Prediction of mechanical properties of healing fractures using acoustic emission

Deterioration of apatite orientation in the cholecystokinin B receptor gene (Cckbr)-deficient mouse femurs

INTRODUCTION

The discrepancy between bone mineral density (BMD), the gold standard for bone assessment, and bone strength is a constraint in diagnosing bone function and determining treatment strategies for several bone diseases. Gastric hypochlorhydria induced by clinically used proton pump inhibitor (PPI) therapy indicates a discordance between changes in BMD and bone strength. Here, we used Cckbr-deficient mice with gastric hypochlorhydria to examine the effect of gastric hypochlorhydria on bone mass, BMD, and preferential orientation of the apatite crystallites, which is a strong indicator of bone strength.

MATERIALS AND METHODS

Cckbr-deficient mice were created, and their femurs were analyzed for BMD and preferential orientation of the apatite c-axis along the femoral long axis.

RESULTS

Cckbr-deficient mouse femurs displayed a slight osteoporotic bone loss at 18 weeks of age; however, BMD was comparable to that of wild-type mice. In contrast, apatite orientation in the femur mid-shaft significantly decreased from 9 to 18 weeks. To the best of our knowledge, this is the first report demonstrating the deterioration of apatite orientation in the bones of Cckbr-deficient mice.

CONCLUSION

Lesions in Cckbr-deficient mice occurred earlier in apatite orientation than in bone mass. Hence, bone apatite orientation may be a promising method for detecting hypochlorhydria-induced osteoporosis caused by PPI treatment and warrants urgent clinical applications.

Mechanics Predicts Effective Critical-Size Bone Regeneration Using 3D-Printed Bioceramic Scaffolds

BACKGROUND

3D-printed bioceramic scaffolds have gained popularity due to their controlled microarchitecture and their proven biocompatibility. However, their high brittleness makes their surgical implementation complex for weight-bearing bone treatments. Thus, they would require difficult-to-instrument rigid internal fixations that limit a rigorous evaluation of the regeneration progress through the analysis of mechanic-structural parameters.

METHODS

We investigated the compatibility of flexible fixations with fragile ceramic implants, and if mechanical monitoring techniques are applicable to bone tissue engineering applications. Tissue engineering experiments were performed on 8 ovine metatarsi. A 15 mm bone segment was directly replaced with a hydroxyapatite scaffold and stabilized by an instrumented Ilizarov-type external fixator. Several in vivo monitoring techniques were employed to assess the mechanical and structural progress of the tissue.

RESULTS

The applied surgical protocol succeeded in combining external fixators and subject-specific bioceramic scaffolds without causing fatal fractures of the implant due to stress concentrator. The bearing capacity of the treated limb was initially altered, quantifying a 28-56% reduction of the ground reaction force, which gradually normalized during the consolidation phase. A faster recovery was reported in the bearing capacity, stiffening and bone mineral density of the callus. It acquired a predominant mechanical role over the fixator in the distribution of internal forces after one post-surgical month.

CONCLUSION

The bioceramic scaffold significantly accelerated in vivo the bone formation compared to other traditional alternatives in the literature (e.g., distraction osteogenesis). In addition, the implemented assessment techniques allowed an accurate quantitative evaluation of the bone regeneration through mechanical and imaging parameters.

Outstanding in vivo mechanical integrity of additively manufactured spinal cages with a novel "honeycomb tree structure" design via guiding bone matrix orientation

BACKGROUND CONTEXT

Therapeutic devices for spinal disorders, such as spinal fusion cages, must be able to facilitate the maintenance and rapid recovery of spinal function. Therefore, it would be advantageous that future spinal fusion cages facilitate rapid recovery of spinal function without secondary surgery to harvest autologous bone.

PURPOSE

This study investigated a novel spinal cage configuration that achieves in vivo mechanical integrity as a devise/bone complex by inducing bone that mimicked the sound trabecular bone, hierarchically and anisotropically structured trabeculae strengthened with a preferentially oriented extracellular matrix.

STUDY DESIGN/SETTINGS

In vivo animal study.

