Unilateral vs. Bilateral Iliac Screw Fixation: Outcomes and Complications

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The clinical outcome of unilateral iliac screw fixation in lumbosacral fusions is uncertain. The clinical and radiographic outcomes of 33 consecutive patients undergoing unilateral iliac screw fixation as part of a lumbosacral fusion were compared with 34 consecutive patients undergoing bilateral iliac screw fixation. Despite having more radiographic signs of iliac screw loosening, patients receiving unilateral iliac screw fixation had similar clinical outcomes with a lower rate of iliac screw removal.


Iliac screw fixation improves construct strength and stability but extends operative times and may require a separate procedure for removal due to irritation. Biomechanical studies have suggested that unilateral iliac screw placement may be as effective as bilateral placement, but no study to date has fully explored the clinical and radiographic outcomes between these two methods.


67 consecutive adults at one center underwent unilateral (n=33) or bilateral (n=34) iliac screw fixation as part of their lumbosacral fusion. Diagnoses were spondylolisthesis (n=8), degenerative scoliosis (n=23), idiopathic scoliosis (n=12), kyphoscoliosis (n=20), and other degenerative disorders (n=4). Clinical outcomes included the visual-analog pain scale (VAS) and the Oswestry Disability Index (ODI) at pre-op, 1 year, 2 year, and latest follow-up.


Unilateral and bilateral groups had similar clinical scores for both VAS (pre-op: 5.0 vs 5.7, p=0.282; 1yr: 3.3 vs 3.4, p=0.989; 2yr: 3.2 vs 4.0, p=0.388) and ODI (pre-op: 25.1 vs 25.0, p=0.932; 1yr: 14.8 vs 17.2, p=0.352; 2yr: 18.5 vs 16.4, p=0.726). All radiographic parameters, including alignment and balance, were similar between the groups except for iliac screw halo signs, which occurred only in unilateral fixation (4[12.1%] vs 0[0%], p=0.05). Complications between unilateral and bilateral groups were similar and included non-union (1[3.0%] vs 1[2.9%]), symptomatic screw (4[12.1%] vs 6[17.6%]), infection (5[15.2%] vs 2[5.9%]), screw fracture (0[0%] vs 2[5.9%]), rod fracture (0[0%] vs 3[8.8%]), spinal imbalance (3[9.1%] vs 1[2.9%]), and screw removal (2[6.1%] vs 4[11.8%]).


Unilateral fixation produces good results in fusions down to the sacrum. Despite having more radiographic signs of screw loosening, the overall rate of union, coronal and sagittal balance, and functional outcomes were not significantly different with unilateral fixation compared with bilateral fixation. Unilateral screws required removal with approximately half the frequency of bilateral fixation.


Unilateral iliac screw fixation produces excellent clinical results that are comparable to bilateral iliac screw fixation.