**Background:** As diagnostic tools and targeted therapies improve for pediatric metabolic bone diseases, affected children have better prognoses and longer lifespans, creating an urgent need for dedicated transition programs to adult care. While transition guidelines exist for conditions like type 1 diabetes and congenital adrenal hyperplasia, there are significant gaps for metabolic bone disorders. This narrative review aims to compile evidence and best practices for transitioning adolescents with primary bone disorders (hypophosphatasia, X-linked hypophosphatemic rickets, osteogenesis imperfecta) and secondary bone disorders (related to hypogonadism, eating disorders, and childhood cancer survivorship) into adult care.
**Methods:** The authors conducted a narrative review synthesizing research and guidelines on transitions of care generally, followed by detailed treatment of specific bone disorders. They reviewed literature on bone health assessment via dual X-ray absorptiometry (DXA), noting critical differences between pediatric and adult standards: children use Z-scores adjusted for height, with preferred sites being total body less head (TBLH) and lumbar spine; adults under 50 also use Z-scores, while menopausal women and men ≥50 use T-scores, with preferred sites including lumbar spine, total hip, and femoral neck. The review incorporates data from clinical trials, cohort studies, registries, and consensus statements.
**Key Results:** For hypophosphatasia (HPP), enzyme replacement therapy with asfotase alfa is approved for severe forms; a non-randomized study in 14 adults showed significant improvement in six-minute walk test and timed up-and-go test. For X-linked hypophosphatemic rickets (XLH), burosumab demonstrated sustained improvements in mobility and pain—mean additional six-minute walking distance after 24 weeks was 23 meters (p<0.001). A cohort of 52 adults with XLH (mean age 41.8 years) showed a link between enthesopathy and decreased quality of life. For osteogenesis imperfecta (OI), bisphosphonates are standard in children but show limited benefit in adults; teriparatide increased BMD at lumbar spine and total hip in a trial (mean age 41 years) but without significant fracture rate difference. In childhood cancer survivors, the IGHG strongly recommends BMD surveillance 2-5 years post-therapy for those with cranial or craniospinal radiation, with repeat at age 25. The St Jude LIFE study found only 5.7% of survivors had BMD Z-score ≤ -2 at median age 31. For eating disorders, lifetime fracture risk in anorexia nervosa is 2- to 7-fold higher than the general population. In Turner syndrome, 50-80% of women not receiving estrogen have low BMC. For Klinefelter syndrome, a meta-analysis showed testosterone replacement increases lumbar spine BMD.
**Clinical Implications:** Adult providers must navigate differences in DXA interpretation across the lifespan, including the transition from Z-scores to T-scores and changes in preferred skeletal sites. Key considerations include continuation of disease-specific therapies (asfotase alfa, burosumab, bisphosphonates) into adulthood, monitoring for complications that emerge with age (enthesopathies in XLH, pseudofractures in HPP, auditory and pulmonary issues in OI), and addressing fertility, pregnancy medication safety, and genetic counseling. Multidisciplinary teams are essential, and resources like the Soft Bones organization, XLH Network, OI Foundation, and Turner Syndrome Foundation provide patient support. The authors identify two major gaps: limited actionable guidelines synthesizing outcome data, and a lack of studies evaluating outcomes in actual transition clinic settings that incorporate existing recommendations.