Exploring the Association Between Platelet Count, the Systemic Immune Inflammation Index, and Fracture Risk in Postmenopausal Women with Osteoporosis: A Cross-Sectional Study

Background/Objectives: Platelets play a role in bone metabolism and fracture healing. This study aimed to investigate the association between platelet indices and the derived systemic immune inflammation index (SII) with fracture risk in postmenopausal women. Methods: Platelet count, mean platelet volume, platelet distribution width (PDW), platelet crit, percentage of large platelets (P-LCR), platelet–lymphocyte ratio, and the SII, calculated as (NxP)/L, where N, P, and L represented neutrophils, platelets and lymphocytes counts, respectively, were evaluated. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry. Results: A total of 124 women (mean age 68.4 ± 9 years) were stratified into two groups based on the median platelet count; the “lower platelet count group” (n = 58) had a count of 200,000 (174,000 to 226,000), while the “higher platelet count group” (n = 66) had a count of 281,500 (256,500 to 308,500). The higher platelet count group showed a higher hip fracture risk (7.4 vs. 4.5%, p = 0.08) and lower lumbar spine BMD (0.773 vs. 0.83 gr/cm2, p = 0.03). By dividing the participants into two groups with higher SSI (950,848.6 ± 746,097.99) (n = 61) and lower SII (355,751.2 ± 88,662.6) (n = 63), the group with the higher SII showed the higher hip fracture risk (7.4 vs. 3.6%, p = 0.01). Univariate regression analysis revealed correlations between chronological age and PDW (r = 0.188, p = 0.047), and P-LCR (r = 0.208, p = 0.03), as well as associations between vitamin D status and P-LCR (r = −0.301, p = 0.034), and between SII and hip fracture risk (r = 0.12, p = 0.007). Conclusions: Platelet count and SII were associated with fracture risk in postmenopausal women undergoing osteoporosis assessment. Given their reproducibility and cost-effectiveness, these markers warrant further investigation in future prospective studies focused on bone fragility.