# CKD-Mineral Bone Disorder

## Introduction

CKD-mineral bone disorder is a systemic disorder of mineral and bone metabolism resulting from chronic kidney disease. It encompasses three interrelated components: biochemical abnormalities involving calcium, phosphorus, PTH, vitamin D, and FGF-23; renal osteodystrophy representing the spectrum of bone pathology; and vascular and soft tissue calcification. The disorder begins early in CKD, with FGF-23 elevation detectable as early as stage G2-3, well before any changes in calcium, phosphorus, or PTH become apparent. CKD-MBD is strongly associated with cardiovascular morbidity, fractures, and mortality.

## Pathophysiology

### Phosphorus Retention and FGF-23

As GFR declines, the capacity for phosphorus excretion decreases, leading to transient hyperphosphatemia. FGF-23, produced by osteocytes, rises very early in CKD at stage G2 as the primary compensatory phosphaturic hormone. FGF-23 acts on the proximal tubule to downregulate NaPi-IIa and NaPi-IIc sodium-phosphate cotransporters, thereby increasing phosphorus excretion. Simultaneously, FGF-23 suppresses 1-alpha-hydroxylase activity, decreasing calcitriol production. FGF-23 requires the co-receptor Klotho, which is expressed in the kidney and parathyroid gland, for its signaling activity. Independently of its phosphaturic effects, FGF-23 is an established risk factor for left ventricular hypertrophy, heart failure, and mortality in CKD patients. Through the compensatory actions of FGF-23, serum phosphorus remains within the normal range until GFR falls below 30 mL/min, with overt hyperphosphatemia occurring in CKD stages G4 and G5.

### Vitamin D Deficiency

Reduced 1-alpha-hydroxylase activity in the failing kidney leads to decreased production of calcitriol (1,25-dihydroxyvitamin D). FGF-23 further suppresses 1-alpha-hydroxylase, amplifying the calcitriol deficit. Deficiency of 25-hydroxyvitamin D, the substrate form, is also prevalent in CKD due to reduced sun exposure, urinary loss of the vitamin D-binding protein complex in proteinuric states, and dietary insufficiency. The resulting low calcitriol levels reduce intestinal calcium absorption, leading to hypocalcemia, which in turn stimulates PTH secretion from the parathyroid glands.

### Secondary Hyperparathyroidism (SHPT)

The parathyroid glands are stimulated by multiple factors in CKD: hypocalcemia, hyperphosphatemia, low calcitriol levels, and reduced Klotho expression. PTH acts to increase bone resorption and calcium release, increase renal phosphorus excretion, and stimulate 1-alpha-hydroxylase, though this last effect is impaired in CKD. Chronic stimulation leads to parathyroid gland hyperplasia, and eventually the hyperplastic glands may develop nodular autonomy, transitioning to tertiary hyperparathyroidism with autonomous PTH secretion independent of calcium regulation. Elevated PTH drives high-turnover bone disease manifesting as osteitis fibrosa cystica.

### Klotho Deficiency

Klotho is an anti-aging protein expressed in kidney tubular cells that serves as the essential co-receptor for FGF-23. Klotho levels decline early in CKD, creating FGF-23 resistance in the kidney and driving further compensatory FGF-23 elevation. Klotho deficiency contributes to vascular calcification, cardiac hypertrophy, and a premature aging phenotype.

<image>Pathophysiology cascade of CKD-MBD starting from declining GFR. Show a timeline from CKD stage G2 to G5. First event: FGF-23 rises (earliest biomarker, stage G2-3) causing phosphaturia and calcitriol suppression. Second: calcitriol declines (stage G3) reducing intestinal calcium absorption. Third: PTH rises (stage G3-4) in response to hypocalcemia, hyperphosphatemia, and low calcitriol. Fourth: phosphorus rises (stage G4-5) when compensatory mechanisms are overwhelmed. Fifth: Klotho declines progressively. Show the downstream consequences: high-turnover bone disease from elevated PTH, vascular calcification from hyperphosphatemia and calcium-phosphorus product elevation, cardiovascular disease from FGF-23 (LVH) and vascular calcification. Include drug intervention points: phosphate binders (reduce Pi absorption), calcitriol/active vitamin D analogs (suppress PTH, increase Ca absorption), calcimimetics (suppress PTH via CaSR), and parathyroidectomy (refractory SHPT).</image>

## Renal Osteodystrophy

### Classification (TMV System)

Renal osteodystrophy is classified by the TMV system, which evaluates three parameters: turnover (high, normal, or low), mineralization (normal or abnormal), and volume (high, normal, or low).

