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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The aim of this study was to compare the resistance to deflection fatigue of a gold alloy used in porcelain-fused-to-metal restorations with and without a brazing joint. Pre-ceramic brazing filler metal was used to join the parent specimens of gold alloy together. The deflection fatigue test was carried out mainly with 0.4 m deflection of the test specimens (n = 5) but to obtain an S-N curve for the specimens, other magnitudes of deflection, i.e. the stress, were also used. When the fracture surface of the test specimens was examined by scanning electron microscopy (SEM), the results showed that the brazing joint in the gold alloy test specimen decreases the fatigue resistance considerably compared to that of specimens without a brazing joint (P = 0.002). SEM examination showed that the failure type of the brazing joint was cohesive and that the brazing filler metal had a more porous structure than the parent gold alloy. These results suggest that, due to the occlusal biting forces in situ, the brazing joints in fixed partial dentures can be fractured by metal fatigue.
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PMID:Brazing joints of gold alloy used in porcelain-fused-to-metal restorations and their resistance to deflection fatigue. 921 89

The authors discuss the significance of interbody fusion for early and long term stability of the lumbar spine. They stress that the aim of the modern spinal instrumentation is to promote bone healing and not to replace fusion. Without fusion every stabilization device will fail in fatigue. The biomechanics of different types of spinal fusion and biomechanical conditions at fusion site are discussed. A history of lumbar interbody fusion including threaded implants (cages) is presented. Interbody cages combine positive properties of tricortical bone graft: the strength of cortical to the bone with improved incorporation properties of cancellous bone. In contrast bone graft their biomechanical performance is far better: they increase strength and stability of osteosynthesis, do not go collapse and resorption, prevent from decrease in disc space height and kyphotic angulation of the fused motion segment, require less bone to achieve fusion. They can be used solely without support of any stabilization system. Preliminary experience in fusion with interbody threaded implants suggest significant efficacy of this method.
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PMID:[Lumbar interbody fusion. Biomechanical significance for the spine]. 1046 37

In most studies performed on motor units in mammalian muscles the division of these units into fast and slow types has been based on the 'sag' visible in the profile of unfused tetanus. The time course of the sag in unfused tetani of fast motor units was analysed in the present study. Fast units of rat medial gastrocnemius muscle were classified as fast fatigable (FF) or fast resistant to fatigue (FR) on the basis of a fatigue index calculated during the standard fatigue test. In middle-fused tetani (fusion index 0.25-0.75), it was observed that for FF motor units the sag was shorter and occurred earlier than for FR units. Moreover, in FF units, the sag was followed by potentiating tension, whereas for FR units this potentiation was weaker or even absent. A tetanus shape index, which expressed the ratio of the area of the first part of the tetanus record (between the tension record and the baseline, from the beginning of tetanus up to the lowest point during the sag in the tension record) to the area under the second part of tetanus (from this lowest point up to the end of the record) was introduced. For FF units, this index ranged from 0.13 to 0.47, whereas for FR units it ranged from 0.54 to 17.8 (with one exception). These results showed that the difference in unfused tetanus expressed in this tetanus shape index could be used as an accurate alternative method of dividing fast units into FF and FR groups. Moreover, the difference in sag time course in FF and FR groups. Moreover, the difference in sag time course in FF and FR units suggests that the metabolism responsible for this contractile phenomenon is significantly different time courses in IIa and IIb muscle fibres, constituting FF and FR units, respectively.
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PMID:Differences in the profile of unfused tetani of fast motor units with respect to their resistance to fatigue in the rat medial gastrocnemius muscle. 1067 16

