Karl J Jepsen, PhD

The primary objective of my research program is to understand how complex adaptive systems like bone establish function during growth and maintain function during aging. Having a better understanding of how complex systems work will benefit efforts to reduce skeletal fracture risk by identifying early predictors of fracture risk and the genetic and environmental factors which impair (or promote) specific components of the functional adaptation process leading to declines (or improvement) in fracture resistance.

My scientific program has advanced a paradigm shift in skeletal biology by establishing “external bone size” (robustness; width relative to length) as a foundational, novel organizer of whole-bone strength across the life course. Beginning with mechanistic, systems-level studies in genetically defined mouse models, we showed that skeletal competence is not captured by any single metric (e.g., bone mass or areal bone mineral density (BMD)), but instead emerges from coordinated, biologically constrained covariation among external geometry, cortical and trabecular structure, and tissue-level material properties. This framework reframed bone as a complex adaptive system in which individuals can achieve similar apparent strength through different trait combinations yet differ in structural reserve and vulnerability to growth conditions, mechanical loading, hormonal change, and aging. Further, this framework provided a hypothesis-generating platform that integrates biomechanics, developmental biology, genetics, and quantitative modeling.

My lab then translated these principles to human populations, demonstrating that robustness-linked trait sets recur across the skeleton and help explain key clinical mismatches, including why patients with similar dual energy X-ray absorptiometry (DXA)-derived BMD can have markedly different fracture susceptibility and trajectories of age-related strength decline. Our studies connected childhood skeletal phenotype to adult structural reserve, clarified phenotype–load mismatch as a driver of stress fractures in otherwise healthy young adults, and identified distinct hip-fracture phenotypes in older women and men characterized by heterogeneous microarchitectural deterioration rather than a single “osteoporotic” pathway. Collectively, our body of work provides a mechanistic rationale for phenotype-informed, precision approaches to fracture-risk assessment, earlier-life risk identification. Moreover, this research will lead to more individualized prevention and treatment strategies beyond density thresholds alone, positioning skeletal fragility to be detected, stratified, and addressed well before catastrophic fracture occurs.

Major accomplishments and emerging clinical impacts include: (1) establishing and validating the external bone size covariation framework as a unifying, life-course model that links developmental history to adult mechanical reserve and late-life fragility; (2) demonstrating, in both preclinical and human studies, that treatment and perturbation responses (e.g., exercise/loading, anabolic stimulation, estrogen deficiency) are phenotype-contingent, supporting more targeted selection and monitoring of therapies rather than “one-size-fits-all” management; (3) providing clinically actionable explanations for high-burden fracture syndromes, including improved mechanistic understanding and risk stratification for stress fractures in young active populations and for hip fractures in older adults; (4) motivating phenotype-rich diagnostic strategies that extend beyond DXA (e.g., incorporating external geometry, cortical/trabecular microarchitecture, and structural trajectories using advanced imaging and multivariate modeling), which can reduce under-recognition of high-risk patients whose BMD appears only modestly reduced; and (5) laying the groundwork for precision bone health management in which earlier identification of constrained, low-strength reserve phenotypes enables proactive prevention (nutrition, activity and load management, longitudinal monitoring) and ultimately shifts care from late-stage fracture response to anticipatory, individualized fracture-risk reduction.