### High-Turnover Bone Disease (Osteitis Fibrosa Cystica)

High-turnover bone disease results from excess PTH, which drives increased osteoclast and osteoblast activity leading to accelerated bone resorption. Histologic findings include peritrabecular fibrosis, so-called "brown tumors" representing cystic lesions, increased osteoclast numbers, and woven bone formation. Radiologic manifestations include subperiosteal resorption, most specifically identified along the radial aspect of the middle phalanges, a "salt and pepper" skull appearance, and brown tumors. Laboratory findings include a very high PTH above 500 pg/mL, elevated alkaline phosphatase, and high bone-specific alkaline phosphatase. Treatment focuses on controlling secondary hyperparathyroidism through phosphate binders, active vitamin D analogs, calcimimetics, and parathyroidectomy when medical management fails.

### Low-Turnover Bone Disease

Adynamic bone disease is now the most common form of renal osteodystrophy in dialysis patients, resulting from over-suppression of PTH by excessive calcium-based binders, vitamin D therapy, or the effects of diabetes on bone. It is characterized by low PTH levels below 100 to 150 pg/mL and low bone alkaline phosphatase. Adynamic bone carries an increased fracture risk and poor bone healing, along with increased vascular calcification risk because the skeleton cannot adequately buffer calcium. Treatment involves reducing calcium load, holding active vitamin D, and targeting a slightly higher PTH of 150 to 300 pg/mL in dialysis patients.

Osteomalacia represents defective mineralization with normal or low bone formation. Historically, aluminum toxicity from aluminum-containing phosphate binders was the primary cause, though this is now rare. Vitamin D deficiency-related osteomalacia remains a consideration. Histologic findings include wide osteoid seams, reduced mineralization fronts, and prolonged mineralization lag time on tetracycline labeling.

### Mixed Uremic Osteodystrophy

Mixed uremic osteodystrophy demonstrates features of both high and low turnover bone disease with variable PTH levels.

| Bone Disease Type | Turnover | Mineralization | PTH Level | Alkaline Phosphatase | Key Histology | Clinical Association |
|------------------|----------|----------------|-----------|---------------------|---------------|---------------------|
| Osteitis fibrosa cystica | High | Normal | Very high (>500 pg/mL) | Elevated | Peritrabecular fibrosis, increased osteoclasts, woven bone | Uncontrolled secondary hyperparathyroidism |
| Adynamic bone disease | Low | Normal | Low (<100–150 pg/mL) | Low | Reduced osteoblasts/osteoclasts, thin osteoid | Over-suppression of PTH (Ca binders, vitamin D excess), diabetes |
| Osteomalacia | Low/Normal | **Abnormal** | Variable | Variable | Wide osteoid seams, reduced mineralization fronts | Aluminum toxicity (historical), vitamin D deficiency |
| Mixed uremic osteodystrophy | Variable | Variable | Variable | Variable | Features of both high and low turnover | Combination of pathologies |

### Bone Biopsy

Transiliac bone biopsy with double tetracycline labeling, consisting of two courses of tetracycline separated by 14 days, remains the gold standard for diagnosing the type of renal osteodystrophy. However, bone biopsy is rarely performed in clinical practice and is reserved for unexplained fractures, atypical biochemical patterns, or pre-parathyroidectomy evaluation.

## Vascular Calcification

### Pathophysiology

Vascular calcification in CKD is not a passive process of calcium-phosphorus deposition but rather an active process that resembles osteogenesis. Vascular smooth muscle cells undergo osteogenic transformation, losing their normal calcification inhibitors including fetuin-A, matrix Gla protein (which is vitamin K-dependent), and pyrophosphate, while gaining expression of osteogenic factors such as Runx2, BMP-2, and alkaline phosphatase. Hyperphosphatemia directly induces this osteogenic transformation of vascular smooth muscle cells through the PiT-1 sodium-phosphate cotransporter. A calcium-phosphorus product exceeding 55 to 72 mg2/dL2 indicates increased calcification risk, though the product alone is an imperfect predictor.

### Types

Two types of vascular calcification coexist in CKD patients. Intimal calcification occurs within atherosclerotic plaques and is associated with plaque instability and cardiovascular events. Medial calcification, also known as Monckeberg sclerosis, produces concentric calcification of the arterial media and is more characteristic of CKD, leading to arterial stiffness, increased pulse pressure, and left ventricular hypertrophy.

### Assessment

Assessment methods include lateral abdominal X-ray for aortic calcification using the Kauppila score, echocardiography for valvular calcification of the aortic and mitral valves, and CT coronary artery calcium scoring using the Agatston score, which serves primarily as a research tool that correlates with cardiovascular events in CKD. Routine screening is not currently recommended but may guide risk discussions.

