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Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

Muscles of the Back

The major muscles of the back are divided into three groups:

Extrinsic muscles Trapezius Elevates and depresses the scapula
Latissimus dorsi Responsible for adduction, internal rotation and extension of the shoulder joint
Rhomboid major and Rhomboid minor Keep the scapula pressed to the thoracic wall
Levator Scapulae Elevates the scapula
Intermediate muscles Serratus posterior superior Elevates the rib cage
Serratus posterior inferior Depresses the rib cage
Intrinsic muscles Spinotransversale muscles (splenius capitus and splenius cervicus) Responsible for extension of the neck
Errector spinae muscles Iliocostalis lies laterally, longissimus is in the middle and spinalis is the most medial one
Transversospinalis Semispinalis lie superficially, multifidus in between the two sets of muscles and rotares are located the deepest

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0


Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

Bony Prominences of the Back

Surface anatomy of the back

Image: “Surface anatomy of the back.” by Henry Vandyke Carter, Henry Gray (1918) in “Anatomy of the Human Body”, Gray’s Anatomy. License: Public Domain

The following bony prominences constitute the surface anatomy of the back:

  • The spinous process of the vertebra in thin lean individuals. The spinous process of cervical vertebra 7, is the most prominent.
  • The acromion and the spine of the scapula.
  • Iliac crest and posterior superior iliac spine of the hip bones.
  • Rib cage in thin lean patients or in patients having a barrel chest.

Triangles of the Back

Triangle of auscultation

Triangle of auscultation

Image: “Posterior dissection of the Visible Human Male showing the triangle of auscultation. The borders and superficial contents of the triangle are highlighted and labeled. Triangle of auscultation in pink.” by Daniel G. Bates – Visible Human Male, dissection created using the VH Dissector and further edited. License: CC BY-SA 3.0

The triangle of auscultation is an area on the back, where the breathing sounds are best audible, due to relative thinning of musculature.

The borders of the triangle are defined by:

  • Trapezius muscle superiorly and medially
  • Latissimus dorsi inferiorly
  • Medial border of the scapula

Inferior lumbar triangle

The inferior lumbar triangle, also known as the petit triangle, lies superficially. The latissimus dorsi forms the medial boundary, the external oblique forms the lateral boundary while the iliac crest forms the inferior boundary. The inferior lumbar triangle is the site of herniation of the abdominal viscera.

Superior lumbar triangle

This is also known as Grynfeltt-Lesshaft triangle and lies deep to the inferior lumbar triangle. The superior lumbar triangle is found in cadavers in a consistent fashion. It is also a common site of herniation of abdominal viscera. Quadratus lumborum muscle lies medial to it, the internal oblique forms lateral margin and 12th rib lies superiorly.

Lumbar Puncture

Lumbar puncture (LP), is an invasive procedure during which cerebrospinal fluid (CSF) is drained from the spinal canal. The CSF can then be analyzed for diagnostic purposes. CSF pressure can also be measured. LP is usually performed after a neurological examination is completed. LP occurs most commonly when there is concern for infection (such as meningitis) or idiopathic intracranial hypertension (pseudotumor cerebri). 

During the LP, the patient is asked to lie in the left lateral position with knees and hips in the flexed position. The intercristal line, which is the line joining the superior parts of the iliac crest, is located and marked. This marks the region between L4 and L5 lumbar vertebrae. A small, non-cutting LP needle, is advanced between the spinous processes of these two vertebrae and CSF is collected and submitted for analysis. If the opening pressure is measured, this is done prior to CSF collection but requires some repositioning of the limbs for accurate measurement.

LP can also be performed in the seated position. One of the potential complications of LP, is post-LP headache. Using a small, non-cutting needle will lower the risk for post-LP headache, also known as “low pressure” headache. While there is less data to suggest that positioning helps minimize post-LP headache, it is still common to ask the patient to remain in the supine position for half an hour after completing the procedure.

Projection of Viscera onto the Back

It is important to know the surface anatomy of various organs and viscera and their projection onto the back. This is useful in various procedures as well as the clinical examination of various body systems.

  • Lungs and visceral pleura roughly lie at T10 while the parietal pleura at T12. This is helpful in thoracocentesis or pleural tap, during which excess fluid is drained from the pleural cavity for therapeutic and diagnostic purposes.
  • Kidneys are retroperitoneal structures and therefore can be easily approached during surgical procedures.
  • The pain of aortic aneurysm and acute pancreatitis radiates to the back. Similarly, the pain of pancreatic cancer may be referred to the back.
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