4.2: Thorax Wall & Diaphragm

Thoracic wall (174 on in 3rd edn)

Thoracic cavity = vertebrae, ribs & sternum; suprapleural membrane roof and diaphragm floor.

-       Ribs are for breathing, not for protecting thoracic viscera. 

Thoracic joints  (170-172)

Ribs articulate with vertebrae in 2 places: heads & tubercles, ® collectively = costovertebral joints. 

-       where ribs meet vertebral bodies there are 2 facets (each synovial jts); one ® rib above, one ® rib of same number.  Between = intra-articular ligament. Joints reinforced ® radiate ligament (central band of the radial ligament ® intravertebral disc).

-       1st rib and last 2 ribs ® own vertebrae only

-       where ribs meet tubercles also 2 facets but only 1 is articular: medial facet ® tip of transverse process of own vertebra).  Lateral facet ® lateral costotransverse ligament.  Structures bonded firmly by costotransverse ligament and superior costotransverse ligament (latter goes from top of rib to undersurface of transverse process.

-       Lower two ribs have no tubercles.

Ribs join costal cartilages ® costochondral joints.  (primary cartilaginous ie unossified rib)

-       upper 7 of these articulate with sternum ® sternocostal joints (1st = primary cartilaginous; next 6 are synovial; 2nd rib articulates with both manubrium and body – separate synovial cavities with intra-articular sternocostal ligament between.)

-       next 3 with each other ® interchondral joints (small synovial jts apart from 9-10 - fibrous)

-       lower 2 ribs are free.

Landmarks for counting in thoracotomy: sternal angle lies opposite the scecond rib; constant.

Manubriosternal & xiphisternal joints

Hyaline cartilage lining with disc of fibrocartilage intervening \ both joints are a symphysis. 

-       30% of manubriosternals develop a fluid filled cavitation ® confused with synovial jt.

Manubriosternal jt moves for respiration.

-       it may become ossified in the elderly, but this usually has little effect on respiration.

Xiphisternal jt ossifies from middle age.

Thoracic muscles (179)

External layer

i)               Posterior serratus (Pl 177) overlie erector spinae (migrated back – still ant rami muscles);

-       SP superior: C6-T2 spinous processes ® angle of ribs 2-5, beneath rhomboids with dorsal scapula nerve and vessels between.  Weak costal elevator for inspiration.

-       SP inferior: T11-L2 spinous processes ® lower four ribs just lateral to their rib angles; beneath lower pole of lat dorsi.  Weak expiration.

ii)             Levator costae: 12 pairs (posterior rami), fan from transverse processes to rib below

iii)            External intercostals: obliquely forward & down like EO.  Run from costotransverse ligaments to costochondral junctions -> anterior intercostal membrane extends to sternum.

Intermediate layer

Internal intercostal muscles - runs up & back like IO.  From sternum to angle of rib ® posterior intercostal membrane extends to superior costotransverse ligament.

Innermost layer (incomplete, aka transversus thoracis group)

i)               subcostals at back: attach to inner rib surfaces

ii)             innermost intercostals at sides (a space between i.i. & subcostal ® nerves and vessels lie in contact with pleura.

iii)            transversus thoracis in front: Pl176, ribs 2 ® 6. 

Intercostal spaces

Neurovascular bundle runs as VAN from above - down, protected by lower border of rib.  Where the nerve crosses the vessels it is always nearer to skin.   Between inner & innermost layers.

Intercostal nerves

Mixed spinal nerve gives off posterior ramus after exiting intervertebral foramen.  Branches thereafter:

(1) collateral branch ® intercostal muscles, parietal pleura, rib periosteum (runs inferior, on top of the rib below)

(2) Lateral cutaneous branch ® lateral thoracic skin

(3) Terminal anterior cutaneous branch ® passes anterior to internal thoracic artery and reaches ventral midline skin. 

Lower 5 intercostal nerves pass down below CM in neurovascular plane to supply anterior abdominal wall. 

1st intercostal nerve

Given off by large T1 anterior ramus as it goes to join brachial plexus.

Is small.  Runs well inside external border of 1st rib in contact with pleura.  Has no cutaneous supply. 

2nd intercostal nerve

Lateral cutaneous branch -> arm as the intercostobrachial nerve.

