show the proximal femur, or the distal 2/3rds of the tibia and fibula.

Views

AP, Lateral, skyline, tib-fib, oblique patella views (not shown)


Abnormal X-rays

Patella Fracture

  • Stable, undisplaced and minimally displaced fractures of the patella are managed in a Zimmer Splint for approximately 6 weeks, keeping the knee completely straight.
  • Stability of the fractures is assessed by the ability to straight leg raise.
  • Displaced or unstable patella fractures require ORIF.


Segond Fracture

  • A segond fracture is a small avulsion type fracture of the lateral aspect of the tibial plateau. The lateral capsular ligament avulses a fragment of bone.
  • While the significance of the actual fracture is not great (conservative management), the presence of a segond fracture is almost always (95%) accompanied by an ACL rupture.


Tibial Tubercle Avulsion

  • In the skeletally immature patient, the tibial tubercle may be avulsed by the pull of the patella tendon.
  • This is usually the culmination of repeated trauma and the patient will often have a history of Osgood Schlatter’s “disease”.
  • Treatment depends on level of displacement, ranging from immobilisation in an extension splint for stable, minimally displaced injuries to ORIF for significantly displaced fractures


Tibial Spine Fractures

  • The anterior cruciate ligament attaches to the anterior tibial spine and the posterior cruciate ligament to the posterior tibial spine.
  • In this film, the ACL has avulsed the anterior tibial spine.
  • Note the presence of a lipohaemarthrosis on the Horizontal Beam view. Fat visible on x-ray usually indicates fracture. The five causes of a haemarthrosis are intra-articular fracture, ACL rupture, PCL rupture, meniscal tear and PFJ dislocation.
  • On CT scan, this injury would likely appear much worse than it does on X-ray. Mx usually entails surgery.


Tibial Plateau Fractures

  • Tibial plateau fractures vary significantly in their appearance.
  • This film shows an obviously displaced tibial plateau fracture which requires ORIF, however these are often subtle, with varying amounts of depression seen on AP films.
  • Depressed fracture of the medial tibial plateau, also requiring ORIF.
  • If the fracture is only minor, with minimal depression, management may not require surgery and just NWB for a length period with some sort of immobilisation (HKB or ZKS)


Maisonneuve Fracture

  • The Maisonneuve fracture is a spiral fracture of the upper third of the fibula associated with a tear of the distal tibiofibular syndesmosis and the interosseous membrane.
  • There is an associated fracture of the medial malleolus or rupture of the deep deltoid ligament.
  • These injuries need orthopaedic attention.