The tension-time area is an estimation of the work performed by contracting motor units. The relationship between tension and frequency of stimulation and between tension-time area and frequency have been studied on 148 single motor units of the rat medial gastrocnemius muscle, under isometric conditions. Motor units were classified as fast fatigable (FF), fast resistant to fatigue (FR) or slow (S). Trains of stimuli of increasing frequency and constant duration were used. For all motor units a half of the maximum tetanic tension corresponded to lower frequencies compared to frequencies at a half of the maximum tension-time area. Moreover, the slopes of tension-frequency and area-frequency curves (change of tension or area per 1 Hz rise in frequency) were higher for slow than for fast motor units. The tension-time area per one pulse was calculated for different frequencies of stimulation. For slow units the maximum area per pulse corresponded to significantly lower frequencies than for fast ones, especially of FF type. However, for all three types of motor units this optimal frequency corresponded to sub-fused tetani with a tension of about 75% of the maximum tension, and with the fusion index slightly over 0.90. The absolute values of the maximum tension-time area per pulse revealed that in one contraction within the tetanus, slow units are generating greater work than FR units. The work performed by FF units is nearly two times larger than for S units, although the tension of slow units is over eight times lower. The presented results reveal that the contraction of slow motor units is much more effective than was suggested based on their low tension.
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PMID:Relationship between the tension-time area and the frequency of stimulation in motor units of the rat medial gastrocnemius muscle. 1089 1

Sacral screw fixation is frequently used for fusion of the lower lumbar spine, but sacral screws appear to offer less secure fixation than lumbar pedicle screws, and failure due to loosening under fatigue loading is common. The aim of this study was to examine in vitro the stability of medial and lateral bicortical and unicortical sacral screw fixation under a physiologically relevant fatigue-loading pattern. Bone mineral density, screw insertion torque, and screw-fixation stiffness were measured prior to cyclic loading between 40 and 400 N compression at 2 Hz for 20,000 cycles. The screw-fixation stiffness was measured every 500 cycles, and the axial pullout strength of the screws was recorded following loading. All of the lateral insertions loosened under the applied loading, but some of the medial insertions remained stable. Medial insertions proved stiffer and stronger than lateral insertions, and bicortical fixations were stronger than unicortical fixations. Bone mineral density and insertion torque were correlated with screw stiffness and pullout strength, although better correlation was found for insertion torque than bone mineral density. Bone mineral density is a good preoperative indicator of sacral screw-fixation strength, and insertion torque is a good intraoperative indicator. An insertion torque greater than 1.5 Nm is suggested as an indicative value for a stable medial unicortical insertion, whereas an insertion torque greater than 2 Nm suggests a stable medial bicortical insertion. It appears that, apart from the choice of technique (screw orientation and depth), minimizing the load on the screws during the initial part of the fusion process is also critical to maintain stability of the fused section and to obtain a solid fusion mass.
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PMID:Loosening of sacral screw fixation under in vitro fatigue loading. 1111 4

Klippel-Feil syndrome is characterized by a variable degree of congenital fusion of the cervical spine. It can exist in association with other mesodermal deformities affecting several systems. Symptomatic cervical disc prolapse in the context of the syndrome is well documented in young adults. We present a case of a 5-year-old girl with the syndrome, who presented with mild motor developmental delay and cervical cord compression from a prolapsed C3/4 intervertebral disc, seen on MR scan. She also had posterior elements fused from C-2 to C-4. She improved after cervical discectomy. The presence of degenerative disc disease at such a young age and at a level mechanically protected by posterior element fusion indicates a congenital primary defect, rather than mechanical stress fatigue.
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PMID:Cervical disc prolapse in childhood associated with Klippel-Feil syndrome. 1121 27

In the present work, we investigate age-dependent changes in isometric endurance in response to repetitive stimulation in single intact fast- and slow-twitch muscle fibers from young and old mice. To examine this issue we performed in vitro experiments in manually dissected EDL and soleus muscle fibers. We examined the force generation capacity of fibers in response to two stimulation protocols characterized by different inter-tetanic intervals, named short (1-s) and long interval (3.65-s). Fatigability was measured according to the fatigue index (FI, ratio between the maximum tension recorded in the last over the first tetanus in a train of pulses), the time course of the FI and sag (gradual decrease in force during a partially fused tetanic contraction). Fibers were classified according to the FI using two different criteria previously used in the literature (first criterion: FI > or = 1, 075-099, 0.5-074 and < 0.5; second criterion: FI > or = 1, 0.75-0.99, 0.25-0.74 and < 0.25). The fatigue index distribution recorded in the population of fibers corresponding to EDL and soleus muscles from young and old mice studied with the short and long interval protocols was not statistically different. In summary, these results support the concept that the decline in mechanical performance with aging is not related with changes in fatigability of individual fast- or slow twitch muscles fibers.
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PMID:Age-dependent fatigue in single intact fast- and slow fibers from mouse EDL and soleus skeletal muscles. 1138 21

