What characterizes grade I furcation involvement?

Grade I furcation involvement is characterized by only soft tissue involvement with a slight loss of supporting bone in the area where the roots of a multi-rooted tooth diverge. At this stage, the furcation is not clinically detectable through probing; rather, it is defined as a slight curvature of the probe into the furcation without any significant penetration. This indicates an early stage of periodontal disease where there is minimal bone loss, allowing for some maintenance of periodontal support. In this context, the condition can often be managed with effective periodontal therapy, as significant structural integrity remains and further progression can potentially be halted.

In contrast, the other options present conditions that are typically associated with more advanced grades of furcation involvement, where the loss of bone is more pronounced or the structural integrity of the furcation is compromised. Hence, they are not descriptive of grade I involvement.