Alignment evaluation using different distal reference points after total knee arthroplasty

Background: To assess alignment for total knee arthroplasty, the center of the ankle has been used as the main reference point for the distal tibia; however, the true load-bearing mechanical axis should be determined as a line from the center of the femoral head to the lowest point of calcaneus. Thus, the purpose of this study was to compare the differences in alignment. Methods: Patients with medial osteoarthritis who underwent primary total knee arthroplasty (Group A, center of ankle reference, or Group C, calcaneal contact reference) were recruited. We determined (1) the total number with calcaneal contact point lateral to the center of ankle and compared (2) percentage of displacement of the load-bearing axis at the level of the knee, (3) anatomical axis angle, (4) mechanical axis angle, and (5) tibial component angle. Results: The study included 94 patients (128 knees), with the calcaneal contact reference point located lateral relative to the center of the ankle in 88.3% (113/128 knees). Using calcaneal contact point references, displacement of the load-bearing axis at the knee was greater (p < 0.0001, 38.7% vs 34.0%), and angles demonstrated significantly valgus alignment (p < 0.0001, 5.6 degrees vs. 4.8 degrees for anatomical axis angle, -3.0 degrees vs. -4.2 degrees for mechanical axis angle, and 89.9 degrees vs. 88.6 degrees for tibial component angle). Conclusions: Varus alignment measured by the ankle reference method might correspond to the neutral alignment by the amount of valgus alignment indicated by the calcaneal reference. Surgeons should take this into account when preoperative planning, performing intraoperative procedures, and during postoperative evaluation.