An alternative method to create extramedullary references in total knee arthroplasty

Purpose: Historically, extramedullary techniques have focused on finding the femoral head center and referred mainly to the anterosuperior iliac spine (the FHC method) in total knee arthroplasty (TKA). The purpose of this study was to evaluate a new extramedullary alignment system that uses neutral overall limb mechanical alignment as an extramedullary reference (the MA method). Methods: A retrospective review of 1018 TKAs (508 FHC group, 510 MA group) was performed to compare the radiographic results obtained using these two methods. There were 75 male patients and 577 female patients with varus osteoarthritis of the knee. Limb and prosthesis alignments in the coronal plane were investigated at 2 months after surgery. Results: The accuracies of postoperative alignments of implanted prostheses were better in the MA group than in the FHC group, as assessed by (1) overall limb alignment (MA group, varus 1.4° ± 2.7°; FHC group, varus 2.2° ± 2.9°), and (2) femoral component coronal alignment (MA group, 89.4° ± 2.3°; FHC group, 88.0° ± 2.7°). Overall limb alignment was within 0° ± 5° of the mechanical axis in 96.1% of the MA group and in 86.6% of the FHC group. Mean femoral component medial angle was within 0° ± 5° in 98.4% of the MA group and in 92.5% of the FHC group. Conclusion: The accuracies of the postoperative radiographic alignments of implanted prostheses were found to be improved when the mechanical axis of the overall lower limb was used as an extramedullary reference. This new reference system would help to achieve correct limb and prostheses alignments during TKA. Level of evidence: III.