Adding sacral anchors through an S1 alar screw and multirod construct as a strategy for lumbosacral junction augmentation: an in vitro comparison to S1 pedicle screws alone with sacroiliac fixation

Aixing PanDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Honghao YangDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Yong HaiDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Yuzeng LiuDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Xinuo ZhangDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Hongtao DingDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Yue LiDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Hongyi LuDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Zihao DingDepartment of Orthopedic Surgery, Beijing Chao-Yang Hospital, Capital Medical University;

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Yangyang XuBeijing Key Laboratory for Design and Evaluation Technology of Advanced Implantable & Interventional Medical Devices, Beijing Advanced Innovation Center for Biomedical Engineering, School of Biological Science and Medical Engineering, Beihang University, Beijing, China

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Baoqing PeiBeijing Key Laboratory for Design and Evaluation Technology of Advanced Implantable & Interventional Medical Devices, Beijing Advanced Innovation Center for Biomedical Engineering, School of Biological Science and Medical Engineering, Beihang University, Beijing, China

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OBJECTIVE

Achieving solid fusion of the lumbosacral junction continues to be a challenge in long-segment instrumentation to the sacrum. The purpose of this study was to test the condition of adding sacral anchors through an S1 alar screw (S1AS) and multirod construct relative to using S1 pedicle screws (S1PSs) alone with sacroiliac fixation in lumbosacral junction augmentation.

METHODS

Seven fresh-frozen human lumbar-pelvic spine cadaveric specimens were tested under nondestructive moments (7.5 Nm). The ranges of motion (ROMs) in extension, flexion, left and right lateral bending (LB), and axial rotation (AR) of instrumented segments (L3–S1); the lumbosacral region (L5–S1); and the adjacent segment (L2–3) were measured, and the axial construct stiffness (ACS) was recorded. The testing conditions were 1) intact; 2) bilateral pedicle screw (BPS) fixation at L3–S1 (S1PS alone); 3) BPS and unilateral S2 alar iliac screw (U-S2AIS) fixation; 4) BPS and unilateral S1AS (U-S1AS) fixation; 5) BPS and bilateral S2AIS (B-S2AIS) fixation; and 6) BPS and bilateral S1AS (B-S1AS) fixation. Accessory rods were used in testing conditions 3–6.

RESULTS

In all directions, the ROMs of L5–S1 and L3–S1 were significantly reduced in B-S1AS and B-S2AIS conditions, compared with intact and S1PS alone. There was no significant difference in reduction of the ROMs of L5–S1 between B-S1ASs and B-S2AISs. Greater decreased ROMs of L3–S1 in extension and AR were detected with B-S2AISs than with B-S1ASs. Both B-S1ASs and B-S2AISs significantly increased the ACS compared with S1PSs alone. The ACS of B-S2AISs was significantly greater than that of B-S1ASs, but with greater increased ROMs of L2–3 in extension.

CONCLUSIONS

Adding sacral anchors through S1ASs and a multirod construct was as effective as sacropelvic fixation in lumbosacral junction augmentation. The ACS was less than the sacropelvic fixation but with lower ROMs of the adjacent segment. The biomechanical effects of using S1ASs in the control of long-instrumented segments were moderate (better than S1PSs alone but worse than sacropelvic fixation). This strategy is appropriate for patients requiring advanced lumbosacral fixation, and the risk of sacroiliac joint violation can be avoided.

ABBREVIATIONS

ACS = axial construct stiffness; AR = axial rotation; BMD = bone mineral density; BPS = bilateral pedicle screw; B-S1AS = bilateral S1AS; B-S2AIS = bilateral S2AIS; IS = iliac screw; LB = lateral bending; ROM = range of motion; S1AS = S1 alar screw; S1PS = S1 pedicle screw; S2AIS = S2 alar IS; SIJ = sacroiliac joint; U-S1AS = unilateral S1AS; U-S2AIS = unilateral S2AIS.
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Illustration from Beck et al. (pp 147–152). © Department of Neurosurgery, Freiburg Medical Center; published with permission.

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