Tensile Yield Strain of Human Cortical Bone from the Femoral Diaphysis Is Constant among Healthy Adults and across the Anatomical Quadrants

The literature suggests that the yield strain of cortical bone is invariant to its stiffness (elastic modulus) and strength (yield stress). However, data about intra-individual variations, e.g., the influence of different collagen/mineral organisations observed in bone aspects withstanding different habitual loads, are lacking. The hypothesis that the yield strain of human cortical bone tissue, retrieved from femoral diaphyseal quadrants subjected to different habitual loads, is invariant was tested. Four flat dumbbell-shaped specimens were machined from each quadrant of the proximal femoral diaphysis of five adult donors for a total of 80 specimens. Two extensometers attached to the narrow specimen region were used to measure deformation during monotonic tensile testing. The elastic modulus (linear part of the stress-strain curve) and yield strain/stress at a 0.2% offset were obtained. Elastic modulus and yield stress values were, respectively, in the range of 12.2-20.5 GPa and 75.9-136.6 MPa and exhibited a positive linear correlation. All yield strain values were in the narrow range of 0.77-0.87%, regardless of the stiffness and strength of the tissue and the anatomical quadrant. In summary, the results corroborate the hypothesis that tensile yield strain in cortical bone is invariant, irrespective also of the anatomical quadrant. The mean yield strain value found in this study is similar to what was reported by inter-species and evolution studies but slightly higher than previous reports in humans, possibly because of the younger age of our subjects. Further investigations are needed to elucidate a possible dependence of yield strain on age.

Biomechanics of the Human Osteochondral Unit: A Systematic Review

The damping system ensured by the osteochondral (OC) unit is essential to deploy the forces generated within load-bearing joints during locomotion, allowing furthermore low-friction sliding motion between bone segments. The OC unit is a multi-layer structure including articular cartilage, as well as subchondral and trabecular bone. The interplay between the OC tissues is essential in maintaining the joint functionality; altered loading patterns can trigger biological processes that could lead to degenerative joint diseases like osteoarthritis. Currently, no effective treatments are available to avoid degeneration beyond tissues' recovery capabilities. A thorough comprehension on the mechanical behaviour of the OC unit is essential to (i) soundly elucidate its overall response to intra-articular loads for developing diagnostic tools capable of detecting non-physiological strain levels, (ii) properly evaluate the efficacy of innovative treatments in restoring physiological strain levels, and (iii) optimize regenerative medicine approaches as potential and less-invasive alternatives to arthroplasty when irreversible damage has occurred. Therefore, the leading aim of this review was to provide an overview of the state-of-the-art-up to 2022-about the mechanical behaviour of the OC unit. A systematic search is performed, according to PRISMA standards, by focusing on studies that experimentally assess the human lower-limb joints' OC tissues. A multi-criteria decision-making method is proposed to quantitatively evaluate eligible studies, in order to highlight only the insights retrieved through sound and robust approaches. This review revealed that studies on human lower limbs are focusing on the knee and articular cartilage, while hip and trabecular bone studies are declining, and the ankle and subchondral bone are poorly investigated. Compression and indentation are the most common experimental techniques studying the mechanical behaviour of the OC tissues, with indentation also being able to provide information at the micro- and nanoscales. While a certain comparability among studies was highlighted, none of the identified testing protocols are currently recognised as standard for any of the OC tissues. The fibril-network-reinforced poro-viscoelastic constitutive model has become common for describing the response of the articular cartilage, while the models describing the mechanical behaviour of mineralised tissues are usually simpler (i.e., linear elastic, elasto-plastic). Most advanced studies have tested and modelled multiple tissues of the same OC unit but have done so individually rather than through integrated approaches. Therefore, efforts should be made in simultaneously evaluating the comprehensive response of the OC unit to intra-articular loads and the interplay between the OC tissues. In this regard, a multidisciplinary approach combining complementary techniques, e.g., full-field imaging, mechanical testing, and computational approaches, should be implemented and validated. Furthermore, the next challenge entails transferring this assessment to a non-invasive approach, allowing its application in vivo, in order to increase its diagnostic and prognostic potential.

Applying a homogeneous pressure distribution to the upper vertebral endplate: Validation of a new loading system, pilot application to human vertebral bodies, and finite element predictions of DIC measured displacements and strains

Image-based personalized Finite Element Models (pFEM) could detect alterations in physiological deformation of human vertebral bodies, but their accuracy has been seldom reported. Meaningful validation experiments should allow vertebral endplate deformability and ensure well-controlled boundary conditions. This study aimed to (i) validate a new loading system to apply a homogeneous pressure on the vertebral endplate during vertebral body compression regardless of endplate deformation; (ii) perform a pilot study on human vertebral bodies measuring surface displacements and strains with Digital Image Correlation (DIC); (iii) determine the accuracy of pFEM of the vertebral bodies. Homogeneous pressure application was achieved by pressurizing a fluid silicone encased in a rubber silicone film acting on the cranial endplate. The loading system was validated by comparing DIC-measured longitudinal strains and lower-end contact pressures, measured on three homogeneous pseudovertebrae of constant transversal section at 2.0 kN, against theoretically calculated values. Longitudinal strains and contact pressures were rather homogeneous, and their mean values close to theoretical calculations (5% underestimation). DIC measurements of surface longitudinal and circumferential displacements and strains were obtained on three human vertebral bodies at 2.0 kN. Complete displacement and strain maps were achieved for anterolateral aspects with random errors ≤0.2 μm and ≤30 μstrain, respectively. Venous plexus and double curvatures limited the completeness and accuracy of DIC data in posterior aspects. pFEM of vertebral bodies, including cortical bone mapping, were built from computed tomography images. In anterolateral aspects, pFEM accuracy of the three vertebrae was: (i) comparable to literature in terms of longitudinal displacements (R2>0.8); (ii) extended to circumferential displacements (pooled data: R2>0.9) and longitudinal strains (zero median error, 95% error: <27%). Circumferential strains were overestimated (median error: 39%). The new methods presented may permit to study how physiological and pathologic conditions influence the ability of vertebral endplates/bodies to sustain loads.

