Martine Pithioux

Fracture following lower limb lengthening in children: a series of 58 patients.

INTRODUCTION Fracture is one of the main complications following external fixator removal used in cases of progressive lower limb lengthening; rates as high as 50% are found in the literature. The aim of this study was to determine the factors influencing this complication. MATERIALS AND METHODS One hundred and eleven cases of lower limb lengthening were performed in 58 patients (40 femurs and 71 tibias). The mean age at surgery was 10.1years old. Lengthening was performed in all cases with an external fixator alone, associated in 39.6% of cases with intramedullary nailing. The patients were divided into three groups according to disease etiology (congenital, achondroplasia and other). The fractures were classified according to the Simpson classification. RESULTS Twenty fractures were recorded (18%). Sixteen fractures were found in patients with congenital disease, four with achondroplasia and none in the group of other etiologies. The fracture was more often in the femur (27.5%) than in the tibia (12.7%). DISCUSSION The rate of fracture is influenced by different factors depending on the etiology of disease. In congenital diseases, the fracture rate is higher when there is lengthening of more than 15% of the initial length and a delay between surgery and the beginning of lengthening of less than 7days. In patients with achondroplasia, the influence of a relative percentage of lengthening is less important than in those with congenital disease. However, to avoid fractures, lengthening should not be started in children under the age of nine. Moreover, lengthening should begin at least 7days after the fixator has been placed. TYPE OF STUDY Retrospective. LEVEL OF EVIDENCE Level IV.

Ratio between mature and immature enzymatic cross-links correlates with post-yield cortical bone behavior: An insight into greenstick fractures of the child fibula

As a determinant of skeletal fragility, the organic matrix is responsible for the post-yield and creep behavior of bone and for its toughness, while the mineral apatite acts on stiffness. Specific to the fibula and ulna in children, greenstick fractures show a plastic in vivo mechanical behavior before bone fracture. During growth, the immature form of collagen enzymatic cross-links gradually decreases, to be replaced by the mature form until adolescence, subsequently remaining constant throughout adult life. However, the link between the cortical bone organic matrix and greenstick fractures in children remains to be explored. Here, we sought to determine: 1) whether plastic bending fractures can occur in vitro, by testing cortical bone samples from childrens fibula and 2) whether the post-yield behavior (ωp plastic energy) of cortical bone before fracture is related to total quantity of the collagen matrix, or to the quantity of mature and immature enzymatic cross-links and the quantity of non-enzymatic cross-links. We used a two-step approach; first, a 3-point microbending device tested 22 fibula machined bone samples from 7 children and 3 elderly adults until fracture. Second, biochemical analysis by HPLC was performed on the sample fragments. When pooling two groups of donors, children and elderly adults, results show a rank correlation between total energy dissipated before fracture and age and a linear correlation between plastic energy dissipated before fracture and ratio of immature/mature cross-links. A collagen matrix with more immature cross-links (i.e. a higher immature/mature cross-link ratio) is more likely to plastically deform before fracture. We conclude that this ratio in the sub-nanostructure of the organic matrix in cortical bone from the fibula may go some way towards explaining the variance in post-yield behavior. From a clinical point of view, therefore, our results provide a potential explanation of the presence of greenstick fractures in children.

Analyzing the anisotropic Hooke's law for children's cortical bone.

Child cortical bone tissue is rarely studied because of the difficulty of obtaining samples. Yet the preparation and ultrasonic characterization of the small samples available, while challenging, is one of the most promising ways of obtaining information on the mechanical behavior of non-pathological children׳s bone. We investigated children׳s cortical bone obtained from chirurgical waste. 22 fibula or femur samples from 21 children (1-18 years old, mean age: 9.7±5.8 years old) were compared to 16 fibula samples from 16 elderly patients (50-95 years old, mean age: 76.2±13.5 years old). Stiffness coefficients were evaluated via an ultrasonic method and anisotropy ratios were calculated as the ratio of C33/C11, C33/C22 and C11/C22. Stiffness coefficients were highly correlated with age in children (R>0.56, p<0.01). No significant difference was found between C11 and C22 for either adult or child bone (p>0.5), nor between C44 and C55 (p>0.5). We observe a transverse isotropy with C33>C22=C11>C44C55>C66. For both groups, we found no correlation between age and anisotropy ratios. This study offers the first complete analysis of stiffness coefficients in the three orthogonal bone axes in children, giving some indication of how bone anisotropy is related to age. Future perspectives include studying the effect of the structure and composition of bone on its mechanical behavior.

