Instrumentation Complications

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CHAPTER 97 Instrumentation Complications

Patrick T. O’Leary, MD, Alexander J. Ghanayem, MD

Spinal instrumentation has greatly evolved over the past half century from simple wiring and suture techniques, to the nonsegmental hook and rod techniques in the 1960s, to contemporary devices such as dynamic anterior cervical plates, motion-sparing devices, and modern pedicle screw-rod systems. What has also evolved is our understanding of spine biomechanics, anatomy, physiology, and biology of the normal and diseased spine and how this relates to the variety of conditions that the spine surgeon will treat. Advances in imaging have also enhanced our ability to understand spinal pathology and provide real-time assistance in the operating room. Thus we can now directly and indirectly decompress the neurologic elements of the spine; stabilize, realign, and/or fuse the spine anteriorly, posteriorly, and anterolaterally; and realign and perform a variety of motion preservation procedures anteriorly and posteriorly. All of these procedures can be performed using traditional open approaches and minimally invasive endoscopic and percutaneous approaches.

The spine surgeon has a large and powerful armamentarium of “tools” at his or her disposal. One needs only to attend a national spine meeting or review earlier editions of this textbook to understand the expansion and evolution of spinal instrumentation. We can now do much more for the patient, as well as for a wider demographic of patients, than in each of the prior decades in the past half century. In the cervical spine, seemingly simple instrumentation systems such as anterior cervical plates have allowed certain patients to avoid cumbersome postoperative braces or halo vests. Fusion rates are also enhanced with anterior cervical plate stabilization. Some physicians will advocate the use of allograft bone with an anterior plate to avoid the painful process of autogenous iliac crest bone harvest for anterior cervical fusion. Unilateral cervical spine traumatic facet injuries with an associated traumatic disc herniation can now be treated with a one-stage anterior decompression and plate stabilization procedure rather than an anterior/posterior procedure. However, with the expanding use and application of spinal instrumentation, the realm of complications expands in parallel. Anterior cervical discectomy with instrumentation can lead to dysphagia and dysphonia, vertebral artery injury, disruption of adjacent disc levels, and other challenges associated with revision anterior cervical procedures. Hence the application of new spinal instrumentation systems is always a balance and requires a thoughtful risk-benefit analysis of the device, its intended consequences, and, more importantly, its unintended consequences.

Classification of Complications

Spinal instrumentation complications can occur for a variety of reasons and clinical scenarios. It can be useful to classify these occurrences in the following manner:

1 Biologic failure related to:

a Infection
b Osteoporosis
c Pseudarthrosis
d Other patient-related challenges

2 Biomechanical failure
3 Error in thought process
4 Error in application

Complications can occur as a result of one or more than one of the above occurrences. Understanding these different processes can help in avoiding and managing complications associated with spinal instrumentation.

Biologic Failure

Spinal fixation and motion-preserving devices need to reside in a bioactive and mechanically challenging environment. Infection surrounding these devices can occur shortly after insertion or many years after the surgical procedure. Wound infection rates are slightly higher in the presence of instrumentation compared with noninstrumented procedures.1,2 For example, a posterior bone-only lumbar fusion may have a smaller infection rate than an instrumented fusion. The infection can be related to the presence of the hardware itself or the increased operative time associated with the instrumentation procedure. Early instrumentation systems were made from stainless steel. The advent of titanium systems theoretically will decrease the affinity of bacteria to the surface of the device.3 However, unintended wear debris may be greater with titanium implants in a developing pseudarthrosis with implant interface micromotion leading to a more robust inflammatory response.4,5 The issue of wear debris as it relates to disc arthroplasty does not seem to be as problematic compared with arthroplasty in a synovial environment.6,7 With a wide range of disc designs and arthroplasty interfaces in use in Europe and only recently approved in the United States, a definitive statement regarding wear debris cannot be made at this time.8 However, infection in the anterior lumbar interbody region can be extremely difficult to approach, drain, and reconstruct.9

The interface between spinal instrumentation systems depends on the quality of the host bone and/or vertebral endplate. Osteoporosis can lead to early fixation failure or implant loosening before an attempted arthrodesis procedure heals. Deficient vertebral endplates or subchondral osteoporosis can lead to interbody device loosening or subsidence.10–12 Motion-sparing devices rely on a solid point of fixation to the spine. Osteoporosis can compromise the interface leading to alterations in the biomechanical performance of the motion-sparing device.

