The Bedrock technique with triangular titanium rods for spinopelvic fixation and sacroiliac joint fusion: is there a learning curve?

purposeThe Bedrock technique involves fusion of the sacroiliac (SI) joint at the time of long construct spinal fusion extending to the pelvis. This is done with triangular titanium rods (TTR) placed parallel and cephalad to S2AI screws, requiring placement of two implants into the small bony corridor in the teardrop of the pelvis. In our initial 21 cases adopting this technique, we found a 7.1% rate of implant malposition requiring intraoperative repositioning. The objective of this follow-up study was to report on our implant malposition rates over a larger number of cases, to determine a potential learning curve, and to report on strategies for minimizing implant malpositioning.

methodsSurgeries in which SI joint fusion was performed concomitant with spinopelvic fixation using the Bedrock Technique with TTRs and CT-navigation between 5/1/2019 and 5/27/2021 were reviewed. Operative reports were analyzed and any intraoperative TTR/S2AI screw malpositions were recorded. Demographic/surgical information, TTR/S2AI specifications, and PROMs were obtained and analyzed along with risk assessment and correlation of failure between cohorts (α < 0.05).

resultsA total of 43 patients were included with a mean length of fusion of 4.8 (1-14 levels). Overall, 86 TTRs and S2AI screws were implanted of which, 4/86 (4.7%) TTRs were repositioned intraoperatively and 1/86 (1.2%) was removed and replaced with allograft, while 0/86 (0%) S2AI screws were malpositioned. All TTR malpositions were medial and/or cephalad. In the initial 21 cases, 3/42 (7.1%) TTRs required intraoperative repositioning; this decreased in the subsequent 22 cases to 2/44 (4.5%) with a non-significant relative risk of 1.57 (p = .664). Malposition was non-significantly negatively correlated to time (r = -.135; p = .39). Our patients showed statistically significant (p < .001) one-year improvements in mean ODI, VAS back, and PROMIS-10 metrics, meeting MCID for ODI and VAS back.

conclusionsPosterior placement of a TTR above an S2AI screw poses technical challenges due to the natural cephalad narrowing of the bony corridor in the teardrop and blunt tip of the implant which predisposes it to medial skiving as it is passes through the SI joint. Our overall intraoperative reposition rate for caudal S2AI screws was 0% compared to 5.8% for cephalad TTRs, which decreased from 7.1 to 4.5% between our first- and second-year cohorts, suggesting a potential learning curve. All malpositions occurred medially and/or cephalad, and over time we transitioned our technique to placing implants as low as possible in the teardrop to minimize this risk.