Computed tomography, histological and ultrasonic measurements of adolescent scoliotic rib hump geometrical and material properties

In Adolescent Idiopathic Scoliosis (AIS), numerical models can enhance orthopaedic or surgical treatments and provide reliable insights into the mechanism of progression. Computational methods require knowledge of relevant parameters, such as the specific geometrical or material properties of the AIS rib, about which there is currently a lack of information. The aim of our study was to determine the geometrical and material properties (Youngs modulus [E] and Poissons ratio [ν]) for AIS rib bones. Twelve ribs extracted during gibbectomy on 15 and 17 year old girls were tested using computed tomography (CT) scanner, histology and ultrasonic scanner. The mean porosity (± standard deviation (SD)) is 1.35 (±0.52)% and the mean (±SD) bone mineral density is 2188 (±19)mmHA/cc. The cortical part of the AIS rib hump is found to be thicker than physiological values in the literature. To mimic the rib hump for an AIS girl, our results suggest that ribs should be modeled as hollow circular cylinders with a 10.40 (±1.02)mm external radius and 7.56mm (±0.75) internal radius, and material properties with a mean E of 14.9GPa (±2.6) and a mean ν of 0.26 (±0.08).

Radiographical texture analysis improves the prediction of vertebral fracture: an ex vivo biomechanical study.

Study Design. Compression biomechanical tests using fresh cadaveric thoracolumbar motion segments. Objective. The purpose of this study was to determine if the combination of bone texture parameters using bone microarchitecture, and bone mineral density (BMD) measurement by dual-energy x-ray absorptiometry provided a better prediction of vertebral fracture than BMD evaluation alone. Summary of Background Data. Bone strength is routinely evaluated using BMD, as measured by dual-energy x-ray absorptiometry. Currently, there is an ongoing debate about the strengths and limitations of bone densitometry in clinical practice. To assess the fracture risk properly, other factors are important to be taken into account such as the macro- and microarchitecture of the bone. Recently, a new high-resolution x-ray device with direct digitization, named bone microarchitecture (BMA, D3A Medical Systems), has been developed to provide a better precision of texture parameters than those previously obtained on digitized films. Methods. Twenty-seven 3-level thoracolumbar motion segments (T11, T12, L1, and L2, L3, L4) of excised spines, obtained at the Anatomy Department of Marseille, were studied using bone microarchitecture to estimate 3 textural parameters: fractal parameter Hmean, co-occurrence matrix, and run-length matrix, dual-energy x-ray absorptiometry to measure BMD, and mechanical compression tests to failure. All specimens were examined by computed tomography before and after compression. The prediction of the vertebral failure load was evaluated using multiple regression analyses. Results. Twenty-seven vertebral fractures were observed with a mean failure load of 2636.3 N (standard deviation, 996 N). Fractal parameter Hmean, co-occurrence matrix, and run-length matrix were each significantly correlated with BMD (P< 0.01) and bone strength (P< 0.01). Combining bone texture parameters and BMD significantly improved the fracture load prediction from adjusted r2 = 0.701 to adjusted r2 = 0.806 (P< 0.01). Conclusion. In these excised vertebrae, the combination of bone texture parameters with BMD demonstrated a better performance in the failure load prediction than that of BMD alone. Level of Evidence: N/A

Determination of mechanical properties of cortical bone using AFM under dry and immersed conditions.

Bone is a hierarchically organized material. At the nanoscopic scale, bone is primarily composed of collagen fibres (diameter = 20-40 nm) and apatite crystals (dimension = 50 x 25 x 3 nm). The mechanical characteristics of bone, like strength, stiffness and Young modulus, are derived from these qualitative and quantitative part of these nanoscale constituents. Atomic force spectroscopy is a technique derived from Atomic Force Microscopy (AFM) which can be used to realize nano-indentation measurements and determine the nanomechanical properties of elements. This technique is increasingly used by research teams (Wallace, 2012). Today, for practical reasons, measurements on bones are mainly performed on dry samples in air and/or on samples included in resin. However, when attempting to mechanically characterize a biological tissue, physiological test conditions are necessary. The aim of this study is therefore to measure the impact of hydration on the nanomechanical properties of bovine bones. Samples were analyzed in both air and PBS solutions.

Characterisation of the difference in fracture mechanics between children and adult cortical bone

Clinical literature describes a specific type of children bone fracture, known as “greenstick facture” which is never encountered for adult bone. Concerning children bone, there is a tremendous lack of mechanical references. Indeed, the few studies which explored the mechanical characteristics of growing process in bone dealt with samples close to cancerous cells [1], or with samples from cadavers [2]. These studies gave dispersive results and did not provide insights to the two different kinds of fracture (i.e. brittle for mature bone and plastic for growing bone). Part of the answer could lie in the evolution of the biochemical composition of cortical bone; indeed, Bala et al. [3] have shown that the elasticity depends on the mineral part of the bone matrix and the plasticity on the organic part (collagen 1). This organic part of cortical bone seems to differ between adult and children. Saito et al. [4] have shown that the main non enzymatic crosslinks in mature bone (PYD+DPD) are different from those of growing bone (DHLNL+HLNL). It seems to us that the difference between plastic growing bone fractures and brittle adult bone fractures could be explained by this difference in the non enzymatic collagen crosslinking. We performed three point microbending tests on children and adult bone to evaluate the mechanical Young’s modulus (Em) and the plastic strain energy (ωp). The results are in agreement with the clinical observations. The goal of this study is to explain these differences in mechanical behaviour between children and adult bone by using a biochemical analysis of the organic part quantifying the composition of the collagen.

