Assessing Lower Limb Alignment: Comparison of Standard Knee Xray vs Long Leg View

Abstract

Background: High tibial osteotomy (HTO) is a well-established and commonly utilized technique in medial knee osteoarthritis secondary to varus malalignment. Accurate measurement of the preoperative limb alignment, and the amount of correction required are essential when planning limb realignment surgery. The hip-knee-ankle angle (HKA) measured on a full length weightbearing (FLWB) X-ray in the standing position is considered the gold standard, since it allows for reliable and accurate measurement of the mechanical axis of the whole lower extremity. In general practice, alignment is often evaluated on standard anteroposterior weightbearing (APWB) X-rays, as the angle between the femur and tibial anatomic axis (TFa). It is, therefore, of value to establish if measuring the anatomical axis from limited APWB is an effective measure of knee alignment especially in patients undergoing osteotomy about the knee.

Methods: Three independent observers measured preoperative and postoperative FTa with standard method (FTa1) and with circles method (FTa2) on APWB X-ray and the HKA on FLWB X-ray at three different time-points separated by a two-week period. Intra-observer and inter-observer reliabilities and the comparison and relationship between anatomical and mechanical alignment were calculated.

Results: Intra- and interclass coefficients for all the three methods indicated excellent reliability, having all the values above 0.80. Using the mean of paired t-student test, the comparison of HKA versus TFa1 and TFa2 showed a statistically significant difference (p<.0001) both for the pre-operative and post-operative sets of values. The correlation between the HKA and FTal was found poor for the preoperative set (R=0.26) and fair for the postoperative one (R=0.53), while the new circles method showed a higher correlation in both the preoperative (R=0.71) and postoperative sets (R=0.79).

Conclusions: Intra-observer reliability was high for HKA, FTal and FTa2 on APWB x-rays in the pre- and post-operative setting. Inter-rater reliability was higher for HKA and TFa2 compared to FTal. The femoro-tibial angle as measured on APWB with the traditional method (FTal) has a weak correlation with the HKA, and based on these findings, should not be used in everyday practice. The FTa2 showed better correlation with the HKA, although not excellent.

Level of evidence: Level III, Retrospective study.

Figure 1. Tibio-femoral angle measurement on APWB x-ray: traditional method (TFa1)

Figure 2. Tibio-femoral angle measurement on APWB x-rays: circles method (TFa2)

Fig.3. Scattered Plot with linear regression showing the correlation between preoperative mechanical axis (full length weight bearing x-rays, HKA) and tibio-femoral angle (as measured on standard AP weight bearing x-rays, TFa1). R=0.26

Fig.4. Scattered Plot with linear regression showing the correlation between postoperative mechanical axis (full length weight bearing x-rays, HKA) and tibio-femoral angle (as measured on standard AP weight bearing x-rays, TFa1). R=0.53

Fig.5. Scattered Plot with linear regression showing the correlation between preoperative mechanical axis (full length weight bearing x-rays) and tibio-femoral angle (as measured on standard AP weight bearing x-rays, TFa2). R=0.71

Fig.6. Scattered Plot with linear regression showing the correlation between postoperative mechanical axis (full length weight bearing x-rays) and tibio-femoral angle (as measured on standard AP weight bearing x-rays, TFa2). R=0.79