Sternal Changes Throughout the Lifespan
An individual's sternum undergoes a series of predictable, yet highly variable, changes from childhood through advanced age. These changes involve the fusion of separate bone segments, the calcification of cartilage, and alterations in bone marrow composition.
The Journey of Sternal Fusion
The sternum is composed of three main parts: the manubrium, the body (composed of four segments called sternebrae), and the xiphoid process. During growth and development, these sections fuse at distinct junctions, and the timing of this fusion is one of the primary indicators studied to understand what the sternum can indicate about age. The process is not uniform and exhibits significant individual and population-level variability.
Fusion of the Sternebrae
The body of the sternum is formed by the fusion of four segments, a process that occurs progressively upward. This process typically starts around puberty and is completed by the mid-20s. By examining the state of these junctions, forensic experts can determine if a person is a juvenile or a young adult. Once these segments are fully fused, however, they are no longer useful for precise age estimation.
Manubrio-Sternal Joint
The manubrio-sternal joint connects the manubrium to the body of the sternum. This joint is highly variable in its fusion timing, often remaining unfused or only partially fused even in advanced age. While some studies show fusion is more common in individuals over 60, it is not a reliable marker for precise age estimation due to its unpredictable nature.
Xiphisternal Joint
The xiphoid process, the pointed end of the sternum, starts as cartilage and ossifies (turns to bone) and fuses to the sternal body later in life, often after age 40.
Variability in Fusion Timing
| Fusion Timing Variations | Aspect | Young Adulthood | Middle Age (40+) | Advanced Age (60+) |
|---|---|---|---|---|
| Sternebrae Fusion | Typically complete. | Fully fused. | Fully fused. | |
| Manubrio-Sternal Joint | Often unfused. | Variable: can be fused, partially fused, or unfused. | Fusion more likely, but still highly variable and may remain unfused. | |
| Xiphisternal Joint | Unfused or cartilaginous. | Often shows ossification and early fusion. | Fully ossified and fused in a high percentage of individuals, but still variable. | |
| Costal Cartilage | Minimal calcification. | Increasing calcification. | Significant calcification, especially on the periphery. | |
| Bone Marrow | High hematopoietic cellularity. | Declining cellularity, increasing fat. | Significantly reduced cellularity, high fat infiltration. | |
| Sternal Morphology | Reaches adult size and shape around 30. | Remains constant in size and shape. | No major morphological change, but density changes. |
Ossification of the Xiphoid Process
The xiphoid process is a small, often pointed projection at the bottom of the sternum. Born as pure cartilage, it begins its transformation into bone through ossification over a lifetime.
The Calcification Timeline
- Childhood and Adolescence: The xiphoid process is primarily cartilaginous.
- Early Adulthood: Ossification begins, but the process can be slow and variable.
- Middle Age (40+): It is common for the xiphoid process to begin fusing to the sternal body. Some studies suggest a mean fusion age of around 45 years.
- Advanced Age (60+): In most cases, the xiphoid process is fully ossified and fused with the sternal body, though it can remain cartilaginous even into old age.
Age-Related Changes in Bone Marrow
Bone marrow is a living tissue with functional changes over a lifetime. In children and young adults, the sternal bone marrow is highly cellular, active in hematopoiesis (blood cell production).
Marrow Involution
With age, the cellularity of bone marrow decreases, and fatty tissue increases. This process, known as marrow involution, is a clear age indicator.
- At birth: Marrow is highly cellular, with approximately 90% hematopoietic tissue.
- By age 30: This cellularity drops to around 50%.
- By age 70: Cellularity may decrease to as little as 30%.
The Aging of Costal Cartilage
The costal cartilages connect the ribs to the sternum. As part of the natural aging process, these cartilages calcify and ossify, becoming stiffer and less flexible.
Calcification Patterns
- Calcification increases with age, particularly after the third decade of life.
- Patterns of calcification can differ between sexes, with some studies suggesting more pronounced calcification in females.
- The calcification is primarily found on the outer surface of the cartilage, which can be observed on radiological scans.
Sternal Morphology and Fractures
Significant morphological changes in the sternum's size and shape occur primarily during childhood and young adulthood. After approximately age 30, the size and shape remain relatively constant. However, the increased incidence of sternal fractures in the elderly is likely due to changes in bone mineral density and cortical thickness, not major morphological shifts.
The Role of the Sternum in Forensic Age Estimation
For forensic anthropologists, the sternum offers clues, but its high variability means it is rarely used as the sole indicator of age at death. Instead, it is used as one piece of evidence, alongside other skeletal markers such as the pubic symphysis, auricular surface, and dental development.
- The presence of fused sternebrae can suggest an individual has surpassed their late teens or early twenties.
- The fusion of the xiphoid process with the sternal body suggests an age above 30, and usually older.
- Conversely, unfused sternal components in an adult can be very misleading due to high individual variation.
Conclusion: Interpreting the Sternum's Secrets
The sternum is a fascinating bone that reflects the passage of time through several age-related changes, including the fusion of its segments, the ossification of its xiphoid process, and changes in its bone marrow. While these changes provide valuable clues for estimating age, especially in forensic contexts, their inherent variability means they must be interpreted cautiously and in conjunction with other indicators.
For a deeper look into human osteology and forensic analysis, refer to resources like the U.S. National Library of Medicine, which provides detailed studies on the aging skeleton and its applications in forensics: NCBI Bookshelf.
