# Personalised High Tibal Osteotomy Surgery Is Accurate: An Assessment Using 3D Distance Mapping
## 1 Introduction
With the coming together of advances in three-dimensional (3D) digital planning and metal additive manufacturing, personalised, or patient specific, HTO has now become a practical reality [31; 32]. Based on these technologies, personalised surgical procedures offer the potential for simplified surgical techniques and increased correction accuracy. In silico trials have demonstrated the safety of 3D-printed personalised HTO, comparable to generic off-the-shelf devices [33], and excellent functional recovery was observed in a preliminary clinical trial [34] on patients treated using a novel fully personalised HTO procedure [32]. This technique uses 3D morphological reconstructions of the patient's knee anatomy from medical imaging to perform customised planning. The final surgical plan generates the design of the surgical instrumentation, i.e., the surgical guide for the osteotomy, and of the fixation plate. The surgical guide and plate are manufactured by 3D printing in medical-grade titanium alloy (Ti6 AL4V) powder. This new surgical procedure overcomes the limitations of traditional HTO techniques, which use generic off-the-shelf plates and can have lower correction accuracy with a negative impact on patient outcomes [31; 35]. To date, the accuracy of this new personalised HTO overall procedure has been evaluated in the preclinical phase and a first-in-human clinical trial,showing better performance than that reported for conventional techniques, especially in terms of postoperative achievement of the preoperative surgical planning [36; 37]. The most used clinical measure of accuracy is the hip-knee-ankle (HKA) angle measured from long-leg radiographs, comparing the achieved change in HKA to the planned change in HKA. The HKA angle change is achieved by changing the orientation of the tibial plateau with the surgical osteotomy. In the personalised procedure, a 3D geometric model is created from the surgical planning process, giving the planned tibial correction and the planned location of the plate. By analysing patient-specific 3D models reconstructed from computed tomography (CT) imaging performed PREoperatively (PRE) and POSToperatively (POST), and the 3D models resulting from the surgical PLAnning process (PLA), the effects of the surgery and the accuracy of achieving the planned osteotomy and plate location can be quantified by distribution of distances between the surfaces of appropriately selected reference and test models. Distance map analysis (DMA), which has been previously utilised in similar contexts [38; 39], can offer a valuable support to these evaluations. Of note, image analysis and segmentation techniques, the related reconstruction of 3D bone geometry, and the identification of anatomical landmarks require special attention and must be approached with care, as much of the accuracy in data processing depends primarily on them [40; 41]. The purpose of the present study was to evaluate the overall 3D accuracy of this new surgical procedure for personalised HTO, using modern imaging techniques, 3D modelling, and DMA. The present analysis is part of the first-in-human clinical trial for this procedure.