Which instrument is not appropriate for removing calculus deposits below the gumline?

The sickle scaler is designed primarily for supragingival scaling; it has a pointed tip that is effective for removing calculus deposits located above the gumline. This design is not suited for navigating the contours and delicate tissues found subgingivally, where there is a higher risk of damaging the periodontal tissues due to the sharpness and angulation of the instrument.

In contrast, both the Gracey curette and the universal curette are specifically designed for subgingival scaling. The Gracey curette has a curved cutting edge that allows for effective scaling within the periodontal pocket, while the universal curette can be used on various surfaces, including both subgingival and supragingival areas. Therefore, among the available instruments, the sickle scaler is the one that is not appropriate for removing calculus deposits that are located below the gumline.