Background This study was designed to analyze three tibial axis reference lines including the anterior tibial cortex (ATC) line,the fibular line (FL),and the anatomical axis of tibia (AAT) line,to determine which line most closely parallels the mechanical axis (MA) of the tibia in the sagittal plane.The clinical relevance of the study is that through finding a reliable landmark on the leg,a surgeon may minimize posterior tibial slope measurement errors thereby and improving the technique for assuring proper alignment of total knee arthroplasty.Methods The material for this study included CT scans of the tibia from 85 consecutive patients and 168 knees (78 without osteoarthritis (OA) and 90 knees with OA).Measurements of the angles between the tibial mechanical axis and each of three reference lines in the sagittal plane were carried out using 3D imaging software.Results Mean angles of 168 knees were as follows:aMT (3.96±0.85)°,aMF (0.70±0.58)°,and aMA (1.40±0.66)°,(aMT:an angle between MA and ATC,aMF:an angle between MA and FL,aMA:an angle between MA and AAT.All abovementioned angles were measured in the sagittal plane of tibia) and the aMF was significantly smaller than the others (P ＜0.0001).The mean value of the medial tibial slope angle vs.the MA was (9.19±3.97)°,and this was significantly larger than the mean lateral slope angle of (6.62±4.23)° (P ＜0.0001).The difference between aMF without OA and with OA was not statistically significant (P=0.5015) and the association between the aMT and aMA was strong (r=0.82,P ＜0.01).Conclusions FL was more closely parallel to the MA of tibia,and more showed less variation between OA and nonOA controls than ATC and AAT lines.Furthermore,the amount of posterior slope in medial plateau was greater than that in lateral plateau.The findings of this analysis suggest that when using the anterior tibial cortex line as is commonly done with extramedullary tibial resection guides,the tibial resection should be sloped approximately four degrees more posteriorly.