Does Convergent Placement of Interlocking Iliosacral Screws into the Body of Vertebra S1 Prevent Screw Migration?

PURPOSE OF THE STUDY To compare the outcomes of parallel and convergent iliosacral screw insertion into the body of vertebra S1 in the treatment of posterior pelvic arch injuries. MATERIAL AND METHODS Radiographs of 120 patients (43 women, 77 men), aged between 14-79 years, treated with iliosacral screw fixation for posterior pelvic ring fractures between 1.1.2009 and 31.12.2019 were reviewed for inclusion in the study. In each case two screws were inserted into the body of vertebra S1. The screws were inserted in either parallel or convergent orientation. Convergent orientation allows the threads of both screws to be interconnected. In this technique, the first screw is inserted into the centre of the body of vertebra S1 as a compression screw. The second screw is inserted as a positioning screw and is placed so that the threads of both screws lock together. We believe that the interlocking of the threads of both screws and contact of the second screw with three cortices (two of the iliac bone and one of the sacrum) increase the stability of the fixation. Migration of loosened screws was measured on radiographs of the pelvis obtained at six weeks and at three, six and twelve months postoperatively. Migration of five millimetres or more within the first six weeks was considered to be clinically significant. Only patients after primary fracture treatment and with a complete one-year follow-up were included in the study. Cases of non-union and failure of osteosynthesis of the anterior pelvic arch and patients with incomplete follow-up were excluded. The incidence of significant screw migration between the two techniques was compared using Fisher's exact test with a 5% level of significance. RESULTS Sixty-three patients (23 women, 40 men) aged 17 to 79 years were included in the study. Parallel screws were used in 24 patients (8 women, 16 men) and convergent screws were used in 39 patients (15 women, 24 men). Clinically significant migration occurred in nine (38%) patients after parallel insertions. In two of these cases there was unstable fixation of the anterior pelvic arch. Migration of convergently placed iliosacral screws occurred in four (10%) cases. In three of these cases this was due to unstable fixation of the anterior pelvic arch. The difference in screw migration between the two groups was shown to be significant (p = 0.0219). DISCUSSION Iliosacral screws ensure sufficient stability of the posterior arch in type B and C pelvic fractures provided that the anterior pelvic arch is stable. Convergent insertion of iliosacral screws may increase the stability of fixation. Minimally invasive surgery with sufficient stability may be advantageous for early treatment of patients after multiple trauma and in elderly patients. The weaknesses of this study are its relatively small number of patients, which prevented reliable statistical analysis of screw migration according to the type of pelvic fractures. The second main limitation is the failure to perform densitometric examination of the skeleton in patients with X-ray proven screw migration for confirmation of osteoporosis as one of the possible causes of fixation failure. CONCLUSIONS The results of the study suggest that convergent insertion of iliosacral screws into S1 is associated with a lower risk of screw migration and subsequent failure of fixation of the posterior pelvic arch. Key words: pelvic fracture, pelvic injury, iliosacral screw, parallel screws, convergent screws, migration of iliosacral screws.