Intensive care patients face health issues that extend beyond their critical illness. A specific area where critical illness may adversely affect the well-being of survivors is increased bone turnover during critical illness, and accelerated bone loss in subsequent years. Critical illness bone loss begins in the first days of critical illness, occurs in both men and women, and is greatest in post-menopausal women. Loss of bone mineral density is significantly greater at both the femur (-2+4.0% vs -0.7+1.1%, p=0.001) and spine (-2.9+4.1% vs -0.2+1.1%, p\<0.001) in women in the year after critical illness compared to age-matched controls. One year after critical illness, 80% of women aged 50-years or greater are classified as osteoporotic or osteopaenic, compared to 71% of the approximately 3.7 million Australian women aged 50 year or greater. In the year after ICU admission a decrease in femur BMD of -1.52% (+ 2.85) is reported in men, which is significantly higher than age adjusted population controls (-0.42% + 1.13, diff -1.10% (95% CI -1.71 to -0.49, p\<0.001). The annual incidence of first fracture in men aged 70 years and over is similar to the annual incidence of fracture in women aged 50 years and over. In addition, there is a dramatic increase in hip fractures as a proportion of all fracture's males aged 70 years and older in the general population. This population is most likely to suffer the major consequence of accelerated bone loss, fragility fracture, and the associated morbidity, loss of quality of life, and economic cost. Older women who survive critical illness have a significantly higher fragility fracture rate compared to community age-matched controls (Intensive Care Unit 4.33 vs control 2.81 per 100 patient years, adj HR 1.7 (95% CI 1.1-2.5), p=0.02).

Bone antiresorptive therapies are effective at reducing bone loss and decreasing fracture risk in non-critically ill populations. Zoledronic acid and denosumab represent antiresorptive agents with established efficacy in adults, and are potential target interventions able to be delivered during critical illness. Denosumab is a human monoclonal antibody directed against Receptor activator of nuclear factor kappa-Β ligand, a central stimulator of osteoclast activity, and is effective for prevention of fractures and bone loss in osteoporosis and malignancy. Zoledronic acid is a bisphosphonate class agent that binds to bone and suppresses bone resorption by entering osteoclasts and inhibiting the enzyme farnesyl pyrophosphate synthase, resulting in disruption of osteoclast attachment to bone surface. In addition to skeletal effects, there are possible mortality benefits associated with the use of antiresorptive medications in populations with increased bone loss.

There is currently insufficient high-quality evidence to support routine, early use of antiresorptive medications in critically ill adults. The Bone Zone trial is a phase III multi-centre randomised placebo-controlled trial of 450 women aged 50-years or greater and men aged 70-years or greater requiring intensive care admission for more than 2 calendar days, to determine the effect of denosumab or zoledronic acid on the prevention of bone loss in the year after critical illness.