Pseudarthrosis may occur in even the most technically well-performed operation in an ideal, young, healthy, nonsmoking patient. If may be difficult to identify the pseudarthrosis in a long, posterior scoliosis fusion until its associated micromotion results in pedicle screw or rod breakage. Although the instrumentation may have failed mechanically, the complication was biologic in its origin. An unplated anterior cervical discectomy and fusion may result in an asymptomatic pseudarthrosis, but a plated pseudarthrosis may result in screw breakage or migration and subsequent dysphagia. If the carpentry of the interbody graft was correct, then the complication is biologic leading to biomechanical failure. If the interbody graft was not properly prepared, the complication is an error in application leading to biologic and subsequent biomechanical failure of the anterior plating system.

Many other patient-related factors contribute to biologic failure and subsequent instrumentation complications. Steroid use, smoking, cancer, prior radiation therapy, multiple trauma, and poor nutrition are all factors than can either adversely affect a patient’s ability to heal a biologic procedure (fusion), diminish bone quality, and/or increase the risk of infection.13–21 Many of these such as steroid use, cancer, and trauma are unavoidable factors that one must encounter in the use of spinal instrumentation. Nutritional status can be assessed and improved if surgery is elective.22 Smoking cessation before spinal reconstructive procedures can improve fusion rates.16 The option should always be explored with patients. The decision of what to do if a patient cannot stop smoking before surgery is a social, ethical, and national health problem that is beyond the scope of this chapter. Needless to say, smoking is a deterrent to the success of a spinal fusion and may contribute to an increased incidence of instrumentation complications.

Biomechanical Failure

It is difficult to discuss biomechanical failure of spinal instrumentation without considering it along with an initiating problem. As mentioned earlier, an instrumented posterior fusion procedure resulting in a pseudarthrosis can lead to pedicle screw or rod breakage (Fig. 97–1).23 With any instrumented fusion, it is a race between failure of the instrumentation and healing of the fusion procedure.

image

FIGURE 97–1 A-B, Anterior-posterior and lateral radiographs 3 years postoperation from an L4 to S1 posterior fusion. The fusion mass appears robust between L4 and L5 but is patchy between L5 and S1. C-D, Anterior-posterior and lateral radiographs of the same patient now 4 years postoperation. The fusion masses appear unchanged, but both S1 screws have broken consistent with a pseudarthrosis at the L5-S1 level. The patient’s symptoms have remained stable with a good surgical outcome.

Morbidly obese patients clearly stress spinal implants more than normal size patients. Here the interface between the implant and host bone becomes important early in the healing process. Later, the interface and the success of the fusion will protect the implant from failure in the form of loosening, migration, and/or breakage.

Spinal instrumentation has become a powerful tool in correcting spinal deformity or reducing and maintaining alignment of spine fractures and dislocations. In scoliosis correction, the instrumentation has to resist the spine’s natural recoil back to its initial deformity secondary to its osseoligamentous components. In addition, pullout of the cranial or caudal end of fixation may fail secondary to these deforming forces even in excellent host bone. Fractures and dislocations may appear statically stabilized, but spinal instrumentation systems are dynamically neutralizing the deforming forces that come into play with the associated traumatic failure of the normal supporting structures. Here, it is a race once again between healing and failure of the interface or fatigue breakage of the implants.

Error in Thought Process

Physicians are commonly, and unfairly, divided into “procedural” and “cognitive” specialists with spine surgeons falling into the procedural group. One of the greatest challenges in spine surgery is matching the correct operative procedure with the right patient—a cognitive skill requiring insight into the patient’s problems, goals, and expectations. Once a patient is deemed a surgical candidate, cognitive skills continue in the preoperative planning process. It is in this process that potential instrumentation complications can be flushed out and avoided once the surgical plan is put into action. Accurate assessment of regional anatomy and preoperative templating, before entering the operating room, is critical.

Understanding regional anatomy requires correlation among the neurodiagnostic studies, one’s knowledge of anatomy, and any patient-specific variabilities that may exist. Placing a screw in bone requires that the bone be of adequate size (i.e., pedicle screws and pedicle diameter) and that the surrounding structures are not at undue risk for injury. The course of the vertebral artery as it passes from C2 to C1 is a classic example of this scenario before placing a transarticular C1-2 screw. One needs to evaluate preoperative sagittal and coronal reformatted computed tomography (CT) scans to determine the course of the vertebral artery. The column of bone accepting the screw should be wide/tall enough for the screw, the C1-2 lateral mass articulations need to be reduced or at least reasonably well aligned, and the vertebral artery should not be high riding or medial so as to lay in the path of the screw. If these criteria are evaluated before surgery, then a proper decision can be made as to whether or not to attempt a C1-2 transarticular screw, as well as plan for alternative surgical options.

