What is the characteristic feature of grade II furcation involvement?

The characteristic feature of grade II furcation involvement is partial penetration of the probe into the furcation area. In this stage of furcation involvement, there is some loss of periodontal support at the furcation but not complete destruction. The probe can be inserted into the furcation area, indicating that there is a partial loss of bone but the furcation is not completely perforated.

This contrasts with other options: grade I involvement is characterized by no significant penetration, grade III would involve complete loss of periodontal support and visibility of the furcation on a radiograph, and grade IV would also show complete visibility of the furcation but with clinical attachment loss that results in an open area. Understanding these distinctions is crucial for diagnosing and planning treatment for periodontal conditions effectively.