Which method is not useful in determining the safety zone above the mandibular canal?

The method that is least useful in determining the safety zone above the mandibular canal is Water's view radiographs. This radiographic technique is primarily utilized for visualizing the maxillary sinuses and the nasal cavity rather than providing specific information about the inferior alveolar nerve canal and the structures in the mandible.

Panoramic radiographs, on the other hand, are widely used in dentistry for assessing the entire jaw and can provide a comprehensive view of the relationship between the implants and the mandibular canal. Reformatted CAT scans offer detailed three-dimensional imaging that allows for precise localization of anatomical structures, making them highly effective for planning implants safely without risking damage to the mandibular canal.

The exposure of a flap, while it might seem invasive, provides direct visual access to the underlying structures and can effectively assess the relationship of the implant site to the mandibular canal. Thus, it is more reliable for surgical planning than a Water's view radiograph, which does not focus on the relevant anatomy in this context.