Bone Turnover Phenotypes for Individualized Care

Abstract

Bone turnover phenotypes represent distinct biological patterns of bone remodeling activity, offering a precision framework to guide individualized care in metabolic bone diseases. This review critically examines current evidence on bone turnover phenotyping, exploring epidemiology, pathophysiological mechanisms, clinical features, diagnostic strategies, therapeutic approaches, and recent advances, with an emphasis on optimizing patient outcomes through tailored intervention.

Introduction

Bone health maintenance is a dynamic interplay between bone resorption and formation, collectively termed bone turnover. Phenotypic classification of bone turnover—ranging from high to low or mixed turnover—enables clinicians to individualize management strategies in osteoporosis and other metabolic bone disorders. Traditional approaches to bone disease have often relied on population-based risk models; however, emerging data underscore the need for a more nuanced, patient-centric approach. Understanding and utilizing bone turnover phenotypes can refine diagnosis, guide therapy selection, and predict response to treatment, ultimately improving clinical outcomes.

Epidemiology / Disease Burden

Globally, metabolic bone diseases such as osteoporosis affect hundreds of millions, with substantial morbidity and mortality attributed to fragility fractures. Epidemiological studies reveal significant heterogeneity in bone turnover rates among affected individuals, with prevalence of high-turnover phenotypes particularly pronounced among postmenopausal women and certain secondary osteoporosis cohorts. Low-turnover states are also clinically relevant, found in populations with aging, chronic kidney disease, and some endocrine disorders. Recognition of these phenotypes is vital, as fracture risk, disease progression, and therapeutic response are closely linked to underlying turnover dynamics.

Pathophysiology

Bone turnover is orchestrated by a balance between osteoclastic resorption and osteoblastic formation. High-turnover phenotypes are characterized by increased osteoclast activity, often driven by hormonal changes (e.g., estrogen deficiency), inflammatory cytokines, or secondary hyperparathyroidism. Conversely, low-turnover phenotypes reflect suppressed bone remodeling, either due to aging, pharmacological suppression (such as prolonged bisphosphonate therapy), or disorders like adynamic bone disease. The molecular pathways involved include RANK/RANKL/OPG axis, Wnt/β-catenin signaling, and sclerostin modulation, all contributing to the dynamic spectrum of bone remodeling rates. Understanding these mechanisms provides a rational basis for individualized therapeutic intervention.

Risk Factors

Risk factors for altered bone turnover are multifaceted. Non-modifiable factors include age, sex, and genetic predisposition, while modifiable influences encompass nutritional status (calcium and vitamin D intake), physical activity, comorbid medical conditions (e.g., diabetes, chronic kidney disease), and medication exposure (e.g., glucocorticoids, aromatase inhibitors). Importantly, secondary causes of high or low bone turnover should be systematically evaluated, as addressing these factors may restore normal remodeling and reduce fracture risk.

Clinical Features

Clinical manifestations of altered bone turnover are often insidious. High-turnover states accelerate bone loss, predisposing to microarchitectural deterioration and increased fracture risk, especially vertebral and non-vertebral fragility fractures. Low-turnover phenotypes may present with subtle symptoms but can lead to accumulation of microdamage, impaired bone quality, and, paradoxically, increased fracture risk in certain contexts. In chronic kidney disease, adynamic bone disease is associated with vascular calcification and cardiovascular morbidity. Hence, a careful clinical assessment is required to identify features suggestive of specific turnover phenotypes for optimal management.

Diagnosis

Diagnosis of bone turnover phenotype integrates clinical, biochemical, and imaging assessments. Bone turnover markers (BTMs), such as serum C-terminal telopeptide (CTX) and procollagen type 1 N-terminal propeptide (P1NP), provide non-invasive insight into resorptive and formative activity, respectively. Dual-energy X-ray absorptiometry (DXA) remains the gold standard for bone mineral density assessment, but does not capture turnover dynamics. Advanced imaging, including high-resolution peripheral quantitative computed tomography (HR-pQCT), may reveal microarchitectural changes. In selected cases, bone biopsy with histomorphometric analysis offers definitive phenotyping, especially in complex or refractory cases, such as renal osteodystrophy.

Treatment & Management

Tailored management based on bone turnover phenotype can optimize therapeutic efficacy while minimizing risks. High-turnover states generally benefit from antiresorptive agents, such as bisphosphonates or denosumab, which suppress excessive bone resorption. In contrast, low-turnover or adynamic bone disease may warrant avoidance of potent antiresorptives, with consideration for anabolic therapies (e.g., teriparatide, abaloparatide) or correction of underlying metabolic derangements. Nutritional optimization, physical activity, and management of secondary causes are universally important. Regular monitoring of BTMs can help assess treatment response and inform adjustments, emphasizing the dynamic nature of bone remodeling in clinical care.

Recent Advances / Emerging Therapies

Recent advances in bone biology have led to the development of novel therapeutics targeting specific pathways involved in bone turnover. Sclerostin inhibitors (e.g., romosozumab) represent a new class of agents with dual effects—increasing bone formation and reducing resorption—offering particular promise in patients with low-turnover osteoporosis or those with inadequate response to conventional therapy. Ongoing research into personalized medicine, incorporating genetic, proteomic, and metabolomic profiling, holds potential to further refine bone turnover phenotyping and therapeutic selection. Additionally, non-invasive imaging techniques and more sensitive BTMs are under investigation to improve diagnostic accuracy and monitoring capabilities.

Guideline Recommendations

Major clinical guidelines increasingly advocate for individualized treatment approaches in metabolic bone disease, emphasizing assessment of bone turnover status. The International Osteoporosis Foundation and Endocrine Society recommend consideration of BTMs and clinical risk factors to inform therapeutic choices. In chronic kidney disease-mineral and bone disorder (CKD-MBD), KDIGO guidelines highlight the importance of distinguishing turnover phenotypes to guide pharmacologic and non-pharmacologic interventions. Adherence to guideline-based, phenotype-driven care improves patient outcomes and reduces the burden of fragility fractures.

Conclusion

Bone turnover phenotyping represents a pivotal advancement in the individualized management of metabolic bone disorders. By integrating clinical assessment with biochemical and imaging modalities, clinicians can identify underlying remodeling dynamics, tailor interventions, and monitor response with greater precision. Continued research into the molecular underpinnings of bone turnover and the development of targeted therapies will further enhance the capacity for personalized care, ultimately improving outcomes for patients with bone disease.