Vickie B. Shim

Development and Validation of Patient-Specific Finite Element Models of the Hemipelvis Generated From a Sparse CT Data Set

To produce a patient-specific finite element (FE) model of a bone such as the pelvis, a complete computer tomographic (CT) or magnetic resonance imaging (MRI) geometric data set is desirable. However, most patient data are limited to a specific region of interest such as the acetabulum. We have overcome this problem by providing a hybrid method that is capable of generating accurate FE models from sparse patient data sets. In this paper, we have validated our technique with mechanical experiments. Three cadaveric embalmed pelves were strain gauged and used in mechanical experiments. FE models were generated from the CT scans of the pelves. Material properties for cancellous bone were obtained from the CT scans and assigned to the FE mesh using a spatially varying field embedded inside the mesh while other materials used in the model were obtained from the literature. Although our FE meshes have large elements, the spatially varying field allowed them to have location dependent inhomogeneous material properties. For each pelvis, five different FE meshes with a varying number of patient CT slices (8-12) were generated to determine how many patient CT slices are needed for good accuracy. All five mesh types showed good agreement between the model and experimental strains. Meshes generated with incomplete data sets showed very similar stress distributions to those obtained from the FE mesh generated with complete data sets. Our modeling approach provides an important step in advancing the application of FE models from the research environment to the clinical setting.

Deformation behavior of the iliotibial tract under different states of fixation

BACKGROUND AND OBJECTIVE The iliotibial tract (tract) is an important structure for the biomechanics of both the hip and knee joint. While a detailed characterization of its mechanical properties might help to better understand its specific role in the load transfer from the pelvis to femur and tibia, determination of those properties is complicated by its particular structure of thin fibers in the fresh state. Moreover, although the tracts mechanical properties are often derived from cadaveric material chemically fixed with either ethanol or formaldehyde, the influence of such fixation methods remains to be elucidated. Aim of this study was to determine Youngs modulus (tensile modulus, YM) of the tract. We hypothesized that either ethanol or formaldehyde fixation would significantly increase the YM compared to the tracts condition in a fresh state. MATERIAL AND METHODS 13 specimens of tract were gained from donators. The ends of the probes were plastinated with resin creating a sharp interface between the clamp and the probe to prevent material slippage. The specimens were measured in their fresh state, under ethanol- and formaldehyde-fixed conditions and re-measured after rinsing with tap water. RESULTS The YM of the fresh probes averaged 397.3N/mm(2) with a standard deviation (SD) of 151.5N/mm(2). The YM of the ethanol-fixed specimens was significantly higher (673.2N/mm(2), SD 328.5N/mm(2), p<0.05). After rinsing with tap water, the YM decreased to 95% of the fresh condition value (377.4N/mm(2), SD 144.5N/mm(2), non-significant change from fresh). After formaldehyde fixation, the YM reached 490.3N/mm(2) (SD 143.0N/mm(2), p<0.05). When the formaldehyde-fixed specimens were rinsed, the YM was 114% of the value of the fresh condition (452.6N/mm(2), SD 115.1N/mm(2), non-significant change from fresh). CONCLUSIONS This study found a significant influence of the chemical fixation method on the YM of the IT tract. If such fixation is required, our results suggest using a treatment with ethanol and subsequent rinsing that results in minimal changes to the tracts YM. Furthermore, plastination of the ends of the specimens could be crucial to allow in vitro determination of valid YM of ligaments data that can then be integrated with confidence in further finite element analyses.

Femoral bone density changes after total hip arthroplasty with uncemented taper-design stem: a five year follow-up study

We measured bone density (BD) changes to assess adaptive bone remodelling five years after uncemented total hip arthroplasty with taper-design femoral component using quantitative computed-tomography-assisted osteodensitometry (qCT). Nineteen consecutive patients (21 hips) with degenerative joint disease were enrolled in the study. A press-fit cup and a tapered uncemented stem ceramic−ceramic pairing were used in all patients. Serial clinical, radiological and qCT osteodensitometry assessments were performed after the index operation and at the one, two and five year follow-ups. At the latest follow-up, the clinical outcome was rated satisfactory in all hips. The radiological assessment showed signs of osteointegration with stable fixation of all cups and stems. Overall, there was evidence of a BD loss at year five (p = 0.004). We estimate that BD loss was between 2.2% and 12.1% in comparison with baseline postoperative values. Progressive loss of BD in the metaphyseal region was observed in all hips. We found unremarkable BD changes of diaphyseal cortical BD throughout the five year follow-up period. qCT osteodensitometry technology allows differentiation of cortical and cancellous BD changes over time. Periprosthetic BD changes at the five year follow-up are suggestive of stable stem osteointegration with proximal femoral diaphysis load transfer and metaphyseal stress shielding.

