Spinal instrumentation has evolved substantially in recent years. In keeping up with this ever-changing landscape, the spine surgeon must be cognizant of the potential complications associated with applying these techniques of spinal fixation. This chapter focuses on enumerating the potential complications associated with placing instrumentation in the subaxial cervical, thoracic, lumbar, and sacral spine in addition to the pelvis. Potential complications can range in severity from a small breach with no sequelae, to an organ or neurovascular injury with conceivably disastrous consequences. This chapter goes on to demonstrate that these complications are avoidable through a meticulous and thorough understanding of the relevant anatomy. Furthermore, this knowledge may be augmented intraoperatively through the use of X-rays, fluoroscopy, or stereotactic guidance. Additionally, these modalities also offer a method of intraoperative verification of hardware placement in order to confirm satisfactory position of instrumentation. Therefore, with a solid anatomical fund of knowledge and the proper intraoperative guidance/verification, the risk of instrumentation-related complications can be maximally mitigated.
Keywordsanterior cervical plating, cervical lateral mass screws, thoracic pedicle screws, lumbar pedicle screws, anterior lumbar interbody fusion, pelvic fixation
During cervical instrumentation, one must be mindful of the location of the vertebral artery in relation to the cervical vertebrae or lateral mass being instrumented.
For thoracic or lumbar pedicle screw placement, palpation of all four walls and floor of the bony channel is a critical portion of the procedure.
The most important factor in preventing instrumentation-related complications is a thorough understanding of the relevant surgical anatomy.
Furthermore, some method of intraoperative verification of hardware should be employed to avoid reoperation and to maximize accuracy.
The treatment of traumatic spinal injuries in the written record dates back as far as the Edwin Smith Papyrus, which was transcribed in the 17th century BC and is theorized to be a copy of a more ancient record dating to sometime between 3000 and 2500 BC. Spine surgery has evolved tremendously from ancient and medieval practices to the modern era, when advances have concentrated on spinal instrumentation and the techniques, trajectories, and locations in which spinal hardware can be placed. In addition, because of the recognition of the significant cost of health care in the United States, the appropriate utilization and delivery of health care in the spine patient has come under thorough scrutiny. Because spinal instrumentation is an integral component of the delivery of care in the modern-day spine patient, an understanding of the complications associated with spinal instrumentation is necessary to improve patient outcomes and reduce cost. This chapter will therefore focus on understanding the major complications associated with instrumenting the subaxial cervical, thoracic, lumbar, and sacral spine as well as the pelvis.
Although what follows will not be a definitive resource on all instrumentation-related complications, topics covered relate to common spinal instrumentation procedures, complications, and technique avoidance. Please note that complications related to the surgical exposure for instrumentation will not be discussed in this chapter as they are beyond its scope.
The cervical spine consists of seven vertebrae and has a lordotic curvature that usually ranges from 16 to 25 degrees from C2 to T2. The vertebral bodies gradually increase in size from C2 to C7 and are usually 17 to 20 mm wide with uncovertebral joints defining the lateral margins. End plates make up the superior and inferior boundaries of the vertebral body and have intervertebral discs above and below, respectively. Lateral masses are connected to the vertebral body by pedicles and transverse processes with an intervening transverse foramen through which courses the vertebral artery. Vertebral arteries enter at C6 in approximately 90% of cases, with levels C3–5 and C7 receiving it in the other 10%. Lateral masses decrease in size traveling down the cervical spine, and the superior and inferior articulating processes of each cervical vertebra make up the superior and inferior borders of the lateral mass, respectively.
Anterior Cervical Instrumentation
Anterior cervical instrumentation-related complications could include dysphagia, hoarseness, plate and screw loosening or fracture, end plate or posterior vertebral cortex disruption/fracture, plate migration, interbody graft migration, esophageal erosion/perforation, and nerve root injury. Anterior cervical plates ideally sit at the midline of the vertebral body between each uncovertebral joint, with the cranial and caudal limits of the plate lying at the midpoint of the vertebral body. The plate should not overlap onto the adjacent disc space and ideally should be at least 5 mm away from the adjacent disc space to prevent the possibility of adjacent segment ossification. Midline plate placement is important so as not to cause screws to be directed too far laterally and cause a neurovascular injury. Screws should be angled 90 degrees to the plate and should be as long as possible, but not so long as to breach the posterior wall of the vertebral body.
