Residency · Residency · Endocrinology

Metabolic Bone Disease - Paget Disease and Osteomalacia

Paget Disease of Bone

Overview

Paget disease of bone is a focal disorder of accelerated and disordered bone remodeling that produces enlarged, mechanically weakened, and highly vascular bone. It stands as the second most common metabolic bone disease after osteoporosis. The prevalence is 1-3% of adults over 55 years, though incidence has been declining in recent decades. The disease demonstrates geographic predilection, being more common in Northern Europe, Australia, and New Zealand, and rare in Asia and Africa. Males are affected slightly more often than females at a ratio of 1.5:1, with an average age at diagnosis exceeding 55 years. Paget disease may be monostotic (affecting a single bone) or polyostotic (involving multiple bones), but it characteristically does not become generalized. The most commonly affected sites are the pelvis (70%), femur (55%), lumbar spine (53%), skull (42%), and tibia (32%).

Pathophysiology

The pathological process in Paget disease is fundamentally osteoclast-driven. Pagetic osteoclasts are dramatically abnormal, containing up to 100 nuclei compared with the normal 3-20, and demonstrating hyperactive resorptive capacity. These cells contain nuclear inclusion bodies that resemble paramyxovirus particles, and measles virus and canine distemper virus have been implicated but not conclusively proven as causative agents.

The disease progresses through three sequential phases. The osteolytic phase is characterized by intense osteoclastic resorption, manifesting radiographically as a flame-shaped advancing front on long bones or as osteoporosis circumscripta on the skull. The mixed phase follows, in which a coupled osteoblastic response produces woven bone that is disorganized and mechanically weak. Histologically, this produces the characteristic "mosaic" pattern of irregularly cemented bone fragments. This phase also features markedly increased bone vascularity. The osteosclerotic phase is the final stage, dominated by dense, enlarged, and deformed sclerotic bone.

Genetically, SQSTM1/p62 mutations are found in 25-50% of familial cases and 5-10% of sporadic Paget disease. The sequestosome-1 protein encoded by this gene participates in the RANK/NF-kappaB signaling pathway, and mutations lead to increased osteoclast sensitivity to RANKL. Other associated genes include CSF1, OPTN, TNFRSF11A (RANK), and VCP.

Clinical Features

Paget disease is asymptomatic in 70-80% of patients and is often discovered incidentally through an isolated elevation of alkaline phosphatase on routine blood work or through abnormal findings on imaging performed for other indications. When symptomatic, bone pain is the most common complaint, described as deep, aching, and constant, often worse at night and localized to the affected site. Skeletal deformity develops with disease progression: bowing of long bones (femur, tibia), skull enlargement (patients may report increasing hat size), and spinal kyphosis. Pathologic fractures occur through pagetic bone, characteristically presenting as "chalk-stick" transverse fractures of long bones or stress fractures on the convex surface of bowed bones.

Neurological complications are clinically significant. Deafness is the most common, resulting from cochlear involvement or auditory nerve compression at the skull base, and may be mixed sensorineural and conductive in nature. Cranial nerve compression can affect the facial nerve (CN VII) and trigeminal nerve (CN V). Spinal stenosis from vertebral enlargement may compress the spinal cord or nerve roots. Basilar invagination (platybasia), in which softened skull base bone permits upward migration of the odontoid process with brainstem compression, is a life-threatening complication.

Cardiovascular involvement, specifically high-output heart failure from increased blood flow through the highly vascular pagetic bone, is rare and occurs only with extensive polyostotic disease involving more than 15% of the skeleton. Secondary osteoarthritis of joints adjacent to pagetic bone (hip, knee) develops from altered biomechanics and joint deformity. The most feared complication is malignant transformation to osteosarcoma, which occurs in fewer than 1% of patients but should be suspected with sudden worsening of pain, development of a soft tissue mass, or rapidly rising alkaline phosphatase. The prognosis of pagetic osteosarcoma is dismal, with 5-year survival below 15%.

Diagnosis

Serum alkaline phosphatase (ALP) is the most useful biochemical marker, correlating with disease extent and activity. It may be normal in monostotic or treated disease, and bone-specific ALP provides greater specificity. Other elevated bone turnover markers include P1NP (formation) and CTX/NTX (resorption), which are useful for monitoring when ALP is normal. Serum calcium and phosphate are usually normal, though hypercalcemia can occur with immobilization or extensive disease.