Surface Analysis of Ti-Alloy Micro-Grooved 12/14 Tapers Assembled to Non-Sleeved and Sleeved Ceramic Heads: A Comparative Study of Retrieved Hip Prostheses

Ti6Al4V titanium alloy (Ti-alloy) sleeved ceramic heads have become widely used in revision surgery when the hip stem is left in situ. This solution guarantees a new junction between the bore of the ceramic head and the Ti-alloy sleeve, regardless of any possible, slight surface damage to the Ti-alloy taper of the stem. However, this solution introduces an additional Ti-alloy/Ti-alloy interface pairing, which is potentially susceptible to mechanically assisted crevice corrosion. This study evaluated both qualitatively and quantitatively the damage that occurred in vivo on Ti-alloy micro-grooved 12/14 tapers of (i) primary implants with non-sleeved ceramic heads (Group 1), (ii) secondary implants with non-sleeved ceramic heads (Group 2), and (iii) secondary implants with sleeved ceramic heads (Group 3). A total of 45 explants-15 for each group, including short-, medium- and long-neck heads-underwent optical evaluation for surface damage (Goldberg scoring), surface roughness analysis, and SEM/EDX analysis. The Goldberg scores did not reveal different patterns in the tapers' surface damage; surface damage was classified as absent or mild (surface damage score ≤2) in 94%, another 94%, and 92% of the analysed regions for Group 1, Group 2, and Group 3, respectively. Small but significant differences in morphological changes occurred in the tapers of the three groups: reductions no greater than a few percentage points in median values of roughness parameters were found in Group 1 and Group 2, while negligible changes were found in Group 3. SEM/EDX analysis revealed little (i.e., a slight increase in the oxygen content) to undetectable changes in the chemical composition on the Ti-alloy surface independently of the group. These results suggest that the Ti-alloy/Ti-alloy sleeve/taper junction is only mildly susceptible to mechanically assisted crevice corrosion. Assembling a sleeved ceramic head, with variable neck lengths up to a "long-neck", to a Ti-alloy micro-grooved 12/14 taper of a stem left in situ does not seem to increase the risk of revision due to trunnionosis, as long as junction stability (i.e., the proper seating of the sleeved ceramic head on the 12/14 taper) is achieved intraoperatively.

Optimization of In Situ Indentation Protocol to Map the Mechanical Properties of Articular Cartilage

Tissue engineering aims at developing complex composite scaffolds for articular cartilage repair. These scaffolds must exhibit a mechanical behavior similar to the whole osteochondral unit. In situ spherical indentation allows us to map the mechanical behavior of articular cartilage, avoiding removal of the underlying bone tissue. Little is known about the impact of grid spacing, indenter diameter, and induced deformation on the cartilage response to indentation. We investigated the impact of grid spacing (range: a to 3a, where a is the radius of the contact area between cartilage and indenter), indenter diameter (range: 1 to 8 mm), and deformation induced by indentation (constant indentation depth versus constant nominal deformation) on cartilage response. The bias induced by indentations performed in adjacent grid points was minimized with a 3a grid spacing. The cartilage response was indenter-dependent for diameters ranging between 1 and 6 mm with a nominal deformation of 15%. No significant differences were found using 6 mm and 8 mm indenters. Six mm and 8 mm indenters were used to map human articular cartilage with a grid spacing equal to 3a. Instantaneous elastic modulus E₀ was calculated for constant indentation depth and constant nominal deformation. E₀ value distribution did not change significantly by switching the two indenters, while dispersion decreased by 5-6% when a constant nominal deformation was applied. Such an approach was able to discriminate changes in tissue response due to doubling the indentation rate. The proposed procedure seems to reduce data dispersion and properly determine cartilage mechanical properties to be compared with those of complex composite scaffolds.

A novel apparatus to assess the mechanical properties of Ankle-Foot Orthoses: Stiffness analysis of the Codivilla spring

Ankle-Foot Orthoses (AFOs) are the most common devices prescribed to support the ankle and restore a quasi-normal gait pattern in drop-foot patients. AFO stiffness is possibly the main mechanical property affecting foot and ankle biomechanics. A variety of methods to evaluate this property have been reported, however no standard procedure has been validated and widely used. This study is reporting the repeatability of a novel apparatus to measure AFO stiffness in ideal frictionless conditions. The apparatus is based on a servo-hydraulic testing machine and allows to apply a displacement-controlled rotation of the AFO shell, simulating the physiological ankle dorsi/plantarflexion movement. The repeatability of the apparatus in measuring AFO stiffness in dorsiflexion and plantarflexion was assessed intra- and inter-session in a sample of standard polypropylene AFOs of different sizes (Codivilla spring). The repeatability of the apparatus in measuring the AFO stiffness was high. The Intra- and Inter-session Coefficient of Variation ranged between 0.02 ÷ 1.3 % and 1.3 ÷ 5 %, respectively. The Intra Class Correlation Coefficient ranged between 0.999 ÷ 1 intra- and 0.993 ÷ 0.997 inter-session. AFOs stiffness was observed to increase with the AFO size. The setup is easy to replicate and can be implemented with any torsion-controlled servo-hydraulic testing machine and has resulted simple to use and flexible enough to adapt to AFOs with different sizes. The frictionless contacts characterizing the apparatus make it possible to measure the ideal AFO stiffness by excluding the effect of the fixation methods to the leg and help to improve the repeatability of measurements.

