non-red nail polish part of the needle, representing the implanted section of the needle, was determined using the measuring tape (Figure 2A). The angles of penetration were measured with a protractor directly on the pictures (Figure 2B).
To achieve our objectives, we relied on ultrasound images, live pictures, and mathematical formulations. Specifically, we calculated that the depth of penetration can be determined by multiplying the sine (sin) angle of entry times the exact length of the needle’s penetration into the skin. Mathematically represented, depth of penetration=sin (angle) x length inserted. (This formula was provided by Mr B Hadjab [Geneva, Switzerland], an engineer not associated with the project).10,11
Using the theoretical mathematical formula depth of penetration=sin (angle) x length inserted, an inserted needle length would be 5.0 mm to 8.0 mm. This length is often used by physicians in the clinical setting, with the classical point by point technique of injection—not the retrograde technique of injection. (In the retrograde technique, the entire needle length [13 mm] is implanted in the skin, and the gel is slowly delivered when the injector withdraws the needle.) Using the formula, sin 30° X 5.0 mm = 2.5 mm and sin 30° X 8.0 mm = 4.0 mm for minimal and maximal injection depth. In 45° angle calculation, sin 45° X 5.0 mm = 3.5 mm and sin 45° X 8.0 mm = 5.7 mm.
With the “blanching technique†of injection14 and inserting only the bevel of the needle, the inserted needle length would be 1.1 to 1.5 mm. This distance is just beyond the length of the needle bevel, requiring precise and delicate injection for point by point HA delivery, rather than for retrograde injection. (With the retrograde injection technique, it is impossible to stay in the superficial dermis all along the injection, even with a bent needle.) Using the same formula for calculation, sin 30° X 1.1 mm = 0.6 mm and sin 30° X 1.5 mm = 0.8 mm for injection depth into the superficial dermis. Depths for injection into the mid dermis are as follows: sin 45° X 1.1 mm = 0.8 mm and sin 45° X 1.5 mm = 1.1 mm.
In 2012, Della Volpe and colleagues published an article that listed the depths of various layers of the skin of various body
areas.7 The Della Volpe measurements in the cheeks and buttocks are shown in Table 1. The investigators in the study reported here assumed that the measurements in the cheeks and the buttocks are roughly equivalent to those in the nasolabial folds. Reference literature with histological data 7-9 shows that the mean thickness of the epidermis is 0.15 mm and the mean thickness of the papillary dermis is 0.20 mm. Thus, to inject into the superficial reticular dermis, the needle has to be placed between 0.351 mm and 1.684 mm.7 For insertion into the mid reticular dermis with its thin skin, the needle has to be placed between 1.35 mm and 3.018 mm.7
Methods of Investigation
Ultrasound
In the 2 ultrasound imaging procedures reported here, the depth of the needle placement was controlled by a specialist in radiology (SB). In the first procedure, the needle was inserted into the superficial reticular dermis.12 In the second procedure, the needle was inserted into the mid reticular dermis.13
In both procedures, the radiologist measured the epidermis and the dermis thickness before needle insertion. After the needle had been inserted, the investigator measured the needle penetration angle, the length of implanted needle section, and the bevel top depth (Figure 2C).
Biopsies
Several studies—two published and one still in preparation--also provide us with histological measurements of epidermis and papillary dermis thickness. 13- 15 In these studies, the thickness of the epidermis and of the papillary dermis—uninfluenced by the injections-- has been measured by a specialist in histopathology (DS). These measurements are similar to the ones determined in the Della Volpe data.7
Live injections
In the first of 2 live injection procedures, the injector (PM) placed the needle on patients in his office for conventional NLF treatment with no assistance from ultrasound. The needle placement, before any injection, was immediately photographed by
