# Personalised High Tibal Osteotomy Surgery Is Accurate: An Assessment Using 3D Distance Mapping
## 2 Materials and Methods
### Imaging, Surgical Planning and Procedure  
Within a four-month window prior to surgery (condition PRE), each patient underwent medical imaging to enable digital surgical planning. This included frontal bi-pedal standing long-leg radiographs, and weight-bearing CT (WBCT) scans of the affected knee in single leg upright stance using a cone-beam CT scanner (OnSight 3D Extremity, Carestream, Rochester, NY, USA). This imaging device uses a rotating X-ray source with three emitters, capturing accurate images with a wide field of view and allowing a volumetric acquisition with a single source rotation (current and energy: 5 mA and 80 kVp; dose: 0.024 mSv; rotation and exposure time: 25 s and 21 s; scan rotation angle: 216\({}^{\circ}\); field-of-view: 216 \(\times\) 216 mm; pixel pitch: 0.13 mm; matrix: 884 \(\times\) 884; scanning slice thickness: 0.26 mm; voxel size: 0.26 isotropic) [45], with an extremely low radiation dose [46; 47]. Relevant CT scan images were then segmented (Mimics Innovation Suite version 22.0, Materialise, Leuven, Belgium) to generate a 3D geometrical model of the intact (i.e., PRE or before the osteotomy surgery) proximal tibial bone surface in STereo-Lithography-interface (STL) format. To generate each 3D bone model, semi-automatic density-based segmentation was used, with cortical bone density set at 400 \(\pm\) 100 HU. The segmented mesh was processed to remove holes resulting in a closed surface mesh. This process relied on three sequential steps: (a) selection of the outer faces by a linked surface faces approach; (b) surface reconstruction using a screened Poisson approach; (c) isotropic re-meshing with a target triangle edge length of 0.8 mm. The PRE model for the tibial bone was subsequently used in the surgical planning stage (i.e., PLA) for the personalised HTO. Note that in the generation of the PLA model, the segmentation software can apply additional iterations of smoothing and decimation of the mesh according to predefined parameters for 'high' export quality to have an optimal reference bone surface in surgical planning. This stage included the definition of the virtual position and orientation of the proximal tibia osteotomy (Figure 1), including position and inclination of the associated fixation screws, as well as the simulation of the amount of wedge opening about the hinge [33; 36]. Using appropriate software for computer-aided modelling and designing (Rhinoceros, Rhino 7 version, Robert McNeel & Associates, Seattle, WA, USA; Geomagic Freeform, 3D Systems, Rock Hill, SC, USA), the PLA geometrical models of the virtual proximal tibial surface after wedge opening and of the corresponding custom surgical guide, as well as of the personalised plate to fix the osteotomy, were designed and generated in STL file format. These PLA models of the guide and plate were then 3D printed (TOKA Personalised HTO, Orthoscape, 3D Metal Printing Ltd., Bath, UK; 3D printer: AM250, Renishaw plc, Wotton-under-Edge, UK) in medical-grade titanium alloy (Ti6 AL4V, ASTM F136 grade 23), delivered to the hospital, and made available for the surgery after steam sterilization [32]. All surgeries were performed by a single surgeon. In addition to the surgical steps of a standard HTO, the present personalised procedure [32] included the fixation of the patient-specific surgical guide to perform the bone osteotomy, the drilling of bone holes to accommodate the screws for the fixation plate, and also the osteotomy opening mechanism, all according to the preoperative planning. During surgery, once the plate was fixed, the resulting wedge-shaped opening of the proximal tibia was filled with bone allograft from the institutional bone bank, followed by the final suture. After surgery, returning to full normal activity was recommended by following a standard rehabilitation program. Postoperatively at six months follow-up, the same preoperative imaging procedure was repeated.