IMPROVED FEMORAL NECK FRACTURE PREDICTIONS USING ANISOTROPIC FAILURE CRITERIA MODELS

Finite element models are widely used to assess long bone strength, implant stability and other clinical problems. In most of the models presented so far in the literature, the bone is taken to be isotropic, and the occurrence of failure is predicted by defining a threshold von Mises stress. However, human bone is found to show orthotropic behavior. Studies so far have focused only on the use of anisotropic criteria in orthotropic models designed to predict the occurrence of human femur failure. The aim of this study was therefore to investigate how specific finite element models for human femora combined with composite failure theories could be used to improve failure predictions in vitro. For this purpose, nine human proximal femora were tested mechanically up to failure under the loading conditions present during the one-leg stance phase in walking. Specific finite element models using various materials to represent the bone were generated for each femur. First, the bone material was modeled in the form of an isotropic brittle material, and the von Mises criterion was used to predict the occurrence of fracture. Second, the bone was modeled as a transversely isotropic brittle material with asymmetric strength characteristics, and the occurrence of fracture was predicted using the Hill and the Tsai–Wu criteria. The results obtained here show that the transversely isotropic model combined with Tsai–Wu and Hill criteria accurately predicted the fracture load (values of R2 = 0.94 and SEE = 10.3% were obtained with the Tsai–Wu criteria and R2 = 0.82 and SEE = 22.9% were obtained with the Hill criteria), while the isotropic model combined with the von Mises criterion overestimated the fracture load, although a good correlation was generally observed with the experimental results (R2 = 0.77, SEE = 30.6%).

Measuring mass density and ultrasonic wave velocity: A wavelet-based method applied in ultrasonic reflection mode

When assessing ultrasonic measurements of material parameters, the signal processing is an important part of the inverse problem. Measurements of thickness, ultrasonic wave velocity and mass density are required for such assessments. This study investigates the feasibility and the robustness of a wavelet-based processing (WBP) method based on a Jaffard-Meyer algorithm for calculating these parameters simultaneously and independently, using one single ultrasonic signal in the reflection mode. The appropriate transmitted incident wave, correlated with the mathematical properties of the wavelet decomposition, was determined using a adapted identification procedure to build a mathematically equivalent model for the electro-acoustic system. The method was tested on three groups of samples (polyurethane resin, bone and wood) using one 1-MHz transducer. For thickness and velocity measurements, the WBP method gave a relative error lower than 1.5%. The relative errors in the mass density measurements ranged between 0.70% and 2.59%. Despite discrepancies between manufactured and biological samples, the results obtained on the three groups of samples using the WBP method in the reflection mode were remarkably consistent, indicating that it is a reliable and efficient means of simultaneously assessing the thickness and the velocity of the ultrasonic wave propagating in the medium, and the apparent mass density of material.

Correlative analysis of vertebral trabecular bone microarchitecture and mechanical properties: a combined ultra-high field (7 Tesla) MRI and biomechanical investigation

Study Design. High-resolution imaging and biomechanical investigation of ex-vivo vertebrae. Objective. The aim of this study was to assess bone microarchitecture of cadaveric vertebrae using ultra-high field (UHF) 7 Tesla magnetic resonance imaging (MRI) and to determine whether the corresponding microarchitecture parameters were related to bone mineral density (BMD) and bone strength assessed by dual-energy x-ray absorptiometry (DXA) and mechanical compression tests. Summary of Background Data. Limitations of DXA for the assessment of bone fragility and osteoporosis have been recognized and criteria of microarchitecture alteration have been included in the definition of osteoporosis. Although vertebral fracture is the most common osteoporotic fracture, no study has assessed directly vertebral trabecular bone microarchitecture. Methods. BMD of 24 vertebrae (L2, L3, L4) from eight cadavers was investigated using DXA. The bone volume fraction (BVF), trabecular thickness (Tb.Th), and trabecular spacing (Tb.Sp) of each vertebra were quantified using UHF MRI. Measurements were performed by two operators to characterize the inter-rater reliability. The whole set of specimens underwent mechanical compression tests to failure and the corresponding failure stress was calculated. Results. The inter-rater reliability for bone microarchitecture parameters was good with intraclass correlation coefficients ranging from 0.82 to 0.94. Failure load and stress were significantly correlated with BVF, Tb.Sp, and BMD (P < 0.05). Tb.Th was only correlated with the failure stress (P < 0.05). Multiple regression analysis demonstrated that the combination of BVF and BMD improved the prediction of the failure stress from an adjusted R2 = 0.384 for BMD alone to an adjusted R2 = 0.414. Conclusion. We demonstrated for the first time that the vertebral bone microarchitecture assessed with UHF MRI was significantly correlated with biomechanical parameters. Our data suggest that the multimodal assessment of BMD and trabecular bone microarchitecture with UHF MRI provides additional information on the risk of vertebral bone fracture and might be of interest for the future investigation of selected osteoporotic patients. Level of Evidence: N /A