17.1.
Anatomy of the Thoracic Wall with Respect to Endoscopic Approaches
17.1.1
Muscles of the Thoracic Wall
The muscles of the pectoral girdle attach the upper limb to the trunk. Of relevance to endoscopic approaches to the thoracic spine are the serratus anterior, the pectora- lis major, and the latissimus dorsi muscles, the latter forming the muscular boundary of the anterior and the posterior axillary line (Fig. 17.1).
The serratus anterior covers the side of the thoracic wall and forms the medial wall of the axilla. It origins widely with its digitations from the first eight ribs in- serting into the scapula. The digitations are bluntly dis- sected during trocar placement.
Fig. 17.1. Muscles of the thoracic wall from a lateral view
The pectoralis major consists of a clavicular head and a sternocostal head, the latter forming the anterior muscular boundary of the anterior axillary line and constitutes the anterior border for trocar placement.
The latissimus dorsi is characterized by its wide ori- gin ranging from the seventh thoracic spinous process with its fleshy origin in the thoracic region down to the sacrum, becoming aponeurotic in the lumbar and sa- cral region. It forms the muscular boundary of the pos- terior axillary line and the posterior border for trocar placement. However, a far posterior access is some- times necessary and blunt dissection of this muscle be- comes inevitable.
The external oblique, part of the anterior abdominal wall, origins with its digitations from the fifth to the twelfth rib and spreads out with its fleshy part inserting into a wide aponeurosis that joins the aponeurosis of the internal oblique below the costal margin. During trocar placement the fibers of the external oblique need to be bluntly dissected.
17.1.2
Mammary Gland
The mammary gland is located in the subcutaneous tis- sue of the anterior thoracic wall and overlies the pecto- ralis major extending laterally and inferiorly to the ser- ratus anterior and the external oblique. It origins quite constantly with its base between midline and midaxil- lary line and from the second to the sixth rib, irrespec- tive of its size. During trocar placement, care must be taken not to injure the mammary gland.
17.1.3
Intercostal Spaces
The intercostal muscles span the ribs and need to be dissected during trocar placement. The external inter- costals run obliquely downward and forward and ex- tend from the superior costotransverse ligament poste- riorly to the costochondral junction anteriorly where they are replaced by the anterior intercostal membrane.
The fibers of the internal intercostals pass obliquely downward and backward, extending anteriorly to the
Fig. 17.2. Right-sided thoracoscopic view of the chest wall showing the internal intercostal muscles, the intercostal neurovascular bundle, and the posterior intercostal membrane
sternum. Posteriorly, they are replaced by the posterior intercostal membrane (Fig. 17.2).
The inner muscular layer is formed by the transverse muscle of thorax at the front, the subcostals at the back, and the innermost intercostal muscle at the side of the rib cage.
Between the internal intercostals and the inner layer, the intercostal neurovascular bundle passes along the inferior rim of each rib (Fig. 17.2). The order from above downward is: intercostal vein, intercostal artery, and intercostal nerve, running in the sulcus costae. The trocars should, therefore, always be placed at the lower boundary of each intercostal space in order to avoid in- jury to the neurovascular bundle. Small collateral branches of nerves and vessels running at the superior rim of the ribs are of subordinate importance and can be ignored.
17.1.4 Diaphragm
The diaphragm is a thin sheet of muscle that originates from the xiphisternum in the front (pars sternalis), from the upper lumbar vertebrae (pars lumbalis) at the back, and from the lower six ribs in between (pars co- stalis).
The diaphragm curves up into two domes, with the right one higher than the left due to the liver. During full expiration the right dome can move up as high as the fourth intercostal space and the left dome to the fifth rib. This must be borne in mind during trocar placement in order not to penetrate the diaphragm, thus endangering liver or spleen on the left.
17.1.5
Anatomic Considerations in Trocar Placement
In thoracoscopic spine surgery, the trocars are usually placed within the axillary lines. It is advisable to mark the borders of the latissimus dorsi posteriorly and the pectoralis major anteriorly to avoid transmuscular tro- car placement, even if a rather posterior access is some- times necessary. However, blunt dissection of the inter- costal muscles and the serratus anterior proximally or the external oblique distally is inevitable in approach- ing the spine thoracoscopically (Fig. 17.1).
The skin incisions should follow the natural tension lines of the skin, running nearly parallel to the ribs. The length of the skin incision varies between 10 and 20 mm, depending on the size of the trocars (normally between 7 and 20 mm). The subcutaneous and muscu- lar tissue is bluntly dissected. The thoracic cavity is en- tered riding on the superior rim of the corresponding rib, perforating the endothoracic fascia and parietal pleura with a blunt clamp (Fig. 17.3). The interpleural space should first be examined with the fingertip to ex- clude any pleural adhesions before the trocar is insert- ed. Flexible ports are widely used since the risk of irri- tation of the intercostal nerves is smaller than with rig- id ones. Normally, the first port is placed in the sixth or seventh intercostal space irrespective of the planned procedure since it gives a good view of the entire hemi- thorax and the risk of injuring the diaphragm and its adjacent organs is minimal. The remaining ports, usu- ally between two and four, are placed under direct tho- racoscopic control.
Fig. 17.3. Perforation of the intercostals muscles, the endothoracic fascia, and the parietal pleura prior to trocar placement (right-sided thora- coscopic view)
17.2.