Accurate assessment of spinal anatomy leads directly to the next step of preoperative planning. Nearly every device placed in the interbody region will usually have a corresponding set of templates, corrected for varying degrees of magnification, to allow templating for the proper implant size. Threaded lumbar interbody cages are a typical implant that requires templating. Dual cages require adequate left to right spacing so that they will be well confined to the interbody region without encroachment of the cauda equina or lumbar root in the neuroforamina.24 They also need to adequately engage the superior and inferior endplates for stability and healing. Potential cage size can be determined from preoperative templating. More important, templating one size larger and smaller than the planned size will give the surgeon insight as to what may be necessary during surgery, what can be attempted should the planned cage size not be correct, and ensure that the proper implants are available at the time of surgery. Other implants/anatomic regions requiring templating on a routine basis are pedicle screws and pedicle diameter, anterolateral plates and vertebral body screws for the thoracolumbar spine, and disc replacements.

Error in Application

Execution of a well-designed and thought-out surgical plan is not without complication. Spinal instrumentation is no exception to this problem. Application errors can be directly linked to errors in thought process by deciding on the incorrect implant for a particular goal in a spinal reconstructive procedure. They can also include basic errors such as misplacing a pedicle screw in a pedicle. Hopefully, this would occur laterally into muscle rather than inferiorly into the foramen or medially into the spinal canal (Fig. 97–2).

image

FIGURE 97–2 Postoperative computed tomography image of a misplaced thoracic pedicle screw entering the spinal canal (left side of image) and displacing the spinal cord. The complication is an error in application.

Lack of intraoperative orientation to the spine can lead to application errors as well. Applying an anterolateral plate at the thoracolumbar junction requires the patient to be in the true lateral position. If the patient has rolled slightly anterior or posterior, vertebral body screws run the risk of entering the spinal canal or injuring the great vessels.25 Loss of midline orientation in the anterior cervical spine can lead to malpositioning of an anterior cervical plate and even vertebral artery injury from vertebral body screws. The surgeon must maintain his or her own three-dimensional orientation while inserting spinal instrumentation in order to prevent injury to surrounding neural, vascular, visceral, and other soft tissues. Intraoperative feedback from live fluoroscopy or image-guidance systems are not substitutes for a thorough understanding of anatomy and surgical landmarks associated with inserting different types of spinal instrumentation systems (Fig. 97–3).

image

FIGURE 97–3 A, Lateral radiograph of a patient who underwent a posterior fusion attempt at L4-S1 and then a latter anterior fusion for a posterior pseudarthrosis. Note the position of the pedicle screws at L4—they appear to be set close to the mobile L3-4 facet joint and traverse the pedicle parallel to its axis. This radiograph would indicate that the pedicle screw start point might be close to the L3-4 facet joint. B-C, Sequential computed tomography images at the L3-4 facet/L4 pedicle level of the same patient. Note that the L4 pedicle screws enter the pedicles and violate the L3-4 facet joints. The complication is an error in application.

Complications by Spinal Region and Implant Type

Anterior Cervical Spine

Cervical plates are the most common implants used in the anterior cervical spine. The use of buttress screws and wire has fallen out of favor over the past 20 years. Anterior plates have evolved from nonlocking, simple bone plates and bicortical screws to systems that have a locking mechanism between the plate and screw while allowing dynamic settling of the plate if the surgeon chooses this option. Contemporary systems tend to use unicortical fixation rather than bicortical screw fixation. Therefore screw penetration into the spinal canal with neurologic injury is a rare complication. Plate fracture is also quite rare. Screw back-out can still occur despite the locking mechanisms designed to prevent this problem. If this occurs, it typically represents abnormal motion at the fusion site leading to a pseudarthrosis (Fig. 97–4). Screws that back out should be removed to prevent esophageal injury, and pseudarthrosis repair should be undertaken if necessary. Occasionally, the screw-plate locking mechanism will remain intact and the plate will lift off the anterior vertebral body surface as the screw-bone interface loosens. This would also be indicative of a pseudarthrosis and should be addressed surgically. Screw breakage can occur with settling of a statically locked, anterior-plated fusion construct. If the plate remains flush with the vertebral body and the fusion heals, then a revision is typically not required.

image

FIGURE 97–4 A, Lateral radiograph performed shortly after surgery of an anterior cervical discectomy and fusion at C5-6, C6-7, and C7-T1. B, This patient developed pseudarthrosis at the C6-7 level with collapse and failure of the locking mechanism between the screw and plate as evidenced by the screw backing out of the plate. The plate is also lifting off the spine. This patient developed recurrent neck pain and dysphagia. The complication includes biologic failure of the fusion and biomechanical failure of the plate-screw interface.