Subcostal nerve

Exits thorax beneath lateral arcuate ligament of diaphragm, below the subcostal vessels. 

Intercostal arteries

Superior intercostal  artery (desc branch of costocervical trunk) ® upper 2 spaces posteriorly (28).  Passes across neck of 1st rib (chain, vein, artery, nerve – from post?).  (‘chain’ is where stellate ganglion is)

Remaining 9 spaces ® separate branch of desc aorta to each = posterior intercostal artery.

Internal thoracic artery ® 2 anterior intercostal arteries to each of upper 6 spaces, musculophrenic artery ® 3 more to 7th, 8th & 9th spaces.  No anterior intercostal arteries to lower 2 spaces.  (See 176)

Intercostal veins

One posterior and 2 anterior veins for each intercostal space.  Anterior veins ® internal thoracic and musculophrenic veins.  (Pl 176)

Posterior veins irregular (Pl226)

Lower 8 ® azygos on R; hemiazygos / accessory hemiazygos on L

1st space ® supreme intercostal vein (® vertebral or brachiocephalic).

2nd, 3rd & sometimes 4th space ® superior intercostal vein (® azygos on R, ® brachiocephalic on L). 

Lymphatics

Follow arteries: ventral ® internal thoracic / parasternal nodes, dorsal ® posterior intercostal nodes. 

Internal thoracic artery (238)

From 1st part of subclavian ® passes down 1 fingerbreadth from border of sternum.   

-       gives 2 anterior intercostals to each space (inferior are small collateral vessel)

Perforating branches run superficially from each space; largest in 2nd & 3rd spaces in females ® breast. 

At termination ® divides into superior epigastric and musculophrenic arteries.

-       latter passes along costodiaphragmatic gutter ® ends by piercing diaphragm and ramifying on its inferior surface (also gives pericardiophrenic br., runs with phrenic n. to pleura/pericardium

-       superior epigastric passes b/n xiphisternum and high costal fibres to enter rectus sheath.
Is separated from pleura tby the transverss thoracic muscle except at the top.

Suprapleural membrane (218-9)

Dense fascial layer (Sibson’s), attached to inner border of 1st rib and cc.

Attaches posteriorly to transverse process of C7 ® muscle fibres of scalenus minimis aka pleuralis.

The cervical dome of pleura attaches underneath.

Gives rigidity to thoracic inlet & prevents puffing up & down of neck structures in respiration.  

The subclavian vessels arch laterally over it.

Diaphragm (180-1)

Development

Composed of 4 structures:

(1) septum transversum (-> central tendon; central covered by pericardium; left and right leaflets ),

(2) dorsal mesentery of oesophagus (® connective tissue around oesophagus)

(3) pleuroperitoneal membranes

(4) lateral body walls -> muscular part

Diaphragm is morphologically a derivative of the innermost body wall layer (\arises in continuity with transversus abdominis) & descends from level of thoracic inlet. 

Muscle may be absent at junction of costal & lumbar parts (attachment of lateral arcuate lig to 12th rib) = site of Bochdalek’s foramen, commonest on L.

Junction of costal & xiphoid elements is potential site of Morgagni’s foramen.

Muscular origins

Arises from posterior xiphi, inner aspect of ribs 7-12, fascial arches of medial & lateral arcuate ligaments, lumbar vertebrae via crura --> muscle fibres pass centrally to insert into central tendon. 

Crura

R from upper 3 lumbar vertebrae, L from upper 2. 
Tendinous fibres from medial edge of each crus unite in front of aorta
= median arcuate ligament. 
Fibres from R
crus run up around oesophagus as a sling. 

Medial arcuate ligament

Thickening of psoas fascia, from body of L1 ® transv process of L1.  From latter, the lateral arcuate ligament runs across to the 12th rib at lateral border of QL (= thickening of anterior layer of lumbar fascia).  

Note that fibres arise from lateral and medial arcuate ligaments, but not from median. 

Diaphragm curves into L & R domes, R highest, extending in inspiration up to nipple level (4th ICS).  Central tendon inseparable from (& embryologically identical to) the fibrous pericardium. 

Openings in diaphragm

Aortic hiatus T12

Between crura; transmits aorta, thoracic duct & azygos/hemiazygos veins (though veins and duct may make their own openings in the R crus). 