A case-driven hypothesis for multi-stage crack growth mechanism in fourth-generation ceramic head fracture

BACKGROUND

Ceramic bearings are used in total hip arthroplasty due to their excellent wear behaviour and biocompatibility. The major concern related to their use is material brittleness, which significantly impacts on the risk of fracture of ceramic components. Fracture toughness improvement has contributed to the decrease in fracture rate, at least of the prosthetic head. However, the root cause behind these rare events is not fully understood. This study evaluated head fracture occurrence in a sizeable cohort of patients with fourth-generation ceramic-on-ceramic implants and described the circumstances reported by patients in the rare cases of head fracture.

METHODS

The clinical survivorship of 29,495 hip prostheses, with fourth-generation ceramic bearings, was determined using data from a joint replacement registry. The average follow-up period was 5.2 years (range 0.1-15.6). Retrieval analysis was performed in one case for which the ceramic components were available.

RESULTS

Clinical outcomes confirmed the extremely low fracture rate of fourth-generation ceramic heads: only two out of 29,495 heads fractured. The two fractures, both involving 36 mm heads, occurred without a concurrent or previous remarkable trauma. Considering the feature of the fractured head, a multi-stage crack growth mechanism has been hypothesized to occur following damage at the head-neck taper interface.

CONCLUSIONS

Surgeons must continue to pay attention to the assembly of the femoral head: achieving a proper head seating on a clean taper is a prerequisite to decrease the risk of occurrence of any damage process within head-neck junction, which may cause high stress concentration at the contact surface, promoting crack nucleation and propagation even in toughened ceramics.

Intercalary reconstruction of long bones by massive allograft: Comparison of construct stability ensured by three different host-graft junctions and two types of fixations in a synthetic femur model

An intercalary segmental allograft is an option for limb salvage in bone tumours. Stable and congruent intercalary reconstructions are a prerequisite for achieving host-graft union. However, a too rigid fixation could increase the risk of late complications correlated with negative bone remodelling. This study compared the reconstruction stiffness achieved by three different host-graft junctions, namely, end-to-end, modified step-cut, and taper. A low-stiffness bone plate was used as the fixation method, except for the taper junction where a low-stiffness intramedullary nail was also used to investigate the effects of different types of fixation on construct stiffness. Composite femora were tested under four loading conditions to determine coronal and sagittal bending stiffness, as well as torsional stiffness in opposite directions. Stiffness values were expressed as a percentage of intact host bone stiffness (%IBS). While a reduction of coronal bending stiffness was found with taper junctions (76%IBS) compared with the high values ensured by end-to-end (96%IBS) and modified step-cut junctions (92%IBS), taper junctions significantly increased stiffness under sagittal bending and torsion in intra- and extra-direction: end-to-end 29%IBS, 7%IBS, 7%IBS, modified step-cut 38%IBS, 20%IBS, 21%IBS, and taper junction 52%IBS, 55%IBS, 56%IBS, respectively. Construct stiffness with taper junctions was decreased by 11-41%IBS by replacing the bone plate with an intramedullary nail. Taper junctions can be an alternative to achieve intercalary reconstructions with more homogeneous and, in three out of four loading conditions, significantly higher construct stability without increasing bone plate stiffness. The risk of instability under high torsional loads increases when taper junctions are associated with a low-stiffness intramedullary nail.

An experimental procedure to perform mechanical characterization of small-sized bone specimens from thin femoral cortical wall

The cortical shell of the femoral neck plays a role in determining the overall neck strength. However, there is a lack of knowledge about the mechanical properties of cortical tissue of the femoral neck due to challenges in implementing accurate testing protocols for the thin shell. Indeed, mechanical properties are commonly derived from mechanical testing performed on tissue samples extracted from the femoral diaphysis, i.e. assuming tissue homogeneity along the femur. The aim of this work was to set up a reliable methodology to determine mechanical properties of bone samples extracted from thin cortical shell of the femoral neck. A three-point bending test was used to determine elastic and post-elastic properties of cortical bone samples extracted from the inferior and superior femoral neck. An optical system was used to monitor the sample deflection. Accuracy was preliminarily evaluated by determining the elastic modulus of an aluminium alloy. A good intra- and inter-sample variability was found on determining aluminium elastic modulus: 1.6% and 3.6%, respectively. Additionally, aluminium elastic modulus value was underestimated by less than 1%. A pilot trial was performed on a human femoral neck to assess the procedure feasibility. A total of 22 samples were extracted from the inferior and superior femoral neck and successfully tested. Preliminary results suggest that mechanical properties of cortical bone tissue extracted from human femoral neck might be side dependent, the superior tissue seems to exhibit better mechanical properties than the inferior one, at least in terms of yield stress and maximum strain. This supposedly different mechanical competence must be further investigated. The proposed procedure makes it feasible to carry out such studies.

