Significance of minimodeling in dialysis patients with adynamic bone disease

Pathophysiology of bone disease in chronic kidney disease: from basics to renal osteodystrophy and osteoporosis

Chronic kidney disease (CKD) is a highly prevalent disease that has become a public health problem. Progression of CKD is associated with serious complications, including the systemic CKD-mineral and bone disorder (CKD-MBD). Laboratory, bone and vascular abnormalities define this condition, and all have been independently related to cardiovascular disease and high mortality rates. The "old" cross-talk between kidney and bone (classically known as "renal osteodystrophies") has been recently expanded to the cardiovascular system, emphasizing the importance of the bone component of CKD-MBD. Moreover, a recently recognized higher susceptibility of patients with CKD to falls and bone fractures led to important paradigm changes in the new CKD-MBD guidelines. Evaluation of bone mineral density and the diagnosis of "osteoporosis" emerges in nephrology as a new possibility "if results will impact clinical decisions". Obviously, it is still reasonable to perform a bone biopsy if knowledge of the type of renal osteodystrophy will be clinically useful (low versus high turnover-bone disease). However, it is now considered that the inability to perform a bone biopsy may not justify withholding antiresorptive therapies to patients with high risk of fracture. This view adds to the effects of parathyroid hormone in CKD patients and the classical treatment of secondary hyperparathyroidism. The availability of new antiosteoporotic treatments bring the opportunity to come back to the basics, and the knowledge of new pathophysiological pathways [OPG/RANKL (LGR4); Wnt-ß-catenin pathway], also affected in CKD, offers great opportunities to further unravel the complex physiopathology of CKD-MBD and to improve outcomes.

Is Adynamic Bone Always a Disease? Lessons from Patients with Chronic Kidney Disease

Renal osteodystrophy (ROD) is a common complication of end-stage kidney disease that often starts early with loss of kidney function, and it is considered an integral part in management of patients with chronic kidney disease (CKD). Adynamic bone (ADB) is characterized by suppressed bone formation, low cellularity, and thin osteoid seams. There is accumulating evidence supporting increasing prevalence of ADB, particularly in early CKD. Contemporarily, it is not very clear whether it represents a true disease, an adaptive mechanism to prevent bone resorption, or just a transitional stage. Several co-players are incriminated in its pathogenesis, such as age, diabetes mellitus, malnutrition, uremic milieu, and iatrogenic factors. In the present review, we will discuss the up-to-date knowledge of the ADB and focus on its impact on bone health, fracture risk, vascular calcification, and long-term survival. Moreover, we will emphasize the proper preventive and management strategies of ADB that are pivotal issues in managing patients with CKD. It is still unclear whether ADB is always a pathologic condition or whether it can represent an adaptive process to suppress bone resorption and further bone loss. In this article, we tried to discuss this hard topic based on the available limited information in patients with CKD. More studies are needed to be able to clearly address this frequent ROD finding.

Performance Status Modifies the Association Between Vitamin D Receptor Activator and Mortality or Fracture: A Prospective Cohort Study on the Japanese Society for Dialysis Therapy (JSDT) Renal Data Registry

Immobilization osteoporosis is characterized by excess bone resorption. Vitamin D receptor activators (VDRA) might have adverse effects in immobilized patients. The objective of this study was to elucidate the impact of performance status (PS) on the associations between VDRA use and outcomes among hemodialysis patients. This is a prospective cohort study. Adults on hemodialysis in the Japanese Society for Dialysis Therapy (JSDT) Renal Data Registry were included. Exposure of interest was the use of VDRA. Outcomes were all-cause mortality and hip fracture. Associations between VDRA use and mortality or hip fractures were examined by Cox and Poisson regression analyses, respectively. Among 208,512 subjects, 128,535 were on VDRA. Poor PS was associated with higher calcium (Ca), lower parathyroid hormone, and higher alkaline phosphatase levels. The association between higher Ca levels and VDRA use was stronger among those with poor PS (p interaction 0.007). Adjusted hazard ratio (HR) (95% confidence interval [CI]) for mortality and incidence rate ratio (IRR) for hip fracture was 1.02 (95% CI, 0.99-1.05) and 0.93 (0.86-1.00) among users of VDRA, respectively. The VDRA use was associated with lower mortality and incidence of hip fractures among subjects with good PS but not among subjects with poor PS (p interaction 0.03 and 0.05). Effect modification by PS was observed for cardiovascular (CV) mortality but not for non-CV mortality. In conclusion, VDRA use was associated with better outcomes only among those with good PS. These results suggest that bone and mineral disorders among hemodialysis patients should be treated differently, depending on their performance status. © 2022 American Society for Bone and Mineral Research (ASBMR).

