In addition to excellent results in the majority of patients undergoing direct pars interarticularis repair, definitive posterior fusion can achieve similar positive results. In those patients failing or not candidates for direct repair, a single level posterior spinal fusion has been advocated and has been described both with and without the use of postoperative immobilization in either brace or cast [19, 22–24]. Retrospective analyses of patients treated with uninstrumented posterolateral fusion for grade I and grade II disease demonstrated fusion rates greater than 83 % and relief of symptoms in more than 75 % [22, 25].

To compare results of posterolateral fusion to direct repair, long-term follow-up in 23 patients treated by Scott’s repair was contrasted with 25 patients treated by posterolateral segmental fusion without instrumentation. At mean follow-up time of 54 months, 87 % of the Scott’s group and 96 % of the fusion group had occasional pain which did not interfere with daily activities or no pain at all. There was no statistical difference in the subjective, clinical, or functional outcome between the two operation groups [26]. At long-term follow-up of 15 years, the previously improved ODI slowly decreased for the direct repair group compared to the segmental fusion which was statistically significant, but only moderately apparent in terms of difference in clinical exam [15]. As such, patients with direct repair may not do as well as those with single level fusion long term, possibly due to the fact that direct repair does not protect the disc of the lytic/olisthetic segment from further degeneration. In comparing posterior vs. posterolateral fusion at 20-year long-term follow-up, nonunion was present in 34 % of patients after posterior fusion and in 13 % after posterolateral fusion, with 14 % of patients reporting back pain often or very often at rest [27].

Comparison of posterior lumbar interbody fusion versus posterolateral fusion with instrumentation in the treatment of low grade isthmic spondylolisthesis showed good or excellent results in 22 (88 %) cases in the PLIF group and 19 (76 %) cases in the PLF group. Fusion rates were 100 % in the PLIF group and 84 % in the PLF group with no difference in the complication rates for each group at 3.3 year mean follow-up [28]. Overall, satisfactory long-term results and patient satisfaction can be anticipated with direct repair as well as posterior/posterolateral fusion techniques for low grade spondylolisthesis.

Surgical management of spondylolisthesis should be reserved for the subset of patients who have failed conservative management who have persistent pain or a limitation to their physical activities. Again using the original treatment guidelines set by Wiltse, skeletally immature patients with slip greater than 50 % are recommended for surgical fusion regardless of the presence or absence of symptoms. For higher grade spondylolisthesis (III and greater), the algorithm of treatment becomes more unclear and has numerous possibilities in terms of fixation type, levels of fusion, and reduction vs. in situ fusion. These cases often present the greatest challenges to the pediatric spine surgeon and are associated with increased risk of complication and morbidity [29, 30]. Here, we will compare results of these interventions including rates of pseudarthrosis, outcomes, and neurologic risk.

Obtaining a thorough history and documentation of physical exam findings, particularly those of nerve root compression (e.g., radiculopathy, weakness, and sensory deficit). In these cases, decompression of the nerve root alone or with fusion (instrumented vs. non-instrumented) is advised. Historically, decompression was performed without concurrent fusion and often resulted in slip progression and pseudarthrosis. This procedure, first described by Gill in 1955 involved removal of the loose posterior element showed good outcomes in early results, but increased slip in 14 % of patients, in long-term follow-up [31]. However, this was difficult to repeat as subsequent studies have shown poorer long-term results with or without slip progression [32, 33]. As such, current recommendations advocate for the use of spinal fusion with instrumentation as an adjunct to any decompression of the posterior elements. In doing so, the surgeon weighs the risk of increased instability against wider decompression of the neural elements. By utilizing instrumentation with fusion of the affected vertebral levels, both stability and adequate release of the involved nerve roots can be obtained. The fusion itself can be performed with or without reduction of the slip with anterior or posterior procedures including posterolateral or interbody fusion.