METHODS

A cage possessing an anisotropic through-pore with a grooved substrate, that we termed "honeycomb tree structure," was designed for guiding bone matrix orientation; it was manufactured using a laser beam powder bed fusion method through an additive manufacturing processes. The newly designed cages were implanted into sheep vertebral bodies for 8 and 16 weeks. An autologous bone was not installed in the newly designed cage. A pull-out test was performed to evaluate the mechanical integrity of the cage/bone interface. Additionally, the preferential orientation of bone matrix consisting of collagen and apatite was determined.

RESULTS

The cage/host bone interface strength assessed by the maximum pull-out load for the novel cage without an autologous bone graft (3360±411 N) was significantly higher than that for the conventional cage using autologous bone (903±188 N) after only 8 weeks post-implantation.

CONCLUSIONS

These results highlight the potential of this novel cage to achieve functional fusion between the cage and host bone. Our study provides insight into the design of highly functional spinal devices based on the anisotropic nature of bone.

CLINICAL SIGNIFICANCE

The sheep spine is similar to the human spine in its stress condition and trabecular bone architecture and is widely recognized as a useful model for the human spine. The present design may be useful as a new spinal device for humans.

Men who stare at bone: multimodal monitoring of bone healing

Knowledge of the physiological and pathological processes, taking place in bone during fracture healing or defect regeneration, is essential in order to develop strategies to enhance bone healing under normal and critical conditions. Preclinical testing allows a wide range of imaging modalities that may be applied both simultaneously and longitudinally, which will in turn lower the number of animals needed to allow a comprehensive assessment of the healing process. This work provides an up-to-date review on morphological, functional, optical, biochemical, and biophysical imaging techniques including their advantages, disadvantages and potential for combining them in a multimodal and multiscale manner. The focus lies on preclinical testing of biomaterials modified with artificial extracellular matrices in various animal models to enhance bone remodeling and regeneration.

A predictive model for fracture in human ribs based on in vitro acoustic emission data

PURPOSE

The aim of this paper is to propose a fracture model for human ribs based on acoustic emission (AE) data. The accumulation of microcracking until a macroscopic crack is produced can be monitored by AE. The macrocrack propagation causes the loss of the structural integrity of the rib.

METHODS

The AE technique was used in in vitro bending tests of human ribs. The AE data obtained were used to construct a quantitative model that allows an estimation of the failure stress from the signals detected. The model predicts the ultimate stress with an error of less than 3.5% (even at stresses 15% lower than failure stress), which makes it possible to safely anticipate the failure of the rib.

RESULTS

The percolation theory was used to model crack propagation. Moreover, a quantitative probability-based model for the expected number of AE signals has been constructed, incorporating some ideas of percolation theory. The model predicts that AE signals associated with micro-failures should exhibit a vertical asymptote when stress increases. The occurrence of this vertical asymptote was attested in our experimental observations. The total number of microfailures detected prior to the failure is N ≈ 100 and the ultimate stress is σ ∞ = 197 ± 62 MPa. A significant correlation (p < 0.0001) between σ ∞ and the predicted value is found, using only the first N = 30 micro-failures (correlation improves for N higher).

CONCLUSIONS

The measurements and the shape of the curves predicted by the model fit well. In addition, the model parameters seem to explain quantitatively and qualitatively the distribution of the AE signals as the material approaches the macroscopic fracture. Moreover, some of these parameters correlate with anthropometric variables, such as age or Body Mass Index. The proposed model could be used to predict the structural failure of ribs subjected to bending.

Real-Time Wireless Platform for In Vivo Monitoring of Bone Regeneration

For the monitoring of bone regeneration processes, the instrumentation of the fixation is an increasingly common technique to indirectly measure the evolution of bone formation instead of ex vivo measurements or traditional in vivo techniques, such as X-ray or visual review. A versatile instrumented external fixator capable of adapting to multiple bone regeneration processes was designed, as well as a wireless acquisition system for the data collection. The design and implementation of the overall architecture of such a system is described in this work, including the hardware, firmware, and mechanical components. The measurements are conditioned and subsequently sent to a PC via wireless communication to be in vivo displayed and analyzed using a developed real-time monitoring application. Moreover, a model for the in vivo estimation of the bone callus stiffness from collected data was defined. This model was validated in vitro using elastic springs, reporting promising results with respect to previous equipment, with average errors and uncertainties below 6.7% and 14.04%. The devices were also validated in vivo performing a bone lengthening treatment on a sheep metatarsus. The resulting system allowed the in vivo mechanical characterization of the bone callus during experimentation, providing a low-cost, simple, and highly reliable solution.

