Final answer:
If the probe tip detects the concavity but cannot enter, it indicates a Grade I furcation. This represents incipient or early bone loss in the multi-rooted tooth's furcation area.
Step-by-step explanation:
If the probe tip can detect the concavity but cannot penetrate the furcation, it would be a Grade I furcation. Furcation grades are used to describe the extent of bone loss in the furcation area of multi-rooted teeth, such as molars. Grade I furcation involves incipient or early bone loss, which allows the probe to detect a concavity in the furcation area but does not permit the probe to enter any significant distance. It is often referred to as a hemifurcation. This type of grading is critical in periodontal diagnostics and treatment planning.