Osteogenesis imperfecta (OI) is an extremely heterogeneous group of heritable connective tissue disorders. Most of the affected patients carry autosomal dominant mutations in the genes encoding for collagen type I, the most abundant protein of the bone extracellular matrix. The resulting phenotypes are extremely broad and have been classified by Sillence and colleagues into four groups according to clinical, radiological and genetic criteria.
More recently, proteins have been described that interact directly or indirectly with collagen biosynthesis and their deficiency result in rare forms of mostly autosomal recessive OI sharing phenotypic features of ‘classical’ types but lacking primary defects in type I collagen. Consequently the Sillence classification has been gradually expanded to include novel forms based on the underlying mutations. The goal of this article is to revisit the actual OI classification and to outline current approaches in categorizing the disorder.