Non-radiative healing assessment techniques for fractured long bones and osseointegrated implant

The paper provides an overview of the fracture healing process of long bones, a review of work that proposed appropriate physical parameters for the assessment of healing and highlights some recent work that reported on the development of non-radiative technique for healing assessment. An overview of the development and monitoring of osseointegration for trans-femoral osseointegrated implant is also presented. The state of healing of a fractured long bone and the stability of osseointegrated implants can be seen as engineering structural components where the mechanical properties are restored to facilitate their desired function. To this end, this paper describes non-radiative techniques that are useful for healing assessment and the stability assessment of osseointegrated implants. The achievement of non-radiative quantitative assessment methodologies to determine the state of healing of fractured long bones and to assess the stability of osseointegrated implant will shorten the patient's rehabilitation time, allowing earlier mobility and return to normal activities. Recent work on the development of assessment techniques supported by the Office of Naval Research as part of the Monitoring of Osseointegrated Implant Prosthesis program is highlighted.

Ultrasound Radiation Force for the Assessment of Bone Fracture Healing in Children: An In Vivo Pilot Study

Vibrational characteristics of bone are directly dependent on its physical properties. In this study, a vibrational method for bone evaluation is introduced. We propose a new type of quantitative vibro-acoustic method based on the acoustic radiation force of ultrasound for bone characterization in persons with fracture. Using this method, we excited the clavicle or ulna by an ultrasound radiation force pulse which induces vibrations in the bone, resulting in an acoustic wave that is measured by a hydrophone placed on the skin. The acoustic signals were used for wave velocity estimation based on a cross-correlation technique. To further separate different vibration characteristics, we adopted a variational mode decomposition technique to decompose the received signal into an ensemble of band-limited intrinsic mode functions, allowing analysis of the acoustic signals by their constitutive components. This prospective study included 15 patients: 12 with clavicle fractures and three with ulna fractures. Contralateral intact bones were used as controls. Statistical analysis demonstrated that fractured bones can be differentiated from intact ones with a detection probability of 80%. Additionally, we introduce a “healing factor” to quantify the bone healing progress which successfully tracked the progress of healing in 80% of the clavicle fractures in the study.

ASSESSMENT OF CRACK INITIATION AND PROPAGATION IN BONE USING ACOUSTIC EMISSION (AE) TECHNIQUES

Acoustic emission (AE) is a nondestructive testing (NDT) technique used for detecting damages, cracks, and leaks in different structures such as metals, composites, wood, fiberglass, ceramics, plastics, etc. In recent years, AE has gained popularity within the field of biomedical applications. The structure of bone is similar to composite materials, therefore, it is advantageous to use NDT technique. Thus, it can be used for monitoring the fracture behavior, crack initiation/propagation, and fatigue detection in bones. The goal of this study was to determine the usefulness of AE techniques in fracture detection phase of bones and to develop an NDT methodology for the monitoring of crack initiation and propagation in bones. This study describes AE activity during fracture of bone tissue under tensile loads. The experiments were carried out in vitro techniques using intact and fracture-simulated bovine tibias. The specimens were loaded to failure in tension using a mechanical testing machine. During the mechanical tests, AE signals were measured and recorded by using AE system processor equipped with two wideband piezoelectric sensors fixed to the surfaces of both ends of the test specimens. By superposing the load–time curve and the cumulative AE event–time curve, AE activities of crack initiation and propagation were identified. In all experiments, the cumulative AE number for each period of time rose up exponentially with the incremental tensile load. Load for AE initiation demonstrated a convincing linear interaction with AE event generation.

