Accuracy and systematic error of patient-specific instrumentation in total ankle arthroplasty: a three-dimensional CT-based registration study

Accurate component positioning remains a critical technical objective in total ankle arthroplasty. Conventional two-dimensional measurements may not fully characterize deviations between the preoperative plan and achieved implant position. This study quantified planned-versus-achieved implant positioning after patient-specific instrumentation-assisted total ankle arthroplasty using a three-dimensional CT-based registration method and validated the precision and accuracy of the registration workflow. We retrospectively analyzed 95 patients who underwent primary patient-specific instrumentation-assisted total ankle arthroplasty using a single implant system. Preoperative implant plans were compared with postoperative CT reconstructions using three-dimensional surface registration. Translational and rotational deviations were calculated for the tibial component, and rotational deviations were assessed for the talar component. Systematic directional bias was evaluated using signed measurements. A cadaveric validation experiment with 11 fresh-frozen below-knee specimens was performed using bead-based registration as the reference standard. For the tibial component, median absolute translational deviations were 0.9, 1.0, and 1.2 mm along the mediolateral, anteroposterior, and proximodistal axes, respectively; 71 of 92 components were within 3 mm in three-dimensional displacement. Median rotational deviations were approximately 3° in the axial plane, 1.5° in the coronal plane, and 4° in the sagittal plane. Signed measurements demonstrated a systematic anterior tilt of approximately 4° in the sagittal plane. The registration method showed a translational precision of 0.03 mm, rotational precision of 0.25°, and mean three-dimensional translational error of 0.24 mm relative to the bead-based reference. Patient-specific instrumentation-assisted total ankle arthroplasty achieved small translational deviations from the preoperative plan, but a consistent sagittal-plane anterior tilt bias was identified. Three-dimensional CT-based registration provides a reproducible framework for quantifying planned-versus-achieved implant positioning after arthroplasty. Level III, retrospective cohort study.