Accuracy of Producing 3D Printed Models From CT Segmentation Renderings of Cadaveric Ankle and Foot Bones and the Kinematics of Loaded Joints in the Ankle and Foot

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Lee, Bonnie Sze Pui
Ankle and foot bones , Rapid-prototype printing , Segmentations , Ankle arthritis , Total ankle arthroplasty , Cadaveric , Plastic models , Kinematics , Computed tomography , Fluoroscopy
Introduction: Total ankle arthroplasty is a relatively new and underdeveloped treatment for ankle arthritis that has disappointing results due to complications from malpositioning and improper sizing of the prosthesis during surgery as well as a lack of knowledge on the kinematics of the ankle and foot. The purpose of this study was two-fold: Part 1 analyzed the accuracy of producing three-dimensional (3D) printed models from computed tomography (CT) segmentation renderings and Part 2 examined the kinematics of the loaded joints in the ankle and foot. Part 1: Methods: CT images were taken from four cadaveric lower limbs and the ankle and foot bones were segmented into 3D virtual models and printed plastic models. A three-way analysis was performed between the CT segmentations, printed models and cadaveric bones. Results: Sub-millimetre accuracy was achieved for the analyses although the CT segmentations were consistently slightly larger than the printed models or cadaveric bones. Part 2: Methods: 3D fluoroscopic images were taken of four loaded cadaveric lower limbs in the plantarflexed, neutral and dorsiflexed positions. The images were segmented to produce 3D virtual models and the translation as well as rotation of the ankle, subtalar and talonavicular joints were analyzed. Results: The ankle joint had little translation (0.4mm-2.5mm) but rotation of the tibiofibular mortise was great in the anterior/posterior (7.2°-51.9°), internal/external (2.1°-24.8°) and medial/lateral (2.2°-31.9°) directions. The talonavicular joint displayed the most translation (0.2mm-7.4mm) and had moderate rotation. The subtalar and talonavicular joints translated and rotated the most during loading of a foot in neutral position indicating that loading induces a significant amount of movement between bones. Conclusions: This study found that 3D models of the ankle and foot bones could be successfully produced from their CT images with sub-millimetre accuracy and should be used for pre-operative planning and during surgeries. This study also reported more translation and rotation in the ankle and foot joints than allowed in current total ankle arthroplasties. As well, the simple action of loading a foot induces a lot of movement in the joints. This kinematic data is valuable to future revisions and designs of total ankle arthroplasty implants.
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