Finite element analysis and computed tomography based structural rigidity analysis of rat tibia with simulated lytic defects

Assessment of Bone Healing: Opportunities to Improve the Standard of Care

➤ Bone healing is commonly evaluated by clinical examination and serial radiographic evaluation. Physicians should be mindful that personal and cultural differences in pain perception may affect the clinical examination. Radiographic assessment, even with the Radiographic Union Score, is qualitative, with limited interobserver agreement.➤ Physicians may use serial clinical and radiographical examinations to assess bone healing in most patients, but in ambiguous and complicated cases, they may require other methods to provide assistance in decision-making.➤ In complicated instances, clinically available biomarkers, ultrasound, and magnetic resonance imaging may determine initial callus development. Quantitative computed tomography and finite element analysis can estimate bone strength in later callus consolidation phases.➤ As a future direction, quantitative rigidity assessments for bone healing may help patients to return to function earlier by increasing a clinician's confidence in successful progressive healing.

The effects of metastatic lesion on the structural determinants of bone: Current clinical and experimental approaches

Metastatic bone disease is incurable with an associated increase in skeletal-related events, particularly a 17-50% risk of pathologic fractures. Current surgical and oncological treatments are palliative, do not reduce overall mortality, and therefore optimal management of adults at risk of pathologic fractures presents an unmet medical need. Plain radiography lacks specificity and may result in unnecessary prophylactic fixation. Radionuclide imaging techniques primarily supply information on the metabolic activity of the tumor or the bone itself. Magnetic resonance imaging and computed tomography provide excellent anatomical and structural information but do not quantitatively assess bone matrix. Research has now shifted to developing unbiased data-driven tools that can predict risk of impending fractures and guide individualized treatment decisions. This review discusses the state-of-the-art in clinical and experimental approaches for prediction of pathologic fractures with bone metastases. Alterations in bone matrix quality are associated with an age-related increase in skeletal fragility but the impact of metastases on the intrinsic material properties of bone is unclear. Engineering-based analyses are non-invasive with the capability to evaluate oncological treatments and predict failure due to the progression of metastasis. The combination of these approaches may improve our understanding of the underlying deterioration in mechanical performance.

The trabecular effect: A population-based longitudinal study on age and sex differences in bone mineral density and vertebral load bearing capacity

BACKGROUND

Approximately 16-24% of postmenopausal women are affected by vertebral fractures, negatively affecting their quality of life. Trabecular and cortical bones in vertebrae decline differently with age, thus having a distinct impact on vertebral failure loads. The purpose of this study was to investigate the effect of trabecular and cortical volumetric bone mineral density loss over time on estimated failure loads; and to evaluate the effect of sex and age.

METHOD

Fracture properties from a cohort of 82 patients were evaluated for L₁-L₃ vertebrae at baseline and 6th year using an image-based method that implements axial rigidity analysis. Cortical and trabecular volumetric bone mineral density were obtained, as well as their individual contribution to total failure load. Regression analyses were performed to determine the effect of age and sex on volumetric bone mineral density and failure loads.

FINDINGS

Decline in trabecular and cortical volumetric bone mineral density, and failure load was sex-dependent (p ≤ 0.0095). Cortical and trabecular volumetric bone mineral density reduced 2.08 (g/cm³)/year and 2.02 (g/cm³)/year, respectively. A 1012 N difference in failure load, ~70% attributed to trabecular bone, was found between men and women of similar age. Over 6 years, this difference increased by 287 N. Areal bone mineral density measured by dual X-ray absorptiometry explained ~60% of the vertebral failure load.

INTERPRETATION

Trabecular bone has a significantly greater effect than cortical bone on the structural integrity and load bearing capacity of vertebrae. This might lead to a higher incidence of fragility fractures in osteoporotic women. Our non-invasive, quantitative computed tomography image-based approach may improve prevention, monitoring, and management of fractures.

Assessment of fracture risk in proximal tibia with tumorous bone defects by a finite element method

There has not been a satisfying method to predict the fracture risk in tumorous bone lesions. To tackle this challenge, we used a finite element method to assess the fracture risk in the proximal tibia (pT) when the size and location of the tumorous defects are varied in bone. Towards this end, the circular cortical defects, mimicking the tumorous lesions by forming cortical window defects, with a diameter (Ф) of 20, 30, 40, or 50 mm, are structured on the anteromedial, lateral, posterior wall of pT, which is located 5, 15, and 25 mm below articular margin, respectively. We found that under walking conditions, the Von Mises Stress of each defective tibia model was larger than that of the intact tibia model and also showed a positive linear correlation with the sizes of the defects. A notable fracture risk was not observed until the defect was Ф30 mm or larger. When the defect emerged, the anteromedial wall resisted fracture risk more than the rest of wall. Our results show that the size and location of the bone tumors are important factors affecting the fracture risk of pT. Our findings will be beneficial to clinicians when deciding what treatment to use for pT lesions.