Masoudi et al. compared outcomes of posterolateral fusion against lumbar interbody fusion using instrumentation using Oswestry low back pain disability scale and the visual analog scale and showed that posterolateral fusion had better clinical results and decreased back pain, but a lower rate of fusion compared to PLIF [34].

In situ fusion of the spine is advocated by many as it serves to prevent further slip and decrease overall pain, but to stabilize the spine for decompression. In situ fusion has shown good results with or without postoperative cast immobilization [35–38]. Long-term follow-up for patients with high grade (<60 %) slip undergoing in situ uninstrumented fusion via anterior, posterior, or circumferential technique was analyzed by Poussa et al. and revealed slightly improved clinical outcomes measured by the ODI in circumferential fusion group, with and associated decrease in lumbar range of motion. Despite the high grade slip and radiologic evidence of nerve root compression, clinical findings of nerve weakness were not usually present [15, 30, 39–41]. Likewise, other groups have advocated for circumferential fusion with addition of a fibular allograft or autograft strut through the S1–L5 vertebral bodies or trans-sacral hollow modular anchorage screws to provide the best opportunity to limit symptomatic pseudarthrosis while obtaining good functional outcomes and limiting slip progression [29, 42–45]. However, even in situ fusion without reduction carries a risk of development of acute cauda equina. In these instances, immediate decompression that includes resection of the posterosuperior rim of the dome of the sacrum and the adjacent intervertebral disc is recommended [46, 47].

For those patients with unsatisfactory results including cauda equina or symptomatic pseudarthrosis, the majority who undergo reoperation achieve good results [37]. Overall, patient satisfaction of greater than 80 % can be expected for those with high grade spondylolisthesis undergoing in situ fusion [48] (Fig. 24.2).

Reduction of high grade spondylolisthesis offers several advantages compared to in situ fusion at the risk of increased surgical time, higher technical demand, and risk of L5 nerve radiculopathy (most common complication) [49]. Several techniques for reduction of high grade slip have been described, and are reserved primarily for patients with severe sagittal imbalance, high mobility of the slip on lateral flexion–extension radiographs, and preoperative radiculopathy requiring wide decompression.

Numerous reduction techniques have been described including single and multiple stage procedures with use of traction, anterior, posterior, combined fusion, as well as interbody fusion. Bradford and Boachie-Adjei described a two-stage procedure with posterior decompression and posterolateral fusion followed by temporary halo-skeletal traction. The second stage then utilized ALIF followed by cast immobilization, resulting in correction of sagittal alignment, solid fusion, and only one incident of persistent back pain with radiculopathy [50]. Similarly good outcomes were obtained using Harrington distraction rods to obtain reduction with concurrent posterolateral fusion followed by anterior lumbosacral fusion using two bicortical wedge-shaped iliac grafts, or pedicular fixation using posterolateral fusion combined with PLIF [42, 51].

A long-term comparison of patients who had undergone in situ fusion vs. in situ with reduction and postoperative cast immobilization showed a decreased rate of progression of slip and lumbosacral kyphosis, but overall similar clinical and functional outcomes. The authors advocated that reduction should only be performed if it results in an improvement in the rate of fusion, functional outcome, or cosmesis [52]. Partial reduction of high grade spondylolisthesis combined with wide decompression of the posterior elements has also been advocated, combined either with circumferential staged anterior and posterolateral fusion, or single stage combined posterolateral and posterior interbody fusion [51, 53–57].^. In a retrospective study, Helenius et al. compared patients treated using posterolateral fusion, anterior fusion, or circumferential fusion without instrumentation for high grade isthmic spondylolisthesis. They concluded that uninstrumented circumferential fusion had improved correction of kyphosis, ODI, and SRS scores [58]. Partial, rather than complete reduction of the sagittal deformity when combined with wide decompression and stable fusion technique through combined anterior/posterior approaches or single posterior approach with interbody fusion, appears to provide stable, safe results with decreased rates of postoperative nerve root compression and cauda equina [57].