Mechanical strength of a femoral reconstruction in paediatric oncology: a finite element study

Comparison of Locked Plates and Blade Plates for Varus Osteotomy of the Proximal Femur by the Finite Element Method

Objective:  The present study compared the difference in load and pressure distribution behavior of the blade plate and locked plate for varus osteotomy of the proximal femur per the finite element method. Methods:  Modeling was performed by scanning a medium-sized left femur with medial valgus deformity made of polyurethane. Results:  The stiffness of the locked plate is higher compared with that of the blade plate. However, this difference was not significant. In addition, the locked plate has proximal locking screws to ensure that the bending moments on the screws are smaller during loading. Conclusion:  In summary, both plates are well-established and effective. However, the study using the finite element method plays a fundamental role in understanding the load and pressure distribution of the implant. Moreover, it opens up new possibilities for further studies, including surgical proposals and customized implant materials.

Predicting bone strength from CT data: Clinical applications

In this review we summarise over 15 years of research and development around the prediction of whole bones strength from Computed Tomography data, with particular reference to the prediction of the risk of hip fracture in osteoporotic patients. We briefly discuss the theoretical background, and then provide a summary of the laboratory and clinical validation of these modelling technologies. We then discuss the three current clinical applications: in clinical research, in clinical trials, and in clinical practice. On average the strength predicted with finite element models (QCT-FE) based on computed tomography is 7% more accurate that that predicted with areal bone mineral density from Dual X-ray Absorptiometry (DXA-aBMD), the current standard of care, both in term of laboratory validation on cadaver bones and in terms of stratification accuracy on clinical cohorts of fractured and non-fractured women. This improved accuracy makes QCT-FE superior to DXA-aBMD in clinical research and in clinical trials, where the its use can cut in half the number of patients to be enrolled to get the same statistical power. For routine clinical use to decide who to treat with antiresorptive drugs, QCT-FE is more accurate but less cost-effective than DXA-aBMD, at least when the decision is on first line treatment like bisphosphonates. But the ability to predict skeletal strength from medical imaging is now opening a number of other applications, for example in paediatrics and oncology.

Comparison of methods for assigning the material properties of the distraction callus in computational models

In silico models of distraction osteogenesis and fracture healing usually assume constant mechanical properties for the new bone tissue generated. In addition, these models do not always account for the porosity of the woven bone and its evolution. In this study, finite element analyses based on computed tomography (CT) are used to predict the stiffness of the callus until 69 weeks after surgery using 15 CT images obtained at different stages of an experiment on bone transport, technique in which distraction osteogenesis is used to correct bone defects. Three different approaches were used to assign the mechanical properties to the new bone tissue. First, constant mechanical properties of the hard callus tissue and no porosity were assumed. Nevertheless, this approach did not show good correlations. Second, random variations in the elastic modulus and porosity of the woven bone were taken from previous experimental studies. Finally, the elastic properties of each element were assigned depending on gray scale in CT images. The numerically predicted callus stiffness was compared with previous in vivo measurements. It was concluded firstly that assignment depending on gray scale is the method that provides the best results and secondly that the method that considers a random distribution of porosity and elastic modulus of the callus is also suitable to predict the callus stiffness from 15 weeks after surgery. This finding provides a method for assigning the material properties of the distraction callus, which does not require CT images and may contribute to improve current in silico models.

A subject-specific integrative biomechanical framework of the pelvis for gait analysis

Analysis of the human locomotor system using rigid-body musculoskeletal models has increased in the biomechanical community with the objective of studying muscle activations of different movements. Simultaneously, the finite element method has emerged as a complementary approach for analyzing the mechanical behavior of tissues. This study presents an integrative biomechanical framework for gait analysis by linking a musculoskeletal model and a subject-specific finite element model of the pelvis. To investigate its performance, a convergence study was performed and its sensitivity to the use of non-subject-specific material properties was studied. The total hip joint force estimated by the rigid musculoskeletal model and by the finite element model showed good agreement, suggesting that the integrative approach estimates adequately (in shape and magnitude) the hip total contact force. Previous studies found movements of up to 1.4 mm in the anterior-posterior direction, for single leg stance. These results are comparable with the displacement values found in this study: 0-0.5 mm in the sagittal axis. Maximum von Mises stress values of approximately 17 MPa were found in the pelvic bone. Comparing this results with a previous study of our group, the new findings show that the introduction of muscular boundary conditions and the flexion-extension movement of the hip reduce the regions of high stress and distributes more uniformly the stress across the pelvic bone. Thus, it is thought that muscle force has a relevant impact in reducing stresses in pelvic bone during walking of the finite element model proposed in this study. Future work will focus on including other deformable structures, such as the femur and the tibia, and subject-specific material properties.

