Anatomy of the thoracic dorsal and ventral ramus
Spinal cord -->
Dorsal nerve root (sensory) & ventral nerve root (motor) -->
Spinal nerve (sensory & motor) -->
- Dorsal ramus of the spinal nerve: joints, muscles and skin of the back.
- Ventral ramus of the spinal nerve: anterolateral trunk & limbs (intercostal nerves & subcostal nerve).
- Rami communicans: spinal nerve --> sympathetic trunk.
Dorsal ramus of the spinal nerve
The dorsal ramus passes through the narrow space between the bony structures and adjacent fibrous tissue (Ishizuka 2012) to innervate the joints, muscles and skin of the back.
The dorsal ramus divides into the:
- Medial (articular) branch of the dorsal ramus: emerges between the joint and medial edge of the superior costotransverse ligament and intertransverse muscle (Standring 2015 pg 945) and then passes between the semispinalis and multifidus (Ishizuke 2012). Innervates the facet joints and multifidus.
Lateral branch of the dorsal ramus: passes underneath the intertransverse ligament (Ishizuke et al 2012) in an interval between the costotransvse ligament and intertransvse muscle before inclining posteriorly on the medial side of the levator costae (Standring 2015 pg 945). The main lateral branch runs caudally, laterally, and dorsally underneath the longissimus muscles and descends approximately two vertebral segments caudally before it pierces the thoracolumbar fascia (Maatman et al 2019). It then divides into the medial (or intermediate) and lateral cutaneous branches:
- Medial (or intermediate) cutaneous branch innervates the longissimus (Saito et al 2006). Penetrates both the thoracolumbar fascia and tendinous portion of the back muscles to innervate the skin (Ishizuka et al 2012).
- Lateral cutaneous branch innervates the iliocostalis (Saito et al 2006). Penetrates only the thoracolumbar fascia to innervate the skin (Ishizuka et al 2012).
Lateral branch of the dorsal ramus forms the posterior cutaneous nerve and superior cluneal nerve.
Dorsal ramus of the spinal nerve: lateral branch
Spinal nerve > dorsal ramus > lateral branch > medial cutaneous branch & lateral cutaneous branch
Dorsal ramus > lateral branch > medial cutaneous branch
T1-6 medial cutaneous branch: between multifidus and semispinalis --> pierces rhomboid --> pierces trapezius --> pierce the thoracolumbar fascia. Supplies the skin by the SP of the same segmental number of the nerve (Aizawa & Kumaki 1996).
T6-12 medial cutaneous branch distributed to the multfidus and longissimus and occasionally the skin near the midline
Dorsal ramus > lateral branch > lateral cutaneous branch
Aizawa & Kumaki (1996) found these nerves extended from T2-4 to T12.
Passes through or beneath the longissimus dorsi to the interval between it and the iliocostales. At T2-3 pierces the rhomboid (Aizawa & Kumaki 1996).
T5/6-12: also give off cutaneous branches which pierce the serratus posterior inferior and latissimus dorsi in line with the angles of the ribs.
Where the lateral cutaneous branch pierces the thoracolumbar fascia is generally at the gap between the longissimus and the iliocostal muscles in the intercostal space. This is one segment lower than the same segmental number of the nerve (Aizawa & Kumaki 1996).
Lateral cutaneous branches are bent at points where the nerve penetrates superficial muscles forming a "Z"-shape (Aizawa & Kumaki 1996):
- Rhomboid: changes course from an infero-lateral to an infero-medial direction.
- Trapezius muscle and latissimus dorsi: changes course from an infero-medial to a lateral direction.
Sometimes the lateral cutaneous branch is sharply pulled in a medial direction by the trapezius muscle as it penetrates the muscle near the median plane (Aizawa & Kumaki 1996).
The lateral cutaneous branches from T12-L3 communicates with one another to form a nerve plexus under the lumbodorsal aponeurosis. This plexus then penetrates the aponeurosis to form several nerve bundles. It then crosses over the iliac crest to supply the skin over the hip as the superior cluneal nerves (Aizawa & Kumaki 1996).
Posterior cutaneous nerve entrapment syndrome (POCNES)
Dorsal ramus > lateral branch > posterior cutaneous nerve
POCNES is pain that can originate from the posterior cutaneous nerve (T7-12) (Maatman et al 2019).
When effected lower down in the thoracoulumbar area this can cause a superior cluneal entrapment syndrome*.
*: dorsal ramus > lateral branch > superior cluneal nerve.
Diagnostically the point of maximum pain is located just lateral to the spinal process in the lower paravertebral region (Maatman et al 2019).
Low back pain caused by an entrapped cutaneous branch of the posterior ramus of the thoracic spinal nerve most commonly originates from T11–12.