Placement of the anterior plate can also be associated with complication. Placing the plate too lateral puts the vertebral artery at risk when drilling for and inserting screws on that side. This was particularly of concern when screws were inserted in a divergent direction. Most contemporary systems make use of convergence to minimize the risk of this occurrence. One must also evaluate each level to be instrumented for vertebral artery anomalies so as to avoid injuring the artery during drilling or screw insertion.26 Placing a plate too cephalad or caudal relative to an adjacent unaffected disc places that disc space at risk for adjacent-level ossification.27 Whether or not this represents a long-term problem remains to be demonstrated.

Anterior odontoid screws have a limited role in the overall treatment of odontoid fractures. Safe screw insertion requires a reducible fracture that is not the reverse oblique pattern, minimal bone comminution, reasonable bone quality for screw purchase, and the ability to keep the odontoid fragment from spinning while drilling and inserting the screw. Fluoroscopic visualization in the anterior-posterior and lateral planes needs to be established before the start of surgery. Patients with large/barrel chests may preclude the surgeon from obtaining a shallow enough angle to drill and insert the screw. Complications therefore include inability to insert the screw, screw cutout, spinning of the odontoid fragment and malreduction, fracture displacement, and spinal canal penetration.28

One set of complications of anterior cervical instrumentation cannot be defined on radiographs—those associated with the surgical approach and the use of power drills and taps. Care must be taken to safely mobilize the midline neck structures and protect the nerves associated with speech and swallowing function. Drill and tap sleeves should be used to prevent soft tissue injury. A common postoperative complication following anterior cervical spine surgery, dysphagia, has been linked to prolonged retractor use.29 Careful attention to adjusting retractors, working efficiently, and attempting to limit retractor time can help to reduce this complication.

Posterior Cervical Spine

The occiput-C1-C2 region has its own unique challenges with reference to the vertebral artery anatomy, unique bony characteristics, and proximity of the brain and spinal cord. Wiring procedures are relatively safe unless sublaminar wire fixation at C1 is considered. Care must be taken not to allow the wire to pass too ventral, causing spinal cord compression. Screw fixation across the C1-C2 facet joint places the vertebral artery at risk. C1 lateral mass screws require assessment of the vertebral and internal carotid artery position, the former as it relates to screw insertion point and the latter as it relates to the final position of the screw tip.30

Subaxial wire fixation using the Bohlman triple-wire technique avoids sublaminar passage of the wire and its associated risk of spinal cord injury. One must take care to pass the wire horizontally through the spinous process in that it is possible to push the wire ventrally into the spinal canal. Subaxial screw fixation is usually accomplished in the lateral masses from C3 to C6 and the pedicles of C7-T1. Insertion angles and screw length need to be monitored to avoid neurologic and vertebral artery injury.

The strength of screw fixation is never better than when it is initially inserted. Marginal fixation secondary to screw or wire cutout or marginal bone quality will usually fail, and alternative plans should be undertaken. Excellent fixation does not obviate the need to properly bone graft the instrumented levels to allow for fusion.

Anterolateral Thoracolumbar Spine

Anterolateral vertebral body screw-rod systems and plating systems represent the bulk of instrumentation systems that are used in the anterolateral thoracolumbar spine. Here orientation and proper patient positioning are the first steps in avoiding aberrant screw placement with potential neurologic or great vessel injury. This is especially true in patients with rotational deformities. It is important before surgery to premeasure the vertebral widths on available imaging studies. In deformity cases, the ends of the construct are areas of stress concentration31 and can pull out or fail. It is important to accommodate adjacent curves and pay attention to sagittal balance in an effort to minimize the risk of transition syndromes.

In the operating room, the anatomy can be deceiving when attempting to locate the posterior aspect of the vertebral body during the anterolateral approach, especially in cases where local trauma, infection, or tumor obscures the anatomy. The posterior screws should be directed anteriorly to minimize risks to the neural structures. Frequent use of fluoroscopic imaging assistance to confirm your orientation can help minimize intraoperative complications. This can also be helpful when assessing the adequacy of a discectomy or corpectomy when the patient is in a lateral decubitus position. As mentioned earlier, it is also important to maintain a “frame of reference,” ensuring that the patient has been maintained in direct lateral posture. Inadvertent problems can occur if the spine/pelvis assumes a more oblique orientation during the surgery.