Thoracoscopic Anatomy
17.2.1
Internal Chest Wall
After entering the thoracic cavity, single-lung ventila- tion is established and the lung slowly collapses. The internal chest wall and its structures become visible (Fig. 17.2). Laterally, the thoracic wall is covered by the innermost intercostal muscles crossing more than one intercostal space. The lower internal chest wall is clothed posteriorly by the subcostal muscles, an incon- stantly developed group of muscles, also spanning more than one intercostal space.
The sloping ribs can be identified by a narrow layer of fatty tissue, but are not directly visible (Fig. 17.3).
Fig. 17.4. Right-sided thoracoscopic view of the midthoracic vertebral column covered by the parietal pleura with the rib heads, the sympathetic trunk, the discs, and the seg- mental vessels draining into the azygos vein visible underneath
More medially, the proximal parts of the ribs become visible with the internal intercostal muscles spanning the intercostal spaces. Posteriorly, they are replaced by the posterior intercostal membrane covering the fibers of the external intercostal muscles. The intercostal neu- rovascular bundles run along the inferior rim of the ribs (Fig. 17.2).
After further collapsing of the lung (either sponta- neously or by manual lung retraction), the heads of the ribs and the anterior vertebral column become accessi- ble (Fig. 17.4). By counting the ribs, the desired level can be identified. However, the first rib is rarely visible since it is surrounded by fatty tissue. It can be found by direct palpation and by localization of the adjacent sub- clavian vessels (Fig. 17.5).
Fig. 17.5. Right-sided thoracoscopic view of the upper thoracic spine showing the first three ribs and the subclavian vein
17.2.2
Costovertebral Joints
The ribs articulate with the vertebral column in two places, i.e., by their tubercles (costotransverse joints) and by their heads (joints of the rib heads). Typically, each rib head possesses two articular facets and articu- lates with two vertebral bodies – the upper rib facet with the lower costal facet of the vertebra above and the lower facet with the upper facet of its own vertebra – spanning the corresponding disc space (e.g., the fourth rib articulates with the vertebral bodies of T3 and T4;
Fig. 17.4). Therefore, the rib head needs to be removed in order to gain access to the epidural space (e.g., as in thoracoscopic discectomy). At T1, T11, and T12, how- ever, the ribs articulate exclusively with their own ver- tebral body (Fig. 17.6). At these levels, removal of the
Fig. 17.6. Right-sided thoracoscopic view of the lower thoracic spine demonstrating the articulation of the eleventh rib with T11
superior portion of the pedicle is sufficient to enter the spinal canal.
Each rib head is attached by ligaments to the vertebral bodies or disc spaces. These structures have to be divided before the proximal part of the rib can be removed, which is necessary to gain access to the epidural space between T2 and T10. The intraarticular ligament links the ridge between the two rib head facets with the outer fiber of the intervertebral disc. The radiate ligament rein- forces the joint capsule and consists of an upper and low- er part, running to the cranial or caudal vertebra, as well as a central part which runs horizontally across the inter- vertebral disc to the anterior longitudinal ligament.
The costotransverse joints are attached by the costo- transverse ligaments, of which the superior band runs to the transverse process of the cranially adjacent vertebra.
Fig. 17.7. Right-sided thoracoscopic view of the superior in- tercostal vein crossing the vertebral body of T4 and draining into the azygos vein
17.2.3 Pleura
The pleura consists of a thin fibrous membrane that clothes the entire thoracic cavity with its parietal and visceral layers. The parietal pleura is attached to the in- ternal thoracic wall by the endothoracic fascia (Fig. 17.3). It covers the vertebral column and the medi- astinum including the vessels and nerves (Fig. 17.4).
The parietal pleura needs to be divided to gain access to the vertebral column. During dissection, it can easily be elevated in order not to injure the segmental vessels or the splanchnic nerves.
17.2.4 Vessels
On the right side, the segmental veins caudal to T4 emp- ty directly into the azygos vein (Fig. 17.4). The second to fourth intercostal veins form the superior intercostal vein that normally crosses T4 before draining into the azygos vein. This typical formation of veins serves as an additional anatomic landmark (Fig. 17.7). The first in- tercostal vein empties directly into the brachiocephali vein. The azygos vein crosses the right main bronchus before joining the superior vena cava, which later di- vides into the left and right brachiocephalic vein. The segmental arteries originate from the thoracic aorta.
On the left side, the upper five segmental veins emp- ty into the accessory hemiazygos vein and the lower in-
to the hemiazygos vein. Both communicate with each other and drain into the azygos vein at the level be- tween T7 and T9. However, neither of the hemiazygos veins are visible due to the descending thoracic aorta that runs close to the vertebral column.
17.2.5
Sympathetic Trunk
The thoracic sympathetic trunk runs just laterally to the vertebral column, crossing the heads of the ribs (Figs. 17.4, 17.7). It originally possessed 12 ganglia, however, due to fusion of adjacent ganglia, there are normally fewer. Branches from each ganglion form the greater splanchnic nerve (fifth to ninth ganglia) and the lesser splanchnic nerve (tenth and eleventh ganglia) (Fig. 17.4). They cross the vertebral column in the lower thoracic spine and join the azygos or the hemiazygos vein before passing the diaphragm.
Suggested Reading
1. Liljenqvist U, Steinbeck J, Halm H, Schröder M, Jerosch J (1996) The endoscopic approach to the thoracic spine. Art- hroskopie 9:267 – 273
2. McMinn RMH (1995) Last’s anatomy, 6th edn. Churchill Livingstone, Edinburgh
3. Regan JJ, McAfee PC, Mack MJ (1995) Atlas of endoscopic spine surgery. Quality Medical Publishing, St Louis, MO
