Biomechanical Strains Research Articles

Effect of passive shoulder exoskeleton support during working with arms over shoulder level.

Musculoskeletal disorders have the highest prevalence of work-related health problems. Due to the aging population, the prevalence of shoulder pain in workers in physically demanding occupations is increasing, thereby causing rising costs to society and underlining the need for preventive technologies. Wearable support structures are designed to reduce the physical work load during physically demanding tasks. Here, we evaluate the physiological benefit of the DeltaSuit, a novel passive shoulder exoskeleton, using an assessment framework that conforms to the approach proposed in the literature. In this study, 32 healthy volunteers performed isometric, quasi-isometric, and dynamic tasks that represent typical overhead work to evaluate the DeltaSuit performance. Muscle activity of the arm, neck, shoulder, and back muscles, as well as cardiac cost, perceived exertion, and task-related discomfort during task execution with and without the exoskeleton were compared. When working with the DeltaSuit, muscle activity was reduced up to 56% (p<0.001) in the Trapezius Descendens and up to 64% (p<0.001) in the Deltoideusmedius. Furthermore, we observed no additional loading on the abdomen and back muscles. The use of the exoskeleton resulted in statistically significant reductions in cardiac cost (15%, p<0.05), perceived exertion (21.5%, p<0.001), and task-related discomfort in the shoulder (57%, p<0.001). These results suggest that passive exoskeletons, such as the DeltaSuit, have the potential to meaningfully support users when performing tasks in overhead postures and offer a valuable solution to relieve the critical body parts of biomechanical strains for workers at high risk of musculoskeletal disorders.

Effects of in vivo fatigue-induced microdamage on local subchondral bone strains

Biomechanical strain is a major stimulus of subchondral bone (SCB) tissue adaptation in joints but may also lead to initiation and propagation of microcracks, highlighting the importance of quantifying the intratissue strain in subchondral bone. In the present study, we used micro computed tomography (μCT) imaging, mechanical testing, and digital image correlation (DIC) techniques to evaluate the biomechanical strains in equine SCB under impact compression applied through the articular surface. We aimed to investigate the effects of in vivo accumulated microdamage in equine SCB on the distribution of mechanical impact strain through the articular cartilage. Under the applied strain of 2.0 ± 0.1% (mean ± standard deviation, n=15) to the articular surface of cartilage-bone plugs, the overall thickness of the SCB developed eSCBOverall = 0.7 ± 0.2% in all specimens. Contours of high strains in specimens without microdamage (NDmg) aligned parallel to the cartilage-bone interface with peak tensile, ϵt, and compressive, ϵc, strains of 0.5 ± 0.3% and 1.2 ± 0.4%, respectively at the time of peak compression (n=7). In damaged specimens (Dmg), contours of high strains aligned with the cracks in the imaged plane with peak strains of ϵt= 1.2 ± 0.8% and ϵc= 3.5 ± 2.2%, respectively (n=7). Microdamage was the main predictor of the normalised compressive and tensile strains across the SCB thickness. Results of multivariable analyses revealed presence of microdamage, distance from the articular surface and TMD were the main predictors of normalised compressive and tensile strain. Strain was greater in the superficial bone, particularly for specimens with microdamage. In vivo fatigue-induced microdamage is an important predictor of local subchondral bone strains.

The Strain Response to Intraocular Pressure Decrease in the Lamina Cribrosa of Patients with Glaucoma