Relationship between bone adaptation and in-vivo mechanical stimulus in biological reconstructions after bone tumor: A biomechanical modeling analysis

BACKGROUND

Biomechanical interpretations of bone adaptation in biological reconstructions following bone tumors would be crucial for orthopedic oncologists, particularly if based on quantitative observations. This would help plan for surgical treatments, rehabilitative programs and communication with the patients. We aimed to analyze the biomechanical adaptation of a femoral reconstruction after Ewing sarcoma according to an increasingly-used surgical technique, and to relate in-progress bone resorption to the mechanical stimulus induced by different motor activities.

METHODS

We created a multiscale musculoskeletal and finite element model from CT scans and motion analysis data at a 76-month follow-up of a patient, to analyze muscle and joint loads, and to compare the mechanical competence of the reconstructed bone with the contralateral limb, in the current real condition and in a possible revision surgery that removed proximal screws.

FINDINGS

Our results showed strategies of muscle coordination that led to differences in joint loads between limbs more marked in more demanding motor activities, and generally larger in the contralateral limb. The operated femur presented a markedly low ratio of physiological strain due to load-sharing with the metal implant, particularly in the lateral aspect. The possible revision surgery would help restore a physiological strain configuration, while the safety of the reconstruction would not be threatened.

INTERPRETATION

We suggest that bone resorption is related to load-sharing and to the internal forces exerted during movement, and the mechanical stimulus should be improved by adopting modifications in the surgical treatment and by promoting physical therapy aimed at specific muscle strengthening.

Subject specific finite element modeling of periprosthetic femoral fracture using element deactivation to simulate bone failure

Subject-specific finite element (FE) modeling methodology could predict peri-prosthetic femoral fracture (PFF) for cementless hip arthoplasty in the early postoperative period. This study develops methodology for subject-specific finite element modeling by using the element deactivation technique to simulate bone failure and validate with experimental testing, thereby predicting peri-prosthetic femoral fracture in the early postoperative period. Material assignments for biphasic and triphasic models were undertaken. Failure modeling with the element deactivation feature available in ABAQUS 6.9 was used to simulate a crack initiation and propagation in the bony tissue based upon a threshold of fracture strain. The crack mode for the biphasic models was very similar to the experimental testing crack mode, with a similar shape and path of the crack. The fracture load is sensitive to the friction coefficient at the implant-bony interface. The development of a novel technique to simulate bone failure by element deactivation of subject-specific finite element models could aid prediction of fracture load in addition to fracture risk characterization for PFF.

Developing CT based computational models of pediatric femurs

The mechanisms of fracture in infants and toddlers are not well understood. There have been very few studies on the mechanical properties of pediatric bones and their responses under fracture loading. A better understanding of fracture mechanisms in children will help elucidate both accidental and non-accidental injuries, as well as bone fragility diseases. The aim of this study is to develop in silico femoral models from CT scans to provide detailed quantitative information regarding the geometry and mechanical response of the femur, with the long term potential of investigating injury mechanisms. Fifteen anonymized QCT scans (aged 0-3 years) were collected and used to create personalized computational models of femurs. The elastic modulus of femur was illustrated at various ages. The models were also subjected to a series of four point bending simulations taking into account a range of loads perpendicular to the femoral shaft. The results showed that mid-shaft cross-section at birth appeared circular, but the diameter in the anteroposterior axis gradually increased with age. The density, and by implication modulus of elasticity at the mid-shaft became more differentiated with growth. Pediatric cortical bone with density close to the peak values found in adults was attained a few weeks after birth. The method is able to capture quantitative variations in geometries, material properties and mechanical responses, and has confirmed the rapid development of bone during the first few years of life using in silico models.

Estimating the density of femoral head trabecular bone from hip fracture patients using computed tomography scan data

The purpose of this study was to compare computed tomography density ( ρCT) obtained using typical clinical computed tomography scan parameters to ash density ( ρash), for the prediction of densities of femoral head trabecular bone from hip fracture patients. An experimental study was conducted to investigate the relationships between ρash and ρCT and between each of these densities and ρbulk and ρdry. Seven human femoral heads from hip fracture patients were computed tomography–scanned ex vivo, and 76 cylindrical trabecular bone specimens were collected. Computed tomography density was computed from computed tomography images by using a calibration Hounsfield units–based equation, whereas ρbulk, ρdry and ρash were determined experimentally. A large variation was found in the mean Hounsfield units of the bone cores (HUcore) with a constant bias from ρCT to ρash of 42.5 mg/cm3. Computed tomography and ash densities were linearly correlated ( R2 = 0.55, p < 0.001). It was demonstrated that ρash provided a good estimate of ρbulk ( R2 = 0.78, p < 0.001) and is a strong predictor of ρdry ( R2 = 0.99, p < 0.001). In addition, the ρCT was linearly related to ρbulk ( R2 = 0.43, p < 0.001) and ρdry ( R2 = 0.56, p < 0.001). In conclusion, mineral density was an appropriate predictor of ρbulk and ρdry, and ρCT was not a surrogate for ρash. There were linear relationships between ρCT and physical densities; however, following the experimental protocols of this study to determine ρCT, considerable scatter was present in the ρCT relationships.

