Mechanical analysis of percutaneous sacroplasty using CT image based finite element models

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Abstract

Sacral insufficiency fractures are an under-diagnosed source of acute lower back pain. A polymethylmethacrylate (PMMA) cement injection procedure called sacroplasty has recently been utilized as a treatment for sacral insufficiency fractures. It is believed that injection of cement reduces fracture micromotion, thus relieving pain. In this study, finite element models were used to examine the mechanical effects of sacroplasty.

Finite element models were constructed from CT images of two cadavers on which sacroplasties were performed. The images were used to create the mesh geometry, and to apply non-homogeneous material properties to the models. Models were created representing the case with and without cement, thus simulating the pre- and post-sacroplasty situation.

The results indicate that the sacrum has a 3D multi-axial state of strain. While compressive strains were the largest, tensile and shear strains were significant as well. Cement in the sacrum reduced strains 40–60% locally around the cement. However, overall model stiffness only increased 1–4%. This indicates that the effects of sacroplasty are primarily local.

Introduction

Sacral insufficiency fracture refers to a stress fracture in a sacrum with diminished strength and elastic modulus due to reduced mineralization of the bone. It was first described as a specific clinical condition by Lourie [1]. The vast majority of cases occur in women, comprising 93% of the cases in the literature reviewed by Weber et al. [2]. Sacral insufficiency fractures usually present clinically as pain in the pelvis, lower back, or buttocks, sometimes with pain extending to the lower limbs [2], [3]. Due to the non-specific nature of the symptoms, sacral insufficiency fractures are frequently misdiagnosed [3]. Conventional radiography often does not show evidence of sacral insufficiency fractures, adding to the possibility of misdiagnosis [2], [4].

Conventional treatment for sacral insufficiency fractures consists of bed rest and analgesics, followed by a return to normal activities over a period of months [3], [5]. Babayev et al. [4] recommends early mobilization with pain control, citing the adverse effects of extended immobility, especially when applied to the elderly. Recently, a cement injection procedure, termed a sacroplasty, has been utilized in the treatment of sacral insufficiency fractures [6], [7]. Such a procedure was first utilized to provide pain relief in patients with bone metastases [8], [9]. Sacroplasty consists of the injection of polymethylmethacrylate (PMMA) cement into the sacrum, where it serves as artificial support in the weakened bone.

Garant [6] reported a single case of sacral insufficiency fracture in a 63 year old woman and Pommersheim et al. [7] reported three cases, involving women aged 71, 74, and 76 years. All four patients suffered from severe lower back pain that affected their daily function. Two were bedridden, one used a wheelchair, and one used a walker. Following sacroplasty, symptoms improved immediately in all four cases. Followups in three patients reported them to be pain free and ambulatory. Thus, in the short term sacroplasty appears to be successful in treating sacral insufficiency fractures, with pain relief and improved function in daily activities [7].

Sacroplasty is a variation of vertebroplasty, a common treatment for vertebral compression fractures. The concepts of the sacroplasty and the vertebroplasty are essentially the same: percutaneous injection of cement into the core cancellous bone for crack stabilization and strength augmentation. However, the sacroplasty presents greater procedural difficulty because the skeletal geometry at the level of the sacrum makes imaging and proper needle placement more difficult [6], [7], [8].

Vertebroplasty is an attractive treatment option for vertebral compression fractures. It is successful in relieving pain, provides rapid results (pain relief within 1 day) and has low complication rates [10], [11]. The pain relief is probably due to mechanical stabilization which eliminates painful micromotion at the fracture site [10], [12].

Sacroplasty is a relatively new procedure and has not been studied as extensively as vertebroplasty. The biomechanics of vertebroplasty have been the subject of numerous experimental studies and several finite element studies, as reviewed by Wilcox [13]. Because of the similarities between the two procedures, vertebroplasty research can provide some general insight into sacroplasty. However, it should be noted that the differences between sacrum and vertebrae are not negligible. For example, the loads undergone by sacrum and vertebrae are not directly comparable.