To measure biomechanical strains in the lamina cribrosa (LC) of living human eyes with intraocular pressure (IOP) lowering. Cohort study. Patients with glaucoma underwent imaging before and after laser suturelysis after trabeculectomy surgery (29 image pairs; 26 persons). Noninvasive imaging of the eye. Strains in optic nerve head tissue and changes in depths of the anterior border of the LC. Intraocular pressure decreases caused the LC to expand in thickness in the anterior-posterior strain (Ezz= 0.94 ± 1.2%; P= 0.00020) and contract in radius in the radial strain (Err=- 0.19 ± 0.33%; P= 0.0043). The mean LC depth did not significantly change with IOP lowering (1.33 ± 6.26 μm; P= 0.26). A larger IOP decrease produced a larger, more tensile Ezz (P < 0.0001), greater maximum principal strain (Emax; P < 0.0001), and greater maximum shear strain (Γmax; P < 0.0001). The average LC depth change was associated with the Γmax and radial-circumferential shear strain (Erθ; P < 0.02) but was not significantly related to tensile or compressive strains. An analysis by clock hour showed that in temporal clock hours 3 to 6, a more anterior LC movement was associated with a more positive Emax, and in clock hours 3, 5, and 6, it was associated with a more positive Γmax. At 10 o'clock, a more posterior LC movement was related to a more positive Emax (P < 0.004). Greater compliance (strain/ΔIOP) of Emax (P= 0.044), Γmax (P= 0.052), and Erθ (P= 0.018) was associated with a thinner retinal nerve fiber layer. Greater compliance of Emax (P= 0.041), Γmax (P= 0.021), Erθ (P= 0.024), and in-plane shear strain (Erz; P= 0.0069) was associated with more negative mean deviations. Greater compliance of Γmax (P= 0.055), Erθ (P= 0.040), and Erz (P= 0.015) was associated with lower visual field indices. With IOP lowering, the LC moves either into or out of the eye but, on average, expands in thickness and contracts in radius. Shear strains are nearly as substantial as in-plane strains. Biomechanical strains are more compliant in eyes with greater glaucoma damage. This work was registered at ClinicalTrials.gov as NCT03267849.

A Potential Role of Acute Choroidal Expansion in Nonarteritic Anterior Ischemic Optic Neuropathy.

PurposeNonarteritic anterior ischemic optic neuropathy (NAION) has been associated with a thickened choroid at the optic nerve head (ONH). Here, we use computational modeling to better understand how choroidal expansion and choroidal geometry influence tissue deformation within the ONH relative to intraocular pressure (IOP) and intracranial pressure (ICP) effects.MethodsUsing a model of the posterior eye that included the sclera, peripapillary sclera, annular ring, pia mater, dura mater, neural tissues, Bruch's membrane, choroid, and lamina cribrosa, we examined how varying material properties of ocular tissues influenced ONH deformations under physiological and supra-physiological, or “pathological,” conditions. We considered choroidal expansion (c. 35 µL of expansion), elevated IOP (30 mm Hg), and elevated ICP (20 mm Hg), and calculated peak strains in the ONH relative to a baseline condition representing an individual in the upright position.ResultsSupra-physiological choroidal expansion had the largest impact on strains in the prelaminar neural tissue. In addition, compared to a tapered choroid, a “blunt” choroid insertion at the ONH resulted in higher strains. Elevated IOP and ICP caused the highest strains within the lamina cribrosa and retrolaminar neural tissue, respectively.ConclusionsAcute choroidal expansion caused large deformations of the ONH and these deformations were impacted by choroid geometry. These results are consistent with the concept that compartment syndrome due to the choroid geometry and/or expansion at the ONH contributes to NAION. Prolonged deformations due to supra-physiological loading may induce a mechanobiological response or ischemia, highlighting the potential impact of choroidal expansion on biomechanical strains in the ONH.

Ocular biomechanics during improvised explosive device blast: A computational study using eye-specific models

