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 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 analyze the relationship between PTS measurement results on short and expanded lateral knee radiographs. Methods The posterior tibial slope was

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... asured 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 posterior tibial slope 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 posterior tibial slope of three degrees, on average. Conclusion The posterior tibial slope measurements in long lateral knee radiographs are more accurate compared to short radiographs. On short lateral knee radiographs only a estimation of the posterior tibial slope can be carried out.