3D-printed saw guides for lower arm osteotomy, a comparison between a synthetic CT and CT-based workflow

Abstract

Background Three-dimensional (3D)-printed saw guides are frequently used to optimize osteotomy results and are usually designed based on computed tomography (CT), despite the radiation burden, as radiation-less alternatives like magnetic resonance imaging (MRI) have inferior bone visualization capabilities. This study investigated the usability of MR-based synthetic-CT (sCT), a novel radiation-less bone visualization technique for 3D planning and design of patient-specific saw guides. Methods Eight human cadaveric lower arms (mean age: 78y) received MRI and CT scans as well as high-resolution micro-CT. From the MRI scans, sCT were generated using a conditional generative adversarial network. Digital 3D bone surface models based on the sCT and general CT were compared to the surface model from the micro-CT that was used as ground truth for image resolution. From both the sCT and CT digital bone models saw guides were designed and 3D-printed in nylon for one proximal and one distal bone position for each radius and ulna. Six blinded observers placed these saw guides as accurately as possible on dissected bones. The position of each guide was assessed by optical 3D-scanning of each bone with positioned saw guide and compared to the preplanning. Eight placement errors were evaluated: three translational errors (along each axis), three rotational errors (around each axis), a total translation (∆T) and a total rotation error (∆R). Results Surface models derived from micro-CT were on average smaller than sCT and CT-based models with average differences of 0.27 ± 0.30 mm for sCT and 0.24 ± 0.12 mm for CT. No statistically significant positioning differences on the bones were found between sCT- and CT-based saw guides for any axis specific translational or rotational errors nor between the ∆T (p = .284) and ∆R (p = .216). On Bland-Altman plots, the ∆T and ∆R limits of agreement (LoA) were within the inter-observer variability LoA. Conclusions This research showed a similar error for sCT and CT digital surface models when comparing to ground truth micro-CT models. Additionally, the saw guide study showed equivalent CT- and sCT-based saw guide placement errors. Therefore, MRI-based synthetic CT is a promising radiation-less alternative to CT for the creation of patient-specific osteotomy surgical saw guides.

Magnetic Resonance Imaging Versus Computed Tomography for Three‐Dimensional Bone Imaging of Musculoskeletal Pathologies: A Review

“…The geometrical accuracy of bone segmentation has been evaluated on long bones and vertebrae in ex vivo studies so that MR segmentation could be compared to the physical bone shape using 3D printing, 47 mechanical contact/optical scanners, 18,[31][32][33] or micro-CT. 31,82 Bone specimens were processed to remove soft tissues, resulting in a potential shrinkage of the gold standard compared to the bone as scanned using MRI and CT. 18,25,33,82 On average, CT segmentation overestimated the actual bone shape, whereas MR segmentation mostly underestimated it, [31][32][33]47 although not consistently. 25,82 Nevertheless, surface distances between the MR-based segmentation and the cadaveric specimen were on average submillimeter, 17,25,31,33,82 with mean absolute surface distances ranging from 0.23 mm to 0.41 mm for MR-segmentations and from 0.15 mm to 0.51 mm for CT-based segmentations. 17,25,31,33 Similarly, root mean square error (RMSE) was mainly submillimeter, 18,31,33 although it could reach 1.2 mm 32 in the knee for MRI models (vs. 0.5 mm for CT models).…”

“…Three‐dimensional bone renderings are gaining popularity in orthopedic care as they provide an overview of the bone morphology, enable kinematic analyses, 30,41 and allow for the patient‐specific design of surgical guides and implants 63,82 . Hence, 3D bone models facilitate the clinical diagnosis and improve surgical outcomes, 26,43,70,83–85 motivating their use in the treatment management of pathologies in the skull, shoulder, and hip 43,83–85 .…”

“…Larger differences were generally observed between the reference and MRI models near the joints in the proximal and distal bone ends, 31,82 although not always with statistical significance 18 . Such differences resulted from the multiple soft tissues present at these locations (muscle, tendons, cartilage, and ligaments) which induce partial volume effects that hinder bone segmentation and warrant manual editing 31,81 .…”

The Role of 3D Printing in Planning Complex Medical Procedures and Training of Medical Professionals—Cross-Sectional Multispecialty Review

“…One such recent example is the so-called synthetic CT (sCT) derived from post-processed MRI images, which is in no way inferior to standard CT. The bone models printed off sCT have been shown to have excellent agreement with ground truth based on distance mapping of the surface, and surgical cutting guide placement error (both x–y–z translation and x–y–z rotation) was also shown to be no different when compared between CT and sCT [ 176 ].…”

Synthetic CT for the planning of MR-HIFU treatment of bone metastases in pelvic and femoral bones: a feasibility study

“…In the last decade, the development of synthetic CT (sCT), i.e., deriving CT-like images from MRI scans, has enabled an MRI-based visualization of osseous tissues for radiotherapy [14] and orthopedic care [15][16][17][18]. In the MRI-HIFU context, such sCT images could be generated from MR images acquired during the treatment session to depict the bone distribution in the treatment position.…”