BackgroundEye injuries comprise 10–13% of civilian improvised explosive device (IED) injuries. The bomb blast wave induces a normal and shear forces on the tissues, causing a large acute IOP elevation. This study calculated the biomechanical stresses and strains in the eye due to IED explosion via eye-specific fluid-structure interaction (FSI) models. MethodsBlast occurred at 2, 3, and 4 m from the front and side of the victim and the weights of the IED were 1 and 2 kg. The ground was covered with the deformable soil to mimic the realistic IED explosion condition and reflect the blast wave. ResultsThe IOP elevation of ∼6,000–48,000 mmHg was observed in the eyes while the highest IOP was occurred with the IED weight and distance of 2 kg and 2 m (front) and the lowest was occurred with the IED weight and distance of 1 kg and 4 m (side). Our findings suggest the importance of the victim location and orientation concerning the blast wave when it comes to ocular injury assessment. IOP elevation of ∼2900 and ∼2700 mmHg were observed in ∼1.6 ms after the blast for the IEDS weight of 2 kg and a victim distance of 2 m in front and side blasts, respectively, in consistence with the literature. Nonetheless, IOPs were considerably higher after ∼1.6 ms due to the merging of the bomb blast wave and its reflection off the ground. ConclusionsThe stresses and strains were highest for the frontal blast. Both side and frontal blasts caused higher stresses and strains at the rectus muscle insertions where the sclera is thinnest and prone to rupture. Blast angle has no considerable role in the resultant IOP. Front blast with a heavier IED resulted a higher stresses and deformations in the eye connective tissues compared to the side blast.

Factors affecting optic nerve head biomechanics in a rat model of glaucoma.

Glaucoma is the leading cause of irreversible blindness and is characterized by the death of retinal ganglion cells, which carry vision information from the retina to the brain. Although it is well accepted that biomechanics is an important part of the glaucomatous disease process, the mechanisms by which biomechanical insult, usually due to elevated intraocular pressure (IOP), leads to retinal ganglion cell death are not understood. Rat models of glaucoma afford an opportunity for learning more about these mechanisms, but the biomechanics of the rat optic nerve head (ONH), a primary region of damage in glaucoma, are only just beginning to be characterized. In a previous study, we built finite-element models with individual-specific rat ONH geometries. Here, we developed a parametrized model of the rat ONH and used it to perform a sensitivity study to determine the influence that six geometric parameters and 13 tissue material properties have on rat optic nerve biomechanical strains due to IOP elevation. Strain magnitudes and patterns in the parametrized model generally matched those from individual-specific models, suggesting that the parametrized model sufficiently approximated rat ONH anatomy. Similar to previous studies in human eyes, we found that scleral properties were highly influential: the six parameters with highest influence on optic nerve strains were optic nerve stiffness, IOP, scleral thickness, the degree of alignment of scleral collagen fibres, scleral ground substance stiffness and the scleral collagen fibre uncrimping coefficient. We conclude that a parametrized modelling strategy is an efficient approach that allows insight into rat ONH biomechanics. Further, scleral properties are important influences on rat ONH biomechanics, and additional efforts should be made to better characterize rat scleral collagen fibre organization.

Periodic Biomechanical Stresses and Strains at Neural Interfaces Modulate Mitochondrial and Metabolic Functionality

Periodic Biomechanical Stresses and Strains at Neural Interfaces Modulate Mitochondrial and Metabolic Functionality

The daily grind: Sex- and age-related activity patterns inferred from cross-sectional geometry of long bones in a pre-Columbian muisca population from Tibanica, Colombia.

Daily activities involve biomechanical strains acting on skeletal structures. This study identifies differences in activity patterns between males and females, and between young, middle, and older aged individuals within an excavated Muisca skeletal sample from the Eastern Andes region of Northern South America. The Tibanica archaeological site (AD 1000-1400) is located at 2600 masl on the Sabana de Bogotá, Colombia. Cross-sectional geometric analysis of femurs from 63 individuals and paired-humerii from 33 individuals was used to examine bone size (TA), strength (J) and diaphyseal shape (Imax /Imin , Ix /Iy ). The findings indicate both age- and sex-related differences in activity patterns. An emphasis on upper body strength and robusticity was observed in the females, while males performed more strenuous work using their lower bodies, suggesting gender-based differences in labor. Men showed significant asymmetry in their humerii, with most showing right-hand dominance for upper body activities, while females showed high levels of humeral symmetry indicating similar levels of biomechanical stress for both arms. Female femoral diaphyseal shape changes with age, suggesting more mobility in youth and decreased mobility in middle and older ages. These results suggest that daily life may have been structured through patterns of routine labor that united and divided particular age and sex groups. Cross-sectional geometry data indicate women likely spent significant time and energy preparing food, especially grinding maize or other foods, while men may have done more long-distance walking potentially to work in agricultural fields or procure other resources.

