Abstract: Objective: To investigate the impact of the orthopedic metal artifact reduction (O-MAR) technique on the measurement of quantitative parameters across various tissues in spectral CT images and define the intrinsic algorithmic bias in a metal-free background. Methods: A CIRS 062M electron density phantom was scanned repeatedly (n = 10) using a Philips IQon Spectral CT. Quantitative results for eight tissue-simulating inserts (inspiratory lung, expiratory lung, adipose, breast, muscle, liver, trabecular bone, and dense bone) were analyzed with and without the O-MAR algorithm. Reconstructed images included conventional polyenergetic images (Con), calcium-suppressed (CaSupp) images (with suppression indices of 25, 50, and 75), effective atomic number (Z_eff) maps, and electron density (ED) maps. Measurements without O-MAR were used as the reference, and Bland–Altman analysis was performed to quantify the mean bias and 95% limits of agreement (95% LoA), supplemented by paired t-tests to evaluate statistical differences. Results: The impact of O-MAR on quantitative measurements exhibited marked tissue specificity: ① Consistency Evaluation: Inspiratory lung, adipose, and breast tissues demonstrated high consistency with minimal bias. In contrast, the dense bone tissue showed significant fluctuations, particularly in CaSupp 75 images, where the mean bias reached 2.05 HU and 95% LoA was substantially wider than that of other tissues. ② Difference Testing: Measurements for inspiratory lung, muscle, and adipose tissues showed significant differences across most image types (P < 0.05). The dense bone tissue was significantly affected in Con and CaSupp 50/75 images (P < 0.05). Conversely, Z_eff and ED maps exhibited the highest numerical stability. Conclusion: In a metal-free background, the O-MAR algorithm introduced tissue-specific systematic quantitative biases, with significant impacts on extremely low-density and high-density tissues. However, quantitative parameters in the Z_eff and ED maps remained stable.