Clinical implementation of finite element models in pelvic ring surgery for prediction of implant behavior: A case report

BACKGROUND Osteosyntheses to stabilize pelvic-ring fractures were developed for younger patients, and are not universally indicated for elderly people. We present the results of parallel-arranged numerical simulations of fixation treatment that an elderly patient with a bagatelle-injured pelvic ring fracture received using a patient-specific finite element model. METHODS The clinical course of an osteosynthetic stabilized pelvic ring fracture, based on an actual case, was numerically simulated using a patient-specific finite element model. FINDINGS A previously validated finite element model of a human pelvis was customized with computed tomography data from a patient with a stabilized pelvic-ring fracture. Numerical simulation was used to analyze primary stability. The clinical process, represented by radiologic examinations, was compared with the results from the finite element simulation. Implant loosening as well as newly-occurring fractures were shown to coincide with regions with the highest stress levels. INTERPRETATION The results from the patient-specific finite element model closely resembled the actual clinical course especially in terms of the location of high strain concentration and subsequent implant loosening. This indicates that patient-specific finite element models have a potential to play an important role in planning osteosynthesis according to biomechanical stability.

Finite element analysis of acetabular fractures—development and validation with a synthetic pelvis

Acetabular fracture presents a challenging situation to trauma surgeons today due to its complexity. Finite element (FE) models can be of great help as they can improve the surgical planning and post surgery patient management for those with acetabular fractures. We have developed a non-linear finite element model of the pelvis and validated its fracture prediction capability with synthetic polyurethane pelves. A mechanical experiment was performed with the synthetic bones and fracture loads and patterns were observed for two different loading cases. Fracture loads predicted by our FE model were within one standard deviation of the experimental fracture loads for both loading cases. The incipient fracture pattern predicted by the model also resembled the actual pattern from the experiment. Although it is not a complete validation with human cadaver bones, the good agreement between model predictions and experimental results indicate the validity of our approach in using non-linear FE formulation along with contact conditions in predicting bone fractures.

A Multiscale Framework Based on the Physiome Markup Languages for Exploring the Initiation of Osteoarthritis at the Bone–Cartilage Interface

The initiation of osteoarthritis (OA) has been linked to the onset and progression of pathologic mechanisms at the cartilage-bone interface. Most importantly, this degenerative disease involves cross-talk between the cartilage and subchondral bone environments, so an informative model should contain the complete complex. In order to evaluate this process, we have developed a multiscale model using the open-source ontologies developed for the Physiome Project with cartilage and bone descriptions at the cellular, micro, and macro levels. In this way, we can effectively model the influence of whole body loadings at the macro level and the influence of bone organization and architecture at the micro level, and have cell level processes that determine bone and cartilage remodeling. Cell information is then passed up the spatial scales to modify micro architecture and provide a macro spatial characterization of cartilage inflammation. We evaluate the framework by linking a common knee injury (anterior cruciate ligament deficiency) to proinflammatory mediators as a possible pathway to initiate OA. This framework provides a “virtual bone-cartilage” tool for evaluating hypotheses, treatment effects, and disease onset to inform and strengthen clinical studies.

Bone remodelling in the natural acetabulum is influenced by muscle force‐induced bone stress

A modelling framework using the international Physiome Project is presented for evaluating the role of muscles on acetabular stress patterns in the natural hip. The novel developments include the following: (i) an efficient method for model generation with validation; (ii) the inclusion of electromyography-estimated muscle forces from gait; and (iii) the role that muscles play in the hip stress pattern. The 3D finite element hip model includes anatomically based muscle area attachments, material properties derived from Hounsfield units and validation against an Instron compression test. The primary outcome from this study is that hip loading applied as anatomically accurate muscle forces redistributes the stress pattern and reduces peak stress throughout the pelvis and within the acetabulum compared with applying the same net hip force without muscles through the femur. Muscle forces also increased stress where large muscles have small insertion sites. This has implications for the hip where bone stress and strain are key excitation variables used to initiate bone remodelling based on the strain-based bone remodelling theory. Inclusion of muscle forces reduces the predicted sites and degree of remodelling. The secondary outcome is that the key muscles that influenced remodelling in the acetabulum were the rectus femoris, adductor magnus and iliacus.