Radiography reveals characteristic findings at each disease phase: cortical thickening, coarsened trabecular pattern, bone enlargement, bowing deformities, osteolytic lesions (including V-shaped advancing fronts in long bones), and sclerotic changes. Bone scintigraphy (Tc-99m MDP bone scan) is the most sensitive modality for detecting the full extent of disease, showing intense focal uptake at pagetic sites, and is valuable for assessing polyostotic involvement at diagnosis, though the findings are not specific and can be mimicked by metastases or fractures. CT and MRI are reserved for evaluating specific complications such as spinal stenosis, basilar invagination, or suspected sarcomatous transformation. Biopsy is rarely needed and is indicated primarily when malignant transformation is suspected.

<image>A multi-panel radiographic illustration of Paget disease. Panel 1: AP pelvis X-ray showing classic pagetic changes in the left hemipelvis - cortical thickening, coarsened trabecular pattern, bone enlargement with protrusion acetabuli, compared to normal right side. Panel 2: Lateral skull X-ray showing "cotton wool" appearance of mixed lytic and sclerotic changes, thickened calvarium, osteoporosis circumscripta in frontal region. Panel 3: AP tibia X-ray showing anterior bowing, cortical thickening, and a flame-shaped lytic advancing front at the distal end. Panel 4: Bone scan showing intense uptake at multiple pagetic sites (pelvis, femur, skull, vertebra) with the rest of skeleton normal. Label each panel with key findings. Use radiographic illustration style.</image>

Treatment

Treatment is indicated for bone pain at a pagetic site, planned orthopedic surgery on pagetic bone, neurological complications, hypercalcemia, high-output heart failure, prevention of complications in weight-bearing or skull/spine sites, and prophylactic treatment of young patients to prevent long-term deformity. Not all asymptomatic patients require treatment; the PRISM trial (2019) demonstrated that early intensive bisphosphonate treatment did not reduce complications or fractures over 3 years compared with symptomatic treatment, though guideline recommendations remain variable.

Bisphosphonates

Zoledronic acid 5 mg as a single IV infusion is the treatment of choice and the most potent available option. It normalizes ALP in more than 90% of patients within 6 months, with prolonged remission lasting 5 or more years in many patients. Risedronate 30 mg orally daily for 2 months is the primary oral alternative, normalizing ALP in 50-70% of patients. Alendronate 40 mg orally daily for 6 months is another oral option. Pamidronate 30-60 mg IV over 2-4 hours for 3 days provides an alternative IV approach. Pre-treatment requirements include ensuring 25(OH)D is at least 30 ng/mL and calcium intake is adequate to mitigate the risk of post-treatment hypocalcemia, particularly with zoledronic acid. Renal function must be assessed, with these agents avoided when eGFR is below 35.

Monitoring

ALP should be monitored every 3-6 months after treatment, targeting normalization. Retreatment is indicated when ALP rises above normal or symptoms recur, and repeat zoledronic acid infusion is effective. Bone-specific ALP or P1NP is useful when total ALP is confounded by liver disease. Calcitonin (salmon calcitonin 100 IU subcutaneously or intramuscularly daily or on alternate days) was the historical standard treatment but is now rarely used due to inferiority compared with bisphosphonates, tachyphylaxis, and concerns about malignancy with chronic use.

Surgical Management

Total joint arthroplasty is performed for secondary osteoarthritis of joints adjacent to pagetic bone. Surgeons must be aware that pagetic bone is highly vascular, increasing intraoperative bleeding risk, and preoperative bisphosphonate therapy may reduce vascularity. Osteotomy addresses severe bowing deformity. Decompressive surgery is indicated for spinal stenosis or basilar invagination. Tumor resection for osteosarcoma often requires amputation and carries a poor prognosis.

Osteomalacia and Rickets

Definition

Osteomalacia is the clinical condition resulting from defective mineralization of newly formed osteoid in adult bone, occurring after growth plate closure. Rickets refers to the same mineralization defect occurring at the growth plate in children before closure, presenting with skeletal deformities and growth retardation. Both conditions result fundamentally from an insufficient calcium-phosphate product at the mineralization front.