Co-deteriorations of anisotropic extracellular matrix arrangement and intrinsic mechanical property in c-src deficient osteopetrotic mouse femur

Osteopetrotic bone shows dissociation between bone mineral density (BMD) and bone strength. In this study, volumetric BMD; preferential orientation of the extracellular matrix (ECM), which is composed of collagen fibers and apatite crystals as bone material quality; and mechanical properties of the src^-/- osteopetrotic and normal mouse femoral cortical bone were analyzed and compared with each other at a bone tissue level. The degree of preferential orientation of ECM along the femoral long axis was significantly decreased in the src^-/- mice femur, suggesting deteriorated bone quality. Young's modulus, as a tissue-level mechanical property analyzed by nano-indentation technique along the long bone direction, also was decreased in the src^-/- mice cortical femur, in spite of the similar volumetric cortical BMD. To the best of our knowledge, this is the first report to demonstrate the synchronous deterioration of Young's modulus and anisotropic ECM organization in the src^-/- osteopetrotic mouse bone. These results indicate that the deterioration of the preferential ECM orientation is one major cause of the impaired mechanical property in the src^-/- mouse bone.

A paradigm shift for bone quality in dentistry: A literature review

PURPOSE

The aim of this study was to present the current concept of bone quality based on the proposal by the National Institutes of Health (NIH) and some of the cellular and molecular factors that affect bone quality.

STUDY SELECTION

This is a literature review which focuses on collagen, biological apatite (BAp), and bone cells such as osteoblasts and osteocytes.

RESULTS

In dentistry, the term "bone quality" has long been considered to be synonymous with bone mineral density (BMD) based on radiographic and sensible evaluations. In 2000, the NIH proposed the concept of bone quality as "the sum of all characteristics of bone that influence the bone's resistance to fracture," which is completely independent of BMD. The NIH defines bone quality as comprising bone architecture, bone turnover, bone mineralization, and micro-damage accumulation. Moreover, our investigations have demonstrated that BAp, collagen, and bone cells such as osteoblasts and osteocytes play essential roles in controlling the current concept of bone quality in bone around hip and dental implants.

CONCLUSION

The current concept of bone quality is crucial for understanding bone mechanical functions. BAp, collagen and osteocytes are the main factors affecting bone quality. Moreover, mechanical loading dynamically adapts bone quality. Understanding the current concept of bone quality is required in dentistry.

Avalanche criticality during compression of porcine cortical bone of different ages

Crack events developed during uniaxial compression of cortical bones cut from femurs of developing pigs of several ages (4, 12, and 20 weeks) generate avalanches. These avalanches have been investigated by acoustic emission analysis techniques. The avalanche energies are power-law distributed over more than four decades. Such behavior indicates the absence of characteristic scales and suggests avalanche criticality. The statistical distributions of energies and waiting times depend on the pig age and indicate that bones become stronger, but less ductile, with increasing age. Crack propagation is equally age-dependent. Older pigs show, on average, larger cracks with a time distribution similar to those of aftershocks in earthquakes, while younger pigs show only statistically independent failure events.

Electrical impedance spectroscopy - a potential method for the study and monitoring of a bone critical-size defect healing process treated with bone tissue engineering and regenerative medicine approaches

The development of strategies of bone tissue engineering and regenerative medicine has been drawing considerable attention to treat bone critical-size defects (CSDs). Notably, new strategies and/or treatment approaches always require appropriate tools to track the healing process so as to evaluate their success. In this paper, we present the development of a novel approach for the non-invasive, yet real-time, monitoring and assessment of bone CSDs treated with biomaterials and biomedical approaches. For this, we employed the technique of electrical impedance spectroscopy (EIS) to quantitatively monitor and assess the changes in electrical impedance, and thus the regeneration process. In our in vitro tests, we examined the biochemical changes of the fracture area and investigated the influence of collagen and hydroxyapatite on the changes in electrical impedance by EIS, thus inferring the changes in bone regeneration and structure. Based on this success, we further demonstrated, in real time, the process of regeneration of the traumatic area in an in vivo rabbit model. Our electrical-impedance data of the experiment groups, i.e., the ones treated with natural coral and bone morphogenetic protein-2 (BMP-2), revealed that each group has its unique impedance graph characteristics, which are directly associated with the degree of regeneration. For comparison, we also employed radiography, gross anatomy, and histological analyses in examination. Our results illustrate that EIS holds considerable potential as a non-invasive tool for monitoring, in real time, the healing of bone CSDs by allowing for quantitatively characterizing the changes of both hydroxyapatite and collagen.

