This bachelor’s thesis deals with the development, production, and clinical validation of a modular contrast radiographic ruler for pediatric orthopedics using 3D printing technology. The thesis addresses the issue of radiographic magnification during X-ray imaging, which prevents the accurate measurement of bone lengths and deformities without the use of a reference calibration tool. The aim was to create a cost-effective, mechanically durable, and non-toxic alternative to commonly available commercial rulers, which often contain toxic lead or are prone to mechanical damage. Fused Deposition Modeling technology and the composite filament Prusament PETG Tungsten 75% were chosen for the prototype production. The work describes the iterative structural design of the modular system utilizing a protective mask and the optimization of multi-material printing parameters. An important part of the thesis is the microscopic and elemental analysis of the composite using Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy methods, which examined the distribution of tungsten particles and the quality of layer fusion, and helped to identify and eliminate printing defects at the material interface. The designed prototypes and calibration phantoms were subsequently tested experimentally and clinically. Clinical trials demonstrated the excellent shielding properties of the material even in thin layers and confirmed that the developed 3D-printed ruler is fully comparable to commercial alternatives, thus representing a safe and functional solution for real clinical practice.