Early radiographic differentiation of infantile tibia vara from physiologic bowing using the femoral-tibial ratio

The authors hypothesized that the ratio of the femoral to tibial metaphysea l-diaphyseal angles (femoral-tibial ratio [FTR]) more accurately differenti ates physiologic bowing from infantile tibial vara than the tibial metaphys eal-diaphyseal angle (TMDA). The purpose of this study was threefold: to de termine the false-negative and false-positive error rate of the FTR and TMD A; to determine to the effect of rotation on the FTR and TMDA; and to deter mine the reliability of the FTR and TMDA measurements. An FTR < 1 resulted in a false-negative error rate of 10% and a false-positive error rate of 7% , whereas a TMDA > 13 degrees resulted in a false-negative error rate of 23 % and a false-positive error rate of 10%. The difference between internal a nd external rotation was not significant for the FTR, whereas it was for th e TMDA. The FTR was found to have good interobserver and intraobserver reli ability (0.78 and 0.98, respectively).