Ventral ramus of the spinal nerve
Anatomy of the intercostal muscles, serratus anterior and endothoracic fascia
Rib tubercles and posterior fibers of the superior costotransverse ligament --> almost costal cartilage: external intercostal membrane --> sternum.
Sternum & cartilage of true ribs & false ribs --> posterior costal angles: internal intercostal membrane --> anterior fibers of superior costotransverse ligament & fascia between the internal & external intercostals.
Davies et al (1932) described an intracostal muscle that is largely co-extensive with the internal intercostal muscle (refer ‘Intercostal nerve’).
Insignificant/absent in highest thoracic levels but more substantial lower down.
It occupies the middle two quarters of the lower intercostals spaces fusing at its inferior insertion with fibres of the internal intercostal (Siddiqi & Mullick 1935).
Posteriorly it may come into contact with the subcostales.
Intercostals intimi are related internally to the endothoracic fascia, parietal pleura and intercostal nerves and vessels.
Functions of the intercostal muscles
External intercostals < posterior portion of the cephalic interspaces contract during inspiration (Hudson et al 2010).
Internal intercostals < caudal interspaces contract during expiration (Hudson et al 2010).
Intercostal intimi contract during expiration.
External intercostals in the lateral portion induces contralateral rotation (Whitelaw et al 1992).
Internal intercostals in the lateral portion induces ipsilateral rotation (Whitelaw et al 1992).
The parasternal intercostals (the intercartilagenous portion of the internal intercostals) induces an ipsilateral rotation and sidebending. Even though they are internal intercostals they have an inspiratory, rather than an expiratory, function (Hudson et al 2010).
The fibers of the transversus thoracis run cephalic and laterally from the posterior aspect of the sternum, xiphoid and costal cartilages r4-7 to the r2-6 costal cartilages.
The fibers of the transversus thoracis contract during expiration, to draw down the costal cartilages (Jelev et al 2011) and contralateral rotation (Hudosn et al 2010). Therefore through reciprocal inhibition contraction of the transversus thoracis diminishes or prevents activation of the parasternal intercostals during contralateral rotation and expiration (Hudson et al 2010).
Attachment: rib 1-8(-10) to the medial boarder of the scapula.
The serratus anterior has three divisions (Webb et al 2018):
(1) Superior division: attaches to the superior angle of scapula (anterior and posteriorly) and rib 1 and 2. These fibers are shorter, thicker, and separated from the rest of the muscle. They, can, in cases, attach on to the fascia covering the intercostal space. They form a bed for the neurovascular structures in the upper axilla (Smith et al 2003).
(2) Middle division: attaches to the medial boarder of scapula and rib 2 and 3.
(3) Inferior division: attaches to the inferior angle of scapula (anteriorly and posteriorly) and rib 4 to 8/10.
The anterior attachments of the levator scapula, rhomboid minor and major overlap the costal surface of the serratus anterior fascia for about three centimeters. This causes the fascia of these muscles to merge with each other (Barihoke & Gupta 1986).
The continuity of the upper fibers of the serratus anterior and rhomboid/levator scapulae has been described as a wide muscular sheet with a deep common fascia (Nguyen & Nguyen 1987).
Action: all fibers protract, upwardly rotate* (Smith et al 2003) and fixes the scapula against the thorax (externally rotates** and posteriorly tips the scapula***). This posterior tipping is countered by an anterior tipping from the superior fibers of the serratus anterior (Smith et al 2003). Just as the serratus anterior rolls the scapula anterior on the rib cage (protracts) it also "lifts the ribs and trunk" up to a fixed scapula (Neumann & Camargo 2019). The serratus anterior is the primary stabilizer of the scapulothoracic articulation. This provides a stable axis for scapular rotation as it provides a solid base of support for upper extremity function (Smith et al 2003). The upward rotation combined with posterior tipping and, to a lesser extent, external rotation of the scapula, functions in increasing or maintain the volume of the subacromial space (Neumann & Camargo 2019).
*: upward rotation of scapula: inferior angle of scapula swings superiorly and laterally around an A-P axis.
**: external rotation of scapula: medial boarder of scapula moves anteriorly around a vertical axis at the AC joint.
***: posterior tipping of scapula: superior aspect of the the scapula moves posterior around a horizontal axis going through the spine of scapula.
Upper fibers: suspend the scapula with the levator scapula and upper fibers of trapezius. They stabilize the scapula during humeral elevation and create an anterior tipping movement of the scapula around an axis parallel to the spine of scapula (Smith et al 2003).
Lower fibers protact the scapula to assist the upper fibers of trapezius in raising the arm above the head.
The endothoracic fascia is the outer lining of the thoracic cavity. It separates the intercostal spaces and ribs from the pleura. It becomes more fibrous over the lung apex as the suprapleural membrane.