Ulnar neuropathy and medial elbow pain in women's fastpitch softball pitchers: a report of 6 cases

Elite-level women's fastpitch softball players place substantial biomechanical strains on the elbow that can result in medial elbow pain and ulnar neuropathic symptoms. There is scant literature reporting the expected outcomes of the treatment of these injuries. This study examined the results of treatment in a series of these patients. We identified 6 female softball pitchers (4 high school and 2 collegiate) with medial elbow pain and ulnar neuropathic symptoms. Trials of conservative care failed in all 6, and they underwent surgical treatment with subcutaneous ulnar nerve transposition. These patients were subsequently monitored postoperatively to determine outcome. All 6 female pitchers had early resolution of elbow pain and neuropathic symptoms after surgical treatment. Long-term follow-up demonstrated that 1 patient quit playing softball because of other injuries but no longer reported elbow pain or paresthesias. One player was able to return to pitching at the high school level but had recurrent forearm pain and neuritis 1 year later while playing a different sport and subsequently stopped playing competitive sports. Four patients continued to play at the collegiate level without further symptoms. Medial elbow pain in women's softball pitchers caused by ulnar neuropathy can be treated effectively with subcutaneous ulnar nerve transposition if nonsurgical options fail. Further study is necessary to examine the role of overuse, proper training techniques, and whether pitching limits may be necessary to avoid these injuries.

Resistive Hysteresis of Carbon Nanofiber Doped Elastomeric Nanocomposite Polymer

Micropatternable flexible elastomeric nanocomposites find application in sensing biomechanical strains and biopotential signals [1-5]. While many polymers have been used to fabricate flexible MEMS polydimethylsiloxane (PDMS), a silicone based elastomer, has been widely used because of its biocompatibility, low cost, low toxicity, high oxidative and thermal stability, optical transparent, low permeability to water, low electrical conductivity, and ease of micropatterning [6-15]. Previously, we had demonstrated fabrication of various PDMS based micropatternable nanocomposite polymers which are either electrically conductive and magnetic in nature [16-17]. We have developed improved electrically and thermally conducuctive micropatternable PDMS based nanocomposite polymer containg milled carbon fibers, prepared by ultrasonically assisted processing technology. The prepared nanocomposite microstructures show interesting resistance hysteresis loop. This paper also discusses the underlying mechanisims that are responsible for resistance hysteresis loop. Acknowledgements: Authors would like to thank Nippon Graphite Fiber Co., Ltd, Japan; for proving milled carbon fiber and technical support for this project.

Equine odontoclastic tooth resorption and hypercementosis

SummaryEquine odontoclastic tooth resorption and hypercementosis (EOTRH) is a progressive and painful disease syndrome that affects aged horses. It affects incisor, canine and infrequently cheek teeth, and is characterised by gingival inflammation, oedema and recession, calculus deposition, feed accumulation, subgingival swellings with or without associated draining tracts, bulbous enlargement of teeth, tooth mobility, associated periodontal disease, tooth fracture and/or tooth loss. The aetiology of EOTRH is likely to be multi‐factorial. However, histopathological findings consistently suggest an aetiological contribution of initial biomechanical stresses and strains, followed by secondary involvement of micro‐organisms. Diagnosis is based on clinical presentation, age and radiographic changes including lytic changes, widening or loss of the periodontal ligament, hypercementosis, tooth fracture and alveolar bone loss. Treatment of EOTRH is not currently possible, but the condition can be managed to some degree, quite successfully for many years in some cases. However, the disease is progressive and invariably results in extraction of the tooth or teeth involved. The prognosis is most probably guarded at best and poor in most cases; however, complete removal of affected teeth carries a good prognosis for an improved quality of life in the short to medium term.

