- Research Article
- Open Access
Toward the Development of Virtual Surgical Tools to Aid Orthopaedic FE Analyses
© Srinivas C. Tadepalli et al. 2010
- Received: 18 May 2009
- Accepted: 28 October 2009
- Published: 14 December 2009
Computational models of joint anatomy and function provide a means for biomechanists, physicians, and physical therapists to understand the effects of repetitive motion, acute injury, and degenerative diseases. Finite element models, for example, may be used to predict the outcome of a surgical intervention or to improve the design of prosthetic implants. Countless models have been developed over the years to address a myriad of orthopaedic procedures. Unfortunately, few studies have incorporated patient-specific models. Historically, baseline anatomic models have been used due to the demands associated with model development. Moreover, surgical simulations impose additional modeling challenges. Current meshing practices do not readily accommodate the inclusion of implants. Our goal is to develop a suite of tools (virtual instruments and guides) which enable surgical procedures to be readily simulated and to facilitate the development of all-hexahedral finite element mesh definitions.
- Cervical Spondylotic Myelopathy
- Hexahedral Mesh
- Bony Surface
- Cervical Laminoplasty
- Surface Definition
Orthopaedic surgeons use both surgical and nonsurgical techniques to treat musculoskeletal trauma, sports injuries, degenerative diseases, infections, tumors, and congenital conditions. Orthopaedic surgical operations are associated with the rearrangement of both hard and soft tissues, oftentimes leading to dramatic changes in structural geometry. The primary objective of a surgical correction is typically the maintenance or restoration of function. Due to the complexity of the anatomy under consideration and the biomechanical behavior of the tissues, the impact of a surgical procedure may not always be predicted on the basis of the surgeon's intuition and experience alone. Computational modeling using individual tomographic data can provide valuable information for surgeons during the planning stage. Specifically, the finite element method provides a means to predict surgical outcome based on factors such as bony cuts (osteotomy), bone fragment/segment repositioning, the addition of instrumentation, and the host tissue response. Countless models have been developed over the years addressing procedures ranging from fusions to total joint replacements [1–14]. Unfortunately, few studies have incorporated patient-specific models. Historically, baseline anatomic models have been used due to the time devoted solely to model development. Moreover, surgical simulations impose an additional level of complexity and accompanying set of challenges. Current meshing practices do not readily accommodate the inclusion of implants. The challenges that accompany traditional modeling techniques are magnified when an implant is to be introduced in the model. Consequently, the time devoted to mesh development increases considerably, and hence such models may often prove impractical.
In pursuit of making patient-specific modeling a reality, we have made advancements in automating the patient-specific bony geometry definitions from CT and/or MR image datasets [15–17] and toward easing the development of corresponding patient-specific finite element (FE) mesh definitions via a custom-written software package, IA-FEMesh . Our goal is to further advance these efforts by developing tools to simulate a variety of surgical procedures, thereby interactively incorporating implants into such models. One day, such models may aid the surgeon in preoperative planning or in the engineering of implant design, ultimately resulting in an improved clinical outcome. Toward that end, our goal is to develop a suite of tools which enable the user to readily simulate a surgical procedure and mesh the resulting structure with an all-hexahedral mesh. Historically, commercial preprocessors were developed for traditional engineering applications where the structures of interest can readily be broken down into geometric primitives, thus making hexahedral mesh development feasible. To capture the geometric complexity of anatomic structures often necessitates the use of a tetrahedral mesh. Hexahedral elements, however, are often preferred for their superior numerical performance as compared to tetrahedral elements [19, 20]. A mathematical argument in favor of the hexahedral element is that the volume defined by one element must be represented by at least five tetrahedral elements, which in turn yields a system matrix that is computationally more expensive. In contrast to the favorable numerical quality of hexahedral meshes, mesh generation is a difficult task.
Herein, we present a general framework for computer-assisted planning of orthopaedic interventions based on finite element modeling via the reconstruction of patient's anatomy from 3D image datasets. To date we have developed a prototype program and an easy to use workflow that interacts with IA-FEMesh, allowing the user to perform a series of surgical manipulations on a bony surface. This tool supports the same datatypes utilized by IA-FEMesh enabling the resulting surfaces to be imported into IA-FEMesh for mesh generation. Herein we demonstrate these surgical capabilities by simulating and meshing a cervical laminoplasty procedure.
To enable the development of patient-/subject-specific models, the generation of an anatomic model begins with a collection of CT and MR images. CT images facilitate the delineation of the bony anatomy while also providing patient-specific material properties, while MR images allow soft tissues such as cartilage, ligaments and tendons, as well as muscles to be defined. The process of delineating the anatomic structures can be performed via a variety of techniques including manual, semiautomated, and fully automated techniques. The ability to define geometrically accurate representations of bony structures has previously been studied by DeVries et al. . While defining the phalanx bones of the hand, good agreement was found between manual raters (Jaccard metric = 0.91) and physical laser scans of the same specimens (surface distance = 0.20 mm). To facilitate the creation of the anatomic models both semi-automated techniques such as the expectation-maximization algorithms  as well as artificial neural networks (ANNs)  have been explored. In this paper, the BRAINS2 software was used to manually segment the regions of interest [22–25]. This software offers a variety of tools to facilitate the delineation of anatomical structures including thresholding, region growing, and clipping. These functions were used in conjunction with the manual editing tools to delineate the bones of the hand, wrist, and spine as shown in the following examples. For the models described in this paper, cadaveric specimens were imaged on a Siemens Sensation 64 slice computed tomography (CT) scanner [matrix = pixels, field of view (FOV) = 172 mm, kilovolts peak (kVp) = 120, current = 94 mA, exposure = 105 mA]. The in-plane resolution for the hand and wrist was 0.34 mm with a slice thickness of 0.40 mm, while the spine was imaged with an in-plane resolution of 0.5 mm and 0.6 mm slice thickness. Once the regions of interest were manually delineated, a surface was generated from the binary segmentation, smoothed via Laplacian smoothing, and exported in STL format from BRAINS2.
