**2. Evidence behind interspinous fusion/fixation devices**

Ex-vivo biomechanical studies have demonstrated that IFDs provide comparable rigidity to PSF in flexion-extension [9]. The data are less clear in lateral bending and axial rotation. Techy et al. in 2013 specifically studied the Aspen interspinous device in comparison to pedicle screw fixation and found that the stability provided by the device was statistically equivalent to both bilateral or unilateral pedicle screw/ rod construct in flexion-extension; however, lateral bending and axial rotation tests showed pedicle screw fixation to have significantly greater stability [9]. In contrast, an earlier biomechanical study by Karahalios et al. in 2010 showed no difference in stability provided by IFDs compared to PSF in flexion-extension, lateral bending or axial rotation [10]. Papp et al. showed IFDs preserve adjacent facet joint anatomy [11]; other studies have even suggested IFDs may reduce load on intervertebral discs and potentially reduce the risk of adjacent segment disease [12, 13]. Yu et al. in 2014 studied their own novel IFD and found that interspinous process fixation combined with posterior lumbar interbody fusion (PLIF) was equivalent in biomechanical stability to bilateral pedicle screw/rod fixation with PLIF [14]. In short, it seems that cadaveric studies have shown IFDs to fare pretty well in restricting motion through flexion-extension comparable to the current gold-standard pedicle screw fixation but are likely unable to stabilize a motion segment against shearing forces.

Tomii et al. studied the S-plate (Kisco DIR, Osaka, Japan) in a series of 15 patients who underwent PLIF and subsequent IFD placement and found no complications and increase in mean JOA scores from 12.1 to 21.9 with a study follow-up period of 1.5–4 years [15]. Kim et al. showed decreased operative time for IFD placement and PLIF versus PS fixation and PLIF (135.8 minutes versus 170.8 minutes) and lower blood loss. The same study also showed decreased visual analog scale (VAS) scores in the immediate post-operative period of IFD and PLIF compared with PS and PLIF (4.6 vs. 7.0) [13]. However, VAS scores at 1 year follow-up showed no significant differences between the two groups. The Korean Oswestry Disability Index (ODI) scores also showed no significant differences between the two techniques [13].

To assess the value of IFDs in fusion rates, Vokshoor et al. [16] analyzed a sub-cohort of 50 patients who underwent IFD with PLIF or TLIF and showed 94% of them showed interspinous process fusion and 86% of those levels showed solid interbody fusion based on Burkus criteria [17]. Kim et al. [13] also studied fusion rates in their paper by either looking at 6-month post-operative flexion-extension films and/or assessing for trabecular bone on the 6 month post-operative CT scan; they found that IFD with PLIF showed a 92.5% fusion rate, which was similar to 91.6% fusion rates for PLIF with PS fixation. The same paper also reported adjacent segment disease in 12.5% of patients who underwent PLIF with IFD versus 36% in PLIF with PS fixation.

Lastly, Panchal et al. [8] in 2016 reported results from the first randomized, prospective, controlled, multi-center trial comparing outcomes from patients receiving anterior (ALIF) or lateral (LLIF) interbody fusion with adjunctive interspinous fusion with the Aspen® MIS device or pedicle screw fixation. Patients were followed pre-operatively and post-operatively at 6 weeks, 3 months, 6 months, 12 months, and even 24 months. The primary study endpoint was the comparison

### *The Aspen MIS Spinous Process Fusion System DOI: http://dx.doi.org/10.5772/intechopen.95941*

of the Oswestry Disability Index (ODI) score from the pre-operative time period to that of the 12-month post-operative time period. The primary hypothesis of the trial was noninferiority of the ODI score change by the Aspen® MIS IFD group (investigation) compared to the pedicle screw fixation group (control).

103 subjects underwent single-level interbody fusion via ALIF or LLIF approach. Sixty-six of them underwent adjunctive interspinous fusion with Aspen MIS spinous process fixation device. Thirty-seven of them were supplemented with pedicle screw fixation. All patients had degenerative disc disease and/or Grade 1 or 2 spondylolisthesis. The trial demonstrated no significant differences between the two groups with respect to patient-reported outcome scores (ODI, SF-36, or VAS) at 1.5, 3, 6, or 12-month time points. Interbody fusion was assessed at 12 months by evaluating computed tomography (CT) scans and scoring them according to the Brantigan, Stelfee, Fraser (BSF) criteria [18]; the authors found no significant difference in the BSF scores, even after adjusting for potential confounders such as anterolateral plating and/or interbody technique. Furthermore, 92% of the patients who had the Aspen® MIS device placed showed bone formation between the device plates bridging the spinous processes [8]. Operative times (47.6 minutes vs. 70.2 minutes), fluoroscopy times (12.2 seconds vs. 58.4 seconds), and blood loss (57.5 cc versus 103.7 cc) were also significantly less between the groups. Notably, no device breakage or dislodgement occurred in the study; however, 6 patients (3.1%) did have spinous process fractures and 3 patients (1.5%) needed to be reoperated due to new or worsening postoperative back and/or leg pain that may have been related to IFD placement.

In short, Panchal et al. was the first randomized multi-center trial to report that interspinous rigid fixation used as a supplement to anterior or lateral interbody fusion techniques is comparable to adjunctive pedicle screw fixation in terms of fusion rates and patient-reported outcomes and has a better intra-operative risk profile.
