Sentence:  The electric gadget is most helpful when there are many crowned <WSD>teeth</WSD> and in individuals who are elderly , bedfast with a chronic disease , or are handicapped by disorders such as cerebral palsy or muscular dystrophy . 
Meaning of the word marked with <WSD> tags:  hard bonelike structures in the jaws of vertebrates; used for biting and chewing or for attack and defense