Effects of Peripapillary Scleral Stiffening on the Deformation of the Lamina Cribrosa.

PurposeScleral stiffening has been proposed as a treatment for glaucoma to protect the lamina cribrosa (LC) from excessive intraocular pressure–induced deformation. Here we experimentally evaluated the effects of moderate stiffening of the peripapillary sclera on the deformation of the LC.MethodsAn annular sponge, saturated with 1.25% glutaraldehyde, was applied to the external surface of the peripapillary sclera for 5 minutes to stiffen the sclera. Tissue deformation was quantified in two groups of porcine eyes, using digital image correlation (DIC) or computed tomography imaging and digital volume correlation (DVC). In group A (n = 14), eyes were subjected to inflation testing before and after scleral stiffening. Digital image correlation was used to measure scleral deformation and quantify the magnitude of scleral stiffening. In group B (n = 5), the optic nerve head region was imaged using synchrotron radiation phase-contrast microcomputed tomography (PC μCT) at an isotropic spatial resolution of 3.2 μm. Digital volume correlation was used to compute the full-field three-dimensional deformation within the LC and evaluate the effects of peripapillary scleral cross-linking on LC biomechanics.ResultsOn average, scleral treatment with glutaraldehyde caused a 34 ± 14% stiffening of the peripapillary sclera measured at 17 mm Hg and a 47 ± 12% decrease in the maximum tensile strain in the LC measured at 15 mm Hg. The reduction in LC strains was not due to cross-linking of the LC.ConclusionsPeripapillary scleral stiffening is effective at reducing the magnitude of biomechanical strains within the LC. Its potential and future utilization in glaucoma axonal neuroprotection requires further investigation.

An MRI-based leg model used to simulate biomechanical phenomena during cuff algometry: a finite element study

Cuff pressure stimulation is applicable for assessing deep-tissue pain sensitivity by exciting a variety of deep-tissue nociceptors. In this study, the relative transfer of biomechanical stresses and strains from the cuff via the skin to the muscle and the somatic tissue layers around bones were investigated. Cuff pressure was applied on the lower leg at three different stimulation intensities (mild pressure to pain). Three-dimensional finite element models including bones and three different layers of deep tissues were developed based on magnetic resonance images (MRI). The skin indentation maps at mild pressure, pain threshold, and intense painful stimulations were extracted from MRI and applied to the model. The mean stress under the cuff position around tibia was 4.6, 4.9 and around fibula 14.8, 16.4 times greater than mean stress of muscle surface in the same section at pain threshold and intense painful stimulations, respectively. At the same stimulation intensities, the mean strains around tibia were 36.4, 42.3% and around fibula 32.9, 35.0%, respectively, of mean strain on the muscle surface. Assuming strain as the ideal stimulus for nociceptors the results suggest that cuff algometry is less capable to challenge the nociceptors of tissues around bones as compared to more superficially located muscles.

Evaluating both Physiological and Biomechanical Strains in Women Using Different Hoe Handle Designs

Physiological and biomechanical strains were investigated in women using four different handle designs for hoe. Electromyography (EMG), energy expenditure, and task workload were measured while using four different hoes: a conventional hoe with 1) the normal (N) handle, 2) the second (S) handle, 3) the T-grip with arm supporter (TA) handle, and 4) the second with T-grip and arm supporter (STA) handle. Each handle strained different regions of the body. The S handle was significantly better for minimizing the erector spinae muscle EMG. The TA handle required less effort on the flexor digitorum profundus (FDP), but more effort on the posterior deltoid. From the STA handle, no interaction was found between the S and TA handle. The results from energy expenditure analysis indicated that the workload in hoeing involves mainly a pushing and pulling motion of the shoulder (r = .923). In comparison with the N handle, the additional handles caused a significantly greater workload, but there was no significant difference between those three handles.