Long term breeding of the Lmna G609G progeric mouse: Characterization of homozygous and heterozygous models

The transgenic Lmna^G609G progeric mouse represents an outstanding animal model for studying the human Hutchinson-Gilford Progeria Syndrome (HGPS) caused by a mutation in the LMNA gene, coding for the nuclear envelope protein Lamin A/C, and, as an important, more general scope, for studying the complex process governing physiological aging in humans. Here we give a comprehensive description of the peculiarities related to the breeding of Lmna^G609G mice over a prolonged period of time, and of many features observed in a large colony for a 2-years period. We describe the breeding and housing conditions underlining the possible interference of the genetic background on the phenotype expression. This information represents a useful tool when planning and interpreting studies on the Lmna^G609G mouse model, complementing any specific data already reported in the literature about this model since its production. It is also particularly relevant for the heterozygous mouse, which mirrors the genotype of the human pathology however requires an extended time to manifest symptoms and to be carefully studied.

Nonunion fracture healing: Evaluation of effectiveness of demineralized bone matrix and mesenchymal stem cells in a novel sheep bone nonunion model

Nonunion treatment has a high rate of success, although recalcitrant nonunion may determine the need for amputation. Therefore, new treatment options are continuously investigated in order to further reduce the risk of nonunion recurrence. This study aimed to (a) develop a new large animal model for bone atrophic nonunion and (b) compare the efficacy of demineralized bone matrix (DBM) and DBM in combination with mesenchymal stem cells (MSC) in the new nonunion model. The new model consists of a noncritical, full-thickness segmental defect created in the sheep tibia, stabilized by an intramedullary nail, and involves the creation of a locally impaired blood supply achieved through periosteum excision and electrocauterization of the stump ends. Six weeks after defect creation, lack of hard tissue callus and established nonunion was observed in all operated tibiae both by radiographic and clinical evaluation. Nonunion was treated with allogeneic DBM or autologous MSC cultivated on DBM particles (DBM + MSC) for 1 day before implantation. Twelve weeks after treatment, radiographic, microtomographic, histologic, and histomorphometric analysis showed the formation of bone callus in DBM group, whereas the fracture healing appeared at an early stage in DBM + MSC group. Torsional strength and stiffness of the DBM group appeared higher than those of DBM + MSC group, although the differences were not statistically significant. In conclusion, a new sheep bone nonunion model resembling the complexity of the clinical condition was developed. DBM is an effective option for nonunion treatment, whereas MSC do not improve the healing process when cultivated on DBM particles before implantation.

Experimentally Achievable Accuracy Using a Digital Image Correlation Technique in measuring Small-Magnitude (<0.1%) Homogeneous Strain Fields

Measuring small-magnitude strain fields using a digital image correlation (DIC) technique is challenging, due to the noise-signal ratio in strain maps. Here, we determined the level of accuracy achievable in measuring small-magnitude (<0.1%) homogeneous strain fields. We investigated different sets of parameters for image processing and imaging pre-selection, based on single-image noise level. The trueness of DIC was assessed by comparison of Young’s modulus (E) and Poisson’s ratio (ν) with values obtained from strain gauge measurements. Repeatability was improved, on average, by 20–25% with experimentally-determined optimal parameters and image pre-selection. Despite this, the intra- and inter-specimen repeatability of strain gauge measurements was 5 and 2.5 times better than DIC, respectively. Moreover, although trueness was also improved, on average, by 30–45%, DIC consistently overestimated the two material parameters by 1.8% and 3.2% for E and ν, respectively. DIC is a suitable option to measure small-magnitude homogeneous strain fields, bearing in mind the limitations in achievable accuracy.

Suturing the myotendinous junction in total hip arthroplasty: A biomechanical comparison of different stitching techniques

BACKGROUND

The repair of the myotendinous junction following total hip arthroplasty is challenging as this region is the weakest part of the muscle structure. This study investigated the mechanical behaviour and the mode of failure of different suturing techniques of the myotendinous junction. A new asymmetrical stitch was compared to two widely used techniques, i.e. the simple stitch (two loops in parallel) and the figure-of-eight stitch.

METHODS

The ovine triceps brachii myotendinous junction was selected as the experimental model. Each technique was sewn in muscle belly on one side and in a polyester belt (no-tendon configuration) or in thin tendon (full configuration) on the other side. The former was chosen to determine the grasping power of the stitch on the muscle despite the tendon quality, the latter to simulate a very thin gluteus medius tendon.

FINDINGS

The new stitch showed a higher ultimate strength (+40%) compared to the two controls in the no-tendon configuration. In the full configuration, no significant increase was observed, although failure of the new stitch always occurred at the tendon side. Furthermore, the new stitch does not alter the stiffness of repair.

INTERPRETATION

The new stitch has a higher grasping power on muscle belly than the single passing-through stitches thanks to the multiple fixation points, which better distribute the load in the tissue. However, such performance can be fully exploited only in the presence of good quality tendons.

Suprapectoral biceps tenodesis: a biomechanical comparison of a new "soft anchor" tenodesis technique versus interference screw biceps tendon fixation

BACKGROUND

The interference screw technique is commonly used in tenodesis of the long head of the biceps for its well-documented robust fixation strength. Some complications may occur after tenodesis with interference screw such as persistent pain, bone fracture and cyst formation. A new technique using a small "soft anchor" has been proposed to avoid the risk of occurrence of the above-mentioned complications associated with the use of the interference screw. However, the proposed technique must provide adequate fixation strength. This study investigated the mechanical performance of the new technique and compared it with interference screw fixation.

METHODS

Fourteen human humeri and proximal biceps were tested after tenodesis using the two techniques. The fixation constructs were cycled 500 times between 20N and 100N at 1Hz to simulate some level of post-operative physical activity. Then, a tensile test to failure was performed to determine the strength of the two tenodesis constructs.

