Chapter 17 17 Anatomic Principles of Thoracoscopic Spine Surgery U. Liljenqvist 17.1. The pectoralis major consists of a clavicular head Anatomy of the Thoracic Wall with Respect and a sternocostal head, the latter forming the anterior to Endoscopic Approaches muscular boundary of the anterior axillary line and constitutes the anterior border for trocar placement. 17.1.1 The latissimus dorsi is characterized by its wide ori- Muscles of the Thoracic Wall gin ranging from the seventh thoracic spinous process The muscles of the pectoral girdle attach the upper limb with its fleshy origin in the thoracic region down to the to the trunk. Of relevance to endoscopic approaches to sacrum, becoming aponeurotic in the lumbar and sa- the thoracic spine are the serratus anterior, the pectora- cral region. It forms the muscular boundary of the pos- lis major, and the latissimus dorsi muscles, the latter terior axillary line and the posterior border for trocar forming the muscular boundary of the anterior and the placement. However, a far posterior access is some- posterior axillary line (Fig. 17.1). times necessary and blunt dissection of this muscle be- The serratus anterior covers the side of the thoracic comes inevitable. wallandformsthemedialwalloftheaxilla.Itorigins The external oblique, part of the anterior abdominal widely with its digitations from the first eight ribs in- wall, origins with its digitations from the fifth to the serting into the scapula. The digitations are bluntly dis- twelfth rib and spreads out with its fleshy part inserting sected during trocar placement. 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- costalsrunobliquelydownwardandforwardandex- 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 Fig. 17.1. Muscles of the thoracic wall from a lateral view downward and backward, extending anteriorly to the 17 Anatomic Principles of Thoracoscopic Spine Surgery 145 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 17.1.5 intercostal membrane (Fig. 17.2). Anatomic Considerations in Trocar Placement The inner muscular layer is formed by the transverse muscle of thorax at the front, the subcostals at the back, In thoracoscopic spine surgery, the trocars are usually and the innermost intercostal muscle at the side of the placed within the axillary lines. It is advisable to mark rib cage. the borders of the latissimus dorsi posteriorly and the Between the internal intercostals and the inner layer, pectoralis major anteriorly to avoid transmuscular tro- the intercostal neurovascular bundle passes along the car placement, even if a rather posterior access is some- inferior rim of each rib (Fig. 17.2). The order from times necessary. However, blunt dissection of the inter- above downward is: intercostal vein, intercostal artery, costal muscles and the serratus anterior proximally or and intercostal nerve, running in the sulcus costae. The the external oblique distally is inevitable in approach- trocars should, therefore, always be placed at the lower ing the spine thoracoscopically (Fig. 17.1). boundary of each intercostal space in order to avoid in- Theskinincisionsshouldfollowthenaturaltension jury to the neurovascular bundle. Small collateral lines of the skin, running nearly parallel to the ribs. The branches of nerves and vessels running at the superior length of the skin incision varies between 10 and rim of the ribs are of subordinate importance and can 20 mm, depending on the size of the trocars (normally be ignored. 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 17.1.4 rib, perforating the endothoracic fascia and parietal Diaphragm pleura with a blunt clamp (Fig. 17.3). The interpleural The diaphragm is a thin sheet of muscle that originates space should first be examined with the fingertip to ex- from the xiphisternum in the front (pars sternalis), clude any pleural adhesions before the trocar is insert- fromtheupperlumbarvertebrae(parslumbalis)atthe ed. Flexible ports are widely used since the risk of irri- back, and from the lower six ribs in between (pars co- tation of the intercostal nerves is smaller than with rig- stalis). id ones. Normally, the first port is placed in the sixth or The diaphragm curves up into two domes, with the seventhintercostalspaceirrespectiveoftheplanned right one higher than the left due to the liver. During procedure since it gives a good view of the entire hemi- fullexpirationtherightdomecanmoveupashighas thorax and the risk of injuring the diaphragm and its thefourthintercostalspaceandtheleftdometothe adjacent organs is minimal. The remaining ports, usu- fifth rib. This must be borne in mind during trocar ally between two and four, are placed under direct tho- placement in order not to penetrate the diaphragm, racoscopic control. thus endangering liver or spleen on the left. 146 Thoracic/Thoracolumbar Spine – General Techniques Fig. 17.3. Perforation of the intercostals muscles, the endothoracic fascia, and the parietal pleura priortotrocarplacement(right-sidedthora- coscopic view) 17.2. More medially, the proximal parts of the ribs become Thoracoscopic Anatomy visible with the internal intercostal muscles spanning the intercostal spaces. Posteriorly, they are replaced by 17.2.1 the posterior intercostal membrane covering the fibers Internal Chest Wall of the external intercostal muscles. The intercostal neu- After entering the thoracic cavity, single-lung ventila- rovascular bundles run along the inferior rim of the tion is established and the lung slowly collapses. The ribs (Fig. 17.2). internal chest wall and its structures become visible After further collapsing of the lung (either sponta- (Fig. 17.2). Laterally, the thoracic wall is covered by the neously or by manual lung retraction), the heads of the innermost intercostal muscles crossing more than one ribs and the anterior vertebral column become accessi- intercostal space. The lower internal chest wall is ble (Fig. 17.4). By counting the ribs, the desired level clothed posteriorly by the subcostal muscles, an incon- can be identified. However, the first rib is rarely visible stantly developed group of muscles, also spanning since it is surrounded by fatty tissue. It can be found by more than one intercostal space. direct palpation and by localization of the adjacent sub- The sloping ribs can be identified by a narrow layer clavian vessels (Fig. 17.5). 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 17 Anatomic Principles of Thoracoscopic Spine Surgery 147 Fig. 17.5. Right-sided thoracoscopic view of the upper thoracic spine showing the first three ribs and the subclavian vein 17.2.2 superior portion of the pedicle is sufficient to enter the Costovertebral Joints spinal canal. Theribsarticulatewiththevertebralcolumnintwo Each rib head is attached by ligaments to the vertebral places, i.e., by their tubercles (costotransverse joints) bodiesordiscspaces.Thesestructureshavetobedivided and by their heads (joints of the rib heads). Typically, before the proximal part of the rib can be removed, each rib head possesses two articular facets and articu- which is necessary to gain access to the epidural space lateswithtwovertebralbodies–theupperribfacet between T2 and T10. The intraarticular ligament links with the lower costal facet of the vertebra above and the the ridge between the two rib head facets with the outer lower facet with the upper facet of its own vertebra – fiber of the intervertebral disc. The radiate ligament rein- spanning the corresponding disc space (e.g., the fourth forces the joint capsule and consists of an upper and low- rib articulates with the vertebral bodies of T3 and T4; er part, running to the cranial or caudal vertebra, as well Fig. 17.4). Therefore, the rib head needs to be removed asacentralpartwhichrunshorizontallyacrosstheinter- in order to gain access to the epidural space (e.g., as in vertebral disc to the anterior longitudinal ligament. thoracoscopic discectomy). At T1, T11, and T12, how- The costotransverse joints are attached by the costo- ever, the ribs articulate exclusively with their own ver- transverse ligaments, of which the superior band runs to tebral