Morphology, Morphometry and Spatial Distribution of Secondary Osteons in Equine Femur.

A high number of differences exist in bone histological features depending on the species, breed, age and bone. Moreover, osteon distribution may vary in the different sides of a bone as a consequence of different biomechanical strains. The aim of this work was to study the distribution and morphology of osteons in different sides of the equine femoral diaphysis with the attempt to correlate them to the main strains operating on them. The following parameters of secondary osteons and Haversian canals were measured in the transverse sections of diaphyses: perimeter, area, minimum and maximum diameter, eccentricity and osteon population density. A typical Haversian tissue was observed with elliptic secondary osteons consisting in about 10 well-defined lamellae surrounding a circular Haversian canal. Quantitative analysis displays a different population density of secondary osteons depending on the side. The caudal and medial sides, where compression strains are higher, have more secondary osteons in comparison with the cranial and lateral sides, where tension strains are prevalent. These data suggest that secondary osteon population density may depend on the predominant strains. Even the elliptical shape of secondary osteons may be related to biomechanical strains, as their major axes are oriented cranio-caudally parallel to prevalent strains.

A biphasic multiscale study of the mechanical microenvironment of chondrocytes within articular cartilage under unconfined compression

Computational analyses have been used to study the biomechanical microenvironment of the chondrocyte that cannot be assessed by in vitro experimental studies; yet all computational studies thus far have focused on the effect of zonal location (superficial, middle, and deep) on the mechanical microenvironment of chondrocytes. The aim of this paper was to study the effect of both zonal and radial locations on the biomechanical microenvironment of chondrocytes in inhomogeneous cartilage under unconfined stress relaxation. A biphasic multiscale approach was employed and nine chondrocytes in different locations were studied. Hyperelastic biphasic theory and depth-dependent aggregate modulus and permeability of articular cartilage were included in the models. It was found that both zonal and radial locations affected the biomechanical stresses and strains of the chondrocytes. Chondrocytes in the mid-radial location had increased volume during the early stage of the loading process. Maximum principal shear stress at the interface between the chondrocyte and the extracellular matrix (ECM) increased with depth, yet that at the ECM–pericellular matrix (PCM) interface had an inverse trend. Fluid pressure decreased with depth, while the fluid pressure difference between the top and bottom boundaries of the microscale model increased with depth. Regardless of location, fluid was exchanged between the chondrocyte, PCM, and ECM. These findings suggested that even under simple compressive loading conditions, the biomechanical microenvironment of the chondrocytes, PCM and ECM was spatially dependent. The current study provides new insight on chondrocyte biomechanics.

Biomechanical and psychosocial occupational exposures: joint predictors of post-retirement functional health in the French GAZEL cohort.

Biomechanical and psychosocial occupational exposures are seldom considered simultaneously and over extended follow-up in occupational epidemiologic studies, although there is some evidence that combined exposures have interactive effects on workers' health during working life. Given high prevalence of functional disability among retirees, research on earlier-life determinants of subsequent functional outcomes can help shape workplace policies and practices. This study investigates whether health effects of combined occupational exposures during working life are observed after individuals retire and are no longer exposed. Analyses were conducted among retirees in the French GAZEL occupational cohort (n=9168). Cumulative exposure during working life to eight biomechanical strains and to one or more reports of psychosocial job strain (high-demand, low-control work) were assessed as predictors of three outcomes: difficulty with physical functioning, role limitations due to physical difficulties, and bodily pain. Individuals were classified by joint exposure to both biomechanical and psychosocial constraints. We modeled risk ratios (RR) between exposure to biomechanical and psychosocial factors at work (separately and in combination) and disability after retirement, and we calculated the relative excess risk due to interaction (RERI) to test whether combined effects departed from additivity. Both psychosocial and biomechanical exposures during working life were independent predictors of the three functional health outcomes. Compared with individuals who had neither biomechanical nor psychosocial exposures, in fully adjusted log-binomial models of the combined effects of biomechanical and psychosocial exposure, among those with low biomechanical exposures, the RR for physical functioning difficulties associated with psychosocial exposures was 1.18 (95% CI 1.01, 1.37). Among those with the highest levels of biomechanical exposures, RR was 1.42 (95% CI 1.21, 1.65) among those with low psychosocial exposures and 1.91 (95% CI 1.61, 2.26) among those with high psychosocial exposures. The two exposure types were modestly super-additive, with an RERI of 0.32 (95% CI 0.00, 0.62) between those with the highest and lowest levels of biomechanical exposures; if the effects were strictly additive, we would have expected an RERI of 0. For the other two outcomes results were similar, although there was no significant departure from additivity. Some effects varied in magnitude by gender. Across the sample, combined biomechanical and psychosocial occupational exposures during working life appear to have additive or perhaps interactive effects on functional health in retirement. However, the relationship is less straightforward among women. Improving overall working conditions may lessen future disability among retirees.

