25 Regional Nuances

10.1055/b-0035-106400

25 Regional Nuances

Multiple factors affect the efficacy of a spinal implant. These include the following: (1) length of the construct; (2) spinal access for decompression, fusion, and instrumentation; and (3) selection of the surgical approach. Each is discussed in the pages that follow.

25.1 Length of Construct

Region-specific anatomy, in large part, dictates the choice of surgical strategies and so the course of the decision-making process. The subaxial spine, from the midcervical to the midlumbar region, is anatomically monotonous. Other than the presence of the rib cage in the thoracic region, small variations in the size and shape of the vertebrae and their bony relationships occur from level to level. Dorsal constructs in this region are not as limited by length as are ventral constructs. Ventral construct length is limited by soft tissue confines and restrictions related to the surgical exposure.

The occipitocervical and the lumbosacral regions, however, are limited by their location at the termini of the spine. Thus, length of construct becomes an issue. Long bending moments cannot be applied to the spine in these regions because of the short moment arms employed. Alternative techniques to achieve fixation must therefore be used.

25.2 Spinal Access

The cervical, thoracic, and lumbar regions are all readily accessible dorsally. The only region that is not truly accessible ventrally is the upper thoracic region. Vascular structures that cross the midline are the main impediments in this region. Vascular obstruction to cervicothoracic spinal access can be lessened by brachiocephalic vein ligation (Fig. 25.1). Differences in paravertebral vascular–spinal relationships in other regions often dictate the true orientation of spinal implant application. A true ventral approach is usually used in the cervical spine because of the ease of access to the ventral spine in this region and the impediments created by the vertebral artery, exiting nerve roots, and sympathetic chain in the paramedian and lateral portions (Fig. 25.2). The low cervicothoracic junction is usually best instrumented via a dorsal approach, in large part because of the anatomical barriers created by ventral vascular structures. However, the upper thoracic spine can often be approached ventrally if the patient‘s anatomy is favorable (Fig. 25.3). As the spine is descended farther, both ventral (by means of the thoracic inlet) and ventrolateral (by means of a thoracotomy or extrapleural thoracotomy) approaches may be used for spinal decompression and instrumentation (Fig. 25.4).

**Fig. 25.1** (A) Ventral cervicothoracic exposure is complicated by vascular structures that cross the midline, as depicted. (B) This limits ventral exposure to approximately T3, even if the innominate vein is ligated.

**Fig. 25.2** Cervical spine and regional anatomical impediments, such as the vertebral artery and sympathetic chain, provide an incentive for using a true ventral instead of a ventrolateral exposure.

**Fig. 25.3** The cervicothoracic region is usually instrumented dorsally. Ventral instrumentation may be precluded by the presence of vascular structures, although ventral decompression and fusion to T2 and T3 can often be achieved through the thoracic inlet if the manubrium and part of the clavicle (*hatched area*) are resected (see Fig. 25.2).

**Fig. 25.4** Below the cervicothoracic region, both ventral and dorsal approaches may be used. (A) True ventral approaches in this region are limited. (B) Ventral–lateral or lateral exposures via a transthoracic or extrapleural approach may be required.

25.3 Selection of Surgical Approach

The decision-making process regarding the most appropriate approach to the spine is often complex. It is clearly influenced by many factors, including the presence of neural compromise; extent of instability, vertebral body comminution, and dispersion of fragments; the presence of deformity, and so on. It is also heavily influenced by region-specific factors. Each region is therefore worthy of further discussion in this regard.

25.3.1 Craniocervical and Upper Cervical Region

The craniocervical and upper cervical region is unique because of its terminal location in the spine; its bony, ligamentous, and anatomical complexity; and the vital importance of upper cervical spinal cord function. Its terminal location on the spine makes it similar in many respects to the lumbosacral region. Fixation to the sacropelvis and fixation to the skull present similar problems regarding ease and security of fixation (Fig. 25.5). Augmenting the integrity of fixation by substantially increasing construct length is not an optimal option in either of these regions. Therefore, increasing the integrity of fixation is critical. In the craniocervical and upper cervical spine, this has been achieved with wires, hooks, screws, and button skull fixation. Transarticular screw fixation has been effectively used to limit C1–C2 rotation, thus enhancing overall stability (see Chapter 20).

