Sagittal Alignment in Total Knee Arthroplasty

BACKGROUND: Sagittal alignment determines the extension and flexion of knee prostheses in total knee arthroplasty (TKA). The definition of the sagittal axes may be different between the Mako TKA system (Stryker) and the conventional manual intramedullary approach. Whether there is any discrepancy between the 2 approaches has not been well studied. METHODS: We retrospectively analyzed 60 full-length computed tomographic (CT) scans of the lower extremities of 54 patients. The femur and tibia were modeled by using Mimics (Materialise). The Mako mechanical axes were determined according to the Mako TKA Surgical Guide. The manual intramedullary axes were determined according to the central axis of the tibial proximal and femoral distal medullary cavities. The femoral, tibial, and combined angular discrepancies were measured in the sagittal plane. RESULTS: On the femoral side, the Mako mechanical axis was more likely to be located in an extended position relative to the manual intramedullary axis (56 of 60 knees). The median angular discrepancy was 2.46° (interquartile range [IQR], 1.56° to 3.43° [range, -1.06° to 5.24°]). On the tibial side, the Mako mechanical axis was likely to be located in a flexed position relative to the manual intramedullary axis (57 of 60 knees). The median angular discrepancy was 2.40° (IQR, 1.87° to 2.84° [range, -0.79° to 4.20°]). The angular discrepancy of the femoral-tibial sagittal angle was 4.63° (IQR, 3.71° to 5.64° [range, 1.20° to 9.02°]). CONCLUSIONS: Compared with manual TKA, the Mako system is more likely to result in a decreased posterior tibial slope and extension of the femoral prosthesis. It may also influence the evaluation of lower-extremity extension and flexion. When using the Mako system, special attention should be given to these discrepancies. LEVEL OF EVIDENCE: Therapeutic Level IV . See Instructions for Authors for a complete description of levels of evidence.