Abstract
Objective
Tibia vara seen in Japanese patients reportedly influences the tibial component alignment when performing TKA. However, it is unclear whether tibia vara affects the component position and size selection. We therefore determined (1) the amount of medial tibial bow, (2) whether the tibia vara influences the aspect ratio of the tibial resected surface in aligning the tibial component with the tibial shaft axis (TSA), and (3) whether currently available tibial components fit the shapes of resected proximal tibias in terms of aspect ratio.
Material and Methods
The study was performed using CT data from 90 lower limbs in 74 Japanese female patients with primary varus knee OA, scheduled for primary TKAs between January 2010 and March 2012. We measured the tibia vara angle (TVA; the angle between the TSA and the tibial mechanical axis), proximal varus angle (PVA; angle between the TSA and the line connecting the center of the tibial eminence and the center of the proximal 1/3 of the tibia) using three-dimensional preoperative planning software [Fig.1]. Then the mediolateral and middle AP dimensions of the resected surface when the tibial component was set so that its center aligned with the TSA was measured. We determined the correlations of the aspect ratio (the ML dimension divided by the AP dimension) of the resected surface with TVA or PVA and compared the aspect ratios to those of five prosthesis designs.
Results
The mean TVA and PVA were 0.6° and 2.0°, respectively. The aspect ratio negatively correlated with both TVA and PVA (r = −0.53 and −0.55, respectively) [Fig. 2, 3]. The mean aspect ratio of the resected surface was 1.48 but gradually decreased with increasing AP dimension, whereas four of the five prostheses had a constant aspect ratio.
Conclusions
The aspect ratio of resected tibial surface was inversely correlated to the degree of tibia vara, and currently available prosthesis designs do not fit well to the resected surface in terms of aspect ratio.
