American Board of Orthodontics (ABO) Practice Exam

On which side are elevated levels of OPG found during tension?

1. Tension side of PDL

2. Compression side of PDL

3. Bone marrow

4. Gingival tissue

The correct answer is: Tension side of PDL

Elevated levels of Osteoprotegerin (OPG) during orthodontic tension are found on the tension side of the periodontal ligament (PDL). OPG is a glycoprotein that plays a critical role in bone remodeling by inhibiting osteoclastogenesis, the process through which bone is resorbed. When tension is applied to the PDL, such as in orthodontic treatment, the body responds by decreasing osteoclast activity, which is necessary to prevent bone loss in the area experiencing tension. This is facilitated by the increased production of OPG. On the tension side, OPG acts to protect the bone from being resorbed in response to the forces applied, thereby promoting bone formation and maintenance. This biological response is essential for the effective movement of teeth through the alveolar bone without compromising bone integrity. In contrast, on the compression side, the opposite reaction occurs, where there is an increase in osteoclast activity and a decrease in OPG, leading to bone resorption to allow for tooth movement. Therefore, recognizing where OPG levels are elevated during tension is key to understanding the mechanisms of tooth movement in orthodontic practice.