Varus/valgus alignment of the femur in total knee arthroplasty. Can accuracy be improved by pre-operative CT scanning?

In a series of 80 consecutive knee replacements, pre-operative scout scans of the femur were taken and the angle between the slope of the worn femur and the mechanical axis was measured. At surgery the angle of the distal femoral cut was adjusted to make it perpendicular to the mechanical axis. Post-operative scans revealed that 97.5% of femoral components were implanted within 2° of the target. The angle between a perfectly positioned virtual intra-medullary rod on the pre-operative films and the mechanical axis showed marked variations between individuals indicating that the use of a fixed angular correction from an intra-medullary rod is unreliable even under perfect circumstances. In practice it is subject to further inaccuracy due to variability in the anatomy of the femur and rod placement. With the advent of robotically assisted surgery, the technical limits of conventional instruments need to be explored in terms of improving accuracy for each individual patient. In terms of the coronal alignment of the femur, this involves some degree of preplanning rather than relying on arbitrary targets based on incorrect assumptions.