Final answer:
The primary factor for choosing periodontal flap surgery over gingivectomy is the need for access to the bony defect, as periodontal flap surgery allows for better management of underlying bone issues and deep pockets.
Step-by-step explanation:
The primary factor for selecting periodontal flap surgery rather than gingivectomy is the need for access to the bony defect. Periodontal flap surgery is chosen when it is necessary to gain access and visibility to thoroughly remove the diseased tissue and repair the bone structure, which is not possible with a gingivectomy. A gingivectomy is more appropriate for treating gingival overgrowth without underlying bone defects.
Pocket depth can also influence the decision, as deeper pockets may be more effectively managed with flap surgery. Subgingival calculus is another concern, as periodontal flap surgery allows better access for calculus removal below the gumline.