A new concept: ‘Relative position between the external force and the bony prominence’ explains location-specific occurrence of superficial injury over an undermining lesion

•Superficial injuries that developed over an undermining lesion of pressure ulcers were examined.•This type of pressure ulcer was termed a bilayer pressure ulcer.•A bilayer pressure ulcer is induced by both pressure and shear force.•Bilayer pressure ulcers were more frequent among pressure ulcers over the sacrum.•Relative position between the external force and bone is critical for development.