Navicular syndrome in sport horses as a result of the disorder of biological bone tissue turnover rhythm: a review

This review summarizes the macroanatomical changes affecting the navicular bone in terms of mechanisms of bone turnover. Many researchers associate the disorders described in this review with limb overloads occurring periodically during training. As a result of intensified and repetitive biomechanical strains, accumulation of microcracks occurs in the bone tissue. It can disrupt the natural rhythm of tissue regeneration. The rhythm comprises a short resorption stage as well as bone formation stage, which lasts several months in horses. The article also dwells on different forms of macroscopic lesions of the navicular bone, which may occur in the uncoupled Basic Multicellular Unit. They lead to the navicular syndrome, which is irreversible and most often eliminates the horse from sport competitions.

In Vitro Performance of Class I and II Composite Restorations: A Literature review on Nondestructive Laboratory Trials—Part II

A literature review was conducted on adhesive Class I and II restorations and nondestructive in vitro tests using the PubMed/Medline database for the 1995-2010 period. The first part of this review has presented and critically appraised selected literature dealing with the quality and in vitro behavior of adhesive Class II restorations using photoelasticity, finite element analysis, and microleakage study protocols. This second part reviews additional parameters, which are deformation and fracture resistance to cyclic loading, shrinkage stress and tooth deformation following restoration placement, bond strength (microtensile, tensile, and shear tests), and marginal and internal adaptation. In addition, a "relevance score" has been proposed that aims to classify the different study protocols according, firstly, to the resulting quality, quantity, and consistency of the evidence and then, secondly, to their potential clinical relevance, as estimated by their ability to simulate oral and biomechanical strains. The highest clinical relevance was attributed to marginal and internal adaptation studies, following cyclic loading in a moist environement. However, a combination of in vitro protocols will have an even greater predictive potential and has to be considered as a crucial preclinical research approach with which to investigate the numerous restorative configurations that cannot be efficiently and rapidly tested in vivo.

Mechanical fatigue and fracture of Nitinol

Nitinol, a near equiatomic intermetallic of nickel and titanium, is the most widely known and used shape memory alloy. Owing to its capacity to undergo a thermal or stress induced martensitic phase transformation, Nitinol displays recoverable strains that are more than an order of magnitude greater than in traditional alloys, specifically as high as 10%. Since its discovery in the 1960s, Nitinol has been used for its shape memory properties for couplings and actuators, although its contemporary use has been in for medical devices. For these applications, the stress induced transformation (‘superelasticity’) has been used extensively for self-expanding implantable devices such as endovascular stents and vena cava filters, and for tools such as endodontic files. Most of these applications involve cyclically varying biomechanical stresses or strains that drive the need to fully understand the fatigue and fracture resistance of this alloy. Here we review the existing knowledge base on the fatigue of Nitinol, both in terms of their stress or strain life (total life) and damage tolerant (crack propagation) behaviour, together with their fracture toughness properties. We further discuss the application of such data to the fatigue design and life prediction methodologies for Nitinol implant devices used in the medical industry.