The sternum forms the central anterior section of the thoracic cage. It is made up of three parts; the manubrium, body and xiphoid process. The sternum is a component of many joints; sternoclavicular, sternocostal, manubriosternal and xiphisternal joints. The manubriosternal joint is an important anatomical surface marking.
Embryologically the sternum is derived from the mesoderm. At the end of the sixth week two paired mesenchymal condensations called sternal bars form in the ventral body wall(1). Laterally these connect to the developing ribs and medially they fuse in the midline in a cranial-caudal direction. Six ossification centers form, one for the manubrium, four for the body of the sternum (known as sternebrae) and one for the xiphoid. Ossification centers appear at intervals from a fetal age of sixty days. The most caudal ossification center in the body forms in the first year after birth while the xiphoid ossification center does not form until between 5 and 18 years after birth(2). All other ossification centers form in utero.
By the mid twenties all four sternebrae have fused together forming a single sternal body, the inferior sternebrae fuse prior to the superior sternebrae. Over an age of forty years the xiphisternal joint may ossify(2).
The sternum is a flat elongated sword shaped bone made up of three parts; cranial to caudal these are the manubrium, sternal body and xiphoid process. The sternum forms the central anterior section of the thoracic cage.
The manubrium is grossly trapezoid in shape and cranially is widened laterally. It is found at the level of the T3-4 thoracic vertebrae(3) . Superiorly and in the midline there is an easily palpable notch known as the suprasternal or jugular notch. Laterally on both sides of this notch but still on the superior aspect can be found the clavicular notches where the sternoclavicualr articulation occurs. On the lateral aspects bilaterally is the attachment site for the costal cartilage of the first rib. At the inferior-lateral tip of the manubrium a demifacet that articulates with the second costal cartilage can be found bilaterally.
The sternal body is the longest part of the sternum and is located at the level of the T5-10 thoracic vertebrae(3). The anterior surface is often marked by transverse ridges where the sternal synchondroses of the sternebrae have fused, remember this process is not complete until the mid twenties of adult life. On the lateral margins of the sternal body can be found the costal notches. At the superior lateral tip can be found a demifacet which articulates with the second costal cartilage. Inferior to the demifacet are four facets for articulation with the third to sixth costal cartilages. Below these four facets on the inferior end of the body can be found a demifacet for articulation with the seventh costal cartilage.
The xiphoid process is the smallest part of the sternum and is found at the level of T10 or 11 vertebrae(3, 4). It demonstrates a wide variety in its shape, it is classically depicted as pointed inferiorly however can be bifid, curved, blunted, widened or perforated. Bilaterally on the upper lateral margin can be found a demifacet which articulates with the seventh costal cartilage.
Name of joint
Type of joint
Manubrium and sternal body
Small amount of angular movement during respiration
(frequently becomes ossified with age)
Sternal body and xiphoid process
Movement possible but no significance of movement
Saddle type synovial
Proximal clavicle, clavicular notch of the manubrium and small part of the first costal cartilage.
Very mobile allowing movement of the clavicle in the following planes; anteroposterior, vertical and some rotation.
First sternocostal joint
Manubrium and first costal cartlage.
No significant movement.
Sternocostal joints II-VII
Plane type synovial
2nd costal cartilage and demifacet of manubrium and sternal body.
3rd-6th costal cartilage and 3rd-6th articular facets of sternal body.
7th costal cartilage and demifacet of sternal body and ziphoid process.
Gliding motion during respiration.
· Anterior sternoclavicular ligament
· Sternocostal ligaments
· Costoxiphoid ligament
· Posterior sternoclavicular ligament
· The superior and inferior sternopericardial ligaments attach to the body of the sternum
· Posterior costoxiphoid ligament
· Sternocleidomastoid: The sternal head of the sternocleidomastoid arises from the upper anterior surface of the manubrium.
· Pectoralis major: The sternocostal head arises from the lateral anterior surface of the sternum.
· Rectus abdominis: Small attachment to the xiphoid process.
· Linea alba: Runs from the xiphoid to the pubic symphysis.
· Sternohyoid: Arises from the manubrium.
· Sternothyroid: Arises from the manubrium.
· Sternocostalis: Arise from the lower part of the body of the sternum.
· Transversus thoracis: Arise from the xiphisternum and inferior part of the body of the sternum.
· Diaphragm: Inserts onto the xiphoid process.
· Transverse abdominis: Inserts onto the xiphoid process.
· Aponeuroses of the abdominal muscles: Attach to the lateral aspect of the xiphoid process.
Derived predominantly from branches of the Internal Thoracic Artery (Internal mammary artery)(5, 6).
Above the level of the manubriosternal joint the skin is supplied by the medial supraclavicular nerve (C3,4), below this the skin is supplied by the intercostal nerves (T2-6)(7).
Key clinical landmarks
Sternal angle (Angle of Louis): This palpable landmark is formed by the manubriosternal joint. It is of clinical significance for the following reasons:
· Is the anterior marker of the thoracic plane.
· Is at the level of the 2nd costal cartilage and thus used for orientation during cardiovascular examination.
Xiphoid: The xiphoid is an important anatomical landmark;
· The xiphisternal joint indicates the inferior limit of the central part of thoracic cavity projected onto the anterior wall of the body.
· It marks the inferior border of the heart and the central tendon of the diaphragm.