Reconstruction of type II+III pelvic resection with a modular hemipelvic endoprosthesis: a finite element analysis study

OBJECTIVE

To conduct a biomechanical study of the whole reconstructed pelvic ring using a modular hemipelvic endoprosthesis.

METHODS

A subject-specific finite-element (FE) model of the whole pelvic ring, including the pelvis, sacrum and main ligaments, was constructed. Type II+III pelvic resection was simulated on the FE model. Then a three-dimensional model of a reconstructed pelvic ring with a modular hemipelvic endoprosthesis was developed, and FE analysis performed to compare the stresses along the bilateral arcuate lines of the reconstructed and normal pelvis in the bipedal standing position. Comparison between bilateral stress distributions along the sciatic notch was also performed. The characteristics of load transmission within the endoprosthesis were also studied.

RESULTS

No significant difference in the stresses along the bilateral arcuate lines was observed between the reconstructed and normal pelvis. The stress distribution on the prosthesis along the sciatic notch paths was significantly greater than that on the unaffected side in the same position. The peak stress of the implant on the S1 vertebral body was 182.9 MPa under a load of 600N. Study of load transfer on the implant showed that the posterior side of the column between the point of iliac fixation and the acetabulum was subject to the greatest stress.

CONCLUSION

This FE study showed that a modular hemipelvic endoprosthesis can restore load transfer between the sacrum and acetabulum on simple standing. Future implant design should aim to decrease the stress concentration and make the biomechanical performance more balanced.

[Femur reconstruction using combined autologous fibula transfer and humeral allograft]

Wide resection far into the femoral metaphysis may be required to treat malignant bone tumors in the pediatric and adolescent patient population. Biological reconstruction using a free, vascularized fibular graft is a well-established surgical technique. A short remaining femoral medullary canal and a relatively small fibula diameter can make fixation of the vascularized bone transfer difficult. Stable fixation and short fusion times, however, can be achieved with the use of an additional humeral allograft and plate osteosynthesis.

MULTIMODAL DISPLAY INTERFACE FOR PLANNING AND MONITORING COMPLEX SKELETAL RECONSTRUCTIONS

The aim of the present work is to present the new software Multimod Data Manager and to show applications in the planning and monitoring of complex skeletal reconstructions in orthopedic oncology. The DataManager allows the full integration of different kind of data, particularly medical imaging data, both static as CT, SPECT or MRI, or time-varying as fluoroscopy or dynamic MRI, with motion analysis data, but also 3D computer and finite element models of bones, joints and also soft tissues. All the data can be visualized with highly interactive and specialized modalities and can be integrated to offer a complete representation of the patient anatomy. Several algorithms are implemented to allow registration and synchronization of the data. A fully 3D environment is offered to the surgeon to navigate inside the medical imaging dataset. To exemplify the use of the software we document in this paper the data processing used to investigate different specific clinical cases in the field of orthopedic oncology.

Direct in vivo strain measurements in human bone-a systematic literature review

Bone strain is the governing stimuli for the remodeling process necessary in the maintenance of bone's structure and mechanical strength. Strain gages are the gold standard and workhorses of human bone experimental strain analysis in vivo. The objective of this systematic literature review is to provide an overview for direct in vivo human bone strain measurement studies and place the strain results within context of current theories of bone remodeling (i.e. mechanostat theory). We employed a standardized search strategy without imposing any time restriction to find English language studies indexed in PubMed and Web of Science databases that measured human bone strain in vivo. Twenty-four studies met our final inclusion criteria. Seven human bones were subjected to strain measurements in vivo including medial tibia, second metatarsal, calcaneus, proximal femur, distal radius, lamina of vertebra and dental alveolar. Peak strain magnitude recorded was 9096 με on the medial tibia during basketball rebounding and the peak strain rate magnitude was -85,500 με/s recorded at the distal radius during forward fall from standing, landing on extended hands. The tibia was the most exposed site for in vivo strain measurements due to accessibility and being a common pathologic site of stress fracture in the lower extremity. This systematic review revealed that most of the strains measured in vivo in different bones were generally within the physiological loading zone defined by the mechanostat theory, which implies stimulation of functional adaptation necessary to maintain bone mechanical integrity.

