Abnormal diaphysis morphology Diaphyseal undertubulation Tubulation refers to the size and shape of tubular bones. In children and adolescents, the modeling process regulates normal bone growth. Final shaft (tube) diameter depends on appositional bone growth and the equilibrium between periosteal and endosteal bone resorption and formation. Undertubulation refers to a broad, widened form of the shafts (diaphyses) of long bones.