Anterior plates should be long enough to capture the superior and inferior screws yet short enough so as not to overlap the mobile disc above and below the instrumented levels. Osteoporosis can lead to loss of fixation and screw cutout into the disc space, plowing through the vertebra, or lifting off/backing out of the vertebral body. Reconstruction of the corpectomy defect can be performed with structural autograft, allograft, composite spacers, linear cages, and expandable cages. They all must bear stress on the vertebral endplate. Nonstructural grafts are more likely to collapse with time. Osteoporosis or damage to the endplate can lead to excessive subsidence and loss of fixation.32,33

Posterior Thoracolumbar Spine

Pedicle screw-based systems are the workhorse instrumentation systems of this region. The benefits over hooks and sublaminar wires are many: three-column fixation allows better pull-out strength and greater control in the sagittal, coronal, and rotational planes due to increased stability to axial, bending, and rotational forces.34–37 Additionally, fewer vertebral motion segments are fused, thereby preserving more motion segments. The added fixation strength may lessen or obviate the need for postoperative bracing. Other benefits include secure fixation after laminectomy or in a situation where the posterior elements are otherwise incompetent and the ability to treat three-column injuries with adequate stability. However, hooks and wires can still be used with or without pedicle screws in modern segmental instrumentation systems and in some cases may be the only option for posterior fixation in a given spinal segment.

The complications related to hooks and sublaminar wires are well studied. Sublaminar wires run the risk of neurologic injury and cutting out of bone by sawing through the lamina. Sublaminar hooks also represent a source of neurologic injury. Hooks should not be placed in regions of canal narrowing secondary to degenerative, traumatic, or deformity-based stenosis.38 Pedicle and transverse process hooks can also be used with less risk of neurologic injury but with the latter not having the biomechanical strength of laminar hooks or pedicle screw systems.

When placing pedicle screws, especially in the thoracic spine, the proximity of the instrumentation to adjacent structures is the major concern. Vaccaro and colleagues39 advanced screws positioned accurately within the pedicle through the anterior cortex in cadaveric specimens to assess the proximity of important neurovascular structures. The following structures are endangered when screws are placed on the right side of the body: superior intercostal vessels (T4-T5), esophagus (T4-T9), azygous vein (T5-T11), inferior vena cava (T11-T12), and the thoracic duct (T4-T12). On the left side of the body, the following structures may be damaged by screws placed through the anterior cortex: esophagus (T4-T9) and aorta (T5-T12). When screws are placed laterally, the lung, segmental vessels, and sympathetic chain are in jeopardy on both sides. The aorta is endangered on the left (T5-T10) with laterally placed screws and the azygous vein on the right (T5-T11). In terms of nerve root and spinal cord proximity to the pedicle, there are consistent differences between the thoracic and lumbar spine. The thoracic nerve roots are approximately evenly spaced between the superior and inferior pedicles due to the more perpendicular angle off of the spinal cord. In the lumbar spine, there is a significant difference in pedicle-nerve root distance with the nerve roots having three to four times more space superior to the pedicle than inferior. The dura directly abuts the medial aspect of the thoracic pedicle in most studies, whereas there is 1 to 2 mm of space in the lumbar spine.

The complications associated with pedicle screws placed in the lumbar spine have been well documented. Reports vary in terms of whether the complication rates correspond to the rate per screw placed or the rate per patient. Lonstein and colleagues40 reviewed almost 4800 pedicle screws—more than 98% within the lumbar spine—with an overall complication rate of 2.4%. On the basis of radiographic analysis, 5.1% of screws extended outside the pedicle or vertebral body. A significant percentage of these screws was intentionally placed through the anterior cortex for bicortical fixation at S1. The most common complication was pain (23%) that was believed to be related to pseudarthrosis or the implant itself. Nerve root irritation was uncommon (0.2%) despite 1% of screws penetrating either medial or inferior to the pedicle wall. In other studies, the rate of implant removal due to pain is relatively low and nerve root irritation is also uncommon (range: 0.2% to 5% in various series).

In the thoracic spine, several studies have analyzed rates of complications. The majority of these studies review pediatric deformity patients. Suk and colleagues41 reported the largest series of thoracic pedicle screws to our knowledge. Of more than 4600 screws investigated, 1.5% of screws were determined to be malpositioned by radiographic analysis. Screw malpositions were lateral in 27%, medial in 6%, superior in 18%, and inferior in 49%. In this series there was one neurologic injury causing transient paraparesis due to a medial pedicle perforation causing a delayed epidural hematoma. Other complications cited include pedicle fractures (0.24%), screw loosening (0.76%), and infections (1.9%). Kim and colleagues42 reported the safety and reliability of a free-hand (anatomic) technique of pedicle screw placement in the thoracic spine. Of the 1118 screws placed without the help of fluoroscopy or radiographs, 400 were randomly evaluated by CT. Of the 400 screws reviewed by CT, 55.8% were totally contained within the pedicle. Three percent of the screws had penetrated the spinal canal by 2 to 4 mm. There were no neurologic or vascular complications.

