Final answer:
The tooth surface is not affected by periodontitis, unlike the other structures mentioned.
Step-by-step explanation:
Periodontitis is a condition that affects the structures surrounding the teeth. However, out of the options provided, the structure that is not affected by periodontitis is the tooth surface. Periodontitis mainly affects the gingiva (gums), periodontal ligament, alveolar bone, and cementum. These structures can experience inflammation, tissue damage, and bone loss due to the infection.