Biological reconstruction of bone defects: the role of the free fibula flap

This review describes the kinds of skeletal bone defects in bones which develop through enchondral ossification. It focuses on the biological reconstruction of those defects according to the two main subtypes, intercalary and osteoarticular. We list the causes of bone defects and outline the different types and configurations that result from them. We then review the currently available reconstructive options according to the patient's age and describe the theoretical options as well. Finally, the history, surgical anatomy and clinical use of the free fibula flap will be reviewed. From our own clinical experience and review of the literature, we conclude that biological reconstruction is, in many ways, superior to alloplastic materials, especially in children, adolescents and young adults.

Biological reconstruction using massive bone allograft with intramedullary vascularized fibular flap after intercalary resection of humeral malignancy

BACKGROUND

Reconstruction after excision of the humeral malignancy is a challenging issue for the reconstructive surgeon. The combined use of a fibular flap and allograft can provide a reliable reconstructive option. This article describes the authors' experience with this technique for the treatment of segmental bone defects after resection of humeral malignancy.

METHODS

From 2005 to 2008, seven patients that had intercalary resection of humeral malignancy underwent reconstruction with an allograft and vascularized fibula construct. Patients were examined clinically and radiographically.

RESULT

The average age at time of operation was 16.7 years. The mean follow-up time was 27.7 months. The average length of the resected humeral segment was 10.6 cm and that of the fibula flap was 13.1 cm. The average time of union of fibula was 20.7 weeks and for union of allograft was 26.3 weeks. Incorporation of the fibula into the allograft was seen in three patients. There were no allograft fractures and no infections. Three patients had surgery-related complications including a temporary radial nerve paralysis in 1, wound dehiscence in 1, and clawed toes in 1. The MSTS average score was 95.2% at final follow-up.

CONCLUSIONS

Intramedullary fibular flaps in combination with massive allografts provide an excellent option for reconstruction of large bony defects after humeral malignancy extirpation. The viability of the fibula is a cornerstone in success of reconstruction that prevents allograft nonunion and result in decreased time to bone healing, leading to earlier patient recovery and return of function.

Mechanical testing of bones: the positive synergy of finite-element models and in vitro experiments

Bone biomechanics have been extensively investigated in the past both with in vitro experiments and numerical models. In most cases either approach is chosen, without exploiting synergies. Both experiments and numerical models suffer from limitations relative to their accuracy and their respective fields of application. In vitro experiments can improve numerical models by: (i) preliminarily identifying the most relevant failure scenarios; (ii) improving the model identification with experimentally measured material properties; (iii) improving the model identification with accurately measured actual boundary conditions; and (iv) providing quantitative validation based on mechanical properties (strain, displacements) directly measured from physical specimens being tested in parallel with the modelling activity. Likewise, numerical models can improve in vitro experiments by: (i) identifying the most relevant loading configurations among a number of motor tasks that cannot be replicated in vitro; (ii) identifying acceptable simplifications for the in vitro simulation; (iii) optimizing the use of transducers to minimize errors and provide measurements at the most relevant locations; and (iv) exploring a variety of different conditions (material properties, interface, etc.) that would require enormous experimental effort. By reporting an example of successful investigation of the femur, we show how a combination of numerical modelling and controlled experiments within the same research team can be designed to create a virtuous circle where models are used to improve experiments, experiments are used to improve models and their combination synergistically provides more detailed and more reliable results than can be achieved with either approach singularly.

Is combining massive bone allograft with free vascularized fibular flap the children's reconstruction answer to lower limb defects following bone tumour resection?

PURPOSE OF THE STUDY

Bone tumours are frequent conditions in children, and their surgical resection may lead to extensive defects which reconstruction is often challenging. Indeed, local conditions do not promote bone healing, and the achieved surgical result requires to be life-lasting. Capanna suggested a reconstruction technique combining massive allograft and free vascularized fibular flap. The first one is intended to withstand mechanical stress, and the second one offers biological and vascular support to improve bone healing and prevent infections.

MATERIAL AND METHODS

We report our experience with this technique when applied to the lower limb in a prospective study including seven children, with a mean follow-up of 44 months.

RESULTS

Bone healing was achieved by one single procedure in 85.7% of the cases, usually 7 months after surgery. Six out of seven patients achieved a final and long-lasting outcome, five of them following a simple surgical history. Partial weight-bearing was post-operatively allowed at about 2 months, full weight-bearing was initiated at about 5.5 months.

