A Jones fracture is a specific break occurring at the base of the fifth metatarsal, the long bone on the outside of the foot leading to the smallest toe. This injury is precisely located at the junction where the wide base of the bone meets the narrower shaft, a region clinically designated as Zone 2. Understanding the mechanism of this fracture is important because it involves a unique application of force that distinguishes it from other breaks in the same bone. This article will focus exclusively on the biomechanical forces and scenarios that cause this particular fracture.
Anatomical Context of the Fifth Metatarsal
The fifth metatarsal is commonly divided into three zones to help clinicians classify and treat injuries, with the Jones fracture isolated to Zone 2. This specific location is known as the metaphyseal-diaphyseal junction, situated approximately 1.5 to 3 centimeters from the bone’s base. The base of the fifth metatarsal also serves as the attachment point for the peroneus brevis tendon, which stabilizes the foot and is a key anatomical landmark.
The structure of the metatarsal base contributes to the vulnerability of the Zone 2 area. This region is considered a vascular watershed area, meaning it is poorly supplied by blood compared to the surrounding bone segments. This limited vascularity is why Jones fractures have a high risk of poor or delayed healing, which is a clinical consideration.
The Specific Forces Causing Injury
The Jones fracture mechanism is characterized by a high-force, indirect load that focuses stress onto the narrow neck of the bone. The injury occurs when the foot is forcibly moved into a specific, combined position. This position involves two primary movements occurring simultaneously: plantar flexion and forefoot adduction.
Plantar flexion means the toes are pointed downward, or the heel is raised, which puts the foot in an unstable position. Forefoot adduction is a movement that turns the forefoot inward toward the body’s midline. This combination of movements creates immense bending stress transmitted through the length of the metatarsal shaft, causing a break in the relatively weaker Zone 2, rather than being caused by a direct muscular pull.
The resulting fracture line is typically transverse or oblique, running through the metaphysis-diaphysis junction and often extending into the joint space between the fourth and fifth metatarsals. The acute break occurs when the force exceeds the structural limits of the bone, causing the shaft to buckle under the compression and bending forces.
Common Injury Scenarios
The acute forces required to cause a Jones fracture are typically generated during dynamic activities that involve rapid changes in direction or body position. The most common scenarios involve the foot being fixed on the ground while the body twists or shifts its weight abruptly.
Examples include athletes making quick, sharp pivots in sports like basketball, soccer, or football, where the foot is planted but the body rotates over it. The injury can also occur when landing awkwardly from a jump, such as coming down on the outside edge of the foot. Simply stepping off a curb incorrectly or rolling the foot with high force can also generate the necessary indirect adduction stress.
Distinguishing the Jones Fracture Mechanism from Avulsion Fractures
The fifth metatarsal is the site of another common injury, the avulsion fracture, which occurs in Zone 1, near the base of the bone. The mechanism of injury for a Jones fracture is fundamentally different from that of an avulsion fracture, even though they occur on the same bone. An avulsion fracture is caused by a sudden, forceful pull from an attached soft tissue structure.
In an avulsion fracture, the peroneus brevis tendon or the lateral band of the plantar fascia pulls a small fragment of bone away from the main structure. This is the classic “rolled ankle” injury, where a strong inversion force causes the tendon to contract violently, tearing off the piece of bone at its insertion point.
The Jones fracture, by contrast, is a “push” or bending injury caused by internal forces transmitted along the metatarsal shaft. The stress is generated by the foot’s position of plantar flexion and adduction, which overloads the bone itself at the metaphyseal-diaphyseal junction. This distinction in mechanism is why the two fracture types are treated differently, as the Jones fracture occurs in the more poorly vascularized portion of the bone.