2D size of trabecular bone structure units (BSU) correlate more strongly with 3D architectural parameters than age in human vertebrae

Bone tissue is continuously remodeled. In trabecular bone, each remodeling transaction forms a microscopic bone structural unit (BSU), also known as a hemiosteon or a trabecular packet, which is bonded to existing tissue by osteopontin-rich cement lines. The size and shape of the BSUs are determined by the size and shape of the resorption cavity, and whether the cavity is potentially over- or under-filled by the subsequent bone formation. The present study focuses on the recently formed trabecular BSUs, and how their 2D size and shape changes with age and trabecular microstructure. The study was performed using osteopontin-immunostained frontal sections of L2 vertebrae from 8 young (aged 18.5-37.6 years) and 8 old (aged 69.1-96.4 years) control females, which underwent microcomputed tomography (μCT) imaging prior to sectioning. The contour of 4230 BSU profiles (181-385 per vertebra) within 1024 trabecular profiles were outlined, and their 2D width, length, area, and shape were assessed. Of these BSUs, 22 (0.5%) were generated by modeling-based bone formation (i.e. without prior resorption), while 99.5% were generated by remodeling-based bone formation (i.e. with prior resorption). The distributions of BSU profile width, length, and area were significantly smaller in the old versus young females (p < 0.005), and the median profile width, length, and area were negative correlated with age (p < 0.018). Importantly, these BSU profile size parameters were more strongly correlated with trabecular bone volume (BV/TV, p < 0.002) and structure model index (SMI, p < 0.008) assessed by μCT, than age. Moreover, the 2D BSU size parameters were positively correlated to the area of the individual trabecular profiles (p < 0.0001), which were significantly smaller in the old versus young females (p < 0.024). The BSU shape parameters (aspect ratio, circularity, and solidity) were not correlated with age, BV/TV, or SMI. Collectively, the study supports the notion that not only the BSU profile width, but also its length and area, are more influenced by the age-related bone loss and shift from plates to rods (SMI), than age itself. This implies that BSU profile size is mainly driven by changes in the trabecular microstructure, which affect the size of the resorption cavity that the BSU refills.

Vitamin D Supplementation for 12 Months in Older Adults Alters Regulators of Bone Metabolism but Does Not Change Wnt Signaling Pathway Markers

Vitamin D status and supplementation regulates bone metabolism and may modulate Wnt signaling. We studied the response of hormonal regulators of bone metabolism, markers of Wnt signaling and bone turnover and bone mineral density (BMD) and bone mineral content (BMC) in a randomized vitamin D intervention trial (12,000 IU, 24,000 IU, 48,000 IU/mo for 1 year; men and women aged >70 years; n = 379; ISRCTN35648481). Associations with total and free 25(OH)D concentrations were analyzed by linear regression. Baseline vitamin D status was (mean ± SD) 25(OH)D: 40.0 ± 20.1 nmol/L. Supplementation dose-dependently increased total and free 25(OH)D concentrations and decreased plasma phosphate and parathyroid hormone (PTH) (all p < 0.05). The procollagen 1 intact N-terminal (PINP)/C-terminal telopeptide (CTX) ratio, C-terminal fibroblast growth factor-23 (cFGF23), and intact FGF23 (iFGF23) significantly increased with no between-group differences, whereas Klotho was unchanged. 1,25(OH)₂D and PINP significantly increased in the 24 IU and 48,000 IU groups. Sclerostin (SOST), osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL), BMD, BMC, and CTX remained unchanged. Subgroup analyses with baseline 25(OH)D <25 nmol/L (n = 94) provided similar results. Baseline total and free 25(OH)D concentrations were positively associated with 1,25(OH)₂D, 24,25(OH)₂D (p < 0.001), vitamin D binding protein (DBP) (p < 0.05), BMD, and BMC (p < 0.05). Associations with PTH (p <0.001), cFGF23 (p < 0.01), and BAP (p < 0.05) were negative. After supplementation, total and free 25(OH)D concentrations remained positively associated only with 24,25(OH)₂D (p < 0.001) and DBP (p < 0.001) and negatively with estimated glomerular filtration rate (eGFR) (p < 0.01). PTH and SOST were significantly associated only with free 25(OH)D. There were no significant relationships with BMD and BMC after supplementation. The decrease in PTH and increase in PINP/CTX ratio suggest a protective effect of supplementation on bone metabolism, although no significant effect on BMD or pronounced changes in regulators of Wnt signaling were found. The increase in FGF23 warrants caution because of its negative association with skeletal and cardiovascular health. Associations of total and free 25(OH)D with biomarkers were similar and known positive associations between vitamin D status and BMD were confirmed. The change in associations after supplementation might suggest a threshold effect. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Interest of Bone Histomorphometry in Bone Pathophysiology Investigation: Foundation, Present, and Future

