Evaluation of anatomic references for tibial sagittal alignment in total knee arthroplasty

The authors aimed to demonstrate the relationship between the sagittal mechanical axis of the tibia and other reference axes of the tibia and fibula in patients with advanced osteoarthritis of the knee joints, and then to identify a reliable landmark in order to minimize posterior tibial slope measurement errors. We evaluated 133 osteoarthritic knees with neutral or varus deformity in 64 female and 8 male patients. Axial computed tomographic images of whole tibiae including knee and ankle joints were obtained and reconstructed using 3-dimensional imaging software. Angles between the mechanical axis (MA), the tibial anatomical axis (TAA), the anterior tibial cortex (ATC) and the fibular shaft axis (FSA) were measured, and then medial and lateral tibial slope angles were measured using all axes. Mean angles between MA and the other anatomical reference lines (TAA, ATC and FSA) were 0.9, 2.2 and -2.1 degrees, respectively. The mean values of lateral tibial slopes with respect to MA, TAA, ATC and FSA were 8.7, 10, 12 and 7.3, respectively, and their intra- and inter-observer reliabilities were higher than those of medial tibial slopes. Although posterior tibial slope change markedly according to the reference axis used, the axes used in conventional TKA showed significant correlations with each other, and thus, may be used safely if differences with the mechanical axis are considered. Moreover, the lateral tibial slope might have advantages over the medial tibial slope in terms of restoration of the natural tibial slope.