2.1. Cutting a Bone via a Planar Cut
An osteotomy, for example, is a surgical operation whereby a bone is cut to shorten, lengthen, or change its alignment. We have developed tools to cut a bone, thereby yielding two distinct bone segments. Moreover, tools have also been introduced to cut away the bony surface in preparation for implant insertion.
2.1.1. Performing an Osteotomy
2.1.2. Removing Bone/Bony Surface
To cut a bone in preparation for an implant, planar cuts are often made with the aid of a guide. Consequently, a 3D plane widget available in VTK has been used. The widget is represented by a plane with four corner vertices and a normal vector. Similar to the box widget, the plane can be moved interactively and positioned precisely with respect to the host bone. Thereafter, the desired bony surface is retained and the open face patched.
2.2. Surface Boolean Operations
2.3. Meshing the Resulting Surface Definition
While the description above outlines the features of the surgical simulation software, this section describes a clinical application used to test the feasibility of using these tools to evaluate patient-specific surgical procedures.
For example, the procedure of choice for decompression of the cervical spine depends on a variety of factors including the source and location of the compression, the number of vertebral segments involved, cervical alignment, and surgeon experience . Consider, for example, cervical laminoplasty. Laminoplasty was originally developed in Japan  to avoid the delayed sequelae of laminectomy without fusion. This procedure initially gained popularity as a treatment for ossification of the posterior longitudinal ligament, but is increasingly being used to treat cases of cervical spondylotic myelopathy. Nevertheless, controversy persists as to whether or not cervical laminoplasty should become the treatment of choice for multilevel cervical stenosis with myelopathy.
Laminoplasty increases the effective diameter of the spinal canal by shifting the laminae dorsally with use of either a single door with a single lateral hinge, or a double door with lateral hinges on both sides. In contrast to laminectomy, laminoplasty retains a covering of the posterior laminar bone and ligamentum flavum over the spinal cord thereby minimizing instability, limits constriction of the dura from extradural scar formation [33, 34], and obviates the need for fusion. Early descriptions of laminoplasty kept the door open with use of suture or wire tethering the spinous process to the hinge side facet joint or capsular tissue . More recent techniques include insertion of an autogenous spinous process graft, allograft bone, or synthetic spacers to keep the door open. Fixation with use of miniplates fixed to the lamina and lateral mass has been reported by multiple authors, without major complications [36–38].
A substantial increase in the spinal canal area (38%) and diameter (29%) was predicted via the FE model, which compared favorably with the measurements obtained experimentally. It was evident from the finite element analysis and cadaveric testing that the introduction of the hinge reduced the strength of the lamina by 5- to 9-fold depending on the direction of loading. The stresses in the region of the hinge exceeded the yield strength of the cortical bone indicative of failure, while the stresses in the laminoplasty constructs (i.e., miniplates) were below the yield strength of titanium. Using these meshing techniques, efforts are currently underway to simulate a multilevel laminoplasty in a C27 model and address the flexibility of the spine postoperatively.
The broad objective of our research plan is to augment IA-FEMesh with a suite of surgical tools, thereby enabling the software to be used to readily simulate/model a variety of surgical procedures. In pursuit of this objective we have developed an easy to use workflow for the manipulation of surfaces representing anatomical structures to simulate surgical procedures. While some of these features exist in other CAD/CAM software (e.g., SOLIDWORKS, VISI) as well as finite element packages (e.g., ABAQUS CAE or ANSYS), implementing a surgical simulation tool within these packages requires a large learning curve, multiple pieces of software with different user interfaces, and significantly more time to generate the model as compared to the workflow presented here. The software presented here provides the same workflow and user interface as utilized in IA-FEMesh allowing users to perform the surgical simulations and generate the model in a matter of minutes.
In future work, we will merge the functionality described in this paper directly into the IA-FEMesh software, providing a complete surgical simulation workflow within a single software package. In addition, we are proposing to develop unique technologies to manipulate the anatomic surface definitions, enhance the multiblock meshing practices, and to improve the resulting mesh definitions. A promising means to improve the mesh definition of both anatomic structures and implants has proven to be feature recognition [43, 44]. This toolkit holds the potential to enable the user to readily simulate surgical interventions, introduce implants, and mesh the resulting models with all-hexahedral elements using multiblock meshing techniques. Our goal is to provide a meshing environment capable of meshing not only anatomic structures, but implants as well. Moreover, establishing the interactions between the two for analysis is imperative. Our long-term goal is to provide a user friendly meshing environment for researchers interested in FE analyses.
Ultimately, these tools and interactions could be coupled with three-dimensional visualization and haptic feedback that could not only serve as a simulation tool, but also a training tool for young physician scientists. This would allow new surgical procedures to be developed and evaluated in mathematical models before transitioning this work to animal models or clinical applications.
The authors gratefully acknowledge the financial support provided in part by an award (R01EB005973) from the National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health and The University of Iowa Presidential Graduate Fellowship.
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