Despite the development of non-invasive methods, bone histomorphometry remains the only method to analyze bone at the tissue and cell levels. Quantitative analysis of transiliac bone sections requires strict methodologic conditions but since its foundation more 60 years ago, this methodology has progressed. Our purpose was to review the evolution of bone histomorphometry over the years and its contribution to the knowledge of bone tissue metabolism under normal and pathological conditions and the understanding of the action mechanisms of therapeutic drugs in humans. The two main applications of bone histomorphometry are the diagnosis of bone diseases and research. It is warranted for the diagnosis of mineralization defects as in osteomalacia, of other causes of osteoporosis as bone mastocytosis, or the classification of renal osteodystrophy. Bone biopsies are required in clinical trials to evaluate the safety and mechanism of action of new therapeutic agents and were applied to anti-osteoporotic agents such as bisphosphonates and denosumab, an anti-RANKL, which induces a marked reduction of the bone turnover with a consequent elongation of the mineralization period. In contrast, an increased bone turnover with an extension of the formation site is observed with teriparatide. Romosozumab, an anti-sclerostin, has a dual effect with an early increased formation and reduced resorption. Bone histomorphometric studies allow us to understand the mechanism of coupling between formation and resorption and to evaluate the respective role of bone modeling and remodeling. The adaptation of new image analysis techniques will help bone biopsy analysis in the future.

Case report: Osteomalacia due to bisphosphonate treatment in a patient on hemodialysis

Background

No publications have reported on osteomalacia in patients receiving intermittent cyclical therapy with etidronate (a bisphosphonate) and undergoing long-term hemodialysis (HD).

Case presentation

We report on a 46-year-old Japanese man admitted to our hospital for further examination of left forearm pain. Maintenance HD was started at age 24 years, and the man had been on HD since then. At age 38 years, surgical parathyroidectomy was performed for secondary hyperparathyroidism; iliac crest bone biopsy performed at the same time showed osteitis fibrosa. The active vitamin D₃ preparation calcitriol was started, and intermittent cyclical etidronate therapy was introduced 2 years later for osteoporosis. At age 45 years, the patient stopped taking calcitriol because of hypercalcemia but continued with etidronate. At age 46 years, a pseudofracture with a Looser zone occurred in the left ulna, and left femur bone biopsy revealed osteomalacia. Etidronate was discontinued, and calcitriol was restarted; open reduction and internal fixation with an angular stability plate were performed. Union of the bone was achieved 10 months after the operation. At age 49 years, a lumber bone biopsy confirmed improved bone morphometry.

Conclusions

We believe that intermittent cyclical etidronate therapy without administration of active vitamin D₃ during long-term HD might have induced osteomalacia, resulting in the ulna insufficiency fracture. Therefore, we propose that administration of active vitamin D₃ is essential to prevent osteomalacia in patients on long-term HD who are receiving bisphosphonates and have potential vitamin D₃ deficiency.

Modeling-Based Bone Formation in the Human Femoral Neck in Subjects Treated With Denosumab

Denosumab is associated with continued gains in hip and spine BMD with up to 10 years of treatment in postmenopausal women with osteoporosis. Despite potent inhibition of bone remodeling, findings in nonhuman primates suggest modeling-based bone formation (MBBF) may persist during denosumab treatment. This study assessed whether MBBF in the femoral neck (FN) is preserved in the context of inhibited remodeling in subjects receiving denosumab. This open-label study enrolled postmenopausal women with osteoporosis who had received two or more doses of denosumab (60 mg subcutaneously every 6 months [Q6M]) per standard of care and were planning elective total hip replacement (THR) owing to osteoarthritis of the hip. Transverse sections of the FN were obtained after THR and analyzed histomorphometrically. MBBF, based on fluorochrome labeling and presence of smooth cement lines, was evaluated in cancellous, endocortical, and periosteal envelopes of the FN. Histomorphometric parameters were used to assess MBBF and remodeling-based bone formation (RBBF) in denosumab-treated subjects (n = 4; mean age = 73.5 years; range, 70 to 78 years) and historical female controls (n = 11; mean age = 67.8 years; range, 62 to 80 years) obtained from the placebo group of a prior study and not treated with denosumab. All analyses were descriptive. All subjects in both groups exhibited MBBF in the periosteal envelope; in cancellous and endocortical envelopes, all denosumab-treated subjects and 81.8% of controls showed evidence of MBBF. Compared with controls, denosumab-treated subjects showed 9.4-fold and 2.0-fold higher mean values of MBBF in cancellous and endocortical envelopes, respectively, whereas RBBF mean values were 5.0-fold and 5.3-fold lower. In the periosteal envelope, MBBF and RBBF rates were similar between subjects and controls. These results demonstrate the occurrence of MBBF in the human FN and suggest that denosumab preserves MBBF while inhibiting remodeling, which may contribute to the observed continued gains in BMD over time after remodeling is maximally inhibited. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Diaphyseal femoral fracture due to severe vitamin D3 deficiency and low parathyroid hormone levels on long-term hemodialysis: a case report

INTRODUCTION

Currently, there are no reports of diaphyseal femoral fracture equivalent to atypical femoral fractures (AFFs) in patients receiving long-term hemodialysis (HD).