To examine changes in contractile properties and mechanisms of fatigue during submaximal nontetanic skeletal muscle activity, in situ perfused soleus (60-min protocol) and extensor digitorum longus (EDL; 10-min protocol) muscles of the rat were electrically stimulated intermittently at low frequency. The partly fused trains of contractions showed a two-phase change in appearance. During the first phase, relaxation slowed, one-half relaxation time increased, and maximal relaxation first derivative of force (dF/dt) decreased. Developed force during the trains was reduced and was closely related to the rate of relaxation in this first phase. During the second phase, relaxation became faster again, one-half relaxation time decreased, and force returned to resting levels between contractions in a train. In contrast, developed force remained reduced, so that peak force of the contractions was 51% (soleus) and 30% (EDL) of control. In the soleus muscle, the changes in contractile properties were not related to ATP, creatine phosphate, or lactate content. The changes in contractile properties fit best with a mechanism of fatigue involving changes in Ca(2+) handling by the sarcoplasmic reticulum.
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PMID:Muscle contractile properties during intermittent nontetanic stimulation in rat skeletal muscle. 1170 82

The aim of this study was to investigate mechanomyograms (MMGs) accompanying unfused tetani of fast fatigable (FF), fast fatigue-resistant (FR) and slow (S) motor units. Signals in the MMG were analyzed during tetanus, which was fused to a variable degree, evoked by electrical stimulation at three frequencies: 20, 40 and 80 Hz. Unfused tetani were characterized by an oscillating tension. Each oscillation in the tension of an unfused contraction was reflected by a parallel pressure wave signal in the MMG. The mean peak-to-peak amplitude of signals in the MMG, the amplitude of oscillation in the tension, the velocity of the tension increase and the fusion index were calculated for the unfused tetanic contraction. The increase in stimulation frequency resulted in an increase in the peak tension, an increase in the fusion of the tetanus, a decrease in the amplitude of force oscillation in the unfused contraction and a decrease in the peak-to-peak amplitude of signals in the MMG. Moreover, it was found that the MMG amplitude was correlated with the amplitude of the three analyzed properties of the unfused contraction. It is concluded that the amplitude of signals in MMGs depends mainly on the dynamic properties of the tetanic contraction, whereas the static component of the contraction (i.e., the level around which the tension oscillates) is not reflected in the MMG.
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PMID:Mechanomyographic signals generated during unfused tetani of single motor units in the rat medial gastrocnemius muscle. 1171 78

Fatigue and potentiation are two forms of force modulation. A general definition of fatigue is "a circumstance where less than the anticipated contractile response is obtained." Fatigue is associated with depressed Ca2+ release and possibly decreased Ca2+ sensitivity. Potentiation results from increased Ca2+ sensitivity due to regulatory light chain phosphorylation. Muscle fatigue and potentiation can coexist, making it difficult to quantify these processes. With repetitive 10Hz stimulation, the developed tension first increases, then decreases. Is fatigue present when developed tension first begins to decrease or when it falls below the developed tension of the first response? Intermittent incompletely fused tetanic contractions for which peak developed tension first decreases, then increases, is another unusual example of fatigue. A third example is when twitch contractions following a tetanic contraction decrease to a level below the pretetanic twitch amplitude, indicating that fatigue may have been coexistent with posttetanic potentiation. These observations illustrate the complexity of detecting fatigue, based on the simple, but commonly accepted definition presented above. Care must be taken in interpreting "before vs. after" contractile responses. Even when the contraction amplitude is greater than the initial response, there is no guarantee that mechanisms associated with fatigue are not present.
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PMID:What is fatigue? 1188 Jun 90


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