The anatomical landmark for the anteroposterior (AP) axis of the proximal tibia and its variability was investigated in order to determine whether a certain landmark could be employed as a reference axis for the proximal tibia after the rotating platform mobile bearing and fixed bearing total knee arthroplasties (TKAs).
Eighty primary osteoarthritic knees were randomized to undergo either rotating platform mobile bearing (Group A, n = 40) or fixed bearing (Group B, n = 40) TKAs, and were followed up for 31 and 30 months, respectively. The AP axes were defined for the distal femur, proximal tibia, ankle, and each TKA component on the reconstructed CT scan and the angles between the distal femoral AP axis and those of each bone or component were estimated. Clinical and radiographic outcomes were evaluated during the follow up.
A significant difference was seen between the preoperative and postoperative rotational position of the proximal tibia relative to the distal femur following rotating platform mobile bearing TKA (P = 0.014) whereas no such difference was seen after fixed bearing TKA. The mean postoperative alignment of the tibia differed between the two groups (Group A:Group B = -2.9:0.2, P = 0.010) and its variability was significantly greater in group A (P < 0.001). There were no differences in the clinical outcomes including range of motion, knee society score, function score, HSS, and WOMAC score as well as the mean postoperative coronal tibiofemoral alignment between the two groups.
The unpredictable change in the rotational axis of the tibia and its broad variability after rotating platform mobile bearing TKA may provide a warning against the use of a fixed landmark for establishing tibial rotational alignment.