Advancements in osteobiologics have improved fusion rates but pseudoarthrosis is still often times reported as the most common postoperative complication [4]. Rates vary from 0 to 45 % and are dependent on radiographic factors, patient factors, and surgical approaches [3, 7, 19, 22]. Molinari et al. reported that 16 % of their patients required repeat interventions for failure of fusion. The most common risk factors for pseudoarthrosis is a high-grade spondylolisthesis especially with significant kyphosis and when treated with a posterior fusion and instrumentation alone [23, 24] (Fig. 26.1). In order to evaluate for pseudoarthrosis, we look at three things: first is increased, second, is a defect in the fusion mass on XR or loss of reduction, and third is loss of fixation-implant breakage. If a robust fusion has occurred, radiographs may be sufficient for determining a diagnosis. Specifically, flexion and extension views will show instability at the vertebral segments in question and orthogonal views may assist in showing a gap in the bone formation in presence of nonunion. If in doubt, a CT scan may be helpful in assessing the fusion mass, particularly if utilizing metal subtraction techniques. Once the location of the pseudoarthrosis has been identified, an attempt should be made at determining the cause. If there is difficulty definitively diagnosing a pseudoarthrosis, the gold standard for identification is direct visualization in the operating room.

The treatment of a symptomatic pseudoarthrosis typically involves further surgical intervention. The explicit plan is detailed according to the diagnosis. For example, in the setting of an infection, irrigation and debridement is required prior to reattempting a fusion. The basic principle is to improve the rigidity of the construct and provide as much assistance to the biologic environment as is technically feasible. For those patients not infected, revision surgery is challenging but must include three basic principles: First, obtain adequate alignment to improve the degree of kyphosis present; second, obtain a circumferential fusion including anterior fusion through a formal anterior approach or via a transforaminal lumbar interbody fusion (TLIF) or posterolateral interbody fusion (PLIF); and finally, obtain stable, rigid fixation (Fig. 26.2).

Use of biologics to improve fusion and decrease pseudoarthrosis has its own complications. Rodgers et al. reported a case report where bone morphogenetic protein-2 (rhBMP2) was packed into an interbody cage at L4–5 level [25]. The patient developed a nonunion, which required revision surgery. It was during this second surgery that the left iliac vein was injured. The patient had an intraoperative cardiac arrest. The inflammatory reaction seen with the use of rhBMP-2 was believed to have caused the scarring and fibrosis surrounding the vessel making it more fragile during dissection. The patient subsequently healed without reported long-term affects.

Complications in this category vary from catastrophic neurologic compromise to resolving paresthesias from intraoperative positioning. According to the SRS database, neurologic deficits occur in 1.3 % of postoperative spondylolisthesis patients which is higher than that for all spine procedures, (1.1 %) but lower than that for scoliosis surgery (1.4 %) [5, 26]. The incidence of neurologic deficit is, in part, dependent on the procedure performed and should be a consideration when evaluating and determining treatment for these patients [27]. These numbers vary depending on the sampled patients and are found to be increased in patients with neuromuscular disorders [28]. Cahill et al. reported on 43 patients of which 5 (12 %) sustained a neurologic deficit. Four of the five patients had deficit at their final follow-up.

When specifically analyzing high-grade spondylolisthesis, there is an even higher rate of neurologic deficit identified in 15–30 % of patients [7, 19, 29]. Recovery most often occurs, with one study demonstrating 10 of 12 patients with complete recovery 18 months postoperatively [8]. Two had incomplete deficit of L5 with three out of five muscle strength in extensor hallucis longus.

Patients with high-grade spondylolisthesis who require a reduction have a complication rate that is nearly two times those who do not require a reduction [3]. While the burden of disease is an important underlying factor, the positioning of the patient on the table is the first time point when an iatrogenic complication may occur. All bony prominences should be appropriately padded. The table pads should be evaluated on a regular basis for cracking or thinning. Patients are typically positioned prone with their lower extremities in maximum extension to allow for ease with the reduction (Fig. 26.3). This extension may place an increased pressure at the site of the anterior superior iliac spines and, by anatomical association, the lateral femoral cutaneous nerve. While resulting postoperative paresthesias are usually transient, parents and patients should be warned about this risk.

There are many theoretical reasons for the increased risk that occurs with a reduction of one vertebral body on another. Sailhan et al. separated the theories into four possibilities [30]. The first is direct pressure of the spinal cord that occurs during decompression and the second is direct pressure on the nerve roots. After or during the reduction tension on the extradural nerves is another factor, while an extruded disc that with extension of the vertebral column, leads to volume reduction of the spinal canal is also a potential factor. Fu et al. reported that 10 % of patients who underwent a reduction ultimately suffered a neurologic deficit in contrast to only 2 % of patients who did not undergo a reduction [3].

Delayed neurologic deficits can also occur. Cahill et al. reported 33 % (4/12) of patients with a neurologic complication occurring several days after surgery during mobilization when early extension of the hips and knees may result in nerve stretch. Sailhan suggested a postoperative protocol to decrease possibility of nerve stretch by flexing hips and knees using pillows in the immediate postoperative period and then gradually extending the hips under careful physical examination [30]. Over the ensuing days, patient’s lower extremities should be placed in more extension unless radicular pain or weakness occurred.

One unique deficit that is specific (but not exclusive) to spondylolisthesis surgery is an L5 nerve root stretch presenting as a foot drop or ankle dorsiflexion weakness with a rate between 5 and 35 % Although not completely understood, two different mechanisms may be in play, the first is the inability to visualize the L5 nerve roots which are often deep and difficult to identify. Chen et al. reported on three out of 118 patients who had L5 nerve root injury [16]. Two were due to poor visualization. They concluded that complete exposure and hemostasis were critical factors in avoiding this complication. The second mechanism is excessive tension on the nerve root which may be due to either inadequate decompression and/or excessive reduction of the L5 vertebral body [31] (Fig. 26.4). Shufflebarger et al. reported on 25 patients undergoing surgery for spondylolisthesis at an average age of 13.5 years. Eleven of the 25 patients were found to have postoperative motor deficits, ten of whom had an L5 stretch injury [18]. All of these were found to have completely resolved within 3 months.