Periodontium is a related term of periodontics.

As nouns the difference between periodontics and periodontium

is that periodontics is the study of supporting structures of teeth—gums, alveolar bone, cementum, and the periodontal ligament—and diseases and conditions that affect them while periodontium is the specialized tissues that both surround and support the teeth, maintaining them in the maxillary and mandibular bones; the tissues including alveolar bone, cementum, gums and periodontal ligament.