Abstract
Background and Aims
Bone disease after kidney transplantation is an issue of growing concern, as prolonged graft survival and older age of recipients necessitate focus on long-term health burdens such as osteoporosis and fractures. Pre-existing type of renal osteodystrophy, post-transplant immunosuppressive treatment, and de novo disturbances of mineral metabolism all contribute to bone disease in kidney transplant recipients. The current pattern of renal osteodystrophy after kidney transplantation is not well characterized. This study reports histomorphometric findings of protocolled bone biopsies in a large cohort of kidney transplant recipients 1 year post-transplant.
Method
Histomorphometric analysis of transiliac bone biopsies with prior tetracycline labelling was performed in 141 kidney transplant recipients. Biochemical measurements included bioactive parathyroid hormone (PTH), total calcium, phosphate, calcidiol, bicarbonate, and sclerostin. Kruskal-Wallis and Wilcoxon signed rank tests were used to evaluate differences across categories and between groups, respectively. Stepwise multivariate linear regression was performed to identify key demographic and biochemical determinants of bone turnover (bone formation rate, BFR), mineralization (mineralization lag time, Mlt), and volume (Bone area, BAr).
Results
Mean age was 57±11 years, 71% were men, and all were Caucasian. Mean eGFR was 49±16 (range 19 to 106) ml/min/1.73 m². Hyperparathyroidism (PTH > 1.5xUNL) was seen in 48%, hypercalcemia (>10.3 mg/dL) in 18%, hypophosphatemia (<2.3 mg/dl) in 12%, and vitamin D deficiency (<15 ng/mL) in 4% of patients.
Categorization of bone turnover, mineralization, and volume is shown in Figure 1. Bone turnover was normal in the vast majority (71%). Patients with low turnover (26%) had received a higher cumulative steroid dose (2.78 vs 2.34g in low vs non-low turnover; p=0.02).
Patients with delayed mineralization (16%) were younger (52 vs 58 yrs, p=0.02) and had received a higher cumulative steroid dose (2.85 vs 2.36g, p=0.003). They had higher levels of PTH (124 vs 53 ng/L, p<0.001), and lower levels of phosphate (2.68 vs 3.18 mg/dL, p<0.001), calcidiol (29 vs 37ug/L, p=0.02), bicarbonate (21.3 vs 23.3 mmol/L, p=0.004), and sclerostin (493 vs 594 pg/mL, p=0.03) compared to patients with normal mineralization. Patients with low bone volume tended to be older (61 vs 56 years, p=0.07).
Independent determinants of BFR were PTH (β=0.68, p<0.001) and cumulative steroid dose (β = -0.22, p=0.02). Determinants of Mlt were phosphate (β=-0.48, p=0.001) and cumulative steroid dose (β=0.18, p=0.004), and determinants of BAr were age (β=-0.15, p=0.002), and BMI (β=0.33, p=0.002).
Conclusion
Bone turnover is normal in the majority of kidney transplant recipients at 1 year post-transplant, despite a high prevalence of hyperparathyroidism. Low levels of bicarbonate, phosphate, and calcidiol may contribute to delayed bone mineralization in kidney transplant recipients.
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