Can the tibial slope be measured on lateral knee radiographs?

Purpose The posterior tibial slope influences both the natural knee stability as well as the stability and kinematics after total knee arthroplasty (TKA). Exact definition of the posterior tibial slope (PTS) requires lateral radiographs of the lower limb. Only lateral knee radiographs are routinely obtained after TKA, however. The purpose of the present study therefore was to analyse the relationship between PTS measurement results on short and expanded lateral knee radiographs. Methods The PTS was measured on 100 consecutive lateral radiographs of the lower limb using the mechanical and three diaphyseal axes with various distances below the tibial plateau. Results Significant differences between PTS results were found for all three diaphyseal axes, with the smallest differences and the strongest correlation for a diaphyseal axis at 16 and 20 cm below the tibial plateau. Using short distances below the tibial plateau (6 and 10 cm) resulted in an overestimation of the PTS of 3°, on average. Conclusion The PTS measurements in long lateral knee radiographs are more accurate compared to short radiographs. On short lateral knee radiographs, only a estimation of the PTS can be carried out. Level of evidence Diagnostic study, Level II.