Quantitative CT with finite element analysis: towards a predictive tool for bone remodelling around an uncemented tapered stem

PurposeWe used quantitative CT in conjunction with finite element analysis to provide a new tool for assessment of bone quality after total hip arthroplasty in vivo. The hypothesis of this prospective five-year study is that the combination of the two modalities allows 3D patient-specific imaging of cortical and cancellous bone changes and stress shielding.MethodWe tested quantitative CT in conjunction with finite elements on a cohort of 29 patients (31 hips) who have been scanned postoperatively and at one year, two years and five years follow-up. The method uses cubic Hermite finite element interpolation for efficient mesh generation directly from qCT datasets. The element Gauss points that are used for the geometric interpolation functions are also used for interpolation of osteodensitometry data.ResultsThe study showed changes of bone density suggestive of proximal femur diaphysis load transfer with osteointegration and moderate metaphyseal stress shielding. Our model revealed that cortical bone initially became porous in the greater trochanter, but this phenomenon progressed to the cortex of the lesser trochanter and the posterior aspect of the metaphysis. The diaphyseal area did not experience major change in bone density for either cortical or cancellous bone.ConclusionThe combination of quantitative CT with finite element analysis allows visualization of changes to bone density and architecture. It also provides correlation of bone density/architectural changes with stress patterns enabling the study of the effects of stress shielding on bone remodelling in vivo. This technology can be useful in predicting bone remodeling and the quality of implant fixation using prostheses with different design and/or biomaterials.

The influence and biomechanical role of cartilage split line pattern on tibiofemoral cartilage stress distribution during the stance phase of gait

This study presents an evaluation of the role that cartilage fibre ‘split line’ orientation plays in informing femoral cartilage stress patterns. A two-stage model is presented consisting of a whole knee joint coupled to a tissue-level cartilage model for computational efficiency. The whole joint model may be easily customised to any MRI or CT geometry using free-form deformation. Three ‘split line’ patterns (medial–lateral, anterior–posterior and random) were implemented in a finite element model with constitutive properties referring to this ‘split line’ orientation as a finite element fibre field. The medial–lateral orientation was similar to anatomy and was derived from imaging studies. Model predictions showed that ‘split lines’ are formed along the line of maximum principal strains and may have a biomechanical role of protecting the cartilage by limiting the cartilage deformation to the area of higher cartilage thickness.

An Efficient and Accurate Prediction of the Stability of Percutaneous Fixation of Acetabular Fractures With Finite Element Simulation

Posterior wall fracture is one of the most common fracture types of the acetabulum and a conventional approach is to perform open reduction and internal fixation with a plate and screws. Percutaneous screw fixations, on the other hand, have recently gained attention due to their benefits such as less exposure and minimization of blood loss. However their biomechanical stability, especially in terms interfragmentary movement, has not been investigated thoroughly. The aims of this study are twofold: (1) to measure the interfragmentary movements in the conventional open approach with plate fixations and the percutaneous screw fixations in the acetabular fractures and compare them; and (2) to develop and validate a fast and efficient way of predicting the interfragmentary movement in percutaneous fixation of posterior wall fractures of the acetabulum using a 3D finite element (FE) model of the pelvis. Our results indicate that in single fragment fractures of the posterior wall of the acetabulum, plate fixations give superior stability to screw fixations. However screw fixations also give reasonable stability as the average gap between fragment and the bone remained less than 1 mm when the maximum load was applied. Our finite element model predicted the stability of screw fixation with good accuracy. Moreover, when the screw positions were optimized, the stability predicted by our FE model was comparable to the stability obtained by plate fixations. Our study has shown that FE modeling can be useful in examining biomechanical stability of osteosynthesis and can potentially be used in surgical planning of osteosynthesis.