FINDINGS

The ultimate strength was 238N (SD 96N) and 172N (SD 58N) for the "soft anchor" and the interference screw, respectively (P=0.14). In two out of seven repetitions in both groups, failure occurred at low load level due to inaccuracies in performing tenodesis. Considering these cases as outliers, the strength values increased up to 290N (SD 40N) and 202N (SD 32N) for the "soft anchor" and the interference screw, respectively (P=0.02).

INTERPRETATION

The "soft anchor" technique provides a fixation strength comparable with the interference screw, but without using a screw. It could be considered as an alternative for suprapectoral biceps tenodesis.

Human bone hardness seems to depend on tissue type but not on anatomical site in the long bones of an old subject

It has been hypothesised that among different human subjects, the bone tissue quality varies as a function of the bone segment morphology. The aim of this study was to assess and compare the quality, evaluated in terms of hardness of packages of lamellae, of cortical and trabecular bones, at different anatomical sites within the human skeleton. The contralateral six long bones of an old human subject were indented at different levels along the diaphysis and at both epiphyses of each bone. Hardness value, which is correlated to the degree of mineralisation, of both cortical and trabecular bone tissues was calculated for each indentation location. It was found that the cortical bone tissue was harder (+18%) than the trabecular one. In general, the bone hardness was found to be locally highly heterogeneous. In fact, considering one single slice obtained for a bone segment, the coefficient of variation of the hardness values was up to 12% for cortical bone and up to 17% for trabecular bone. However, the tissue hardness was on average quite homogeneous within and among the long bones of the studied donor, although differences up to 9% among levels and up to 7% among bone segments were found. These findings seem not to support the mentioned hypothesis, at least not for the long bones of an old subject.

Compressive behaviour of child and adult cortical bone

In this study, cortical bone tissue from children was investigated. It is extremely difficult to obtain human child tissue. Therefore, the only possibility was to use bone tissue, free from any lesion, collected from young bone cancer patients. The compressive mechanical behaviour of child bone tissue was compared to the behaviour of adult tissue. Moreover, two hypotheses were tested: 1) that the mechanical behaviour of both groups is correlated to ash density; 2) that yield strain is an invariant. Small parts of the diaphysis of femora or tibiae from 12 children (4-15 years) and 12 adults (22-61 years) were collected. Cylindrical specimens were extracted from the cortical wall along the longitudinal axis of the diaphysis. A total of 107 specimens underwent compressive testing (strain rate: 0.1 s(-1)). Only the specimens showing a regular load-displacement curve (94) were considered valid and thereafter reduced to ash. It was found that the child bone tissue had significant lower compressive Young's modulus (-34%), yield stress (-38%), ultimate stress (-33%) and ash density (-17%) than the adult tissue. Conversely, higher compressive ultimate strain was found in the child group (+24%). Despite specimens extracted from both children and adults, ash density largely described the variation in tissue strength and stiffness (R(2)=in the range of 0.86-0.91). Furthermore, yield strain seemed to be roughly an invariant to subject age and tissue density. These results confirm that the mechanical properties of child cortical bone tissue are different from that of adult tissue. However, such differences are correlated to differences in tissue ash density. In fact, ash density was found to be a good predictor of strength and stiffness, also for cortical bone collected from children. Finally, the present findings support the hypothesis that compressive yield strain is an invariant.

The effect of vacuum mixing and pre-heating the femoral component on the mechanical properties of the cement mantle

We investigated the effect of pre-heating a femoral component on the porosity and strength of bone cement, with or without vacuum mixing used for total hip replacement. Cement mantles were moulded in a manner simulating clinical practice for cemented hip replacement. During polymerisation, the temperature was monitored. Specimens of cement extracted from the mantles underwent bending or fatigue tests, and were examined for porosity. Pre-heating the stem alone significantly increased the mean temperature values measured within the mantle (+14.2 degrees C) (p < 0.001) and reduced the mean curing time (-1.5 min) (p < 0.001). The addition of vacuum mixing modulated the mean rise in the temperature of polymerisation to 11 degrees C and reduced the mean duration of the process by one minute and 50 seconds (p = 0.01 and p < 0.001, respectively). In all cases, the maximum temperature values measured in the mould simulating the femur were < 50 degrees C. The mixing technique and pre-heating the stem slightly increased the static mechanical strength of bone cement. However, the fatigue life of the cement was improved by both vacuum mixing and pre-heating the stem, but was most marked (+ 280 degrees C) when these methods were combined. Pre-heating the stem appears to be an effective way of improving the quality of the cement mantle, which might enhance the long-term performance of bone cement, especially when combined with vacuum mixing.

Anisotropy and inhomogeneity of the trabecular structure can describe the mechanical strength of osteoarthritic cancellous bone

Osteoarthritic cancellous bone was studied to investigate the development of this pathology, and the functional changes it induces in the bone. In order to predict how the morphological alterations of the tissue induced by the pathology can change the mechanical properties of the structure, two different strategies have been used in the literature: (1) emphasising the influence of structural anisotropy; (2) stressing the highly inhomogeneous characteristics of cancellous bone. The aim of the present study was to verify the theory that mechanical strength of osteoarthritic cancellous bone depends both on tissue anisotropy and inhomogeneity. Twenty-five specimens were extracted from osteoarthritic femoral heads, along selected directions, and analysed by means of a microtomograph. The same specimens were mechanically tested in compression to determine the mechanical strength. The most representative structural parameters, confirmed by a stepwise analysis, were used to define four models to describe the measured mechanical strength. The models were applied neglecting (global analysis) or considering (local analysis) tissue inhomogeneities to verify whether the correlation with ultimate stress could be improved. The coefficient of determination increased from 0.53, considering only bone volume fraction, up to 0.88, combining it with off-axis angle and normalised eigenvalue. A further improvement was found performing a local analysis (R(2)=0.90), which corresponded to a decrease of 17% in the residual error. The proposed approach of considering both tissue anisotropy and inhomogeneity improved the accuracy in predicting the mechanical behaviour of cancellous bone tissue and should be suitable for more general loading conditions.