A review of the literature shows few finite element models of the sacrum. Dawson et al. [14] modeled the entire pelvis to examine loading during lateral impact fractures. Garcia et al. [15] modeled the entire pelvis, including sacrum, to examine the stability of pelvic ring fracture fixation methods. No finite element models have examined the effect of sacroplasty.

Vertebroplasty has been the subject of a variety of finite element models [16], [17], [18], [19], [20], [21], [22] which have examined diverse aspects of the vertebroplasty procedure. Liebschner et al. [16] examined the effects of cement volume and placement by inserting ‘cement’ cylinders into a model of a vertebral body. Tack et al. [17] also examined the effect of cement volume, along with the importance of patient bone mineral density (BMD) in determining necessary injection volumes. Polikeit et al. [18] and Baroud et al. [19] examined the effect of vertebroplasty on load transfer between levels with similar two-vertebra models. Keller et al. [22] and Kosmopoulos and Keller [21] have examined vertebroplasty using two-dimensional finite element models that model cancellous bone at the trabecular level.

CT images are commonly used in the creation of finite element models of bone. They provide accurate information on bone geometry, can be interpreted to provide mechanical properties, and allow the modeling of bones in vivo [23]. All the models of vertebroplasty above used CT images to define bone geometry.

By using the distribution of mineral density provided by the CT image, an estimate of the bone inhomogeneity can be made in FE models. This is essential as cancellous bone differs from point to point within the same bone. The material properties of cancellous bone vary greatly and depend on factors such as anatomic location and bone density [24].

The goal of this study was to evaluate the mechanical effects of sacroplasty using finite element modeling. Specifically we sought to estimate: (1) the overall stiffening effect of sacroplasty on the sacrum, (2) the changes made to the mechanical environment of the local cemented area, and (3) assuming that sacroplasty lowers strains in the cemented area, if it raises strains elsewhere enough to cause damage in other locations. Finite element models of the upper pelvis were created based on CT images of two cadavers on which sacroplasty had been performed. The CT images were used both to define the mesh geometry and to apply non-homogeneous material properties to the meshes. The material properties applied to the mesh were modified to create models representing the sacrum with and without cement present. Comparisons between the models were primarily based on the principal strains. This was done both on a whole model scale and locally. Local strain comparisons were performed in regions of interest defined in the locations of cement in the sacrum.

Section snippets

Materials and methods

Researchers at Wake Forest University provided CT Images of sacroplasties from an earlier study [25]. Sacroplasty procedures were performed on two cadavers. As the specimens were anatomic cadavers, the ages and medical histories of the specimens are unknown. However, a radiologist saw no evidence of osteoporosis upon viewing the scans. About 3–7 ml of cement, mixed with barium sulphate for opacity, was injected on each side of the sacrum, as shown in Fig. 1. After the sacroplasty, CT scanning

Results

The deflections at the sacral promontory were less in both cadavers with the inclusion of cement compared to without. However, the reduction was small, ranging from 1.79 to 3.90% less as shown in Table 1. Thus, the cement had a small effect on the overall model stiffness.

Contrary to the effect on the overall model, the inclusion of cement had a significant impact in the local region of the cement. The strain magnitudes in the regions of interest were reduced by the inclusion of cement. Fig. 9

Discussion

Both compressive and tensile strains were significant in the regions of interest. The minimum principal (compressive) strains in the regions of interest were generally of larger magnitude than the maximum principal (tensile) strains. However, the magnitude of the tensile strains was significant, generally about 50–90% of the magnitude of the compressive strain. Because significant tensile and compressive principal strains were present, shear strains were also significant at the regions of

Acknowledgement

We thank Dr. Pearse Morris, Wake Forest University Baptist Medical Center, for providing the CT images used in this study.

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