Biomechanical Characteristics of Osteoporotic Fracture Healing in Ovariectomized Rats: A Systematic Review

Biomechanical tests are widely used in animal studies on osteoporotic fracture healing. However, the biomechanical recovery process is still unknown, leading to difficulty in choosing time points for biomechanical tests and in correctly assessing osteoporotic fracture healing. To determine the biomechanical recovery process during osteoporotic fracture healing, studies on osteoporotic femur fracture healing with biomechanical tests in ovariectomized rat (OVX) models were collected from PUBMED, EMBASE, and Chinese databases. Quadratic curves of fracture healing time and maximum load were fitted with data from the analyzed studies. In the fitted curve for normal fractures, the predicted maximum load was 145.56 N, and the fracture healing time was 88.0 d. In the fitted curve for osteoporotic fractures, the predicted maximum load was 122.30 N, and the fracture healing time was 95.2 d. The maximum load of fractured femurs in OVX rats was also lower than that in sham rats at day 84 post-fracture (D84 PF). The fracture healing time was prolonged and maximum load at D84 PF decreased in OVX rats with closed fractures. The maximum load of Wister rats was higher than that of Sprague-Dawley (SD) rats, but the fracture healing time of SD and Wister rats was similar. Osteoporotic fracture healing was delayed in rats that were < = 12 weeks old when ovariectomized, and at D84 PF, the maximum load of rats < = 12 weeks old at ovariectomy was lower than that of rats >12 weeks old at ovariectomy. There was no significant difference in maximum load at D84 PF between rats with an osteoporosis modeling time <12 weeks and > = 12 weeks. In conclusion, fracture healing was delayed and biomechanical property decreased by osteoporosis. Time points around D95.2 PF should be considered for biomechanical tests of osteoporotic femur fracture healing in OVX rat models. Osteoporotic fracture healing in OVX rats was affected by the fracture type but not by the strain of the rat.

In vivo standardization of bone ultrasonometry of the clavicle

OBJECTIVE

The assessment of fracture union includes physical examination and radiographic imaging, which depend on the examiner's experience. The development of ancillary methods may avoid prolonged treatments and the improper removal of implants. Quantitative bone ultrasonometry has been studied for this purpose and will soon be included in clinical practice. The aims of the present study were to assess the feasibility of using this technique on the clavicle and to standardize its in vivo application.

METHODS

Twenty adult volunteers, including 10 men and 10 women without medical conditions or a previous history of clavicle fracture, underwent axial quantitative ultrasonometric assessment using transducers in various positions (different distances between the transducers and different angulations relative to the clavicle).

RESULTS

Similar values of wave propagation velocity were obtained in the different tested set-ups, which included distinct distances between the transducers and angular positions relative to the clavicle. There were significant differences only in the transducers positioned at 0° and at 5 or 7 cm apart.

CONCLUSIONS

The use of bone ultrasonometry on the clavicle is feasible and the standardization of the technique proposed in this study (transducers placed at 45° and at 7 cm apart) will allow its future application in clinical trials to evaluate the healing process of diaphyseal fractures of the clavicle.

Utilization of bone impedance for age estimation in postmortem cases

In the field of Forensic Medicine the number of unidentified cadavers has increased due to natural disasters and international terrorism. The age estimation is very important for identification of the victims. The degree of sagittal closure is one of such age estimation methods. However it is not widely accepted as a reliable method for age estimation. In this study, we have examined whether measuring impedance value (z-values) of the sagittal suture of the skull is related to the age in men and women and discussed the possibility to use bone impedance for age estimation. Bone impedance values increased with aging and decreased after the age of 64.5. Then we compared age estimation through the conventional visual method and the proposed bone impedance measurement technique. It is suggested that the bone impedance measuring technique may be of value to forensic science as a method of age estimation.

