Technical and surgical causes of outliers after computer navigated total knee arthroplasty.

Background: Navigated total knee arthroplasty (TKA) improves implant and limb alignment but outliers continue to exist. This study aimed to determine the technical and surgical causes of outliers.

Methods: This retrospective cohort study included 208 patients who had undergone navigated TKA. Limb and implant alignment indices were measured on post-operative CT scans: mechanical femoro-tibial angle (MFTA); coronal femoral angle (CFA); coronal tibial angle (CTA); sagittal femoral angle (SFA); and sagittal tibial angle (STA). Values outside 0°±3° for MFTA and SFA, 90°±3° for CFA, CTA and STA were considered outliers. Intra-operative navigation data and CT scans were evaluated to categorize the causes of sagittal and coronal plane outliers into hip centre error; ankle centre error; heterogeneous tibial cement mantle; malalignment accepted by surgeon; suboptimal knee balance; and no obvious explanation.

Results: Of the 1040 measurements (five per TKA), the overall incidence of outliers was 10.4% (n = 108). Femoral component outliers (CFA + SFA, n = 51) were all attributable to hip centre error. Tibial component outliers (CTA + STA, n = 43) were attributable to ankle centre error (n = 6), heterogeneous cement mantle (n = 20), malalignment accepted by the surgeon (n = 6) and no obvious cause (n = 11). MFTA outliers were attributable to hip centre error (n = 4) or suboptimal knee balance (n = 10).

Conclusions: Surgeon related errors can be minimized by a meticulous operative technique. These results indicate scope for additional technical improvement, especially in hip centre acquisition, which may further reduce the incidence of outliers.