A bone fracture is a break in the continuity of a bone that occurs when a physical force exceeds the bone’s structural capacity. Bones are layered structures, possessing a dense outer shell and an inner core. For clinicians, specifying the damaged layer is important for determining the severity of the injury and the appropriate treatment. The term “cortical fracture” refers to a break involving this specific hard outer layer of bone.
Understanding Cortical Bone Structure
Bone tissue is organized into two primary types: cortical and cancellous bone. Cortical bone, also known as compact bone, forms the dense, strong exterior of the bone and accounts for approximately 80% of the total skeletal mass. This hard outer layer provides the primary resistance to mechanical stress and gives bone its smooth, solid appearance.
The cortex is thickest in the mid-shaft sections of long bones, known as the diaphysis, where it is needed most for support and weight-bearing. Cortical bone provides structural support for the entire body, protects internal organs, and offers levers for muscle attachment and movement. In contrast, cancellous bone (trabecular bone) is the spongy, porous inner layer found primarily at the ends of long bones and within the vertebrae. Cancellous bone is lighter and more metabolically active than the cortex, but it does not provide the same degree of load-bearing strength.
Distinguishing Features of a Cortical Fracture
A cortical fracture is a disruption that occurs specifically within the dense, compact outer shell of the bone. This break can range from a tiny crack to a complete separation of the outer layer. A key distinction is that a cortical fracture may not involve the underlying cancellous bone or the entire circumference of the bone.
These injuries are often categorized as incomplete fractures, meaning the fracture line does not fully traverse the entire bone. Examples include a greenstick fracture, common in children, where the break occurs through one side of the cortex while the opposite side merely bends. Another incomplete break is a buckle fracture (torus fracture), where the cortex wrinkles or bulges outward but does not fully separate.
A break that extends through the entire thickness of the cortex and into the marrow space is classified as a complete fracture. Complete fractures are described by the pattern they create, such as a transverse break straight across the bone or a spiral fracture resulting from a twisting force. Fractures are also classified as non-displaced, where the broken fragments remain in alignment, or displaced, where the fragments have shifted out of their normal position.
Common Mechanisms of Cortical Injury
Cortical fractures occur when mechanical forces exceed the high compressive and tensile strength of the compact bone tissue.
High-Impact Trauma
One of the most common causes is high-impact trauma, such as a fall or a direct blow, which delivers a sudden, powerful force to the bone. This acute force causes the cortex to fail immediately, often resulting in a clean break or a comminuted fracture with multiple fragments.
Rotational Forces
Another mechanism involves rotational or twisting forces, which are damaging to the rigid structure of the cortex. These torsional stresses can cause the bone to break in a spiral pattern, where the fracture line wraps around the shaft of the bone. This is common in sports injuries or accidents where a limb is suddenly twisted while the foot remains planted.
Repetitive Stress
A distinct cause is repetitive stress, which leads to a stress fracture, typically originating as micro-breaks in the cortex. Unlike acute trauma, this injury results from chronic, low-level forces that accumulate faster than the bone can repair itself through remodeling. Stress fractures often present as hairline cracks in the cortex and are common in athletes involved in endurance activities.
Diagnosis and General Treatment Approaches
Diagnosing a cortical fracture typically begins with a physical examination followed by medical imaging. The standard first step is a plain X-ray, which is usually sufficient to confirm the presence of a fracture and assess its alignment and location. X-rays clearly show the dense, white outline of the cortical bone and any breaks within it.
In cases where the fracture is subtle, such as a non-displaced stress fracture or a hairline crack, a more detailed scan may be required. Computed Tomography (CT) scans offer a three-dimensional view, providing a clearer picture of the fracture configuration and the extent of cortical damage. Magnetic Resonance Imaging (MRI) is also utilized for detecting occult fractures, providing better soft tissue detail and helping to differentiate between a bone bruise and a true stress fracture.
General treatment focuses on stabilization and immobilization to allow the bone to heal naturally. Non-displaced fractures are often managed with a cast, splint, or brace, which restricts movement and keeps the bone fragments in their correct position. This immobilization allows the body’s natural bone remodeling process to bridge the break in the cortex.
If the cortical fracture is severely displaced or unstable, surgical intervention may be necessary to realign the fragments. This often involves open reduction and internal fixation, where the surgeon uses plates, screws, or rods to hold the cortical pieces together. Recovery involves a period of rest followed by physical therapy to restore strength and range of motion to the affected limb.