CASE REPORT

A 56-year-old Japanese man receiving long-term HD for 34 years was admitted to our hospital due to a delay in postoperative healing. The patient began maintenance hemodialysis at 22 years of age. The patient then underwent surgical parathyroidectomy (PTX) for secondary hyperparathyroidism at 43 years of age, which resulted in decreased levels of parathyroid hormone (PTH). Thereafter, this patient's serum 1,25(OH)₂ D₃ level was very low because active vitamin D₃ derivative was not administered. At 54 years of age, a transverse fracture of the femoral shaft equivalent to AFF occurred. Surgery with open reduction and internal fixation using intramedullary nailing was performed; however, the delay of postoperative healing continued for 16 months. A left iliac crest bone biopsy was performed and showed osteoid-like lesion and an increase of woven bone. The patient received active vitamin D₃ derivative and recombinant human PTH (1-34) derivative. Twenty-nine months after the first surgery, a reoperation was performed. Simultaneously, a right iliac crest bone biopsy was performed. Bone morphometrical improvement was confirmed. Six months after resurgery, the bone union was achieved. Severe vitamin D₃ deficiency and decreased levels of PTH may induce a higher osteoid state and an increase of woven bone, which may then attribute to the development of diaphyseal femoral fracture and impairment of postoperative bone healing. It is hypothesized that treatment with active vitamin D₃ and teriparatide acetate may be a therapeutic option via the accelerated formation of lamellar bone for refractory diaphyseal femoral fracture of long-term dialysis.

The bone remodelling cycle

The bone remodelling cycle replaces old and damaged bone and is a highly regulated, lifelong process essential for preserving bone integrity and maintaining mineral homeostasis. During the bone remodelling cycle, osteoclastic resorption is tightly coupled to osteoblastic bone formation. The remodelling cycle occurs within the basic multicellular unit and comprises five co-ordinated steps; activation, resorption, reversal, formation and termination. These steps occur simultaneously but asynchronously at multiple different locations within the skeleton. Study of rare human bone disease and animal models have helped to elucidate the cellular and molecular mechanisms that regulate the bone remodelling cycle. The key signalling pathways controlling osteoclastic bone resorption and osteoblastic bone formation are receptor activator of nuclear factor-κB (RANK)/RANK ligand/osteoprotegerin and canonical Wnt signalling. Cytokines, growth factors and prostaglandins act as paracrine regulators of the cycle, whereas endocrine regulators include parathyroid hormone, vitamin D, calcitonin, growth hormone, glucocorticoids, sex hormones, and thyroid hormone. Disruption of the bone remodelling cycle and any resulting imbalance between bone resorption and formation leads to metabolic bone disease, most commonly osteoporosis. The advances in understanding the cellular and molecular mechanisms underlying bone remodelling have also provided targets for pharmacological interventions which include antiresorptive and anabolic therapies. This review will describe the remodelling process and its regulation, discuss osteoporosis and summarize the commonest pharmacological interventions used in its management.

Remodeling- and Modeling-Based Bone Formation With Teriparatide Versus Denosumab: A Longitudinal Analysis From Baseline to 3 Months in the AVA Study

There has been renewed interest of late in the role of modeling-based formation (MBF) during osteoporosis therapy. Here we describe early effects of an established anabolic (teriparatide) versus antiresorptive (denosumab) agent on remodeling-based formation (RBF), MBF, and overflow MBF (oMBF) in human transiliac bone biopsies. Postmenopausal women with osteoporosis received subcutaneous teriparatide (n = 33, 20 μg/d) or denosumab (n = 36, 60 mg once/6 months), open-label for 6 months at 7 US and Canadian sites. Subjects received double fluorochrome labeling at baseline and before biopsy at 3 months. Sites of bone formation were designated as MBF if the underlying cement line was smooth, RBF if scalloped, and oMBF if formed over smooth cement lines adjacent to scalloped reversal lines. At baseline, mean RBF/bone surface (BS), MBF/BS, and oMBF/BS were similar between the teriparatide and denosumab groups in each bone envelope assessed (cancellous, endocortical, periosteal). All types of formation significantly increased from baseline in the cancellous and endocortical envelopes (differences p < 0.001) with teriparatide (range of changes 2.9- to 21.9-fold), as did MBF in the periosteum (p < 0.001). In contrast, all types of formation were decreased or not significantly changed with denosumab, except MBF/BS in the cancellous envelope, which increased 2.5-fold (difference p = 0.048). These data highlight mechanistic differences between these agents: all 3 types of bone formation increased significantly with teriparatide, whereas formation was predominantly decreased or not significantly changed with denosumab, except for a slight increase in MBF/BS in the cancellous envelope. © 2017 American Society for Bone and Mineral Research.

