USING PREOPERATIVE MRI FOR DETERMINING ROTATIONAL ALIGNMENT OF THE FEMORAL COMPONENT IN TOTAL KNEE ARTHROPLASTY AND EVALATION OF THE ACCURACY – AN IN-VIVO STUDY -

Abstract

Correct rotational alignment of the femoral prosthesis in total knee arthroplasty is important for correct patella tracking, patellofemoral joint contact forces, varus-valgus positioning in flexion, and the avoidance of anterior femoral notching. But achieving correct femoral rotation can be difficult, and there are reports of highly variable rotational alignment with the use of fixed surgical landmarks to determine femoral rotation. Minimal invasive technics makes it more difficult to identify these surgical landmarks. Computer assisted surgery may increase the accuracy of coronal and sagittal positioning but probably does not increase the accuracy of rotational positioning. We used preoperative MRI to aid us in determining femoral rotation preoperatively and used that information to implant our femoral components and evaluated the results.

We measured the angular difference between the surgical epicondylar axis and the posterior condylar axis of twenty patients preoperatively using MRI images and then used that angle to implant the femoral component. For a second group of twenty patients, computer assisted balanced flexion gap technic (Aesculap. Orthopilot system) was used to determine the rotational alignment of femoral components. CT scans were taken postoperatively and the accuracy of the rotational alignment was analyzed for both groups.

The ranges of error were as follows;

1. Preoperative MRI Group, 8degrees (3 degrees IR to 5 degrees ER).

2. Gap technic group, 21degrees (11 degrees IR to 10 degrees ER).

If an error of more than 5 degrees from neutral alignment is defined as an outlier, 2 in the pre-operative group and 6 in the GAP method group would fall in the outlier zone.

In conclusion, using preoperative MRI to determine the femoral rotational alignment and then using that information to implant the femoral component could aid in avoiding errors in rotation positioning of the femoral prosthesis. It is a simple and effective method to avoid rotational positioning errors with no learning curve.