Functional restoration and risk of non-union of the first metatarsocuneiform arthrodesis for hallux valgus: A finite element approach

First metatarsocuneiform arthrodesis is one of the surgical interventions to correct hallux valgus, especially those with hypermobile first ray. There is lacking of biomechanical investigations to assess this operation. The objective of this study was to explore the functional restoration and the risk of non-union after the surgery via finite element analysis. A three-dimensional foot model was constructed from a female aged 28 via magnetic resonance imaging. Thirty bones and encapsulated bulk tissue were modeled. Walking stance was simulated by the gait analysis data of the same participant. Parts of the first metatarsal and cuneiform were resected and the bone graft was assigned with the same stiffness as adjacent bones to resemble the surgery of first metatarsocuneiform arthrodesis. The third principal stress of the first metatarsal at midstance (25% stance) and push off (60% stance) was increased by 76% and 139% respectively after the operation, while that of the second metatarsal was decreased by 14% and 66%. The operation reduced the medial deviation of the first metatarsal head by about 3.5mm during initial push off (60% stance). Besides, the bone graft could experience tensile stress inferiorly (26.51MPa). In conclusion, the increase of stress on the first metatarsal and the reduced medial excursion of the first metatarsal head after the simulated operation reflected that metatarsocuneiform arthrodesis could restore the load-bearing function of the first ray. However, inter-fragmentary compression could not be guaranteed. The appropriate course of hardware and non-weight-bearing protocol should be noted and further investigated.

Degree of biological apatite c-axis orientation rather than bone mineral density controls mechanical function in bone regenerated using recombinant bone morphogenetic protein-2

The aim of the present study was to assess the bone regeneration process in defects introduced into rabbit long bones, which were regenerated with controlled release of recombinant bone morphogenetic protein-2 (rBMP-2). The orientation of the biological apatite (BAp) c-axis and bone mineral density (BMD) were compared as predictors of bone mechanical function. A 20-mm-long defect was introduced in rabbit ulnas, and 17 µg of rBMP-2 was controlled-released into the defect using a biodegradable gelatin hydrogel as the carrier. In the bone regeneration process, two characteristic phases may have been governed by different factors. First, new bone formation actively occurred, filling the bone defect with newly formed bone tissue and increasing the BMD. This process was regulated by the strong osteoinductive capacity of rBMP-2. Second, after filling of the defect and moderate BMD restoration, preferential BAp c-axis orientation began to increase, coincident with initiation of remodeling. In addition, the BAp c-axis orientation, rather than BMD, was strongly correlated with Young's modulus, an important index of bone mechanical function, particularly in the later stage of bone regeneration. Thus, preferential BAp c-axis orientation is a strong determinant and predictor of the mechanical function of tissue-engineered bone. Therefore, analysis of BAp preferential c-axis orientation in addition to measurement of BMD is crucial in assessment of bone mechanical function.

Measurement of mechanical properties with respect to gap healing in a rabbit osteotomy model using echo tracking

The most important issue in the assessment of fracture healing is to acquire information about the restoration of the mechanical integrity of bone. Echo tracking (ET) can noninvasively measure the displacement of a certain point on the bone surface under a load. Echo tracking has been used to assess the bone deformation angle of the fracture healing site. Although this method can be used to evaluate bending stiffness, previous studies have not validated the accuracy of bending stiffness. The purpose of the present study is to ensure the accuracy of bending stiffness as measured by ET. A four-point bending test of the gap-healing model in rabbit tibiae was performed to measure bending stiffness. Echo tracking probes were used to measure stiffness, and the results were compared with results of stiffness measurements performed using laser displacement gauges. The relationship between the stiffness measured by these two devices was completely linear, indicating that the ET method could precisely measure bone stiffness.

Measurement of mechanical properties on gap healing in a rabbit osteotomy model until the remodeling stage

BACKGROUND

The most important issue in the assessment of fracture healing is to acquire information about the restoration of the mechanical integrity of bone. Many researchers have attempted to monitor stiffness either directly or indirectly for the purpose of assessing strength, as strength has been impossible to assess directly in clinical practice. The purpose of this study was thus to determine the relationship between bending stiffness and strength using mechanical testing at different times during the healing process.

