Accuracy of Patient-Specific Instruments Resections In Vivo in Total Ankle Arthroplasty on Postoperative Weightbearing CT Scan

Abstract

Submission Type: Total Ankle Arthroplasty Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose Total ankle arthroplasty (TAA) using patient-specific instruments (PSI) has increased in popularity with evidence for decreased operative duration, fluoroscopy usage, and increased implant placement accuracy on postoperative radiograph (XR). XR is notably inferior to weightbearing computed tomography (WBCT) in measuring implant placement. To date, no comparisons have verified the accuracy of PSI in vivo utilizing WBCT. Our study aims to quantify the accuracy and precision of PSI between the preoperative template and postoperative implant location utilizing WBCT. Primary outcome will identify >2° angular deviation in any plane for talus and tibial resections. Secondary outcome will identify implant placement positioning deviation >2mm of translation in any plane from plan. Correlation and regression analyses were performed to identify variables that correlate to resection or implant placement deviation. Methods: This was a single-surgeon, single implant consecutive patient retrospective study where preoperative and postoperative WBCTs were obtained prospectively. WBCT was performed via standardized protocol preoperatively and approximately 3 months postoperatively per manufacturer specifications. TAA was performed by the senior author with the same low-profile implant (Exactech Vantage) for all cases. Patients or specific implant components were excluded if conventional instruments or separate manufacturers were utilized. Preoperative template resections for a neutral total ankle replacement were planned by the primary surgeon. WBCT images were segmented per protocol. Talus and tibial resection analysis was performed in computer-automated fashion on postoperative segmented WBCTs (MATLab, FreeForm) and is described as 4 possible deviations from plan (cut height in mm, sagittal slope, varus/valgus deviation, axial rotation in deg). Implant placement factors independent of PSI guides are described as center-of-mass translations (mm). Statistical analysis of data collected included student t-test and linear regression analyses. Results: Twenty-four patients were included. Four talus implants were excluded per intraoperative surgeon discretion (deviation from PSI plan, use of conventional instruments). Preoperative mean coronal plane deformity was −2.5° (range, -18.7 valgus to 15 varus) and mean preoperative tibial slope was 0 +/- 1.7. All mean tibial and talus resection measurements were within deviation limits, except tibial sagittal slope height (mean deviation 2.6° dorsiflexed). Mean tibia and talus implant position was within deviation limits in all planes, except talus axial rotation (2.7 +/- 5.9° internal rotation) and talus implant slope (mean deviation 5.6° dorsiflexed). Preoperative coronal plane deformity (r = -0.7) and tibial anatomic axis (r=0.66) were significantly correlated with mediolateral tibial implant position. 79% of resection and implant position measurements were within deviation limits. Conclusion: Total ankle replacement performed with PSI resulted in highly accurate tibial and talus resections in 7/8 axes when analyzed on WBCT. Implant placement also matched the plan in resulting in 9/12 categories with +/2mm/deg deviation, and 11/12 within 3mm/3deg. The most common error was sagittal plane resection, and talus sagittal slope had the highest degree of variability likely because of the curved implant design. Preoperative deformity did not correlate with postoperative deviation identifying that PSI guides can be successfully used for ankle deformity correction. These findings demonstrate more granularity although also agree with previous data utilizing postoperative radiographs.