It is continuous:
- Superiorly with the prevertebral fascia (Feigl 2015 & Natale et al 2015) via the scalene minimus.
- Inferiorly with the endoabdominal fascia via traverseing the medial and lateral arcuate ligaments, aortic hiatus and oesophageal hiatus by forming the phrenico-esophageal ligament (Apaydin et al 2008).
The endothoracic fascia splits to enclose the subcostales and intercostal muscles*. At the anterior border of the intercostal muscle the fascia covers the inner surface of the internal intercostal muscle and splits again to enclose the transversus thoracis muscle before fusing with the periosteum of the sternum (Siddiqi & Mullick 1935).
*: this splitting of the endothoracic fascia is an alternative to the description of other authors who describe the endothoracic fascia as simply being continuous with the most internal component of the investing fascia of the intercostal muscles and the adjacent layer of periosteum (epimysium). The confusion as to whether the endothoracic is separate to, or splits to enclose the intercostal muscles (and transversesus thoracis and subcostales) maybe clarified by Stecco et al (2017) explanation. These authors found the visceral fascia of the thorax cannot be isolated from the muscular fascia of the intercostal muscle, as the two are fused, forming the endothoracic fascia.
Ventral rami of the spinal nerves
Spinal cord > dorsal nerve root (sensory) & ventral nerve root (motor) > spinal nerve (sensory & motor) > dorsal ramus & ventral ramus
There are twelve pairs of thoracic ventral rami:
1-11: intercostal nerve.
12: subcostal nerve.
Each ventral rami is connected with the sympathetic trunk via the grey and white rami communicans.
The intercostal nerves enter the corresponding intercostal space between the posterior intercostal membrane and the parietal pleura. The intercostal intimi does not exist at this point and only the parietal pleura is present on the inner side. As the intercostal nerve travels forward in the subcostal groove it travels between the intercostal intimi and internal intercostal muscles (Olamikan et al 2016).
Davies et al (1932) described the intercostal nerve being separated from the pleura by two soft tissue strata:
- A thin layer of muscle these authors termed the intracostal muscle. This muscle is largely co-extensive with the internal intercostal, but becomes more membranous in its anterior part and superior attachment to the inner (upper) lip of the subcostal groove.
- Extra-pleural stratum consisting of subcostal muscles posteriorly, transversus thoracis anteriorly and the connecting fascial plane between these two muscles.
This may be the fibrous sheath described by Olamikan et al (2016) that contained the intercostal nervs and vessels.
T3-6 intercostal nerves: run in the intercostal space between the internal intercostal and intercostal intimi.
T7-11 intercostal nerves: run superficial to either the transversus thoracis or transversus abdominis muscles (Nguyen (2018).
The intercostal nerves are distributed primarily to the thoracic and abdominal wall. However they also innervate the diaphragm and may also supply the pleura and the peritoneum.
T1-6 intercostal nerves
Intercostal nerves 1-2: upper limb and thorax.
T1 ventral ramus, often receives a connecting branch from T2 ramus, and splits into two branches:
- Brachial plexus.
- Intercostal nerve which terminates as the first anterior cutaneous nerve of the thorax.
In approximately 60% of individuals, there is a linkage of the brachial plexus to the first and/or second intercostal nerve and stellate ganglion, known as Kuntz’s nerve. This nerve carries sympathetic fibers to the brachial plexus without passing through the sympathetic trunk (Zaidi & Ashraf 2010).
Intercostal nerves 2-6: pass forwards in the intercostal spaces.
Posteriorly: travels between the pleura and external intercostal membrane. Mainly runs between internal intercostal and subcostales/intercostal intimi. Anteriorly near the sternum they cross anterior to transversus thoracis, pierce internal intercostals, external intercostal membrane and pectoralis major to terminate as the anterior cutaneous nerve of the thorax.
Second anterior cutaneous nerve may send branches of the supraclavicular nerves (cervical plexus).
- Muscular branch: intercostals, serratus posterior superior, transversus thoracis.
- Collateral branch.
- Lateral cutaneous nerve. The lateral cutaneous branch of each nerve has a short course between the internal and external intercostal muscles before finally piercing the external intercostal (Davies et al 1932) and serratus anterior (Maatman et al 2017) to innervate the skin at the mid-axillary line. Maatman et al (2017) identified the nerve piercing the external intercostal and serratus anterior as a site of entrapment ('Lateral Cutaneous Nerve Entrapment Syndrome' 'LACNES').
Except for first and second intercostal nerve (first intercostal nerve does not always give off a lateral cutaneous branch) each lateral cutaneous nerve divides into an anterior rami (skin over pectoralis major/upper external oblique) and posterior rami (skin over latissimus dorsi/scapula).
