Knee radiographs obtained after trauma should evaluate for fracture, dislocation, and soft tissue injury.
Evaluate tibiofemoral alignment.
Lateral Radiographs should demonstrate overlapping femoral condyles and weight bearing surface projecting over mid tibia.
Displacement of the tibia with respect to the femur may be secondary to subtle tibial plateau fracture, ligamentous injury, or dislocation-relocation.
For posterior traumatic dislocation of knee, CTA lower extremity is recommended post reduction to assess for popliteal artery injury (stenosis, occlusion, dissection), especially if asymmetric lower extremity pulses are noted post reduction.
Evaluate position of the patella relative to femoral condyles. Ensure the patella is well seated in the intercondylar groove, better assessed on sunrise views, if available.
If high or low riding patella is suspected, use Insall-Salvati ratio which is calculated by dividing patellar tendon length by patella length.
Patella alta (too high) >1.2 cm
Patella baja (too low) <0.8 cm
Assess for normal radiographic appearance of quadriceps and patellar tendons. Abnormal thickening and irregular contour with soft tissue swelling suggests injury.
Trace cortices and scan the medullary spaces to assess for fracture (fig a).
Look very closely for irregularity or step off at the tibial plateau (fig b).
Familiarize yourself with Schatzker classification.
Look for ossific fragments about the joint space which may represent ligament avulsion fractures. Common examples of these include tibial spine avulsion, Segond fracture (fig c), or arcuate fracture.
On lateral view, also look specifically for femoral condyle impaction fracture, commonly described as deep lateral sulcus sign (fig d). Which is commonly seen with ligamentous injury.
Insufficiency fractures occur in the setting of demineralized bone quality (disuse osteopenia, osteoporosis) and may appear as linear sclerosis due to trabecular impaction.
Since these fractures can be radiographically occult, MRI should be recommended for elderly patients unable to bear weight or with persistent pain following injury.
Left: Generalized demineralization. Right: Subtle linear band of sclerosis with right medial tibial metaphysis with adjacent cortical buckling, consistent with insufficiency fracture.
Check for joint effusion on lateral projection, identified by soft tissue/fluid density in the suprapetallar region between the suprapatellar and prefemoral fat pads (arrows).
Effusion following trauma, especially in the absence of degenerative change, can indicate soft tissue and ligamentous injuries.
Lipohemarthrosis is identified as a fat-fluid level within the effusion, suggesting intra-articular fracture and extrusion of fatty marrow. This is most easily on cross table lateral radiographs.
Example report negative for traumatic injury:
FINDINGS:
- No acute fracture. Normal knee alignment. No joint effusion.
IMPRESSION:
- No acute fracture.