How Normal Are the Abnormalities in the Human Skeleton?

FROM THE LECTURE SERIES: How We Move: The Gross Anatomy of Motion

By Elizabeth A. MurrayMount St. Joseph University

Do you have any idea how many bones there are in the human body? The number actually varies with age. There’s a general pattern of growth and development in which the skeleton starts as other types of connective tissue, including the cartilage. Then, throughout our prenatal and postnatal development, some of that cartilage begins to ossify, which means it transitions to bone.

Human bone close-up, isolated on black background
The development of a typical long bone is only finished after the diaphysis is fused to the epiphyses. (Image: bdavid32/Shutterstock)

Development of a Bone

A typical long bone—like the humerus of the upper arm or femur of the thigh—doesn’t just ossify in a single bony element. First, the shaft, called the diaphysis in anatomy, starts turning into bone, becoming what’s known as the bone’s primary center of ossification. Then, the two ends, called epiphyses, turn to bone, becoming secondary ossification centers. 

This process leaves bridges of cartilage between the shaft and its two ends. These are the bone’s growth plates—you’ve probably heard of those; they are the regions where the bone continues to grow in length during childhood.

Eventually, when the bone has achieved its genetically programmed length—provided good health and nutrition—the growth plates close up, fusing the diaphysis to the epiphyses and resulting in a single bone. Long bones of the body grow that way; the skull and other types of bones form from other soft tissues in different patterns of ossification.

Different Bone Counts

Ultrasound of a fetus
A fetus in its third trimester can have up to 800 separate bones. (Image: Igor Dolgov1974/Shutterstock)

But back to the bone count. When these long bones are developing and still composed of multiple different bony elements, technically we actually have far more than the typical 206 bones that we’re told we have. 

In the third trimester of fetal life, if all ossification centers are counted, at the highest point the number is about 800 tiny ‘bones’—bones in quotes there, since they would be better described as 800 separate ossification centers, or bits of bone in formation. Some of these bony elements fuse together in utero, resulting in about 450 ‘bones’ at birth—but sources vary on the exact numbers.

In fact, over anatomical history, sources have varied significantly even in the total bone count in the adult skeleton. The total attributed from Galen’s work was 248, whereas Vesalius counted 307, and the modern number is 206. It wasn’t that the number changed through the past few thousand years but depended on how they counted—like counting the sternum as one bone or two. 

But even after the adult skeleton is formed, and the last growth region at the medial end of the clavicle closes, the number of bones can still change. For instance, arthritis can fuse vertebrae, the 22 bones in the skull often fuse together in old age, and arthritis or injury can fuse small elements like finger and toe bones. So, technically once that happens, an individual doesn’t really have 206 separate bones anymore.

This article comes directly from content in the video series How We Move: The Gross Anatomy of MotionWatch it now, on Wondrium.

Abnormality in the Human Body Is Quite Normal

X-ray of a fractured left clavicle
Injuries and bone fractures can potentially lead to the fusion of two bones that were previously separate. (Image: Puwadol Jaturawutthichai/Shutterstock)

Still, aside from the changes that occur with age, 206 is an average number of bones in the normal adult. Individuals can be born with extra bones or fewer bones; it’s not even all that rare—in fact, unusual skeletal features can help get a person identified. For example, we may suspect who the person is from other clues—like an ID card, or simply the location where they’re found. 

And if we can find an x-ray of that suspected person that includes the unusual feature seen in the morgue, we can match those up and get a head start on identification, long before the DNA results are back. In fact, for any given anatomical feature, there’s what one could call the 70% rule: It’s estimated that only 70% of us have the by-the-book pattern for a given structure, meaning that 30% of people have anomalies. 

So, if 100 people are reading this now, that means that around 70 of them have 206 bones, and 30 of them probably have some variation in bone number—and not just from age or maybe an injury that fused bones, but congenitally, from their own genetics and development. And when you consider how many different anatomical structures we each have, and that approximately 30% of us have anomalies for each one of those, it’s no wonder we see so much diversity in ourselves and in each other. 

And despite years of teaching gross anatomy, one will still see things they’ve never seen before. But we can usually look up the anomaly and find it in reference lists of so-called normal variants, which is kind of an oxymoron. Some of these unusual features are common enough that science has statistics for how often they occur. As some say in gross anatomy, “The only constant in anatomy is variation!”

Common Questions about the Abnormalities in the Human Skeleton

Q: How does a typical long bone form?

In a human body, a typical long bone’s formation starts with the shaft or diaphysis turning into bone. This shaft is the primary ossification center and later the two ends become the secondary ossification centers, also called epiphyses. On each side of the primary ossification center are the growth plates which eventually stop growing and the diaphysis and the epiphyses will fuse together.

Q: How many bones does a human body have on an average?

The average amount of bones in an adult human body is 206.

Q: What are the factors that decide how many bones a person has?

The amount of bones in the human body is always affected by age. But it can also be affected by injuries that result in the fusion of some bones, or illnesses like arthritis which fuse vertebrae. Another important factor is genetic differences. It’s estimated that around 30% of the population don’t have the typical amount of bones one might predict.

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