Final answer:
The best instrument for scaling and root planing the specified area of tooth #18 is the Gracey curette, due to its design allowing for precision in area-specific scaling and root planing tasks.
Step-by-step explanation:
The appropriate instrument for scaling and root planing the distal aspect of the mesial-buccal root of tooth #18 is the Gracey curette. The Gracey curette is specifically designed with a lower blade angle which is ideal for adapting to specific surfaces of the teeth, making it optimal for accessing and instrumenting the roots effectively. In contrast, the other instruments listed have more generalized uses and may not provide the same level of precision for this task. Curettes can be used for general scaling, but Gracey curettes have area-specific designs. An ultrasonic scaler could be used for initial debridement but lacks the precision of a Gracey curette for fine scaling and root planing. A periodontal probe is used for measuring pocket depths and is not designed for tooth scaling or root planing.
In this case, a Gracey curette designed for posterior teeth, such as the Gracey 13/14, which is specifically made for the distal surfaces of molars, will provide the best combination of access and angulation. Using a Gracey curette allows the clinician to reach difficult areas while maintaining the efficacy of the scaling and root planing procedure.
For precise and effective scaling and root planing of the distal aspect of the mesial-buccal root of tooth #18, the Gracey curette is the best choice. Its design is specifically tailored for the task, allowing for more targeted and efficient treatment.