Increased implant thickness in mechanically aligned off-the-shelf total knee replacement compared to bone resection

Purpose: Total knee arthroplasty (TKA) with mechanical alignment (MA) aims to align the leg neutrally, optimising stability and load distribution. To achieve this, bone cuts are performed, modifying the constitutional alignment of the knee and determining a mismatch between the implant thickness and the bone resection across the various compartments of the knee (bone–implant mismatch). This study aims to quantify the bone–implant mismatch in TKA with MA using an off-the-shelf implant and investigate how this ratio is influenced by the preoperative coronal knee alignment phenotype. Methods: Data from 100 patients who underwent primary off-the-shelf TKA with MA with a medial pivot design implant from January 2021 to September 2023 were analysed. Preoperative alignment phenotype was determined via long-leg weightbearing radiographs. During surgery, bone resections were measured with a caliper for each compartment. Bone–implant mismatch was determined for each compartment, analysing differences between medial and lateral compartments and the influence of preoperative alignment phenotype. Results: The overall implant thickness was 14.2% greater than the bone resection. In the patello-femoral compartment the implant thickness was 16.0% ± 19.0 less than the bone resection. Bone-implant mismatch was observed in the tibio-femoral joint across all the compartments with a significant difference between medial and lateral (p < 0.001). No significant differences in bone–implant mismatch were found based on preoperative alignment phenotype. Conclusions: In mechanically aligned off-the-shelf TKA, implant thickness exceeds bone resection by an average of 14.2%, with a consistent bone–implant mismatch observed in all tibiofemoral compartments—significantly greater medially than laterally. Conversely, in the patellofemoral joint, bone resection surpasses implant thickness. Preoperative coronal alignment phenotype does not significantly influence this mismatch. These findings highlight a systematic discrepancy between bone resections and implant geometry, suggesting potential benefits from adjusted surgical techniques or implant design modifications to improve anatomical congruence and joint kinematics. Level of Evidence: Level III.