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AJNR - American Journal of Neuroradiology

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Investigating Sacroplasty: Technical Considerations and Finite Element Analysis of Polymethylmethacrylate Infusion into Cadaveric Sacrum

C.T. Whitlow^a, S.K. Yazdani^b, M.L. Reedy^a, S.E. Kaminsky^a, J.L. Berry^b and P.P. Morris^a

^a Division of Radiologic Sciences, Department of Radiology, Wake Forest University School of Medicine, Winston-Salem, NC
^b Department of Biomedical Engineering, Wake Forest University School of Medicine, Winston-Salem, NC

View larger version (58K): [in this window] [in a new window]   Fig 1. Intact (A) and fractured (B) FEMs of a hemisacrum are shown, which were constructed by using densitometric CT data from a single imaged cadaveric pelvis. In the models (A, B), a system of individual elements interconnected by a meshwork of discrete nodes has been assigned elasticity parameters approximating that of bone. Boundary and loading conditions applied to the intact sacral fracture and cement-augmented sacral fracture models are demonstrated (A). Hatch marks (A) represent boundary conditions that were applied to simulate anchoring of the sacrum at the sacroiliac joint. Yellow vector lines (A) indicate loading conditions, which were defined as a 35-kg force, approximating one half of body weight above the first sacral vertebra. Arrowheads (A) demonstrate the sagittal plane along which movement could occur in a 1-legged-stance paradigm. An asterisk and solid arrow (B) indicate sacral fracture origin and the point of fracture propagation, respectively. Color-coded transformation of FEA data demonstrates the amount of maximal principal stress experienced by the hemisacrum after the application of a 35-kg load, both before (C) and after (D) fusion at a point along the fracture (open arrow) designed to simulate sacroplasty. Each color represents kilopascals of maximal principal stress according to the calibration scale provided (E), with red corresponding to the lowest and white corresponding to the highest levels of maximal principal stress. Note that the point of fracture fusion (open arrow, D) appears to subsume a portion of the stress generated by the 35-kg load, thereby attenuating maximal principal stress that surrounds the site of fracture propagation, as compared with the prefusion model (C).

View larger version (140K): [in this window] [in a new window]   Fig 2. Fluoroscopic (A, C) and corresponding volume-rendered 3D reconstructions of CT images (B, D) of a hemisacrum from a single cadaveric pelvis, demonstrating anatomic landmarks for needle placement (arrow in image C indicates needle) and PMMA injection before (A, B) and after (C, D) sacroplasty. The sacrum is shown in a left posterior oblique orientation (A–D), with the beam manipulated parallel to the L5-S1 disk space1 and ipsilateral sacroiliac joint.3 Fluoroscopy (C) and CT (D) demonstrate PMMA within the superior-lateral sacral ala in an area bounded by the following: first, a line connecting the lateral edge of the posterior foraminal openings,2 and second, a line superimposed on the medial edge of the sacroiliac joint.3

View larger version (89K): [in this window] [in a new window]   Fig 3. 3D reconstructed CT images (A–C) of a hemisacrum from a single cadaveric pelvis are shown in a left posterior oblique orientation. Anatomic landmarks used for injection overlie the sacrum and correspond to the following: first, a line superimposed on the medial edge of the sacroiliac joint,1 and second, a line connecting the lateral edge of the posterior foraminal openings.2 A dotted line demonstrates the outline of the most superior sacral foramen. Note that PMMA spreads beyond the foraminal landmark2 and is appreciated within the sacral foramen (white arrow) and intravascular space (black arrow, B). Additional reformatting of the CT images (C) reveals the isolation of PMMA attenuation from sacral bone and more clearly shows PMMA tracking beyond the posterior foraminal landmark2 into the sacral foramen (solid arrow) and intravascular space (open arrow).

View larger version (9K): [in this window] [in a new window]   Fig 4. The effect of simulated sacroplasty on fracture gap micromotion in an FEM of the fractured hemisacrum is shown. In this simulation, single nodes on each side of the sacral fracture are selected near the point of fracture propagation and internodal distance measured after the application of a 35-kg load pre- and postsacroplasty. Fracture-gap micromotion is decreased by 48% after simulated sacroplasty, as compared with the nonfused fractured sacrum.