Posterior Cervical Instrumentation
Posterior cervical instrumentation complications can include vertebral artery injury, dural injury, nerve root injury, pedicle/lateral mass fracture, and screw/rod fracture or pullout. Subaxial posterior cervical instrumentation is most often accomplished with lateral mass screws, with several techniques for screw placement available. The Magerl technique is a common method that involves having a starting point of 1 mm medial and 1 to 2 mm superior from the center point of a cervical lateral mass and then directing the screw 25 degrees laterally and 30 degrees superiorly, or in parallel with the superior articulating process and that allows for bicortical bony purchase. Pedicle screws carry a greater risk of neurovascular injury due to small cervical pedicles with vertebral arteries laterally and the spinal canal medially, but are used occasionally if lateral mass screws are not an option.
The thoracic spine consists of 12 vertebrae with associated rib articulation. The thoracic spine is naturally kyphotic due to the angulation of its vertebral bodies, with a normal kyphotic curvature ranging from 20 to 50 degrees. Thoracic facets are coronally oriented in the upper levels and become progressively more sagittal in orientation caudally, down to T12. Thoracic spine pedicles become smaller from T1 to T4, with the smallest thoracic pedicles between T4 and T6, and then once again enlarge gradually down the thoracic spine to T12. Pedicle angles also significantly vary in the thoracic spine with an angle of approximately 30 degrees at T1 to 5 degrees or perpendicular at T12. The thoracic spine has two distinct transition zones: cervicothoracic and thoracolumbar. The cervicothoracic junction is a more abrupt transition zone than the thoracolumbar junction because of the relatively rigid upper thoracic spine and mobile cervical spine. In relation to the thoracic spine, important anatomic structures such as the esophagus, lungs, sympathetic chain, diaphragm, heart, thoracic duct, and great vessels sit in close proximity and bear significant consideration. An important artery to consider in lower thoracic spinal approaches is the artery of Adamkiewicz. This radicular artery most often enters from the left from T8 to L2, with T9 to T11 being the most likely location, and supplies the ventral spinal cord from the lower thoracic spinal cord to the conus. Each of these structures poses a possible site of injury during posterior instrumentation.
Lateral/Anterior Thoracic Instrumentation
A detailed discussion on technique avoidance and complications of anterior and lateral thoracic instrumentation is outside the scope of this chapter. However, complications can include injury to closely related anatomic structures such as the heart, lungs, esophagus, great vessels, sympathetic plexus, and thoracic duct.
Posterior Thoracic Instrumentation
Complications associated with thoracic pedicle instrumentation include spinal cord injury, dural tear, nerve root injury, pedicle breach/fracture, and injury to a major anatomic structure related to the thoracic spine as listed above.
There are several approaches to the posterior thoracic spine for a variety of indications. However, thoracic pedicle screws, which traverse the vertebral body, are the most commonly employed instrumentation technique. Thoracic pedicle screw starting points vary from T1 to T12 with upper thoracic levels being at the junction of the mid transverse process and lamina at the lateral pars, mid thoracic being at the junction of the proximal edge of the transverse process and lamina and lateral to the midpoint of the base of the superior articulating process, and at the junction of the bisected transverse process and lamina in the lower thoracic spine. The axial trajectory of a thoracic pedicle screw is vital because if it is too medial, the spinal canal can be violated, resulting in spinal cord injury, whereas if it is too lateral, vital anatomic structures as listed above may be injured. The appropriate angle at T1–2 should be approximately 30 degrees, whereas at T3–12 it should be approximately 20 degrees. These angles should be appropriately contoured based on preoperative imaging and thoracic pedicle level with an understanding that there is a significant difference in angulation from T1 to T12. Pedicle breach is a known potential complication of all pedicle screws, and in the thoracic spine a medial breach is considered safe if less than 2 mm, probably safe between 2 and 4 mm, and questionably safe if there are no electrophysiologic changes in intraoperative monitoring between 4 and 8 mm of medial pedicle wall breach. If the thoracic pedicle is breached laterally, up to 6 mm is considered acceptable; however, individual anatomy varies, and once again, it is important to consider proximally related structures as listed above. Sagittal angulation is also an important consideration with possible trajectories being more anatomic or straight on. The term anatomic refers to screws following the natural angle of the pedicle, which in the thoracic spine is posteriorly more rostral and anteriorly, more caudal. The term straight on refers to a screw trajectory that runs parallel to the surface of the superior end plate. One study comparing the two trajectories demonstrated that with straight-on screws, there is a 27% greater pullout strength and a 39% increase in maximum insertional torque when compared with anatomic screws. Being able to find and clearly visualize thoracic spine transverse processes, laminae, and facets is critical when inserting thoracic pedicle screws to avoid complications.
