TYPES OF KNEE INJURY
There are mainly five types of injuries (pneumonic - PCL Direct Hit): Pivot-shift, Clip, Lateral patellar dislocation, Dashboard and Hyperextension.1. PIVOT SHIFT INJURY:
Non-contact injury
Skiers, American football players
Knee valgus - Femur internally rotated - Tibia externally rotated
MR:
- Edema in posterolateral tibial plateau and midportion of the lateral femoral condyle
- ACL disruption
- Posterior capsular tear
- Acuate ligament tear
- Posterior horn of lateral or medial meniscus tear
- MCL injury
- Osteochondral impactions of tibial plateau, lateral femoral condyle
2. CLIP INJURY:
Pure valgus stress applied to knee while knee in mild flexion of 10°–30°
American football players
MR:
- Edema in lateral femoral condyle and smaller area involving medial femoral condyle
- O’Donoghue triad: MCL injury (commonly proximal portion), ACL disruption, Medial meniscal tear
3. LATERAL PATELLAR DISLOCATION:
Teenagers and young athlets
Shallow trochlear groove
Femur rotates internally on fixed tibia while knee is flexed - contraction of quadriceps - lateral dislocation of the patella
MR:
- Edema in inferomedial patella and anterior aspect of lateral femoral condyle. Rarely, edema may in adductor tubercle of medialfemoral condyle secondary to avulsion of MPFL
- Medial retinaculum, MPFL (femoral attachment) and medial patellotibial ligament injury
- Vastus medialis injury (best seen on sagittal)
- Osteochondral injuries of patella or lateral femoral condyle
4. DASHBOARD INJURY:
Force to anterior aspect of proximal tibia while knee is flexed - tibia forced posteriorly relative to femur
Most common injury resulting in disruption of PCL (PCL is tight and ACL is lax when knee is in 90° flexion)
MR:
- Edema in anterior aspect of proximal tibia
- PCL disruption (usually intact ACL)
- Posterior joint capsule rupture
5. HYPEREXTENSION INJURY:
Impaction of anterior femoral condyle against anterior aspect of tibial plateau
MR:
- Kissing contusion of anterior proximal tibia and distal femur (If valgus force with hyperextension, kissing contusions are seen medially)
- ACL or PCL disruption
- Meniscal injury
- Popliteal neurovascular structure injury
- Complete disruption of posterolateral complex
- Gastrocnemius injury
Difference between pivot shift injury and lateral patellar dislocation:
Lateral patellar dislocation: Edema of lateral femoral condyle is located more anteriorly and peripherally.
Pivot shift injury: Edema of lateral femoral condyle is located more centrally or posteriorly. With less flexion, the femoral edema is located more anteriorly, but it will be associated with edema of the posterior tibial plateau rather than with edema of the inferomedial patella.
Reference:
Timothy G et al. Bone Contusion Patterns of the Knee at MR Imaging: Footprint of the Mechanism of Injury. RadioGraphics 2000; 20: 135.
