The distance from the extramedullary cutting guide rod to the skin surface as a reference guide for the tibial slope in total knee arthroplasty

• Computer simulation studies were performed on 100 knees.
• We make the distance from the skin to the rod equal to the cutting block depth.
• The angle between the mechanical axis and the virtual guide rod was evaluated.
• Ninety-nine percent of the knees were within three degrees of deviation from the mechanical axis.

BackgroundAlthough sagittal tibial alignment in total knee arthroplasty (TKA) is important, no landmarks exist to achieve a reproducible slope. The purpose of this study was to evaluate the clinical usefulness of the distance from the guide rod to the skin surface for the tibial slope in TKA.MethodsComputer simulation studies were performed on 100 consecutive knees scheduled for TKA. The angle between the line connecting the most anterior point of the predicted tibial cut surface and the skin surface 20 cm distal to the predicted cut surface (Line S) and the mechanical axis (MA) of the tibia in the sagittal plane was measured.ResultsThe mean (±SD) absolute angle difference between the Line S and the MA was 0.9° ± 0.7°. The Line S was almost parallel to the MA in the sagittal plane (95% and 99% within two degrees and three degrees of deviation from MA, respectively).ConclusionThe guide rod orientation is a surrogate for the tibial cut slope because the targeted posterior slope is usually built into the cutting block and ensuring the rod is parallel to the MA in the sagittal plane is recommended. Therefore the distance between the skin surface and the rod can be a useful guide for the tibial slope.Level of evidenceII