Bone Mineral Density and Microarchitecture in Patients With Autosomal Dominant Osteopetrosis: A Report of Two Cases

The aim of this case study is to describe changes in areal bone mineral density (aBMD) by dual-energy X-ray absorptiometry (DXA) scan, as well as volumetric bone density and microarchitecture by high-resolution peripheral quantitative computed tomography (HR-pQCT) in two patients with autosomal dominant osteopetrosis (ADO) and compare with 20 healthy subjects. We describe a 44-year-old male patient with six low-impact fractures since he was age 16 years, and a 32-year-old female patient with four low-impact fractures on her past history. Radiographic changes were typical of ADO. Consistent with the much higher aBMD, total volumetric BMD (average bone density of the whole bone, including trabecular and cortical compartments) at distal radius and tibia (HR-pQCT) was more than twice the mean values found in healthy subjects in both patients. Trabecular number and thickness were higher, leading to an evident increase in trabecular bone volume to tissue volume. Also, an enormous increase in cortical thickness was found. Most important, a great heterogeneity in bone microstructure of the affected patients was evident on HR-pQCT images: islets of very dense bone were interposed with areas with apparent normal density. The increase in aBMD, volumetric BMD, and most indices of trabecular and cortical bone, associated with the great heterogeneity on bone tridimensional microarchitecture, reflect the accumulation of old and fragile bone randomly distributed along the skeleton. These alterations in bone microstructure probably compromise bone quality, which might justify the high prevalence of low-impact fractures in patients with ADO, despite abnormally elevated BMD.