PMC5390651_01
Preoperative plain radiograph and computed tomography. Anteroposterior long leg standing image of both legs (a), anteroposterior one leg standing image of the left knee (b), lateral one leg standing image of left knee (c), skyline view of patella at 30 degrees of knee flexion (d), and preoperative computed tomography of the left knee (e).
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Skyline view radiographs showed the patella was dislocated to the lateral side of the lateral femoral condyle, the central ridge was absent, and the trochlear groove was hypoplastic (sulcus angle was 169 ) (Figures 1(a)-1(d)). Computed tomography (CT) scan showed that the tibial tubercle-trochlear groove distance was 22 mm, and the surgical epicondylar axis (SEA) was rotated 7.6  externally from the posterior condylar axis (PCA) (Figure 1(e)).
The data of navigation system. The screenshot of navigation system in the distal femur (a) and the proximal tibia (b).
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The medial parapatellar approach was used. The distal femur and proximal tibia were cut vertically to the mechanical axis using navigation system (VectorVision CT-free knee; BrainLAB, Munich, Germany). The distal femur was cut 0.5 degrees in varus and 1.5 degrees in flexion and the proximal tibia was cut 0.5 degrees in varus and 5.0 degrees in posterior slope when we confirmed the precision of osteotomy with navigation system (Figures 2(a) and 2(b)).
Postoperative plain radiograph. Anteroposterior long leg standing image of both legs (a), anteroposterior one leg standing image of left knee (b), lateral one leg standing image of left knee (c), skyline view of patella at 30 degrees of knee flexion (d), 60 degrees of knee flexion (e), and 90 degrees of knee flexion (f).
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Postoperative plain radiographs showed that FTA was 174 , HKA was neutral, and the patella was located on the femoral component groove (Figure 3).