Osteon classification in human fibular shaft by circularly polarized light

Pattern of osteonal classification referred to collagen fiber orientation in a human fibula was studied. Along the length of the whole bone, serial 100-microm parallel cross sections were obtained and analyzed by circularly polarized light microscopy. The distribution of 3 different kinds of osteons (transversal, alternated and longitudinal-hooped) depending on their collagen fiber orientation were analyzed to determine the effect of length, anatomical quadrant and cortical thickness. Referring to length, the 3 types of osteons appeared with approximately the same relative frequency, but when the symmetry of the fibula was considered and the length was expressed as distance from the bone mid-section, the alternated osteons are predominant in the mid-section. On the contrary, moving toward the epiphyses, the balance is inverted, and transversal together with longitudinal-hooped types become more frequent than the alternated type. Osteonal pattern distribution was not significantly affected by the anatomical quadrant of each section or by the thickness of the cortex. This observation seems to confirm that along the fibula shaft the orientation of collagen fibers within the osteons is consistent with the loading conditions which the bone undergoes.

Microindentation on cortical human bone: Effects of tissue condition and indentation location on hardness values

The hardness of cortical human bone has been measured on osteons in different conditions. However, no data are reported in the literature regarding the effect of cortical tissue condition and indentation location on the measured hardness values. This study aimed to investigate whether the hardness of the human cortical bone evaluated by micro-indentation is influenced, first, by the tissue condition and, second, by the distance of the indentation from the edge of the Haversian canal. Two femura were collected from a subject without musculoskeletal disease. The Vickers hardness was measured by means of microindentation (applied load, 100 gf) on osteons with a cross-section greater than 200 μm. The tests were performed on wet and embedded tissue at different distances from the Haversian canal edge (30—150 μm). No significant differences were found in hardness values between the two contralateral femura. Embedded tissue was significantly harder (12 per cent) than wet tissue. No significant differences were found in hardness values measured at different distances from the Haversian canal edge except for those closer than 60 μm. Therefore, indentations cannot be performed on osteons small in cross-section, since the distance from the closer pore has to be controlled. They should be performed on wet tissue, to avoid an offset in the measured hardness.

Effect of stem preheating on the fatigue behaviour of bone cement around hip prostheses

Tensile fatigue behaviour of bone cement specimens obtained from cement mantles moulded in vitro, simulating the surgical scenario, was investigated. The effect of stem preheating, before its insertion into the cement dough, on specimen fatigue life was studied. A commercial bone cement was selected for this study. Bone cement mixing was conducted in air, following the manufacturer's instructions, and injected simulating the clinical practice. Two conditions were considered: stem maintained at the surgical room temperature (23 degrees C), and stem preheated to 45 degrees C. Four repetitions of the whole procedure were performed for each condition obtaining a total of 32 specimens. All specimens underwent fatigue testing (stress ratio, 0; maximum tensile stress, 15 MPa) until failure. Both two-parameter and three-parameter Weibull distributions were initially used to analyse the fatigue life data set. However, the two-parameter distribution was chosen for both groups on the basis of the coefficient of determination used to test the goodness of fit. Stem preheating seems to have a negligible effect on fatigue behaviour of the studied bone cement in the low range of fatigue lives (up to 10(4)). However, above this number of cycles, stem preheating seems to reduce the probability of failure. These findings are discussed in the text.

Repeatability of experimental procedures to determine mechanical behaviour of ligaments

The aim of this study was to investigate intra- and interspecimen repeatability of an experimental procedure, which determines elastic and viscoelastic properties of knee ligaments. The collateral ligaments from sheep were used and the repeatability was evaluated in terms of the coefficient of variation. The results indicated a good intraspecimen repeatability (the coefficient of variation generally less than 5%), whereas the interspecimen repeatability was lower (coefficient of variation of about 50%). In conclusion, since the intraspecimen coefficient of variation was low the test procedure was assumed to be repeatable.

The effects of embalming using a 4% formalin solution on the compressive mechanical properties of human cortical bone

BACKGROUND

The use of formalin fixed bone tissue is often avoided because of its assumed influence on the mechanical properties of bone. Fixed bone tissue would minimise biological risks and eliminate preservation issues for long duration experimental tests. This study aimed to determine the short- and long-term effects of embalming, using a solution with 4% formalin concentration, on the mechanical properties of human cortical bone.

METHODS

Three-millimetre cylindrical specimens of human cortical bone were extracted from two femoral diaphyses and divided in four groups. The first group was used as control, the remaining three groups were left in the embalming solution for 48 h, 4 week, and 8 week, respectively. Compressive mechanical properties, hardness and ash density were assessed. The last was used to check the homogeneity among the four groups.

FINDINGS

No significant differences were found among the four groups in yield stress, ultimate stress and hardness. The specimens stored for 8 week in the embalming solution had significant lower Young's modulus (-24%), higher yield strain (+20%) and ultimate strain (+53%) compared to the other groups.

INTERPRETATION

On a short-term perspective, embalming did not affect the compressive mechanical properties, nor hardness of human cortical bone, whereas a long-term preservation (8 week) did significantly affect Young's modulus, yield strain and ultimate strain in compression. Preserving bone segments for up to 4 week in an embalming solution with low formalin concentration seems to be an interesting alternative when collecting and/or managing fresh or fresh-frozen bone segments for biomechanical experiments is not possible.

