The Sternum's Developmental Journey
Ossification and Fusion in Early Life
The sternum is not a single bone from birth but a complex structure that undergoes significant transformation during childhood and adolescence. In fact, what we know as the breastbone begins as several segments of cartilage. A study in the National Institutes of Health's database highlighted that the most dramatic changes in sternum morphology occur in childhood and young adulthood as the bone ossifies.
- Sternebrae Fusion: The main body of the sternum is composed of four segments known as sternebrae. These segments start fusing together over time, a process typically completed by the mid-twenties, though the specific timing and pattern of this fusion are highly variable.
- Xiphoid Process Development: The smallest, triangular-shaped piece at the bottom of the sternum, the xiphoid process, is primarily cartilage until midlife.
Midlife Calcification and Fusion
As adults age, the changes continue, though they are less about overall growth and more about hardening and fusion. The xiphoid process, for example, begins to calcify and turn to bone around the age of 40 and often fuses with the main body of the sternum by age 60, though this can occur earlier or later. Some studies show this fusion generally occurs after 35-40 years, but there is wide individual variation. Similarly, the junction between the manubrium (the top segment) and the body of the sternum can also undergo a degenerative fusion, particularly in older females.
How the Sternum Interacts with the Aging Thoracic Cage
While the sternum's local geometry may not change dramatically after adulthood, its position and function are heavily influenced by broader changes in the thoracic cage. A 2017 study found that while local sternal geometry in females did not significantly change with age, its anterior placement was altered by shifts in the spine and ribs.
Spinal Curvature and Sternum Placement
With age, an increase in the natural outward curvature of the spine (kyphosis) occurs, especially in older women. This change in spinal alignment directly impacts the geometry of the entire chest, pushing the sternum further forward. This can lead to a flatter, more elongated rib cage, which changes the mechanical properties and function of the chest.
Changes in Rib Shape and Movement
Related to spinal changes, the ribs also undergo modifications. Ribs in older individuals can become more elongated and flatter, with their ends extending further forward. The connective tissues and joints also lose elasticity, which reduces the excursion of the ribs and sternum during breathing. This compromises the 'pump-handle' and 'bucket-handle' movements necessary for full respiratory capacity.
The Impact on Biomechanics
For senior adults, these combined skeletal and connective tissue changes mean the thorax is less flexible. This can increase the risk of injury, such as a sternal fracture, during a traumatic event like a car accident, as the more rigid structure is less able to absorb impact than a younger, more flexible one.
Comparison of Age-Related Sternal Changes
| Characteristic | Childhood/Adolescence | Young Adulthood | Older Adulthood |
|---|---|---|---|
| Sternebrae | Multiple cartilaginous segments | Segments fuse, forming one body | Fully fused, potentially with persistent lines of fusion |
| Xiphoid Process | Primarily soft, flexible cartilage | Begins to ossify and harden | Fully calcified, often fused with the body of the sternum |
| Manubriosternal Joint | Moves relatively freely | Stable, but can be a site of degenerative fusion | Can fuse, especially in females, through a degenerative process |
| Thoracic Cage Shape | More rounded; developing | Mature and stable shape | Flattens and elongates, with sternum moving anteriorly |
| Chest Wall Mobility | High degree of flexibility | Optimal respiratory excursion | Decreased range of motion during respiration |
The Role of Gender in Sternal Changes
Studies have shown that some age-related changes in the sternum and broader thoracic cage can differ by gender. For example, the rate of change in rib shape and angulation with age can be more pronounced in males, while the degenerative fusion of the manubriosternal joint appears to be more common in older females. Similarly, the anterior displacement of the sternum due to spinal changes is a well-documented finding in elderly females. For more information on age-related musculoskeletal changes, you can read this article.
Conclusion: A Lifetime of Transformation
In conclusion, the sternum is far from a static structure throughout life. It undergoes a remarkable journey from flexible cartilage to a complex, fused bone. While its size and shape stabilize after developmental ossification, its relationship with the broader thoracic skeleton continues to evolve. Changes in spinal curvature, rib geometry, and joint flexibility subtly but significantly alter its position and function, especially in later years. These shifts are a natural part of the aging process, influencing respiratory mechanics and even susceptibility to injury. Understanding these changes is crucial for appreciating the complexities of the human body and its adaptation over a lifetime.