Evidence for Ongoing Modeling-Based Bone Formation in Human Femoral Head Trabeculae via Forming Minimodeling Structures: A Study in Patients with Fractures and Arthritis

Bone modeling is a biological process of bone formation that adapts bone size and shape to mechanical loads, especially during childhood and adolescence. Bone modeling in cortical bone can be easily detected using sequential radiographic images, while its assessment in trabecular bone is challenging. Here, we performed histomorphometric analysis in 21 bone specimens from biopsies collected during hip arthroplasty, and we proposed the criteria for histologically identifying an active modeling-based bone formation, which we call a "forming minimodeling structure" (FMiS). Evidence of FMiSs was found in 9 of 20 specimens (45%). In histomorphometric analysis, bone volume was significant higher in specimens displaying FMiSs compared with the specimens without these structures (BV/TV, 31.7 ± 10.2 vs. 23.1 ± 3.9%; p < 0.05). Osteoid parameters were raised in FMiS-containing bone specimens (OV/BV, 2.1 ± 1.6 vs. 0.6 ± 0.3%; p < 0.001, OS/BS, 23.6 ± 15.5 vs. 7.6 ± 4.2%; p < 0.001, and O.Th, 7.4 µm ± 2.0 vs. 5.2 ± 1.0; p < 0.05). Our results showed that the modeling-based bone formation on trabecular bone surfaces occurs even during adulthood. As FMiSs can represent histological evidence of modeling-based bone formation, understanding of this physiology in relation to bone homeostasis is crucial.

Acute and Stress-related Injuries of Bone and Cartilage: Pertinent Anatomy, Basic Biomechanics, and Imaging Perspective

Bone or cartilage, or both, are frequently injured related to either a single episode of trauma or repetitive overuse. The resulting structural damage is varied, governed by the complex macroscopic and microscopic composition of these tissues. Furthermore, the biomechanical properties of both cartilage and bone are not uniform, influenced by the precise age and activity level of the person and the specific anatomic location within the skeleton. Of the various histologic components that are found in cartilage and bone, the collagen fibers and bundles are most influential in transmitting the forces that are applied to them, explaining in large part the location and direction of the resulting internal stresses that develop within these tissues. Therefore, thorough knowledge of the anatomy, physiology, and biomechanics of normal bone and cartilage serves as a prerequisite to a full understanding of both the manner in which these tissues adapt to physiologic stresses and the patterns of tissue failure that develop under abnormal conditions. Such knowledge forms the basis for more accurate assessment of the diverse imaging features that are encountered following acute traumatic and stress-related injuries to the skeleton. (©) RSNA, 2016.

Alendronate does not prevent long bone fragility in an inactive rat model

The lack of estrogen and inactivity are both important in the pathogenesis of osteoporosis in elderly women, and there have been no appropriate rodent studies to examine the effects of common bisphosphonates on these two components separately. We compared the efficacy of alendronate (ALN) on the long bones of aged female rats, which were sedentary, estrogen deficient, or both. The rats were either forced to remain in a sitting position or allowed to walk in standard cages with or without ALN administration. The 8-week experimental period began 5 weeks after ovariectomy or sham surgery. Parameters of the hindlimb bones were determined by a three-point bending test, peripheral quantitative computed tomography, microfocus computed tomography, confocal laser Raman microspectroscopy, and dynamic histomorphometry. Regardless of ovariectomy, ALN was ineffective against the deterioration of breaking stress caused by sitting even though the trabecular bone mineral density was significantly higher in the sitting-ALN groups. Toughness was significantly deficient in the ovariectomy sitting-ALN group. This was in agreement with the bone geometry with a greater marrow space. Sitting also increased the mineral-to-matrix ratio and the carbonate-to-phosphate ratio, both indicative of aged bone. A greater loss of proteinaceous amide intensity compared with mineral intensity resulted in an increased mineral-to-matrix ratio in the presence of ALN. Sitting resulted in deficits in the quality and the geometry of cortical bone, resulting in fragility. The use of bisphosphonates, such as ALN, may provide a therapy best suited for osteoporotic individuals whose daily activity is not limited.

Histomorphometric analysis of minimodeling in the vertebrae in postmenopausal patients treated with anti-osteoporotic agents

Minimodeling is a type of focal bone formation that is characterized by the lack of precedent bone erosion by osteoclasts. Although this form of bone formation has been described for more than a decade, how anti-osteoporotic agents that are currently used in clinical practice affect the kinetics of minimodeling is not fully understood. We performed a bone morphometric analysis using human vertebral specimens collected from postmenopausal patients who underwent spinal surgery. Patients were divided into three groups according to osteoporosis medication; non-treated, Eldecalcitol (ELD, a vitamin D derivative that has recently been approved to treat patients with osteoporosis in Japan)-treated, and bisphosphonate-treated groups. Five to six patients were enrolled in each group. There was a trend toward enhanced minimodeling in ELD-treated patients and suppressed of it in bisphosphonate-treated patients compared with untreated patients. The differences of minimodeling activity between ELD-treated and bisphosphonate-treated patients were statistically significant. The present study suggests that ELD and bisphosphonates have opposite effects on minimodeling from one another, and show that minimodeling also takes place in vertebrae as has been described for the ilium and femoral head in humans.

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We aimed to evaluate the potential effects of a new vitamin D3 analog, Eldecalcitol and BPs on minimodeling in humans using surgical specimens.

