The Anatomy of the Sternum

Known as the Breastbone

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The sternum is a long and flat bone which protects the underlying muscles, organs, and important arteries within the chest. This includes the lungs, heart, and stomach, along with all of their intricate blood vessels, muscles, and cartilage. The sternum also acts as a joining structure to the upper ribs on either side of the body. This bone is sometimes cracked during cardiopulmonary resuscitation (CPR) as part of the sternum is located directly above the heart. The sternum must be cut through during operations on the heart including open-heart surgery, making the rehabilitation for that procedure more intensive than most others. 

Anatomy

The sternum is a long, flat bone measuring around 6 inches. There are three main parts of the sternum: the manubrium, body, and the xiphoid process. The manubrium refers to the wide rectangular portion at the top of the sternum. The long and flat part which makes up most of the sternum is the body. At the end of the sternum, there is a small point called the xiphoid process. This process is much more narrow and significantly thinner than the rest of the sternal body.

The sternum as a whole has been compared to an upside-down sword due to the rectangular part at the top resembling a handle. The remainder of the sternum is flat and long, similar to the blade of a sword with a tip looking like the xiphoid process at the end.

The entire xiphoid process consists of cartilage until it gradually turns to bone by the age of 40.

Due to the stark differences in the size and strength of the xiphoid process in comparison to the rest of the sternum, it poses a safety risk during CPR, when it is at risk of being broken. During such life-saving efforts, the individual providing CPR is likely unaware if a fracture occurs. A broken xiphoid process followed by continued CPR can cause this sharp portion of the sternum to penetrate one of the underlying organs. This can lead to a damaged stomach, punctured lung, or damage to the heart and/or associated blood vessels.

Anatomical variations of the sternum include varying sizes of the sternal angle. This often has little impact on function, but can vary between individuals. Approximately 30% of the population lacks a xiphoid process. Other variations include a two-pronged xiphoid process and the presence of a sternal foramen. A sternal foramen is a separation between the two sides of the sternum and results from poor development. This foramen appears as a small tunnel in the sternum and can often be mistaken by doctors as a result of trauma.

Function

The sternum connects the first six ribs in the middle of the chest while serving as a strong protector of the stomach, heart, and lungs which lie below. The xiphoid process specifically acts as an insertion point for the tendons of the diaphragm, rectus abdominis, and transverse abdominis muscles.

The sternum does not assist with any visible range of motion to the chest or torso. However, cartilaginous connectors between the sternum and each of the upper six ribs assist with minor motions which occur with each breath. Each inward breath requires the lungs to fill with air and the intercostal (or rib) muscles to contract. All of these motions require some internal room to function appropriately.

Associated Conditions

One of the most common conditions to affect bones is a fracture, with the sternum being no exception to this. A fractured sternum can cause swelling and pain when breathing, coughing, or laughing, along with difficulty breathing. It is also common for a fractured sternum to cause grinding bones with each arm movement.

Due to the stark differences in the size and strength of the xiphoid process in comparison to the rest of the sternum, it poses a safety risk during CPR, when it is at risk of being broken. During such life-saving efforts, the individual providing CPR is likely unaware if a fracture occurs. A broken xiphoid process followed by continued CPR can cause this sharp portion of the sternum to penetrate one of the underlying organs. This can lead to a damaged stomach, punctured lung, or damage to the heart and/or associated blood vessels.

Another condition related to the sternum is costochondritis, which is inflamed rib cartilage which connects with the sternum. This pain can be local to the sternal area or can spread to the arms or shoulders in severe cases. Costochondritis can cause sharp, stabbing rib pain and tenderness to any of the first three ribs. Redness and warmth to the sternal and rib area may also be noted.

The sternum can also be affected as a result of a scapula (shoulder blade) injury, a clavicle (collarbone) injury, or a hernia. The top of the sternum joins with the collarbone to allow for normal range of motion and muscle use. Injury to the collarbone may cause the sternum to swell, ache, pop, or click with significant movement. The presence of a hernia or muscle strain to the chest and torso muscles can cause pain, bruising, and tenderness in the sternal area. These symptoms can also affect someone’s ability to breathe easily, causing some limited motion and pain to the sternal area.

Digestive and respiratory issues can also impact the function of the sternum. Heartburn, stomach ulcers, and gastroesophageal reflux disorder (GERD), also known as acid reflux, can cause pain to the sternal area if left untreated. Lung disorders such as pneumonia, bronchitis, and inflamed pleura can cause sternal pain and difficulty breathing.

Treatment

Most bone fractures must follow a particular protocol for rehabilitation. However, sternal fractures are an exception to this since they do not assist in direct, gross range of motion. The protocol for sternal fractures is to manage pain and allow rest to encourage healing.

Sternal fractures often take up to three months to heal, with pain levels easing after this point as well. The prognosis for return of function after a sternal fracture is excellent, as it is rather simple to immobilize a bone with such little functional motion. One of the main complications which can result from a sternal fracture is an infected incision.

Sternal precautions include weight restrictions along with limitations on arm movements. Some of the main ways to maintain sternal precautions include placing a pillow on your chest when coughing, not lifting heavy items, using your legs to stand up from a chair, stretching your shoulders, and not using your arms to get out of bed. These instructions all relieve the pressure and strain on abdominal muscles and the sternum.

Since sternal fractures often arise after a traumatic incident such as a major car accident or blunt trauma to the body, there are often other injuries which are simultaneously rehabilitated. However, an individual with no other injuries requiring hospitalization can successfully be rehabilitated from a sternal fracture from the comfort of their own home. Anatomical variations and conditions related to the sternum often do not impact function, meaning there are no associated treatment protocols for the sternum in such instances.

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