What change is observed in the bone of area #21?

In the context of dental anatomy and specifically when examining the changes in bone surrounding a tooth, area #21 refers to the maxillary right central incisor. When evaluating the changes observed in this area, particularly due to various factors such as periodontal disease, tooth movement, or alveolar bone remodeling, a change in height is significant.

Changes in height could be attributed to the loss of supporting bone caused by periodontal issues, which often leads to a reduction in the vertical dimension of the alveolar process. This can be visualized as a change in the bone architecture that supports the tooth, impacting how well the tooth is retained and influencing overall dental health. The concept of height change is especially relevant in the context of prosthetic considerations, orthodontic treatment, or periodontal health.

In contrast, changes in length, density, or no change would not typically pertain to the common observations made in this specific area during an assessment of periodontal health or dental implants. Thus, "change in height" accurately captures the likely alterations seen in the bone surrounding the maxillary central incisor in clinical settings.