Background/Purpose: Varus alignment is one factor of medial osteoarthritis (OA) of the knee. According to the concept of the constitutional varus, the bone growth disturbance at the medial proximal metaphysis of the tibia results in proximal tibia vara. In this situation, the tibia is bent at the proximal metaphysis. Therefore, the tibial articular surface (TAS) tilts in varus position and shifts medially. The medial shift of TAS may influence the value of the Hip-knee-ankle (HKA) angle because the ankle joint shifts laterally relative to the TAS. The purpose of this study was to assess the extent of the medial shift of TAS in knees with medial OA, and to assess the effect of this medial shift of TAS on the value of HKA angle, and to discuss the importance of the medial shift of TAS for the onset of medial OA of the knee.

Methods: This study consisted of 116 knees with medial OA. 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˚. On the anteroposterior view radiograph of the tibia, an anatomical axis (AA), a mechanical axis (MA) and a tibial plateau tangent were drawn. AA was the central line of the tibial shaft. MA is the line between the center of the tibial spines notch and the center of the tibial plafond (ankle joint). Two angle parameters and two distance parameters were measured. Those were angle between AA and MA (Angle A-M) (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 MA on the tibial plateau (Distance A-M) (the value was positive when MA located medial to AA), and the length of MA.

Results: The mean (±SD) Angle A-M and Angle plateau was 1.0˚ ± 0.6˚ (range, -0.1˚ to 3.0˚) and 8.2˚ ± 2.9˚ (1.0˚ to 17.0˚), respectively. The mean Distance A-M and length of MA was 5.6mm ± 3.4mm (-1.4mm to 16.1mm) and 347.3mm ± 3.4mm (300.7mm to 408.2mm), respectively. The correlation coefficient between Angle Plateau and Distance A-M was 0.62. The more proximal tibia had varus deformity, the more TAS shifted medially. The maximum Distance A-M was 16.1mm. In this case, HKA angle underestimated varus deformity up to 3˚. The coronal alignment of the lower extremity is expressed in the two-dimensional coordinate system. HKA angle is constructed by three points (hip, knee and ankle) in the system on condition that AA and MA match on the tibia. The difference between MA and AA cannot be assessed by HKA angle. The medial shift of the TAS would increase the mechanical loading in the medial side of the knee.

Conclusion: The knees with proximal tibia vara have medial shift of the tibial articular surface. HKA angle under-estimates varus deformity in such knees. The medial shift of the tibial articular surface should be taken into account for one factor of the medial OA of the knee, and for the treatment of the medial OA.

« Back to 2017 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-medial-shift-of-the-tibial-articular-surface-is-one-factor-of-medial-osteoarthritis-of-the-knee/