Cortical and trabecular bone in pediatric end-stage kidney disease

Cortical bone density by quantitative computed tomography mirrors disorders of bone structure in bone biopsy of non-dialysis CKD patients

Bone biopsy is still the gold standard tool to evaluate either trabecular or cortical bone, though the quantitative computed tomography of the vertebrae (QCT), a non-invasive technique, could be useful to evaluate bone structure in patients with chronic kidney disease (CKD). Cortical bone microstructure derangements have been associated with poor outcomes in the general population. An association between trabecular bone density, assessed by QCT, and bone volume and microarchitecture by histomorphometry, has been previously documented. This relationship has not yet been fully evaluated in cortical bone in the CKD scenario. The aim of this study was to evaluate the relationship among vertebrae density measured by QCT, structural histomorphometric parameters of cortical bone and biochemical and hormonal data in 50 CKD stage 2-5ND patients. This was a post hoc analysis of a cross-sectional study where cortical porosity and cortical thickness were analyzed in undecalcified bone samples from the iliac crest. The cortical bone density was obtained by QCT from the thoracic vertebrae. The patients were 52 ± 10 years, 68% men, 30% diabetes and the estimated glomerular filtration rate 34 ± 16 mL/min/1.73 m2. Cortical porosity was 4.6% (3.6; 6.6) and cortical thickness was 578.4 ± 151.8 μm, while cortical bone density was 149.2 ± 58.3 HU. Cortical density correlated with cortical thickness (p = 0.001) but not with cortical porosity (p = 0.30). Higher porosity was associated with older age (p = 0.02), higher levels of PTH (p = 0.04) and lower renal function (p = 0.03), while smaller thickness was associated with higher levels of PTH (p = 0.02). Lower density was associated with older age (p = 0.02) and higher levels of PTH (p = 0.01). In conclusion, cortical bone density measured by QCT was able to mirror the cortical thickness of bone biopsy in pre-dialysis CKD patients. In addition, PTH action on cortical bone can be already seen in this population.

Serum PTH, PTH1R/ATF4 pathway, and the sRANKL/OPG system in bone as a new link between bone growth, cross-sectional geometry, and strength in young rats with experimental chronic kidney disease

The progression of chronic kidney disease (CKD) in children is associated with deregulated parathyroid hormone (PTH), growth retardation, and low bone accrual. PTH can cause both catabolic and anabolic impact on bone, and the activating transcription factor 4 (ATF4), a downstream target gene of PTH, is related to its anabolic effect. Osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL) are PTH-dependent cytokines, which may play an important role in the regulation of bone remodeling. This study aimed to evaluate the impact of endogenous PTH and the bone RANKL/OPG system on bone growth, cross-sectional geometry and strength utilizing young, nephrectomized rats. The parameters of cross-sectional geometry were significantly elevated in rats with CKD during the three-month experimental period compared with the controls, and they were strongly associated with serum PTH levels and the expression of parathyroid hormone 1 receptor (PTH1R)/ATF4 genes in bone. Low bone soluble RANKL (sRANKL) levels and sRANKL/OPG ratios were also positively correlated with cross-sectional bone geometry and femoral length. Moreover, the analyzed geometric parameters were strongly related to the biomechanical properties of femoral diaphysis. In summary, the mild increase in endogenous PTH, its anabolic PTH1R/ATF4 axis and PTH-dependent alterations in the bone RANKL/OPG system may be one of the possible mechanisms responsible for the favorable impact on bone growth, cross-sectional geometry and strength in young rats with experimental CKD.

Bone evaluation in paediatric chronic kidney disease: clinical practice points from the European Society for Paediatric Nephrology CKD-MBD and Dialysis working groups and CKD-MBD working group of the ERA-EDTA

Mineral and bone disorder (MBD) is widely prevalent in children with chronic kidney disease (CKD) and is associated with significant morbidity. CKD may cause disturbances in bone remodelling/modelling, which are more pronounced in the growing skeleton, manifesting as short stature, bone pain and deformities, fractures, slipped epiphyses and ectopic calcifications. Although assessment of bone health is a key element in the clinical care of children with CKD, it remains a major challenge for physicians. On the one hand, bone biopsy with histomorphometry is the gold standard for assessing bone health, but it is expensive, invasive and requires expertise in the interpretation of bone histology. On the other hand, currently available non-invasive measures, including dual-energy X-ray absorptiometry and biomarkers of bone formation/resorption, are affected by growth and pubertal status and have limited sensitivity and specificity in predicting changes in bone turnover and mineralization. In the absence of high-quality evidence, there are wide variations in clinical practice in the diagnosis and management of CKD-MBD in childhood. We present clinical practice points (CPPs) on the assessment of bone disease in children with CKD Stages 2-5 and on dialysis based on the best available evidence and consensus of experts from the CKD-MBD and Dialysis working groups of the European Society for Paediatric Nephrology and the CKD-MBD working group of the European Renal Association-European Dialysis and Transplant Association. These CPPs should be carefully considered by treating physicians and adapted to individual patients' needs as appropriate. Further areas for research are suggested.