In summary, a few trends with regard to pedicle screw complications appear to exist in the literature. Although the thoracic vertebrae have smaller pedicle dimensions and lack a significant “safe zone” from neurologic injury, the rate of neurovascular complications is surprisingly low. This may be due in part to the fact that root injuries in the thoracic level may not be detected even when present. The higher relative thickness of the medial pedicle wall correlates with the lower incidence of medial wall fractures and thus lower risk of neurologic injuries. Furthermore, despite several anatomic studies suggesting that the midthoracic pedicles may be too small to allow pedicle screw placement, this does not appear to be the case. Pedicle expansion, especially in the pediatric population, appears to compensate for the small pedicle diameters. Also, to our knowledge, the close proximity of the pleura, aorta, and azygous system has yet to present as a common problem outside of a few isolated reports. The risk to the anterior structures can be offset by carefully avoiding penetration of the anterior or lateral cortex.

Lumbar Interbody Fixation

Lumbar interbody devices can be placed via anterior or posterior approaches. All of these devices require preoperative templates to determine the implant size and extent of disc exposure to permit safe insertion. Interbody fusion devices placed via a posterior interbody route (posterior lumbar interbody fusion [PLIF]) place the cauda equina and nerve roots at risk during insertion because of the retraction required to insert the device (cage).43,44 Implants need to be large enough to engage the disc endplates but not too large to force over-retraction of the neurologic elements. Transforaminal cage insertion (transforaminal lumbar interbody fusion, TLIF) places the exiting and traversing nerve roots on the side of insertion at less risk than the PLIF approach. Both PLIF and TLIF procedures destabilize the posterior tension band supporting structures and require supplement posterior stabilization.45–47

Anterior devices require mobilization of the abdominal contents and, most commonly, the left iliac vein. The use of paired, stand-alone interbody cages requires preoperative templating to ensure that cage height is sufficient to engage the vertebral endplates while the combined cage width does not exceed the margins of the disc space and protrude into the spinal canal/neuroforamina (Fig. 97–5).22 Depth of insertion depends on the implant: interbody cages are usually placed so that the anterior portion of the cage resides just under the anterior margin of the vertebral body. Countersinking the cage can result in subsidence and subsequent pseudarthrosis, as well as loss of foraminal height resulting in radicular symptoms. Disc arthroplasty usually requires that the prosthesis is placed posteriorly in the disc space—usually at the posterior vertebral margin so as to optimize motion. Excessive endplate resection or damage during the discectomy, as well as osteoporosis, can lead to subsidence of interbody devices as well.

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FIGURE 97–5 A, Anterior-posterior radiograph of an anterior interbody fusion with a pair of cages. Note that the cage on the left side of the photograph is laterally positioned and extends farther lateral than the pedicle. B-C, Axial computed tomography and sagittal reformatted images reveal that the laterally placed cage extends beyond the confines of the disc space and extends into the neuroforamina. It also appears that a disc fragment may have been pushed into the foramina.

Other Implant-Related Complications

Although this chapter is mainly dedicated to instrumentation complications, the authors feel that a review of complications associated with the use of bone morphogenetic proteins in spinal fusion surgery should be included. INFUSE (Medtronic), recombinant human bone morphogenetic protein-2 (rhBMP-2), was approved by the U.S. Food and Drug Administration (FDA) in 2002 for use in a threaded lumbar interbody cage for anterior lumbar interbody fusion. It was approved as a medical device. This is the only current FDA-approved indication for rhBMP-2 in spine surgery. Nonetheless, rhBMP-2 has been used for years in what has come to be called “off-label” applications. The potential benefits include increased fusion rates and minimizing and/or eliminating autologous bone graft harvest and its associated morbidity.48–50

With increasing use, however, have come some device-specific adverse events. rhBMP-2 used for interbody arthrodesis in cages inserted through a posterior approach (transforaminal lumbar interbody fusion/TLIF) has been linked to an increased incidence of radiculitis in both standard open and minimally invasive approaches.51,52 The exact mechanism creating the radiculitis remains unknown. Rihn and colleagues51 reported a reduction in the incidence of radiculitis in the rhBMP-2 TLIF cohort with the use of a hydrogel sealant.

