Tibial joint line correction and bone resection thickness influence the extension soft tissue balance in functionally aligned robotic arm-assisted cruciate-retaining total knee arthroplasty

The aim of this study was to investigate the influence of alignment and joint line correction on soft tissue balance after functionally aligned (FA) robotic arm-assisted cruciate-retaining (CR) total knee arthroplasty (TKA). This retrospective analysis included 59 patients who underwent unilateral FA CR-TKA. Knee phenotypes, including joint line obliquity (JLO) and arithmetic hip–knee–ankle angle (aHKA), and an intra-operative surgical plan using a robotic system were examined. The joint component gap balance at extension and flexion was measured using an offset-type tensor after insertion of the femoral trial components, and the correlations between gap balance and knee phenotypes were statistically evaluated. Correlations between gap balance and clinical outcomes were also evaluated. The aHKA and its correction were positively correlated with extension gap balance, but not with flexion gap balance. The tibial JLO correction angles were positively correlated with extension gap balance. The tibial lateral bone resection thickness was positively correlated with the extension gap balance and the tibial JLO correction angle. Flexion gap balance was not correlated with tibial JLO correction angle or external rotational angle relative to the femoral posterior condylar axis, and the two factors were positively correlated with each other. No significant correlations were found between clinical outcomes and gap balance. Tibial JLO correction and tibial lateral bone resection thickness were associated with extension gap balance in FA CR-TKA. Knee phenotypes and tibial resection thickness should be considered when surgeons attempt to balance the extension gap in CR-TKA. • Arithmetic hip–knee–ankle angle and its correction correlated with extension gap balance. • Tibial joint line obliquity angle (JLO) and its correction positively correlated with extension gap balance. • Flexion gap balance was not correlated with tibial JLO correction.