Do CAS measurements correlate with EOS 3D alignment measurements in primary TKA?

Purpose Objective of this study was to compare intraoperative computer‐assisted surgery (CAS) alignment measurements during total knee arthroplasty (TKA) with pre‐ and postoperative coronal alignment measurements using EOS 3D reconstructions. Methods In a prospective study, 56 TKAs using imageless CAS were performed and coronal alignment measurements were recorded twice: before bone cuts were made and after implantation of the prosthesis. Pre‐ and postoperative coronal alignment measurements were performed using EOS 3D reconstructions. Thanks to the EOS radiostereography system, measurement errors due to malpositioning and deformity during acquisition are eliminated. CAS measurements were compared with EOS 3D reconstructions. Varus/valgus angle (VV), mechanical lateral distal femoral angle (mLDFA) and mechanical medial proximal tibial angle (mMPTA) were measured. Results Significantly different VV angles were measured pre‐ and postoperatively with CAS compared to EOS. For preoperative measurements, mLDFA did not differ significantly, but a significantly larger mMPTA in valgus was measured with CAS. Conclusion Results of this study indicate that differences in alignment measurements between CAS measurements and pre‐ and postoperative EOS 3D are due mainly to the difference between weight‐bearing and non‐weight‐bearing position and potential errors in validity and reliability of the CAS system. EOS 3D measurements overestimate VV angle in lower limbs with substantial mechanical axis deviation. For lower limbs with minor mechanical axis deviation as well as for mMPTA measurements, CAS measures more valgus than EOS. Eventually the results of this study are of clinical relevance, since it raises concerns regarding the validity and reliability of CAS systems in TKA. Level of evidence IIb.