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We found that minimodeling is readily observed in the humans.

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We found that patients who had been treated with Eldecalcitol prior to surgery tend to have increased minimodeling activity.

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Our data indicate that Eldecalcitol can activate minimodeling in patients with post-menopausal osteoporosis while bisphosphonates have negative or little impact on this form of bone formation.

Bone loss in chronic kidney disease: Quantity or quality?

Chronic kidney disease (CKD) patients experience bone loss and fracture because of a specific CKD-related systemic disorder known as CKD-mineral bone disorder (CKD-MBD). The bone turnover, mineralization, and volume (TMV) system describes the morphological bone lesions in renal osteodystrophy related to CKD-MBD. Bone turnover and bone volume are defined as high, normal, or low, and bone mineralization is classified as normal or abnormal. All types of bone histology related to TMV are responsible for both bone quantity and bone quality losses in CKD patients. This review focuses on current bone quantity and bone quality losses in CKD patients and finally discusses potential therapeutic measures.

Bone histomorphometry in a long-term hemodialysis patient with hypoparathyroidism and sarcoidosis

A bone biopsy specimen in a long-term hemodialysis patient with sarcoidosis coexisting with severe hypoparathyroidism has demonstrated that a persistent near physiological level of 1,25-dihydroxyvitamin D3 contributes to the preservation of bone remodeling and has the potential to retard the development of vascular calcification and atherosclerosis. Sarcoidosis-related hypercalcemia and hypoparathyroidism, which is characterized by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) overproduction, is rarely seen in hemodialysis patients. Herein, we describe a 60-year-old Japanese woman on hemodialysis for 35 years who presented with malaise and hypercalcemia. Severe hypoparathyroidism without parathyroidectomy and a preserved 1,25(OH)2D3 level were detected. Computed tomography showed bilateral axillary lymphadenopathy and minimal aortic and soft tissue calcification. The axillary node biopsy led to a definite diagnosis of sarcoidosis. A bone biopsy specimen obtained from the right iliac crest showed remodeling of normal lamellar bone with scalloped cement lines and clear double labeling by tetracycline on fluorescence microscopy. Histomorphometric analysis revealed that the bone formation rate was preserved (30.0 %/year), together with a decrease of osteoid volume (5.75 %) and fibrous volume (0 %), indicating that the patient did not have adynamic bone disease and only showed mild disease. This is the first documented case of sarcoidosis-related hypercalcemia associated with severe hypoparathyroidism in a long-term hemodialysis patient who underwent bone histomorphometry. Our findings suggest that, in hemodialysis patients with sarcoidosis coexisting with severe hypoparathyroidism, a persistent near physiological level of 1,25(OH)2D3 contributes to the preservation of bone remodeling and has the potential to retard the development of vascular calcification and atherosclerosis.

Epidemiology and mortality of hip fracture among patients on dialysis: Taiwan National Cohort Study

Chronic kidney disease increases the risk for hip fractures. Hip fractures are associated with increased mortality, decreased quality of life, and higher economic burden. To determine whether dialysis modality is associated with a higher incidence of hip fractures in patients with end-stage renal disease (ESRD), we used the Taiwan National Health Insurance Research Database to examine the records of 51,473 patients who began dialysis between 1999 and 2005. The patients were followed until death, transplantation, dialysis cessation, or 31 December 2008. The follow-up period was (mean±SD) 4.14±2.48 years. The cumulative incidence rate of hip fracture was calculated using Kaplan-Meier methods. Predictors of hip fracture were determined using Cox models. During the study period, 1903 patients had a hip fracture. The overall incidence rate of hip fracture was 89.21/10,000 patient-years. Patients on hemodialysis (HD) had a 31% higher incidence of hip fracture than those on peritoneal dialysis (PD) (HR 1.31, 95% CI: 1.01-1.70). Patients ≥65 years old had more than 13 times the risk of a hip fracture than did those 18-44 years old (HR: 13.65; 95% CI: 10.12-18.40). Other factors that increased the risk of a hip fracture were a prior hip fracture (HR: 1.44; 95% CI: 1.15-1.80), osteoporosis (HR: 1.24; 95% CI: 1.07-1.45), DM (HR: 1.66; 95% CI: 1.51-1.83), and liver cirrhosis (HR: 1.37, 95% CI: 1.15-1.64). The overall in-hospital mortality rate was 3.2%. The cumulative survival rates after a hip fracture were 74.6% at one year and only 29.6% at seven years. Our findings supported the notion that being on HD is a risk for hip fracture. Additionally, old age, female gender, a prior hip fracture, osteoporosis, DM and liver cirrhosis were also risk factors for hip fracture in patients with ESRD and undergoing dialysis.

Is gastrectomy-induced high turnover of bone with hyperosteoidosis and increase of mineralization a typical osteomalacia?