Biological and biomechanical effects of vancomycin and meropenem in acrylic bone cement

Antibiotic-loaded bone cement is extensively used in joint arthroplasty, but increasing bacteria resistance against common antibiotics has lead to a demand for alternative drugs. However, bone cement containing new additives must be characterized both biologically and mechanically. This study evaluated elution kinetics, antibacterial activity, and mechanical properties for cement loaded with vancomycin and/or meropenem. The presence of meropenem broadened the antibacterial spectrum and enhanced the elution of vancomycin. The mechanical properties were negatively affected by 1.0 g of vancomycin, but these detrimental effects were acceptable when only 0.5 g of vancomycin were added to a cement containing 0.5 g of meropenem. Further investigations on this formulation with adjusted antibiotic amounts are, however, necessary to reach the optimal compromise between the antibacterial and the mechanical properties of the bone cement.

Multiscale investigation of the functional properties of the human femur

The mechanical strength of human bones has often been investigated in the past. Bone failure is related to musculoskeletal loading, tissue properties, bone metabolism, etc. This is intrinsically a multiscale problem. However, organ-level performance in most cases is investigated as a separate problem, incorporating only part (if any) of the information available at a higher scale (body level) or at a lower one (tissue level, cell level). A multiscale approach is proposed, where models available at different levels are integrated. A middle-out strategy is taken: the main model to be investigated is at the organ level. The organ-level model incorporates as an input the outputs from the body-level (musculoskeletal loads), tissue-level (constitutive equations) and cell-level models (bone remodelling). In this paper, this approach is exemplified by a clinically relevant application: fractures of the proximal femur. We report how a finite-element model of the femur (organ level) becomes part of a multiscale model. A significant effort is related to model validation: a number of experiments were designed to quantify the model's sensitivity and accuracy. When possible, the clinical accuracy and the clinical impact of a model should be assessed. Whereas a large amount of information is available at all scales, only organ-level models are really mature in this perspective. More work is needed in the future to integrate all levels fully, while following a sound scientific method to assess the relevance and validity of such an integrated model.

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.

An accurate estimation of bone density improves the accuracy of subject-specific finite element models

An experimental-numerical study was performed to investigate the relationships between computed tomography (CT)-density and ash density, and between ash density and apparent density for bone tissue, to evaluate their influence on the accuracy of subject-specific FE models of human bones. Sixty cylindrical bone specimens were examined. CT-densities were computed from CT images while apparent and ash densities were measured experimentally. The CT/ash-density and ash/apparent-density relationships were calculated. Finite element models of eight human femurs were generated considering these relationships to assess their effect on strain prediction accuracy. CT and ash density were linearly correlated (R(2)=0.997) over the whole density range but not equivalent (intercep t <0, slope >1). A constant ash/apparent-density ratio (0.598+/-0.004) was found for cortical bone. A lower ratio, with a larger dispersion, was found for trabecular bone (0.459+/-0.100), but it became less dispersed, and equal to that of cortical tissue, when testing smaller trabecular specimens (0.598+/-0.036). This suggests that an experimental error occurred in apparent-density measurements for large trabecular specimens and a constant ratio can be assumed valid for the whole density range. Introducing the obtained relationships in the FE modelling procedure improved strain prediction accuracy (R(2)=0.95, RMSE=7%). The results suggest that: (i) a correction of the densitometric calibration should be used when evaluating bone ash-density from clinical CT scans, to avoid ash-density underestimation and overestimation for low- and high-density bone tissue, respectively; (ii) the ash/apparent-density ratio can be assumed constant in human femurs and (iii) the correction improves significantly the model accuracy and should be considered in subject-specific bone modelling.

Fracture properties of an acrylic bone cement

This study investigated experimentally the fracture properties, i.e., the fatigue strength, the resistance to crack propagation and the fracture toughness, of an acrylic bone cement (Cemex RX). The mean endurance limit was determined following the staircase method. The endurance limit was estimated at 9.2 MPa. The fatigue crack propagation rate was measured according to the ASTM E647 standard. The equation of the line fitting the crack growth per cycle (da/dN) versus the stress-intensity factor range (delta K), in a log-log graph, was used to calculate the empirical constants of Paris' law for the selected bone cement: da/dN (m/cycle) = 3.56 x 10(-7) x delta K (MPa x m1/2)5.79. This power-law relationship described well (R2 = 0.96) the growth rate in the stable crack growth region, i.e., in the mid delta K range. The fracture toughness K(IC) of the bone cement was determined according to the ASTM E399 standard. The K(IC) mean value was 1.38 MPa x m1/2. These experimental results provide the set of necessary inputs for numerical studies aimed to investigate the damage accumulation process in the mantle fixing cemented prostheses.

Structural parameters and mechanical strength of cancellous bone in the femoral head in osteoarthritis do not depend on age

For normal bone, aging has been associated with a decrease of both density and failure strength, and with the development of pathologies such as osteoporosis. Conversely, it has been reported that another common disease, osteoarthritis, may alter these age-related changes in cancellous bone, suggesting that it may have a protective role against osteoporosis and the correspondent fracture risk. It was reported that in the principal compressive region of the femoral head in osteoarthritis the bone density does not depend on age. However, it is not clear if this independence on age of the cancellous bone density corresponds also to a reduced dependence on age of the strength to failure. The present work examined cancellous bone from the principal compressive region of the femoral head of 37 patients having severe osteoarthritis. The aim was (1) to investigate the dependence on age of both the structural parameters and the ultimate stress and (2) to investigate the relationships between the ultimate stress and the structural parameters. Using X-ray microcomputed tomography, three-dimensional structural parameters, such as bone volume fraction, direct trabecular thickness and structure model index were calculated. Then the specimens were compressed to failure to determine the ultimate stress. It was found that none of the investigated structural parameters did depend on age, and also the ultimate stress did not depend on age (p>0.05 for all regressions on age). In addition, the ultimate stress was significantly correlated with the structural parameters, primary with the minimum bone volume fraction and the average bone volume fraction (R(2)=0.95 and R(2)=0.84, respectively). These findings show that severe osteoarthritis or a related factor may change the age dependences of both the structural parameters and the mechanical properties usually reported for normal cancellous bone. These results suggest for this pathology to have a protective role against the age-related decrease in density, the age-related deterioration of the microarchitecture and the age-related decrease of the failure strength for the cancellous bone in the principal compressive region of the human femoral head.

