Bone-Preserving Robotic Conversion of Medial UKA to TKA: A Step-by-Step Technique

Converting a medial unicompartmental knee arthroplasty (UKA) to total knee arthroplasty (TKA) presents challenges in bone preservation, alignment, and soft-tissue balance. Robotic assistance enables three-dimensional CT-based planning, precise bone preparation, and real-time ligament balance assessment, thereby supporting a bone-preserving approach. We describe a stepwise workflow using the Mako system (Stryker) to convert a failed medial UKA to a condylar-stabilizing (CS) Triathlon TKA within a functional alignment framework. Pre-explantation registration on the in situ components maintains accuracy despite potential metallic artifacts. The polyethylene insert is briefly removed solely to access reference surfaces, then registration landmarks are acquired directly on the metal components along the marked central sagittal axis. The insert is reinserted to enable dynamic intraoperative balance evaluation. After component removal, a small medial tibial or femoral defect can be filled with autologous cancellous bone graft from the resected bone surfaces. Definitive cementless components are implanted without stems or augments, and the patella is resurfaced. This technique provides a reproducible robotic workflow for UKA-to-TKA conversion in selected cases with preserved bone stock and stable soft-tissue balance and facilitates accurate and reproducible conversion with optimal bone preservation.