The lumbar spine classically consists of five vertebral bodies with an overall lordotic curvature of 20 to 65 degrees. Approximately 67% of lumbar lordosis occurs from L4 to the sacrum. Lumbar vertebral bodies and pedicles increase in size from L1 to L5, and the facet joints are sagittally oriented and allow for significant resistance to rotational forces. Relevant surgical anatomy associated with the lumbar spine includes the abdominal compartment as well as the retroperitoneum and their structures such as: bowel, aorta, inferior vena cava, kidneys, and ureters. The lower lumbar spine can also occasionally contain transitional anatomy where the L5 vertebral body can display anatomic variations in degree of fusion to the sacrum. There is a prevalence of 4% to 30% of a lumbosacral transitional vertebrae and is important to recognize when counting vertebral levels for surgery.
Anterior/Lateral Lumbar Instrumentation
As in the Thoracic Spine section, a detailed discussion on technique avoidance and complications of anterior and lateral lumbar instrumentation is outside the scope of this chapter. However, complications can include injury to closely related anatomic structures such as bowel, aorta, inferior vena cava, kidneys and ureters, pancreatitis, retrograde ejaculation, dural injury, and nerve root injury.
Posterior Lumbar Instrumentation
The most common form of posterior lumbar instrumentation includes pedicle screws and rods. The typical entry point for a lumbar pedicle screw is the junction of the pars interarticularis with the transverse process and mammillary process. The most common trajectory of a lumbar pedicle screw is lateral to medial in the axis of the pedicle. However, another option is the cortical bone trajectory, which follows a medial to lateral and caudal to rostral pathway through the pedicle. Complications associated with lumbar pedicle instrumentation can include pedicle breach/fracture, dural injury, nerve root injury, and vascular injury. Although lumbar pedicles are robust, there still exists a risk of breach with resultant injury. Therefore a thorough exposure of the posterior elements is necessary for safe access to the pedicle.
The sacrum consists of five vertebrae, which fuse together by the time a human reaches adulthood. The sacrum articulates with four different bones: the lumbar spine rostrally, the coccyx caudally, and bilaterally with the ilium through the sacroiliac joint. As in the lumbar spine, the sacrum can display transitional anatomy with a “lumbarized” sacral vertebral body. An important anatomic consideration regarding instrumentation is the fact that S1 pedicles tend to be large and full of cancellous bone, which can limit screw purchase within pedicles. Relevant posterior pelvic anatomy in relation to instrumentation includes: the posterior superior iliac spine, acetabulum, and sciatic notch, all of which contain the superior gluteal artery and sciatic nerve.
Posterior Sacral/Pelvic Instrumentation
The sacrum is most commonly instrumented at S1 and S2. Sacral screws can be directed through the pedicle into the sacral vertebral body and laterally into the sacral ala as well as through the sacral ala and into the iliac bone. The S1 pedicle is the trajectory most commonly instrumented; however, because of its large cancellous bony component, screws are at risk for pullout. Therefore S1 screws are often placed in a way to engage as much cortical bone as possible, which can lead to anterior breach of the screw. The entry point for an S1 pedicle screw is the inferolateral margin of the L5/S1 facet and directed anteromedially through the pedicle and toward the sacral promontory. However, complications related to S1 instrumentation are screw pullout and sacral fracture; to help prevent these complications, constructs are often extended to S2 and the ilium. S2 screws are often directed laterally through the sacral ala and into the iliac bone (S2AI screw) to allow for a pelvic point of fixation, for instrumentation to remain in line and not require an offset connector. This also allows for less of a lateral dissection for exposure of the iliac screw start point. The start point of an S2AI is 1 mm inferolateral to the S1 dorsal foramen. The screw is angulated toward the greater trochanter and ~30 degrees anterior to the floor. One concern with the S2AI screw is the violation of the sacroiliac joint; however, it is still unknown whether this has a clinically significant adverse effect. Finally, iliac screws are also an important consideration for helping to improve the caudal strength of a construct while fusion occurs by unloading the pressure off of S1 screws. Potential complications associated with instrumenting the sacral spine include damaging the structures anterior and lateral to it, especially when considering that sacral screws are as long as possible to properly engage cortical bone and are liable to breach anteriorly. Potential structures at risk include the iliac veins and arteries, nerve roots, and rectum. The pelvic screw start point correlates with the posterior superior iliac spine, and screws are directed to the cortical bone directly above the greater sciatic notch and toward the anterior inferior iliac spine. Potential complications while inserting pelvic screws include breaching cortical bone anteriorly or posteriorly, breaching the greater sciatic notch and causing injury to the superior gluteal artery or sciatic nerve, or breaching into the acetabulum.