Six-month outcomes from a randomized controlled trial of minimally invasive SI joint fusion with triangular titanium implants vs conservative management

In our trial of patients with chronic SIJ pain, improvements in LBP, disability scores, physical function and quality of life were superior in subjects receiving minimally invasive SIJF using triangular titanium implants compared to CM. Differences in these outcomes occurred soon after treatment initiation and were statistically significant between the two groups at all postoperative time points.

Our findings both replicate and extend previous studies. In previously published case series [36, 37, 38, 39], systematic reviews [40, 41], a prospective multicenter clinical trial [42], and a recently published randomized clinical trial of similar design conducted in the USA [43], similar improvements in SIJ pain, self-rated limitations in activities due to pain (Oswestry Disability Index) and quality of life were observed in participants undergoing SIJF. Our study provides an additional, independent confirmation that the improvements after surgery are clinically important and statistically superior to those seen with continued conservative care. Similar to previous studies, preplanned subgroup analyses revealed no predictors of poor responses after surgical fusion.

Our randomized trial differed in design and intervention compared to a US randomized trial; in the US study, non-surgical management included intraarticular SIJ steroid injections and radiofrequency (RF) ablation. Instead, our trial included only PT and adequate information and reassurance, consistent with European guidelines for pelvic girdle pain [29]. We note that although SIJ steroid injections and RF ablation are not commonly delivered in Europe, many trial participants had already undergone such treatments.

Subjects in our cohort had marked reduction in baseline quality of life compared to the general population, with EQ-5D scores substantially lower than population controls. At 3 and 6 months, minimally invasive SIJF resulted in improved EQ-5D scores (postoperative means of 0.69 and 0.74) that were similar to postoperative values observed for other low back pain surgical procedures [44]. In contrast, EQ-5D score improvements in the CM group were minimal. The improvements in quality of life seen in our study mirror those seen in a prior randomized trial [43]. Our study, combined with the prior randomized trial and 4- and 5-year outcomes from both European [39] and US [45] cohorts, suggest that minimally invasive SIJ fusion can be added to the portfolio of spine surgeries proven safe and effective that European surgeons can offer their patients.

Minimal clinically important differences (MCID) are often used to assess the clinical significance of study findings as they may be distinct from statistical differences. Although they were developed for degenerative spinal conditions other than SIJ and the effects of SIJF, the MCID for improvement in chronic back pain is approximately 20 % when measured by VAS [46] and that for ODI is approximately 13–15 points [47]. For EQ-5D, the MCID is less well defined, with changes of 0.15–0.46 reported [44, 48]. Observed mean values in our study exceeded these MCID values for pain, ODI and EQ-5D, and response rates were markedly higher in the SIJF group compared to CM. Improvements in EQ-5D TTO and VAS in our study were similar to those seen in the Swedish spine registry for other spine surgeries [34].

Our results extend findings from prior studies in important ways. First, we included two functional assessments [walking distance and physical functioning (ASLR)], both of which have not been previously reported. Both measures showed improvement in the SIJF group but not in the CM group. Second, CM was provided per European treatment guidelines, meaning that the control group intervention may have been more standardized than prior trials. Our findings extend prior trials and serve to validate the procedure overall.

Surgical revision is an important clinical outcome. To date, only one trial subject has undergone revision surgery after SIJF; in this case, the implant was placed too close to the sacral nerve root, a known risk, and this subject’s new radicular pain improved on repositioning the implant. The risk of early revision of this implant is approximately 1 % and the risk of revision at 4 years is approximately 3.6 % [49], a revision rate that is low compared to standard open surgical procedures in the spine [50, 51]. No unanticipated adverse events occurred.

Our study has several limitations. Because the intervention was not blinded, we cannot rule out the possibility that knowledge of the treatment assignment might have influenced patient responses to questions, which could have contributed to the greater improvements seen in the SIJF group. However, other potential biases—e.g., the fact that 11 of the 18 patients in the SIJF group diagnosed with bilateral pain received only unilateral SIJF, might have decreased the improvements in the SIJF group due to under treatment. Moreover, blinding is not done in standard clinical settings, so our results may be more generalizable to what can be expected in standard practice compared to a blinded trial.

While we included patients for whom the SIJ had been identified as the primary cause of LBP, we cannot exclude that patients with other contributory sources of LBP, e.g., facet arthropathy or degenerative disc disease, have been enrolled. In these patients the LBP due to the SIJ pathology might have been reduced; however, the other causes of the LBP might not have been addressed adequately. It is reasonable to suppose that both arms of the study are equally affected by this potential problem.

Second, although CM was patterned after European guidelines for pelvic girdle pain, which recommend treatment individualized to patient needs, non-surgical care provided to patients in our trial may have varied across centers. Physical therapy may be helpful in post-partum pelvic girdle pain [45], most of which is likely to emanate from the SIJ, but the target population in our study differs from the cited study. Finally, our report includes 6-month data only; the study continues to 24 months of planned follow-up. However, one-year data from other prospective trials of the same device/patient population [42, 43], as well as longer-term data from retrospective cohorts [39, 45] suggest sustained effectiveness.