Overview
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.
Gross Anatomy
Development
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).
Macro-anatomy
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.
Articulations
|
Name of joint |
Type of joint |
Articulation between |
Movement |
|
Manubrosternal joint |
Secondary cartilaginous |
Manubrium and sternal body |
Small amount of angular movement during respiration |
|
Xiphisternal joint |
Primary cartliginous (frequently becomes ossified with age) |
Sternal body and xiphoid process |
Movement possible but no significance of movement |
|
Sternoclavicular joint |
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 |
Fibrocartilaginous |
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. |
Ligamentous
attachments
Anterior attachments
·
Anterior sternoclavicular
ligament
·
Sternocostal ligaments
·
Costoxiphoid ligament
Posterior attachments
·
Posterior sternoclavicular
ligament
·
The superior and inferior
sternopericardial ligaments attach to the body of the sternum
·
Posterior costoxiphoid ligament
Muscle
attachments
Anterior:
·
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.
Posterior:
·
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.
Lateral:
·
Aponeuroses of the abdominal
muscles: Attach to the lateral aspect of the xiphoid process.
Blood
Supply
Derived predominantly from branches of the
Internal Thoracic Artery (Internal mammary artery)(5, 6).
Nerve
Supply
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.
Clinical Anatomy
Sternal fracture
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).
Median sternotomy
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.
Interosseous access
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.
Quick Anatomy
Key Facts
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).
Aide-Memoire
Summary
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.
References
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