Curved Beam Computed Tomography based Structural Rigidity Analysis of Bones with Simulated Lytic Defect: A Comparative Study with Finite Element Analysis

In this paper, a CT based structural rigidity analysis (CTRA) method that incorporates bone intrinsic local curvature is introduced to assess the compressive failure load of human femur with simulated lytic defects. The proposed CTRA is based on a three dimensional curved beam theory to obtain critical stresses within the human femur model. To test the proposed method, ten human cadaveric femurs with and without simulated defects were mechanically tested under axial compression to failure. Quantitative computed tomography images were acquired from the samples, and CTRA and finite element analysis were performed to obtain the failure load as well as rigidities in both straight and curved cross sections. Experimental results were compared to the results obtained from FEA and CTRA. The failure loads predicated by curved beam CTRA and FEA are in agreement with experimental results. The results also show that the proposed method is an efficient and reliable method to find both the location and magnitude of failure load. Moreover, the results show that the proposed curved CTRA outperforms the regular straight beam CTRA, which ignores the bone intrinsic curvature and can be used as a useful tool in clinical practices.

Biomechanics and mechanobiology of trabecular bone: a review

Trabecular bone is a highly porous, heterogeneous, and anisotropic material which can be found at the epiphyses of long bones and in the vertebral bodies. Studying the mechanical properties of trabecular bone is important, since trabecular bone is the main load bearing bone in vertebral bodies and also transfers the load from joints to the compact bone of the cortex of long bones. This review article highlights the high dependency of the mechanical properties of trabecular bone on species, age, anatomic site, loading direction, and size of the sample under consideration. In recent years, high resolution micro finite element methods have been extensively used to specifically address the mechanical properties of the trabecular bone and provide unique tools to interpret and model the mechanical testing experiments. The aims of the current work are to first review the mechanobiology of trabecular bone and then present classical and new approaches for modeling and analyzing the trabecular bone microstructure and macrostructure and corresponding mechanical properties such as elastic properties and strength.

Predicting the stiffness and strength of human femurs with real metastatic tumors

BACKGROUND

Predicting patient specific risk of fracture in femurs with metastatic tumors and the need for surgical intervention are of major clinical importance. Recent patient-specific high-order finite element methods (p-FEMs) based on CT-scans demonstrated accurate results for healthy femurs, so that their application to metastatic affected femurs is considered herein.

METHODS

Radiographs of fresh frozen proximal femur specimens from donors that died of cancer were examined, and seven pairs with metastatic tumor were identified. These were CT-scanned, instrumented by strain-gauges and loaded in stance position at three inclination angles. Finally the femurs were loaded until fracture that usually occurred at the neck. Histopathology was performed to determine whether metastatic tumors are present at fractured surfaces. Following each experiment p-FE models were created based on the CT-scans mimicking the mechanical experiments. The predicted displacements, strains and yield loads were compared to experimental observations.

RESULTS

The predicted strains and displacements showed an excellent agreement with the experimental observations with a linear regression slope of 0.95 and a coefficient of regression R(2)=0.967. A good correlation was obtained between the predicted yield load and the experimental observed yield, with a linear regression slope of 0.80 and a coefficient of regression R(2)=0.78.

DISCUSSION

CT-based patient-specific p-FE models of femurs with real metastatic tumors were demonstrated to predict the mechanical response very well. A simplified yield criterion based on the computation of principal strains was also demonstrated to predict the yield force in most of the cases, especially for femurs that failed at small loads. In view of the limited capabilities to predict risk of fracture in femurs with metastatic tumors used nowadays, the p-FE methodology validated herein may be very valuable in making clinical decisions.

Computed tomography-based rigidity analysis: a review of the approach in preclinical and clinical studies

The assessment of fracture risk in patients afflicted with osseous neoplasms has long presented a problem for orthopedic oncologists. These patients are at risk for developing pathologic fractures through lytic defects in the appendicular and axial skeleton with devastating consequences on their quality of life. Lesions with a high risk of fracture may require prophylactic surgical stabilization, whereas low-risk lesions can be treated conservatively. Therefore, effective prevention of pathologic fractures depends on accurate assessment of fracture risk and is a critical step to avoid debilitating complications. Given the complex nature of osseous neoplasms, treatment requires a multidisciplinary approach; yet, little consensus regarding fracture risk assessment exists among physicians involved in the care of these patients. In order to improve the overall standard of care, specific criteria must be adopted to formulate consistent and accurate fracture risk predictions. However, clinicians make subjective assessments about fracture risk on plain radiographs using guidelines now recognized to be inaccurate. Osseous neoplasms alter both the material and geometric properties of bone; failure to account for changes in both of these parameters limits the accuracy of current fracture risk assessments. Rigidity, the capacity to resist deformation upon loading, is a structural property that integrates both the material and geometric properties of bone. Therefore, rigidity can be used as a mechanical assay of the changes induced by lytic lesions to the structural competency of bone. Using this principle, computed tomography (CT)-based structural rigidity analysis (CTRA) was developed and validated in a series of preclinical and clinical studies.