**2. Pertinent arm anatomy**

The subcutaneous fat layer of the arm is divided into two layers: a superficial or areolar layer and a deep or lamellar layer, with the latter being more prone to fat accumulation [2]. With the arm by the patient's side, Hoyos [4] divides the arm into four regions: anterior, posterior, internal (adjacent to body), and external (away from body) (**Figure 1**). The major fat composition of the anterior, external, and internal regions is a thin layer of superficial fat. Pinch test in the internal region is commonly less than 1 cm and aggressive liposuction is typically avoided to prevent contour deformities [5]. The major neurovascular structures travel in the internal region, close to the bicipital groove, but are located deep to the deep fascia and away from zones of liposuction [5, 6]. The posterior region contains a distinct deep layer of fat that can vary in thickness with patient body mass index (BMI) and is focal to arm lipocontouring. The arm shape is primarily determined by the muscular composition of the deltoid, biceps, triceps muscles, and the overlying fat [4]. The ideal arm contour is defined by convexities and concavities, highlights and shadows that correspond to underlying musculature. With the arm and elbow abducted at 90 degrees, shape of the anterior arm is defined by the deltoid and biceps muscle mass convexities. The shape of the posterior arm is more intricate, with triceps muscle mass creating a convexity in the midportion and concavities proximally at

#### **Figure 1.**

*Fat distribution and zones of contour assessment in the arm, according to Hoyos [4]. The arm is divided into 4 zones: anterior (1), external (2), posterior (3), and internal (4). Fat is divided into a superficial (areolar) and a deep (lamellar) layer in the arm. The posterior zone (3) is prone to fat accumulation in the deep layer. The remaining zones are largely comprised of superficial fat layer and less susceptible to fat accumulation in normal BMI ranges. (Image reprinted with permission).*

muscle insertion and distally owing to the flat triceps tendon [4]. Arm contouring techniques, whether by liposuction or excisional surgery, aim to enhance these ideal aesthetic arm shapes, while respecting the patient's overall body habitus to create a desired and harmonious silhouette.