Causes

Vitamin D-Related

Vitamin D deficiency is the most common cause of osteomalacia globally, resulting from inadequate sunlight exposure, dietary insufficiency, malabsorption (celiac disease, inflammatory bowel disease, bariatric surgery, pancreatic insufficiency), liver disease, or chronic kidney disease. 1-alpha-hydroxylase deficiency (vitamin D-dependent rickets type 1A/VDDR1A), caused by autosomal recessive CYP27B1 mutations, produces low 1,25(OH)2D despite adequate 25(OH)D and responds to calcitriol replacement. Vitamin D receptor mutations (VDDR type 2/hereditary vitamin D-resistant rickets) involve autosomal recessive VDR mutations causing end-organ resistance to calcitriol; this condition often presents with alopecia and is partially responsive to high-dose calcium infusions. CYP24A1 mutations impair 24-hydroxylase-mediated vitamin D inactivation, causing accumulation of active vitamin D and infantile hypercalcemia.

Phosphate-Related

X-linked hypophosphatemia (XLH) is the most common genetic cause of rickets, resulting from PHEX gene mutations that lead to elevated FGF23, causing renal phosphate wasting and low 1,25(OH)2D. It presents in childhood with rickets, short stature, and dental abscesses, and persists into adulthood as osteomalacia. Tumor-induced osteomalacia (TIO) is a paraneoplastic condition in which mesenchymal tumors (often small and occult) produce FGF23, causing acquired hypophosphatemic osteomalacia in adults with severe phosphate wasting and low 1,25(OH)2D; surgical tumor resection is curative. Autosomal dominant hypophosphatemic rickets (ADHR) results from FGF23 gain-of-function mutations. Fanconi syndrome, involving proximal tubular dysfunction with wasting of phosphate, glucose, amino acids, and bicarbonate, has multiple causes including multiple myeloma, medications (tenofovir, ifosfamide), Wilson disease, and cystinosis.

Other Causes

Severe dietary calcium restriction, even with adequate vitamin D, causes osteomalacia, particularly in developing countries. Aluminum toxicity, historically associated with dialysis, prevents mineralization through aluminum deposition at the osteoid-mineral interface. Hypophosphatasia, caused by autosomal recessive ALPL (tissue-nonspecific alkaline phosphatase) mutations, produces characteristically low ALP with elevated PLP (pyridoxal-5'-phosphate) and variable severity, from perinatal lethal to mild adult forms. Treatment with asfotase alfa (enzyme replacement) is available. Chronic metabolic acidosis (renal tubular acidosis) and very high-dose oncologic bisphosphonates can also impair mineralization.

Clinical Features of Osteomalacia

The clinical presentation of osteomalacia centers on diffuse bone pain, which is often misdiagnosed as fibromyalgia. The pain is characteristically worse with weight-bearing. Proximal muscle weakness, particularly when phosphate-related, produces a characteristic "waddling gait." Insufficiency fractures and pseudofractures (Looser zones) are pathognomonic findings. In children with rickets, the manifestations include widened growth plates, bowing of the legs (genu varum or valgum), rachitic rosary (costochondral junction enlargement), craniotabes (softened skull), delayed dentition, and short stature.

Diagnosis

Laboratory findings differ by etiology. In vitamin D-related osteomalacia, 25(OH)D is low, calcium and phosphate may be low or normal, PTH is elevated (secondary hyperparathyroidism), ALP is elevated, and urinary calcium is low. In phosphate-related forms (XLH, TIO), phosphate is low, FGF23 is elevated, 1,25(OH)2D is low or inappropriately normal, calcium is normal, and ALP is elevated.

Radiographically, Looser zones (pseudofractures) are pathognomonic: bilateral, symmetric lucent bands perpendicular to the cortical surface at stress points (medial femoral neck, pubic rami, scapulae, ribs), with surrounding sclerosis. Additional findings include generalized osteopenia, indistinct cortices, and coarsened trabecular pattern. Bone biopsy with tetracycline labeling from the iliac crest remains the gold standard, showing increased unmineralized osteoid, widened osteoid seams, and reduced mineral apposition rate on fluorescent tetracycline labels, though it is rarely performed in practice. For tumor-induced osteomalacia, FGF23 is elevated, and 68Ga-DOTATATE PET/CT or 18F-FDG PET/CT is used for tumor localization, as the causative tumors are often small mesenchymal lesions in the extremities or sinuses.

Treatment

Vitamin D Deficiency

Treatment involves ergocalciferol or cholecalciferol 50,000 IU weekly for 6-8 weeks as a loading phase, followed by maintenance at 1500-2000 IU daily. Calcium supplementation at 1000-1500 mg daily is provided concurrently. Monitoring of 25(OH)D, calcium, and PTH guides therapy, targeting 25(OH)D above 30 ng/mL. For malabsorption states, higher doses (up to 5,000-10,000 IU daily) or calcitriol 0.25-1 mcg daily may be necessary.