Oesophageal hiatus T10

In muscle of R crus, to L of midline.  Oesophagus attaches to diaphragm by fibrous phreno-oesophageal ligament.  V
agal trunks, asc branch of L
gastric artery and accompanying veins & lymphatics also pass through. 
If hiatus needs to be enlarged, do so laterally and posteriorly; avoid the left hepatic vein; or open anteriorly avoiding the pericardium.

Foramen for IVC T8

To R of midline in central tendon.  R phrenic nerve pierces the central tendon alongside it.

Other structures piercing diaphragm

Gr, lesser & least splanchnic nerves ® each crus

L phrenic nerve ® L dome

Extraperitoneal lymph vessels from abdominal surface pass through to nodes on thoracic surface. 

Innervation

Phrenic nerve (C3,4,5) is sole motor supply to diaphragm, from centrally via radiating fibres \ peripheral circumferential or radial incisions denervate least amount. 
Sensory fibres from intercostal nerves supply peripheral diaphragm.

R crus fibres looping around to L of oesophagus supplied by L phrenic.

Blood supply

Central mass from inferior phrenic vessels from aorta; costal margin by lower 5 intercostals and subcostal arteries. 

Actions

Inspiration, abdominal straining.  Deep breath ® descent of central tendon from T8 ® T9 level. 

Effects of deep breath on openings:

IVC foramen pulled open ® ­venous return

Oesophageal hiatus tends to close ® Ýreflux

Aortic opening unaffected (behind diaphragm)

In straining main contribution of diaphragm is splinting; generation of ­intra-abdominal P is mainly by lateral wall muscles. 

Thoracic Movements and Respiration (183)

All 3 diameters are increased by inspiration

-       manubriosternal hinge (~7o) and rib elevation ensures AP expansion.  If this joint stiffens, no thoracic expansion is possible.

-       the opposite effect to expansion/contraction occurs in abdomen with normal breathing (passive in normal thoracic breathing); in children and women thoracic movement is > abdominal.

Diaphragm = principle muscle of inspiration.

-       passive recoil of lungs/rib cage ® expiration, (abdo muscle contraction domes diaphragm)

Externals = inspire; Internals = expire

Intercostals = more important in stiffening the chest wall.

In maximal inspiratory effort: scalenes, and SCM elevate 1st rib & manubrium

-       quadratus lumborum assists by fixing 12th rib (or even depressing it)

-       chest wall muscles, eg pecs can contribute to expansion when arms fixed.

In maximal expiratory effort: lat dorsi and abdominal muscles contract to squeeze thoracic volume

Thoracotomy


Pass through rhmboids, trapezius, lat dorsi posteriorly, serratus laterally and pec major and minor anteriorly.

- see pectoral girdle notes for these m's.


Posterolateral approach
Right-side used for approaching oesophagus, as aortic arch curves over oesophagus in L chest
(Although L would be better for distal oesophagus)

     Patient in lateral position, upper arm lifted to head on a rest.
Incision links 3 points: 1) 3 fingers below nipple, 2) two fingers below inf angle of scapula; 3) midpoint b/n medial scapula and vertebral column; can keep going backwards
Divides traps, rhomboids, lats and seratus;
Then go through an intercostal space or excision of rib


Anterolateral 

-       patient supine, arm elevated on a board abducted.

Straight incision above a chosen rib, usually 5th or 6th.
  From sternal edge to axilla; will angle upwards as this occurs

- goes throub mec major; reflected upwards; avoid internal thoracic medially
-
periosteum then stripped and pleural cavity punctured with a blunt instrument; then carry posteriorly keeping on top of rib below to avoid nerves.
Open ribs slowly over 2-3mins

-       Nerve to serratus (long thoracic) is in danger over that muscle ® often severed.

Median sternotomy: whole length of sternum split vertically.  Retrosternal tissues freed and diaphragm dissected at its origin.

-       avoid damage to pleural sacs, remembering that right sac may extend beyond mid-line.

Thoracoabdominal: through 8th or 9th rib bed then dividing costal cartilage and continuing the incision across the superior abdominal wall.

-       incise diaphragm peripherally or radially towards point of phrenic n entry to avoid damage.