Background Varus knee is one factor of medial osteoarthritis of the knee. According to the concept of the constitutional varus1), the bone growth disturbance at the growth plate in the medial proximal metaphysis of the tibia results in proximal tibia vara. In this situation, the tibia is bent at the proximal metaphysis and the tibial articular surface (TAS) may be shift medially. The medial shift of the TAS will increase the mechanical loading in the medial side of the knee. The medial shift of TAS will also influence the value of the Hip-knee-ankle (HKA) angle because the tibial plafond that is the end point of the mechanical axis (MA) of the tibia will shift laterally.
Objectives The purpose of this study was to assess the extent of the medial shift of TAS in knees with medial osteoarthritis, and to assess the effect of this medial shift of TAS on the value of HKA angle.
Methods This study consists of 116 knees with medial osteoarthritis. The mean age was 75.3 years old. The mean standing femorotibial angle (FTA: lateral angle between femoral and tibial anatomical axes) was 183.6°. The anatomical axis (AA) was the central line of the femoral and the tibial shaft. On the anteroposterior view radiograph of the tibia, AA, MA and tibial plateau tangent were drawn. MA is the line between the center of the tibial spines notch and the center of the tibial plafond. Two angle parameters and two distance parameters were measured. Those are angle between AA and MA (Angle AA-MA) (the value was positive when MA located medial to AA), angle between the tibial plateau tangent and the line perpendicular to AA (Angle plateau), distance from AA to the center of the tibial spines notch on the tibial plateau (Distance AA-MA) (the value was positive when Point M located medial to AA), and the length of MA.
Results The mean (±SD) Angle AA-MA and Angle plateau was 1.0° ± 0.6° and 8.2° ± 2.9°, respectively. The mean Distance AA-MA and length of MA was 5.6±3.4mm and 347.3±3.4mm, respectively. Figure 1 shows the relationship between Angle Plateau and Distance AA-MA. The correlation coefficient between two parameters was 0.62. The more proximal tibia had varus deformity, the more the tibial articular surface shifted medially. The maximum Distance AA-MA was 16.1mm. In this case, HKA angle underestimated varus deformity up to 3°.
Conclusions The knees with proximal tibia vara have medial shift of the tibial articular surface. There are three factors to influence the evaluations of the medial osteoarthritis of the knees. Firstly, varus knees have higher loading condition in medial compartment because of proximal tibia vara and medial shift of the tibial articular surface. Secondly, HKA angle under-estimates varus deformity in knees with medial shift of the tibial articular surface. Thirdly, FTA cannot reveal the exact loading condition, either. Even with the same FTA, it is clear that loading in the medial compartment is larger in knees with medial shift of the tibial articular surface compared with knees with simple medial osteoarthritis. The medial shift of the tibial articular surface should be taken into account for one factor of medial osteoarthritis of the knee.
Bellemans J, et al. The Chitranjan Ranawat award: is neutral mechanical alignment normal for all patients? The concept of constitutional varus. Clin Orthop Relat Res. 2012;470(1):45–53.
Disclosure of Interest None declared