## Management

### Phosphorus Control

Dietary phosphorus restriction to 800 to 1000 mg per day is the first step, with emphasis on phosphorus bioavailability. Organic phosphorus from meat and dairy is 40 to 60 percent absorbed. Plant-based phosphorus in the form of phytate has lower bioavailability at 20 to 40 percent. Inorganic phosphorus additives found in processed foods and cola beverages have greater than 90 percent bioavailability and represent the major dietary contributor to hyperphosphatemia, making patient education on reading food labels essential.

Phosphate binders taken with meals are the mainstay of phosphorus management. Calcium-based binders including calcium carbonate and calcium acetate are effective and inexpensive but carry risks of hypercalcemia and vascular calcification; total elemental calcium from binders should be limited to less than 1500 mg per day. Sevelamer, available as carbonate or hydrochloride, is a non-calcium, non-aluminum binder that also binds bile acids and may lower LDL; the INDEPENDENT trial in incident hemodialysis patients showed survival benefit over calcium carbonate. Lanthanum carbonate is a potent binder, though concerns exist about long-term tissue accumulation that have not been proven harmful. Sucroferric oxyhydroxide is an iron-based binder with high potency and low pill burden that does not significantly increase iron stores. Ferric citrate also serves as an iron-based binder while simultaneously functioning as an oral iron supplement that increases iron stores and reduces intravenous iron requirements. Aluminum hydroxide remains the most potent binder but is limited to short-term use of less than 4 weeks due to aluminum toxicity causing osteomalacia, encephalopathy, and anemia.

| Phosphate Binder | Type | Elemental Ca Load | Pill Burden | Key Advantage | Key Limitation | Special Feature |
|-----------------|------|-------------------|-------------|---------------|----------------|-----------------|
| Calcium carbonate | Calcium-based | High (40% eleite Ca) | Moderate | Inexpensive, effective | Hypercalcemia, vascular calcification | Limit total elemental Ca <1500 mg/day |
| Calcium acetate | Calcium-based | Moderate (25% elemental Ca) | Moderate | More potent per mg Ca than carbonate | Same Ca risks as carbonate | Slightly better phosphorus binding per Ca load |
| Sevelamer (carbonate/HCl) | Non-calcium, non-aluminum | None | High | No Ca load; lowers LDL (bile acid binding) | GI side effects, cost | INDEPENDENT trial: survival benefit over Ca binders |
| Lanthanum carbonate | Non-calcium, non-aluminum | None | Low (chewable) | High potency, low pill burden | Tissue accumulation concern (unproven harm) | Chewable tablets; not absorbed systemically |
| Sucroferric oxyhydroxide | Iron-based | None | Low | Very high potency, low pill burden | GI side effects, dark stool | Does not significantly increase iron stores |
| Ferric citrate | Iron-based | None | Moderate | Dual function: binds phosphorus + oral iron supplement | GI side effects, iron overload potential | Increases iron stores, reduces IV iron need |
| Aluminum hydroxide | Aluminum-based | None | Low | Most potent binder available | Aluminum toxicity (osteomalacia, encephalopathy, anemia) | **Short-term use only (<4 weeks)** |

Standard hemodialysis removes approximately 800 to 1000 mg of phosphorus per session with thrice-weekly treatment, which is insufficient to match daily dietary phosphorus intake of 1000 to 1500 mg. Extended or daily dialysis schedules improve phosphorus control.

### PTH Management

KDIGO targets for PTH vary by CKD stage. In CKD G3-5 non-dialysis patients, PTH should be maintained within the normal range, with trends rather than absolute values guiding management. In CKD G5D dialysis patients, the target PTH range is 2 to 9 times the upper limit of normal, approximately 130 to 600 pg/mL, avoiding extreme suppression or elevation.

Active vitamin D analogs include calcitriol, which directly suppresses PTH gene transcription but carries risks of hypercalcemia and hyperphosphatemia; paricalcitol, a selective VDR activator with less hypercalcemia than calcitriol; doxercalciferol, a pro-drug requiring hepatic activation; and alfacalcidol.

Calcimimetics include cinacalcet, an allosteric activator of the calcium-sensing receptor on the parathyroid gland that increases its sensitivity to calcium and thereby suppresses PTH. The EVOLVE trial in 2012 studied cinacalcet versus placebo in hemodialysis patients; the primary composite endpoint did not reach statistical significance at p=0.11, but significance was achieved after lag-censoring adjustment, and the rate of parathyroidectomy was significantly reduced. Etelcalcetide is an intravenous calcimimetic peptide administered three times weekly with hemodialysis that achieves more potent PTH suppression than cinacalcet with fewer gastrointestinal side effects. Side effects of calcimimetics include hypocalcemia, nausea, vomiting, and risk of QT prolongation with hypocalcemia.

