*Purpose: Bone loss and mineral abnormalities are frequent in kidney transplant recipients (KTRs) and associated with a high risk of fracture and cardiovascular mortality. In daily clinics, the detection of bone abnormalities after transplantation includes bone biomarkers and imaging technique (dual energy X-ray absorptiometry (DEXA)) to assess respectively the bone turnover and the bone mineral density (BMD), but with limitations. The high-resolution peripheral quantitative computed tomography (HR-pQCT) provides additional noninvasive information on bone microarchitecture and BMD with better distinction between cortical and trabecular area. The goal of our study is to evaluate the evolution of bone structure using HR-pQCT compared to standard technique in a prospective cohort of KTRs.

*Methods: All patients referred for a single kidney transplant at our hospital with no history exposure to antiresorptive agents were eligible for inclusion (NCT04713774). Participants underwent baseline and 3-month biomarkers analysis, BMD measurements by DXA. HR-pQCT images were obtained of the distal radius and tibia using the XtremeCT device with standard protocols. HR-pQCT assessed quantitative measurement of the volumetric density of trabecular and cortical bone as well as bone structure (trabecular number or thickness or cortical porosity).

*Results: 26 patients were prospectively included. Mean age was 57.3+/-12.1 years. Mean dialysis vintage was 27.5+/-16.4 months before transplantation. Bone biomarkers showed significant decrease at 3 months after transplantation. PTH decreased from 221.72ng/l to 59.6 ng/l (p<0.0001), P1NP from 211ug/l to 72ug/l (p<0.013), and BLAP from 23ug/l to 13ug/l (p=0.042). BMD was measured by DXA and HR-pQCT at 7 days [6; 8] and 102 days [90; 113] after transplantation. We observed a significant reduction of BMD by DXA at the hip site from 0.868g/cm² to 0.856g/cm² (p=0.02), but not at the lumbar site. The HR-pQCT analysis demonstrated a significant reduction of the trabecular BMD from 152.62 mg HA/ccm to 150.80 mg HA/ccm (p<0.0001) at tibia site and from 159.09 mg HA/ccm to 156.25 mg HA/ccm (p<0.0001) at radius site. No change in bone structed have been observed at 3 months post transplantation with the HR-pQCT analysis.

*Conclusions: HR-pQCT is sensitive enough to show a significant decrease of BMD at trabecular site, as soon as 3 months after transplantation compared to DEXA at the lumbar spine. However, no change on bone structure nor cortical bone volume has been observed. The sensibility of this technique might be higher than DEXA. The rapid assessment of bone structure (3 months post transplantation) might be too soon to evaluate such abnormalities. Detecting properly rapid changes in bone density, as soon as 3 months after renal transplantation seems feasible. The impact on bone health management needs to be further studied.

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