Anatomical Continuation Between the Sub-Superficial Musculoaponeurotic System Fat and Retro-Orbicularis Oculi Fat: The True Nature of the Retro-Orbicularis Oculi Fat.

Objective: This study aims to analyze the anatomical location and continuation between the retro-orbicularis oculi fat (ROOF) and sub-superficial musculoaponeurotic system fat (subSMAS fat; named "innominate fascia") by comparing their layered structures, thereby letting us suggest a safe minimally invasive procedure guideline for the forehead and temple. Methods: Ultrasonographic scanning was performed from the upper medial eyebrow to the lateral side of the superior temporal line in 109 volunteers. Hematoxylin and eosin staining was performed on five specimens at the same area as ultrasonographic scanning. Then, four embalmed cadavers were dissected from the orbicularis oculi, frontalis muscle, superficial temporal fascia, and SMAS to confirm the location and continuation pattern of the ROOF and subSMAS fat. Results: On ultrasonography and histological images, there was a continuous fatty layer from the ROOF to the subSMAS fat. When dissecting, the ROOF, which was located deep below the frontalis muscle, also continued to the subSMAS fat, passing through the superior temporal line in the upper temporal region. Conclusions: This study confirmed that the subSMAS fat is an anatomical fat structure that is continuous with the ROOF. Since the subSMAS fat layer is known as a less vascular area, it is considered a safer layer to avoid serious complications, and injecting accurately into the subSMAS fat layer has been a goal of clinicians. Based on this study, a cannula will safely approach the subSMAS fat through the ROOF injection that named "forehead-downward approach."