Intra and post-operative accuracy assessments of two different patient-specific instrumentatio systems for total knee replacement.

Purpose: The aim of this study is to assess and compare the accuracy of two different patient-specific instrumentation (PSI) systems for total knee replacement, both intra-operatively for bone preparation and post-operatively for final component alignment. Methods: Twenty-five patients were treated according to a computer tomography (CT)-based PSI system (group A) and 25 to a magnetic resonance imaging (MRI)/X-ray-based system (group B). Alignments on the three anatomical planes and resection thickness at the cutting blocks and at the resulting bone cuts were recorded intra-operatively by a standard surgical navigation system. Alignments of the prosthetic components and mechanical axis were also measured post-operatively on radiographs. These measurements at both the femur and tibia were compared with those of the corresponding pre-operative planning, considering discrepancies larger than 3° as outliers. Results: In both groups, the mean absolute differences between pre-operatively planned alignments and corresponding intra- and post-operative measurements ranged from a minimum of 1.2° to a maximum of 2.9° in all three anatomical planes. In both groups and in both femur and tibia, the plane with the smallest percentage of outliers was the coronal, maximum 17 %. The comparison between two groups was statistically significant (p = 0.02) in the femoral sagittal plane, where group B showed smaller alignment discrepancies at the cutting blocks. Conclusions: Both PSI systems showed good alignments in the coronal plane in all stages. For a few measurements, a better performance was observed in the MRI/X-ray-based system than in the CT-based system. Level of evidence: I.