Other adverse events include ectopic bone formation. This has been reported to be both asymptomatic and symptomatic. Joseph and Rampersaud reported the incidence of heterotopic bone formation following minimally invasive TLIF or PLIF with rhBMP-2 to be 20.8% versus 8.3% in the non-BMP group.53 In the rhBMP-2 group, one patient developed bone within the epidural space and the other four developed bone in the foramen. The authors did not detect any adverse clinical events associated with the development of heterotopic bone.

Recent retrospective case series suggest that ectopic bone formation following TLIF/PLIF with rhBMP-2 can be symptomatic. Wong and colleagues54 reported on five cases in which BMP was used with a PLIF/TLIF and resulted in ectopic bone within the spinal canal and potential neurologic compromise. The findings were confirmed by CT. Three of the five patients had revision surgery for symptomatic neural compression following initial surgery. Ectopic bone was found within the canal in each case, adherent to the neural structures. According to the authors of the study, each patient has at least partial improvement in symptoms postoperatively, suggesting that the ectopic bone formation was, at least in part, contributing to symptoms.54 Chen and colleagues55 reported on four cases following minimally invasive TLIF with BMP in which the patients developed delayed, symptomatic neural compression related to ectopic bone formation.

Adverse events associated with rhBMP-2 use in the anterior cervical spine have been well-documented. In 2006 Smucker and colleagues56 reported on clinically significant adverse events associated with rhBMP-2 use for anterior cervical arthrodesis. Of patients in the rhBMP-2 group, 27.5% developed significant swelling events (P < 0.0001) compared with 3.6% in the non-rhBMP-2 group. Swelling complications included delay in discharge, severe dysphagia, reintubation, and visible neck swelling. Swelling occurred, on average, on postoperative day 4.2. Most cases of swelling occurred in primary surgeries. Three patients were treated for irrigation and débridement for a swollen surgical site without evidence at time of exploration of hematoma or fluid collections. No patient demonstrated an actual compromised airway on exploration.

Since this study, other reports have surfaced regarding adverse events associated with swelling in the anterior cervical spine. Butterman57 reported a 50% incidence of neck swelling presenting as dysphagia in a series of anterior cervical fusions with allograft and BMP, compared with 14% in the iliac crest bone graft cohort. In a study citing the efficacy of rhBMP-2 in anterior cervical arthrodesis with use of a PEEK spacer, Tumialan and colleagues58 reported a 7% (14 patients) incidence of clinically significant dysphagia. The authors suggest lowering the dose of rhBMP-2 and placing it only within the PEEK spacer to limit symptomatic dysphagia.58

The reported complications of “off-label” use of rhBMP-2 in spinal surgery raise many questions including optimal dosing/concentration regimen and appropriate delivery mechanism. Judicious use of recombinant human bone morphogenetic proteins is prudent in spinal surgery, especially for “off-label” applications. Patients should be informed of specific risks associated with its use.

Instrumentation Complications and Standard of Care

Instrumentation complications will invariably occur for each and every spine surgeon. We are treating a wider variety and technically more challenging group of patients today than we have in each of the preceding decades over the past half century. Systems are more complex and powerful in their ability to move the spine. Patients are older, and challenges that a surgeon may not have addressed in the past are entering the surgical realm. Societal expectations from health care providers are also increasing with the advent of new technology. Complications and high patient expectations can result in significant tension between the surgeon and patient and their family. Many physician behaviors can help address this including preoperative teaching that includes a discussion of potential complications and avoiding overconfident behavior with the patient before surgery. When complications occur, the physician must be willing to confront them and the patient directly and address the issues with an eye toward proper patient care and re-education of the patient and their family. Terms such as “complication,” “problem,” and “error” become interchanged with “mistake,” “negligent,” and “malpractice.” In today’s medico-legal environment, these terms may lead to issues regarding standard of care. The definition of standard of care varies: We would offer that the standard of care implies how a reasonably qualified physician responds to a set of problems or challenges. Therefore biologic- and biomechanical-related complications, as well as errors in thought process and application of spinal instrumentation, represent objective problems that need to be addressed in the course of caring for a patient and his or her associated problem. Fortunately, the vast majority of complications do not represent standard of care violations, especially if they are addressed in a timely and proper manner.

