Biomechanical evaluation of conventional internal contemporary spinal fixation techniques used for stabilization of complete sacroiliac joint separation: a 3-dimensional unilaterally isolated experimental stiffness study.

STUDY
DESIGN: Comparative 3-dimensional biomechanical testing.
OBJECTIVE: To compare 5 fixation techniques, 3 using screws or screw and plates and 2 spinal, used for stabilization of complete unilateral sacroiliac dislocation in composite models. SUMMARY OF BACKGROUND DATA: Harrington compression rods have been used for posterior iliosacral stabilization. Recently, the use of compact spinal instrumentation has been introduced for stabilization of iliosacral joint separation to achieve immediate and permanent stability, allowing early mobilization. To the authors' knowledge, no comparative mechanical studies between commonly used internal fixation techniques and contemporary spinal instrumentation have been performed.
METHODS: Fifteen identical composite models of the left hemipelvis and sacrum were used to simulate consistently the "worst-case scenario" of complete unilateral sacroiliac dislocation. Subgroups of 3 models each were used to apply 5 (A-E) alternative fixation iliosacral joint fixation techniques: 1 multiaxial 7.5 mm Cotrel-Dubousset screw inserted in the posterior superior iliac spine and connected with a long Cotrel-Dubousset horizontal rod with 6.5 mm multiaxial Cotrel-Dubousset screws inserted bilaterally in the S1 pedicles (technique A); 1 multiaxial 7.5 mm Cotrel-Dubousset titanium pedicle screw inserted in the posterior superior iliac spine and connected with a short horizontal Cotrel-Dubousset-rod to a 6.5 mm multiaxial Cotrel-Dubousset-screw inserted to the ipsilateral S1 pedicle (technique B); 1, 6.5 mm cancellous AO-screw (technique C); 2, 6.5 mm cancellous AO screws (technique D); and 2 dynamic stainless steel compression plates (technique E) placed anteriorly. Constructs were biomechanically tested. The ilium was unilaterally rigidly fixed, the sacrum was put horizontal in the mediolateral direction with a forward tilt of 30 degrees (close to physiologic conditions) in the sagittal plane, and a vertical quasi-static compressive load ranging from 0 to 500 N was applied on the endplate of S1, reproducing a "worst case" loading scenario. Construct stiffness, frontal plus sagittal kinematics, and iliosacral joint gap size for all 5 techniques were measured.
RESULTS: The construct stiffness (N/mm +/- standard deviation) ranged for model: A, 121 +/- 18; B, 78 +/- 10; C, 168 +/- 13; D, 193 +/- 42; and E, 145 +/- 4. All other parameters exhibited minor variations between the different techniques of fixation: at the 400 N load level, the maximum iliosacral gap globally ranged 0.9-2.8 mm, the maximum mediolateral sacral tilt ranged 1.3-2.4 degrees, and the maximum anteroposterior sacral tilt ranged 0.6-3.0 degrees.
CONCLUSIONS: The iliosacral fixation with 2 6.5 mm AO-cancellous screws for complete sacroiliac dislocation demonstrated the highest stiffness and the short spinal instrumentation the poorest stiffness. All other fixation techniques could be generally considered of equivalent stability value.