Duchenne muscular dystrophy (DMD) causes progressive muscle weakness and loss of ambulation. While glucocorticoid (GC) therapy improves motor function, many boys sustain fractures due to osteoporosis. Recently, i.v. bisphosphonate (i.v.BP) therapy has shown promise in the treatment of DMD-related osteoporosis. At the same time, bone histomorphometry revealed lowered bone volume and significant reductions in bone formation pre-i.v.BP treatment, and a further drop after 2 years therapy.
Given these findings, the purpose of this study was to evaluate the bone matrix mineralization density distribution (BMDD) in cancellous (Cn.) and cortical (Ct.) compartments on paired transiliac bone biopsies from five boys with DMD using quantitative backscatter electron imaging compared to healthy controls. The baseline biopsy was performed at a mean±S.D. age of 12.8±1.2 years, and the second was carried out between 2.1 and 2.8 years after the start of i.v.BP (pamidronate or zoledronic acid) therapy.
The typical (mode) Ca concentration in cancellous bone was high at baseline (Cn.CaPeak+5.6%, P<0.05) and even higher after i.v.BP treatment (Cn.CaPeak+7.2%, P<0.001) compared to reference, while the mineralization heterogeneity was normal before and decreased after i.v.BP (Cn.CaWidth −15%, P<0.01). Paired sample comparisons after i.v.BP showed non-significant increases in Cn.CaPeak in all but one boy, and decreases in Cn.CaWidth in all patients. Ct.BMDD variables were not different from controls at both timepoints.
Our findings suggest higher degrees of bone matrix mineralization in boys with DMD even before i.v.BP therapy, which we hypothesize is due to GC therapy. Although i.v.BP treatment effects failed statistical significance (low sample size), the comparison to normal suggests i.v.BP treatment further increases the magnitude, but decreases the heterogeneity of mineralization consistent with the earlier histomorphometric bone formation findings. Bone fragility in these boys with DMD appears to result mainly from reductions in bone volume and is further associated with elevated bone matrix mineralization.