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What Happens When A Bone Breaks (Fractures) And Heals?

Bone Breaks And Heals

We might think of bones as being dry, rigid structures. But actually, bones, like all the other tissues in our body are dynamic living things. They are part of our body’s unique structure which has to bridge our needs for both stability and mobility. Our bones are highly adaptable and their structure reflects the individual stresses that we put on them. The ultimate stress for our bones is a break. But, our bones are amazing in that, even after most breaks, they can fully heal. Read on to learn more about the process that happens when a bone breaks and then heals.

Bone composition: What is bone made of?

Let’s start with a little anatomy. Our bones are a type of connective tissue. Like all types of connective tissue, bone tissue is made up of an extracellular matrix and the cells and fibers which exist within that matrix. The extracellular matrix of bone is mostly composed of collagen and various other types of proteins. Several types of bone cells exist within that matrix: mesenchymal stem cells (sometimes called osteoprogenitor cells), osteoblasts, osteoclasts, and osteocytes. The roles of stem cells, osteoblasts, and osteoclasts are particularly important in healing a broken bone.

Each bone cell type has a role in bone formation and maintenance. Stem cells are a type of cell that divide and differentiate into osteoblast cells. Osteoblasts are responsible for the creation of bone tissue. They secrete the extracellular matrix that bone cells live within. When osteoblasts mature, they become osteocytes. These are our main mature bone cells. Finally, osteoclasts are responsible for breaking down bone tissue to release minerals and nutrients that we store in our bones until our body needs to reabsorb them. They also respond to the stresses that we put on our bones and continually reshape them to our individual needs.

Why does a bone break?

Most bones break due to a traumatic incident. That happens with a serious fall or other type of accident such as a car accident. Sports are also a common source of the type of injuries that cause a bone to break. Think of the significant impact of football players running into each other or a soccer player getting a strong kick to the shin. Overuse also causes some types of fractures. Long-distance runners, for example, can put so much loading stress on the bones in their feet and legs that cracks form.

Types of fractures

The technical name for a break in a bone is a fracture. There are many different types. Doctors categorize them by several factors. One of those is whether the bone goes through the skin. A closed or simple fracture means the bone doesn’t go through the skin. An open or compound fracture refers to a broken bone that punctured the skin. If it is described as a complete break, then the fractured bone is in two separate pieces. A partial break describes a broken bone that is not completely broken into more than one piece.

Fractures are also described by whether or not the broken sections of bone are lined up after the break. When sections of bone line up it's called a non-displaced break. And when the sections are not lined up, it’s called a displaced break. 

Doctors also describe fractures by the many different shapes that a broken bone can take. A transverse fracture is a break that goes straight across a bone. An oblique fracture is one that goes at an angle across the bone. A spiral fracture is a break that goes in a spiral around the bone.

When a bone breaks on one side, but only bends rather than breaking on the other side, it’s called a greenstick. Fractures are referred to as segmental if the same bone is broken in two places. They are called comminuted if the bone is broken into three or more pieces. And, they are called compression fractures if the bone was crushed. Finally, a very thin crack in a bone is called a stress fracture or a hairline fracture.

What happens during bone healing?

Primary bone healing pathway

Our bodies are incredibly adaptable even in situations like a bone fracture. There is more than one pathway to healing a broken bone. In primary, or direct healing, the path to healing is as the name suggests more direct, but it requires certain conditions. In that situation, the two ends of the bone need to be directly lined up, touching each other, and stabilized at the site of the break. If all of those conditions are met, then it is possible for new bone to be directly created to heal the break. However, those conditions are more often not met and in that case, our body does what is called secondary, or indirect, bone healing.

Secondary bone healing pathway

Secondary healing of a fracture is the more common situation. In this case, the broken bone goes through several stages in order to complete the healing process. Immediately after you break a bone, blood clots start forming. The blood clot helps fill the gap created at the site of the broken bone. Our body also initiates an inflammatory response. White blood cells move into the area and other aspects of our immune response help clean the area, preparing it for further stages of healing.

There is overlap of course between all of the stages of healing. Generally, however, the next step is that mesenchymal stem cells create a soft fibrocartilaginous callus to replace the blood clot. Then osteoblasts derived from the stem cells create a hard callus to replace the fibrocartilaginous network. Finally, mature bone is formed and shaped at the site of the break. Any excess bone that formed during the early part of the healing process is broken down by osteoclasts. While most steps in this healing process happen in the days to weeks immediately after a bone is broken, this last step can take months to years to complete. 

What factors affect healing time?

The type and location of the fracture as well as the age and health of the person affect how quickly a bone heals. Other specific factors related to the type of break that slow healing include things like a particularly large gap between the ends of the broken bone, an open fracture, a fracture that is in the transverse direction (Claes, 2021), and/or instability at the site of the break (ElHawary et al., 2021). Sufficiently stabilizing the area of the break, in particular, to avoid shearing movement helps support healing (Claes, 2021). Other factors that could slow healing include an inadequate blood supply to the injured area, smoking, and either an excessive or insufficient immune response (ElHawary et al., 2021).  

Conclusion

Bones are living tissue and are constantly adapting to the stresses we put on them in daily life. Bones are an especially amazing tissue because when we experience the ultimate bone stress, a break, our bones can heal! However, the process that happens when a bone heals can be slow. It involves multiple stages and can take months to years to fully complete.

References

Claes, L. 2021. Improvement of clinical fracture healing – What can be learned from mechano-biological research? Journal of Biomechanics. 115:110148.

ElHawary, H., A. Baradaran, J. Abi-Rafeh, J. Vorstenbosch, L. Xu, J.I. Efanov. 2021. Bone healing and inflammation: Principles of fracture and repair. Seminars in Plastic Surgery. 35(3):198-203.

Lin X., S. Patil, Y.G. Gao, A. Qian. 2020. The bone extracellular matrix in bone formation and regeneration. Front Pharmacol. 11:757.

Marsell, R. and T.A. Einhorn. 2011. The biology of fracture healing. Injury-International Journal of the Care of the Injured. 42:551-555.

Mohamed, A.M. 2008. An overview of bone cells and their regulating factors of differentiation. Malays J Med Sci. 15(1):4-12.

Sheen J.R., A. Mabrouk, V.V. Garla. Fracture Healing Overview. [Updated 2023 Apr 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK551678/

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