The effect of ankle rotation on cutting of the tibia in total knee arthroplasty.

BACKGROUND

Extramedullary alignment guides are commonly used to prepare the tibia during total knee arthroplasty. One disadvantage is that the guide is easily affected by the position of the ankle joint. The tibia may have a rotational mismatch between its proximal and distal ends. We hypothesized that a rotational mismatch might cause incorrect positioning of an extramedullary alignment guide and evaluated such a mismatch on the predicted postoperative coronal alignment of the tibia.

METHODS

Fifty-three osteoarthritic knees with varus deformity in fifty-one patients were evaluated with use of computerized tomography scans before total knee arthroplasty. We defined one anteroposterior axis of the ankle joint and five different anteroposterior axes of the proximal aspect of the tibia using three-dimensional bone models from the computerized tomography data. We measured the rotational angle between the anteroposterior axis of the ankle joint and the proximal part of the tibia. The distal end of the extramedullary guide was placed in front of the center of the ankle joint (on the line of the extended anteroposterior axis of the ankle joint), and the proximal end was placed on the line of the extended anteroposterior axis of the proximal part of the tibia. We established spatial coordinates to evaluate the effect of the rotational angle on the predicted postoperative coronal alignment of the tibia and calculated the presumed tibial coronal alignment.

RESULTS

The rotational angle was positive (3.6 degrees to 19.7 degrees) for all of the anteroposterior axes of the proximal aspect of the tibia, indicating that the ankle joint was externally rotated relative to the proximal part of the tibia. The predicted tibial coronal alignment was varus (0.5 degrees to 5.1 degrees) for all of the anteroposterior axes of the proximal part of the tibia.

CONCLUSIONS

When an extramedullary alignment guide is used to prepare the tibia in total knee arthroplasty, varus alignment of the tibial component can occur because of a rotational mismatch between the proximal part of the tibia and the ankle joint.