METHODS

Unilateral, transverse, mid-tibial osteotomies with a 2-mm gap were performed in 28 rabbits. The osteotomy site was stabilized using a double-bar external fixator. The animals were divided into four groups (n=7/group/time point; 4, 6, 8 and 12 weeks). A series of images from micro-computed tomography of the gap was evaluated to detect the stage of fracture healing and a 4-point bending test was performed to measure stiffness and strength. Relative stiffness and strength values were also acquired from calculation of ratios relative to those of the non-osteotomized contralateral bones.

FINDINGS

Formation of cortex and medullary canal at the gap was seen in the 12-week group and would represent the remodeling stage. In addition, the relationship between stiffness and strength remained almost linear until at least 12 weeks. However, stiffness recovered much more rapidly than strength.

INTERPRETATION

Strength was not fully restored until the later stages of fracture healing. However, the current study demonstrated that stiffness could be monitored as a surrogate marker of strength until at least the remodeling stage.

Biomechanical evaluation of regenerating long bone by nanoindentation

It is crucial to measure the mechanical function of regenerating bone in order to assess the mechanical performance of the regenerating portion as well as the efficiency of the regeneration methods. In this study, nanoindentation was applied to regenerating and intact rabbit ulnae to determine the material properties of hardness and elasticity; viscoelasticity was also investigated to precisely evaluate the material properties. Both intact and regenerating bones exhibited remarkable viscoelasticity manifested as a creep behavior during load hold at the maximum load, and the creep was significantly greater in the regenerating bone than the intact bone. The creep resulted in an overestimation of the hardness and Young's modulus. Hence, during nanoindentation testing of bones, the effect of creep should be eliminated. Moreover, the regenerating bone had lower hardness and Young's modulus than the intact bone. The nanoindentation technique proved to be a powerful approach for understanding the mechanical properties of regenerating bone.

Ability and limitation of radiographic assessment of fracture healing in rats

The purpose of the present study was to assess whether clinicians are actually able to evaluate the mechanical status of fracture healing from radiograms. Fifteen orthopaedic surgeons evaluated the radiograms of experimentally produced femur fractures in rats and predicted mechanical strength (%) of the affected side compared to the unaffected control side. Following this, actual mechanical strength of the affected and control side was determined by a three-point bending test. The median of the strength in the transverse fracture model predicted from radiograms was 33% (2 weeks), 72% (4 weeks), 88% (6 weeks), 84% (8 weeks), and 89% (12 weeks). The actual measured recovery ratio of mechanical strength (exp/control x 100) was 36%, 76%, 93%, 89%, and 106% in each observation period respectively. The tendency was almost the same in a comminuted fracture model. The mean recovery rate determined by interpretation of the surgeons correlated linearly to the actual measured mechanical strength determined by mechanical testing (R(2): 0.80 in transverse fracture, 0.60 in comminuted fracture). Clinicians demonstrated that a comparatively good evaluation of the mechanical status of fracture healing is possible from radiograms up to approximately 80% recovery. However, they tended to make less accurate, weaker assessments at the final stages. In conclusion, radiograms may be inadequate for evaluation of fracture healing completion.

Monitoring the mechanical properties of healing bone

Fracture healing is normally assessed through an interpretation of radiographs, clinical evaluation, including pain on weight bearing, and a manual assessment of the mobility of the fracture. These assessments are subjective and their accuracy in determining when a fracture has healed has been questioned. Viewed in mechanical terms, fracture healing represents a steady increase in strength and stiffness of a broken bone and it is only when these values are sufficiently high to support unrestricted weight bearing that a fracture can be said to be healed. Information on the rate of increase of the mechanical properties of a healing bone is therefore valuable in determining both the rate at which a fracture will heal and in helping to define an objective and measurable endpoint of healing. A number of techniques have been developed to quantify bone healing in mechanical terms and these are described and discussed in detail. Clinical studies, in which measurements of fracture stiffness have been used to identify a quantifiable end point of healing, compare different treatment methods, predictably determine whether a fracture will heal, and identify factors which most influence healing, are reviewed and discussed.

