An Intra-Operative Assessment of the Axial Rotational Positioning of the Tibial Component Using Patient-Specific Instrumentation Prophecy

Abstract

Category: Ankle, Ankle Arthritis Introduction/Purpose: Ankle replacements (TAR) have evolved significantly over the past 10 years. While most TAR systems reference the medial gutter to set axial tibial component rotation, newer CT scan-based patient specific instrumentation (PSI) allows any rotational position to be set, and this is commonly set to bisect the medial and lateral gutter lines. It is still not yet known which alignment technique is optimal. Methods: We initiated a prospective cohort analysis of 22 consecutive cases using the Prophecy® 3D PSI guide (Wright Medical Technologies, Memphis, TN). There were four foot and ankle fellowship trained surgeons practicing from two referral centers. Every procedure had an intra-operative photograph taken from the knee looking down to the ankle after both the attachment of the PSI guide to the tibia and the insertion of the standard medial gutter fork. The axial rotational angle between both was measured and compared to the Prophecy® pre-operative plan. Statistical analysis was performed using the student’s T-test for paired distribution and descriptive statistics given the number of cases (N=22). Results: The primary outcome of this study was to assess the correlation between the pre-operative medial gutter line drawn on the Prophecy plan and the intra-operative medial gutter fork angle. The Pearson R correlation value was 0.54 (p = 0.53). The average difference between the pre-operative medial gutter line on the Prophecy® plan and the intra-operative gutter fork was -0.46 [95% CI -2.04 – 1.10]. This could mean that the medial gutter fork is on average slightly more internally rotated than templated medial gutter line. However, the average of the absolute differences between the preoperative medial gutter line and the intraoperative medial gutter fork was 2.69 degrees. Of the 22 cases, 11 were within 2 degrees and 17 were within 4 degrees. Conclusion: The medial gutter line on Prophecy® PSI pre-operative TAR plans is a moderately good representation of the intra- operative medial gutter fork position used in standard TAR instrumentation. Based on this, using the Prophecy® default which sets the tibial implant axial rotation to bisect the medial and lateral gutter lines will typically lead to greater external rotation of the tibial component compared to standard TAR instrumentation which aligns the tibial component to the medial gutter fork. Further work needs to be done to better understand which of these axial alignment options, or potentially others, is optimal.