Final answer:
A furcation probe tip that detects concavity but cannot penetrate the furcation indicates a Grade I furcation. This reflects early bone loss detectable by a probe without significant space for the probe to enter between the roots.
Step-by-step explanation:
If a furcation probe tip can detect the concavity but cannot penetrate the furcation, this would be a Grade I furcation. In dental terminology, Grade I furcation involvement is characterized by the presence of early bone loss which can be detected by a probe. The probe can feel the concavity of the furcation area, but there isn't a significant loss of bone, and thus the probe cannot enter the space between the roots.
In comparison, a Grade II furcation allows the probe to enter the space but not pass completely through. Grade III furcation permits the probe to pass completely through the furcation, whereas Grade IV has similar characteristics to Grade III but with the furcation visible due to gingival recession.