DISCUSSION

A low complication rate was reported despite the extent of the disease and the type of the surgical procedure. Capanna's combined reconstructive technique appears very efficient in the management of massive bone defects following tumour resection in children's lower limb.

LEVEL OF EVIDENCE

Level IV. Retrospective therapeutic study.

A new meshless approach for subject-specific strain prediction in long bones: Evaluation of accuracy

BACKGROUND

The Finite Element Method is at present the method of choice for strain prediction in bones from Computed Tomography data. However, accurate methods rely on the correct topological representation of the bone surface, which requires a massive operator effort, thus restricting their applicability to clinical practice. Meshless methods, which do not rely on a pre-defined topological discretisation of the domain, might greatly improve the numerical process automation, but currently their application to biomechanics is negligible.

METHODS

A meshless implementation of an innovative numerical approach based on a direct discrete formulation of physical laws, the Cell Method, was developed to predict strains in a cadaver femur from Computed Tomography data. The model accuracy was estimated by comparing the predicted strains with those experimentally measured on the same specimen in a previous study. As a reference, the results were compared to those obtained with a state-of-the-art finite element model.

FINDINGS

The Cell Method meshless model predicted strains highly correlated with the experimental measurements (R2=0.85) with a good global accuracy (RMSE=15.6%). The model performed slightly worse than the finite element one, but this was probably due to the need to sub-sample the original data, and the lower order of the interpolation used (linear vs parabolic).

INTERPRETATION

Although there is surely room for improvement, the accuracy already obtained with this meshless implementation of the Cell Method makes it a good candidate for some clinical applications, especially considering the full automation of the method, which does not require any data pre-processing.

Multiscale modelling of the skeleton for the prediction of the risk of fracture

BACKGROUND

The development of a multiscale model of the human musculoskeletal system able to accurately predict the risk of bone fracture is still a grand challenge. The aim of this paper is to present the Living Human Project, to describe the final system and to review the achievements obtained so far. The Living Human musculoskeletal supermodel is conceived as the interconnection of five interdependent sub-models: the continuum, the boundary condition, the constitutive equation, the remodelling history and the failure criterion sub-models.

METHODS

Methods are available to develop accurate subject-specific finite element models of bones that can incorporate the subject's tissue-density distribution and empirically derived constitutive laws. Anatomo-functional musculoskeletal models can be registered with gait analysis data to predict muscle and joint forces acting on the patient's skeleton during gait. These are the boundary conditions for the continuum models that showed an average error of 12% in the prediction of the failure load. Still, the entire supermodel is defined as a collection of procedural macros to predict the risk of fracture and should be improved.

FINDINGS

Even with these limitations, the organ-level model already found some clinically relevant applications, especially in the analysis of joint prostheses. Also, the body-organ level multiscale model finds some clinical applications in paediatric skeletal oncology. The tissue- and the cell-level models are not yet fully validated. Thus, they cannot be safely used in clinical applications.

INTERPRETATION

The continuum sub-model is the most mature model available. More powerful methods are needed for the generation of anatomo-functional musculoskeletal models. Muscle force prediction should be improved, investigating new probabilistic approaches to identify the neuro-motor strategy. The changes of the tissue properties in the various regions of the skeleton and predictive remodelling models should be included. An adequate information technology infrastructure should be developed to support collaborative work and integration of different sub-models.

A modified method for assigning material properties to FE models of bones

The aim of the present study is to compare the results from subject-specific finite element analysis (FEA) of a human femur to experimental measurements, using two different methods for assigning material properties to the FE models. A modified material mapping strategy allowing for spatial variation of material properties within the elements and Young's modulus surface corrections is presented and compared to a more conventional strategy, whereby constant material properties are assigned to each element. The accuracy of the superficial stress-strain predictions was evaluated against experimental results from 13 strain gauges and five different load cases. Both methods predicted stresses with acceptable accuracy (R(2) = 0.92, root mean square error, RMSE < 10%), with the conventional method performing slightly better. The modified method performed better in strain prediction (R(2) = 0.85, RMSE = 23% versus R(2) = 0.79, RMSE = 31%).

Mathematical relationships between bone density and mechanical properties: a literature review

BACKGROUND

In many published studies, elastic properties of bone are correlated to the bone density, in order to derive an empirical elasticity-density relationship. The most common use of these relationships is the prediction of the bone local properties from medical imaging data in subject-specific numerical simulation studies. The proposed relationships are substantially different one from the other. It is unclear whether such differences in elasticity-density relationships can be entirely explained in terms of methodological discrepancies among studies.

METHODS

All relevant literature was reviewed. Only elasticity-density relationships derived from similarly controlled experiments were included and properly normalized. The resulting relationships were grouped according to the most important methodological differences: type of end support during testing, specimen geometry, and anatomical sampling location.