Pearls

1 Smoking cessation before spinal fusion procedures can improve arthrodesis rates.
2 Before placement of a C1-2 transarticular screw, it is imperative to delineate the course of the vertebral artery in order to ensure safe passage of the screw.
3 A true lateral position is essential for correct placement of thoracolumbar plates.
4 Pedicle screws afford excellent fixation options and can enhance correction and fusion rates in spinal deformity.
5 RhBMP-2 enhances fusion rates and can obviate the need for autograft harvest.

Pitfalls

1 Smoking significantly increased pseudarthrosis risk.
2 A vertebral artery with medial course through the body of C2 may preclude safe passage of a C1-2 transarticular screw.
3 Without a true lateral view/appropriate orientation, application of a thoracolumbar plate can lead to inadvertent screw placement into the spinal canal or near the great vessels.
4 A number of structures can be potentially compromised by misplaced pedicle screws including visceral, vascular, and neurologic structures.
5 RhBMP-2 can be associated with significant adverse swelling events when used off-label in the anterior cervical spine. Additionally, reports of radiculitis and ectopic bone formation have also been associated with rhBMP-2 use in posterior lumbar spine surgery.

Key Points

1 Instrumentation complications have a variety of etiologies including biologic failure, biomechanical failure, error in thought process, and error in application.
2 Biologic failure. Infection rates increase with the use of instrumentation. Smoking contributes to marked increases in pseudarthrosis rates. Other patient factors including osteoporosis, steroid use, prior radiation therapy, diabetes, and others can increase chances of biologic failure through a variety of mechanisms.
3 Biomechanical failure. In arthrodesis procedures, without the presence of a bony fusion, instrumentation is ultimately likely to fail. Examples of this include rod, screw, or plate breakage. In addition, poor bone quality may lead to limited purchase, failure at the bone-screw interface, or pull-out/toggle and weakening of the construct.
4 Error in thought process and error in application. Careful preoperative planning is required in order to safely and successfully place spinal instrumentation. Much of this information is gleaned from preoperative radiographic studies including plain radiographs, CT, and MRI. When placing implants, it is critical to be aware of surrounding anatomy and have adequate exposure for reference.
5 The use of bone morphogenetic protein has increased in the past few years. It increases fusion rates but can be associated with a variety of complications including radiculitis, ectopic bone formation, and significant swelling.

Key References

1 Arens S, Schlegel U, Printzen G, et al. Influence of materials for fixation implants on local infection. An experimental study of steel versus titanium DCP in rabbits. J Bone Joint Surg Br. 1996;78:647-651.

In this animal study, the authors tested the biocompatibilities of two different metals in an animal tibia model. The authors report a significantly higher infection rate for steel plates (75%) versus titanium plates (35%).

2 Andersen T, Christiensen FB, Laursen M, et al. Smoking as a predictor of negative outcome in lumbar spinal fusion. Spine. 2001;26:2623-2628.

At 2 years’ follow-up a statistically significant increased risk of nonunion in those patients who smoked more than 10 cigarettes a day was demonstrated. The authors concluded that cigarette smoking has a deleterious effect on fusion rates and overall patient satisfaction.

3 Curylo LJ, Mason HC, Bohlman HH, Yoo JU. Tortuous course of the vertebral artery and anterior cervical decompression: a cadaveric and clinical case study. Spine. 2000;25:2860-2864.

This anatomic study offers an excellent description of both normal and aberrant vertebral artery anatomy and reports clinically useful landmarks relative to the vertebral arteries for spine surgeons.

4 Kim YJ, Lenke LG, Bridwell KH, et al. Freehand pedicle screw placement in the thoracic spine: is it safe? Spine. 2004;29:333-342.

More than 3200 thoracic pedicle screws inserted via freehand technique with use of anatomic landmarks were reviewed, and 577 were randomly evaluated with CT. Overall there was a 6.2% cortical breach, with 1.7% of the screws having a medial breach of the pedicle wall. There were no cases of neurologic, vascular, or visceral complications in the 3204 screws evaluated.

5 Smucker JD, Rhee JM, Singh K, et al. Increased swelling complications associated with off-label usage of rhBMP-2 in the anterior cervical spine. Spine. 2006;31(24):2813-2819.

The authors reviewed the use of rhBMP-2 in anterior cervical spine surgery and demonstrated a 27.5% incidence in clinically significant swelling events versus 3.5% in the non-rhBMP-2 group. Significant swelling events included delay in discharge, severe dysphagia, reintubation, and visible neck swelling.

References

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3 Arens S, Schlegel U, Printzen G, et al. Influence of materials for fixation implants on local infection. An experimental study of steel versus titanium DCP in rabbits. J Bone Joint Surg Br. 1996;78:647-651.

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