Dependence of mechanical compressive strength on local variations in microarchitecture in cancellous bone of proximal human femur

Human cancellous bone is a heterogeneous material. Despite this, most of the published studies report correlations between mechanical properties and morphometric parameters averaged on the whole specimen. This work investigated whether local variations in morphometric parameters were linked to the localized failure regions of cancellous bone. Additionally, it was examined whether local values of morphometric parameters can predict the ultimate stress better than the average bone volume fraction (BV/TV). Cylindrical cancellous bone specimens extracted along the primary compressive group of human femoral heads were studied. These were microCT-imaged to assess the morphometric parameters, compressed to determine the ultimate stress, and rescanned by microCT to visualize the failure region. Failure involved slightly less than half of the free height of the specimens. Significant differences were found in the morphometric parameters calculated in the failure and in the non-failure regions. The cross-sections containing minimum BV/TV values were those most often located inside the failure region (83%, p<0.001). Regression analysis confirmed that variations in BV/TV best describe variations in ultimate stress (R2=0.84) out of the averaged morphometric parameters. The prediction of ultimate stress increased when minimum or maximum values of the morphometric parameters were taken, with the highest prediction found by considering the minimum BV/TV (R2=0.95). In conclusion, due to the heterogeneity of cancellous bone, there may exist regions characterized by a different microarchitecture, where the bone is weaker and consequently is more likely to fail. These regions mostly contain minimum values in BV/TV, which were found to predict ultimate stress better than average BV/TV.

Mechanical testing of cancellous bone from the femoral head: experimental errors due to off-axis measurements

The aim of this study was to verify whether a misalignment between the testing direction and the trabecular main direction has a significant effect on the compressive behaviour of cancellous bone. Ten cylindrical specimens were extracted from femoral heads with a misalignment to the trabecular main direction of approximately 20 degrees. Each specimen was paired with a specimen extracted aligned with the main direction of the trabeculae on the basis of the closest bone volume fraction, obtaining two groups, one 'aligned' and one 'misaligned'. The average off-axis angle was 6.1 degrees and 21.6 degrees for the 'aligned' and 'misaligned' group, respectively. The specimens underwent micro-tomographic analysis, compressive testing, micro-indentation testing and ashing. No significant differences were found in histomorphometric parameters, hardness and ash density between the two groups, whereas significant differences were found in Young's modulus and ultimate stress: both parameters, measured for the 'misaligned' group, were about 40% lower than those measured for the 'aligned' group. These results demonstrate a great effect of the angle between the testing direction and the main direction of the trabecular structure on the compressive behaviour of cancellous bone. This angle should be reduced as much as possible (in the present work the average value was 6.6+/-3.3 degrees), in any case measured, and always reported together with the mechanical parameters of cancellous bone.

The effect of tissue condition and applied load on Vickers hardness of human trabecular bone

Hardness of trabecular human bone, evaluated by microindentation testing, has generally been measured on embedded tissues. It was known that this was not ideal but it had been preferred to other conditions (e.g. wet or dehydrated) as the trabeculae could withstand the applied load and the measurements were reliable. The aim of this study was to investigate if the tissue condition of the specimen and the applied load would alter the hardness values measured by Vickers microindentation. Vickers hardness values of human trabecular bone from the femoral head, prepared in three different ways (wet, dry and embedded) and tested with two different loads (50 and 25 gf), were measured. No significant difference was found between the two different loads. However, in several cases the 50 gf indentations had to be redone because they were too large or the trabecula broke locally. Even if the outlines of the indentations on wet bone were slightly less marked than the ones done on dehydrated or embedded bone, it was possible to measure the hardness. Significant differences of Vickers hardness values were found between the three preparations: the hardness increased passing from wet to dried (10%) and from wet to embedded (35%). Whereas the variation coefficient of the three tissue conditions were comparable. In conclusion, it is recommended to test human trabecular bone in a wet condition as it better represents the in vivo condition. Furthermore the use of a 25 gf load is suggested, allowing hardness measurements on almost all trabeculae without breaking them.

Radiopacity of tantalum-loaded acrylic bone cement

Radiopacifying agents are commonly added to bone cements to enhance the visibility of the cement in radiography. The radiopacifiers usually employed may, however, have undesired effects on the mechanical properties of the cement. A potentially new radiopacifier is tantalum, which in the present work was evaluated in terms of radiopacity. Bone cements containing different percentages of tantalum were compared with plain bone cement as well as with formulations containing different percentages of the commonly used radiopacifier barium sulphate. The radiopacity was assessed quantitatively and qualitatively, by measuring with a digital densitometer the optical density of the cement on X-ray films, and consulting the expertise of ten orthopaedic surgeons. It was found that tantalum does present radiopacity, but not as high as barium sulphate under the specific conditions applied to this study.