FINDINGS

Even after normalization with respect to strain rate and densitometric measurement unit, substantial inter-study differences do exist, and they can only be partially explained by the methodological differences between studies.

INTERPRETATION

Some recommendations are made for the application of elasticity-density relationships to subject-specific finite element studies. The importance of defining a standardized mechanical testing methodology for bone specimens is stressed, and some guidelines that emerged from the literature are proposed. To identify density-elasticity relationships suitable for use in subject-specific FE studies, the development of a benchmark study is also proposed, where the elasticity-density relationship is taken as the variable under study, and a numerical model of known numerical accuracy predicts experimental strain measurements.

Virtual palpation of skeletal landmarks with multimodal display interfaces

The 3D location of skeletal landmarks on CT datasets is an important procedure, used in many research and clinical contexts. The standard procedure involves the segmentation of the CT images, the creation of a 3D surface bone model, and the location of the landmarks on this surface. However, the segmentation is time-consuming and requires skilled operators and sophisticated software. The aim of the present study is to evaluate the efficacy of a multimodal display interface to direct volumetric interactive visualization in performing a virtual palpation task. An expert operator used the CT dataset of a patient's thigh region to locate 14 femoral skeletal landmarks. This operation was repeatedly performed using different CT data representation; the accuracy and repeatability were compared to those achievable with the conventional procedure based on the segmented 3D surface. When a multimodal display interface (formed by an orthogonal slice, RXCT and interactive isosurface views) was used to perform the virtual palpation directly on the CT data, the average coordinates of the landmarks did not differ significantly from those located on the 3D surface, and the measurement repeatability was actually better with the multimodal display of the volumetric data than with the 3D surface. Thus, we can conclude that skeletal virtual palpation can be performed directly on the CT dataset, as far as the virtual palpation is performed with a multimodal display interface.

Micro-computed tomography prediction of biomechanical strength in murine structural bone grafts

Correlating massive bone graft strength to parameters derived from non-invasive imaging is important for pre-clinical and clinical evaluation of therapeutic adjuvants designed to improve graft repair. Towards that end, univariate and multivariate regression between measures of graft and callus geometry from micro-CT imaging and torsional strength and rigidity were investigated in a mouse femoral graft model. Four millimeter mid-diaphyseal defects were grafted with live autografts or processed allografts and allowed to heal for 6, 9, 12, or 18 weeks. We observed that allograft remodeling and incorporation into the host remained severely impaired compared to autografts mainly due to the extent of callus formation around the graft, the rate and extent of the graft resorption, and the degree of union between the graft and host bone as judged by post-mechanical testing analysis of the mode of failure. The autografts displayed greater ultimate torque and torsional rigidity compared to the allografts over time. However the biomechanical properties of allografts were equivalent to autografts by 9 weeks but significantly decreased at 12 and 18 weeks. Multivariate regression analysis demonstrated significant statistical correlations between combinations of the micro-CT parameters (graft and callus volume and cross-sectional polar moment of inertia) with the measured ultimate torque and torsional rigidity (adjusted R(2)=44% and 50%, respectively). The statistical correlations approach used in this mouse study could be useful in guiding future development of non-invasive predictors of the biomechanical properties of allografts using clinical CT.

Subject-specific finite element models can accurately predict strain levels in long bones

The prediction of the stress-state and fracture risk induced in bones by various loading conditions in individual patients using subject-specific finite element models still represents a challenge in orthopaedic biomechanics. The accuracy of the strain predictions reported in the literature is variable and generally not satisfactory. The aim of the present study was to evaluate if a proper choice of the density-elasticity relationship can lead to accurate strain predictions in the frame of an automatic subject-specific model generation strategy. To this aim, a combined numerical-experimental study was performed comparing finite element predicted strains with strain-gauges measurements obtained on eight cadaver proximal femurs, each instrumented with 15 rosettes mostly concentrated in the bone metaphyses, tested non-destructively in vitro under six different loading scenarios. Three different density-elasticity power relationships were selected from the literature and implemented in the finite element models derived from computed tomography data. The results of the present study confirm the great influence of the density-elasticity relationship used on the accuracy of numerical predictions. One of the tested constitutive laws provided a very good agreement (R(2)=0.91, RMSE lower than 10% of the maximum measured value) between numerical calculations and experimental measurements. The presented results show, in addition, that the adoption of a single density-elasticity relationship over the whole bone density range is adequate to obtain an accuracy that is already suitable for many applications.