Racial differences in bone histomorphometry in children and young adults treated with dialysis

BACKGROUND

Healthy African-Americans are known to have greater bone mineral density and decreased risk of fracture when compared to Caucasians. In fact, comparisons of bone histomorphometry in healthy South African children and adults reveal greater cortical thickness in Black subjects as compared to White. How these differences are reflected in the bone of American children and young adults on dialysis is unknown.

METHODS

Using tetracycline-labeled, iliac crest bone biopsies obtained in prior research protocols in pediatric and young adult dialysis patients, we compared trabecular and cortical parameters between non-Hispanic African-American subjects and non-Hispanic Caucasian subjects matched by age and gender. A linear regression model controlled for trabecular turnover and mineralization was used to further investigate the association of race with cortical thickness.

RESULTS

The matched cohort consisted of 52 subjects-26 African-American and 26 Caucasian. Turnover, mineralization and volume parameters in trabecular bone did not show significant differences between racial groups. Characterizing subjects by renal osteodystrophy type did not show a statistically significant difference although Caucasian patients had double the prevalence of mineralization defects. Consistent with this was a trend toward better mineralization parameters in African-Americans including shorter osteoid maturation time and lower osteoid volume. A sub-cohort of patients with cortical measures demonstrated greater median (IQR) cortical thickness in African-Americans (541 μm [354, 694]) than in Caucasians (371 μm [336, 446], p = 0.08). In a linear regression model controlling for trabecular turnover and mineralization, African-American subjects had 36.2% (95% CI 0.28 to 85.1%, p = 0.048) greater cortical thickness as compared to White subjects. There was no significant difference in cortical porosity.

CONCLUSIONS

Although likely limited by sample size, our findings suggest that, similar to findings in populations of normal children, African-American race in pediatric and young adults on dialysis is associated with greater cortical thickness. Additionally, there was a trend toward greater mineralization defects in Caucasian children. Both findings require further exploration with larger patient samples in order to thoroughly explore these racial differences and the implications on CKD-MBD treatment.

Pediatric bone evaluation with HR-pQCT: A comparison between standard and height-adjusted positioning protocols in a cohort of teenagers with chronic kidney disease

BACKGROUND

High-resolution peripheral quantitative computed tomography (HR-pQCT) evaluates different components of bone fragility. The positioning and length of the region of interest (ROI) in growing populations remain to be defined.

METHODS

Using HR-pQCT at the ultradistal tibia, we compared a single-center cohort of 28 teenagers with chronic kidney disease (CKD) at a median age of 13.6 (range, 10.2-19.9) years to local age-, gender-, and puberty-matched healthy peers. Because of the potential impact of short stature, bone parameters were assessed on two different leg-length-adjusted ROIs in comparison to the standard analysis, namely the one applied in adults. The results are presented as median (range).

RESULTS

After matching, SDS height was -0.9 (-3.3;1.6) and 0.3 (-1.4;2.0) in patients and controls, respectively (P<0.001). In younger children (e.g., prepubertal, n=11), bone texture parameters and bone strength were not different using standard analysis. However, using a height-adjusted ROI enabled better characterization of cortical bone structure. In older patients (e.g., pubertal, n=17), there were no differences for height between patients and controls: with the standard evaluation, cortical bone area and cortical thickness were significantly lower in CKD patients: 85 (50-124) vs. 108 (67-154) mm² and 0.89 (0.46-1.31) vs 1.09 (0.60-1.62) mm, respectively (both P<0.05).

CONCLUSIONS

Adapting the ROI to leg length enables better assessment of bone structure, especially when height discrepancies exist between controls and patients. Larger cohorts are required to prospectively validate this analytic HR-pQCT technique.

Cortical bone analysis in a predialysis population: a comparison with a dialysis population

This study was designed to characterize cortical bone by histomorphometry in a predialysis population, to correlate turnover, mineralization, and volume between cortical and trabecular bone, and to compare the findings with those in patients treated with maintenance dialysis. We evaluated cortical bone by histomorphometry in 16 patients with stage 3 or stage 4 chronic kidney disease and in 16 dialysis patients. Dual-energy X-ray absorptiometry (DXA) of the distal end of the forearm was performed in seven predialysis patients, and the findings correlated with histologic parameters. Predialysis patients compared with dialysis patients showed increased cortical bone thickness (p = 0.027) and decreased osteonal bone formation rate (p = 0.020) and adjusted apposition rate (p = 0.018), mainly for external cortical bone. In this predialysis population, trabecular bone volume positively correlated with external cortical porosity (r = 0.723, p = 0.003), external cortical thickness (r = 0.569, p = 0.034), and external osteonal accumulation (osteonal osteoid thickness, r = 0.530, p = 0.05; osteonal osteoid volume to bone volume ratio, r = 0.921, p < 0.001; and osteonal osteoid surface to bone surface ratio, r = 0.631, p = 0.016). These correlations were not observed in the internal cortical bone. Cortical osteonal mineralization surface negatively correlated with DXA Z and T scores and bone mineral density for the distal end of the forearm. The osteonal bone formation rate of both internal cortical bone and external cortical bone correlated with Z score (r = -0.975, p = 0.005 and r = -0.880, p = 0.021 respectively). We found no significant correlations between cortical thickness or porosity and DXA parameters for either external cortical bone or internal cortical bone. Our results suggest that a greater degree of kidney disease is associated with thinner cortices, eventually contributing to the higher fracture rate observed in the chronic kidney disease population. In predialysis patients, parathyroid hormone seems to have a modulating effect on both trabecular bone and cortical bone, particularly in external cortical bone.