**Fig. 25.5** (A, B) The cervicothoracic and lumbosacral regions are alike regarding their terminal spinal location. This poses problems with the length of the moment arm applied (d and d’).

The bony and ligamentous integrity and complex anatomical relationships in this region (Fig. 25.6) both hamper and assist the surgeon. If disrupted, these relationships (and integrity) must be restored, usually at the expense of motion. Failure to account for this may have significant consequences, particularly regarding the prevention of catastrophic injury to the spinal cord.

**Fig. 25.6** The complex ligamentous anatomy and anatomical relationships of the cervical spine in (A) an anteroposterior and (B) a dorsolateral view.

The surgical options in the craniocervical and upper cervical region include ventral and dorsal approaches and their variants. Transoral decompression almost always necessitates the use of an instrumentation–fusion construct, usually applied dorsally. Although biomechanical studies have implied that a limited destabilization effect occurs following odontoidectomy in some cases, the ability of such a spine to resist high stresses and loads is clearly suspect. Therefore, a transoral decompression almost always necessitates the use of a dorsal instrumentation construct.

25.3.2 Middle to Lower Cervical Spine

The subaxial cervical spine is not as anatomically complex as the craniocervical and upper cervical spine. It is, in fact, quite anatomically monotonous and very similar in this regard to the remainder of the subaxial spine. The subaxial spine is readily exposed both ventrally and dorsally. The ease of circumferential access facilitates both decompression and stabilization in this region.

Both ventral and dorsal region-specific stabilization techniques have been fraught with difficulties. Therefore, combined strategies have been recommended in selected cases. ¹ ^, ² The need for these, however, may be infrequent if biomechanical principles are appropriately applied (see Chapter 29 and Fig. 25.7). ¹ ^, ²

**Fig. 25.7** A combined ventral and dorsal construct employed for gross spinal instability as a result of involvement with rheumatoid arthritis at both the cervicomedullary junction and the subaxial cervical spine.

25.3.3 Cervicothoracic Region

The cervicothoracic region is exposed to significant stress because of its transitional nature and its configuration. Unfortunately, this region is difficult to decompress ventrally and is difficult to stabilize, as well. Ventral exposure by either a manubriectomy or a transsternal approach is limited by mediastinal vascular structures. These structures, as well as the narrowness of the exposure attained, limit transthoracic strategies in this region. Therefore, the lateral extracavitary and dorsolateral approaches may often be the most relevant approaches for the ventral decompression of lesions in the region of the cervicothoracic junction if the lesions are located below the reach of a ventral approach. ³ A ventral transcervical approach, with or without manubriectomy, may also be useful but may not provide adequate access for caudally located lesions in the cervicothoracic region (Fig. 25.8). The preoperative decision-making process can be facilitated by employing magnetic resonance imaging to help determine the feasibility of surgical trajectories. ⁴

**Fig. 25.8** (A) Ventral exposure of the cervicothoracic region is limited by mediastinal vascular structures and the sternum. The manubrium and sternum are usually not the limiting factor (*dashed line*). (B) Therefore, the lateral extracavitary and dorsolateral approaches are often employed in this region (*arrow*).

Stabilization is usually achieved dorsally. Upper thoracic vertebrae are often too small and narrow (with a heart-shaped configuration) to accept laterally placed implants (Fig. 25.9). Furthermore, the spinal curvature (lordosis transitioning to kyphosis) often precludes the transcervical application of true ventral strategies.

**Fig. 25.9** Vertebral bodies in the upper thoracic spine are usually small and heart-shaped, as depicted. The application of ventral instrumentation is limited not only by the midline vascular structures and the sternum but also by the obligatory suboptimal trajectory to the vertebral body attained.

Dorsal instrumentation techniques are complicated by the risk for sublaminar cervical hook or wire placement, by the relatively weak nature of lateral mass screw fixation sites, and by implant design limitations. Vendors have provided user-friendly implants for this region since the early 1990s. In this respect, care must be taken to ensure alignment of the cervical and thoracic components of an implant if they are to be attached at the time surgery. Lack of appropriate consideration of this phenomenon can place significant stress at the bone–metal juncture (Fig. 25.10). A bone–screw interface that is not stressed at the time of screw insertion and implant assembly and tightening is yet to be achieved. The surgeon must therefore aggressively limit the stressing of implant–bone interfaces during the assembly and tightening process.