Subject-specific finite element models of long bones: An in vitro evaluation of the overall accuracy

The determination of the mechanical stresses induced in human bones is of great importance in both research and clinical practice. Since the stresses in bones cannot be measured non-invasively in vivo, the only way to estimate them is through subject-specific finite element modelling. Several methods exist for the automatic generation of these models from CT data, but before bringing them in the clinical practice it is necessary to assess their accuracy in the predictions of the bone stresses. Particular attention should be paid to those regions, like the epiphyseal and metaphyseal parts of long bones, where the automatic methods are typically less accurate. Aim of the present study was to implement a general procedure to automatically generate subject-specific finite element models of bones from CT data and estimate the accuracy of this general procedure by applying it to one real femur. This femur was tested in vitro under five different loading scenarios and the results of these tests were used to verify how the adoption of a simplified two-material homogeneous model would change the accuracy with respect to the density-based inhomogeneous one, with special attention paid to the epiphyseal and metaphyseal proximal regions of the bone. The results showed that the density-based inhomogeneous model predicts with a very good accuracy the measured stresses (R(2)=0.91, RMSE=8.6%, peak error=27%), while the two-material model was less accurate (R(2)=0.89, RMSE=9.6%, peak error=35%). The results showed that it is possible to automatically generate accurate finite element models of bones from CT data and that the strategy of material properties mapping has a significant influence on its accuracy.

Predicting the subject-specific primary stability of cementless implants during pre-operative planning: Preliminary validation of subject-specific finite-element models

Pre-operative planning help the surgeon in taking the proper clinical decision. The ultimate goal of this work is to develop numerical models that allow the surgeon to estimate the primary stability during the pre-operative planning session. The present study was aimed to validate finite-element (FE) models accounting for patient and prosthetic size and position as planned by the surgeon. For this purpose, the FE model of a cadaveric femur was generated starting from the CT scan and the anatomical position of a cementless stem derived by a skilled surgeon using a pre-operative CT-based planning simulation software. In-vitro experimental measurements were used as benchmark problem to validate the bone-implant relative micromotions predicted by the patient-specific FE model. A maximum torque in internal rotation of 11.4 Nm was applied to the proximal part of the hip stem. The error on the maximum predicted micromotion was 12% of the peak micromotion measured experimentally. The average error over the entire range of applied torques was only 7% of peak measurement. Hence, the present study confirms that it is possible to accurately predict the level of primary stability achieved for cementless stems using numerical models that account for patient specificity and surgical variability.

Gait performance in an original biologic reconstruction of proximal femur in a skeletally immature child: a case report

Biologic reconstruction of the femoral head by a sophisticated autotransplantation of the proximal growing fibula associated with a massive bone allograft has been performed in a 4-year-old girl affected by Ewing's sarcoma. The child was treated in 1997 and then followed: clinical and functional tests were performed 2, 3, 4, 6, and 7 years after surgery. Gait and specific motor tasks were assessed by means of motion analysis instruments. The patient was followed by a specific rehabilitation program, aimed at controlling load on the treated limb, resuming good muscle function, and recovering a physiological pattern of movement during daily routine activities. The outstanding radiographic evolution 8 years after surgery with the peroneal head progressively remodeled as a femoral head, and the more than satisfactory gait pattern observed at last follow-up, makes this study a keystone of experience and knowledge to be applied to other patients.

The Use of Massive Bone Allograft with Intramedullary Free Fibular Flap for Limb Salvage in a Pediatric and Adolescent Population

BACKGROUND

Long segmental bony defects after tumor extirpation can pose difficult problems for the reconstructive surgeon. Capanna and colleagues have described a technique that places a free fibular flap within the intramedullary canal of an allograft for reconstruction of large intercalary bony defects. This article describes the authors' long-term follow-up with this technique for the treatment of large segmental bone defects in a pediatric population.

METHODS

Over a 6-year period, seven patients underwent bony reconstruction with an allograft and vascularized fibular construct. All reconstructions were performed for lower extremity salvage after tumor extirpation. Grafts were evaluated for viability with bone scans 10 days postoperatively. Radiologic and clinical evaluations were performed on all patients. Time to union was recorded through evaluation of plain radiographs. Patients' charts were evaluated for postoperative complications.

RESULTS

There were two female and five male patients with an average age of 10.5 years. The average follow-up time was 36 months (range, up to 72 months). Limb salvage was 100 percent, with all bone scans positive at 10 days. Two nonunions at the allograft interface were treated successfully with a secondary bone graft. The average time to complete bony union of the fibula and allograft to the native bone was 9 months. There were no allograft fractures and no infections. One patient developed nonunion at the donor leg syndesmosis site. Average final knee motion was 110 degrees. All patients returned to ambulation.

