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Osteoporosis-associated fragility fractures of the pelvic ring (FFP) are fundamentally different from pelvic fractures of young patients and their surgical management presents a challenge. Little is known about the biomechanical stability of various osteosynthesis procedures for fragility fractures and care strategies are mostly based on retrospective studies. The aim of the present biomechanical study is to evaluate the stability of various osteosynthesis options in osteoporosis-associated fractures of the anterior and posterior pelvic ring under cyclic loading, measured by the stiffness, plastic deformation and maximum load capacity until the onset of osteosynthesis failure.
Eighteen embalmed osteoporotic cadaver pelvis specimens (median age 84.41 years (SD: 6.29), eight female and ten male) were prepared and randomized based on the t-score (-4.66, SD: 1.57) and stiffness. Then they were biomechanically tested per experimental group under axial cyclic loading simulating a one-leg stand. The study is divided into three sub-steps in which various fracture models and osteosynthesis methods were tested on a total of seven experimental groups. Part 1 included experimental group 1 (n=18, native pelvis), group 2 (n=18, pelvis with isolated sacrum fracture in the left ventral massa lateralis, FFP type IIa fracture according to Rommens), group 3 (n=9, pelvis with sacrum fracture and cement-augmented sacroiliac screw) and group 4 (n=9, pelvis with sacrum fracture and ipsilateral anterior pelvic fracture). For part 2, the anterior pelvic ring of one half 18 pelvises from experimental group 5 (pelvis with sacrum fracture and cement-augmented sacroiliac screw + ipsilateral anterior pelvic fracture of the superior and inferior of the pubic bone, FFP type IIB fracture according to Rommens) was stabilized with external fixator (group 6, n=9) and the other half with internal fixator (group 7, n=9). All test groups of part 1 and 2 were loaded cyclically 20 times with a load increase of 10 to 100 Newtons. Finally, in Part 3, the failure test of experimental Group 6 (External Fixator) and 7 (Internal Fixator), beginning at 100 N, increased cyclic loading by 50 N every 10 cycles until osteosynthesis failure occurred.
Part 1: The stiffness of the native pelvis significantly decreased (p = 0.05) from 35.05 N/mm (SD: 16.84) to 27.95 N/mm (SD: 7.36) after the addition of the isolated sacrum fracture. By cement-augmented sacroiliac screw osteosynthesis, stiffness could be increased significantly (p=0.01) to 35.38 N/mm (SD: 10.95). Stiffness decreased again to 32.46 N/mm (SD: 11.02), but not significantly (p=0.09) due to generation of the ipsilateral anterior pelvic ring fracture.
Part 2: Mean stiffness and plastic deformation both were significantly better in the internal fixation (group 7) than in the external fixation (group 6) [mean stiffness: 43.69 N/mm (SD: 18.39) against 26,52 N/mm (SD: 9.76), p=0.029); mean plastic deformation: 0.37 mm (SD: 0.23) vs. 0.71 mm (SD: 0.26), (p = 0.011)].
Part 3: In the failure test there was no significant difference between internal and external fixator [average load: 506.25 N (SD: 129.39) vs 461.11 N (SD: 147.43), p = 0.515), however, a trend in favor of the internal fixator could be observed.
Cement augmented iliosacral screw osteosynthesis has been proven in an isolated unilateral sacrum fracture and significantly increases stability. In addition, stabilization with the iliosacral screw seems to be sufficient in osteoporoseassociated FFP type IIb fractures with combined anterior and posterior involvement.
The assessment of stability, as measured by stiffness and plastic deformation, in pelvic osteoporotic FFP-type IIB fractures shows significantly better results for the internal fixator (a submuscular minimally invasive novel screw-and-rod system) than for the external fixator (both in combination with the sacroiliac screw osteosynthesis) and offers an interesting alternative with advantages in clinical use (higher patient comfort, no pin infections and suitable in obese patients).
However, both osteosynthesis methods are without significant difference concerning maximum load capacity, presumably because, due to the less flexible osteoporotic bone quality, the higher stiffness of the internal fixator caused completion of the sacral fracture through the posterior cortex in some specimens.
For the treatment of fragility fractures of the pelvic ring, further biomechanical and prospective studies are needed.