### Parathyroidectomy

Parathyroidectomy is indicated for severe secondary hyperparathyroidism refractory to medical therapy, typically with PTH above 800 pg/mL accompanied by hypercalcemia, hyperphosphatemia despite binders, calciphylaxis, or severe symptoms. The surgical approach involves either subtotal parathyroidectomy removing 3.5 glands or total parathyroidectomy with auto-transplantation to the forearm. Post-operative hungry bone syndrome, characterized by profound hypocalcemia, hypophosphatemia, and hypomagnesemia, requires aggressive intravenous calcium supplementation and close monitoring.

<image>Treatment algorithm for CKD-MBD management based on CKD stage. Show three columns for CKD G3-4, CKD G5 non-dialysis, and CKD G5D (dialysis). For each stage, show target ranges for phosphorus (2.5-4.5 mg/dL), calcium (8.4-9.5 mg/dL), and PTH (stage-specific targets). Show the treatment pyramid for each stage. CKD G3-4: check and replace 25(OH)D, dietary phosphorus restriction, phosphate binders if hyperphosphatemic. CKD G5 non-dialysis: add active vitamin D analog if PTH rising, intensify phosphate binder therapy. CKD G5D: full arsenal including phosphate binders, active vitamin D or calcimimetics (cinacalcet or etelcalcetide), and parathyroidectomy for refractory SHPT. Include monitoring frequency: phosphorus and calcium every 3-6 months in G3-4, monthly in G5D; PTH every 6-12 months in G3-4, every 3-6 months in G5D.</image>

### Calciphylaxis (Calcific Uremic Arteriolopathy)

Calciphylaxis is a rare but life-threatening condition characterized by calcification and thrombosis of dermal arterioles, producing painful violaceous skin lesions with central necrosis. Risk factors include ESRD on dialysis, warfarin use, obesity, diabetes, hypercoagulability, high calcium-phosphorus product, and high PTH. Diagnosis is clinical, supported by skin biopsy showing arteriolar medial calcification, intimal fibrin thrombi, and panniculitis, though biopsy carries a risk of non-healing wounds. Treatment is multimodal: warfarin should be immediately discontinued as it depletes matrix Gla protein which is a calcification inhibitor; intravenous sodium thiosulfate at 25 grams in 100 mL normal saline over 30 to 60 minutes at the end of hemodialysis three times weekly is the mainstay of therapy; non-calcium phosphate binders should replace any calcium-based binders; cinacalcet should be used for PTH control; wound care and pain management are essential; and parathyroidectomy should be considered in hyperparathyroid patients. Mortality is extremely high at 30 to 80 percent at 1 year, with sepsis from infected wounds being the most common cause of death.

## Key Clinical Pearls

- FGF-23 is the earliest biomarker of CKD-MBD, rising in CKD G2-3 before any changes in calcium, phosphorus, or PTH; clinical FGF-23 testing is not yet standard practice but informs our understanding of disease pathogenesis
- Adynamic bone disease is now the most common form of renal osteodystrophy in dialysis patients; over-suppression of PTH with calcium-based binders and active vitamin D is the main driver — maintain PTH 2-9x normal on dialysis
- Inorganic phosphorus additives in processed foods have >90% bioavailability and are the major dietary contributor to hyperphosphatemia; patient education on reading food labels is essential
- Calciphylaxis is a medical emergency with 30-80% mortality; sodium thiosulfate is the mainstay of treatment, and warfarin should be immediately discontinued (depletes matrix Gla protein, a calcification inhibitor)
- The choice of phosphate binder matters: calcium-based binders contribute to vascular calcification and should be limited; non-calcium binders (sevelamer, lanthanum, sucroferric oxyhydroxide) are preferred in patients with vascular calcification or adynamic bone disease

## References
1. KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of CKD-MBD. *Kidney Int Suppl*. 2017;7(1):1-59.
2. Gutierrez OM, Mannstadt M, Isakova T, et al. Fibroblast Growth Factor 23 and Mortality among Patients Undergoing Hemodialysis. *N Engl J Med*. 2008;359(6):584-592.
3. Block GA, Raggi P, Bellasi A, et al. Mortality Effect of Coronary Calcification and Phosphate Binder Choice in Incident Hemodialysis Patients. *Kidney Int*. 2007;71(5):438-441.
4. EVOLVE Trial Investigators. Effect of Cinacalcet on Cardiovascular Disease in Patients Undergoing Dialysis. *N Engl J Med*. 2012;367(26):2482-2494.
5. Nigwekar SU, Thadhani R, Brandenburg JG. Calciphylaxis. *N Engl J Med*. 2018;378(18):1704-1714.