CONCLUSION

Intramedullary free fibular flaps in combination with massive bony allografts provide an excellent option in the pediatric population for reconstruction of large bony defects after tumor extirpation.

Use of a Vascularized Fibula Bone Flap and Intercalary Allograft for Diaphyseal Reconstruction after Resection of Primary Extremity Bone Sarcomas

BACKGROUND

The standard treatment for primary bone sarcomas of the extremities has become chemotherapy and limb salvage surgery. However, the difficulties in achieving reliable long-term healing with allograft reconstruction have led us to use vascularized fibula transfer to enhance healing.

METHODS

From 1992 to 2003, 14 vascularized fibula transfers were performed at our institution for bone reconstruction in 12 patients with bone sarcoma. Free vascularized fibula transfers were performed in 13 cases, and a pedicled vascularized fibula transfer in one case. The mean age was 25 years (range, 6 to 71 years). Locations included the femur (n = 10), humerus (n = 1), and tibia (n = 3). The mean length of the vascularized fibula transfer was 17.4 cm (range, 10 to 24 cm). Indications for use of a vascularized fibula transfer included allograft nonunion (n = 8), and primary diaphyseal bone defect reconstruction combined with an intercalary allograft (n = 6). For all allograft nonunions, a vascularized fibula transfer was used with an onlay technique. For segmental bone defects, an intramedullary technique was used in three cases and an onlay technique in two cases.

RESULTS

The overall mean time for bone union after a vascularized fibula transfer was 8.6 months (range, 3 to 24 months): 10 months (range, 5 to 24 months) for patients with allograft nonunions, and 6 months (range, 3 to 8 months) for patients who underwent immediate segmental bone reconstruction. All but one patient had successful bone union. One patient with persistent nonunion required a second vascularized fibula transfer. The mean time from initial limb salvage surgery to full use of the reconstructed limb without restrictions was 28 months (range, 13 to 45 months) for patients treated with a delayed vascularized fibula transfer for an allograft nonunion and 6 months (range, 3 to 8 months) for patients who underwent immediate reconstruction with a vascularized fibula transfer combined with an allograft.

CONCLUSIONS

The use of a vascularized fibula transfer combined with an intercalary allograft to reconstruct bone defects after tumor resection can prevent allograft nonunion and result in decreased time to bone healing, leading to earlier patient recovery and return of function.

Prediction of fracture callus mechanical properties using micro-CT images and voxel-based finite element analysis

Assessment of fracture healing is a common problem in orthopaedic practice and research. To determine the effectiveness of certain treatments, drugs, mechanical loads, or rehabilitation regimes, the strength of the fracture callus must be determined. Both clinically and experimentally, there is a need to noninvasively and quantitatively evaluate fracture callus quality during healing. The objective of this study was to develop a method to assess fracture stiffness using micro-computed tomography (micro-CT) and finite element analysis. The method was developed and validated with plastic phantoms of various cross sections and known material properties, tested experimentally in four-point bending and torsion. The method was then applied to fractured rat femurs after 3 and 4 weeks of healing tested experimentally in torsion (50 femurs total). Micro-CT scans were made of the fracture calluses to determine three-dimensional geometry and material properties for the finite element models. Experimentally measured torsional rigidities were compared to finite element solutions. Finite element model predictions of callus rigidity correlated significantly better with experimental torsional rigidity than other common measures of healing progress such as callus area, bone mineral density, or area moment of inertia. Using FEA to predict mechanical properties of the callus could prove to be a useful tool in fracture-healing studies.

Kinematic study of a reconstructed hip in paediatric oncology

In 1997, a large portion of the femur of a four-year-old child affected by a Ewing's sarcoma was reconstructed with an innovative technique that used a massive bone allograft, in conjunction with a vascularised fibula autograft that was directly articulated within the acetabulum. The aim of the present study was to assess the kinematic behaviour of the reconstructed hip during flexion, once the acute remodelling process observed after the operation had ceased. A few additional CT slices of the hip joint region, in a flexed position, were taken at month 33 of the follow-up. The helical axes relative to the neutral-flexion motor action were estimated: their relative positions, with respect to the anatomical femoral heads, were compared, and the translation of the anatomical head centres was estimated. The angles spanned by the two femurs were almost equal, as were the translations along the respective helical axis. The main difference between the two femurs was the distance between the estimated femoral head centres and the relative helical axes. This resulted in a non-negligible translation of 2.9 mm of the fibula head inside the acetabulum during flexion, significantly higher than the 0.5 mm found for the intact contralateral femur. The results showed that, although the transplanted fibula grew and remodelled during the follow up, the action of the reconstructed hip joint still cannot be described as a ball-and-socket.