Evolution of bone disease after kidney transplantation: A prospective histomorphometric analysis of trabecular and cortical bone

AIM

Post-transplant bone disease results from multiple factors, including previous bone and mineral metabolism disturbances and effects from transplant-related medications. Bone biopsy remains the gold-standard diagnostic tool.

METHODS

We aimed to prospectively evaluate trabecular and cortical bone by histomorphometry after kidney transplantation. Seven patients, willing to perform follow-up bone biopsy, were included in the study. Dual-X-ray absorptiometry and trans-iliac bone biopsy were performed within the first 2 months after renal transplantation and repeated after 2-5 years of follow-up.

RESULTS

Follow-up biopsy revealed a significant decrease in osteoblast surface/bone surface (0.91 ± 0.81 to 0.47 ± 0.12%, P = 0.036), osteoblasts number/bone surface (0.45 (0.23, 0.94) to 0.00/mm(2) , P = 0.018) and erosion surface/bone surface (3.75 ± 2.02 to 2.22 ± 1.38%, P = 0.044). A decrease in trabecular number (3.55 (1.81, 2.89) to 1.55/mm (1.24, 2.06), P = 0.018) and increase in trabecular separation (351.65 ± 135.04 to 541.79 ± 151.91 μm, P = 0.024) in follow-up biopsy suggest loss in bone quantity. We found no significant differences in cortical analysis, except a reduction in external cortical osteonal eroded surface (5.76 (2.94, 13.97) to 3.29% (0.00, 6.67), P = 0.043). Correlations between bone histomorphometric and dual-X-ray absorptiometry parameters gave inconsistent results.

CONCLUSIONS

The results show a reduction in bone activity, suggesting increased risk of adynamic bone and loss of bone volume. Cortical bone seems less affected by post-transplant biological changes in the first years after kidney transplantation.

The role of bone biopsy for the diagnosis of renal osteodystrophy: a short overview and future perspectives

Chronic kidney disease (CKD) patients present specific bone and mineral metabolism disturbances, which account for important morbidity and mortality. The term renal osteodystrophy, classically used for the nomination of CKD-associated bone disorder, has been limited to the histologic description of bone lesions, requiring the use of bone biopsy. Biochemical markers and imaging tools do not adequately predict the complex bone changes that are observed in renal osteodystrophy. Parathyroid hormone, which is a universally used biomarker of bone turnover in clinical practice, lacks specificity and sensitivity. Therefore, tetracycline double-labelled transiliac bone biopsy, with bone histology and histomorphometric evaluation, remains the best clinical tool to discriminate bone turnover and to evaluate the other dimensions of renal osteodystrophy. This review will focus on the value of classic bone histomorphometric analysis of trabecular bone in CKD patients and unfold new perspectives of this diagnostic tool, including cortical bone evaluation and bone tissue immunohistochemistry.

Fractures in chronic kidney disease: pursuing the best screening and management

PURPOSE OF REVIEW

Osteoporotic fractures are common and cause increased sickness and death. Men and women with chronic kidney disease (CKD) are at particularly high risk of osteoporotic fractures. Currently, however, there are no guidelines concerning noninvasive methods to assess fracture risk in CKD. Further, approved treatments to prevent fractures in otherwise healthy men and women are only recommended for use with caution in those with CKD. This review focuses on the recent data that support the use of noninvasive methods to assess fracture risk in CKD and highlights new therapies that could be used in fracture prevention in CKD.

RECENT FINDINGS

Data from prospective studies demonstrate that low bone mineral density predicts fracture in CKD patients. Post-hoc analyses demonstrate that agents approved for the treatment of postmenopausal osteoporosis (bisphosphonates, denosumab and teriparatide) when given to those with CKD are well tolerated and potentially efficacious with respect to fracture risk reduction.

SUMMARY

To date, patients, and nephrologists taking care of them, have largely ignored fracture risk assessment and treatment in CKD. This should change given recent data. Further studies are needed, specifically bone histomorphometric studies, which will increase our understanding of CKD-mineral bone disease (MBD) pathophysiology, and randomized clinical trials of therapy in patients with CKD.