X-Linked Hypophosphatemia (XLH)

Conventional therapy consists of oral phosphate (1-3 g daily in 4-5 divided doses) combined with calcitriol (0.25-1 mcg daily). While this prevents rickets and osteomalacia, it does not fully correct the disease and is complicated by nephrocalcinosis and secondary hyperparathyroidism from phosphate loading. Burosumab (Crysvita), an anti-FGF23 monoclonal antibody, has transformed XLH management. FDA-approved for children 6 months and older and adults with XLH, it is dosed at 1 mg/kg subcutaneously every 2 weeks (children) or every 4 weeks (adults, maximum 90 mg). Burosumab normalizes phosphate, improves 1,25(OH)2D levels, heals rickets (radiographic improvement in 94% at 40 weeks), reduces pain and stiffness in adults, improves walking ability, and reduces fractures. Side effects include injection site reactions, restless leg syndrome, tooth abscess, and hypersensitivity. Phosphate and 1,25(OH)2D must be monitored to avoid hyperphosphatemia, and burosumab cannot be combined with conventional phosphate/calcitriol therapy due to the risk of hyperphosphatemia and nephrocalcinosis.

Tumor-Induced Osteomalacia

Curative treatment is surgical excision of the FGF23-producing tumor, making localization the critical step. 68Ga-DOTATATE PET/CT is the most sensitive imaging modality for this purpose. When the tumor cannot be found or is not resectable, conventional therapy with phosphate and calcitriol or burosumab (approved for TIO in adults) provides medical management.

<image>A comparison panel of two metabolic bone diseases. Left panel (Osteomalacia): show a femur X-ray with Looser zone (pseudofracture) at the medial cortex - a bilateral symmetric lucent line perpendicular to cortex with surrounding sclerosis. Include a bone biopsy inset showing thick unmineralized osteoid seams (pink) surrounding mineralized bone (blue/green on Goldner trichrome stain). Show laboratory panel: low phosphate, low 25(OH)D, elevated ALP, elevated PTH. Right panel (Paget Disease): show a pagetic tibia with anterior bowing, cortical thickening, and coarsened trabecular pattern. Include a bone biopsy inset showing mosaic cement line pattern (irregular cement lines creating jigsaw puzzle appearance). Show laboratory panel: normal calcium/phosphate, markedly elevated ALP, normal PTH. Use radiographic and histologic illustration style with clear labeling.</image>

Key Clinical Pearls

• Paget disease is often diagnosed incidentally via an isolated elevated alkaline phosphatase on routine labs; always consider Paget in an older patient with unexplained ALP elevation (check bone-specific ALP, radiographs of common sites, consider bone scan)
• A single dose of zoledronic acid 5 mg IV is now the standard treatment for active Paget disease; it normalizes ALP in >90% of patients with remission lasting 5+ years, making it both highly effective and convenient
• Osteosarcoma complicating Paget disease is rare (<1%) but devastating; suspect if sudden worsening of pain at a pagetic site, soft tissue mass, or rapidly rising ALP; prognosis is very poor
• Osteomalacia due to vitamin D deficiency is commonly misdiagnosed as fibromyalgia; diffuse bone pain with proximal weakness, elevated ALP, and low 25(OH)D should prompt consideration of osteomalacia
• Burosumab (anti-FGF23) has transformed the management of X-linked hypophosphatemia; it normalizes phosphate physiologically rather than through phosphate loading, avoiding the complications of conventional therapy (nephrocalcinosis, secondary hyperparathyroidism)
• In tumor-induced osteomalacia, 68Ga-DOTATATE PET/CT is the most sensitive imaging modality for localizing the causative FGF23-secreting tumor, which is often small and located in extremities or sinuses

References

1. Singer FR, et al. "Paget's Disease of Bone: An Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab. 2014;99(12):4408-4422.
2. Ralston SH, et al. "Clinical, Biochemical, and Radiographic Effects of Aminohydroxypropylidene Bisphosphonate Treatment in Paget's Disease of Bone (PRISM)." J Bone Miner Res. 2019;34(7):1167-1175.
3. Minisola S, et al. "Osteomalacia and Vitamin D Status: A Clinical Update." JAMA. 2023;329(15):1296-1306.
4. Imel EA, et al. "Burosumab versus Conventional Therapy in Children with X-Linked Hypophosphataemia." Lancet. 2019;393(10189):2416-2427.
5. Florenzano P, et al. "Tumor-Induced Osteomalacia." Calcif Tissue Int. 2021;108(1):128-142.

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