Gastrectomy (GX) is thought to result in osteomalacia due to deficiencies in Vitamin D and Ca. Using a GX rat model, we showed that GX induced high turnover of bone with hyperosteoidosis, prominent increase of mineralization and increased mRNA expression of both osteoclast-derived tartrate-resistant acid phosphatase 5b and osteocalcin. The increased 1, 25(OH)2D3 level and unchanged PTH and calcitonin levels suggested that conventional bone and Ca metabolic pathways were not involved or changed in compensation. Thus, GX-induced bone pathology was different from a typical osteomalacia. Gene expression profiles through microarray analysis and data mining using Ingenuity Pathway Analysis indicated that 612 genes were up-regulated and 1,097 genes were down-regulated in the GX bone. These genes were related functionally to connective tissue development, skeletal and muscular system development and function, Ca signaling and the role of osteoblasts, osteoclasts and chondrocytes. Network analysis indicated 9 genes (Aldehyde dehydrogenase 1 family, member A1; Aquaporin 9; Interleukin 1 receptor accessory protein; Very low density lipoprotein receptor; Periostin, osteoblast specific factor; Aggrecan; Gremlin 1; Angiopoietin-like 4; Wingless-type MMTV integration site family, member 10B) were hubs connected with tissue development and immunological diseases. These results suggest that chronic systemic inflammation might underlie the GX-induced pathological changes in bone.

Bone disease in elderly individuals with CKD

Bone disease can lead to significant morbidity and mortality for those who are afflicted by it, irrespective of etiology. Two very prevalent causes of bone disease that contribute to this are osteoporosis and chronic kidney disease (CKD). The modern era has seen important advances in the understanding and management of these processes, but in elderly patients with CKD it remains a complex issue that has yet to be clearly defined. Changes in mineral metabolism that accompany the loss of renal function result in a spectrum of bone disease that occurs concomitantly with bone loss secondary to aging. As such, the traditional paradigms used to manage bone disease may not be appropriate for these patients. With the aging dialysis population, a better understanding of these 2 processes and their interplay deserves more attention.

Microarchitecture, the key to bone quality

Bone has the ability to adapt its shape and size in response to mechanical loads via a process known as modelling in which bones are shaped or reshaped by the independent action of osteoblasts and osteoclasts. Remodelling is a process that maintains mechanical integrity of the skeleton, allowing it to selectively repair and replace damaged bone. During adulthood, bone remodelling is the dominant process; after the age of 40 years, the age-related decline in bone mass increases the risk of fracture, especially in women. Osteoporosis is defined as a reduction in bone mass and an impairment of bone architecture resulting in thinning and increased cortical porosity, bone fragility and fracture risk. As new products and methods have been developed, focusing on bone fragility, effective and sensitive non-invasive means able to detect early changes in bone fragility process have also been developed. Due to limitations in assessing fracture risk and response to therapy, the evaluation of bone mineral contents by bone densitometry is progressively replaced by new non-invasive and/or non-destructive techniques able to estimate bone strength, providing structural information about the pathophysiology of bone fragility by quantitative assessments of macro- and microstructural bone features. DXA and volumetric QCT quantify bone macrostructure, whereas high-resolution CT, microCT, high-resolution MR and microMR assess bone microstructure. Knowledge of bone microarchitecture is a clue for understanding osteoporosis pathophysiology and improving its diagnosis and treatment; the response of microarchitecture parameters to treatment should allow assessment of the real efficacy of the osteoporosis therapy.

Catabolic properties of microdissected human endosteal bone lining cells

Bone lining cells cover > 80% of endosteal surfaces of human cancellous bone. Current research assigns to them a dual role: (1) as a biological membrane regulating exchange of substrates between the bone fluid compartment and the extracellular fluid of bone marrow and (2) as a signaling link between the osteocytic network as mechanical receptor and the osteoclastic cell pool for local induction of bone resorption. Furthermore, a catabolic role has been considered. We therefore examined the presence of matrix-metalloproteinases (MMPs) and their physiological tissue inhibitors (TIMPs) as putative proteolytic elements. Firstly, human cancellous bone from 60 patients was examined by immunofluorescence with antibodies against MMPs and TIMPs. Secondly, we applied laser-assisted microdissection (LMD) to isolate bone lining cells from frozen sections of human trabecular bone. mRNA analysis was performed using a single-cell PCR protocol. Three laser microdissection systems were tested: the new generation of Leica LMD and P.A.L.M. laser pressure catapulting (LPC) were compared to P.A.L.M. laser microdissection and micromanipulation (LMM). In a few pooled cell profiles, mRNA of MMP13, MMP14, TIMP1, and CBFA-1 was clearly detected. By immunofluorescence MMP13 and -14 as well as TIMP1 and -2 were strongly present in lining cells, while MMP2, TIMP3, and TIMP4 showed weak or negative signals. Although the functional impact of these enzymatic components remains open, there is additional evidence for a catabolic function of lining cells. The new diode-laser microdissection with LMD and LPC proved to be especially suitable to gain new insights into the properties of bone lining cells.

