The human foot is a complex structure made of 26 bones, 33 joints, and over 100 tendons, muscles, and ligaments, designed to absorb shock and support the entire body’s weight. When people refer to the “outside” of the foot, they are pointing to a specific, highly functional region that plays a distinct role in balance and movement. This outer edge is structurally different from the inside arch and the sole. The specialized language of anatomy helps to precisely define this area, which is frequently prone to injury, particularly during athletic activities or missteps.
The Correct Anatomical Term
The formal anatomical term for the outside of the foot is the Lateral Aspect or Lateral Border. This designation uses the directional term “lateral,” which in anatomy describes a position or structure located farther away from the midline of the body. Conversely, the inner edge of the foot, containing the main arch, is known as the Medial Aspect or Medial Border.
The lateral border extends from the heel bone, or calcaneus, to the little toe, and it is sometimes referred to as the fibular border, named for the fibula bone in the lower leg. This side of the foot forms the lateral longitudinal arch, which is noticeably flatter and lower than the high, dynamic arch on the medial side. The lateral aspect contacts the ground more readily and is designed to act as a stable, weight-bearing column.
Key Structures Supporting the Lateral Foot
The lateral aspect is composed of a tightly integrated group of bones, tendons, and ligaments that work together to maintain stability. The bony foundation includes the calcaneus, the cuboid bone in the midfoot, and the fifth metatarsal, which is the long bone connecting to the little toe. The base of the fifth metatarsal is particularly important because it features a prominent bump that serves as an attachment point for tendons and is a common site for certain fractures.
Running along this outer edge are the peroneal tendons, which are formally known as the fibular tendons. These include the peroneus longus and the peroneus brevis, which pass behind the ankle’s outer bump, the lateral malleolus. The primary role of these tendons is to facilitate eversion, the motion that turns the sole of the foot outward, and to stabilize the ankle against excessive inward rolling.
The ankle joint’s stability on the lateral side is secured by a set of three strong, yet frequently injured, ligaments. These are the Anterior Talofibular Ligament (ATFL), the Calcaneofibular Ligament (CFL), and the Posterior Talofibular Ligament (PTFL). These ligaments connect the bones of the lower leg to the bones of the foot and act to limit excessive movement, particularly the inward rotation of the foot.
Understanding Common Lateral Foot Injuries
The anatomy of the lateral foot, while designed for stability, makes it susceptible to specific mechanisms of injury. The most frequent injury in this region is the inversion ankle sprain, which occurs when the foot rolls inward, forcefully stretching or tearing the lateral ankle ligaments. The ATFL is typically the first and most commonly injured ligament in this type of sprain, often followed by the CFL in more severe cases.
Another injury specific to the lateral side involves the fifth metatarsal bone. A Jones fracture is a break in the shaft of the fifth metatarsal, an area known for having a limited blood supply, which can make healing difficult. An avulsion fracture, also common in this bone, occurs when a sudden, forceful pull from the attached peroneal brevis tendon yanks a small piece of bone away from the main structure.
The peroneal tendons themselves can become inflamed, a condition known as peroneal tendonitis, typically resulting from overuse, repetitive stress, or uncorrected biomechanical issues like high arches. This condition causes pain and swelling along the outer side of the ankle and midfoot, often worsening with activity.