Posterior intra-articular fixation stabilizes both primary and secondary sacroiliac joints: a cadaveric study and comparison to lateral trans-articular fixation literature

The goal of our study was to compare the effects of unilateral and bilateral fixations on sacroiliac joint mobility between a posterior and a lateral approach during single-leg stance in a cadaveric multidirectional bending flexibility model [10]. Our results indicate that posterior approach has a similar performance in stabilizing the SI joint during flexion–extension motions, and superior performance in stabilizing the SI joint during lateral bending and axial rotation motions, compared to published data using the lateral approach. We analyzed the intact motions of the joint between our samples and those of the previous research and found no significant differences in either primary, secondary, or pooled joint samples. Our intact results are in line with those of previous investigations which report the range from 4.5 ± 3.3 degrees to 2.3 ± 1.4 degrees for flexion–extension loading, 1.5 ± 1.5 degrees to 1.1 ± 0.8 degrees during left/right lateral bending, and 2.9 ± 2.1 degrees to 1.7 ± 0.8 degrees during left/right axial rotation [8, 9, 11,12,13].

The current and previous studies were performed using the same physiological model, preserving the pubic symphysis, to maintain an intact pelvic ring [10]. The posterior and lateral techniques produced similar motion reductions in flexion–extension, after unilateral and bilateral treatments. However, in axial rotation and lateral bending, the posterior approach generated 4.5 and 4.9 times more mean percent motion reductions upon unilateral treatment and subsequently 0.9 and 0.6 more mean percent motion reductions upon bilateral treatment.

Unilateral fixation using the posterior approach reduced the primary and secondary joints’ mobility in flexion–extension, lateral bending, and axial rotation. Lindsey et al. [10] using the lateral approach reduced the primary and secondary joints’ mobility in only flexion–extension by 46% and 22%, respectively. The previous study reported that the mobility of the pubic symphysis appeared to be the reason that unilateral joint fixation did not reduce motion of the contralateral joint significantly [10]. Our results are somewhat similar, as while reductions in the mobility of the joints were significant in flexion–extension, lateral bending, and axial rotation when pooled together, reductions in mobility of the ipsi and contralateral joint alone in flexion–extension were not significant upon unilateral fixation. This stabilizing effect may be due to the compressive effect which the force generated by the distraction interference of the ipsilateral joint (in the posterior approach) may have on the contralateral joint as described in Fig. 10. This underscores the importance of intra-articular placement of the stabilizing implant, as placement in the recess of the joint at the level of the PSIS as described for the DIANA method may eliminate this stabilizing effect, as the width of the recess makes contralateral stabilization difficult [30]. At the same time, ipsilateral fixation has been shown to be difficult when the implant is placed close to the axis of rotation [31]. While this remains a pitfall of the DIANA technique, the LinQ posterior technique is not placed in the same location as the DIANA method. DIANA places the implant into the joint’s recess, in the region of the interosseous ligament, and enters at the level of the PSIS, which is where the axis of the joint motion is located. In contrast, the allograft implant is placed much lower, i.e., below the PSIS, and is thus farther away from the joint’s axis of rotation.

Fig. 10figure 10

Illustration of pelvis showing contralateral joint stabilization by means on ipsilateral distraction interference

Bilateral fixation using the posterior approach maintained the reduction in the primary and secondary joints’ mobility in flexion–extension, lateral bending, and axial rotation. Lindsey et al. [10] using the lateral approach reported a 45% and 75% decrease in the primary and secondary joints’ mobility, respectively, from the intact joints’ motion in only flexion–extension, upon bilateral fixation. It is also reported that bilateral fixation maintains the reduced mobility of the primary joint in flexion–extension [10]. Our results are also similar, as bilateral fixation maintained the reduced joint mobility introduced by unilateral fixation in flexion–extension, lateral bending, and axial rotation. However, while we observed significant changes upon bilateral fixation during lateral bending and axial rotation in the secondary joint, we did not observe significant changes in the mobility of the secondary joint in flexion–extension.

The standard pure moment multidirectional bending flexibility model has been consistently and reliably utilized by many investigators for evaluating spinal fusion techniques [32, 33]. However, while it is representative of in vivo motions, such as rise-to-stand, rotation, and bending, it does not accurately represent complexity of typical combined in vivo loading. To mitigate the influence of bone deformation on the range of motion results, optical markers were placed as close as possible to the tested sacroiliac joint [34]. Although the statistical power of our analyses was high in our pooled joint analyses (78–83%), they were low to moderate in our independent primary and secondary joint analyses (42–70%). While low sample sizes are common with many cadaver-based investigations, at the time of this study, specimen availability was severely impacted due to the prevalence of SARS-CoV-2 infection in cadavers [35]. While the amount of motion reduction required to promote fusion is not known, the degrees of motion reduction (1°–2°) obtained are similar between the techniques evaluated in this study. The results were not compared with respect to variations in implant placement. While it would be important to highlight the effectiveness of the implant in its ideal placement, this study aimed to not control the exact location of the implant any more than the guidance provided in the surgical technique. This was to ensure that results were clinically representative of the population utilizing this posterior technique, and thus, an average quantification of performance across these variations is a more realistic evaluation of actual clinical biomechanical performance. It is also important to note as shown in Fig. 5 that both the right and left implants followed identical trajectories. The differences noted are thus due to the asymmetry between the left and right joints, which may not be a significant determinant of biomechanical performance in a bilateral construct, compared to the placement trajectory, and insertion position. As in the instance of left–right asymmetry shown in Fig. 5, the biomechanical performance was identical, at 64%, 65%, and 53% motion reduction in the left joint, and 72%, 68%, and 57% motion reduction in the right joint, during flexion–extension, lateral bending, and axial rotation, respectively. This further underscores the importance of clinicians transitioning from placement within the recess (DIANA) to placement below the recess (LinQ) while taking the gradual learning curve into consideration. Although this study describes the initial stability of this approach, the current model cannot simulate biological changes over time, such as time to fusion, subsidence, or creep. However, investigators of this posterior approach have recently reported efficacies ranging from 66.5 to 76.5% [36, 37]. Investigators have also reported in a multicenter case series, in which the posterior intra-articular technique (LinQ) was utilized as a salvage therapy for the lateral trans-articular technique (iFuse) patients who did not show pain improvement and fusion after 20 ± 8 months postoperation [38]. The investigators reported 77 ± 11% reduction in pain scores upon salvage therapy using the posterior intra-articular technique (LinQ) in all patients after 10 ± 6 months postoperation with evidence of bony bridging.

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