Normal bone anatomy and physiology

This review describes normal bone anatomy and physiology as an introduction to the subsequent articles in this section that discuss clinical applications of iliac crest bone biopsy. The normal anatomy and functions of the skeleton are reviewed first, followed by a general description of the processes of bone modeling and remodeling. The bone remodeling process regulates the gain and loss of bone mineral density in the adult skeleton and directly influences bone strength. Thorough understanding of the bone remodeling process is critical to appreciation of the value of and interpretation of the results of iliac crest bone histomorphometry. Osteoclast recruitment, activation, and bone resorption is discussed in some detail, followed by a review of osteoblast recruitment and the process of new bone formation. Next, the collagenous and noncollagenous protein components and function of bone extracellular matrix are summarized, followed by a description of the process of mineralization of newly formed bone matrix. The actions of biomechanical forces on bone are sensed by the osteocyte syncytium within bone via the canalicular network and intercellular gap junctions. Finally, concepts regarding bone remodeling, osteoclast and osteoblast function, extracellular matrix, matrix mineralization, and osteocyte function are synthesized in a summary of the currently understood functional determinants of bone strength. This information lays the groundwork for understanding the utility and clinical applications of iliac crest bone biopsy.

Bone formation by minimodeling is more active than remodeling after parathyroidectomy

Bone formation using the process known as minimodeling forms only lamellar bone in the absence of prior bone resorption even in uremic patients. In patients undergoing parathyroidectomy for secondary hyperparathyroidism, we compared the contribution of minimodeling to remodeling during the change in bone volume. Iliac bone biopsies were used to measure parameters related to minimodeling and remodeling before, at 3 to 4 weeks and 10 to 12 weeks after parathyroidectomy. Osteoblast surface due to minimodeling was greater than the entire bone osteoblast surface before and at 10 to 12 weeks after parathyroidectomy, but not 3 to 4 weeks after surgery. Minimodeling significantly increased osteoid volume 3 to 4 weeks after parathyroidectomy. The rate of change of osteoid volume by minimodeling was greater than that of osteoid volume during the first 3 to 4 weeks after surgery, indicating osteoid formation was more active at the minimodeling surface than at the entire bone surface. Furthermore, higher mineral apposition rates at the minimodeling sites than at remodeling sites yielded increased minimodeling bone volume at 10 to 12 weeks after surgery. Our results show that bone formation by minimodeling is more active than by remodeling and accounts, in part, for the increase of bone volume following parathyroidectomy.

Adynamic bone in patients with chronic kidney disease

Adynamic bone in patients with chronic kidney disease (CKD) is a clinical concern because of its potential increased risk for fracture and cardiovascular disease (CVD). Prevalence rates for adynamic bone are reportedly increased, although the variance for its prevalence and incidence is large. Differences in its prevalence are largely attributed to classification and population differences, the latter of which constitutes divergent groups of elderly patients having diabetes and other comorbidities that are prone to low bone formation. Most patients have vitamin D deficiency and the active form, 1,25-dihydroxyvitamin D, invariably decreases to very low levels during CKD progression. Fortunately, therapy with vitamin D receptor activators (VDRAs) appears to be useful in preventing bone loss, in part, by its effect to stimulate bone formation and in decreasing CVD morbidity, and should be considered as essential therapy regardless of bone turnover status. Future studies will depend on assessing cardiovascular outcomes to determine whether the risk/reward profile for complications related to VDRA and CKD is tolerable.

Minimodeling Reduces the Rate of Cortical Bone Loss in Patients With Secondary Hyperparathyroidism

BACKGROUND

Secondary hyperparathyroidism often causes progressive cortical thinning because of increased bone resorption at the endocortical surface and increases cortical porosity because of increased resorption at the intracortical surface. Because bone formation by minimodeling has not yet been reported in cortical bone, we investigated the effects of cortical minimodeling on the decrease in rate of bone loss.

METHODS

Thirty-five patients with secondary hyperparathyroidism were enrolled. Remodeling and minimodeling parameters at the endocortical and periosteal surfaces, as well as at the intracortical surface, were measured. Relationships between remodeling parameters and minimodeling parameters at each surface were investigated by using linear regression analysis. Cortical bone specimens were classified into 3 groups according to cortical width and cortical porosity values. Relationships of minimodeling parameters at the endocortical surface with cortical width and at the intracortical surface with cortical porosity were investigated.

RESULTS

Some minimodeling parameters showed positive correlations with serum parathyroid hormone levels and remodeling parameters. Minimodeling bone volume at the endocortical surface was greater in the narrow-cortical-width group than wide-cortical-width group, possibly slowing the progression of cortical thinning. Minimodeling volume at the intracortical surface was greater in the high-porosity than low-porosity group, possibly slowing the progression of intracortical resorption space enlargement. Minimodeling of the periosteal surface was found in 1 specimen.

CONCLUSION

Results show enhanced cortical minimodeling in patients with secondary hyperparathyroidism, possibly representing the decrease in rate of cortical bone loss.