Sternal fractures are found in 3.7% of patients who attend hospital following a road traffic collision(8). The classic mechanism behind sternal fractures is blunt chest trauma (steering wheel) and deceleration injuries (seat belt), however they have been observed in forced thoracic flexion with a thoracic spine wedge fracture(8-10). This may occur due to seatbelt injury, impact on vehicle steering wheels, sporting impacts, falls in the elderly and post menopausal women or those on long term steroids have an increased risk(9).
It is important to identify whether there is an isolated sternal fracture or other associated injuries present. A myriad of injuries are associated with sternal fractures and can be divided into 1) Soft tissue injuries (including cardiac) 2) Injuries to the chest wall and 3) Injuries to the spine, appendages and cranium. It is important to recognize associated injuries as mortality increases to 24-45% in such cases and management of the sternal fracture may no longer be the clinical priority(9).
Once associated injuries have been excluded (never forget an ECG and cardiac enzymes) an isolated sternal fracture can be diagnosed and the mortality rate is much lower at <1%(10). Classic clinical findings will be of anterior chest pain, point tenderness, swelling and/or deformation, the patient may also be reluctant to take deep breaths due to pain. Sternal fractures are most often transverse in nature and will be visualized on lateral radiographs, some have used ultrasound for diagnosis however computer tomography is the preferred modality(11). This is because it provides excellent visulisation of the fracture and can aid in the exclusion of associated chest trauma.
Management of isolated sternal fracture is most often conservative and once adequate analgesic control has been achieved the patient can be discharged home, ensuring ECG and cardiac enzymes are normal prior to discharge(8, 10). Analgesic control must be sufficient to enable deep breathing minimising subsequent risk of chest infection. A small proportion may require surgical fixation if there is significant displacement, overlap of segments, pulmonary or cardiac compromise and this may be achieved using sternal wires or plate fixation(9, 10).
Posterior sternoclavicular dislocation
Posterior sternoclavicular dislocation is a rare but potentially life threatening event due to possible mediastinal injury. Injury to the great vessels, trachea, oesophagus, subclavian artery and vein, brachiocephalic artery and vein, cervical root compression, laryngeal nerve and lung injury have all been reported(12-14). It is therefore an emergency.
The sternoclavicular joint has the least bony stability of all major joints in the body and gains its stability from its surrounding ligaments, however only 3% of dislocations occur at this joint of which anterior dislocations are 2 to 3 times more common(14-16). The posterior sternoclavicular ligament offers more support than the anterior sternoclavicular joint and this may explain the distribution of anterior and posterior dislocations(12) . Dislocation typically occurs following direct trauma or lateral compressive force at the shoulder. Fusion of the medial clavicular epiphysis is incomplete before age 25 years thus true dislocation before this age is rare and posterior displacement may be considered a Salter Harris type I fracture(12).
Presentation may be varied from the unconscious patient to a complaint of pain over the sternoclavicular region worsened by abduction at the shoulder making it at times difficult to diagnose clinically(12, 14). Thus it is important to have a high suspicion of injury if the mechanism is appropriate. Posterior dislocation can be difficult to see on plain film radiographs thus computer tomography is the preferred imaging modality, this also allows visualization of potential soft tissue injury and if indicated CTA can be performed(12).
All posterior dislocations must be reduced. This can be closed or open. Even during closed reduction due to the potential life threatening complications many advocate undertaking this in an operating theatre with appropriate surgical specialties to hand and ICU support available for post-operative care(12, 14, 16). If closed reduction is obtained this is usually stable, however if this is not stable or not obtained open reduction must be performed(16).
The median sternotomy is a common incision for cardiac procedures. The following gives an overview of the technique for performing a median sternotomy(17-19).
· Skin incision from just below the sternal notch to the tip of the xiphoid process.
· Divide the pectoral fascia and interclavicular ligament.
· Mobilise tissues to one side of the xyphoid process.
· The sternum can now be sawn down the midline.
· Periosteal bleeding points are controlled with cautery.
· Bone wax or towels can be used to cover the divided surfaces of the sternum.
· Retract the sterum progressively dividing the sternopericardial ligaments and soft tissue attachments on the posterior surface.
Vascular access is a fundamental principle for successful resuscitation in the acutely unwell patient. However this can be challenging taking time and potentially affecting outcome most notably in major trauma patients and cardiac arrest. In recent years adult intraosseous delivery of fluids/drugs has become commonplace when conventional access has failed. The intraosseous route can be considered as a non-collapsing vein.
The sternum is an accepted site for intraosseous access. The semi-automated device FAST-1 (Pyng Medical Corporation, Richmond, British Columbia, Canada) is widely used with rapid and high success rates(20). In the case of this device access is gained into the manubrium allowing continuation of chest compressions after access has been gained.
The proximal tibia and head of the humerus are other sights of intraosseous access and are more commonly performed in hospital than sternal access. For these the EZ-IO (Teleflex, Wayne, Pennsylvania, USA) is often used, remember these should not be used to gain sternal intraosseus access.
Development: Mesoderm structure.
Blood supply: Branches of the internal thoracic artery.
Nerve supply: Medial supraclavicular nerve above the sternal angle and intercostal nerves below (cutaneous supply).
The sternum makes up the anterior midline component of the thoracic cage and is made up of three parts. It forms an important attachment point for other structures and provides protection to the organs underlying it.
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