Lateral cutaneous nerve of second intercostal (intercostobrachial) nerve: pierces the external intercostal and serratus anterior (Chang et al 2018), travels through the axilla, joins the medial cutaneous nerve of the arm and pierces deep fascia of the arm to communicate with the posterior cutaneous branch of the radial nerve. Supplies skin of the upper posterior and medial parts of the arm.
Koizumi & Horiguchi (1992) described a branch of the second third or fourth lateral cutaneous nerve adhering to the fascia of the external intercostal muscle. It then pierces the origin of the pectoralis minor to communicate with a pectoral nerve which originates from the loop composed of the lateral and medial pectoral nerves.
T6-12 intercostal nerves
Ventral rami T6-11 (intercostal nerve) and T12 (subcostal) with L1 (iliohypogastric and ilioinguinal nerves) supplies the muscles and skin of the anterior abdominal wall.
Intercostal nerves 7-10: approaching the costal margin (7-10 costal cartilages) they course between the digitations of the diaphragm and transversus abdominis.
Subcostal nerve (T12) passes along the inferior boarder of r12 behind the lateral arcuate ligament and kidney and anterior to the upper part of the quadratus lumborum.
All these nerve run within thin layers of fascia formed between the transverse abdominis and internal oblique where they branch and interconnect with other nerves.
They enter the rectus sheath at the lateral margin and pass posterior to the rectus abdominis, where they intercommunicate and then pierce the rectus abdominis from its posterior aspect.
Muscular branches: transversus abdominis, internal oblique, external oblique, rectus abdominis and pyramidalis.
Sensory branches: costal part of the diaphragm and the parietal peritoneum.
Cutaneous branches: skin of the lateral and anterior abdominal wall. The subcostal nerve (T12) also supplies the skin of the anterior gluteal region and groin.
Anterior cutaneous nerve entrapment syndrome (ACNES)
The anterior abdominal wall is innervated by the ventral rami of the T6–L1 spinal nerves (Shian & Larson 2018).
The terminal branches of the T7–T12 ventral rami (intercostal nerves) enter the lateral posterior rectus abdominis at a 90-degree angle through a fibrous neurovascular channel, progressing anteriorly through the muscle and anterior rectus sheath to become the anterior cutaneous nerves of the abdomen. Once those nerves reach the overlying aponeurosis, the nerves again change course at a 90-degree angles beneath the skin (Sian & Larson 2018).
L1 nerve bifurcates into the iliohypogastric and ilioinguinal nerves; the iliohypogastric nerve pierces the external oblique aponeurosis superior to the superficial inguinal ring, whereas the ilioinguinal nerve passes through the inguinal canal to emerge through the superficial inguinal ring (Sian & Larson 2018).
These nerves are usually anchored at three sites (Maatman et al 2019):
- Posteriorly where the posterior branches of the thoracic nerve originate at the back.
- Laterally at the flank where the lateral branch originates.
- Anteriorly at the anterior abdominal wall where the nerve enters the rectus abdominis muscle.
Mole et al (2017) found the anterior cutaneous nerve (T7-12 intercostal nerves) anchored at:
- Spinal cord.
- Dorsal branch of the spinal nerve.
- Where the lateral cutaneous branch originates.
- Entering the point of the anterior cutaneous branch into the posterior rectus sheath.
- Skin (Mol et al 2017)
Sian & Larson (2018) identified the most common location of entrapment being at the lateral border of the rectus abdominis. Mol et al (2017) described various plexiform interconnections between anterior branches of intercostal nerves at this level possibly extending over multiple dermatomal zones.
Mol et al (2017) described the most likely anatomical site of entrapment is the point where the neurovascular bundle enters the posterior rectus abdominis sheath and makes a nearly 90° turn, piercing through the rectus muscle, eventually reaching the skin.
A branch of T9 travels beneath the internal oblique and runs within the posterior rectus sheath through a resistant fibrous foramen. At this level, the posterior rectus sheath is a fusion of the deep internal oblique abdominal aponeurosis and the transverse abdominal aponeurosis. These authors described that this structure might correspond with the description of a fibrous ring.
These authors also found llioinguinal–iliohypogastric nerve (L1) entrapment as a common cause of lower abdominal pain in patients with a history of lower abdominal surgery (Sian & Larson 2018).
Diagnostically depending on the site of entrapment depends on where the point of tenderness is (Maatmen et al 2019).
Differential diagnosis of intercostal pain (Olamikan et al 2016)
The most common causes of intercostal neuralgia:
- Damage-related pain: postherpetic neuralgia, thoracic surgery and diabetic thoracic neuropathy.
- Direct nerve injury (physical trauma or post surgery): stretching (e.g. expanding gravid uterus, entrapment) and inflammation.
- Entrapment can be caused by neoplasm, sarcoidosis and pleural mesothelioma.
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