The extramedullary guide of the proximal tibia resection should be seen straight in front during total knee...

If surgeons see the shaft of the extramedullary guide from lateral to the guide during preparation of the proximal tibia resection during total knee arthroplasty, the tibial component may be implanted in varus position in the frontal plane. In order to clarify the effect of the angle of the surgeons' sight relative to the sagittal plane and the posterior slope angle of the resected surface on varus position of the tibial component in the frontal plane, mathematical analysis was performed. Three-dimensional coordinate system was utilized so that the central axis of the tibial shaft on the Z-axis and the shaft of the guide were skew. The relationship between two lines was analyzed solving equations on three dimensional planes. When the posterior slope angle is 10 degrees, and if surgeons see the shaft of the guide 10 degrees, 20 degrees and 30 degrees lateral to the sagittal plane, and the shaft and the central axis of the tibial shaft would seem to be parallel, the true varus tilt angle of the shaft on the frontal plane is 1.8 degrees, 3.7 degrees and 5.8 degrees, respectively. The extramedullary guide should be seen straight in front of the guide.