Measurement of bone electrical impedance in fracture healing

BACKGROUND

Although external fixation is widely used for fractures and limb lengthening, evaluation of the time for removing the external fixator is dependent upon radiographic examinations and clinical findings, and a useful method has yet to be established clinically. This study aimed to measure the bone electrical impedance (Z values) non-invasively by using external fixation pins as electrodes, and clarify the relationship with bone union.

METHODS

Thirty rabbits received the external fixation at the right tibia and were assigned to a control group (group C; n = 5) and a fractured group (group F; n = 25). Z values were measured once a week following surgery. The animals of group F were assigned to 5 groups (weeks 2, 3, 4, 5, and 6 after osteotomy, each n = 5). The resistivity (rho) of the electrical property between electrodes was measured prior to euthanasia, and fracture cross-sectional area (FrA) of the conduction pathway and maximum bending stress (Bmax) were measured following excision of the tibia.

RESULTS

Although Z values in group F increased through 5 weeks after surgery, Z values in group C remained constant at 3 weeks, and significant differences were observed between groups at 4, 5, and 6 weeks. The rho values and FrA in group F decreased through 5 weeks; while Bmax increased, reaching a plateau at 5 weeks.

CONCLUSIONS

Narrowing of conduction pathway due to the decrease in the contour of fracture area accompanying bone remodeling resulted in an increase of Z values. Both Z values and Bmax in group F reached a peak at 5 weeks, this was believed to be the optimal time for removal of external fixation. These results suggest that measurement of Z values makes it possible to evaluate bone union.

A new method for evaluation of fracture healing by echo tracking

Assessment of bone healing on radiographs depends on the volume and radio-opacity of callus at the healing site, but is not necessarily objective, and there are differences of judgment among observers. To overcome this disadvantage, a clinical system was developed to quantify the stiffness of healing fractures of the tibia in patients by the echo tracking (ET) method in a manner similar to a three-point bending test. The purpose of this study was to ensure that the ET system could clinically assess the progress, delay or arrest of healing. The fibular head and the lateral malleolus were supported. A 7.5-MHz ultrasound probe was placed on the proximal and distal fragments and a load of 25 N was applied. Five tracking points were set along the long axis of the ultrasound probe at intervals of 10 mm. With a multiple ET system, two probes measured the displacement of five tracking points on each of the proximal and distal fragments of the tibia, thereby detecting the bending of the two fragments generated by the load. ET angle was defined as the sum of the inclinations of the proximal and distal fragments. Eight tibial fractures in seven patients treated by a cast or internal fixation were measured over time. In patients with radiographically normal healing, the bending angle decreased exponentially over time. However, in patients with nonunion, the angle remained the same over time. It was demonstrated that the ET method could be clinically applicable to evaluate fracture healing as a versatile, quantitative and noninvasive technique.

Ultrasonic monitoring of bone fracture healing

Quantitative ultrasound has attracted significant interest in the evaluation of bone fracture healing. Animal and clinical studies have demonstrated that the propagation velocity across fractured bones can be used as an indicator of healing. Researchers have recently employed computational methods for modeling wave propagation in bones, aiming to gain insight into the underlying mechanisms of wave propagation and to further enhance the monitoring capabilities of ultrasound. In this paper, we review the relevant literature and present the current status of knowledge.

Biomechanical monitoring of healing bone based on acoustic emission technology

Acoustic emission testing is a well-established method for assessment of the mechanical integrity of general construction projects. The purpose of the current study was to investigate the usefulness of acoustic emission technology in monitoring the yield strength of healing callus during external fixation. Thirty-five patients with 39 long bones treated with external fixation were evaluated for fracture healing by monitoring load for the initiation of acoustic emission signal (yield strength) under axial loading. The major criteria for functional bone union based on acoustic emission testing were (1) no acoustic emission signal on full weightbearing, and (2) a higher estimated strength than body weight. The yield strength monitored by acoustic emission testing increased with the time of healing. The external fixator could be removed safely and successfully in 97% of the patients. Thus, the acoustic emission method has good potential as a reliable method for monitoring the mechanical status of healing bone.