Letter to the Editor Response: Failure of Cup Screw Interface in the Trident II Acetabular Cup in Total Hip Arthroplasty

We would like to thank van der Lelij et al. for their review of our manuscript, "Failure of Screw/Shell Interface in Trident II Acetabular System in Total Hip Arthroplasty." Importantly, they call attention to the manufacturing differences between the Trident II Tritanium and Trident II hydroxyapatite (HA) acetabular shells (Stryker, USA). Despite the Tritanium and HA shells being in the same Trident II acetabular system, their manufacturing process is vastly different. The radiostereometric analysis study by van der Lelij et al. further cautions against generalizing the manufacturing processes of acetabular shells within the same implant brand portfolio. [[1]van der Lelij T.J.N. Marang-van de Mheen P.J. Kaptein B.L. Koster L.A. Ljung P. Nelissen R.G.H.H. et al.Migration and clinical outcomes of a novel cementless hydroxyapatite-coated titanium acetabular shell: two-year follow-up of a randomized controlled trial using radiostereometric analysis.Bone Joint J. 2024; 106-B: 136-143https://doi.org/10.1302/0301-620X.106B2.BJJ-2023-0862.R1Crossref Scopus (0) Google Scholar] While likely not intentional, manufacturers' lack of disclosure of the differences in manufacturing processes and mechanical properties of similar devices within the same brand portfolio may lead to erroneous assumptions by clinicians and researchers. The 2019 Trident II acetabular system design rational document by Stryker, USA, was used as a reference during the preparation of our original manuscript. [[2]Stryker. Trident II acetabular system design rationale 2019.https://www.stryker.com/us/en/joint-replacement/products/trident-ii.htmlDate accessed: October 10, 2021Google Scholar] When reviewing the design rational document, additive manufacturing is repeatedly mentioned and promoted; however, there is no mention of forging and machine finishing for the HA shells. This would lead to the flawed assumption that all the Trident II shells were produced using additive manufacturing. Of note, at the time of publication, the design rational document had been removed from the manufacturer's website. While van der Lelij et al. criticize our manuscript in their radiostereometric analysis study for our assumption that a uniform manufacturing process was used across the Trident II line, the focus of our manuscript was not a formal mechanical analysis of the shell. Rather, we sought to call attention to a mode of failure in the Trident II shell and to inspire future investigations, which it did. We decided to pursue publication for our case series because failures were occurring that were previously undocumented, and the implications that could occur from these failures could result in catastrophic failure or patient injury. It is with help from the orthopaedic community that we can further our understanding of implant survivorship, and we again commend van der Lelij et al. for clarifying the manufacturing process used in the different acetabular shells. We hope the Letter to the Editor by van der Lelij et al. serves to enhance and not detract from the primary focus of our manuscript to demonstrate failure of the screw/shell interface in the Trident II acetabular system. The authors declare there are no conflicts of interest. For full disclosure statements refer to https://doi.org/10.1016/j.artd.2024.101367. Download .pdf (.48 MB) Help with pdf files Conflict of Interest Statement for Cochran Download .pdf (.03 MB) Help with pdf files Conflict of Interest Statement for Ulrich Download .pdf (.16 MB) Help with pdf files Conflict of Interest Statement for Zondervan