The earliest evidence for indirect decompression came from the development of interspinous spacers. Interspinous spacers were developed to treat lumbar spinal stenosis. Distraction of the intervertebral segment stretches the ligamentum flavum and increases the central canal. Also, distraction of the disc space increases the diameter of the neuroforamen. This effectively relieves pressure on the traversing and exiting nerve roots to relieve patients of their neurogenic claudication and radiculopathy symptoms. In 2004, Zuckerman et al. published the results of their prospective study on the X-STOP [24]. They randomized 191 patients in a prospective study comparing the X-STOP versus nonoperative care. At 6 months, the success rates were 52 and 9 %, respectively, and at 1 year, 59 and 12 %. The authors determined that the X-STOP was a significant improvement over nonoperative therapies at 1 year with a success rate comparable to published reports for decompressive laminectomy, but with considerably lower morbidity. In another study by Siddiqui et al., MRI studies were performed on patients before and after surgery [25]. Significant increase in the dimensions of the neural foramen and canal area were demonstrated after surgery. The problem with the interspinous spacers is they would erode through the spinous processes over time, and patients would have recurrent symptoms as the distraction was lost [26, 27]. Hence, interspinous spacers demonstrated the ability to decompress the spinal canal via indirect decompression, but the concern was that the effect may not be long lasting.

With lateral interbody fusion, indirect decompression would be achieved by placement of the interbody spacer, and the benefit would be maintained via the pedicle screws and ultimately the fusion. Currently, there is growing evidence that indirect decompression is a reasonable option in patients undergoing lateral interbody fusion. In a study by Oliveira et al., the authors performed MRI studies on patients before and after surgery [23]. Substantial dimensional improvement was evidenced in all radiographic parameters, with increases of 41.9 % in average disc height, 13.5 % in foraminal height, 24.7 % in foraminal area, and 33.1 % in central canal diameter. Two patients (9.5 %) required a second procedure for additional posterior decompression and/or instrumentation. So, the authors concluded that the LIF procedure provided enough decompression of central and/or lateral stenosis in a minimally disruptive fashion. This then avoids the need for the direct resection of posterior elements and the associated morbidity. The authors did note that indirect decompression may be limited in cases of congenital stenosis and/or locked facets. Its effect may also be reduced by postoperative subsidence and/or loss of correction.

In another study by Elowitz et al., the authors performed a similar study on 25 consecutive patients [22]. Fifteen patients had grade I spondylolisthesis. VAS for back pain intensity improved from 7.74 to 2.07. VAS for leg pain intensity improved from 7.24 to 1.87. Radiographic evaluation in 20 treated levels (15 patients) found an increase in dural sac dimension of 54 % in the anterior-posterior plane and 48 % in the medial-lateral plane. The calculated area of the dural sac increased an average of 143 %. In contrast to the interspinous spacers, these results also seem to be maintained because a fusion is concurrently performed. In a study by Castellvi et al., the authors performed MRIs on 158 consecutive patients preoperatively and 1 year postoperatively [19]. Increases in disc height (67 %, p < 0.001), foraminal area (24–31 %, p < 0.001), and canal area (7 %, p = 0.011) measured immediately postoperatively were sustained at 1-year follow-up. VAS pain score and ODI both improved (p < 0.001) at 3 months and were maintained at 1 year.

In one more paper by Kepler et al., the authors evaluated patients preoperatively and postoperatively clinically and with CT scans [21]. The authors noted that average foraminal area increased approximately 35 % after cage placement without variation based on cage position. This was correlated with statistically significant improvements in ODI scores. Overall, ODI scores improved from an average of 32.8 ± 9.8 (range 16–44) preoperatively to 19.8 ± 9.8 (range 2–37) postoperatively. This meets the criteria for minimum clinically significant differences. The amount of indirect decompression was enough to result in a meaningful clinical improvement in terms of ODI scores. Hence, the amount of indirect decompression was not only statistically significant; it also led to increased functional capabilities in these patients.

However, the benefits of indirect decompression may be lost if the implant subsides through the endplates. In a study by Nemani et al., the authors performed a retrospective study on 117 patients who had stand-alone LIF [28]. A total of 10.3 % of patients who underwent stand-alone lateral lumbar interbody fusion ultimately required revision surgery. The most common reason for surgery was for persistent radiculopathy and symptomatic implant subsidence. Average time to revision was 10.8 months. A similar study was also performed by Marchi et al. [29]. At 12 months, 70 % in the standard group and 89 % in the wide group had grade 0 or I subsidence, and 30 % in the standard group and 11 % in wide group had grade II or III subsidence. Subsidence was detected early (6 weeks), at which point it was correlated with transient clinical worsening, although progression of subsidence was not observed after the 6-week time point. Moreover, subsidence occurred predominantly (68 %) in the inferior endplate. Fusion rate was not affected by cage dimension or by incidence of subsidence. Hence, subsidence is a concern with stand-alone LIF, and the addition of pedicle screws to provide additional stability is a good consideration.