An exploratory study on the influence of CT slice spacing on the positioning accuracy of 3D-printed patient-specific instruments designed for a distal radius osteotomy

Abstract Background Computed tomography (CT) images with slice spacing up to 3 mm are generally sufficient for accurate 3D reconstructions of patient anatomy. For designing patient-specific instruments, particularly for forearm and wrist osteotomies, higher-resolution imaging (slice spacing of 0.4–1.25 mm) is commonly used. However, it remains unclear whether CT slice spacing meaningfully affects the surface fit of 3D-printed patient-specific instruments. This in-vitro study quantified the impact of CT slice spacing on the positional accuracy of instruments for the distal radius. Methods Data from 12 patients were analysed. Each had a high-resolution CT image (0.625 mm slice spacing), from which a reference 3D radius model and a patient-specific instrument were designed. The original images were downsampled to slice spacings of 1.0, 1.25, 1.5, 1.875, and 2.5 mm to generate corresponding radius models and instrument bases. An experienced hand surgeon, blinded to slice spacing, fixated each 3D-printed instrument base onto its reference 3D-printed radius model in a simulated surgery. The assembled constructs were scanned, and the translation and rotation deviations of each lower-resolution instrument position were measured relative to the gold-standard instrument from the original scan. Results Mean rotational deviations across all slice spacings remained consistently low (< 1°), with the largest of 0.56° around the Z-axis at 2.5 mm. Translational deviations were similarly small (< 0.2 mm), with the greatest mean deviation of − 0.14 mm along the Z-axis at 1 mm. No systematic trend toward increasing error with decreasing image resolution was observed. Conclusion CT slice spacing had a generally small effect on the positional accuracy of 3D-printed patient-specific instruments for the distal radius in vitro. These results indicate that clinically acceptable instrument accuracy may be achieved across a broad range of CT slice spacing commonly used in musculoskeletal imaging, potentially reducing the need for high-resolution protocols in selected clinical scenarios.