التقاطعات الصافنية المأبِضية كلتاهما تعملان بكفاءة لمنع ارتداد السائل اللمفاوي

Here's the improved translation considering the details you've provided:

Instead of: التقاطعات الصافنية المأبِضية كلتاهما تعملان بكفاءة دون انعكاس تدفق

Use: الصمامات في كلا الملتقى الوريدي الصافن المأبضي سليمة ولا يوجد ارتجاع للدم.

Explanation of the changes:
"الصمامات" (valves) instead of "تعمل بكفاءة" (working efficiently): As you correctly pointed out, "competent" in this context refers specifically to the functionality of the valves within the veins. Therefore, mentioning the valves directly provides a more accurate representation of the medical finding from the ultrasound imaging.

"سليمة" (healthy/intact) instead of "تعمل بكفاءة" (working efficiently): This further clarifies that the valves are functioning as they should and preventing backflow of blood.
"ارتجاع" (reflux) instead of "انعكاس" (reflection): While both words can imply a backward flow, "ارتجاع" is the more precise medical term used to describe the backflow of blood in veins due to faulty valves.